Silylene dimesitylsilylene

When alkynyldisilanes 13a and b were photolyzed in the presence of freshly generated dimesitylsilylene (Mes2Si ), the silylene added to the Si=C double bond of 1-silaallenes 14a and b to form disilacyclopropanes 15a and b (Scheme 5). Even without the independently generated silylene, photolysis of 13b produced 15b in 8% yield, but compound 13a gave only traces of 15a. In the case of 15b, the dimesitylsilylene most likely originated from silacyclopropene 16. 

Other authors have observed similar behavior of silenes with silylene sources. Thus Ishikawa202 reported the addition of dimesitylsilylene to the silicon-carbon double bond of a silaallene, yielding the disilacyclopropene shown in Eq. (34). 

When silylenes are generated photochemically in hydrocarbon matrices in the presence of electron-pair donors, they may form Lewis acid-base complexes that act as intermediates in the silylene dimerization to disilenes.3233 In a typical example, Mes2Si(SiMe3)2 was photolyzed in 3-meth-ylpentane (3-MP) matrix containing 5% of 2-methyltetrahydrofuran. At 77 K, dimesitylsilylene (Amax 577 nm) was formed. When the matrix was... 

There is much evidence for silylenes reacting as Lewis bases, but complexes of silylenes acting as a Lewis acid are now well established (Fig. 14.3, Table 14.2). These complexes are also described as silaylides, R2Si —X+. Formation of silylene complexes with Lewis bases is conhrmed by a strong blue shift of the n-p transition. Matrix isolated dimesitylsilylene reacts with carbon monoxide to form the complex shown in Eq. The complex absorbs at 354 nm. 

The only exception to these uniform spectroscopic properties of silylene-Lewis base complexes, i.e., the hypsochromic shift on complexation, is silacarbonyl ylide 27, the absorption maximum of which is shifted to wavelengths longer than that of free dimesitylsilylene (20a) 31 an explanation of this observation is yet to come. 

In a similar study, dimesitylsilylene was generated photochemically from Mes2Si(SiMe3)2 in a soft matrix, 4 1 isopentane 3-MP, in the presence of bases, at 77 K. The absorption frequencies in nm for the complexed silylene with various bases were A-rnclhy I pyrrolidine, 324 pyrrolidine, 334 piperidine, 328 quinuclidine, 330 n-Bu3P, 338 THF, 320 thiophene, 315. Agreement with the values in Table 4 is rather good143. 

Although the currently available results still do not provide a uniform scheme, they do clearly indicate that silylenes bearing bulky substituents such as 2, and also dimesitylsilylene [14], xmdergo [2+1]-cycloadditions to the double bonds of 1,3-dienes rather than [4+l]-cycloadditions. In contrast, the behavior of disilenes towards the C=C double bonds of alkenes and conjugated dienes is still not clear. While additions of the stable tetraaryldisilenes to such double bond systems are still unknown [1], the marginally stable disilene 3 is able, in individual cases, to take part in both [2+2]- and [4+2]-cycloaddition reactions. 

Silylenes 1 are highly reactive homologues of the carbenes, and we have been interested to compare the reactivity and primary products of the oxidation of these divalent species. In principal one can expect two different primary adducts of a silylene and molecular oxygen the formal "end-on" adducts silanone 0-oxide 2 or "side-on" adducts dioxasilirane 3 (Scheme 1). It was shown by numerous matrix studies [8-12], experiments in solution using time resolved spectroscopy [13-16], and a preparative scale synthesis in solution [17] that triplet as well as singlet carbenes yield carbonyl 0-oxides as the primary oxidation products, while dioxiranes are products of secondary photolysis. Ando et al. reported on the synthesis of the silanone 0-oxide 2e by the reaction of dimesitylsilylene le and O2 in solid argon [1]. This is so far the only experimental evidence for a silanone O-oxide. 

The fact that the vinylsiliranes can be isolated is worthy of note since the previously reported reactions of practically all silylenes including those bearing very voluminous substituents had given rise to [ 1 +4] cycloaddition products exclusively. Only with dimesitylsilylene, generated by photolysis of 2,2-dimesitylhexamethyltrisilane, was it possible to characterize mixtures of [1+2] and [1+4] cycloadducts, but no pure vinylsilirane was isolated [6],... 

The generation and reactions of silylenes (silanediyls) have again attracted attention. The decay of dimesitylsilylene,... 

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