The synthesis and potential applications of asymmetric silacycles
Although the use of silicon-based reagents has undergone rapid development during the last twenty years, the application of organosilicon chemistry to asymmetric synthesis has been somewhat slower to develop. The many problems associated with the use of 'Si-centred' chiral organosilicon compounds has led to the application of 'C-centred' chiral organosilicon compounds. This work has been aimed at the synthesis and application of cyclic silicon species. Routes towards the synthesis of medium-sized rings have been investigated as a potential application of enantiomerically pure silacycles. This work has led to the discovery of an unusual tandem cycloaddition-bond fragmentation reaction of 3-(dienylacyloxy)cycloalk-2- en-l-ones, which affords a-tetralone as the principal product. Most work has been directed at the synthesis of asymmetric silacycles. Two routes have been explored. Firstly, the double asymmetric hydrosilylation of dienes, catalysts based on many transition metals were used but little evidence of hydrosilylation was observed. The second route is that of the double asymmetric hydroboration of divinylsilanes. Asymmetric stoichiometric hydroboration led to products of moderate to high enantiomeric excess, whilst rhodium-catalysed hydroboration led to high yields of the achiral syn isomer. The diastereoselectivity has been found to vary according to the length of the tether between two phosphine ligands, with maximum diastereoselectivity being observed for butanodiphosphines. NMR studies have investigated the possibility that this is related to the stability of a divinylsilane-diphosphine rhodium complex. Finally, the formation of a variety of silacycles has been attempted. Boron- redistribution of the product of hydroboration with (-)-diisopinocampheylborane has been shown to occur with retention of stereochemistry and subsequent carbonylation led to the formation of asymmetric silacyclohexanones. Oxidation of the hydroboration product led to the formation of a silyldiol species. Reactions of this silyldiol have provided the basis for encouraging preliminary attempts at the formation of other heterosilacycles.