New chiral P,N-ligands and their application in the copper-catalyzed enantioselective conjugate addition of dialkylzinc reagents to enones

NEW CHIRAL P,N-LIGANDS AND THEIR APPLICATION IN THE COPPER-CATALYZED ENANTIOSELECTIVE CONJUGATE ADDITION OF DIALKYLZINC REAGENTS TO ENONES P. Scafato, G. Cunsolo, S. Labano, C. Rosini Dipartimento di Chimica, Università degli Studi della Basilicata Via N. Sauro 85, 85100 Potenza E-mail: scafato@unibas.it Copper-phosphoramidite complexes are often the catalysts of choice for the highly enantioselective transfer of functionalized or unfunctionalized alkyl groups to cyclic and acyclic enones [1]. Starting from enantiopure BINOL and the flexible biphenyl amine 1, we synthesized the new phosphoramidite 2, in which the chirality is only due to atropisomerism, because we saw the possibility that 2 could work as a tropos catalyst, according to the concept of asymmetric activation introduced by Mikami [2]. The catalytic efficiency of 2 has been tested on some asymmetric conjugate additions of dialkylzinc reagents to different enones and compared with that of the analogous Feringa’s ligand 3 [3], in which the possibility of the asymmetric activation is not possible. Interestingly, while 2 and 3 provide comparable results in the addition of diethylzinc to chalcone and 2-cyclohexenone, in the case of the addition of dimethylzinc to 2-cyclopentadecenone the results are quite different. In fact, the ligand 2 provides (-)-muscone, a valuable ingredient of the perfume industry, in 84% ee, while 3 gives a much lower ee (57%). [1] (a) Feringa, B.L. Acc. Chem. Res., 2000, 33, 346; (b) Alexakis, A.; Benhaim, C. Eur J. Org. Chem 2002, 3221. [2] Mikami,K.; Yamanaka, M. Chem. Rev.,2003, 103, 3369. [3] Arnold, L.A.; Imbos, R; Mandoli, A.; de Vries, A.H.M.; Naasz, R.; Feringa, B.L. Tetrahedron, 2000, 56, 2865.