Temperature-dependent stereoselectivity and hydrogen deuterium kinetic isotope effect for .gamma.-hydrogen transfer to 2-hexyloxy radical. The transition state for the Barton reaction

Abstract

The diastereomers of 5-deuterio-2-hexanol, prepared stereospecifically from acetol and lactic acid by using fermentation procedures, have been subjected to the Barton reaction with Ag/sub 2/CO/sub 3//Br/sub 2/ in pentane over the temperature range -8.65 to 30.05/sup 0/C. The derived 2,5-dimethyltetrahydrofuran deuterium incorporation yields both the C-5 diastereotopic hydrogen stereoselectivity and the isotope effect (k/sub H//k/sub D/) for the intramolecular hydrogen-transfer step. The isotope effect is classically temperature dependent, 5.82 (+- 0.07), 30.05; 6.01 (+- 0.22), 20.55; 6.81 (+- 0.17), 10.55; 6.90 (+- 0.08), 1.65; 7.47 (+- 0.49), -8.65/sup 0/C, while the stereoselectivity is small, 1.23 (1.65/sup 0/C), and temperature independent. Molecular mechanics calculations show that conformations with linear arrangements of C/sub 5/, H, and O show small steric energy differences between the diastereotropic hydrogens at C-5. Calculations of isotope effects and their temperature dependencies for model transition states match the experimental data for only C/sub 5/, H, and O angles greater than 150/sup 0/.