UV spectrum and decay kinetics of transient methylsilene produced in the 193 nm photolysis of gaseous 1-methyl-1-silacyclobutane

A Combined Experimental and Theoretical Study of the Kinetics and Mechanism of the Addition of Alcohols to Electronically Stabilized Silenes: A New Mechanism for the Addition of Alcohols to the SiC Bond

The stabilized silene 1,1-bis(trimethylsilyl)-2-adamantylidenesilane (4) has been generated by photolysis of a novel trisilacyclobutane derivative in various solvents and studied directly by kinetic UV spectrophotometry. Silene 4 decays with second-order kinetics in degassed hexane solution at 23 degrees C (k/epsilon = 8.6 x 10(-6) cm s(-1)) due to head-to-head dimerization. It reacts rapidly with oxygen [k(25 degrees C) approximately 3 x 10(5) M(-1) s(-1)] but approximately 10 orders of magnitude more slowly with methanol (MeOH) than other silenes that have been studied previously. The data are consistent with a mechanism involving reaction with the hydrogen-bonded dimer of the alcohol, (MeOH)(2) (k = 40 +/- 3 M(-1) s(-1); k(H)/k(D) = 1.7 +/- 0.2). The stable analogue of silene 4, 1-tert-butyldimethylsilyl-1-trimethylsilyl-2-adamantylidenesilane (5), reacts approximately 50 times more slowly, but via the same mechanism. The mechanism for addition of water and methanol (ROH; R = H, Me) to 4, 5, and the model compound 1,1-bis(silyl)-2,2-dimethylsilene (3a) has been studied computationally at the B3LYP/6-31G(d) and MP2/6-31G(d) levels of theory. Hydrogen-bonded complexes with monomeric and dimeric methanol, in which the Si=C bond plays the role of nucleophile, have been located computationally for all three silenes. Reaction pathways have been characterized for reaction of the three silenes with monomeric and dimeric ROH and reveal significantly lower barriers for reaction with the dimeric form of the alcohol in each case. The calculations indicate that 5 should be approximately 40-fold less reactive toward dimeric MeOH than 4, in excellent agreement with the approximately 50-fold difference in the experimental rate constants for reaction in hexane solution.

Substituent effects on the reactivity of the silicon-carbon double bond

Laser flash photolysis of various organosilicon compounds such as aryl-, vinyl-, and alkynyldisilanes, silacyclobutanes and silacyclobutenes, and alpha-silylketenes and -diazomethanes leads to the formation of reactive silenes which can be detected directly in solution, allowing detailed studies of the kinetics and mechanisms of their reactions with nucleophiles. Over 30 transient silenes have now been studied by these methods, providing the opportunity to systematically assess the effects of substituents at silicon and carbon on the reactivity of the Si=C bond.

The one- and two-photon photochemistry of benzylsilacyclobutanes, acyclic benzylsilanes, and 1,1,2-triphenylsilacyclobutane

The photochemistry of several α-silylbenzyl compounds has been investigated in hexane and in methanol solution. Direct photolysis of 1-benzyl-1-methylsilacyclobutane (1) in methanolic hexane solution produces 1-propyl-1-methyl-2,3-benzosilacyclobutene (6) in quantitative yield, by a sequential two-photon process involving the photoactive isotoluene derivative 1-methylene-6-(1-methylsilacyclobutyl)-2,4-cyclohexadiene (13a), which has been identified on the basis of its 1H NMR and UV absorption spectra. In contrast, direct irradiation of 1-benzyl-1-phenylsilacyclobutane (2) under similar conditions results in the formation of a complex mixture of products consistent with the competing formation of 1-benzyl-1-phenylsilene and benzyl- and 1-phenylsilacyclobutyl radicals. The silene is a transient which has been detected directly by laser flash photolysis of 2 (λmax = 315 nm, τ ~ 4.5 µs). Free radical formation is shown to be due to secondary photolysis of a second primary product, 1-methylene-6-(1-phenylsilacyclobutyl)-2,4-cyclohexadiene (13b), which has also been detected and identified by static UV absorption (λmax = 335 nm) and 1H NMR spectroscopy. In a reaction with some analogy to the acid-catalyzed desilylation of allylsilanes, both 13a and 13b can be intercepted in neutral or acidic methanol solution to yield toluene and 1-methyl- or 1-phenyl-1-methoxysilacyclobutane, respectively. Direct photolysis of benzyldimethylphenylsilane (4) also leads to the formation of the corresponding isotoluene derivative, while benzyltrimethylsilane (3) exhibits negligible photoreactivity. The endocyclic benzylsilane 1,1,2-triphenylsilacyclobutane (5) is shown to undergo competing [2 + 2]-cycloreversion and [1,3]-silyl migration to yield a bicyclic isotoluene analogue, which reacts rapidly with methanol to yield the acyclic methoxysilane reported previously to be the main product of photolysis of this silacyclobutane in methanol solution. Relative quantum yields for isotoluene formation from photolysis of 1-4 and absolute rate constants for methanolysis of several of these compounds under neutral and acidic conditions have also been determined.Key words: photochemistry, organosilicon, benzylsilane, silacyclobutane, silene, kinetics, isotoluene.

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

Direct photolysis of 1-phenylsilacyclobutane and 1-phenyl-, 1-(2-phenylethynyl)-, and 1-(4'-biphenylyl)-1-methylsilacyclobutane in hexane solution leads to the formation of ethylene and the corresponding 1-arylsilenes, which have been trapped by photolysis in the presence of methanol. Quantum yields for photolysis of the three methyl-substituted compounds have been determined to be 0.04, 0.26, and 0.29, respectively, using the photolysis of 1,1-diphenylsilacyclobutane Φsilene = 0.21) as the actinometer. The corresponding silenes have been detected by laser flash photolysis; they have lifetimes of several microseconds, exhibit UV absorption maxima ranging from 315 to 330 nm, and react with methanol with rate constants on the order of (2-5) × 109 M-1 s-1 in hexane. Absolute rate constants for reaction of 1-phenylsilene and 1-methyl-1-phenylsilene with water, methanol, tert-butanol, and acetic acid in acetonitrile solution have been determined, and are compared to those of 1,1-diphenylsilene under the same conditions. With the phenylethynyl- and biphenyl-substituted methylsilacyclobutanes, the triplet states can also be detected by laser flash photolysis, and are shown to not be involved in silene formation on the basis of triplet sensitization and (or) quenching experiments. Fluorescence emission spectra and singlet lifetimes have been determined for the three 1-aryl-1-methylsilacyclobutanes, 1,1-diphenylsilacyclobutane, and a series of acyclic arylmethylsilane model compounds. These data, along with the reaction quantum yields, allow estimates to be made of the rate constants for the excited singlet state reaction responsible for silene formation. 1-Methyl-1-phenylsilacyclobutane undergoes reaction from its lowest excited singlet state with a rate constant 10-80 times lower than those of the other three derivatives. The results are consistent with a stepwise mechanism for silene formation, involving a 1,4-biradicaloid intermediate that partitions between product and starting material.Key words: silene, silacyclobutane, photochemistry, biradical.