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M.C. White, Chem Mechanism Week of September th, Ligand Exchange Mechanisms Associative ligand substition is often called square planar substition because e, d square planar complexes generally undergo ligand substitution via an associative mechanism the MNu bond is formed before the MX bond breaks. The intermediate is e and therefore provides a lower energy route to the product than a e intermediate formed via dissociative substitution the MX bond is fully broken before the MNu bond begins to form. Analogous in many ways to SN reactions. Nu empty, nonbonding pz orbital can act as an acceptor orbital for the e density of the incoming nucleophile Nu L L M L X L L M L X L eM NiII, PdII, PtII, RhI, IrI. e intermediates L L M Nu X L L X eL M Nu L L M L Nu Dissociative ligand substitution is most favored in coordinatively saturated e complexes e.g. d tetrahedral, d octahedral. In the dissociative mechanism, the MX bond is fully broken before the MNu bond forms thereby avoiding an energetically unfavorable eintermediate. Analogous in many ways to SN reactions. Note that in all ligand substition processes, there is no oxidation state change at the metal center. L L M L L eM RuII, CoIII, RhIII, IrIII X L X L L L Nu L M L Nu L eL LMLLe M.C. White, Chem Structure amp Bonding Week of September , MO Description of bonding in ML square planar Metal ValenceOrbitals au e Rule pz au eu px py au LML LL Linear Combinations of Ligand Donor Orbitals The square planar geometry is favored by d metals e.g. Ni II, Pd II, PtII, Ir I, RhI. A stable electronic configuration is achieved at e, where all bonding and non bonding orbitals are filled. Spinpaired compounds display diamagnetic behavoir i.e. weakly repelled by magnetic fields and may be readily characterized by NMR. eu LUMO ag ag s ag dz When combining orbitals, the resulting MOs must be symmetrically dispersed between bonding and antibonding. y Thus, combining orbitals i.e. ags requires one of the orbitals to be nonx bonding. ag bg eg bg n HOMO eg bg LL LL bg dxy n bg eg dxz dyz bg dxy In a square planar ligand field the degenerate d orbitals split into orbitals of ag, b g, eg, and bg symmetries. The degenerate p orbitals split into orbitals of eu and au symmetries. eu ag M.C. White, Chem Mechanism Week of September th, Associative Substitutionthe nucleophile Nu Rate d PtCl k PtCl k NuPtCl dt k first order rate constant that arises from substition of leaving group by solvent. k secondorder rate constant for bimolecular attack of Nu on metal complex. Cl N N PtII Cl MeOH, rt Cl N N PtII Nu Cl e e Nu MeOH CH CO CO F relative rate lt lt lt Nu relative rate NN Basicity of the incoming ligand nucleophile plays only a minor role in its reactivity for soft metal centers. In general, the softest i.e. most polarizable nucleophiles react fastest with soft metals like PtII via associative substitution. Steric hinderance at the nucleophile i.e. picoline vs pyridine can retard the rate of substition. NHN CH OEtN lt N H ClNH BrICHCN CH OP PhSPh P Et P ,.x.x.x.x.x.x M.C. White, Chem Mechanism Week of September th, Associative Substitution Sterics Sterically shielding the positions above and below the plane of the square planar complex can lead to significant decreases in the rates of associative substition. EtP EtP PtII Cl , rt N k , M sec EtP EtP PtII Py Cl EtP EtP PtII Cl , rt N k M sec EtP EtP PtII Py Cl EtP EtP PtII Cl , rt N k M sec EtP EtP PtII Py Cl Pearson J. Chem. Soc. . M.C. White, Chem Mechanism Week of September , Associative Substitution Sterics as the steric bul k of the imine backbone increases, the aryl groups become more rigidly locked perpendicular to the square plane making their ortho substituents more effective at blocking the axial sites above and below the plane. N Pd N CH BAr N Pd N CH N associative second order rate constants for ethylene exchange were examined by HNMR in CDCl at o C BAr N Pd CH BAr k too fast to measure even at o C. k L/mol/sec k