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Redox Reaction of the Pd 0 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
Author(s) -
Tsarev Vasily N.,
Wolters Dennis,
Gais HansJoachim
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200902739
Subject(s) - chemistry , medicinal chemistry , ligand (biochemistry) , desymmetrization , nucleophile , catalysis , carbonate , enantioselective synthesis , tsuji–trost reaction , stereochemistry , palladium , organic chemistry , biochemistry , receptor
The Pd 0 complex 1 that bears the Trost ligand 2 undergoes a facile redox reaction with 1,4‐biscarbonates 5 b – d and rac ‐ 22 under formation of the diamidato–Pd II complex 7 and the corresponding 1,3‐cycloalkadienes 8 b – d . The redox deactivation of complex 1 was the dominating pathway in the reaction of 5 b – d with HCO 3 − at room temperature. However, at 0 °C the six‐membered biscarbonate 5 b , 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 6 b with ≥99 % ee in 66 % yield. An increase of the catalyst loading in the reaction of 5 b with 1 and HCO 3 − afforded the bicyclic carbonate 12 b (96 % ee , 92 %). Formation of carbonate 12 b involves two consecutive inter‐ and intramolecular substitution reactions of the π‐allyl–Pd II complexes 16 b and 18 b , respectively, with O‐nucleophiles and presumably proceeds through the hydrogen carbonate 17 b as key intermediate. The intermediate formation of 17 b is also indicated by the conversion of alcohol rac ‐ 6 b to carbonate 12 b upon treatment with HCO 3 − and 1 . The Pd 0 ‐catalyzed desymmetrization of 5 b with formation of 12 b and its hydrolysis allow an efficient enantioselective synthesis of diol 13 b . The reaction of the seven‐membered biscarbonate 5 c with ent ‐ 1 and HCO 3 − afforded carbonate ent ‐ 12 c (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 5 c catalyzed by 1 gave the Pd II complex 7 in high yield besides carbonate 12 c .