54. "Platinum-Catalyzed Asymmetric Alkylation of Bis(isitylphosphino)ethane: Stereoselectivity Reversal in Successive Formation of Two P-C Bonds."  Chapp, T. W.; Glueck, D. S.; Golen, J. A.; Moore, C. E.; Rheingold, A. L. Organometallics 2010, 29, 378-388.

Abstract

Alkylation of the bis(secondary) phosphine IsHP(CH₂)₂PHIs (1; Is = isityl = 2,4,6-(i-Pr)₃C₆H₂) with 2-(bromomethyl)naphthalene using 10 mol % of the catalyst precursor Pt((R,R)-Me-DuPhos)(Ph)(Cl) and the base NaOSiMe₃ selectively yielded meso-IsP(CH₂Ar)(CH₂)₂P(CH₂Ar)(Is) (2; Ar = 2-naphthyl; dr = meso/rac ratio = 3.4:1). Half-alkylated IsP(CH₂Ar)(CH₂)₂PH(Is) (3), an intermediate in this reaction, was prepared from 1 by deprotonation (s-BuLi) and alkylation with 2-(chloromethyl)naphthalene. Analysis of the observed diastereo- and enantioselectivity in the Pt-catalyzed alkylations of 1 and 3 yielded quantitative information on the stereoselectivity of both P-C bond-forming steps. The first alkylation (1 --> 3) resulted in diastereoselective formation of a tertiary phosphine stereocenter (~2:1 ratio). In the second alkylation (3 --> 2), however, both (R_P)-3 and (S_P)-3 (the label refers to the configuration of the tertiary phosphine) selectively formed meso-2, instead of (R,R)-2 or (S,S)-2, respectively (the ratios were ca. 3:1 and 7:1). Thus, the tertiary phosphine in 3 favored alternation of stereochemistry in the alkylation of the secondary phosphine (substrate control with negative cooperativity). Platinum-catalyzed alkylation of IsPH(CH₂)₂OSi(i-Pr)₃ (6) gave IsP(CH₂Ar)(CH₂)₂OSi(i-Pr)₃ (9) in a 1.5:1 enantiomeric ratio (er). A related reaction of IsPH(CH₂)₂OSiMe₃ (4) gave a mixture of IsP(CH₂Ar)(CH₂)₂OR (R = SiMe₃ (7); R = H (8)), while alkylation of IsPH(CH₂)₂OH (5) gave 8 in about 2:1 er. Thus, the nature, and even the absolute configuration, of the pendant group X three bonds from the reactive phosphorus center in the substrates IsHP(CH₂)₂X (X = PHIs (1), P(CH₂Ar)(Is) (3), OSiMe₃ (4), OH (5), OSi(i-Pr)₃ (6)) had a strong influence on the selectivity of Pt-catalyzed phosphorus alkylation. Possible mechanistic explanations for this substrate control are discussed.