A new class of chiral Cz-symmetric bis(phospho1ane) ligands has been prepared and used in rhodium-catalyzed asymmetric hydrogenation reactions. We describe a practical, one-pot procedure which utilizes enantiomerically pure 1,4-diol cyclic sulfates 4 for the preparation of a homochiral series of 1,2-bis(phospholano)ethanes 1 and 1,2-bis-(phospho1ano)benzenes (DuPHOS) 2. Cationic rhodium complexes bearing these new ligands behave as very efficient catalyst precursors for the asymmetric hydrogenation of a broad range of a-(N-acy1amino)acrylate (enamide) substrates 5. Significantly, a variety of unnatural and nonproteinaceous a-amino acid derivatives 6 were obtained directly with enantioselectivities approaching 100% ee when using the DuPHOS ligands 2. Substrate-to-catalyst ratios of 10 000 were routinely used, and ratios as high as 50 000 were demonstrated in these reactions. Details of the DuPHOS-Rh-catalyzed hydrogenations are discussed.Asymmetric phosphine ligands have played a dominant role in the development of novel transition-metal-catalyzed enantioselective syntheses.' Notwithstanding the high selectivities observed in certain applications using some of the more successful asymmetric phosphines such as DIPAMP,Z CHIRAPHOS,3 and BINAP: there are many reactions of interest where catalysts bearing these phosphines perform rather poorly in terms of efficiency and enantioselectivity. Such failure indicates the need for alternative asymmetric ligands and/or catalysts.Toward this goal, our research has been focused on the design of new chiral ligands for use in transition-metal-based asymmetric catalysis. We recently outlined our approach and described the synthesis of new electron-rich Cz-symmetric bis(phospho1ane) and C3-symmetric tris(phospho1ane) ligands-' These studies indicated the potential utility of Cz-symmetric 1 ,2-bis(phospholano)ethane ligands of type 1. The reported ligand preparation, however, was inefficient for cases other than the dimethylsubstituted diphospholane (Le., la, R = Me). We herein report details of an improved synthetic procedure that allows facile preparation of the homochiral series of Cz-symmetric 1,2-bis-(phospho1ano)ethanes 1 (BPE, R = Me, Et, Pr, and i-Pr). The evolutionary nature of our ligand design subsequently led us to prepare the analogous homochiral series of 1 ,2-bis(phospholano)benzene ligands 2 (DuPHOS, R = Me, Et, Pr, and i-Pr).sWe have found that the new ligands 1 and 2 afford efficient catalysts for the highly enantioselective hydr0genation5-~ and t Present address: hydrosilylation1° of various unsaturated substrates. In particular, enantioselectivities approaching 100% ee have been observed in the Rh-catalyzed hydrogenation of a wide range of a-(N-acy1amino)acrylates. Importantly, the high efficiency, selectivity, and substrate generality exhibited by these catalysts provide a practical route to a variety of enantiomerically pure unnatural and nonproteinaceous a-amino acid derivatives. The details of these hydrogenation studies are described herein. Results and Disc...

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Takaya, H.; Ohta, T.; Sayo, N.; Kumobayashi, H.; Akutagawa, S.; Inoue, S.; Kasahara, I.; Noyori, R. J. Am. Chem. SOC. 1987, 109, 1596,4129. (2) (a) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345 and references therein. (b) Takahashi, H.; Sakuraba, s.; Takeda, H.; Achiwa, K. Fryzuk, M. D.; James, B. R.; Kang, G.-J.; Rettig, S. J. Inorg. Chem. 1991, 30, 5002 and references therein. (e) Bakos, J.; Orosz, A.; Heil, B.; Laghmari, M.; Lhoste, P.; Sinou, D. J. Chem. Soc., Chem. Commun. 1991,1684. ligands have the general structure (R = Me, Et, i-Pr): (4) Burk, M. J. J . Am. Chem. SOC. 1991, 113, 8518. The DuPHOS ( 5 ) (A) Domiano, P.; Musatti, A.; Nardelli, M.; Pelizzi, C.; Predieri, G.

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