Development of mitsunobu reagents recyclable by aerobic oxidation and the application to catalytic mitsunobu reactions

Abstract

The Mitsunobu reaction is widely used for transformation of hydroxy groups into various functional groups and inversion of the stereochemistry of secondary alcohols in organic synthesis. The Mitsunobu reaction, however, has some serious problems that inhibit its application of the reaction to practical synthesis on large scales. Especially, the reaction requires hazardous azo reagents such as diethyl azodicarboxylate (DEAD) and produces large amounts of undesired waste that sometimes complicate purification of the desired products. We have recently developed catalytic Mitsunobu reactions using new azo reagents recyclable by iron-catalyzed aerobic oxidation. Ethyl 2-(3,4-dichlorophenyl)hydrazinecarboxylate and ethyl 2-(4-cyanophenyl)hydra-zinecarboxylate (or their azo forms) were identified as good catalysts by our systematic investigation. The modified catalytic reaction using these azo reagents was applicable to broad scope of substrates. High thermal stability of the reagents was shown by SC-DSC analysis, suggested low explosive nature of these reagents.