Highly diastereoselective palladium-catalyzed cyclizations of 3,4-allenylic hydrazines and organic halides-highly stereoselective synthesis of optically active pyrazolidine derivatives and the prediction of the stereoselectivity

Pyrazolidines containing two chiral centers, an interesting class of heterocyclic compounds possessing a range of biological activities, have been prepared highly diastereoselectively (up to 95:5) through asymmetric Pd(OAc)₂-catalyzed cyclizations between the easy available optically active allenylic hydrazines and organic halides in THF in the presence of (R,R)-Bn-Box (L2) as the ligand. It was observed 1) that in most cases (3R,5S)-pyrazolidines were obtained in good yields with very high enantiopurities (>99%) and high diastereoselectivities (up to 95:5) in the presence of (R,R)-Bn-Box (L2), 2) that aryl halides containing electron-donating or -withdrawing groups, heteroaryl, and 1-alkenyl iodides are all suitable substrates for this diastereoselective cyclization, 3) that the absolute configurations of the newly formed chiral centers in the pyrazolidines depend on the structure of substrate 1, and 4) that the enantioand diastereopurities of the trans-pyrazolidines are co-controlled by the chiralities of the chiral catalysts and the substrates. A model for prediction of the enantiopurities of the products and the diastereoselectivities of the reactions based on an HPLC study of the starting hydrazines and the products was established.