Intramolecular CH Oxidative N-Allylation of C-Homoallylhydrazones to 5-Vinyl-2-pyrazolines Catalyzed by the [Pd(OAc)2]/BIMINAP System
Résumé
A catalytic procedure is described for intramolecular benzoquinone-mediated CH oxidative N-allylation of C-homoallylhydrazones to 5-vinyl-2-pyrazolines. Depending on the steric and electronic demand of the substituents at the allylic moiety of the substrate, other six or seven membered cyclic hydrazones were obtained in moderate yields. As compared to other Pd(OAc)2/phosphine combinations, the optimal catalytic conditions are based on the Pd(OAc)2/BIMINAP system [BIMINAP=formal contraction of the acronyms BIMIP=2,2′-bis(diphenylphosphino)-1,1′-bibenzimidazole and BINAP=2,2′-bis(diphenylphosphino)-1,1′-binaphthyl in a 10 % ratio. In a preliminary attempt, the use of the enantiopure (R)-BIMINAP ligand was shown to provide the corresponding 5-vinyl-2-pyrazoline in 75 % yield and 30 % ee in the (+)-enantiomer, whereas the analogous (R)-BINAP ligand afforded the same product in only 47 % yield and 10 % ee. The results highlight the specific value of the electron-poor atropochiral BIMINAP ligand in catalysis and, in particular, oxidative catalysis.