THE RACEMIZATION OF ALLENE, A 2-STEP REAC TION

For four vinyl- and phenyl-substituted allenes racemization enthalpies have been determined, which are 2-6 kcal . mol(-1) below the estimate derived from the parent system by correction for the additional radic al stabilization of the intermediate allyldiradical 2(I), indicating t he growing importance of the carbene type structure 2(II) for the inte rmediate. - The racemization of 1,3-dimethylallene 1b at temperatures > 300 degrees C in the presence of NO allows the trapping of an interm ediate, which is considered to be the orthogonal allyl diradical 2b. F rom the trapping kinetics (307-388 degrees C), the reversible bimolecu lar addition of NO at low temperatures (225-285 degrees C) and the str ength of the C-NO bond an enthalpy well for the singlet diradical of 2 .7 kcal . mol(-1) and a singlet-triplet splitting of 5.6 kcal . mol(-1 ) is derived with the triplet being the groundstate. - The postulated intermediate diradical 2 can be stabilized electronically by phenyl an d vinyl groups or by strain when incorporated in an eight-membered rin g. Thermolyses of the optically active 1,2,5-cyclooctatriene 18 in the presence of oxygen allows to determine the enthalpy well of the corre sponding diradical 25 to be 13.1 kcal . mol(-1).