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

Abstract

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.

Steric Effects on Silene Reactivity. The Effects ofortho-Methyl Substituents on the Kinetics and Mechanisms of the Reactions of 1,1-Diarylsilenes with Nucleophiles

“…We suspect, but cannot yet confirm (vide infra), that these transients are related to the phototransposition chemistry, as we have not observed behavior of this type in any of the arylsilacyclobutane derivatives whose photochemistry we have studied previously. 11,[17][18][19] The phototransposition of substituted benzene derivatives is thought to proceed via excited singlet state isomerization to a benzvalene derivative, which is then transformed into the aromatic isomer via tripletsensitized (quantum chain) decomposition. 13 In the case of 6c, as with any unsymmetrically disubstituted benzene derivative, the initial isomerization can in principle lead to six distinct benzvalene isomers, of which two (9c and 10c in eq 4) would yield 8c upon triplet-sensitized decomposition; the other four leave the ortho-substituent unchanged from its initial position.…”

Intramolecular Nucleophile-Induced Photorearrangements and Silene Formation from ano-(Methoxymethyl)phenylsilacyclobutane

“…This species is proposed to undergo competing loss of ethylene to yield silene 2 and migration of the methoxy group from the benzylic carbon to silicon, the driving force for which is the formation of the relatively strong Si-O bond in the product proposed to be a secondary photolysis product, while dialkoxysilane 7 is the product expected from (dark) reaction of 8 with methanol. 9 The considerably lower yield of 6 in the photolysis of 1 in methanol (compared to that in hexane) is consistent with a lower pseudophotostationary state concentration of 8 due to its reaction with the solvent to form 7. Additional evidence that 6 is formed via secondary photolysis of 8 was obtained by photolyzing a hexane solution of 1 simultaneously with 254 and 350 nm light sources.…”

“…The UV and NMR spectra of 8, its lifetime, and photochemical and ground-state reactivities are analogous to those of other 1-methylene-6-silyl-2,4-cyclohexadiene derivatives that have been reported. 9 The rearrangement of 9 to 10 can be viewed as an intramolecular homolytic substitution resulting from attack of the silicon radical center at oxygen. While we are unaware of any precedent for this process in silicon (mono)radical chemistry, preliminary DFT calculations indicate that the analogous rearrangement of a silyl radical closely related to 9 is exothermic by ca.…”

A gas-phase electron diffraction and quantum chemical investigation of the molecular structure of 1-bromosilacyclobutane

“…In numerous papers it has been shown that the thermolysis or flash photolosis of silacyclobutanes leads to either intermediate or stable divalent silicon compounds like silenes R 2 Si@C and silylenes R 2 Si: [8][9][10][11][12][13][14][15]. Silacyclobutanes have been recently utilized as carbanion pump in anionic living polymerization for the synthesis of block copolymers [16,17].…”