Brook silene

Only a few photoelectron spectra of silenes have been measured. Silene 25 has a first vertical ionization energy of ca. 8.85 eV, in agreement with the theoretical value of 8.95 eV30. The assignment is further supported by the observed fine structure of the first band which is similar to that found for ethene. The first ionization potential of the stable Brook silene 150 is 7.7 eV3, similar to what is found for Me2Si=CH2 2 (7.98 eV)276. 

The Brook silene 28, produced photochemically as shown in equation 18, dimerizes to yield a mixture of the 1,2-disilacyclobutane 29 and the acyclic ene-dimer 3064, the common mode of dimerization for the large majority of l,l-bis(trialkylsilyl)silenes that have been studied to date12. Conlin and coworkers determined the absolute rate constant for dimerization of 28 in cyclohexane solution, k, tm = 1.3 x 107 M 1 s 1 at 23 °C65. Arrhenius activation parameters for the reaction were determined over the 0-60 °C temperature range. The values obtained, a = 0.9 0.4 kJmol 1 and log(A/M 1 s 1) = 7 1, are consistent with the stepwise mechanism for head-to-head dimerization originally proposed by Baines and Brook (equation 19)64, provided that the rate of reversion of the... 

DMB with the Brook silene 28, which yields the Diels-Alder adduct 67 as the major product (equation 52)65. 

A few years later, Apeloig and Kami reported ab initio calculations of the structure, frontier molecular orbital energies and Mulliken charge distributions in a series of substituted silenes of structure RHSi=CH2 and H2Si=CHR, which indicated that all four of the substituents in the Brook silenes contribute to the reduction in Si=C bond polarity that was suggested to give rise to their unusual stability58. Their systematic study led to the conclusion that Si=C bond polarity is reduced by the presence of [Pg.995]

Brook, Adrian, G., and Brook, Michael, A., The Chemistry of Silenes. 

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. 

In view of the evident reactivity of the Brook-type silenes toward carbonyl compounds and the fact that these silenes were prepared by the photolysis of acylsilanes, it is natural to ask why the silenes apparently did not react with their acylsilane precursors. This question has been answered recently. On the one hand, as shown in Scheme 19, the silene Ph2Si=C(OSiMe3)Ad apparently did add in a [2 + 2] manner to its acylsi-... 

Brook et al. 5X1 observed such reactions during the formation of siienes by photolysis. Using radiation with A > 360 nm, they photolyzed acylsi-lanes such as 127, which bears a mesityl group attached to the carbonyl carbon. On prolonged photolysis of the initially formed silene 128, the C—H bond of the ortho methyl group of the mesityl group added to the silicon-carbon double bond to form the benzocyclobutane 129. Alternatively a 1,5-H shift would lead to the species 130, which would also yield the benzocyclobutane on electrocyclic rearrangement. 

The bulky, stable silenes of Brook et al. (104,122-124,168) and Wiberg et al. (166,167) have been the only systems capable of being studied by nuclear magnetic resonance (NMR) spectroscopy to date. Table III lists the 13C and 29Si chemical shifts and the relevant coupling constants of these compounds. 

The double bond in silenes is strongly polarized. They react with phosphorus ylides, as shown by Brook and MacMillan,45 like alkenes with the strongly polar C=C bond. Therefore, it is reasonable to suggest that the reaction also occur through the betaine intermediate (12) (Scheme 6). 

In the first stereochemical study, Brook has observed nonstereospecific addition of methanol to certain isolable silenes65,66. Although the precise stereochemistry of the products was not established, a 1/3 mixture of syn/anti or anti/syn isomers was obtained (equation 13). The results indicate a nonconcerted process for the addition of alcohol to silenes. Wiberg has proposed a two-step mechanism involving an initial formation of a silene-alcohol complex, in accord with the formation of nucleophilic adducts, followed by proton migration from the alcohol to the carbon of the silene (equation 14)59,61. This mechanism may be compatible with the results obtained by Brook and coworkers, if rotation around the silene s Si—C bond occurs faster than the proton migration. 

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. 

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