Highly Controlled Chemoselectivity of Tin Enolate by Its Hybridization State. Anionic Complex of Tin Enolate Coordinated by Tetrabutylammonium Bromide as Halo Selective Reagent

Abstract

The control of reactivity and/or selectivity of organotin(IV) enolates has been studied by means of a high coordination method in which the hybridization state of tin was effectively changed. An anionic tin complex, five-coordinated tin enolate 1(h), was formed by the coordination of bromide anion from tetrabutylammonium bromide (Bu4NBr) to neutral four-coordinated tin(IV) enolates 1(e). The highly coordinated enolates which attained a marked change in chemoselectivity have higher nucleophilicity to organic halides. In addition, they showed low nucleophilicity toward carbonyl moieties by the coordination of the bromide anion, whereas carbonyl addition readily proceeds using the usual four-coordinated tin enolate. NMR studies revealed the generation of HMPA- or bromide-anion-coordinated tin enolates in equilibrium between four-coordinated enolate 1c(e) and five-coordinated enolate 1c(h), showing a considerable upfield shift δ(119Sn) and increasing values of the coupling constants 1J(119Sn−13C). In detailed NMR analyses of highly coordinated tin enolate, it was shown that the contribution of highly coordinated species 1(h) to the equilibrium between 1(e) and 1(h) became more significant as the added amount of ligands such as Bu4NBr or HMPA was increased. Considering these results, it was assumed that the bromide-anion-coordinated tin enolate was generated, which showed a unique reactivity and selectivity. The effective control of chemoselectivity in the intermolecularly competitive reaction between organic halides and carbonyl compounds was demonstrated using two types of tin enolates, four-coordinated enolate and highly coordinated anionic enolate; halide displacement reaction exclusively took place using the bromide-anion-coordinated enolate, and the usual four-coordinated enolates reacted with only carbonyl compounds.