Dimerization, silenes

UV photolysis (Chapman et al., 1976 Chedekel et al., 1976) and vacuum pyrolysis (Mal tsev et al., 1980) of trimethylsilyldiazomethane [122]. The silene formation occurred as a result of fast isomerization of the primary reaction product, excited singlet trimethylsilylcarbene [123] (the ground state of this carbene is triplet). When the gas-phase reaction mixture was diluted with inert gas (helium) singlet-triplet conversion took place due to intermolecular collisions and loss of excitation. As a result the final products [124] of formal dimerization of the triplet carbene [123] were obtained. 

The second example of a bis-silene involved the thermolysis of the benzo-l,2-disilacyclobutane 2, which formed the bis-silene 3 that subsequently dimerized in the unusual manner shown in Eq. (3).95,96... 

The former silene is not stable at room temperature, owing to its tendency to dimerize in the absence of other reactants, but it can be stored as its [2+2] cycloadduct with A/-trimethylsilylbenzophenonimine, from which it is cleanly and easily liberated on mild thermolysis.99 The latter silene arose from the loss of LiX from the compound (r-Bu)2SiX-CLi (SiMe3)2, which subsequently underwent a spontaneous 1,3-silicon-to-silicon methyl shift. 

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. 

Subsequently, other members of the family of siienes (Me3Si)2Si= C(OSiMe3)R have been prepared, where R = Me, Et, i-Pr, CH2Ph, bicyclooctyl, CEt3, 1-methylcyclohexyl, and Mes. The first four siienes listed were not stable in inert solvents, and hence were not observable by NMR spectroscopy, since they rapidly reacted intermolecularly to give linear and/or cyclic head-to-head dimers.87 This is illustrated in Eq. (14) for the benzyl compound where the initially formed silene 5 yielded the cyclic head-to-head dimer 6 as well as the linear head-to-head dimer 7. The latter four siienes were all relatively stable and were characterized by NMR spectroscopy.105... 

Dimerization is a special case of [2+2] cycloaddition with silenes it has been observed to occur in both a head-to-tail and in a head-to-head manner, yielding 1,3- or 1,2-disilacyclobutanes. These two cases will be discussed separately below. 

The mechanisms by which silenes dimerize, both head-to-tail and head-to-head, have been the subject of intense interest to theoreticians for as long as the dimerization process has been known. Siedl, Grev, and Schaefer167 calculated that the barrier between the parent H2Si=CH2 68 and its head-to-tail dimer 69 was 5.2 kcal mol-1, with the product head-to-tail dimer 69 being 79 kcal mol-1 more stable than reactants. The head-to-head dimer 70 was found to be 19.8 kcal mol-1 less stable than 69. 

When the Me3C group in the above system (see Eq. 25) was replaced by the bulkier Ad group, no dimer was formed and the silene was isolated... 

When phenyl (Ph) groups replaced both Me3Si groups, again a rather unstable 1,2-disilacyclobutane dimer appeared to be formed,90 as shown by NMR data but when f-butyl replaced a Me3Si group, the silene failed to dimerize.87 Thus, it is evident that whether or not head-to-head [2 + 2] cyclodimerization occurs depends on the bulk of the substituents on both sp2-hybridized silicon and carbon. 

The second form of head-to-head dimerization involved the formation of a linear (as distinct from a cyclic) species in which two molecules of silene form a silicon-silicon bond. If this follows the pathway suggested above in Eq. (25), the resulting 1,4-diradical must then disproportionate by hydrogen abstraction, forming a molecule saturated at one end and unsaturated at the other. Recent examples are given in Eq. (27).86... 

In the past decade, Ishikawa et al. have investigated the photochemistry of aryldisilanes.66 73 76 84 Lately, dimers derived from these unusual silenes have been observed from the first time.78 Thus, photolysis of 1,4-bis(penta-methyldisilyl)benzene 76 in hexane presumably gave rise to the silene 77 but, on workup employing crystallization, two head-to-head stereoiso-meric dimers 78 and 79 were obtained in about 45% yield in a 1 1 ratio. These were said to be formed from the silenecyclohexadienes 77 by the two alternative pathways shown in Scheme 12. Related dimers were also... 

Analogous behavior was followed by the phenyl-substituted silene 156. The initially formed silene 157 underwent 1,3-methyl migration to give the silene 158, which then dimerized in a head-to-tail manner to yield three different stereoisomeric dimers 159, two of which were characterized by crystal structures. Again, the exchange of trimethylsilyl and trimethylsi-loxy groups at the ends of the Si=C bond occurred, followed by 1,3-methyl silicon-to-silicon rearrangements. The steps are summarized in Eq. (54). 

Dimerization is one of the most common reactions of silenes, particularly in the absence of an effective trapping agent. 

The reason for the observed head-to-head dimerization is of some interest. Unlike simple silenes, where the double bond is significantly polarized, Sis+=C8- it is believed that the siloxysilenes are not significantly polarized overall, because of the effects of the substituents, which... 

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