L/mol/sec Brookhart JACS . BAr N Pt N CH R Ruffo OM . M.C. White, Chem Mechanism Week of September th, Brookhart Polymerization Catalysts BAr N Pd N CH N BAr N Pd CH Polymer Mw , BAr Polymer Mw , BAr R N Pd N R CH BAr RN R N Pd polymer propagation CH Pd CH insertion NR NR High Mw polymers H hydride elimination R N Pd N R H BAr R N Pd N R H BAr R associative displacement BAr RN BAr CH CH N Pd N R H Pd N R H Low Mw polymers Brookhart JACS . M.C. White/Q.Chen Chem Mechanism Week of September th, to Cp via Slipped Cp Intermediate Cp PCH OC OC Re CO oC OC CO slipped Cp H Cp PCH OC OC Re PCH OC CO Re PCH e proposed quotslippedquot Cp intermediate e e Based on the observation that the rate of reaction of with PCH to form depends on both the concentration of and PCH , an associative mechanism was proposed. To account for associative substition at a formally coordinatively and electronically saturated center, the authors propose an quotslippedquot Cp intermediate that forms concurrently with phosphine attack. Casey OM . M.C. White/M.W. Kanan Chem Mechanism Week of September th, to Ring Folding WII CO CO WII CO CO e e e complexes with cyclopentadienyl, aryl, indenyl ligands may undergo quotassociativequot substitution avoiding an energetically unfavorable eintermediate via ligand rearrangement from to or even cyclopentadienyl and indenyl. Haptotropic rearrangement may take the form of ring quotslippagequot where the ring is acentrally bonded to the metal and its aromaticity is disrupted or ring quotbendingquot where the conjugation of the system is broken. . a nonbonding distance . Huttner J. Organometallic Chem. . M.C. White, Chem Mechanism Week of September th, Ligand Exchange Dissociative Mechanism O RhI O erate of ethylene exchange via associative displacement at oC is sec slower at oC erate of ethylene exchange via dissociative displacement at oC is x sec BDE kcal/mol E RhI RhI RhI The ratedetermining step in a dissociative ligand substition pathway is breaking the ML bond. Because of the late, productlike transition state for forming the coordinatively unsaturated intermediate in such a process, the ML BDE is a good approximation of the activation energyEA. EA BDE rxn coordinate PPh mmol . RhI RhI PPh RhI PPh . k x sec . . C a fold increase in the concentration of nucleophile does not significantly affect the rxn rate. Results are consistent with a mechanism where the ratedetermining step is ethylene dissociation and is not affected by the concentration of the nucleophile. o e e e Cramer JACS . rate d LRhC H k LRhCH dt M.C. White, Chem Mechanism Week of September th, Ligand dissociation sterics P Fe tBu tBu tBu tBu P Pd P P Fe Fe P Pd P P tBu tBu Fe catalyst quotresting statequot P quotineffective catalystquot L tBu tBu dissociative L tBu tBu P Fe P tBu tBu Pd catalytically active species The steric bulk of the bidentate phosphine ligand is thought to weaken the PdP bond, thereby favoring ligand dissociation required to form the catalytically active species. Cl HN also aniline, piperidine NaOtBu NH also aryl Br, I, OTs Hartwig JACS . M.C. White, Chem Mechanism Week of September th, Ligand dissociation or hv coordinatively and electronically unsaturated complexes capable of oxidatively adding into unactivated CH bonds. RhI OC e CO hv or CO OC RhI OC e RhIII H e Lightpromoted ligand dissociation MCO hv bond order bond order MCO H RhI OC proposed intermediate Bergman JACS . M.C. White, Chem Mechanism Week of September th, Ligand dissociationweakly coordinating solvents First generation Crabtree hydrogenation catalyst PMePh Ir PMePh PF cat. R R H, solvent S A glimpse into the catalytic cycle PF H IrIII S PMePh dissociative displacement Ph MeP IrIII S PF H R PMePh Ph MeP IrIII S PMePh H H PF H PhMeP H S S catalytically active species R Solvent S O Turnover Frequency TOF CHCl TOF mol reduced substrate/mol catalyst/h Crabtree Acc Chem Res . M.C. White, Chem Mechanism Week of September th, Noncoordinating solvents no such thing N N N H B N N Cl HC N Ir CH