Redox reaction of the Pd0 complex bearing the Trost ligand with meso-cycloalkene-1,4-biscarbonates leading to a diamidato Pd(II) complex and 1,3-cycloalkadienes: enantioselective desymmetrization versus catalyst deactivation.
The Pd(0) complex 1 that bears the Trost ligand 2 undergoes a facile redox reaction with 1,4-biscarbonates 5b-d and rac-22 under formation of the diamidato-Pd(II) complex 7 and the corresponding 1,3-cycloalkadienes 8b-d. The redox deactivation of complex 1 was the dominating pathway in the reaction of 5b-d with HCO(3)(-) at room temperature. However, at 0 degrees C the six-membered biscarbonate 5b, catalytic amounts of complex 1, and HCO(3)(-) mainly reacted in an allylic alkylation, which led to a highly selective desymmetrization of the substrate and gave alcohol 6b with > or = 99% ee in 66% yield. An increase of the catalyst loading in the reaction of 5b with 1 and HCO(3)(-) afforded the bicyclic carbonate 12b (96% ee, 92%). Formation of carbonate 12b involves two consecutive inter- and intramolecular substitution reactions of the pi-allyl-Pd(II) complexes 16b and 18b, respectively, with O-nucleophiles and presumably proceeds through the hydrogen carbonate 17b as key intermediate. The intermediate formation of 17b is also indicated by the conversion of alcohol rac-6b to carbonate 12b upon treatment with HCO(3)(-) and 1. The Pd(0)-catalyzed desymmetrization of 5b with formation of 12b and its hydrolysis allow an efficient enantioselective synthesis of diol 13b. The reaction of the seven-membered biscarbonate 5c with ent-1 and HCO(3)(-) afforded carbonate ent-12c (99% ee, 39%). The Pd(0) complex 1 is stable in solution and suffers no intramolecular redox reaction with formation of complex 7 and dihydrogen as recently claimed for the similar Pd(0) complex 9. Instead, complex 1 is rapidly oxidized by dioxygen to give the stable Pd(II) complex 7. Thus, formation of the Pd(II) complex 10 from 9 was most likely due to an oxidation by dioxygen. Oxidative workup (air) of the reaction mixture stemming from the desymmetrization of 5c catalyzed by 1 gave the Pd(II) complex 7 in high yield besides carbonate 12c.
Tsarev
Vasily N
VN
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany.
Wolters
Dennis
D
Gais
Hans-Joachim
HJ