Silylenes hexadienes

Silylated triphosphanes and triphosphides, synthesis, 31 188-194 yields, 31 194 Silylenes, 29 2-6 addition reactions, 29 4-6 to butadiene, 29 4 to ethylene, 29 4 to hexadienes, 29 5 mechanism, 29 4 nitric oxide scavenging, 29 4 complexes, 25 37, 51, 116, 118 as catalyst intermediates, 25 118 extrusion from disilanes, 25 114, 118 halides, 3 225 from hydridosilanes, 25 14 insertion into element-hydrogen bonds, 29 3-4... 

Silylene addition to dienes was first reported in 1968 (Scheme 17) (68JA3438). The l-silacyclopent-3-ene produced is again thought to result from a silirane intermediate, as a concerted 1,4-cycloaddition can be ruled out. Thermally generated silylene SiH2 does not add stereospecifically to trans,trans- and cis,trans-2,4-hexadiene but gives a mixture of the two silacyclopentenes (6 1 and 2 1 ratio) in each case (Scheme 18) (74JA6198). 

We have recently studied the addition of SiHj and Sip2 to trans-2- rans-4-hexadiene and its cis-cis and cis-trans isomers. A steric effect which decreases the reactivities of these hexadienes in comparison with that of butadiene is again observed. The relative reactivities of trans-l-transA-hexadiene toward silylenes in comparison with those of butadiene are around 0.6, while the corresponding values for c/s-2-c/s-4-hexadiene are only around 0.3. 

Overall, for the addition of silylenes to alkynes to give 1,4-disilacyclo-hexadienes, the operation of mechanism b seems to be firmly established. The complete operation of mechanism a is probably in doubt, but the conversion of silacyclopropene to disilacyclobutene (patch c) is still possible. However, since the primary steps of both mechanisms, the addition of silylene to n bonds and the dimerization of silylenes, are both well established, the relative importance of the two mechanisms should be kinetically controlled by the concentration of the silylenes. At some lower concentration of the reacting silylenes, the silacyclopropene route may actually prevail. 

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