1,1-organoboration of alkynylsilicon, -germanium, -tin and -lead compounds

Abstract

In the course of 1,1-organoboration reactions the metal-carbon bond in 1-alkynylmetal compounds (L n MCCR 1) is cleaved by a triorganoborane (R 3B), followed by a selective new CC bond formation via a 1,2 shift of a group R from boron to carbon. Mono-1-alkynyl derivatives of silicon, germanium, tin and lead react with R 3B via 1,1-organoboration to give organometallic-substituted alkenes in high yield and stereoselectively, in most cases, with the R 2B group and M (MSi, Ge, Sn, Pb) in cis position at the CC bond. These alkenes can again be used for 1,1-organoboration reactions which lead either to allenes or to dienes and 3-borolenes. The same type of reaction can be applied to di-1-alkynylmetal derivatives. The first step in the twofold 1,1-organoboration is an intermolecular 1,1-organoboration, followed by an intramolecular 1,1-vinyloboration which leads exclusively, in the case of MSi and Ge, to siloles and germoles. In the same way stannoles and plumboles can be prepared; however, depending on R (e.g. R iPr) in R 3B and on R 1 (e.g. R 1Me) in the alkyne other heterocycles (e.g. 1,4-stannabora-2,5-cyclohexadiene or 1,4-plumbabora-2,5-cyclohexadiene derivatives) may also be obtained. Tetra-1-alkynyl derivatives of silicon and germanium react with R 3B to give selectively spiro compounds with two silole or germole rings. Starting from Sn(CCR 1) 4 and R 3B various types of spiro compounds are obtained, depending on R and R 1. In all compounds derived from 1,1-organoboration reactions numerous reactive element-carbon bonds are available for further transformations. The mechanism of the 1,1-organoboration is revealed by the nuclear magnetic resonance spectroscopic analysis in solution and in the solid state as well as by direct structural characterization of several zwitterionic intermediates in which a cationic triorganotin or triorganolead fragment is weakly coordinated to the CC bond of an alkynylborate.