ABSENCE OF INTERSYSTEM CROSSING IN 1,4-DIDEHYDROBENZENE

The rate of singlet-to-triplet intersystem crossing in 1,4-didehydrobe nzene (the biradical produced as a reactive intermediate in the therma l cycloaromatization of enediynes), cannot be increased by the applica tion. of an external magnetic field. The rate of product formation and the distribution of stable products of 2,3-di-n-propyl-1,4-didehydrob enzene thermolysis is unchanged at magnetic flux densities in the rang e 0-2000 G and at 66 000 G. Similarly. the rate of thermolysis of an u nsymmetrical enediyne is insensitive to magnetic field flux in the sam e range. This finding precludes the modulation of enediyne reaction ra tes in pharmaceutical and synthetic pursuits.