Silenes adducts formed

The addition of N-(trimethylsilyl)benzophenone imine to 1,1-dimethyl-2,2-bis(trimethylsilyl)silene (54)243 proceeds in a fashion quite analogous to the reaction of benzophenone, but is much slower. Both the 2 + 2 (56) and the 4 + 2 (55) cycloadducts have been isolated (equation 74). The former is formed at only about half of the rate of the latter but dominates in the equilibrium. As described in Section III.A.2.C, the small amount of 54 present in the equilibrium can be trapped by other added reagents so that 56 represents a convenient means of storing the silene 54, which cannot be stored neat since it rapidly dimerizes244,246. Another such store is provided by the silenate adduct of 54 with NMe3, which reacts with N-(trimethylsilyl)benzophenone imine to yield 55 and 56266. 

The evidence is the formation of the expected methanol adducts of the silanone and of the silene when methanol is present in the irradiated mixture. In an inert solvent the expected 2 4-2 cycloaddition product of the silanone and the silene is formed quantitatively76. If the suspected silene and silanone transients were present in the solution in a truly free form, one might expect the formation of some silene dimer and some silanone... 

A few routes to new silenes, usually involving flash vacuum pyrolysis at high temperatures, have been reported. Silenes were proposed as the result of the thermal expulsion of trimethylmethoxysilane, or a similar volatile fragment, from the starting material but frequently, proof that the silenes proposed to account for the observed products were in fact formed was not provided.116 119 The other thermal route employed was the retro-Diels-Alder regeneration of a silene from an adduct with an aromatic compound—often a 9,10-anthracene or 1,4-naphthalene adduct or, in some cases, a 1,4-benzene adduct, as illustrated in Eq. (19).120... 

Jones218 has described an unusual photochemically initiated rearrangement of a silene-anthracene adduct to a silene which is part of an eight-membered ring (Eq. 60). Photolysis of the adduct 190 was believed to form the silaallylic diradical 191, whose canonical form 192 affords the... 

Photolysis of acyldisilanes at A > 360 nm (103,104) was shown, based on trapping experiments, to yield both silenes 22 and the isomeric siloxy-carbenes 23, but with polysilylacylsilanes only silenes 24 are formed, as shown by trapping experiments and NMR spectroscopy (104,122-124) (see Scheme 4). These silenes react conventionally with alcohols, 2,3-dimethylbutadiene (with one or two giving some evidence of minor amounts of ene-like products), and in a [2 + 2] manner with phenyl-propyne. Ketones, however, do not react cleanly. Perhaps the most unusual behavior of this family of silenes is their exclusive head-to-head dimerization as described in Section V. More recently it has been found that these silenes undergo thermal [2 + 2] reactions with butadiene itself (with minor amounts of the [2 + 4] adduct) and with styrene and vinyl-naphthalene. Also, it has been found that a dimethylsilylene precursor will... 

The silene 124 is probably formed as its THF adduct and can be trapped by, e.g., 1,3-dimethyl 2,3-butadiene to give a [4+2] cycloadduct. The attempt to liberate the silene 124 from its donor adduct results in the formation of a disilacyclobutane 125. This is ascribed to the prolonged life-time of the intermediate 359 formed by the methyl migration in the silene (equation 96), which allows for a hydrogen migration to take place. 

With the dihydropyranyl-substituted phenyldisilane 240 Ishikawa and coworkers found the formation of both possible types of silenes 241 and 242 they were formed and trapped by methanol giving adducts 401-403 (equation 120)134. 

Brook -type silenes have been found to undergo [4+2] and [2+2] cycloadditions with imines R2C=NR to give silatetrahydroisoquinolines or silaazetidines, respectively248. If formed, the [4 + 2] adducts generally rearrange slowly in the dark or more rapidly when photolysed to the thermodynamically more stable [2 + 2] isomers. 

Silaacrylate 305 undergoes reactions with ketones165. The initial coordination of the ketones is reminiscent of the donor adducts to silenes. The products 520-522 are formed by an ene or a formal [2 + 2] cycloaddition reaction, depending on the substituents on the ketones (equation 177). 

Wiberg and coworkers published relative rate constants and the products of reaction of silene 6 with a number of alkenes and dienes in ether solution at 100 °C6 106-108. These data are listed in Table 2 along with an indication of the type of product formed in each case. As is the norm in Diels-Alder additions by more conventional dienophiles, the rate of [2 + 4]-cycloaddition of 6 to dienes increases with sequential methyl substitution in the 2- and 3-positions of the diene, as is illustrated by the data for 2,3-dimethyl-1,3-butadiene (DMB), isoprene and 1,3-butadiene. The well-known effects of methyl substitution at the 1- and 4-positions of the diene in conventional Diels-Alder chemistry are also reflected with 6 as the dienophile. For example, lruns-1,3-pen tadiene reacts significantly faster than the f/.v-isorrier, an effect that has been attributed to steric destabilization of the transition state for [2 + 4]-cycloaddition. In fact, the reaction of c/s-l,3-pentadiene with 6 yields silacyclobutane adducts, while the trans-diene reacts by [2 + 4]-cycloaddition108. No detectable reaction occurs with 2,5-dimethyl-2,4-hexadiene. The reaction of 6 with isoprene occurs regioselectively to yield adducts 65a and 65b in the ratio 65a 65b = 8.5 (equation 50)106,107. 

Following studies of our research group [16-19] and later of other work groups [20], silenes with polar double bonds form adducts with donors D, which like halogenides, ethers, or amines possess atoms with a free, that is a nonbonding electron pair (Eq. 2) ... 

Products derived from the two silenes (163) and (164), formed by competing 1,3-trimethylsilyl migrations, were obtained on irradiation of l-(3,4-dihydro-2H-6-pyranyl)-1-phenyltetramethyl-disilane (165)Separate studies have shown that silenes are intermediates in the photorearrangement of analogous benzenoid disilanes. 1,4-Bis(pentamethyldisilanyl)naphthalene (166), however, behaves in a different fashion, and in the absence of a trapping agent is converted on irradiation into the trisubstituted naphthalene (167) In the presence of methanol, the adduct (168) is also obtained the proposed pathway is outlined in Sotaeme 8. 

The polar nature of the silicon-carbon bond is reflected in the fact that d(Si=C) in 8 is significantly shorter than the sum of the double-bond radii of the participant atoms (1.74 A)19. The charge distribution in the Si=C bond leads to the ability of silene 8 to form surprisingly stable adducts with weak Lewis bases, in which the unsaturated silicon atom is coordinated. An X-ray structure is available for the adduct 9 with THF67. As may... 

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