Photochemical Generation and Characterization of Di-tert-butylsilylene (SitBu2) in Solution

Abstract

Di-tert-butylsilylene (SitBu2) has been detected directly in solution and its reactivity characterized by laser flash photolysis methods. Laser photolysis of 7,7-di-tert-butyl-7-silabicyclo[4.1.0]heptane (5) affords a transient product that exhibits λmax ≈ 520 nm and decays on the microsecond time scale, concurrent with the growth of a second, much longer-lived species exhibiting λmax = 290, 430 nm. The two species are assigned to di-tert-butylsilylene (SitBu2) and its disilene dimer, tetra-tert-butyldisilene (tBu2Si═SitBu2, 8), respectively. Transient absorption spectra recorded by laser photolysis of hexa-tert-butylcyclotrisilane (7) are consistent with the prompt formation of both SitBu2 and 8, the former appearing as a weak shoulder absorption on the red edge of the intense absorption band due to the disilene. Absolute rate constants were determined for the reactions of SitBu2 with a variety of representative silylene substrates in hexanes at 25 °C, including methanol, triethylsilane, acetic acid, acetone, molecular oxygen, and four aliphatic alkenes (1-hexene, cyclohexene, cis-cyclooctene, and 2,3-dimethyl-2-butene). Rate and(or) equilibrium constants for Lewis acid–base complexation of the silylene with diethyl ether, tetrahydrofuran, tetrahydrothiophene, and diethyl- and triethylamine were also determined, along with the UV–vis absorption spectra of the corresponding Lewis acid–base complexes. Rate constants for the reactions of dimethylsilylene (SiMe2) and dimesitylsilylene (SiMes2) with cis-cyclooctene, 2,3-dimethyl-2-butene, and 1-hexene are also reported, enabling a comprehensive assessment to be made of steric effects on the reactivities of simple (transient) dialkyl- and diarylsilylene derivatives in solution. The kinetic data show the reactivity of SitBu2 to be intermediate between that of SiMe2 and SiMes2 under similar conditions, with the (2 + 1)-cycloaddition reactions with alkenes exhibiting the largest variations in rate constant as a function of substitution at Si. Absolute rate constants for the reactions of tetra-tert-butyldisilene (8) with O2 and acetone are also reported.