56.  "Effects of Linker Length on the Rate and Selectivity of Platinum-Catalyzed Asymmetric Alkylation of the Bis(isitylphosphino)alkanes IsHP(CH₂)_nPHIs (Is = 2,4,6-(i-Pr)₃C₆H₂, n = 1–5)." Chapp, T. W.; Schoenfeld, A. J.; Glueck, D. S. Organometallics 2010, 29, 2465-2473.

Abstract

Catalytic asymmetric alkylation of the bis(secondary phosphines) IsHP(CH₂)_nPHIs (1a-e, n = 1-5, Is = isityl = 2,4,6-(i-Pr)₃C₆H₂) with benzyl bromide using the base NaOSiMe₃ and the catalyst precursor Pt((R,R)-Me-DuPhos)(Ph)(Cl) gave the bis(tertiary phosphines) Is(PhCH₂)P(CH₂)_nP(CH₂Ph)Is (2a-e, n = 1-5) via the intermediates Is(PhCH₂)P(CH₂)_nPHIs (4a-e, n = 1-5). The rates of these reactions depended strongly on n, in the order 1a < 1b < 1c ~1d-1e. The bulkier bis(secondary phosphine) Mes*HP(CH₂)₂PHMes* (5, Mes* = 2,4,6-(t-Bu)₃C₆H₂) did not undergo catalytic alkylation under these conditions. The alkylation selectivity also depended on n. Alkylation of 1b was meso-selective, while alkylation of 1a,c-e was rac-selective, occurring with similar diastereoselectivity and enantioselectivity for the longer linkers (1c-e). The product ratios suggested that the catalyst controlled the selectivity for 1d,e, while substrate control operated for ethano-bridged 1b, with negative cooperativity. Substrate control also likely occurred for 1a, for which competition from the background alkylation was significant. Analysis of the observed diastereo- and enantioselectivity for Pt-catalyzed alkylation of 1c and the mixed secondary/tertiary phosphine IsHP(CH₂)₃P(CH₂Ph)Is (4c) yielded quantitative information on the selectivity of both P-C bond-forming steps, which was consistent with predominant catalyst control, altered slightly by the influence of the substrate.