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IJC-B Vol.44B(10) [October 2005] >


Title: Transition states for hydride and methyl 1,2-migrations in carbene rearrangements to alkenes: An AM1 SCF-MO study
Authors: Dkhar, Peter G S
Lyngdoh, R H Duncan
Keywords: Carbene-alkene rearrangements
hydride and methyl 1,2-migrations
AM1 SCF-MO method
transition state stability
“early” and “late” transition states
Issue Date: Oct-2005
Publisher: CSIR
IPC CodeInt.Cl.7 C 07 C
Abstract: The AM1 SCF-MO theoretical method to investigate transition states for hydride and methyl 1,2-migrations in the rearrangement of 27 different singlet carbenes to alkenes or their analogues has been used. This study focuses only on qualitative trends regarding structural effects upon reaction facility, which include the effects of (a) bystander group(s) at the migration origin, (b) various substituent groups at the migration terminus, (c) varying ring size in cyclic carbenes, and (d) methyl migration instead of hydrogen. Calculated AM1 activation energies for the 1,2-shift are much larger than those calculated by more sophisticated regimes. However, the qualitative trends follow those obtained from highly accurate theoretical methods, and also follow the expectations of chemical intuition. Hydride migration is predicted to be kinetically favoured by branching at the migration origin, but thermodynamically preferred from electronegative atoms than from carbon atoms. Methyl migration is predicted to be kinetically less favourable than hydride migration. The transition state geometries are described in detail, all centering around a three-membered cyclic moiety in line with a concerted one-step mechanism for the rearrangement reaction. These are described as “early” or “late”, these descriptions being linked to the various structural features present.
Page(s): 2138-2148
ISSN: 0975-0983(Online); 0376-4699(Print)
Source:IJC-B Vol.44B(10) [October 2005]

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