Carbon-silicon double bonds

Silenes with a wide range of structures have been synthesized over the years, from the simplest possible compound, H2Si=CH2, which has a transient existence, to species that contain four complex groups attached to the ends of the silicon-carbon double bond, e.g., (Me3Si)MesSi=C... 

The stable silene Me2Si=C(SiMe3(SiMe(r-Bu)2) was first reported as a stable complex with THF,34 and its crystal structure showed the length of the silicon-carbon double bond as 1.747 A. Subsequently, it was possible to remove the THF and isolate the uncomplexed silene, which had a noticeably shorter Si=C bond length of 1.702 A.29 Further investigation showed that stable complexes of this or closely related silenes with trimethyl- or ethyldimethylamine, pyridine, and fluoride ion were also readily formed and moderately stable.31141... 

Bands in the region 950-1100 cm"1 have been attributed to the Si=C stretching frequency of the simple silenes, the frequency reported depending on the experimental method, the methodology employed in the calculations, and the substituents present on the ends of the silicon-carbon double bonds there is generally good agreement between these observations and the appropriate calculations. The material has been well summarized.6,153... 

Several further calculations of various properties of the silicon-carbon double bond have been reported. As in the previous decade the strength of the silene bond and the reasons for it have continued to attract attention. 

Prior to 1985, much had been learned about the chemistry of the silicon-carbon double bond through the study of the reactions of silenes with a wide variety of reactants. Thus it was known that all silenes studied reacted readily with alcohols (particularly methanol) by regiospecific addition across the ends of the Si=C bond in which the MeO group became attached to silicon and the alcoholic H to carbon, as in Eq. (22). 

A wide variety of other polar reagents had also been shown to add across the silicon-carbon double bond, and trimethylmethoxysilane (see Eq. 22) had been found to be another rather useful reagent for trapping reactive silenes. 

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). 

Sakurai et al. have provided what is probably the most important mechanistic finding in the area of intermolecular additions of silenes in recent years, namely a detailed proposal for the mechanism of alcohol addition to the silicon-carbon double bond.68 A cyclic silene 116 was synthesized in the presence of various amounts of methanol and other alcohols, and varying proportions of methanol adducts 117 and 118 were obtained. It was concluded that the methanolysis involved two steps, the first being the association of the oxygen lone pairs with the sp 2-hybridized silicon atom of the silene. The second step, proton transfer, could occur in two ways. If the proton was transferred from the complexed methanol molecule (path a) its delivery would result in syn addition. However, if a second molecule of methanol participated (path b), it would deliver its proton... 

Methanol and other alcohols also add across the silicon-carbon double bond component of silaallenes. Thus, when (Me3Si)PhC=C=Si(Si Me3)Mes was generated at 140°C in the presence of methanol, the adduct (Me3Si)PhC=CHSi(OMe)(SiMe3)Mes was obtained, among other products.72... 

Other Additions to the Silicon-Carbon Double Bond... 

Ever since their discovery in 1967, there has been interest in the kinds of rearrangements that silenes might undergo and curiosity about the behavior of the silicon-carbon double bond as compared to that of the carbon-carbon double bond. 

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. 

In the less than three decades since silenes were first described by Gusel nikov and Flowers, an impressive amount of knowledge concerning silenes and the behavior of the silicon-carbon double bond has been discovered and reported. Several hundred papers have been published dealing with silenes in one context or another, and it is clear that the status of silenes has changed from that of rare oddity to a not uncommon occurrence. Much has been learned about their reactions, although much remains to be learned about the finer details of the mechanisms of some of their reactions. 

The silyl radicals formed in the initial scission appear to undergo further reactions, which may be complex. A possible secondary reaction is hydrogen transfer from an alpha carbon atom to give Si-H and a silicon carbon double bond (21)... 

When similar photolysis of 11 in the presence of MeOD was carried out, again the product whose NMR reveals the resonance due to the Si-H proton was observed. The relative ratio of the Si-H and CH3-0 protons was identical with those of the products obtained in the presence of non-deuterated methanol. The formation of the methoxysilyl group can be understood by the addition of methanol across the silicon-carbon double bonds. H NMR spectra of all photoproducts obtained from the photolyses of 11 in the presence of methanol reveal no resonances attributed to the cyclohexadienyl ring protons. This indicates that the photochemical degradation of the polymer 11 gives no rearranged silene intermediates, but produces... 

This review describes the current status of silenes (silaethylenes, silaethenes), molecules which contain a silicon-carbon double bond. The heart of the material is derived from a computer-based search of the literature which we believe reports all silenes that have been described to date, either as isolated species, chemically trapped species, proposed intermediates (in reactions where some experimental evidence has been provided), or as the result of molecular orbital calculations. Ionized species... 

Prior to 1966-1967, based on numerous attempts to synthesize molecules containing a silicon-carbon double bond (150-152) the firm position had been reached that, if they could be formed at all, silenes were (would be) too unstable to survive. It was believed that the it bond would be very weak because of poor overlap, due to the differences in energy and the longer bond distances between the adjacent 2p and 3p orbitals of carbon and silicon, respectively (153,154). In 1967 Gusel nikov and Flowers (155) reported experiments which reopened the entire issue of it bonding... 

There have now been four experimental determinations of a silicon-carbon double bond length. The first of these was a gas phase electron diffraction study of 1,1-dimethylsilene (173). This study was the subject of much controversy since the experimentally determined bond length, 1.83 A, was much longer than the one predicted by ab initio calculations (1.69-1.71 A, see below) (159). Since the calculations were carried out at a relatively high level of theory and the effects of electron correlation on determining the Si=C bond length were considered, the validity of the data extracted from the electron diffraction study is in serious doubt. 

The second determination of a silicon-carbon double bond length came from the X-ray crystal structure of l,l-bis(trimethylsilyl)-2-(trimethyl-siloxy)-2-(l-adamantyl)-l-silaethene (1) (122). Again, the experimentally... 

The silicon-carbon double bond has attracted the attention of theoretical chemists who have now performed a large number of high quality calculations on various properties of siienes. It is gratifying to observe that there is close agreement between experiment and calculation in most properties investigated, as summarized below. 

Despite the differing levels of calculations, the same general conclusions were reached. The silicon-carbon double bonds in 1-silaallene (1.69 A) and 2-silaallene (1.70 A) are shorter than in isolated silenes at the same level of theory. This trend is also observed in the analogous carbon series. 1-Silaallene is thermodynamically more stable than 2-silaallene by 21 kcal/mol (22). Intuitively, this is what would have been expected, realizing the low ability of silicon to participate in multiple bonds. As may be expected from simpler systems (i.e., H2Si=CH2)(i97), silylene isomers (for example, structures 8 and 9) are considerably more stable (approximately IS kcal/mol) than their silaallene counterparts. 

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