NMR Studies of Five-Coordinate Tin Enolate: An Efficient Reagent for Halo Selective Reaction toward .alpha.-Halo Ketone or .alpha.-Halo Imine

Abstract

NMR studies of tin enolates (1), in the presence of HMPA, revealed the presence ofa five-coordinate O-stannyl enolate 1(h) which contributes to upfield shifs of Sn peaks in the 119 Sn NMR spectrum and increased coupling constants J( 119 Sn- 13 C), compared with the four-coordinate tin enolate 1(e). The tautomeric equilibrium between C-stannyl ketone 1(k) and O-stannyl enolate 1(e) was changed by the addition of HMPA, the percentage of enol form being increased. The resulting five-coordinate tin enolates 1(h) showed high reactivity and selectivity for halide di- splacement in reactions with α-halo ketones (2). The tin enolates, when coordinated by Bu 4 NBr, effected a selective reaction with α-halo imines (5) to give a variety of γ-imino ketones (6), which were subsequently hydrolyzed to 1,4-diketones or cyclodehydrated to substituted pyrroles (9)