KINETIC-STUDIES OF THE CYCLIZATION OF THE 6,6-DIPHENYL-5-HEXENYL RADICAL - A TEST OF THE ACCURACY OF RATE CONSTANTS FOR REACTIONS OF HYDROGEN TRANSFER AGENTS

Cyclization of the 6,6-diphenyl-5-hexenyl radical (1) to the diphenylcyclopentylcarbinyl radical (2) was studied by indirect and direct methods. Indirect kinetic studies were accomplished by the PTOC-thiol method using hydrogen atom transfer trapping from thiophenol and from tert-butylthiol as the competing basis reactions over the temperature range of -47 to +44-degrees-C. Tributyltin hydride trapping at 22-degrees-C also was studied. There was excellent agreement in the rate constants for cyclization determined with the various trapping agents showing that the bimolecular rate constants for hydrogen transfer from these agents are equivalently placed on a kinetic scale. The Arrhenius function for cyclization of 1 was log (k(r)/8-1) = 10.40-3.63/2.3RT. In direct studies, radical 1 was produced by photolysis of the PTOC ester and the diacyl peroxide derived from 7,7-diphenyl-6-heptenoic acid. The rate constant for cyclization of 1 at 22-degrees-C was 4.5 X 10(7) s-1. The good agreement between the direct kinetic value and the value of 5.1 X 10(7) s-1 calculated from the above Arrhenius function confirms that the rate constants for hydrogen atom transfer from the trapping agents are accurate. The rate constant for cyclization of 1 at room temperature is about one-half as great as that for ring opening of the cyclopropylcarbinyl radical, and primary alkyl radical 1 should prove useful as a radical clock for timing fast radical reactions.