Correspondence diagram for dimerization of silacyclopropene

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. 

In their discussion of sigmatropic rearrangements, Woodward and Hoffmann state For the analysis of these reactions correlation diagrams are not relevant since it is only the transition state and not the reactants or products which may possess molecular symmetry elements. [1, p. 114] This is something of an overstatement. For example, they could hardly have meant it to apply to 1,5-hexadiene, which is no less symmetrical than any transition state that can be assumed for its Cope rearrangement. 

The syn and anti isomers of hexadiene have a mirror plane and a twofold rotational axis respectively, each of which bisects the a bonds broken and formed in the rearrangement. Woodward and Hoffmann themselves set up a correlation diagram in C2v between hexadiene - treated as if it were planar - and an infinitely separated pair of allyl radicals [1, p. 149], in an attempt to rationalize the stereochemical course of the reaction. Dewar [2] rejects their attempt as forced , adding It is clear that no simple interpretation is possible in terms of orbital correlations.  

1 Correspondence Between Reactant and/or Product and TV ansition Structure 

West and his coworkers [3, 4] have followed the isomerization of the gem tetra-substituted disilene, Ar2Si=SiAr2, in which Ar and Ar are distinguishable aryl groups with similar steric and electronic properties, e.g. Ar = mesityl and Ar 2 = 2,6-xylyl. The two 1,2-isosubstituted isomers Z and E are formed at different rates. Having provisionally identified the first product as the Z isomer, the authors propose that the 1,1-isosubstituted molecule equilibriates with its 1,2-isomers by a stepwise mechanism An initial dyotropic shift via a bicyclobutanelike transition state to the Z isomer is followed by slower conversion to the E isomer, presumably by rotation about the SiSi double bond. 

---

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

---