M.C. White Chem 153 Hydrogenation -140 Week of october 15. 2002 Wilkinson,'s original report Wilkinson's Catalyst Ph3p H,(I atm), benzene,rt Functionality tolerated Ph groups indicates that the metal hydride intermediate is primarily H2:D2(1:1) covalent in character (lacks hydridic properties characteristic Minimal H/D scrambling in the product is of strongly ionic M-H). See dicative of formation of a dihydrometal ON Structure& Bonding pg 28 intermediate that transfers both of its hydrido 6.1% Ethylene is not hydrogenated under these conditions butstoichiometric hydrogen transfer from preformed dihydride complex occurs. Data indicates that formation of an ethylene/ Rh(CI(PPh3b complex inhibits hydrogen PhaP P Ph3P,, aPE activation by the complex This implies that dihydride H(I atm), benzene, rt formation precedes olefin PPh cycle The stereochemical outcome of this experiment indicates that the mechanism involves stereospecific cis hydrometallation of the unsaturated substrate followed by stereospecific reductive elimination from the resulting alkenyl (or alkyl) hydrido species Phap PPh3 C3h7- CH3 H,(50 atm), benzene CH, CH3 hexane HO, C COH D2(I atm), benzene H 2C COH 20°C cis- hexene trans-hexene meso compound major product observed (>20:1) Wilkinson Chem Soc(A)1966, 1711M.C. White, Chem 153 Hydrogenation -140- Week of October 15, 2002 Wilkinson's original report: Wilkinson’s Catalyst Ph3P Rh(I) Ph3P PPh3 Cl cat. H2 (1 atm), benzene, rt quantitative Investigations into the reactivity of (PPh3)RhCl uncovered its high activity as a homogeneous hydrogenation catalyst. This was the first homogeneous catalyst that compared in rates with heterogeneous counterparts. Wilkinson J. Chem. Soc. (A) 1966, 1711. O C N NO2 O R O OR O OH Functionality tolerated Ph3P Rh(I) Ph3P PPh3 Cl Cl Rh(III) Ph3P H PPh3 H PPh3 Ph3P Rh(I) Ph3P PPh3 Cl Data indicates that formation of an ethylene/ Rh(Cl)(PPh3)3 complex inhibits hydrogen activation by the complex. This implies that dihydride formation precedes olefin complexation in the catalytic cycle. cat. H2 (1 atm), benzene, rt Ethylene is not hydrogenated under these conditions but...stoichiometric hydrogen transfer from preformed dihydride complex occurs. + rt + Ph3P Rh(I) Ph3P PPh3 Cl C3H7 CH3 C3H7 CH3 H H Compatibility with carbonyl groups indicates that the metal hydride intermediate is primarily covalent in character (lacks hydridic properties characteristic of strongly ionic M-H). See Structure & Bonding pg. 28. H2 (50 atm), benzene 20oC cis- hexene: trans-hexene (>20:1) + hexane HO2C CO2H Ph3P Rh(I) Ph3P PPh3 Cl D2 (1 atm), benzene 20oC H H D D HO2C H H CO2H The stereochemical outcome of this experiment indicates that the mechanism involves stereospecific cis hydrometallation of the unsaturated substrate followed by stereospecific reductive elimination from the resulting alkenyl (or alkyl) hydrido species. meso compound major product observed Ph3P Rh(I) Ph3P PPh3 Cl cat. H2: D2 (1:1) H H 50% D D 43.9% H D 6.1% Minimal H/D scrambling in the product is indicative of formation of a dihydrometal intermediate that transfers both of its hydrido ligands to the unsaturated substrate