Wiberg-type silenes

The Wiberg -type silenes like 92, available through salt elimination reactions from 93, react with nonenolisable aldehydes, ketones and the corresponding imino derivatives to give in a first step donor adducts 9459, which are then transformed to the [2 + 2] and [2 + 4] cycloadducts 95 and 96, respectively (equation 21)60-62. These cycloadducts may liberate the silene 92 upon heating and it can be trapped by suitable reagents. 

Wiberg -type silenes which can be easily cleaved thermolytically to the silene and the imine60-64. 

The synthesis of the Wiberg -type silenes is primarily achieved through the metalation of halogenotrisilylmethanes 122 with subsequent mild thermal salt elimination from the metalated species 92-LiX, 97-LrX and 104-LiX . The silenes formed, 92, 97 and 104, rearrange to the silenes 92a, 97a and 104a or are trapped by suitable reagents (equation 29). In the case of R = t-Bu (104a) the silene is metastable and its strnctnre could be determined by X-ray diffraction. ... 

The complex reaction sequence shown in equation 34 might provide some rationalization. The formation of the silylcarbene 141 is suggested, based on experimental results from related reactions , but there is no evidence for the formation of 141 nor for a silylene intermediate. Thus, the transformation 137 142 might proceed via a dyotropic rearrangement as well. The facile 1,3-methyl shift in 2-trimethylsilylsilenes which interconverts 142 139 is well known from Wiberg -type silenes . 139 (R = i-Bu) is stable in solution at room temperature over days and isomerizes only slowly to 140 (R = t-Bu) which rapidly dimerizes giving a 1,3-disilacyclobutane . 

Two comprehensive studies of the reactions of Wiberg type and of the Jones-Auner type silenes with dienes will be described immediately below, without separating the results into the separate subsections of [2 + 4], [2 + 2], and ene reactions. The overall results of their investigations, given in one location, will allow a better, appreciation of the effects that the substituents on the silene or diene have in determining which reaction takes place. 

The reactions with quadricyclane, shown in Eq. (31), gave products identical to those formed by the same silene reacting in a [2 + 2] manner with norbornene. Mixtures of exo endo isomers were frequently observed. Again, only silenes of the Auner type have been studied with this reagent,51-53,185,188 so it is not known whether the Wiberg- or Brook-type silenes will undergo this mode of cycloaddition. 

The facile photochemical sigmatropic 1,3-trimethylsilyl shift in polysilylacylsilanes from silicon to oxygen (equation 33) was utilized historically to prepare the first relatively stable silenes3 86 87. Silenes prepared by isomerization of acylpolysilanes bear, due to the synthetic approach, a trimethylsiloxy group at the sp2-hybridized carbon and relatively stable silenes of this type have in addition also at least one trimethylsilyl group at the silicon. These substituents strongly influence the physical properties and the chemical behaviour of these silenes. This is noticeable in many reactions in which these Brook -type silenes behave differently from simple silenes or silenes of the Wiberg type. 

Simple silenes readily couple to yield the head-to-tail dimCTs, 1,3-disilacyclobuta-nesi.2>l5,l57 pjjg dimerization is extremely facile and silenes bearing only small alkyl groups dimerize in an argon matrix even at 40 K, i.e., the dimaization proceeds at a diffusion controlled rate. Bulky substituents slow down the dimaization rate and allow the isolation of stable silenes. The head-to-taU dimerization (equation 98) is the predominant dimerization path for silenes, including those of the Auner-Jones and Wiberg type . ... 

Recently, Wiberg and coworkers have answered the question whether the formation reaction for silenes of the type 126 can be analogously extended for silene types 127 and 12878. 

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. 

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