Dyotropic shift

The above dyotropic mechanism (Fig. 2) can be invoked to explain the pronounced differences in rate within the series of compounds studied. Fluorenyl and structurally similar substrates are not only electronically well suited for dyotropic shifts owing to their electron-withdrawing ability 10), they are also flat, reducing steric hindrance at the backside of the carbon atom to a minimum 18. In contrast, two phenyl groups sterically prevent smooth backside attack (19). The xanthenyl system is sterically less demanding, but electronically unfavorable owing to the... 

Q 1. i-Pr substimted trans (I) and cis (V) P-lactones undergo Type 1 dyotropic shifts to give different products as shown below. Explain the mechanism of their formation. 

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. 

Since 1,4 sigmatropic shifts [e.g., Eq. (6)] of silyl groups have been known for some time (30), it was tempting to test the feasibility of dyotropic 1,4 processes. 

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 . 

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