Page 931 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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SECTION 10.6
Sigmatropic
Rearrangements

1,3-suprafacial 1,3-suprafacial 1,5-suprafacial 1,5-suprafacial
retention inversion retention inversion
Hückel, 4 electrons Mobius, 4 electrons Hückel, 6 electrons Mobius, 6 electrons
antiaromatic aromatic aromatic antiaromatic
forbidden allowed allowed forbidden

Fig. 10.32. Classification of sigmatropic shifts of alkyl groups with respect to basis set orbitals.

10.6.2.1. Computational Characterization of Transition Structures for [1,3]-, [1,5]-,
and [1,7]-Sigmatropic Shifts There have been a number of computational studies
aimed at providing information about the TSs of the sigmatropic rearrangements.
Chamorro and co-workers characterized the TS for prototypical [1,3]-sigmatropic shifts
of hydrogen and methyl. 216 The 1,3-hydrogen shift is an antarafacial process, whereas
the methyl migration is a suprafacial process that occurs with inversion at the alkyl
group. The corresponding nuclear positions are depicted in Figure 10.33.
Computational studies on [1,5]-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 217 to be in the range
35–37 kcal/mol, in good agreement with the experimental value of 36.3. 218
Lee and co-workers compared the activation barrier and TS structure for
suprafacial migration of BH ,CH , and NH at several levels of theory. 219
3
2
2

H7

H1 C4

H2 H3 H3
H2
C2
C2
C1 H1
C1 C3 H5
H4
H1 C2 H4
H3
H4 H2
Fig. 10.33. Nuclear positions and for antarafacial [1,3]-sigmatropic migration of hydrogen
and for suprafacial [1,3]-sigmatropic migration of methyl with inversion (B3LYP/6-
311++G(d,p)), Reproduced from J. Phys. Chem. A, 106, 11533 (2002), by permission of
the American Chemical Society.

216 E. Chamorro, J. C. Santos, B. Gomez, R. Contreras, and P. Fuentealba, J. Phys. Chem. A, 106, 11533
(2002).
217
I. Alkorta and J. Elguero, J. Chem. Soc., Perkin Trans., 2, 2497 (1998); B. S. Jursic, Theochem, 423,
189 (1998); N. J. Saettel and O. Wiest, J. Org. Chem., 65, 2331 (2000).
218 W. R. Roth and J. Konig, Liebigs Ann. Chem., 699, 24 (1966).
219
J. Y. Choi, C. K. Kim, C. K. Kim, and I. Lee, J. Phys. Chem. A, 106, 5709 (2002).