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CHAPTER 10 ELIMINATION REACTIONS

I. Overview! elimination reactions can occur by transfer of a group from the ,, carbon atom

of a covalent ligand and the group can transfer to the metal center or to an

incoming reagent

! hydrogen elimination processes are usually slower than hydrogeneliminations => observed only when hydride elininations are inhibited are

absent

! elimination reactions usually require reaction of an external reagent or aresimply an intramolecular C-H activation process

II. -Hydride Elimination from Metal-Alkyl Complexes! forms a hydride and a coordinated olefin from a single alkyl group requires an open coordination site at the metal prior to the C-H bond

cleavage step

occurs most readily when a syn coplanar arrangement of the metal andhydride groups can be achieved

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! hydrogen elimination from square planar Pd and Pt complexes usually occursby 2 pathways: one occurring after generation of an open coordination site cis to

the alkyl group and one that occurs from the four coordinate square planar

species

! hydrogen elimination from these complexes in the absence of added ligandoccurs mainly by a 3 coordinate complex (unless the complex contains PMe2Ph)

! hydrogen elimination from square planar metallacyclic complexes is slowerthan that from acyclic alkyl complexes since the former are less able to adopt a

syn coplanar geometry

metallacyclic complexes are thermally more stable than their acyclicanalogues and decompose by pathways that do not include hydride

eliminations

III. Factors Affecting the Rate of -Hydride Elimination

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! the less stable isomer can be formed because of the geometric requirement of the-hydrogen elimination step

! EWG slow the rate of -hydride elimination

! in general, an increase in steric hindrance decreases the rate of hydrideelimination because it disfavors the structure containing the syn coplanar

arrangement of the metal, -H, and the two linking carbons; in addition, the

generation of 2 ligands from 1 increases the coordination number and thus the

amount of steric hindrance

! in general, an increase in electron donation increases the rate of -H eliminationIV. -Hydride Elimination from Metal Alkoxides and Amides

! hydride eliminations from thiolate and phosphide complexes are rare becausethe C=S and C=P bonds are very weak

! occur by migratory deinsertion! ligand dissociation to generate a 14 electron intermediate is needed for both

hydrogen elimination processes to occur

! C-H bond cleavage is the RDS of hydride elimination from the amidocomplexes

! C-H bond cleavage can be reversible or irreversible during hydride eliminationfrom Ir-OR complexes depending on the concentration of added ligand that

displaces the ketone product

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V. -Hydrocarbyl Eliminations! fastest from highly electrophilic early transition metal centers! most alkyl eliminations from late transition metals involve the cleavage of a

strained ring

! formed by a combination of -H elimination and -alkyl elimination

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! the initial set of -alkyl elimination products undergo a series of transformationsto generate a complex set of products

! the Zr and Hf complexes react with similar rates and the differences in theenthalpic and entropic parameters offset each other

! cleavage of the C-C bond to the -carbon of the late metal-alkyl complexes isslower and less common that that of early d0metals

! require an open coordination site

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VI. -Alkyl and -Aryl Eliminations from Alkoxido and Amido Complexes! require open coordination sites

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! analogous oxametallacycles containing bidentate, rather than monodentate,ligands do not undergo -alkyl elimination

! occur via migratory deinsertions

VII. -Halide and Alkoxide Elimination! chloride elimination is particularly fast from -chloroalkyl complexes of early

transition metals, Ni, Pd

! elimination of fluoride and alkoxide groups is also known when these groupsare present on the carbons of the alkyl ligands in hard d0metal alkyl complexes

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VIII. -Hydrogen Eliminations and Abstractions! generates a carbene and a hydride ligand from an alkyl ligand or a carbene ligand

and alkane from two alkyl ligands

! most common method for generating Schrock-type alkylidene complexes (noheteroatoms)

! usually slower than -hydride eliminations and are most common for complexesthat cannot undergo -hydride eliminations (neopentyl, benzyl, methyl,etc)

! occurs most often among early transition metals such as d0complexes of group 4and 6 metals since these can access the high oxidation state needed for the

formation of an alkylidene hydride product and the absence of d electrons does

not favor formation of a stable olefin complex resulting from -hydride

elimination

! steric hindrance also facilitates hydrogen eliminations since the M -C-C angleincreases as an alkyl group is converted to an alkylidene ligand

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relief of steric hindrance

! the immediate product of -H elimination usually isnt observed directly reductive elimination of a hydride and an alkyl group to form an alkane

frequently occurs

in other cases, a carbene forms from a dialkyl complex of a d 0metal thatcannot accommodate the additional valency required to form an alkylidene

and a hydride ligand; this pathway requires a 4-centered transition state

when the valency of the metal allows either pathway to occur, eliminationsand hydrogen abstractions are hard to distinguish

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CHAPTER 11 NUCLEOPHILIC ATTACK ON COORDINATED LIGANDS

I. Fundamental Principles! can be divided into 2 groups: nucleophilic attack at the metal center and

nucleophilic attack at a ligand coordinated to the metal

! coordination of an unsaturated molecule to a transition metal reverses itsreactivity from that of a weak nucleophile to that of an electrophile

