Silirenes dimerization

Subsequent evidence from the thermolysis of the silirene (29), which gave the two silins (30) and (31) as the major products even in the presence of another alkyne, tends to support a mechanism involving silirene dimerization (77JOM(l42)C45). Indeed, the presence, albeit in trace amounts, of pentaphenyltrimethylsilylbenzene (32) supports a silacyclopentadiene intermediate (Scheme 38), while the 1,2-disilacyclobutene (33) and diphenylacetylene give the disilin in only 1.2% yield (Scheme 39) (78JOM(162)C43). 

Irradiation of (38) with nitriles induces dimerization of the silirene with nitrile insertion to give the azadisilabicyclo[3.2.1]octa-3,6-diene (39), except with acrylonitrile, when the isoskeletal bicyclo[3.3.0]octa-3,6-diene (40) also results. It seems reasonable to suppose they form through [2+2] and [4 + 2] addition (Scheme 44) (78CC80). By way of contrast,... 

The first step of the retro-reaction involves loss of silylene 79, which could be trapped with 1-pentyne to give the known silirene 81 (equation 125). In the absence of a trapping agent, 79 recondenses to 77, probably by first dimerizing to the disilene Ar2Si=SiAr2 followed by 2 +1 cycloaddition to give 77 (equation 126). From the principle of microscopic reversibility, the fact that silylene is formed in the retro-reaction leads to the conclusion that 79 must also be an intermediate in the cycloaddition reaction. 

Of the more specialized methods for the formation of acyclic disilenes, two new methods deserve mention. Kira and coworkers reported that photolysis of the silirene 1 afforded the diaminosilylene 223. In contrast to theoretical predictions24-27, the silylene 2 apparently does not undergo dimerization to afford the bridged dimer 3 as is usually preferred in the presence of electronegative substituents but rather, according to the results of variable-temperature electronic spectroscopy, furnishes the disilene 4 (equation 4)23. 

The reaction mechanism for dimerization of the silirene intermediate has been a controversial subject for many years 3, 4, 7, 45, 49, 58, 86). It is now generally accepted that in the case of dialkylsilylene, (T-dimerization is probably the true mechanism. Dichlorosilylene is expected to behave similarly. 

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