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Indian Journal of Chemistry -Section B (IJC-B) >
IJC-B Vol.44B [2005] >
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 Code: | Int.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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