Impact of Host Flexibility on Selectivity in a Supramolecular Host-Catalyzed Enantioselective aza-Darzens Reaction.

A highly enantioselective aza-Darzens reaction (up to 99% ee) catalyzed by an enantiopure supramolecular host has been discovered. To understand the role of host structure on reaction outcome, nine new gallium(III)-based enantiopure supramolecular assemblies were prepared via substitution of the external chiral amide. Despite the distal nature of the substitution in these catalysts, changes in enantioselectivity (61 to 90% ee) in the aziridine product were observed. The enantioselectivities were correlated to the flexibility of the supramolecular host scaffold as measured by the kinetics of exchange of a model cationic guest. This correlation led to the development of a best-in-class catalyst by substituting the gallium(III)-based host with one based on indium(III), which generated the most flexible and selective catalyst.

A Nanovessel-Catalyzed Three-Component Aza-Darzens Reaction.

It has been previously demonstrated that nanovessels can be highly competent catalysts providing high rate accelerations and unique selectivity to the organic transformations which they mediate. However, for supramolecular assemblies to be considered a standard reagent in organic synthesis they must first demonstrate the ability to catalyze increasingly complex transformations. Herein, we report a three-component Aza-Darzens reaction that generates N-phenylaziridines, catalyzed by a supramolecular host, that provides the stereoisomer opposite to the one generated in bulk solution (trans vs cis). This transformation constitutes a rare catalytic three-component coupling within a supramolecular assembly, providing a supramolecular solution to a synthetically challenging transformation.

Homeomorphic Isomerization as a Design Element in Container Molecules; Binding, Displacement, and Selective Transport of MCl2 Species (M = Pt, Pd, Ni).

The dibridgehead diphosphine ((CH2)14)3 P (1) can rapidly turn inside-out (homeomorphic isomerization) to give a mixture of in,in and out,out isomers. The exo directed lone pairs in the latter are able to scavenge Lewis acidic MCl2; cagelike adducts of the in,in isomer, trans- Cl2(P((CH2)14)3 P) (M = 2/Pt, 3/Pd, 4/Ni), then form. The NiCl2 unit in 4 may be replaced by PtCl2 or PdCl2, but 2 and 3 do not give similar substitutions. U-tubes are charged with CH2Cl2 solutions of 1 (lower phase), an aqueous solution of K2MCl4 (charging arm; M = Pt, Pd), and an aqueous solution of excess KCl (receiving arm). The MCl2 units are then transported to the receiving arm until equilibrium is reached (up to 22 d). When the receiving arm is charged with KCN, transport is much faster (ca. 100 h) and higher K2MX4 equilibrium ratios are obtained (≥96≤4). Analogous experiments with K2PtCl4/K2PdCl4 mixtures show PdCl2 transport to be more rapid. A similar diphosphine with longer methylene chains, P((CH2)18)3P, is equally effective. No transport occurs in the absence of 1, and other diphosphines or monophosphines assayed give only trace levels.