PMe BArf The first isolated chloromethanemetal complex. There are also similar complexes formed with CHCl, and Cl CH that have been characterized by NMR. Bergman JACS . M.C. White, Chem Mechanism Week of September th, Oxidative Addition/Reductive Elimination Oxidative Addition OA metal mediated breaking of a substrate bond and formation of or new ML bonds. OA requires removal of electrons from the metals d electron count. This is reflected in a two unit increase in the metals oxidation state. The formation of or new ML bonds is accompanied by an increase in the metals coordination number by or units respectively. The latter results in a unit increase in the electron count of the metal complex e.g. e to e. Currently, OA of low valent, electron rich metals to polar substrates is the best way to form MC bonds within the context of a catalytic cycle. The term oxidative addition confers no information about the mechanism of the reaction. LMn quotNuquot oxidative addition R X quotE quot R LMn X or L Mn R X reductive elimination Reductive elimination RE microscopic reverse of oxidative addition where two ML bonds are broken to form one substrate bond. RE results in the addition of two electrons into the metal d electron count. This is reflected in a two unit decrease in the metals oxidation state. The breaking of ML bonds is accompanied by a decrease in the metals coordination number by units. The result is a unit decrease in the electron count of the metal complex e.g. e to e. The two ML bonds undergoing reductive elimination must be oriented cis to each other. Currently, RE is the most common way to form CC bonds via transition metal complexes. General OA Mechanisms Nucleophilic displacement generally for polar substrates Concerted generally for nonpolar substrates A B LxM n A B centered TS LxM n A LxM B n LxM n A LxMn X A X LxMn A X cis addition Radical both nonpolar and polar LxMn A C A LxMn A C LxMn C M.C. White, Chem Mechanism Week of September th, Oxidative Addition Metal Complex electron rich metals in low oxidation states, with strong donor ligands and a site of coordinative unsaturation. d, tetrahedral, e gt d, ML, e gt d, ML, square planar, e e.g. Ni , Pd, Pt tBu tBu tBu tBu P Fe P Pd tBu tBu P Fe Fe Pd P tBu tBu Cl Fe tBu tBu P Pd P tBu tBu Cl PP tBu tBu tBu tBu OA L d, ML , square planar, e gt d, ML, octahedral, e e.g. RhI, IrI Ir I H H IrIII PCy N PCy N PF H e eSubstrates two groups segregated into nonpolar and polar. Currently, the most facile way to form CM bonds is with polar substrates e.g. alkyl, aryl, and vinyl halides. Nonpolar substrates RH Polar substrates RX where X I, Br, Cl, OTf H H, RSiH, R BH, RCHH, R H , H , , R OH HX, RCHX, X , X,R OX M.C. White/M.S. Taylor, Chem Mechanism Week of September th, Transition Metal Oxidation States Ti V Cr Mn Fe Co Ni Cu Zr Nb Mo Tc Ru Rh Pd Ag Hf Ta W Re Os Ir Pt Au Observed positive oxidation state Most stable oxidation state aqueous solution Mingos Essential Trends in Inorganic Chemistry Oxford University Press, . M.C. White, Chem Mechanism Week of September th, OA Concerted centered nonpolar substrates HHMH complex backbondinggtgt donation oxidative addition MH metal dihydride complex intermolecular binding of a substrate via its bond to a metal complex. complexes are thought to be along the pathway for oxidative addition of nonpolar substrates to low valent, erich metal complexes. Analogous to the DewarChattDuncanson model for olefin metalbonding, bonding is thought to occur via a way donoracceptor mechanism that involves donation from the bonding electrons of the substrate to empty orbital of the metal and backbonding from the metal to the orbitals of the substrate. These bonding principles have been applied to nonpolar bonds such as HH, CH, SiH, BH and even CC bonds. H L xM n H . LxM n H LxMn H . complex H H backbondinggtgt donation LxMn H gt. H metal dihydride Concerted mechanism complex formation precedes an early little bond breaking, centered transition state where strong backbonding