Photochemistry of sila- and silylnorbornenes: Charge transfer photochemistry of pi-conjugated di- and trisilanes
Abstract
Mono- and Disilacyclopentenes undergo photochemical addition of alcohols to give adducts attributable to the intermediacy of $\beta$-silyl carbocations. Evidence suggests these $\beta$-silyl cations arise through protonation of strained ground state trans-silacyclopentenes produced via E,Z-photoisomerization. Two rigid bicyclic silacycloalkenes were designed to preclude the Z,E-photoisomerization, thus providing opportunities to study excited state reactivity towards photoprotonation by alcohols. The 214 nm photolysis of 1-silanorbornene in MeOH or (CH$\sb3)\sb3$COH resulted in (1,3-C) migration to form 3-alkoxy-3-methyl-3-silabicyclo(4.1.0) -heptanes with quantum yields of 0.053 and 0.062. The efficiency for photoprotonation was estimated to be $\Phi<$ 0.01 for methanol and neglible for tert-butyl alcohol. (1,3-C) photorearrangement was observed upon irradiation 2-trimethylsilylnorbornene in pentane and (CH$\sb3)\sb3$COH. In CH$\sb3$OH(D) photoprotonation formed minor product 2-trimethylsilylnorbornene via hydride transfer. Photoprotonation assumed greater importance in $\rm CF\sb3CH\sb2OH$ and resulted in desilylation to norbornene and addition of alcohol to a mixture of three alcohol adducts. Direct $\rm\sp1LE\to\ \sp3LE$ intersystem crossing is known to be inefficient in the parent system benzobicyclo(2.2.2) octadienone, it displays characteristic singlet and triplet photoreactivity that can be distinguished from each other. Photochemical studies of disilanyl-substituted benzobicyclo(2.2.2) octadienone were intended to obtain evidence for the sequence of electron transfer $\rm\sp1LE\to CT\to\ \sp3LE.$ Disilanyl-substituted benzobicyclo-(2.2.2) octadienone was synthesized by a regioselective route utilizing 6$\sp\prime$-bromo-2-naphthol. Despite efforts to promote the charge transfer process, the electron transfer from the disilyl group did not compete with singlet excited state ketene extrusion. Certain substituted aryldisilanes exhibit, in addition to fluorescence of the $\pi,\pi*$ locally excited (LE) state, a longer wavelength emission ascribable to an intramolecular charge transfer (CT) excited state. The direction of the electron transfer is from the $\sigma$ Si-Si bond to the aromatic $\pi$ system, a twisted intramolecular charge transfer state (TICT) is proposed. Studies have characterized the chemistry of $\pi,\pi*$ LE and CT states of an (arylcyclopentenyl)-pentamethyldisilane bearing a good electron acceptor aryl group. For comparison, an analog arylcyclopentenyl-heptamethyltrisilane was synthesized and photolyzed. Four products were observed. Of these, 1-(p-cyanophenyl)-2-(dimethylsilyl)cyclopentene, tricyclic 6-cyano-1,1-dimethyl-1-sila-2,3-cyclopentenoindene were previously observed from the analogous disilane. The two major photoproducts of the trisilane were SiH compound 1-(p-cyanophenyl)-2-(tetramethyldisilyl)cyclopentene from cleavage of the terminal Si-Si bond, and the internal Si-Si bond cleavage product 1-(p-cyanophenyl)-2-(dimethylmethoxysilyl)cyclopentene. Deuterium labeling and mechanistic studies are planned for further studies.
This paper has been withdrawn.