Platinum-catalyzed asymmetric alkylation of a secondary phosphine: Mechanism and origin of enantioselectivity

The catalyst precursor Pt((R,R)-Me-Duphos)(Ph)(Cl) (1) mediated asymmetric alkylation of the secondary phosphine PHMe(Is) (2; Is = 2,4,6-(i-Pr)(3)C6H2) with benzyl bromide in the presence of the base NaOSiMe3 to yield enantioenriched PMeIs(CH2Ph) (3). A mechanism for the catalysis has been proposed, on the basis of studies of the individual stoichiometric steps. The terminal phosphido complex Pt((R,R)-Me-Duphos)(Ph)(PMeIs) (4) was formed by proton transfer from 2 to the silanolate ligand in Pt((R,R)-Me-Duphos)(Ph)(OSiMe3) (5), which was generated from 1 (or from Pt((R,R)-Me-Duphos)(Ph)(Br) (7)) and NaOSiMe3. The silanolate complex 5 reacted with water to yield Pt((R,R)-Me-Duphos)(Ph)(OH) (6); both 6 and 7 were crystallographically characterized. The stoichiometric reaction of 4 and benzyl bromide in toluene gave the bromide 7 and 3. In more polar solvents these compounds were in equilibrium with the cation [Pt((R,R)-Me-Duphos)(Ph)(PMeIs(CH2Ph))][Br] (8-Br), the major Pt complex present during catalysis, which was isolated as the BF4 salt. Treatment of 8-BF4 with 2 and NaOSiMe3 yielded phosphine 3 and regenerated phosphido complex 4. This reaction does not appear to proceed via phosphine ligand substitution on 8-BF4 to yield the secondary phosphine complex cation [Pt((R,R)-Me-Duphos)(Ph)(PHMe(Is))][BF4] (12). Instead, treatment of 8-BF4 with NaOSiMe3 gave phosphine 3 and 5, which then reacted with 2 to yield 4. The crystal structure of the major diastereomer of 8-BF4 showed that the major enantiomer of 3 formed by catalyst precursor 1 had an R-P absolute configuration. Low-temperature NMR studies on the major diastereomer of phosphido complex 4 were consistent with the RP solid-state structure. Thus, the major enantiomer of phosphine 3 appeared to be formed from the major diastereomer of intermediate 4, and enantioselectivity was determined mainly by the thermodynamic preference for one of the rapidly interconverting diastereomers of 4, although their relative rates of alkylation were also important (Curtin-Hammett kinetics).