Diastereoselectivity in the reaction of RCH2C[CH2P(Ar)(Li)]3 with electrophiles: Enhancement of diastereoselective control by η3-coordination in RCH2C[CH2P(Ar)(Li)]3Mo(CO)3

Article abstract of DOI:10.1002/cber.19971301006

Following a procedure developed for RCH₂C[CH₂P(Ar)(Li)]₃ (la) as the starting compound, various tripod ligands RCH₂C-[CH₂P(Ar)₂]₃ (1) have been transformed into the trilithiotriphosphides RCH₂C[CH₂P(Ar)(Li)]₃ by reductive cleavage of their P-Ar bonds by metallic lithium. The triphosphides are readily protonated to produce RCH₂C[CH₂P(Ar)(H)]₃ (2). Reaction of RCH₂C[CH₂P(Ar)(Li)]₃ with various electrophiles R'-Hal leads to the two diastereomers of RCH₂C[CH₂P-(Ar)(R′)]₃ (6) with an (RRR/SSS : RRS/SSR) ratio close to the statistical value of 1:3, except when Ar = Ph and R′ = Bzl, where the RRS/SSR diastereomer is obtained almost exclusively. In contrast, the reaction of RCH₂C[CH₂P(Ar)(Li)]₃)-Mo(CO)₃ (4) with electrophiles R′-Hal tends to favour the formation of the homochiral RRR/SSS diastereomers. The triphosphide coordination compounds 4 are available by two different routes: either the complexes {RCH₂C[CH₂P-(Ar)(H)]₃]Mo(CO)₃, obtained from 2 and (CH₃CN)₃Mo(CO)₃, are deprotonated by MeLi, or the trilithiotriphosphides RCH₂C[CH₂P(Ar)(Li)]₃ are reacted with (CH₃CN)₃Mo(CO)₃ to produce 4 in high yields. The ratio in which the two diastereomeric forms of 5 are obtained depends on the nature of the electrophile: the greatest diastereomeric discrimination is obtained for Ar = Ph, R = Ph,′ = Bzl, where the homochiral RRR/SSS enantiomeric pair is produced in a fourfold excess relative to the RRS/SSR pair. Two-dimensional NMR spectra and simulations of one-dimensional spectra are used to ascertain the diastereomeric excess in each case. X-ray analyses of three compounds of type 5 (5d, Ar = Ph, R = H,′ = Bzl; 5k, Ar = 3,5-Me2C6H3, R = H,′ = Bzl; 51, Ar = Ph, R = Ph,′ = Bzl) indicate the remarkable conformational stability of the tripod metal scaffolding, with the conformations observed for these three compounds in three different solid-state environments being closely similar, even with respect to the torsional arrangement of the phosphorus-bound benzyl groups. WILEY-VCH Verlag GmbH,.