DIASTEREOSELECTIVITY IN THE REACTION OF RCH2C[CH2P(AR)(LI)](3) WITH ELECTROPHILES - ENHANCEMENT OF DIASTEREOSELECTIVE CONTROL BY ETA(3)-COORDINATION IN (RCH2C[CH2P(AR)(LI)](3))MO(CO)(3)

Following a procedure developed for H3CC[CH2P(Ph)(2)](3) (1a) as the s tarting compound, various tripod ligands RCH2C[CH2P(Ar)(2)](3) (1) hav e been transformed into the trilithiotriphosphides RCH2C[CH2P(Ar) (Li) ](3) by reductive cleavage of their P-Ar bonds by metallic lithium. Th e 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) wit h 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 ob tained 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 tri-phosphide coordination compounds 4 are available by two different routes: either the complexes {RCH2C[CH2P(Ar)(H)](3)}Mo(CO)(3) obtained from 2 and (C H3CN)(3)Mo(CO)(3), are deprotonated by MeLi, or the trilithiotriphosph ides RCH2C[CH2P(Ar) (Li)](3), are reacted with (CH3CN)(3)Mo(CO)(3) to produce 4 in high yields. The ratio in which the two diastereomeric fo rms of 5 are obtained depends on the nature of the electrophile: the g reatest diastereomeric discrimination is obtained for Ar = Ph, R = Ph, R' = 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 ascert ain the diastereomeric excess in each case. X-ray analyses of three co mpounds of type 5 (5d, Ar = Ph, R = H, R' = Bzl; 5k, Ar = 3,5-Me2C6H3, R = H, R' = Bzl; 51, Ar = Ph, R = Ph, R' = Bzl) indicate the remarkab le conformational stability of the tripod metal scaffolding, with the conformations observed for these three compounds in three different so lid-state environments being closely similar, even with respect to the torsional arrangement of the phosphorus-bound benzyl groups.