CARBONYL PHOSPHONYLATION VIA [1(N),3(E)]-BIFUNCTIONAL REAGENTS - PROBING MECHANISTIC AND REACTIVITY FEATURES THROUGH CHEMICAL AND ISOTOPIC LABELING

A range of organophosphorus(III) esters of the general form {DIAM}PX(SiR(3))(n){X = 0, n = 1; X = N, n = 2; R(3) = Me(3), Ph(3), (t)BuMe(2), Et(3); DIAM = N,N'-(CH(2)NMe)(2) and N,N'-CH2(CH(2)NMe)(2)} has been prepared via reaction of {DIAM}PCl with R(3)SiOH or LiN(SiMe(3))(2) respectively. These esters will phosphonylate aldehydes via the Abramov reaction to afford alpha-siloxyphosphonate esters cleanly and in high yields. The mechanism of the Abramov reaction using {DIAM}POSiR(3) reagents has been investigated by (i) O-18 isotopic labelling experiments which reveal that reaction proceeds with exclusive [O-Si] rather than [P-O] bond cleavage which, in turn, supports a mechanism with overall retention of configuration at phosphorus, (ii) double crossover experiments which support intramolecular silyl-group transfer and (iii) manipulations of the electron-donating properties of the carbonyl substrate which suggest that [P-C] bond formation is rate determining. Further tuning of the phosphonylation reaction is possible by manipulating (a) the nature of the phosphorus-coordinated donor atoms in the chelate ring. (b) the size and rigidity of the chelate ring. (c) the ester residue donor atom X and (d) the silicon substituents R.