Type I dyotropic rearrangement

Good examples of Type I dyotropic rearrangements involving C—C bond as the stationary surface are provided by the interconversion of vicinal dibromides in cyclohexane and cyclohexanone ring systems (Scheme 6.7). 

A special term ( dyotropic reaction see also Ref 222.374) proposed to denote the noncatalyzed processes in the course of which two o-bonds migrate simultaneously and intramolecularly, and a general theory of such reactions was developed In accordance with the definitions given in Ref. the rearrangement of ion (77) by the concerted route should be considered as a dyotropic reaction of type I. 

The complex reaction sequence shown in equation 34 might provide some rationalization. The formation of the silylcarbene 141 is suggested, based on experimental results from related reactions , but there is no evidence for the formation of 141 nor for a silylene intermediate. Thus, the transformation 137 142 might proceed via a dyotropic rearrangement as well. The facile 1,3-methyl shift in 2-trimethylsilylsilenes which interconverts 142 139 is well known from Wiberg -type silenes . 139 (R = i-Bu) is stable in solution at room temperature over days and isomerizes only slowly to 140 (R = t-Bu) which rapidly dimerizes giving a 1,3-disilacyclobutane . 

A similar rearrangement is observed between tetrasilyldisilenes 34 and 35 [Eq. (56)].14 The activation free energy for the rearrangement from 35 to (Z)-34 (or ( )-34) is 17.4kcalmol-1 at 283 K, which is ca. 1.7 kcal mol-1 larger than that for the /i,Z-isomerization and 7.7 kcal mol-1 smaller than those for the dyotropic type rearrangement of tetraaryldisilenes.96... 

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