Computed C2 reaction path for dimerization of cyclopropene

This closely analogous reaction takes a surprisingly different course. [33] At low temperatures, silylenes react with acetylenes to form silacyclopropenes, but at higher temperatures, the usual products are l,4-disilacyclohexa-2,5-dienes, which are presumably formed by dimerization of the low-temperature products. 

It had been suggested [35] that the dimerization proceeds stepwise, via a silatricyclohexane intermediate. The formation of the syn isomer of silatricyclo-hexane should be considerably more facile than that of the anti isomer, but the substitution pattern of the dimerization product speaks against the incursion of either syn- or anh-TCH as an intermediate. [36] When dimethylsilylene is generated in a mixture of 2-butyne and diphenylacetylene, the first step produces two symmetrically substituted molecules R2SiC2A2 and R2SiC2B2 (A = CH3 B = CeHs). Dimerization of either across the double bond would produce a mixed dimer of disila-TCH - and eventually of disilacyclohexadiene - in which one Si atom is flanked by two A substituents and the other by two of B. This isomer is not observed in the mixed isomer that is observed, each Si atom is flanked by one A and one B. 

A correspondence diagram for the stepwise sequence is shown in Reference [34, Fig. 6]. 

The simplest mechanism consistent with these findings involves direct formation of the disilahexadiene by a W.-H, forbidden [ 2a + J-cycloaddition across the CSi single bonds. As is clear from the correspondence diagram for this reaction, displayed in Fig. 7.14, the single orbital mismatch is removed by a big displacement that takes the reactants into thus bypassing the barrier imposed by orbital symmetry in D2/1. 

Construction of an orbital correlation digram in will illustrate once more the distinction between a correlation diagram and a correspondence diagram The detailed pairwise connections beween pairs of orbitals across the diagram 

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Figure 7.13. Computed C2 reaction path for dimerization of cyclopropene.The numbering of the atoms follows that of Chemical Abstracts for tricyclo[3.1.0.0 ]hexane. The slight computational instability near r4s = 1.7A is due to a discontinuity in the limited Cl procedure used.

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