RADIATION-INDUCED DIMERIZATION OF 1,3-CYCLOHEXADIENE . SOLVENT EFFECTS AND FORMATION FO DIELS-ALDER DIMERS BY A CATIONIC CHAIN MECHANISM

The radiation-induced dimerization of 1,3-cyclohexadiene occurred by two simultaneous mechanisms: mechanism 1 produced mainly the endo and exo Diels-Alder products of 1,4,1 ʹ,2ʹ addition; mechanism 2 produced mainly the cis-anti-cis and cis-syn-cis isomeric products of 1,2,1 ʹ,2ʹ addition. Both mechanisms were sensitized by the aprotic solvents benzene, n-hexane, cyclohexane, and di-n-propyl ether, and were inhibited by the protic solvent ethanol. The relative efficiencies of the solvents for the sensitization of both mechanisms were: benzene > n-hexane ≈ cyclohexane > propyl ether. Mechanism 1 involved a cationic chain reaction in benzene, and probably also in the other aprotic solvents. In all the aprotic solvents the yield of the Diels-Alder products went through a maximum as the CHD concentration was increased. Thus mechanism 1 was inhibited by a large concentration of CHD. The maximum yield of the major Diels-Alder dimer I was larger if the maximum occurred at a lower CHD concentration. The values of G(I)max and [CHD]max in each solvent were as follows: benzene, 11, 0.03 M; n-hexane, 6, 0.5 M; cyclohexane, 6, 0.7 M; di-n-propyl ether, 3, 4 M. There was no evidence of a chain in mechanism 2 and the yields of the corresponding dimers were relatively small. For example, in a 0.092 M solution of CHD in benzene, the G value of dimers from mechanism 1 was 16.3, while that from mechanism 2 was only 1.8. It appears that triplet-state CHD molecules were the immediate precursors of the dimers from mechanism 2, and that roughly half of the triplet CHD molecules resulted directly or indirectly from neutralization reactions. © 1969, American Chemical Society. All rights reserved.