Journal of the American Chemical Society p. 4091 - 4096 (1981)
Update date:2022-08-17
Topics:
Lockhart, Thomas P.
Bergman, Robert G.
Two approaches have been used to investigate the spin state(s) of 1,4-dehydrobenzene produced in the solution thermolysis of diethynyl olefins.One method relies on the "spin correlation effect" which postulates a relationship between the spin state of a caged radical pair and the ratio of cage and escape reactions (C/E) which may occur in the pair.When the 2,3-di-n-propyl-1,4-dehydrobenzene biradical (4) abstracts hydrogen from 1,4-cyclohexadiene, a radical pair is generated.If a mixture of 1,4-cyclohexadiene-d0 and-d4 is employed, it is possible, by performing a VPC-MS analysis, to determined the C/E ratio leading from the radical pair to the reduced product, o-dipropylbenzene (10).When this method was applied to the reaction of (Z)-4,5-diethynyl-4-octene (3), C/E was found to be 0.6, independent of the concentration of 1,4-cyclohexadiene (between 0.1 and 10 M) in the chlorobenzene reaction solution.This result indicates the presence of the singlet state of 4 in the reaction of 3.Additional support for this analysis came from the reaction of 3,4-dimethyl-1,5-hexadiyn-3-ene (11) in hexachloroacetone solvent in a 1H NMR probe.The single polarized signal (emission) observed is attributed to the major product of the reaction, 1,4-dichloro-2,3-dimethylbenzene (12), obtained by chlorine abstraction from the solvent.The interpretation of this result indicates solvent trapping of the singlet state of the intermediate 2,3-dimethyl-1,4-dehydrobenzene, consistent with the chemical trapping study.These experimental approaches indicate that at least a substantial portion of the products formed from 1,4-dehydrobenzenes at elevated temperatures arise from the singlet state of the biradical.This suggests that either the singlet is the ground state or, if the triplet is lower in energy, the rate of intersystem crossing from the singlet must be <=109 s-1 at 200 deg C.
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