Photoreaction of 3-Allyloxy-2-pentamethyldisilanylpyridine

Abstract

It is well established that the photoreactions of phenyldisilanes proceed through four pathways: 1) formation of a silaethene, Si=C, 2) formation of a silatriene via 1,3-trimethylsilyl radical migration, 3) elimination of a silylene, and 4) nucleophilic cleavage of a Si-Si bond in the photoexcited state since the first report on the photolysis of phenylpentamethyldisilane and (p-tolyl)pentamethyldisilane by Ishikawa et al. in 1975. However, relatively little is known about the photoreactions of ortho-substituted phenylpentamethyldisilanes. Recently, we have found that the photoreactions of various ortho-substituted phenylpentamethyldisilanes afforded the novel intramolecular photoproducts via silatriene intermediates. As part of our ongoing investigations for the organic synthesis of silicon-having heterocyclic compounds, we were, now, interested in the pyridine ring instead of the phenyl ring as an aromatic ring. And, we would like to report detailed photochemical investigations of 3-allyloxy-2-pentamethyldisilanylpyridine 2, aza analogue of (2-allyloxyphenyl)pentamethyldisilane, since novel photoproducts from the photoreaction of 2 via new intermediates are expected. The starting 3-allyloxy-2-pentamethyldisilanylpyridine 2 was prepared by the reaction of pentamethyldisilanyl chloride with 3-allyloxy-2-pyridyl magnesium bromide in tetrahydrofuran as shown in Scheme 1. In this reaction, a compound 3 formed from the allyl group cleavage of 2 was also obtained. Irradiation of 2 in deaerated benzene with 300nm UV light afforded a novel photoproduct 4 (9%yield) in addition to 5 (15%yield) along with several decomposition products of unknown structure as shown in Scheme 1, when 92% of 2 was photolyzed. Indeed, the atomic charges of 2-, 3-, and 4pyridyl radical site (+0.015, −0.268, and −0.007, respectively) have been calculated at the BPW91/cc-pVDZ// BPW91/cc-pVDZ. In this paper, it is revealed that 2pyridyl radical with a partial positive charge behaves like an electrophilic radical, but 3and 4-pyridyl radical behave like a nucleophilic radical. And, the relative energies of 2-, 3-, and 4-pyridyl radical (0, 26, and 21 kJ/mol, respectively) have been calculated by G3B3. Therefore, it is revealed that 2-pyridyl radical is more stable than the 4-isomer, which is in turn more stable than 3-isomer in this paper. These above results suggested that the photoproduct 4 is formed from the reaction of electrophilic silylene and 2 mol of nucleophilic, unstable 3-pyridyl radical 9, formed via 1,4radical shift from the electrophilic, stable 2-pyridyl radical 8 (Scheme 2). In this case, it is thought that the reaction of nucleophilic 4-pyridyl radical and electrophilic silylene was not occurred because of the steric hindrance of the allyoxy group. The formation of 5 is explained by the allyl group cleavage from the radical intermediates, 8 or 9 as shown in Scheme 2. The structure of the photoproduct 4 was determined by various physical methods. The H NMR spectrum of the photoproduct 4 shows the characteristic 3,5-disubstituted pyridine structure. The two protons (2 and 6 positions in the