A Mechanistic Study of Alcohol Addition to 1,1-Diphenylsilene

Abstract

<p> The photochemistry of 1,1-diphenylsilacyclobutane has been studied by steady-state and nanosecond laser flash photolysis (NLFP) techniques in order to investigate the mechanism of reaction of a transient silene with alcohols. NLFP of 1,1-diphenylsilacyclobutane (I) in acetonitrile solution at 325 nm results in the formation of 1,1-diphenylsilene (II) derived from excited-state [2+2]-cycloreversion of the silacyclobutane. The transient has a lifetime of 100 ns - 1.2 μS in acetonitrile solution at room temperature, depending on the presence of water in the solvent. Steady-state photolysis in the presence of various σ-bonded nucleophiles such as methanol results in the generation of a single product in each case, consistent with addition of the nucleophile to the silene (II). Absolute rate constants (kq) have been determined by NLFP techniques for reactions of the silene with water, methanol, ethanol, tert-butyl alcohol, 2-propanol, 2,2,2-trifluoroethanol, 1,1,1,3,3,3-hexafluoroisopropanol, and glacial acetic acid. The rate constants (kq) illustrate the following characteristics: linear quenching plots characteristic of second order kinetics, observation of small but detectable deuterium kinetic isotope effects, and an inverse Arrhenius temperature dependence. Two possible mechanisms will be discussed based on these results. (See Mechanisms in Thesis)</p>