Nitronyl and imino nitroxides: Improvement of Ullman's procedure and report on a new efficient synthetic route

The synthesis of nitronyl and imino nitroxides has been reexamined with the aim of both increasing yields and of offering opportunities for new struct ures. The conditions for the formation of 2,3-bis(hydroxyamino)-2,3-dimethy lbutane, the key intermediate of Ullman's route, have been carefully studie d, and a new procedure is proposed, which affords the free base in a very p ure form and up to 60% yield. Full characterization of this intermediate in cluding an X-ray crystal structure is presented. An alternative synthetic r oute through 2,3-diamino-2,3-dimethylbutane and the corresponding imidazoli dines which bypasses the delicate synthesis of the bis(hydroxyamino) compou nd is described. It is shown that 3-chloroperbenzoic acid is an effective o xidant for the transformation of adequately substituted imidazolidines into nitronyl nitroxides, which are obtained in high yield. An illustration of the potentialities of this new route, a new nitronyl nitroxide with two eth yl substituents in positions 4 and 5 of the imidazoline ring, is reported. The scope and limitations of the two routes are discussed.