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CO, olefins, dienes, arenes react with nucleophiles when coordinated tometals due to the flow of electron density from the ligand to the metal

! unsaturated ligands of more electron deficient metals are more reactive towardsnucleophilic attack than are unsaturated ligands of more electron rich metals

! the presence of spectator -acceptor spectator ligands on the metal facilitatenucleophilic attack by accommodating the increase in charge

! attack at the ligand is generally favored for complexes that are coordinativelysaturated or cannot accommodate an additional ligand for steric or electronic

reasons

! nucleophilic addition to coordinated ligands does not change the total number ofelectrons but may or may not change the oxidation state (no change for attack at

neutral ligands, reduction for attack at anionic ligands)

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II. Nucleophilic Attack on CO and RNC Complexes

! stable products! many relatively weak nucleophiles add to coordinated CO and RNC! RNC ligands produce stable metal-carbene complexes but require stronger

nucleophiles due to its lower electrophilicity

! most occur by direct nucleophilic attack on the coordinated ligand without priorcoordination of the nucleophile to the metal

! the susceptibility of M-CO ligands to nucleophilic is inversely related to theamount of backbonding

rate of nucleophilic attack: [M-CO]+>[M-CO] > [M-CO]- rate decreased by EDG

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! organic halides and tosylates tend to react at the metal center while harder andmore reactive alkylating reagents react at O to generate carbene complexes

! the hydroxycarbonyl complexes generated from attack by OH-are unstable andextrude CO2to produce metal hydrides ; when the anionic hydride is acidic

enough to be deprotonated by the hydroxide, a 2 electron reduction of the metal

occurs

III. Nucleophilic Attack on Carbene and Carbyne Complexes

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! cationic carbene complexes undergo simple addition processes to generateneutral products while neutral complexes react either by addition or by addition +

eliminations

! nucleophilic additions to carbenes are restricted to cationic Fischer carbenecomplexes or Fischer carbene complexes that possess ancillary ligands that

stabilize negative charges

high valent, early metal Schrock type carbenes are usually nucleophilic at thecarbene carbon and thus react with Lewis bases and nucleophiles

IV. Nucleophilic Cleavage of M-C Bonds! nucleophilic attack on bound alkyl ligands is slower than attack on unsaturated

ligands bound through a single atom or on unsaturated ligands bound through

multiple atoms

lack of low-lying LUMO at the alkyl ligand and the partial negative chargethat typically lies at carbon bound to a transition metal

! reaction rate depends on the amount of electron density at or the electrophilicityof the alkyl carbon and on the stability of the metal fragment that would serve as

the leaving group

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! attack of a nucleophile on an alkyl group is most common when the alkyl groupis bound to an electron poor and high valent metal center

occur by mechanisms similar to SN2 sensitive to solvent polarity, steric effects, and produces an inversion of

configuration

! metal benzyl complexes also react with nucleophiles with an inversion ofconfiguration

! metal acyl complexes react with nucleophiles to produce carboxylic acidderivatives

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the fastest reactions occur with acyl groups bound to transition metals thatare electrophilic and that are good leaving groups

! allyl groups undergo more rapid nucleophilic attack than any of the -bondedgroups 1allyl groups are much less reactive towards nucleophiles than 3allyl

groups

V. Nucleophilic Attack on 2Unsaturated Hydrocarbon Ligands! coordination of an olefin to an electron poor metal causes the coordinated olefin

to react with nucleophiles because coordination leads to a net flow of electron

density from the unsaturated hydrocarbon to the metal

! nucleophilic attack occurs most readily on unsaturated hydrocarbons bound tometal centers in relatively high oxidation states possessing a positive charge or

EW ancillary ligands

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! an increase in reactivity of unsaturated hydrocarbons with electrophiles isobserved upon coordination to electron rich metal centers

their olefin complexes are best described as metallacyclopropane complexes large degree of M-C bond character react with electrophiles

! the Davies-Green-Mingos guidelines predict the most kinetically favorableposition for nucleophilic attack on coordinated, unsaturated hydrocarbons in 18e

cationic complexes

even or odd according to n open or closed according to acyclic or cyclic nucleophilic attack on open polyenes tends to occur at a terminal carbon

atom

VI. Nucleophilic Attack on 2Olefin Complexes! usually, nucleophilic attack occurs onto the face of the olefin that is opposite the

metal without coordination of nucleophile to the metal prior to attack

! symmetric metal olefin complexes are deactivated towards nucleophilic attackbut slippage of the olefin creates a LUMO that can interact with the incoming

nucleophile

! nucleophilic attack occurs mostly at the more substituted terminus of substitutedolefins since these carbons bear more positive charge

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! limited to mono or disubstituted olefins since more highly substituted olefinscoordinate only weakly to metals

displacement of the more substituted olefins from the metal by thenucleophile is a major competing reactions (esp. if the nucleophile is also a

good ligand for the metal)

