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COMMUNICATIONS 243 K. 4282 Reflections were independent and unique, and 2162 with I > 3 OOa(1) (28,,, = 55") were used for the structure solution. The hydrogen atoms at NI and at Ru could be localized and were refined isotropically, and the remaining hydrogen atoms were calculated from ideal geometries. fixed, and included in thecalculation ofthe structural factor. R = 0.050, Rw = 0.067, w' = (o'(E) +0.0025F2)-'. Rigaku AFC7R diffractometer (graphite monochromator, MoK.). The structures were solved with PATTY and DIRDIF94 [Sb]. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. . Copies of the data can be obtained free of charge on application to The Director, CCDC, 12 Union Road, Cambridge CB2 lEZ, UK (fax' Int. code +(1223)336-033;

for kp as high as 120 L/(mol s) at 0 °C. Considering the assumptions made, this value should be taken as a lower limit.In conclusion, the anionic polymerization of e-caprolactone in tetrahydrofuran represents a living ring-chain equilibrium system. The product distribution is essentially determined by the entropy term, the lower cyclics being favored over the linear polymers at higher dilution, as expected from the Jacobson-Stockmayer theory. The very fast equilibration, as shown in this paper, should be considered in studies concerned with either or both the thermodynamics of the ring-opening polymerization and the statistics of chain conformations of the species involved.

Examples of developments in asymmetric hydrogenation from various perspectives, in an effort to improve efficiency, are reported. Discussed in this Account are (1) the improved synthesis of BINAP ligands, (2) the design of SEGPHOS ligands for higher enantioselectivity, (3) a new protocol with fewer reaction steps to synthesize beta-aminoesters, and (4) a novel asymmetric hydrogenation mediated by a copper catalyst.

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