Mechanistic Studies of the Reactions of Silicon−Carbon Double Bonds. Addition of Alcohols to 1,1-Diphenylsilene

Abstract

The addition of water, aliphatic alcohols, and acetic acid to 1,1-diphenylsilene (generated by photolysis of 1,1-diphenylsilacyclobutane) has been studied in polar solvents using steadystate and nanosecond laser flash photolysis techniques. Absolute rate constants and (selected) deuterium kinetic isotope effects for the addition of water, methanol, ethanol, 2-propanol, tert-butyl alcohol, and acetic acid have been determined at 23°C in acetonitrile solution. Silene quenching follows a linear dependence on quencher concentration over the range investigated in all cases and proceeds with rate constants which vary over a range 4.1 × 10 8 -1.6 × 10 9 M -1 s -1 . The rate constants exhibit small primary deuterium kinetic isotope effects in all cases except acetic acid. Rate constants for addition of methanol, tertbutyl alcohol, and acetic acid have also been determined in hexane and THF solution. The transient absorption spectrum of the silene is broadened and red-shifted markedly in the latter solvent compared to that in acetonitrile and hexane, consistent with the formation of a solvent complex. Steady-state competition experiments have been carried out with various pairs of alcohols and water. The product ratios agree with the corresponding relative rate constants for water, methanol, and ethanol. Those for methanol/tert-butyl alcohol are significantly different from the rate constant ratio but approach it at very low total alcohol concentrations. The results are consistent with a two-step mechanism involving reversible formation of a silene-alcohol complex, followed by intracomplex proton transfer. The latter is rate-determining in all cases but acetic acid, for which it is proposed that complexation is the rate-determining step for reaction. Proton transfer from the complex to a second molecule of alcohol competes with the intracomplex pathway at higher alcohol concentrations for all cases but tert-butyl alcohol and acetic acid.

Steady state and time-resolved spectroscopic studies of the photochemistry of 1-arylsilacyclobutanes and the chemistry of 1-arylsilenes

“…Quantum yields for disappearance of 4-6 were determined by 254 nm merry-go-round photolysis of deoxygenated 0.03-0.04 M solutions in hexane containing methanol (0.05 M), using 1,1-diphenylsilacyclobutane (1) as actinometer. Photolysis of 1 in methanolic hexane solution affords ethylene and diphenylmethoxymethylsilane (16) as the only detectable products (2, 4, 11) -the latter due to trapping of 1,1-diphenylsilene (2) -and is characterized by a quantum yield of Φ 2 = 0.21 ± 0.03 (11). Quantum yields were calculated from the relative slopes of plots of concentration vs. photolysis time, which are shown in Fig.…”

“…1,1-Diphenylsilacyclobutane (1) affords 1,1diphenylsilene (2; eq. [1]) with reasonable efficiency (Φ 2 = 0.21), and time-resolved experiments have shown that silene formation occurs on the nanosecond time scale and cannot be quenched by saturation of the solution with oxygen or other triplet quenchers (11). The reactive excited state is thus most likely the lowest excited singlet state, which in these cases is predominantly of benzenoid π,π* character.…”

“…Silacyclobutanes are well known to yield products consistent with the formation of silene-reactive intermediates upon photolysis in solution or the gas phase, or upon pyrolysis at high temperatures (1)(2)(3)(4)(5)(6)(7)(8). These compounds have proven to be tremendously useful as photochemical precursors for transient silenes, in experiments aimed at detecting the silenes directly and studying the kinetics and mechanisms of their reactions with alcohols and other "silenophiles" (9)(10)(11)(12)(13)(14)(15)(16)(17). Photochemical silene formation proceeds with useful efficiencies from silacyclobutanes that bear alkyl, vinyl, ethynyl, silyl, and phenyl substituents at silicon, and possess UV absorption maxima between 190 and 250 nm (2,4,12,13,(17)(18)(19)(20).…”

Silenes: Connectors between classical alkenes and nonclassical heavy alkenes

“…The Ph-Si C -conjugation can also be attenuated by twisting the phenyl rings out of the plane of the silene moiety. Increased number of ortho-methyl groups bonded to the phenyl substituents of Ph 2 Si CH 2 (59) weakened the conjugation as indicated by a blueshift in λ max of 30 nm when going from 59 to 61a-61f [101,102].…”

Kinetics and Mechanism of the Addition of Aliphatic Amines to Transient Silenes

“…This behavior is also analogous to what has been observed previously for the addition of oxygen-centered nucleophiles to these two silenes. 15,18,21 In the case of oxygen-centered nucleophiles, we have recently shown that these trends are due, at least to some extent, to the effects of weak complexation of the free silene…”