VII. Nucleophilic Attack on Square Planar Pd(II) Diene and Allene Complexes! dienes can bind to metals in a 2or 4fashion

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VIII. Nucleophilic Attack on 2-Alkyene Complexes! produces -vinyl complexes! there are fewer stable alkyne complexes of higher oxidation state or cationic

metals than olefin complexes since they are most susceptible to nucleophilic

attack

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! the trans stereochemistry of the -vinyl group suggests that nucleophilic attackoccurs external to the metal and does not involve prior coordination of MeOH

trans attack of the nucleophile on the coordinated acetyleneIX. Reactions of 2-Arene Complexes

! 2 coordination leads to de-aromatization of the arene, thus isolating the dieneunit for reactions

the chemistry of 2-arene complexes resemble those of dienes

X. Nucleophilic Attack on Imine and Aldehyde Complexes! coordination of imines and aldehydes to Lewis acids enhances their reactivity

with nucleophiles

! rearrangement of the 2-aldehyde to the 1form precedes attack of CN-at thealdehydic carbon

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! hydride additions occur with high stereoselectivity for addition to one face of thealdehyde over the other to form an amido complex as the major product;

deprotonation of the N-H hydrogen produces an iminyl or azavinylidene as the

minor product

XI. Nucleophilic Attack on Polyhapto (3- 6) Ligands! usually occurs at the face of the allyl ligand opposite the metal to produce a

neutral 2olefin complex in which the metal has been reduced by 2 electrons

! reaction occurs most readily for electron poor allyl complexes! the ligands trans to the two termini of an allyl group affect the rate of attack attack often occurs trans to the ligand that is more donating or has a higher

trans influence

lengthening of the M-C bond trans to this ligand and a greater positive chargeof the carbon more weakly bound to the metal

! in accord with the Davies-Green-Mingos rule, attack occurs at the terminalcarbon of the allyl group and occurs at the open allyl group rather than at the

closed 5Cp group

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! even though products are usually formed from attack at the less substitutedposition, the regiochemistry can be directed to the more substituted position by

electronic effects on the allyl group imparted by steric properties, bite angle,

donor atoms and ancillary ligands

! complexes generated from electron accepting ligands form the most branchedproduct

! nucleophilic addition to the central carbon is also favored by the presence ofsignificant positive charge at this site

XII. Nucleophilic Attack on 4Diene Complexes! cationic 4diene complexes should be among the most reactive substrates

towards nucleophilic attack and should undergo preferential attack at a terminal

position according to the DGM rule

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XIII. Nucleophilic Attack on 5Dienyl Complexes! reactions of electrophiles at Cp ligands are more common! reaction occurs exclusively at the terminal carbon to generate a new 4diene

complex

! attack occurs exclusively from the face opposite the metal and exclusively at theterminal position of the dienyl system

XIV. Nucleophilic Attack on 6Arene and Cycloheptatrienyl Complexes! coordination to EW metals makes the arene more electron poor and converts it

from a nucleophile to an electrophile

coordination lowers the pKa for deprotonation of both arene and benzylichydrogens

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coordination makes unsaturated groups conjugated with the arene moreelectrophilic

benzylic carbocations and benzylic radicals of coordinated arenes are morestable than benzylic carbocations and radicals of free arenes

allows nucleophilic attack to occur on the arene unit

! the steric effect of the metal fragment leads to attack on the side of the areneopposite the metal

can be used to control the relative stereochemistry of the substituents in theproduct

the position of attack is controlled by the electronic properties of thesubstitutents (meta vs para)

steric effects discourage ortho attack

! unstabilized nucleophiles like organolithium reagents tend to add irreversiblywhile stabilized ones like malonates tend to add reversibly

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! as predicted by the DMG rule, nucleophilic attack on cationic metal complexesoccurs on the complexed arene, not on the cycloheptadienyl ligand

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CHAPTER 14 PRINCIPLES OF CATALYSIS

! a catalyst is a substance that increases the rate of a chemical reaction withoutitself being consumed

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it is eventually regenerated after a series of transformations and can thus beused in substoichiometric amounts

! catalysts reduce the free energy of the highest energy transition states and therebyincrease the rate of reaction

they dont change the energies of the reactants and products they only change the reaction rates, not the thermodynamics or equilibrium

constants

! catalysts decrease the activation energy relative to that of the uncatalyzedreaction by stabilizing the transition state

binds one or more of the reactants and remains bound through the transitionstate; dissociation of the product either regenerates the starting catalyst

directly or generate a species that will be converted to the starting catalyst

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! catalysts can also lower the energy of the transition state by creating a completelynew reaction pathway

! in catalyzed reactions, the reaction typically occurs by more steps but theactivation energy of each of the individual steps is lower than the activation

energy of the uncatalyzed process => lower overall

! a catalyst precursor is a complex or mixture of compounds that gives rise to acomplex that lies on the catalytic reaction pathway; drawn off the catalytic cycle

because it leads to the true catalyst

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! irreversible decomposition of the catalyst is drawn in a step external to thecatalytic cycle

! reactions outside the cycle can retard the catalytic process by siphoning thecatalyst away from the productive steps; often, dissociation of dative ligands

must occur for stable complexes to enter the catalytic cycle

! cocatalysts or promoters are additional reaction components that are added insmall amounts to enhance the rates and selectivities of catalytic reactions

