Silacyclobutanes: Head‐to‐Head Dimerization Versus Anionic Polymerization — α‐Silyl Substituted Carbanions as Reactive Intermediates

Reacting 2-neopentyl substituted silacyclobutanes 1a,b with MeLi/HMPA (hexamethylphosphoric triamide) anionic polymerization to give polymers 3a,b plays only a minor role for product formation. Instead, the head-to-head dimers 2a,b are isolated as main products. Their formation is explained by a complex reaction mechanism, in which various carbanionic, highly reactive intermediates are discussed. Obviously, the bis-a-silyl substituted carbanions 10a,b are remarkably stable, as can be concluded from 29Si NMR spectroscopic investigations at low temperature and from the products formed by trapping reactions with alcohols.