A library of 20 monodentate phosphoramidite ligands has been prepared and applied in rhodium-catalyzed asymmetric hydrogenation. This resulted in the identification of two ligands, PipPhos and MorfPhos, that afford excellent and in several cases unprecedented enantioselectivities in the hydrogenation of N-acyldehydroamino acid esters, dimethyl itaconate, acyclic N-acylenamides, and cyclic N-acylenamides. In addition, a method for the parallel enantioselectivity determination of eight acylated amines is presented.

Co2+ + 2H20 Co(OH)2 + 2H+ (2) Co(OH)2 + Ru(III) T=5 Co(OH)2+ + Ru(II) (3) k-i Co(OH)2+ + Ru(III) -^p• Co02+ + Ru(II) + H20 (4) CoQ2+ ~Co2+ + H202 (5) Co02+ + Co(II) CoOCo4+ppt. (6) 2Ru(III) + H202 -2Ru(II) + 02 + 2H+ (7) as part of the catalytic sequence.23,8 The net reaction, Ru(II) formation and Co(III) precipitation, found when [Ru(III)] and[Co(II)] are equal, results from eq 2-4 and 6 (or its equivalent).9,10The diminished catalyst activity at high Ru(III) is also ascribed to eq 6; this reaction converts part of the Co(IV) and Co(II) to an inactive Co(III) species each cycle. Thus the effective catalyst concentration is diminished at high [Co(II)] and high [Ru- (III)] / [Co(II)] ratios. The production of peroxide in eq 5 is chemically reasonable and receives some support from electrochemical studies of Co(II).11 1The oxidation of H202 by Ru-(bpy)33+ (eq 7) is sufficiently rapid (ktff = 5 X 102 M'1 s'1 at pH 73) when [Ru(II)] > 5 [Ru(III)] to quantitatively oxidize H202to 02.The sequence eq 2-5 and 7 provides a mechanism for Co(II) catalysis of eq 1. It gives rise to a rate law of the observed form when reaction 4, oxidation of Co(III) to Co(IV), is the slow step preceded by equilibria 2 and 3. Provided that Co2+ and Co(OH)2+ are the dominant forms of Co(II) and Co(III), respectively, a is equal to nK2K3kA where n is 4 when reaction 7 is rapid. From the Co(H20)63+/2+ reduction potential (1.86 V12) and estimated Con+ hydrolysis constants,13 K2K3¡[H+]2 is ~1 X 10"2 at pH 7 (i.e., K2 = 10"18•8 and £°(Co(OH)2+/Co(OH)2) = 1.1 V), giving kA {=a/4K2K3) ~1 X 106 7M"1 s'1 and k¡ > 100 s'1. The formation of Co(IV) is thus implicated to be rate determining in the Co(II) catalysis of eq 1 at pH ~7. At present the rate constant for the reaction of Co(IV) with water (hydroxide ion) can only be estimated as >100 s'1 at pH 7. Future experiments may provide greater insight into the details of 0-0 bond formation on Co(IV).Acknowledgment. We thank E. Norton for performing the cobalt analyses. This work was carried out at Brookhaven National Laboratory under contract with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences.

The biologically important amino acid statine, (3S,4S)-4-amino-3-hydroxy-6-methylheptanoic acid, as well as optically active statine analogues, are readily accessible in the ester form by simple reduction of the corresponding N,N-dibenzyl P-keto esters using NaBH4 followed by deprotection.Chiral p-amino alcohols are biologically and pharmacologically interesting compounds.1 An example is statine (l), a constituent of the naturally occurring small peptide pepstatin which is a strong inhibitor of such aspartic proteinases as pepsin, renin, and cathepsin.2 Since renin inhibitors are currently of great interest in the treatment of hypertension and congestive heart failure, considerable efforts have gone into the synthesis of statine and statine analogues.3Since the (3S,4S)-configuration is an essential requirement for biological activity, stereoselective routes are required. The most efficient approach to date involves the conversion of t-butoxycarbonyl (Boc) protected L-amino acids into the corresponding P-keto esters (2) followed by reduction to (3). Unfortunately, common achiral reducing agents such as NaBH4 or NaCNBH3 lead to mixtures of diastereoisomers (3)/(4) or to the wrong (3R74S)-diastereoisomer (4) , and bulky reagents such as K(Bus)~BH afford <25% of (3).3-5 Therefore, chiral reducing agents had to be applied, e.g., enzymes5 or optically active Wilkinson catalysts.6

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