Silenes rearrangement

References to the silenes so prepared since 1985 are listed in Table I. None of these silenes was stable, most undergoing head-to-tail dimerization to give 1,3-disilacyclobutanes in the absence of trapping reagents. Some interesting spontaneous silene-to-silene rearrangements were observed,52 which will be described in Section IV.E. 

Grobe15 has described the pyrolysis of 1 -methyl-1 -vinyl- and 1,1 -diviny 1-1-silacyclobutanes 166 which led to the formation of methylvinylsilene and divinylsilene, respectively. Under the experimental conditions used, it was suggested that the silenes rearrange to exo-methylene- 1-silacyclo-propanes 167 which extrude methylsilylene or vinylsilylene, respectively. In support of this proposal, when the reactions were carried out in the presence of 2,3-dimethylbutadiene, the anticipated silylenes were trapped as their respective l-silacyclopent-3-enes 168. 

Scheme 14.18). The silylene-silene rearrangement 27 28 is nearly thermoneutral, with the silene being slightly more stable. The photolysis of a-diazo compounds (30) is the only frequently used reaction path to silenes (31) via a carbene-silene... 

The following examples in this section illustrate that the silylcarbene-to-silene rearrangement and subsequent silene reactions are common also under nonmatrix conditions. The early research on this topic was reviewed in 197937. 

When two carbene functions are separated by one or more silicon atoms, one can expect them to enter independently the usual inter- or intramolecular reactions. Among the intramolecular reactions, extensions of those which have been discussed in Sections III.B and IILC are particularly appealing, namely silylcarbene-to-silene rearrangement at one or both carbene centers and intramolecular carbene dimerization to form a C,C double bond and thus an unsaturated silaheterocycle. 

The dramatic influence of a methyl group on the reaction pathway is exemplified by carbene 163 which rearranges to a vinylsilane. In the absence of this methyl group, however, the silylcarbene-to-silene rearrangement (equation 16) occurs48,49. 

An analogous mechanistic scheme (equation 87) has been proposed for the flash vacuum pyrolysis of dimethylsilyl(trimethylsilyl)thioketene148 (256). The pyrolysis of bis(trimethylsilyl)thioketene (257) leads to a more complicated product mixture (equation 88). With 47% conversion, a mixture of trimethylsilylacetylene, 1-trimethylsilyl-1-propyne, bis(trimethylsilyl)acetylene, (trimethylsilyl)thioketene, 2,2,4,4-tetramethyl-2,4-disila-l-thietane and 2,2,4,4-tetramethyl-2,4-disila-l,3-dithietane was obtained. All products can be rationalized, however, by the assumption that carbene 258 undergoes not only a silylcarbene-to-silene rearrangement (as in the preceding two cases) but also isomerization to 2-thiirene and insertion into a methyl-C, H bond. 

A photochemical approach via a silylene-to-silene rearrangement was followed by Fink and coworkers in their synthesis of silacyclobutadienes in a 3-methylpentane matrix at low temperatures161. Irradiation of the cyclopropenyltrisilane 294 gives the relatively stable cyclopropenylsilylene 295. 295 can be efficiently converted to silacyclobutadiene 296 by irradiation into the visible absorption band of the silylene (equation 72)161 162. 

II. SILYL CARBENE-TO-SILENE REARRANGEMENT AND RING FORMATION A. Decomposition of Monosilanyl Diazo Compounds... 

During a photochemical silene-to-silene rearrangement, Brook and coworkers mentioned the possibility that both the two groups, Me3Si and Me3SiO, migrated simultaneously across a Si=C double bond (equation 4)30. 

As mentioned earlier, few absolute kinetic data exist for unimolecular reactions of silenes, largely because most of the known silene rearrangements occur under high temperature pyrolytic conditions and are difficult to measure directly8,10,12. 

Relatively few absolute rate constants have been reported for the silene rearrangements that are known to occur at or near room temperature, such as the interconversion of 1,1-di-terf-butyl-2,2-bis(trimethylsilyl)silene (4) and its kinetically stable isomer 5 via 1,3-methyl migration (equation 6)27-29. The analogous migration in the simpler dimethyl derivative of this type (6) occurs with a half-life ti/2 30 min at 120 °C, as measured by monitoring the scrambling of the —CD3 groups in 6-d(, (equation 7)30. 

In contrast to the carbene and carbenoid chemistry of simple diazoacetic esters, that of a-silyl-a-diazoacetic esters has not yet been developed systematically [1]. Irradiation of ethyl diazo(trimethylsilyl)acetate in an alcohol affords products derived from 0-H insertion of the carbene intermediate, Wolff rearrangement, and carbene- silene rearrangement [2]. In contrast, photolysis of ethyl diazo(pentamethyldisilanyl)acetate in an inert solvent yields exclusively a ketene derived from a carbene->silene->ketene rearrangement [3], Photochemically generated ethoxycarbonyltrimethyl-silylcarbene cyclopropanates alkenes and undergoes insertion into aliphatic C-H bonds [4]. Copper-catalyzed and photochemically induced cyclopropenation of an alkyne with methyl diazo(trimethylsilyl)acetate has also been reported [5]. 

Keywords silacyclobutanes, pyrolysis, silenes, rearrangement, 3,4-benzo-1-silacyclobutenes, 2-silaindanes, B3LYP/6-31G calculations... 

The second type of bonding situation that favors a silylene-to-silene rearrangement is a vinylsilylene structure. Examples in which this reaction is believed to occur are discussed in Section III.B.l.b in connection with silylene-to-silene-to-silylene rearrangements207,225,227. 

Another reaction perhaps involving a vinylsilylene-to-silene rearrangement is the formation of methylethynylsilane in the pyrolysis of 1-chloro-l-vinyltetramethyldisilane (equation 66)224. 

Thermal silylcarbene-to-silene rearrangements have been known for a long time1. The pyrolytic product from trimethylsilyldiazomethane, 1,1,2-trimethylsilene, was trapped in an argon matrix230, and the pyrolysis of bis(trimethylsilyl)diazomethane126 was reported to produce fair amounts of 2,4-bis(trimethylsilyl)hexamethyl-l,3-disilacyclobutane, the expected dimerization product of 2-(trimethylsilyl)-1,1,2-trimethylsilene. A second product was the disilane expected from an ene addition of one... 

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