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! the efficiency of a catalytic reaction can be quantified by the TON (turnovernumber) and TOF (turnover number)

the TON is the number of moles of product generated per mole of catalyst the TOF is the TON per unit time

! the rate constants of different steps of a catalytic cycle are different, but the netrates of each step of a catalytic process are identical once the system has reached

steady state

the rate of each step is then proportional to the rate constant and theconcentrations of reagents and species that lie on the catalytic cycle

! the step with the smallest pseudo first order rate constant is the turnover limitingstep and controls the efficiency of the catalytic process since the rate cannot

exceed k[cat][reagent]

! a homogeneous catalyst in the same phase as the reactants => discrete, molecularspecies

! heterogeneous catalysts are in a different phase as the reagents => solidHOMOGENEOUS CATALYSTS HETEROGENEOUS CATALYSTS

rapid rates for the diffusion of reagents andheat in the solution phase

low cost, easy separation from products, highstability, recyclability

ability to initiate reactions with discretecompounds that can be characterized by

solution phase methods

often catalyze oxidation and reductionreactions

ability to control a 3D architecture created by

ligand geometries => higher regioselectivity,diastereoselectivity, enantioselectivity

higher thermal stability

easy separation

more reactive slow rates because of low effectiveconcentration

difficult separation lack of precise 3D architecture andnonuniformity in the local structures and

properties of catalytic sites

inability to regenerate decomposed catalyst low selectivity

low thermal stabilitypotential catalyst contamination

! it is hard to definitively distinguish homogeneous catalysts from heterogeneousones

! light scattering and TEM can be used to identify the presence of nanoclusters insolution that appear to be homogeneous

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! elemental Hg has been shown to poison heterogeneous catalysts by blockingpores of high surface area solids

reactions catalyzed by heterogeneous catalysts are typically retarded byadding Hg whereas those catalyzed by homogeneous catalysts are unaffected

suggestive but not definitive! dibenzo[a,e]cyclooctetraene binds to soluble complexes to poison their catalytic

activity but does not bind as well to metal surfaces

reactions catalyzed by homogeneous catalysts are unaffected by this additivewhile those catalyzed by heterogeneous catalysts are

suggestive, but not definitive

! kinetic measurements provide the most definitive data to distinguish betweenhomogeneous and heterogeneous catalysts when the particulate matter can be

observed visually via light scattering or TEM

if the observed solid or nanoclusters catalyze the reaction, then the reactionwill occur with a sigmoidal profile in which the rate increases as the solid or

nanoclusters form

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! there is no correlation between reaction at the Si and Re face and the (S) and (R)configurations or between reaction at a pro-(S) and a pro-(R) group and the (S)

and (R) configuration

! stereoselectivity can be quantified with optical rotation, %ee, er

! the reaction of a prochiral substrate with achiral reagents in the presence of anasymmetric catalyst occurs preferentially at a prochiral face, at one of two

enantiotopic groups or with one of two enantiomers in a racemic mixture

! the reaction of a chiral catalyst with a prochiral substrate gives rise todiastereomeric transition states

different spatial relationships between the substrate, reagent, and catalystresult in diastereomeric transition states that have unequal energies

! small changes in activation energies results in dramatic differences in %ee 1.38kcal/mol difference gives rise to a 10:1 ratio of products

! the enantioselectivity determining step controls the enantioselectivity and occursin the first irreversible step that takes place through diastereomeric transition

states

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! asymmetric induction can occur when a chiral catalyst reacts directly with aprochiral substrate without substrate coordination to the catalyst prior to the

enantioselectivity determining step

2 diastereomeric steps

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! when prochiral substrates bind to chiral catalysts in step separate from theenantioselectivity determining step, diastereomeric complexes possessing

different energies result

! Curtin Hammett conditions: when interconversion of diastereomeric complexesis significantly faster than their reaction to form product, the enantioselectivity

determining step is the reaction to form product

! Curtin Hammett principle: when competing reaction pathways begin fromrapidly interconverting isomers, the product ratio is determined by the relative

heights of the highest energy barriers leading to the 2 different products

enantioselectivity is controlled by the relative energies of the twodiastereomeric transition states rather than by the stabilities of the two

diastereomeric intermediates

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! asymmetry is transmitted when chiral catalysts bind substrate and reactpreferentially with one of its prochiral faces or when it prevents reaction at one of

the prochiral faces by blocking it (steric biasing)

! catalysts containing C2symmetric ligands are most selective because of the smallnumber of metal substrate adducts and transition states available to them due to

their symmetry

! quadrant diagrams are models used to predict the facial stereoselectivity incatalyst substrate complexes and transition states subject to steric biasing

! in the quadrant model, the environment around the metal is divided intoquandrants in which the horizontal dividing line is congruent with a plane in the

catalyst; the shaded diagonal quadrants are spaces occupied by substituents on

the ligands that extend forward while unshaded areas correspond to less hindered

space => conformational preferences

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! the means by which metal complexes of chiral ligands block quadrants dependson the nature of the ligand and the M-L adduct

