Sigmatropic Shifts of Hydrogen and Alkyl Groups

Orbital symmetry analysis of the 1,5-sigmatropic shift of hydrogen leads to the opposite conclusion. The relevant frontier orbitals in this case are the hydrogen Is and 1/ 3 of the pentadienyl radical. The suprafacial mode is allowed, whereas the antarafacial mode is forbidden. The suprafacial shift corresponds to a geometrically favorable six-membered ring. 

Sigmatropic migration involving alkyl group shifts can also occur. 

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.  

Lee and co-workers compared the activation barrier and TS structure for suprafacial migration of BH2, CHj, and NH2 at several levels of theory.  

Sigmatropic shifts of hydrogens have been systematically examined in cyclic systems. Early studies demonstrated equilibration among the methylcyclopenta-dienes and methylcycloheptadienes by 1,5-hydrogen shift.  

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