The kinetics, stereochemistry, and deuterium isotope effects in the α-pinene pyrolysis.: Evidence for incursion of multiple conformations of a diradical

Pyrolysis of optically active alpha-pinene gave 95% racemic limonene (dipentene), alloocimine, racemic alpha-pinene, alpha-pyronene. Activation parameters are reported. Pyrolysis of (S) syn-6-trideuteriomethyl alpha-pinene at 256.7degreesC for 2400 s gave dipentene with twice as much deuterium as hydrogen transfer with k(H)/k(D) = 1.49 and alloocimine with a Z and E trideuteriomethyl ratio of ca. 5 with k(H)/k(D) = 0.89. The isotope effect on loss of starting material was 1.16. Separation of the enantiomers of alpha-pinene from 3600 s pyrolyses at 256.7degreesC followed by NMR analysis revealed that the ratio of the R-syn to R-anti to S-anti isomers is 4.6:3.7:1 at roughly two half-lives. Kinetic analysis reveals that the previously proposed mechanism for all conversions involving slow interconversion of two diradicals with C-s symmetry is not consistent with the distribution of the alpha-pinene isomers, particularly the formation of more suprafacial-retention product (R-anti) than bond-rotated isomer (S-anti). Inclusion of another C-s species (ignoring the deuteriums) that would be intermediate between the originally proposed C-s species, appears more consistent with the observations. (C) 2002 Elsevier Science Ltd. All rights reserved.