Dienes with silenes

Two reactions have come to be extensively used with silenes, arising from the need to trap the short-lived species cleanly and in high yield, as evidence either of their formation or of the extent of their formation. These are the addition of alcohols, usually methanol, across the double bond to yield an alkoxysilane, and the Diels-Alder reaction with a diene, often 2,3-dimethylbutadiene. Each is an example of the two different types of addition to the Si=C double bond. 

Ene-additions of alkenes and dienes to silene 6 are considerably slower than [2 + 4]-cycloadditions. cA-Substitution in the ene component of the reaction causes a small acceleration in rate relative to fraws-substitution, as illustrated in Table 2 by the relative rate constants for reaction of 6 with cis- and rraws-2-butene. Reaction with cis, trans-2,4-hexadiene produces only a single adduct (66 equation 51), corresponding to selective ene-reaction with the cA-methyl group in the diene. 

Absolute kinetic data have been reported for four of the characteristic bimolecular reactions of disilenes 1,2-addition of alcohols and phenols (equation 72), [2 + 2]-cycloaddition of ketones (equation 73), [2 +4]-cycloaddition of aliphatic dienes (equation 74) and oxidation with molecular oxygen (equation 75). As with silenes, the addition of alcohols has been studied in greatest detail. 

When carbene 21 is generated from the diazo precursor photochemically in solution, it reacts with added alcohols by 0,H insertion (equation 55), in contrast to the gas-phase copyrolysis where the silene intermediate is trapped . Similarly, photolysis of 20 in the presence of 2,3-dimethylbutadiene gives mainly vinylcyclopropane 184, while after copyrolysis of 20 and the same diene one finds that most of the vinylcyclopropane rearranged to the cyclopentene, together with the [4-1-2] cycloaddition product of diene and silene 22 °. Furthermore carbene 21, generated photochemically or thermally (at 117-148 °C) in solution, undergoes [1 4-2] cycloaddition to alkynes to give cyclopropenes 185. 

It was also well established that silenes could take part as the dienophile in Diels-Alder reactions. In many cases, particularly with unsymmetric dienes such as isoprene, the reactions were not clean because, in addition to formation of the [2+4] cycloadduct 61, the possibility exists for the formation of it regioisomer 62, products of an ene reaction 63, and conceivably the [2+2] cycloaddition product 64, as shown in Eq. (23). Wiberg... 

Recent study of silenes has clearly established their ability to act as dienophiles in [2 + 4] Diels-Alder-type reactions involving 1,3-dienes. However, it has also been clearly demonstrated that products of the ene reaction commonly accompany the [2 + 4] cycloadducts and may on occasion be the major products. In addition, unlike those in carbon chemistry, [2 + 2] cycloadditions are often observed to occur in competition with the above processes—not only from reactions of silenes with dienes... 

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 overall combination of these effects identifies isoprene as the linear diene that reacts most rapidly in a [2 + 4] manner with the silene Me2 Si=C(SiMe3)2 (involving formation of the favored 3-methyl-substituted... 

Table IX lists the data available for the reactions of three typical silenes, Cl2Si=CHCH2CMe3, Cl2Si==C(SiMe3)CH2CMe3, and Ph2Si=CHCH2-CMe3, with a variety of dienes.

Products from the Reaction of Selected Silenes R2Si=CR CH2CMe3 with Dienes (% Yields)... 

The relative rates of reaction of the silene Me2Si=C(SiMe3)2 with a series of amines, alcohols, phenols, thiophenols, dienes, and alkenes were obtained174 and are reported in Table VIII Section IV.C. 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

These studies proved that the reactions of 22 with butadienes and propenes take place both regioselectively and stereoselectively and are accelerated by electron-donating groups on propenes and butadienes (e.g., 2-methylpropene in relation to propene) and retarded by increasing bulkiness of substituents in 1,4- or 1,3-positions. As in the case of alkenes and silenes, the reactions of 22 occur in a concerted way and are HOMO (dienes or enes)-LUMO (dienophiles or enophiles) controlled.31 However, some small differences are observed between germene 22 and the analogous... 

When phenyl(trimethylsilyl)diazomethane (20) is pyrolyzed in the gas phase, typical reactions of carbene 21 can be observed (see Section III.E.4). However, copyrolysis with alcohols or carbonyl compounds generates again products which are derived from silene 2239,40 (equation 6). Thus, alkoxysilanes 23 are obtained in the presence of alcohols and alkenes 24 in the presence of an aldehyde or a ketone. 2,3-Dimethylbuta-l,3-diene traps both the carbene (see Section ni.E.4) and the silene. 

This question can be answered positively for the sterically shielded silenes 129 and 130. They not only form dimers, but can be trapped by dienes. In the case of 131, no dimers or trapping products with dienes are obtained. The formation of the products from the reaction t-Bu2Si(Br)—C(Br)H2 +LiR, however, is tentatively explained by the authors by the participation of a silene along the reaction pathway. 

Ishikawa and coworkers investigated the relative ease of migration of a Me3Si group to vinyl and phenyl groups in precursor compounds containing both groups133. In the case of the irradiation of compound 237 in the presence of methanol, silene 238 was found to be the major reactive intermediate, while with acetone and 2,3-dimethylbuta-l,3-diene the number and nature of different ene products can only be explained by the existence of silatriene 239 (equation 59). 

In this section we will summarize bimolecular reactions of silenes with alkenes, alkynes and dienes which might be regarded nominally as cycloaddition reactions. 

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