Silylenes reactions

Pyrolysis of polysilanes played an important role in the discovery of silylene reactions. Through the pyrolysis of alkoxydisilanes in the presence of diphenylacety-lene, Atwell and Weynberg obtained a product regarded as a dimer of dimethylsilylene adduct. 1,2-Shift of a methoxy group in disilanes takes place under relatively mild conditions (Scheme 14.1). ... 

R. Becerra and R. Walsh, Kinetics and mechanisms of silylene reactions a prototype for gas-phase acid/base chemistry, in Research in Chemical Kinetics (Eds. R. G. Compton and G. Hancock), Vol. 3, Elsevier, Amsterdam, 1995, p. 263. 

III. YLIDE AND SMALL-RING FORMATIONS BY SILYLENE REACTIONS WITH HETEROATOMS AND CUMULENES... 

Extrusion of a silylene from a silirane or silirene is of course the inverse of silylene addition to alkenes or alkynes, respectively. The reversibility of most silylene reactions allows the inverse of addition to 1,3-dienes to also be employed as a silylene source. The first such reaction was reported by Chernyshev and coworkers, who found that transfer of SiCl2 units from l,l-dichlorosilacyclopent-3-enes was a unimolecular process and hence was likely to consist of silylene extrusion and readdition (equation 38)82. Dimethylsilylene extrusion has been found in the pyrolysis of silacyclopentenes and other products of... 

Despite the short lifetimes of most silylenes, improvements in flash photolysis techniques for their generation and time-resolved spectroscopic detection methods in the past decade have made possible direct kinetic measurements on the reactions of silylenes. The purpose of these kinetic studies has been to elucidate the mechanisms of silylene reactions. While considerable work remains to be done, transition state structures and activation barriers are emerging from these experiments, and aspects of silylene insertion and addition mechanisms have been revealed that were not uncovered by product studies and were, indeed, unexpected. 

Becerra and Walsh have presented a clear exposition of the mechanistic problems addressed by kinetic studies on silylenes and the conclusions that have been drawn to date323. Absolute rate studies have begun to throw light on the questions How do the structures and energies of silylenes affect their reactivity What are the transition state structures What is the balance between the electrophilic and nucleophilic character of the attacking silylene in the transition state What roles do steric and electronic effects play in silylene reactions ... 

In this model the reaction rate decreases with increasing temperature because dissociation of the intermediate complex to the reactants has a higher barrier (i.e. via TSi), than does the rearrangement of the complex (i.e. via TS2) to the product of the silylene reaction. Thus, as the temperature increases, product formation is disfavored relative to regeneration of the silylene from the intermediate complex. 

The reactions of transient silylenes are so rapid that most of the limited mechanistic information that has been obtained over the past quarter-century has been through indirect means. Direct measurements of silylene reaction rates by kinetic spectroscopy in the past decade have yielded important new insights. One can predict with some confidence an explosion of mechanistic studies of silylenes employing fast spectroscopies capable of providing more structural information than traditional electronic absorption and emission techniques. The nearly universal reversibility of silylene reactions remains to be fully exploited through kinetic studies of retro-reactions. The mechanisms of most silylene reactions remain to be fully elucidated, and this task will increase in urgency as silylenes see more use in synthesis. 

The C NMR spectra confirm these conclusions. It is interesting to note that the corresponding inethoxy compound was isolated by Atwell et al. [3] after disproportionation of tetramethoxy-dimethyldisilane. The formation of the branched silane was considered as a proof for a silylene reaction mechanism which is also discussed for our system. 

The majority of silylene reactions are insertions that immediately reestablish a tetracoordinated silicon. The most frequently observed insertions are into Si-O,... 

During the 1960s and 1970s, clean methods were developed for the generation of silylenes [5]. It required many years, however, for woricers in the field to recognize the connection between the most useful reactions for the production of silylenes, extrusion from disilanes and siliranes, and reactions that efficiently consume them, insertions into cr-bonds and addition to r-bonds. Yet staring us in the face was a phenomenon quite different from carbene chemistry nearly all silylene reactions are reversible [6]. This provides a compact framework within which to present a selection of the reactions that lead to silylenes and those reactions that consume them, as shown in Scheme 1. 

---