CHAPTER 15

! [Rh(PPh3)3Cl] is Wilkinsons catalyst and it catalyzes the cis hydrogenation ofalkenes

! mildly oxygen sensitive => some of the PPh3reacts with oxygen to becomePOPh3and the resulting catalyst becomes dinuclear [Rh(PPh3)2Cl]2with bridging

Cl, producing different rates, regioselectivities, and stereoselectivities

! steric effects affect olefin binding constants but when the olefin binds toostrongly, it can siphon the catalyst away from the cycle and change the

mechanism

! undergoes hydrogenation without isotopic scrambling

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! involves a dihydride intermediate in which both hydrides are transferred to thesame alkene and a migratory insertion => cis

! selective olefin reduction in the presence of esters, ketones, and nitroarenes

! cationic Rh complexes with the general formula [RhL2(H)2(S)2]+also catalyze thehydrogenation of olefins

! cationic Rh complexes with more ED ligands and bidendate ligands are moreactive

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! the active species is generated from [Rh(alkene)2(H)2]+by hydrogenation of thecoordinated alkene

! the cationic catalyst generates products with little olefin isomerization while theneutral catalyst produced isomerization products

! Crabtrees catalyst is [Ir(COD)py(PCy3)]+and is a precatalyst in which hydrogenreduces COD to form cationic Ir(III) hydride complexes ; unaffected by oxygen

and its activity relies on the use of relatively noncoordinating solvents

activity improved by using bulky, extremely weakly coordinating anions! it catalyzes the hydrogenation of hindered olefins in reduces them in the presence

of many other functional groups

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Crabtrees catalyst is much more reactive than the neutral, more hindered Rhand Ru catalysts towards more substituted alkenes ; cationic Ir complexes are

unusually reactive towards tetrasubstituted alkenes

! [Ru(PR3)n(H)(Cl)] catalysts selectively catalyze the hydrogenation of terminalalkenes

Typical Hydrogenation Mechanisms

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! Rh complexes containing phosphine ligands catalyze the hydrogenation ofolefins by mechanisms that involve dihydride intermediates

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! the accumulation of products outside the dotted enclosure reduces the rate of thecatalytic reaction

! as can be seen above, the overall hydrogenation reaction by Wilkinsons catalystcan be divided into: addition of hydrogen to RhClL3, the reaction of RhH2ClL3

with the olefin by migratory insertion, and reductive elimination of the reduced

product => hydrogen first pathway

CHAPTER 22 POLYMERIZATION AND OLIGOMERIZATION OF OLEFINS

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! the physical properties of polypropylene depend heavily on the relativeconfigurations of the methyl groups in the polymer chain

! isotactic => all syn, highly crystalline! syndiotactic => all anti, highly crystalline! atactic => random relative stereochemistry, amorphous

! olefins can join in a head to head or head to tail fashion! the stereochemistry of the polymer involves the relative configuration of diad

units and can be controlled by catalyst or the last inserted monomer

"#$%&'()*(&*

)()*(&*

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! ansa-metallocene systems (two indenyl groups linked by a bridge) are used toobtain highly stereoregular polymers ; the linker between the two indenyl groups

restricts ligand rotation and forces a C2or Csorientation of the indenyl groups

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! polymerization catalysts can be converted to oligomerization catalysts byreducing their steric properties

! olefin oligomerization by group 4 metal catalysts are among the most usedindustrially

! stronger electron donation increases the rate of -elimination versus olefininsertion and oligomers are formed instead of polymer

! olefin dimerization can occur by two general mechanisms, one involving olefininsertion and the other involving a metallacyclic intermediate

! the coupling of acrylates to form predominantly linear dimers occurs in thepresence of a variety of Pd, Ru, and Rh catalysts

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! for all catalysts, the polymerization and oligomerization of conjugated dienes arethought to proceed through (3-allyl) metal complexes as intermediates; these

intermediates can be formed by insertion of the diene into the M-H or M-R bond

! the equilibrium between the 3and 1forms depends strongly on the nature ofthe other ligands and is a major factor in determining the course of the reaction

! many complexes with low valent metals (esp. Ni(0) and Pd(0)) couple butadieneto form the corresponding bis-( 3-allyl) complex

! Ziegler type catalysts are most commonly used for the commercial production ofpolybutadiene and polyisoprene; the selectivity of these processes are strongly

metal-dependent; however, all involve (3-allyl)metal complexes as the active

species

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! the cyclotrimerization of butadiene to cyclododecatriene is also catalyzed by Ni(0)

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! the insertion can occur from an 1-allyl complex or from an 3-allyl complex the formation of 1,2 or 1,4-polybutadiene depends on whether migratory

insertion occurs at the terminal or internal carbon of the 3-allyl ligand

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CHAPTER 21 METATHESIS OF OLEFINS AND ALKYNES

! olefin metathesis reactions cleave C=C bonds or triple bonds and reassemblethem to generate products with new C=C bonds or triple bonds ; requires catalyst

and is largely controlled by thermodynamics

! there are six classes of olefin metathesis

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! Schrock catalysts contain Mo! Grubbs catalysts contain Ru