results in oxidative addition of the bound substrate to the metal. The concerted mechanism is thought to operate primarily for nonpolar substrates i.e. HH,CH, SiH, BH with electron rich, low valent metals. The spectroscopic identification of metal dihydrogen complexes with HH bond distances stretched between the nonbonding . and dihydride extremes gt. provides strong support for this mechanism with H. M.C. White/M.W. Kanan Chem Mechanism Week of September th, Dihydrogen complexes PR OC W OC PR H CO H . The first stable dihydrogen metal complex was isolated by Kubas. The lengthened HH bond . is greater than the HH bond length in free H .. This is thought to arise from metal backbonding into the HH orbital. Kubas Acc. Chem. Res. . M.C. White/M.W. Kanan Chem Mechanism Week of September th, Dihydrogen complexes . H H N Os N H OAc HHN NH The HH bond distance is thought to be a measure of the metals ability to backdonate its electrons. Low oxidation state metal complexes such as OsHen OAc with strong and donor ligands are very effective backbonders as evidenced by the very long HH bond in their MH complex. Taube JACS . M.C. White, Chem Mechanism Week of September th, spCH OA via complex intermediates H H M C complex C backbondinggtgt donation oxidative addition M Hydridoalkylmetal complex regioselectivity sp CH gt o spCHgt o sp CH gtgtgt o sp CH. There is both a kinetic and thermodynamic preference to form the least sterically hindered CM bond. Kinetic preference activation barrier to complex formation is lower for less sterically hindered CH bonds and bonds with more s character. Thermodynamic preference stronger CM bonds are formed see Structure and Bonding, pg. . Evidence in support of a complex intermediate CD D C CD CD RhI OC eCO hv flash, Kr K CO OC CO v cm D RhI Kr OC CD DC CD RhI DC OC DC CD RhIII D CD CD CO v cm complex CO v cm Bergman JACS . M.C. White, Chem Mechanism Week of September th, spCH concerted vs. radical crossover experiment evidence in support of a concerted mechanism. IrI H C OC D IrI OC eOC CO hv CO OC IrI D OC IrIII D IrI H IrIII OC H D complexes D Less than of the crossover products were observed by HNMR. This may be indicative of a minor radical pathway. HC D IrII OC H OC IrII D OC IrIII H OC IrIII D Bergman JACS . D M.C. White, Chem Mechanism Week of September th, Agostic interactions intramolecular complex HMCRR An agostic interaction is generally defined as an intramolecular complex that forms between a metal and a CH bond on one of its ligands. Brookhart JOMC CO H RuII PhP mol O CO O O H O O O OEt RuPPh Toluene, reflux SiOEt O O O PPh Y O OO H Ru PPh PPh H Y OEt O PPh PPh OA O O O H PPh O OEt O RuII PPh PPh The strategy of identifying substrates that can act as transient metal ligands has led to the only synthetically useful examples of CHgt CM CH activation to date. Like all substrate directed reactions, the scope of such processes is limited. O O O OEt SiOEt O O O Trost JACS M.C. White, Chem Mechanism Week of September th, OA spCspC C CR M C R complex backbondinggtgt donation oxidative addition R M R C metal dialkyl complex Even though BDEs of CC bonds are lower than those of analogous CH bonds e.g. CH CH kcal/mol vs. CH H kcal/mol, transition metal mediated OAs into CC bonds are much more rare than those for analogous CH bonds. Formation of the complex is kinetically disfavored by steric repulsion between the metal complex and the carbon substituents and by the high directionality of the spCspC bond that localizes the bonding orbital deep between the carbon nuclei. Milstein and coworkers are able to overcome the kinetic barrier by approximating the CC bond at the metal center. PtBu C H RhCl oC, tol NEt PtBu RhI Cl oC k x /sec NEt stable o C observed by H NMR PtBu RhIII NEt only product observed no CH activation product Cl Milstein JACS . M.C. White, Chem Mechanism Week of September th, OA spCspC PtBu RhIII NEt only product observed no CH activation product Cl Milstein OM . M.C. White, Chem Mechanism Week of