Mechanistic Aspects of the Complexation, Chalcogen Abstraction and Sigma Bond Insertion Reactions by Transient Silylenes and Germylenes in Solution

Abstract

<p>The complexation reactions of silylenes (SiMe₂, SiPh₂ and SiMes₂) and germylenes (GeMe₂, GePh₂ and GeMes₂) with a series of O-, S-, N- and P-donors have been studied in hexanes solution. The equilibrium constants for complexation of SiMes₂ and GeMes₂ with 7 Lewis bases were determined, and demonstrate that the silylene is more Lewis acidic than the germylene by ca. 1 kcal mol^-1. Diethyl ether reacted with the six tetrellylenes with equilibrium constants that decrease in the order SiPh₂ > SiMe₂ > GePh₂ > GeMe₂ > SiMes₂ > GeMes₂, establishing a trend in the Lewis acidities of the silylenes and germylenes. Experimental results are complemented by calculated (G4) binding enthalpies of the MMe₂-donor complexes, which were found to correlate with Drago’s E and C parameters leading to the classification of SiMe₂ and GeMe₂ as borderline soft Lewis acids.</p> <p>A number of sigma-bond insertion reactions by transient silylenes was examined, namely the O-H, N-H and Si-O insertion reactions with alcohols, amines and siloxanes, respectively. In all cases the reactions were found to proceed via a two step mechanism in which the first step is a reversible formation of a Lewis acid-base complex. The second step was found to be a catalytic H-migration in the reactions with alcohols and amines; the catalysis by the alcohol is at least 10⁴ times faster than that by the amine. Complexes of silylenes with alkoxysilanes and siloxanes transform into the final products via a unimolecular [1,2]-silyl migration.</p> <p>Chalcogen abstraction reactions by silylenes (SiMe₂, SiPh₂, SiTmp₂ and SiMes₂) and germylenes (GeMe₂ and GePh₂) from oxiranes (cyclohexene oxide (CHO) and propylene oxide (PrO)) and thiiranes (cyclohexene sulfide (CHS) and propylene sulfide (PrS)) were investigated by laser flash photolysis and steady-state photolysis methods. The results indicate that the reaction proceeds via a two step mechanism, in which the first step is a reversible complexation followed by a unimolecular decomposition of the complex to yield products of chalcogen abstraction, namely alkenes and the corresponding R₂M=X transients (R = Me, Ph, Tmp and Mes, M = Si or Ge, X = O or S). Diphenylsilanethione was directly detected and identified on the basis of its spectra and reactivity with amines and alcohols. The O- and S- abstraction by silylenes proceed with ca. 50% efficiency; in contrast, no evidence for O-abstraction by GeMe₂ from CHO could be found, while propene was formed in ca. 35% yield in the reaction of GeMe₂ with PrS.</p>