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AM1 and polarized‐π frontier molecular orbital (PPFMO) studies of facial selectivity in hydrogen transfer to substituted adamantyl radicals
Author(s) -
Huang Xiao Ling,
Dannenberg J. J.
Publication year - 1993
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610061206
Subject(s) - chemistry , radical , propene , molecular orbital , photochemistry , computational chemistry , homo/lumo , hydrogen bond , molecular geometry , molecule , organic chemistry , catalysis
Fully optimized transition states for hydrogen transfer from methane and propene to substituted 2‐adamantyl radicals were calculated using the AM1 molecular orbital method. Methane and propene were chosen to provide an early (propene) and a late (methane) transition state. For 5‐substituted radicals (F, Cl, Ph, CH 3 , CF 3 ) the enthalpic differences between syn and anti reactions was found to be small [<0·1 kcal mol −1 (1 kcal = 4·184 kJ)]. Other radicals with nitrogen or boron included directly in the adamantyl fragment showed greater selectivities, with aza substitution favoring syn and bora substitution favouring anti attack, as did the 4,9‐difluoro derivative. The calculated selectivities are all in qualitative agreement with experimental results (where available) on bromine atom transfer. PPFMO calculations showed the polarization of the SOMO to be generally in accord with the AM1 results, whereas the polarization of the LUMO's was less indicative. The bond orders for the bonds syn and anti to the incipient CH bond in the adamantyl fragments indicated that the anti bonds were always weaker than those syn , in agreement with the suggestion by Cieplak that has been used to explain the experimental selectivities. However, the bond lengths of the incipient CH bonds are always shorter for the side of preferred attack, in apparent agreement with the suggestion by Anh.