the identity of the turnover-limiting step will depend upon the relative olefin and hydrogen concentrations. Neither of these rate constants is particularly large in comparison to other f-element catalysts. Thus, hydrogenolysis is turnover-limiting in the analogous mechanism for (Cp,2LuH)2-catalyzed hydrogenation of 1-hexene, and k2 = 7.7 X 101 23 M"1 s'1 (&t ÍOMÓ3 M~* s"1).1011 Olefin addition is turnover-limiting for the corresponding hydrogenation of cyclohexene and ~2.3 X 10"2 M"1 s'1.10"1 The present results show that k{ is sterically very sensitive, and the simple transposition of OCH(?-Bu)2 -* O-f-Bu affects a ca. 103 increase in kx. Our earlier results12 show that k2 is sensitive to the electrophilicity at the metal center (which is depressed by OR introduction) and the Th-C bond enthalpy. As judged by variation in OR, k2 is (not surprisingly) sterically rather insensitive. The greater hydrogenolytic reactivity of U-C bonds vis-á-vis Th-C bonds apparently reflects electronic factors that do not dominate An-H olefin insertion chemistry.

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Watson and Crick discovered the DNA double helix in 1953 and realized immediately that its replication involved a templated synthesis.1 Their work inspired Todd to suggest in 1956 that organic templates might one day be used to control laboratory syntheses in a

Molecular knots remain difficult to produce using the current synthetic methods of chemistry because of their topological complexity. We report here the near-quantitative self-assembly of a trefoil knot from a naphthalenediimide-based aqueous disulfide dynamic combinatorial library. The formation of the knot appears to be driven by the hydrophobic effect and leads to a structure in which the aromatic components are buried while the hydrophilic carboxylate groups remain exposed to the solvent. Moreover, the building block chirality constrains the topological conformation of the knot and results in its stereoselective synthesis. This work demonstrates that the hydrophobic effect provides a powerful strategy to direct the synthesis of entwined architectures.