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! olefin metathesis occurs by a sequence of [2+2] cycloaddition and cycloreversionreactions

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! cyclic olefins possessing ring strain are used as monomers for olefin metathesisbecause the thermodynamics will favor the ring-opened materials

! the overall mechanism for ROMP is the reverse of RCMP

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- alkyne metathesis cleaves the C-C triple bonds in two alkynes to form two new alkynes;it is used to make conjugated polymers with interesting electronic properties and to

synthesize macrocycles with E or Z alkenes after reduction ; it is catalyzed by metal

alkylidenes

- the intermediate of alkyne metathesis involves a metallacyclobutadiene complex

-alkyne cross metathesis can be used to synthesize unsymmetrical alkynes

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- ring closing alkyne metathesis + reduction can be used to synthesize macrocyclic naturalproducts containing cis olefins

- enyne metathesis generates dienes from alkynes and alkenes by splitting the alkene inhalf and adding them across the alkyne ; high heterofunction tolerance when used with

Grubbs catalyst

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- enyne metathesis can be used with olefin metathesis to form bicyclic products

- the mechanism of enyne metathesis involves the formation of an alkylidene complex andthe subsequent formation of a metallocyclobutene complex

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CHAPTER 17 CATALYTIC CARBONYLATION

! all the group 9 and 10 metals, in combination with iodide, are active catalysts formethanol carbonylation

! 3rdrow transition metals are lesscommonly used in industrial

processes because they form stronger

M-L bonds, which decreases the rate

of migratory insertion of CO

! => promoters required! however, the stronger M-L bonds

also means that the rate of oxidative

addition is faster

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! hydroformylation is a metal catalyzed reaction in which an olefin, CO, and H2react to produce an aldehyde; also called the oxo process

addition of formaldehyde elements across the C=C bond! common side reactions include alkene hydrogenation, aldehyde hydrogenation,

and alkene isomerization

! to obtain desired product is n-butyraldehyde, the n:i (normal to iso or the l/b forlinear to branched ratio) ratio of aldehydes is maximized

! hydroformylations catalyzed by HCo(CO)4are typically run at high temperaturesand high pressures of a 1:1 mixture of CO:H2(syn gas)

! HCo(CO)4is exceptionally acidic, TBP 18e- complex that is synthesized by

reacting H2and Co2(CO)8

! high CO pressure is needed to prevent the formation of higher Co clusters and ofmetallic Co

! the rate of hydroformylation depends only on [CO]-1and [H2]

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increasing the pressure of the 1:1 syn gas mixture has little effect on the ratebut prevents catalyst decomposition

! the catalytic cycle involves a succession of 18e and 16e species

! hydroformylation of terminal alkenes catalyzed by HCo(CO)4in the absence ofany added ligands usually occurs to form a 3-4:1 n:i ratio

the regioselectivities and rates can be improved by adding phosphines the electron donating property of phosphine ligands increases the Co-CO

bond strength by increasing backbonding, allowing the reaction to be

conducted at lower CO pressures and higher temperatures without catalyst

decomposition

higher n:i ratios

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! however, the electron donating property of the added phosphine ligand alsoincreases the rate of hydrogenation of the initially produced aldehydes to form

alcohols

react much more slowly and produce hydrogenated alkene products assignificant side products

different catalytic species (no alkylcobalt complexes; only phosphinesubstituted cobalt carbonyl dimers and hydrides complexes)

! the formation of linear aldehydes from internal olefins occurs by insertion of theolefin into a Co-H to form a branched alkyl species, followed by isomerization of

the branched alkyl species to the linear alkyl

! the phosphine modified cobalt systems are good catalysts for alkeneisomerization, producing n:i ratios that are the same as those for reactions

initiated with terminal or internal alkenes

! in the absence of added phosphorus ligands, the Rh-catalyzed hydroformylationof alkenes is very fast but unselective (1:1 n:i ratio)

! Wilkinsons catalyst could be used as a catalyst precursor for hydroformylationat room temperature and atmospheric pressure and was highly selective for n-

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aldehydes (n:i >10); no alkene hydrogenation, alcohol formation or alkene

isomerization reactions were observed; the active catalyst is HRh(CO)2(PPh3)2

! water soluble Rh-triarylphosphine catalysts circumvent the problem of separatingand recycling the valuable Rh catalyst

the reaction is carried out in a biphasic system in which the catalyst resides inthe aqueous phase and the product resides in the organic phase and can be

removed from the catalyst without distillation

the catalyst is generated from the product of sulfonation of PPh3to produceP(C6H4-m-SO3

-Na+)3, which binds to Rh to generate a highly water soluble

catalyst

easy separation of catalyst and product without distillation at temperaturesthat might decompose the Rh catalyst

! the hydroformylation of geminally disubstituted alkenes has been shown to formlinear products with high selectivity

