Catalytic Enantioselective Rotation of Watermill‐Shaped Dinuclear Pd Complexes

Abstract

The catalytic enantioselective rotation of double-paddled, watermill-shaped palladium complexes is described in this report. The achiral syn-and chiral anti-forms of binuclear transbis(salicylaldiminato)palladium(II) complexes doubly linked with polymethylene bridges (1) were synthesized and subsequently characterized by spectroscopic and single crystal XRD analyses. The heptamethylene-linked complex 1 b undergoes highly controllable rotation between the achiral syn and chiral antiforms at ambient temperature, while the hexamethylene-linked analogue 1 a does not exhibit this rotational motion under the same conditions, due to its shorter bridges, and 1 c (with octamethylene linkers) undergoes uncontrollable rapid rotation even at lower temperatures. The kinetics of the dynamic rotation of syn-1 b to the anti-isomer in solution were examined on the basis of 1 H NMR analyses. The data showed that the rotational rate can be greatly modified by varying the solvent or adding a carboxylic acid. The enantioselective rotation of syn-1 b can be induced using a catalytic amount of (2R,3R)-2,3bis(3,5-dimethylbenzoyl)tartaric acid (2 a) to afford (R)-anti-1 b, with an initial enantiomeric excess of 63 % ee and an equilibrium value of 27 % ee. Curve fitting of data for the parallel reversible transformations of syn-1 b to (R) and (S)-anti-1 b gave the forward and reverse first-order rate constants k 1 , k À 1 (for syn to (R)-anti), k 2 and k À 2 (syn-to (S)-anti), and verified that the enantioselective nature of the present catalytic enantioselective rotation is controlled kinetically during the initial stage (k 1 > k 2 ) and thermodynamically in the equilibrium state (k 1 /k À 1 > k 2 /k À 2 ). The specific changes induced in the transition states in response to the use of various solvents and adducts are discussed herein on the basis of the activation parameters for the rotation of syn-1 b, as determined from plots of the kinetic data as functions of time.