Sigmatropic shifts 3.5 -, computational studies

The oldest reaction involving a rearrangement of an organozinc compound is probably the Reformatsky reaction54 55. It took one hundred years until its key step could be classified as a [3,3)-sigmatropic shift ( metallo-Claisen rearrangement) on the basis of computational studies (equation 51)56. 

Diphenylpyrrolidine (77) catalyses the enantioselective cy-chlorination of aldehydes.299 Mechanistic and computational studies suggest that - in contrast to pre- viously proposed mechanisms involving direct formation of the carbon-electrophile bond - iV-chlorination occurs first, followed by a 1,3-sigmatropic shift of chlorine to the enamine carbon. The product iminium ion is then hydrolysed in the ratedetermining step. 

Computational studies on [l,5]-sigmatropic hydrogen shifts are also in agreement with the qualitative predictions based on orbital symmetry relationships. The TS shows aromatic character. Activation energies are calculated to be in the range 35-37 kcal/mol, in good agreement with the experimental value of 36.3. ... 

Computational studies of competing [3,3]- and [3,5]-sigmatropic shifts and intramolecular 4 -I- 2-cycloaddition of 1,3,7-octatriene have been reported (Scheme 27). ... 

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