Carbonyl phosphonylation via[1N,3E]-bifunctional reagents. Probing mechanistic and reactivity features through chemical and isotopic labelling

Abstract

A range of organophosphorus(III) esters of the general form {DIAM}PX(SiR₃)_n{X = O, n= 1; X = N, n= 2; R₃= Me₃, Ph₃, ^tBuMe₂, Et₃; DIAM =N,N′-(CH₂NMe)₂ and N,N′-CH₂(CH₂NMe)₂} has been prepared via reaction of {DIAM}PCI with R₃SiOH or LiN(SiMe₃)₂ respectively. These esters will phosphonylate aldehydes via the Abramov reaction to afford α-siloxyphosphonate esters cleanly and in high yields. The mechanism of the Abramov reaction using {DIAM}POSiR₃ reagents has been investigated by (i)¹⁸O 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 silylgroup 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.