September th, OA with CspX bonds aryl and vinyl halides LxM n X oxidative addition LxM n X R LxM n RX oxidative addition LxM n X Note retention of stereochemistry in OA to vinyl halides Rate of OA X IgtBrgtClgtgtF Three main mechanisms to consider for this process . Concerted process with unsymmetrical, minimallycharged, centered transition state . S NArlike with highly charged transition state . Singleelectron transfer processes with oppositelycharged, radical intermediates Csp L xM n X LxM n X X LxMn M.C. White, Chem Mechanism Week of September th, Hammet plots linear free energy relationships a valuable mechanistic probe See Carey and Sunberg Part A, rd Edition. pp . Recall the effect of e withdrawing groups and e donating groups on the acidity of benzoic acid resulting from stabilization and destabilization of the carboxylate anion, respectively COH COH COH COH COH COH COH MeOC CF COMe Me Me OH pKa . . . . Substituent group . . . meta ...... para . . . by definition . . . COH Define pKaH pKax CF COMe Cl H OMe Me OH X Because these same substituents can often similarly stabilize or destabilize polar transtition states for reactions involving aryl substrates, a linear relationship can exist between and reaction rate for such processes. This type of quotlinear free energy relationshipquot can lend valuable insight into the charge characteristics of the transition state for various reactions. M.C. White, Chem Mechanism Week of September th, Hammet plots linear free energy relationships a valuable mechanistic probe See Carey and Sunberg Part A, rd Edition. pp . . pNO mNO . pCl log k/ko pMe pOMe H mNH slope . positive indicates buildup of negative charge in the TS log k/ko slope . negative indicates buildup of positive charge in the TS ... . .. pNH . Cl Cl EtO COEt HO O OH EtO COH HH OH OH X HO X X X HCl XX M.C. White, Chem Mechanism Week of September , Oxidative addition of aryl chlorides to tristriphenylphosphinenickel pCN pCOPh pCOMe mCN . log k/ko mCOMe mCl mMe H pMe mOPh pCl ... pOMe pOPh . . Cl LxMn Csp X PPhNi X LxMn X X or PPhNi Cl Foa and Cassar J. Chem. Soc Dalton . M.C. White, Chem Mechanism Week of September th, OA CspXalkyl, allyl, and benzyl halides Nucleophilic displacement generally for polar substrates A L xM n X LxM n A X LxMn A X quot Stereochemistry is the single most valuable type of mechanistic evidence in reactions that make or break bonds to tetrahedral carbon.quot G.M. Whitesides JACS . COMe PPhPd mol MeO C OAc MeO C inversion MeO C Na PhPPdII COMe OAc inversion COMe COMe COMe MeO C net retention Trost Acc. Chem. Res. . Mes P Pd P Ph OH Mes II OTf Mes mol P Pd II P Ph OTf Ph H N NHPh NH Mes Ozawa JACS . M.C White/M.W. Kanan, Chem Mechanism Week of September th, Transmetalation Definition and Utility Definition the transfer of an organic group from one metal center to another. The process involves no formal change in oxidation state for either metal. LnM R LnM X LnM X LnM R Transmetalation Commonly used transmetalation reagents and their associated crosscoupling reaction Transmetalation is often a reversible process, with the equilibrium favoring the more ionic MX bond. Subsequent reactivity of one LnMR species can drive the equilibrium in one direction. This is often exploited in crosscoupling reactions, where a transmetalated intermediate undergoes a reductive elimination to generate a new organic product. Subsequent oxidative additions generates a new substrate for transmetalation reductive elimination R R R n Lr M X oxidative addition R n Ln M R RM XM XM RM M is typically group Reagent LiR, MgXR RZrClCp RZnCl RCuLn RSnR RBOR RBBN RSiR AlR, AlX R vinyl, aryl, allyl, alkyl vinyl, alkyl vinyl, aryl, alkyl alkynyl, aryl vinyl, aryl, alkynl vinyl, aryl alkyl aryl, vinyl ,alkyl alkyl Xcoupling reaction Kumada Negishi e.g. Sonagashira Stille Suzuki SuzukiMiyaura Hiyama LrM X transmetalation LrM R M Lr n In generalthe rates of transmetalation of R follow the order alkynylgt aryl,vinylgtalkyl M Ni, Pd RX M.W. Kanan, Chem Mechanism