Dimerization of Silacyclopropenes

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 

---

An analysis of the dimerization of silacyclopropene was carried out by Orbital Correspondence Analysis in Maximum Symmetry (OCAMS). This analysis showed that the dimerization of silacyclopropene follows an allowed pathway <82JOM(240)129>. 

Figure 7.14. Correspondence diagram for dimerization of Silacyclopropene (D2/1). (The asymmetry introduced by the substitutents is ignored)...

For mechanism a, there is no doubt that silylenes could add to alkynes to give silacyclopropenes. What is at issue is whether or not the dimerization of the silacyclopropene takes place. The evidence is as follows, (i) Stable silacyclopropenes have been recently prepared by Conlin and Gaspar from the reactions of SiMcj with 2-butyne, and by Seyferth and coworkers from the reactions of SiMcj with bis(trimethylsilyl)acetylene, and by Sakurai and coworkers from the photolysis of (pentamethyldisilanyl) phenylacetylene ... 

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. 

The first 1,2-disilacyclobutene (82) was prepared in 1973 by the gas phase reaction of dimethylsilylene and 2-butyne (73JOM(52)C21). It probably results through silylene insertion into the intermediate silacyclopropene (Section 1.20.3.4), but silylene dimerization followed by addition to the alkyne is also suggested (76JA7746), since (82) is formed in good yield if the disilene is generated directly (Scheme 127) (78JOM(162)C43). 

The palladium chloride-(PPh3)2 catalyzed photoisomerization of a disilylbutadiyne 102 has recently been described. It was shown that isomerization of the diyne initially led to the substituted silacyclopropene 103, which subsequently dimerized to three isomers of 1,4-disilacyclohexadiene 104, (equation 14). A mechanism was proposed. 

Thermolysis of 29 in presence of bis(trimethylsilyl)acetylene at 60°C yields bis(silacyclopropene) 30 (61%), which is unstable to moisture and air oxidation <1997JA3629>. Thermolysis of 29 in a degassed sealed tube produced the dimer disilabenzvalene 34 in 87% (Scheme 36). 

Silacyclopropene (66) was converted to its dimer (67) in 67% yield under mild thermal conditions in the presence of a catalytic amount of [PdCl2(PEt3)2] (Equation (24)) (77CC352, 820M1473). No other isomers of disilacyclohexa-2,5-diene were detected. 

A comparison of the three dimerizations discussed in the latter part of this chapter illustrates nicely the interplay between symmetry and energy The presence of an additional tt bond in cyclobutadiene offers enough energetic advantage to concerted closure of the four-membered ring to syn-TCOD for it to take precedence over the more general - and no less allowed - stepwise pathway via a transoid biradical. Cyclopropene, with just the one 7r-bond, behaves like a normal alkene and finds the latter pathway more convenient. Silacyclopropene starts off along a similar pathway, but makes use of the relative weakness of the CSi bonds to react in an entirely different manner, but one that is still consistent with the requirements of orbital symmetry conservation. 

The formation of 1,4-disilacyclohexadiene from the addition of silylene to alkynes could be explained by two separate mechanisms as indicated in the scheme in equation (117). Mechanism a involves the addition of SiXY to the acetylene to give a substituted silacyclopropene, which subsequently dimerizes to give the disilacyclohexadiene compound. Mechanism b first calls for the... 

---