Study of Steric and Electronic Effects on the Reactivities of Diarylsilylene in Solution by Laser Flash Photolysis Methods

Abstract

<p>1, 1, 1,3,3,3-Hexamethyl-2,2-bis(3,4,5-trimethylphenyl)trisilane (<strong>43</strong>) has been synthesized and its photochemistry studied by steady state and nanosecond laser flash photolysis (NLFP) methods in solution. Photolysis of <strong>43</strong> in the presence of methanol leads to the formation of methoxybis(3,4,5-trimethylphenyl)silane (<strong>53</strong>) as the major product, resulting from the reaction of the alcohol with the reactive intermediate, bis(3,4,5-trimethylphenyl)silylene (<strong>1</strong>, SiTmp₂).</p> <p>The reactivity of SiTmp₂ with various substrates such as methanol, acetone, CCl₄, acetic acid, triethylsilane, cyclohexene, 1,3-dienes, bis(trimethylsilyl)acetylene (BTMSE), and O₂ has been examined by NLFP techniques, and the results are compared to analogous data for diphenylsilylene (<strong>3</strong>). In almost all cases it was found that electronic factors reduce the absolute rate constants by a factor of ca. 2, except for the reactions with triethylsilane and BTMSE where a factor of ca. 5 rate retardation was observed. This indicates that in general the inductive electron-donating effect associated with six methyl groups on the aromatic rings has only a small effect on diarylsilylene reactivity. The reactivity of SiMes₂ (<strong>2</strong>) with some substrates such as alcohols, CCl₄, acetic acid, isoprene and BTMSE has also been studied. The rate constants for the reactions of SiMes₂ and SiTmp₂ with each substrate were compared to provide a quantitative assessment of steric effects of the ortho methyl groups in diarylsilylene reactivity. It has been observed that for substrates like acetone, acetic acid and O₂ there is almost no sensitivity to steric effects, it is moderate in the cases of triethylsilane, CCl₄ and methanol and high in the cases of alkenes, dienes and alkynes. This study is the first quantitative assessment of the individual effects of steric hindrance and electronics on the reactivities of diarylsilylenes.</p>