Week of September th, Transmetalation Mechanism The mechanism for transmetalation is the leaststudied of the basic reaction steps. In a simple picture, the metal accepting the R group is the electrophile and the MR bond being transferred is the nucleophile. MR bond formation may or may not be simultaneous with MX bond formation, depending on the nature of X and the actual complexes involved. MX RM MM RX With this model, increasing the nucleophilicity of R by altering the ligands on M and increasing the electrophilicity of M through its ligands will facilitate the transmetalation step. For weakly nucleophilic transmetalation reagents, an added nucleophile or base often facilitates the transmetalation. RR R R LnPd R Transmetalation with the Suzuki coupling often requires added base OR RO B RO OR B OR OR LnPd II X Suzuki Chem. Rev. . F is thought to activate the organosilicon reagent for transmetalation via formation of a nucleophilic pentavalent silicate in a Hiyama coupling R SiRnFn TBAF R SiRnFn R R LnPd LnPd X R Hiyama Tet. Lett. . M.W. Kanan, Chem Mechanism Week of September th, Transmetalation with advanced intermediates SuzukiMiyaura OBn O BnO O BnO H H H O OTBS H BBN BnO THF, rt BnO H O H H O OTBS BBN OBn O H OBn H O O OBn BnO H OBn O H OTBS TfO O O H H BnO H OHOHO OBn O M aq. Cs CO, DMF PdPh, C OBn Synthetic studies on Ciguatoxin Takakura ACIEE, M.C. White/Q. Chen Chem Mechanism Week of September th, Reductive Elimination Reductive elimination is a key transformation in transition metal mediated catalysis, often representing the product forming step in a catalytic cycle. General trend for reductive elimination from d square planar complexes H M H gt M Csp H gt M Csp H gt M H gt Csp Csp M Csp Best overlap . Orbitals with more s character are less directional and lead to better overlap in the transition state for reductive elimination RE. Note cis orientation of the ligands is required for RE to occur. Worst overlap . H o. H o. . H o. . H Pd .o. . CH . CH o. Pd Pd Pd . H metal dihydride H Transition state TS Calculated E . kcal/mol PdH bond is stretched only in TS Pd CH CH . hydridoalkylmetal complex Transition state TS Calculated E . kcal/mol o . Pd CH CH Transition state TS Calculated E . kcal/mol PdH bond is stretched in TS metal dimethyl Goddard JACS . Dedieu Chem. Rev. . Computational studies suggest that the spherical symmetry of the s orbitals of H allows the simultaneous breaking of the ML bonds while making the new bond of the product. M.C. White Chem Mechanism Week of September th, RE Bite Angle Effects RE can be promoted by Increasing the bite angle of the ligand Increasing electrophilicity of metal center e.g. acids Ligand dissociation TMS P CHTMS Pd II P CN oC P Pd II P C N P CHTMS P PdII CN k Ph P CHTMS PdII CN Ph P Pd P Ph II CHTMS CN O Ph P Pd II CHTMS CN P Ph bite angle P Ph O bite angle o k . x bite angle o k . x bite angle o k . x Large bite angles of diphosphines have been shown to enhance the rates of reductive elimination from square planar complexes presumably by bringing the two departing ligands closer together. Moloy JACS . M.C. White Chem Mechanism Week of September th, RE Acid Effects RE can be promoted by Increasing the bite angle of the ligand Increasing electrophilicity of metal center e.g. acids Ligand dissociation O Ni O I Bu PentZn FC O II O O mol FC O O NiII O O Bu Pent Bu mol possible intermediate yield, h w/out acid , h Knochel ACIEE . M.C. White, Chem Mechanism Week of September th, Migratory Insertion/Deinsertion Alkyl, H CosseeArlman Mechanism for alkyl migration to a coordinated olefin CH Zr CH Zr C H Zr H Brintzinger catalyst for Ziegler Natta polymerizations no oxidation state change at M e from the electron count The bonding electrons of the olefi n are used in bond formation with a Malkyl . Formation of the new CC and MC bonds are thought to occur simultaneously with breaking of the bond and alkylM bond through a centered concerted transition state. Migratory insertion of a hydride into a