Absolute rate constants for the reaction of a series of ring-substituted 1,1 -diphenylsilene derivatives with methanol, tert-butanol, and acetic acid in acetonitrile solution have been determined using nanosecond laser flash photolysis techniques. The three reactions exhibit small positive Hammett ρ-values at 23 °C, consistent with a mechanism involving initial, reversible nucleophilic attack at silicon to form a σ-bonded complex that collapses to product via rate-limiting proton transfer. Deuterium kinetic isotope effects and Arrhenius parameters have been determined for the reactions of 1,1-di-(4-methylphenyl)silene and 1,1-di-(4-trifluoromethylphenyl)silene with methanol, and are compared to those for the parent compound. Proton transfer within the complex is dominated by entropic factors, resulting in negative activation energies for reaction. The trends in the data can be rationalized in terms of variations in the relative rate constants for reversion to reactants and proton transfer as a function of temperature and substituent. A comparison of the Arrhenius activation energies for reaction of acetic acid with 1,1-diphenylsilene (Ea = +1.9 ± 0.3 kcal/mol) and the more reactive di-trifluoromethyl analogue (Ea = +3.6 ± 0.5 kcal/mol) suggests that carboxylic acids also add by a stepwise mechanism, but with formation of the complex being rate determining. Keywords: silene, substituent effects, kinetics, Arrhenius, flash photolysis.
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.
Absolute rate constants for the reaction of acetone with
phenylsilene, 1-methyl-1-phenylsilene, and a series of ring-substituted 1,1-diphenylsilene
derivatives have been
determined in polar and nonpolar solvents using nanosecond laser flash
photolysis
techniques. The reaction (which affords the corresponding silyl
enol ether) proceeds
significantly faster at 23 °C in hydrocarbon solvents than in
acetonitrile in all cases, but
the Hammett ρ-values defined by the data for the substituted
1,1-diphenylsilenes are larger
in isooctane (ρ ≈ +1.5) than in acetonitrile (ρ ≈ +1.1).
Deuterium kinetic isotope effects
and Arrhenius parameters have been determined for the reactions of
1-methyl-1-phenyl-,
1,1-diphenyl-, 1,1-bis(4-methylphenyl)-, and
1,1-bis(4-(trifluoromethyl)phenyl)silene in
hexane
and acetonitrile. All but
1,1-bis(4-(trifluoromethyl)phenyl)silene exhibit
negative activation
energies for reaction. The trifluoromethyl derivative, the most
reactive in the series, exhibits
a positive E
a in acetonitrile and a curved
Arrhenius plot in hexane. The results are consistent
with a mechanism involving initial, reversible formation of a
silene−ketone complex which
collapses to product by rate-controlling proton transfer. The
trends in the data can be
rationalized in terms of variations in the relative rate constants for
reversion to reactants
and hydrogen transfer as a function of temperature, substituent, and
solvent. The differences
between acetonitrile and hydrocarbon solvents are rationalized as due
to the effects of the
strong solvation of the free silene by the nitrile solvent.
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