Tropos ligand for the asymmetric addition of phenylboronic acid to enones

TROPOS LIGAND FOR THE ASYMMETRIC ADDITION OF PHENYLBORONIC ACID TO ENONES P. Scafato, F. Caprioli, L. Pisani, C. Rosini Dipartimento di Chimica, Università degli Studi della Basilicata, via N. Sauro, 85; Potenza. e-mail: pisani.laura@virgilio.it The asymmetric conjugate addition of arylboronic acids to activated olefins is nowadays an efficient method for the stereoselective construction of C(sp2)-C(sp3) bonds, given the large spectrum of substrates which can be used and the availability of many different chiral ligands.1 In this context, phosphoramidites certainly constitute a class of versatile and easily tunable ligands.2 We report herein the asymmetric addition of phenylboronic acid to different cycloalkenones with both the tropos phosphoramidite (S)-L1 and the non tropos (S)-L2 ligand. Different enantioselectivities have been obtained, mainly depending on the ring size of the substrates, with (S)-L1 affording higher e.e. values than (S)-L2. These results can been explained reasoning that, only for the tropos phosphoramidite, a “chiral pocket” is formed when the ligand links to the metal atom. References: 1) (a) Fagnou, K.; Lautens, M. Chem. Rev. 2003, 103, 169-196; (b) Hayashi, T.; Yamasaki, K. Chem. Rev. 2003, 103, 2829-2844. 2) a) Boiteau, J.-M.; Imbos, R.; Minnaard, A. J.; Feringa, B. L. Org. Lett. 2003, 5, 681-684. 3) P. Scafato, G. Cunsolo, S. Labano, C. Rosini, Tetrahedron 2004, 60, 8801-8806.