Allylic selenoxide 2,31-rearrangements transition state

Like allyl sulfoxides, allylic selenoxides rearrange via a highly ordered five-membered transition state. The arguments, already presented for the allyl sulfoxide rearrangement (Section 4.11.2.1.2.), apply for the rationalization of the high E selectivity of double-bond formation. Table 7 shows some examples7,8,12-15 for the strong preference for E double bonds (see also reference 2, Table V-2, p 148). Trisubstituted (A)-allyl alcohols are also obtained from allyl selenides with a substituent at C-2 of the allylic moiety (entries 8-10)7,8. 

The influence of a stereogenic center at C-l seems to override that of the stereogenic heteroatom. In selenoxides monosubstituted at C-1, the R1 substituent will occupy the equatorial position. One of the diastereoniers will rearrange via the exo, the other via the endo, transition state to give the same allylic alcohol, e.g., for the (C)-alkene as shown overleaf. 

DFT has been used to model the endo and exo transition states for [2,3]-sigmatropic rearrangement of allylic aryl-selenoxides and -selenimides. °... 

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