coordinated olefin the microscopic reverse of hydride elimination is thought to procede via the same mechanism. For metal alkyls, the equilibrium lies to the right, whereas for metal hydrides it lies to the left. Early TS Grubbs and Coates Acc. Chem Res. . Late TS M.C. White, Chem Mechanism Week of September th, Hydride Elimination A significant decomposition pathway for metal alkyls is hydride elimination which converts a metal alkyl into a hydrido metal alkene complex. BAr CH Pd II N R H BAr CH PdII N R H RN R R N Pd N R H BAr hydride elimination N CH donative agostic interaction concerted transition state hydride elimination can occur when cis to the alkyl group there exists is a site of coordinative unsaturation on the metal which corresponds to a site of electronic unsaturation empty metal orbital. the MCCH unit can take up a coplanar conformation which brings the hydrogen in close enough proximity to the metal to form an agostic interaction. the metal is electrophilic resulting in an agostic interaction that is primarily electron donative in nature i.e. donationgtgtbackbonding. HMC donation gtgt backbonding complex M.C. White, Chem Mechanism Week of September th, Wacker Oxidation R R PdCl / O HCl HO PdCl CuCl ClPd CuCl Pd HCl H Pd Cl OH O R OH R HClPd R H OH R R OH HO Binding specificity terminal olefins Regioselectivity o carbon Remote functionality tolerated protonolysis OH R ClPd hydride elimination Commericial production of acetaldehyde M.C. White Chem Mechanism Week of September th, Hydride Elimination Computational studies suggest that the higher energy of the NiII vacant d orbital . hartree with respect to that of the analogous PdII complex . hartree results in a weaker donative agostic interaction with the CH bond. The energetically optimized geometries of the agostic complexes show a greater lengthening of the CH bond in the PdII complex than in the NiII complex, indicative of greater donation in the former. These computational results are consistent the experimentally observed greater stability of Ni alkyls towards hydride elimination that Pd alkyls and can be rationalized based on the greater electronegativity of PdII vs NiII as reflected in their respective second ionization potentials. . H NiII H second ionization potential NiII . eV recall sp CH is . PH . H Pd II H second ionization potential PdII . eV PH Morokuma JACS . M.C. White, Chem Mechanism Week of September th, Migratory Insertion/Deinsertion CO Mechanism for CO insertion via alkyl migration to coordinated CO O CO . o H C . o Pd II PH H . o H C O C .o Pd II PH H Pd II PH H CHC alkyl groups migrate preferentially over hydrides Computational studies suggest that in the TS, the methyl group has moved up half way towards the carbonyl. no oxidation state change at M e from the electron count Morokuma JACS . Experimental evidence also suggests that carbonyl insertion occurs via alkyl migration not CO migration hydride elimination RP RP Pd II CO PR RP OC Pd II CO insertion RP O C Pd II H RP O C Pd II H reductive elimination H O cisdiethyl complex RP Pd II PR CO PR RP Pd II CO CO insertion RP Pd II reductive elimination C O O transdiethyl complex Yamamoto Chem. Lett. . M.C. White, Chem Mechanism Week of September th, Migratory Insertion/Deinsertion CO RP PtII OC monomer R ROC Cl RP Pt II Cl Pt Cl dimer II Cl COR no dimer observed Ph lt Me lt CHPh lt Me lt Ph lt Et Anderson Acc. Chem. Res. . Electron donating substituents on aryl R groups promote migrations whereas electron withdrawing substituents inhibit them. R Monomer Dimer N MeO Me Cl NC Cross J. Chem. Soc., Dalton Trans. , . M.C. White Chem Mechanism Week of September th, Heck Arylation OMe I MeO N O O OMe PdPPh cat. NEt, CHCN o C MeO N O PPh I Pd MeO H PPh OMe NCOMe migratory insertion MeO reasonable intermediates O MeO I PPh Pd OMe O NCOMe associative displacement oxidative addition OMe I PdL O OMe NCOMe NCOMe MeO O N O N hydride elimination MeO MeO OMe MeO MeO N O yield FR Danishefsky JACS . O NCO Me O N O NCOMe OCONH OMe