Bis-silenes

While the decomposition of silacyclobutanes as a source of silenes has continued to be studied in the last two decades, the interest has largely focused on mechanisms and kinetic parameters. However, a few reports are listed in Table I of the presumed formation of silenes having previously unpublished substitution patterns, prepared either thermally or photo-chemically from four-membered ring compounds containing silicon. Two cases of particular interest involve the apparent formation of bis-silenes. Very low-pressure pyrolysis of l,4-bis(l-methyl-l-silacyclobutyl)ben-zene94 apparently formed the bis-silene 1, as shown in Eq. (2), which formed a high-molecular-weight polymer under conditions of chemical vapor deposition. 

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... 

Finally, Eq. (28) documents a unique situation in which a bis-silene was observed to undergo both head-to-tail and head-to-head [2 + 2] cyclodimerizations. Photolysis of the bisdiazoalkane 80 in benzene apparently... 

Some unusual behaviour was displayed by the benzodisilacyclobutane 84 as described by Ishikawa et al.95 When thermolyzed, it appeared to form the quinodimethane bis-silene species 85 shown in Scheme 13, as confirmed by trapping reactions with f-butyl alcohol, alkynes, or aldehydes, all of which added in a 1,4-manner (see Scheme 13). In the absence of a trapping reagent, 85 decomposed, but not to 86 as claimed earlier.95 ... 

The photochemical decomposition of bis(silyldiazomethyl)tetrasilane 320 produces one silene group to give 321 followed by intramolecular [2 + 3] silene-diazo cycloaddition via 322 to give the bicyclic compound 323 as final product, while thermal decomposition gives a bis-silene 324 which then undergoes head-to-tail dimerization168,169 to... 

Ando and coworkers have described unusual photochemical behavior of a bis-diazo compound 263 which on photolysis gave rise to a bis-silene 264135. In inert solvents this spontaneously underwent two modes of reaction, criss-cross (head-to-tail) addition and parallel (head-to-head) addition, leading to bridged disilacyclobutanes 265 and 266 (Scheme 47). 

As another example, photolysis in inert solvents of compound 275, where four SiMe2 groups bridged the two diazo functions, led to the bis-silene 276, which reacted by either head-to-head or head-to-tail pathways resulting in the formation of the bicyclic compounds 277 and 278, respectively. Alternatively, [2 -f 3] cycloaddition gave the intermediate 279 which led to the formation of the bicycloadduct 280 (Scheme 49). 

The successful synthesis of the acetylene-linked bis(ger-menes) posed a question whether analog compounds with conjugated Si=C double bonds could be isolated by the reaction of a silylene with 1,3-diynes. Although attempted reactions can postulate the existence of intermediary compounds consisting of bis(silenes) linked by an acetylene unit, no stable compounds having Si=C-C=C-C=Si conjugated systems have been obtained by such a synthetic strategy. ... 

More complex systems such as 537 have been examined recently. For instance, bis silene 538 is obtained which, in the absence of trapping agents, leads to the bicyclic compound 539 28. 

Bravo-Zhivotovskii D, Dobrovetsky R, Nemirovsky D, Molev V, Bendikov M, Molev G, Botoshansky M, Apeloig Y (2008) The synthesis and isolation of a metal-substituted bis-silene. Angew Chem Int Ed 47 4343... 

---