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! reactions of many olefins containing EWG at the C=C bond react to formbranched products since the branched alkyl complex is more stable when it bears

an EWG on the carbon

! asymmetric hydroformylation uses C2symmetric biphosphites! the atropisomeric ligand (S,R)-BINAPHOS generates a Rh complex that

catalyzes the hydroformylation of vinylarenes with high n:i selectivities and high

%ee

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! the copolymerization of CO and olefins forms polyketones

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CATALYSTS FOR ASYMMETRIC HYDROGENATION

I. Aromatic Bisphospines with Axial Chiral Backbones- the first enantioselective hydrogenations were achieved with P-chiral

bisphosphines

- the scope was increased with the use of aromatic phosphines with axial chiralbackbones

- the dihedral angle of the backbone of the ligand can influence enantioselectivity

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- the substituents on the P-bound aryl groups (esp. on the 3rdand 5thpositions) canincrease the enantioselectivity by enhancing gearing of these groups and limiting

the degrees of freedom

- the electronic properties of the backbone can affect selectivitiesII. Compounds with Chiral Ferrocenyl Backbones

- ferrocene units have planar chirality when they have 2 different substituents onthe same Cp ligand

- these are highly active and selective catalysts for many enantioselectiveprocesses; they do so without the C2symmetry of P-chiral ligands and BINAP

- these ligands contain a carbon stereocenter and a chiral ferrocene

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III. P-chiral phosphines

SHARPLESS EPOXIDATION

- an enantioselective reaction to prepare 2,3-epoxyalcohols from primary andsecondary allylic alcohols- the stereochemistry of the resulting epoxide is determined by the diastereomer of

the chiral tartrate diester used

- t-butyl hydroperoxide is used as the oxidizing agent- titanium tetra(isopropoxide) as catalyst- high %ee- drawback: requires allylic alcohol

diethyl tartrate

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- the structure of the catalyst is still uncertain- the putative catalyst was determined using X-ray techniques

- selectivity decreases with larger R1but increases with larger R2and R3- the product from 1,2-allylic diols cannot be predicted with this model- a given dialkyl tartrate will preferentially add to the same face independent of

substitution on the olefin

- the Sharpless epoxidation can also give kinetic resolution of a racemic mixture ofsecondary 2,3-epoxyalcohols

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Examples

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FLUXIONALITY

I. Ring Whizzing

- 3 1H NMR resonances are seen at low T (AABB multiplet for the 4 olefinic protons at6-6.5ppm, a singlet for the 5-Cp ring at 3-3.5ppm, and another for the C1 proton)

- only 2 singlets of almost equal intesntiy are seen at room T => ring whizzing of the 1-Cpring around Fe

- not because of dissociation + recombination- 1,2 or 1,3-shifts

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- the indenyl system would rearrange rapidly if a direct 1,3-shift occurs via a 3-allylintermediate or transition state

- a direct 1,2-shift would be suppressed in the indenyl system since it would have to gothrough a high energy o-xylene intermediate

- therefore, if the molecule is fluxional, it would do so via the 1,3 process; however, themolecule is nonfluxional, which means that the 1,2-shift is the pathway for the 1-Cp

rings fluxionality

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II. Interchange of 1-Cp and 5-Cp Rings

- at low T, 2 sharp singlets are observed at 5.9ppm and 5.2ppm; both broaden when T isincreased => the two 1-Cp rings undergo ring whizzing to give 1 peak and the 5-Cp

ring gives the other peak

- at high T, only 1 sharp singlet is observed => the 5-Cp and 1-Cp rings are exchanging- the molecule can undergo 5-Cp and 1-Cp ring exchange because it is coordinately

unsaturated (16e) => does not occur in 18e complexes

- the exchange is also observed when one of the ligands in the complex has the ability toshuttle between two structures having a difference of 2 electrons (such as NO)

III. Allyl Complexes- anti protons (1-3ppm) are closer to the metal => shielded and high field relative to the

syn (2-5ppm) protons; the proton at the middle is at 4-6.5ppm

- since the molecule is rigid, the 3 types of protons produce distinctive chemical shifts- the syn and anti protons do not couple with each other but couple with the proton on the

central carbon => the syn and anti protons appear as doublets while the central proton

appears at a multiplet

- upon raising the temperature, the two doublets collapse into one and the multipletbecomes a quintet => only 2 types of protons (4 terminal and 1 nonterminal hydrogen)

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IV. Allene Complexes- 3 signals : the lowest value is for the a and b protons while the highest is for the c proton- the coupling constant between a and b is smaller than in free allene- 4 signals may be observed if the symmetry is lower

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V. Scrambling of CO groups- the interconversion of permutational isomers and geometric isomers in mononuclear

complexes (example: Fe(CO)5)

- the interconversion of bridging and terminal arrangements and the migration andscrambling of CO groups in binuclear and polynuclear complexes => arises because the

energy of the M2(CO)2system does not vary much over an entire range of configurations

- 2 signals at low T (cis, trans) and 1 signal at R.T => interconversion between the cis andtrans forms at R.T

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- the cis/trans interconversion results in an overall exchange of bridging and terminal COgroups

