Rearrangements of Silenes

A number of molecular rearrangements of silenes have been reported, 

A rearrangement of some interest involving a very simple silene is that of the interconversion of 1-methylsilene and dimethylsilylene [Eq. (17)]. 

Barton has reported a wide variety of elegant studies in which various silenes or silylenes have been created, usually thermally, and their subsequent rearrangements investigated in terms of the observed products of trapping (51,53,65,145). It has been clearly established that interconversion between silenes and silylenes, especially where H atoms or Me3Si groups migrate, are facile processes. In some cases, radicals can be the precursors to silenes (65). 

A related scrambling of groups in a silene has also been reported by Eaborn (143) to explain the structure of compounds isolated from the thermolysis of tris(trimethylsilyl)fluorodiphenylsilylmethane at 450°C, where Me and Ph groups freely interchange between silicon atoms [Eq. (19)]. A related rearrangement is probably also involved in the photochemical silene-to-silene isomerizations derived from acylpolysilanes described earlier. 

It seems obvious that groups attached to the ends of a silicon-carbon double can scramble under a variety of conditions, but the limited observations to date vary so widely in experimental conditions required that it is not at all clear what the specific requirements are for this process to occur. 

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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. 

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... 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

Finally, research in our group has shown that a wide variety of polysilylacylsilanes consistently undergo very clean photochemical 1,3-rearrangements of silyl groups from silicon to oxygen and yield silenes, some of which are remarkably long-lived, and two of which have been crystallized [Eq. (10)] (104,122-124). 

Two families of silenes warrant special comment. The first arises from the photolysis of a number of aryldisilanes as investigated by Kumada et al. (97-102), who reported that a rearrangement occurs to give products formulated as silenes, as shown in Eq. (16). 

Rearrangement of a silene to a silylene (40) via migration of a Me3Si group has been suggested as a step in the gas-phase silylene to silylene rearrangement. Labeling experiments, however, have indicated an alternative mechanism (Scheme 14.23). ... 

Rearrangements of disilanes to a-silylsilenes are well established and are involved in the exchange of substituents between a silylene center and the adjacent silicon.Pulsed flash pyrolysis of acetylenic disilane (41) gave rise to the acetylenic silene (42), which subsequently rearranged to the cyclic silylene, 1-silacyclopropenylidene (43). Irradiation of the cyclic silylene resulted in the isomerization to the isomeric 42, which itself could be photochemically converted into the allenic silylene (44). Both 42 and 43 also were reported to isomerize on photolysis to the unusual (45), which was characterized spectroscopically (Scheme 14.24). 

The primary product from the reaction of 97 with Me3Si—N=CPh2 is the [2+4] adduct 102, which, above 80 °C, equilibrates with the [2+2] adduct 103.102 as well as 103 serve as silene sources when heated (equation 23)63,64. Trapping reactions and rearrangements of 97 are discussed in more detail below. 

The attempted synthesis of 104 from its LiF adduct by salt elimination leads exclusively to the silene 104a65,66. 104a can be reacted with benzophenone to give the [2 + 4] and [2 + 2] cycloadducts 105 and 10667. The [2 + 4] cycloadduct of silene 104 cannot be obtained directly. The adducts 105 and 106, however, rearrange to the thermodynamically more stable 107, probably via the donor adducts 108, 109 and the free silenes 104 and 104a (equation 24). 

Silenes are formed by rearrangement of silylcarbenes. If polysilylated diazomethanes 298-300 are employed, a selective migration of a silyl group to the carbene centre occurs and silenes 301, 92 and 302 are formed (equations 74-76)164. The outcome of trapping reactions is independent of the mode of silene generation photochemical and pyrolytic methods give the same results. 

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