Competing reactions of hypercoordinate silicon dichelates

Abstract

AbstractNeutral hexacoordinate silicon complexes derived from hydrazide chelating ligands with imino‐donor groups, and their pentacoordinate ionic dissociation products, undergo facile intramolecular aldol‐type condensation catalyzed by their chloride counterion leading to formation of a third chelate ring. In analogous silacyclobutane dichelates, in the absence of halide counterion, a similar uncatalyzed rearrangement takes place, accompanied by opening of the four‐membered ring. In the absence of α‐protons necessary for the condensation, the four‐membered ring residue adds directly to one of the imino‐carbon atoms forming a new CC bond and closing a different chelate ring. This latter addition to the imino carbon is the preferred reaction pathway, even in the presence of 12 α‐protons, when cyanide ion replaces the chloride counterion and acts as nucleophile. The cyanide reactivity is rationalized in terms of the HSAB concept. An unusual intramolecular rearrangement involving the migration of a t‐butyl group from silicon to carbon, while enabling the unprecedented attachment of a third hydrazide chelating agent, leading to a hexacoordinate trichelate complex, is presented. Copyright © 2008 John Wiley & Sons, Ltd.