On the origin of cis selectivity in the cyclization of N-protected 2-substituted 3-aza-5-hexenyl radicals: a density functional study.
Cyclization of the N-dimethylphosphinoyl-2-methyl-3-aza-5-hexenyl radical has been studied at the UB3LYP/6-31+G(d)//UB3LYP/6-31G(d) hybrid density functional level. The corresponding radical precursor has been synthesized and found to give cis/trans ratios of up to 10/1 in reductive radical cyclizations. The relative energies of reactant and transition state conformers were determined. In discord with the Beckwith-Houk model, it has been found that chair-axial transition states, which lead to cis products, are lowest in energy, rationalizing the observed experimental diastereoselectivity.
Shanks
David
D
Department of Organic Chemistry, Institute of Chemistry, Box 599, Uppsala University, 751 24, Uppsala, Sweden.
Berlin
Stefan
S
Beşev
Magnus
M
Ottosson
Henrik
H
Engman
Lars
L