A Mechanistic Study of the Reactions 1,1-Diarylsilenes and 1-Silastyrenes

Abstract

<p>The chemistry of 1,1-diarylsilenes, 1-phenylsilene and 1-methyl-1-phenylsilene has been studied using nanosecond laser flash and steady-state photolysis techniques. The silenes were generated by photochemically induced [2+2] cycloreversion of 1,1-diarylsilacyclobutane, 1-phenylsilacyclobutane and 1-methyl-1-phenylsilacyclobutane, respectively. Steady-state photolysis in the presence of alcohols, methoxytrimenthylsilane, water and acetic yields the corresponding alkoxy-, hydroxy-, or acyloxysilane in high yield consistent with the trapping of the silene by ROH or silyl ether. Steady-state photolyses in the presence of acetone afford silyl enol ethers exclusively the product of an ene reaction between the silene and acetone. Nanosecond laser flash photolysis of air saturated hexane, acetonitrile or tetrahydrofuran solutions of the silacyclobuanes leads to readily detectable transient absortions in the 310-330 nm range which have lifetimes of 2-4 μs and have been assigned to the corresponding silenes. Incresing phenyl substitution at the silenic silicon atom results in red-shifts in the silene absorption maxima consistent with increasing conjugation of the chromophore. The transient absorption spectrum of 1,1-diphenylsilene in the THF solution is broadened and red-shifted compared to that in hexane and aceonitrile, consistent with the formation of a silene-THF complex. Absolute rate constants for reaction of the silenes with alcohols, trimethylmethoxysilane, acetic acid and acetone have been determined by NLFP techniques. Silene quenching follows a linear dependence on quencher concentration over the range investigated in all cases and proceeds with the rate constants which vary over the range 10⁷-10⁹ M⁻¹s⁻¹. The absolute rate constants for reaction with alcohols, trimethylmethoxysilane and acetic acid are slightly faster for phenyl- and methylphenysilene than for diphenylsilene. Deuterium kinetic isotope effects for reactions of the 1-silastyrenes (1,1-diphenylsilene, 1-phenylsilene and 1-methyl-1-phenylsilene) with methanol, t-butanol and acetone are consistent with a primary effect and rate determining proton transfer. Acetic acid addition is not subject to a deuterium kinetic isotope effect. Steady-state competition experiments between 1,1-diphenylsilene and variousu alcohols and water have been carried out. The product ratios agree with the corresponding relative constants for water, methanol and ethanol. Those for methanol/t-butanol are significantly different from the rate constant ratio but approach it at very low total alcohol concentrations. Reactions of the 1,1-diarysilenes with alcohols, acetic acid, acetone and trimethylmethoxysilane afford small positive Hammett ρ-values consistent with initial nucleophilic attack by oxygen on silicon. Deuterium isotope effects and arrhenius parameters have been determined for the reactions of 1,1-di-(4-methylphenyl)silene, 1,1-diphenylsilene and 1,1-di-(4-trifluriomethylphenyl)silene with methanol, acetone and acetic acid. Methanol and acetone additions are characterized by negative activation energies, which are suggested to result from entropy dominated proton transfer within a reversibly formed silene-nucleophile complex. The kinetic isotope effects for methanol and acetone addition decrease with increasing electron withdrawing ability of the diaryl substituent and temperature. Acetic acid addition proceeds with a positive activation energy that increases in magnitude with increasing electron withdrawing ability of the substituent. This observation is consistent with a small enthalpic barrier to complex formation. The above results are consistent with a two-step mechanism involving reversible formation of a zwitterionic complex, followed by intramolecular proton (or SiMe₃⁻) 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. The trends in kinetic isotope, substituent and temperature effects are rationalized in terms of variations in the relative rate constants for reversion of the complex to reactants and proton transfer as a function of substituent and temperature. In the case of alcohol additions, proton transfer from the complex to a second molecule of alcohol competes with the intracomplex proton transfer pathway at high alcohol concentrations, for all cases except t-butanol and acetic acid.</p>