- the bridging COs are inequivalent because one is cis to Cp and one is cis to CO; theterminal CO should appear as a doublet since it is coupled to one Rh; the bridging CO

should appear as a triplet since they are coupled to two Rh atoms

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WILKINSONS CATALYST

- classes of hydrogenation catalysts: those without M-H bond (initiated by OA of H2),those with M-H bond (not initiated by OA of H2), and f block/early transition metal

hydrides (no OA)

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- terminal alkynes are hydrogenated more rapidly than terminal alkenes- conjugated dienes are hydrogenated more slowly than isolated alkenes- internal and branched alkenes/alkynes are hydrogenated more slowly than terminal ones- substrates containing polar functional groups are hydrogenated more rapidly than those

without since polar functional groups help the olefin coordinate better with the catalyst

- cationic Rh complexes are more active than Wilkinsons catalyst => more electrophilicand thus favors alkene coordination

- the active form of the cationic Rh complexes is generated after COD is hydrogenated andthen detaches from the metal, freeing up 2 coordination sites that are then weakly

coordinated by solvent molecules

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- the cis isomer is preferentially hydrogenated

- homogeneous catalysts provide much higher selectivity than heterogeneous catalysts

- phosphine ligands used for asymmetric hydrogenations can be categorized into thefollowing types: diphosphines with chiral phosphorus atoms (DIPAMP), diphosphineswith chiral backbones (DIOP), atropisomeric ligands (BINAP), those with chiral

substituents on P (DuPhos), and ligands with planar symmetry (Josiphos)

- atropisomers are stereoisomers resulting from hindered rotation about single bonds wherethe steric strain energy barrier to rotation is high enough to allow for isolation of the

conformers (tropos = turn, a=not); they display axial/planar chirality and have ortho

substituents that cause steric repulsion

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- the enantioselective step is the OA of hydrogen to the square planar diastereomericsubstrate complex that are in rapid dissociation equilibrium; the major enantiomer of theproduct arises from the minor substrate catalyst diastereomer

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HYDROFORMYLATION

- the aldehyde produced is the kinetic product; alkanes are the thermodynamic product

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- d[aldehyde]/dt = k[alkene][Co][pH2]/[pCO]- the inverse dependence on CO concentration agrees with the mechanistic requirement of

CO dissociation from the 18e complex HCo(CO)4

- however, if CO pressure is not high enough, metallic Co becomes deposited; increasingthe CO pressure decreases the rate of hydroformylation and reduces the extent of alkene

isomerization

- the general relative reactivity of alkenes for hydroformylation is as follows:

- substituting CO for an electron donating PR3 group increases thermal stability of thecatalyst and reduces the CO pressure needed; however, it also increases the hydridic

nature of H, which could reduce the aldehyde to an alcohol

- halides inhibit hydroformylation- Rh phosphine catalysts for hydroformylation have the following rate dependence:

R=[alkene][Rh][pH2]

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- larger phosphine ligand bite angles favored higher n/iso ratios- the use of phosphites instead of phosphines also lead to higher n/iso ratios

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- chiral aldehydes are formed only when the addition of H2/CO to the alkene occurs in theMarkovnikov manner

- in enantioselective hydroformylation, higher iso/n ratios are preferred because the nisomer would be achiral

THE ACETIC ACID PROCESS & WACKER PROCESSES

- the OA of Me-I is the rate determining step; CO insertion is faster than OA

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- in the Cativa processes substitutes Rh for Ir; the Ir catalyst is more stable and moresoluble in the reaction medium

- an iodide ligand is transferred from the anionic Ir complex to the promoter, creating afree site on the catalyst so that the CO can bind; in addition, the OA of R-X is muchfaster than to Rh

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- in the Wacker process, the rate follows the equation: R= k[PdCl42-][C2H4]/[Cl-]2[H+]

OLEFIN METATHESIS

- the Mo catalyst (Schrocks catalyst) has high activity, allowing it to react with internaland terminal olefins and open low strain cyclic olefins; however, it is highly oxophilic,

moisture and oxygen sensitive, and has low functional group tolerance

- the Ru catalyst (Grubbs catalyst) has a higher preference for C=C bonds and have higherfunctional group tolerance; in addition, they do not require air and moisture free

conditions

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- the first generation Grubbs catalyst has phosphine groups while the second generationGrubbs catalyst has a NHC group => more reactive

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- the cycloaddition between two alkenes to give cyclobutanes is symmetry forbidden andoccurs only photochemically => d orbitals on the metal alkylidene fragment breaks this

symmetry and the reaction becomes facile

- the driving force for ROM is the release of ring strain

- the driving force for RCM is the formation of a thermodynamically stable ring

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- in ROMP, strained cyclic olefins to produce stereoregular and monodisperse polymersand copolymers; proceeds by the same mechanism as normal olefin metathesis except

that the new olefin generated remains attached to the catalyst as part of a growing

polymer chain

- enyne metathesis, is a reaction between an alkyne and an alkene that forms a 1,3 dieneproduct; can be either intermolecular and intramolecular

intermolecular EM

intramolecular EM

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mechanism

example