Kinetics of thermal racemization of (2S,3S)-1-13C-1,2,3-d3-cyclopropane followed by vibrational circular dichroism spectroscopy

Vibrational circular dichoism spectra recorded for (2S,3S)-1-13C-1,2,3-d3-cyclopropane and for mixtures of it and the three related stereoisomers prepared through gas-phase thermal stereomutation reactions at 407°C lead to the rate constant for racemization: kα = (4k1 + 4k12) = (3.12 ± 0.04) x 10-5 s-1. This and the rate constant measured for geometrical equilibration between the two chiral and the two achiral stereoisomers of 1-13C-1,2,3-d3-cyclopropane, ki = (8k1 + 4k12) = (4.63 ± 0.20) x 10-5 s-1, give two equations in two unknowns, and allow one to solve for one-center (k1) and two-center (k12) epimerization rate constants for cyclopropane stereomutations. They are nearly equal, a clear indication of closely competitive reaction pathways.Key words: cyclopropane stereomutations, thermal epimerizations, chirality through deuterium and carbon-13 labeling, vibrational circular dichroism.