ИСТИНА

Unique activity of cyclopalladated compounds (CPC's) in classical homogeneous catalysis has stimulated use of their chiral representatives as enantioselective catalysts. However, by contrast with impressive achievements in CPC-catalyzed asymmetric Overman's rearrangements, cross-coupling catalysis by chiral palladacycles is poorly developed, with numerous reported failures in this field. Our research is devoted to development of an enantioselective Suzuki cross-coupling reaction catalyzed by palladacycles. First of all, we have found that only the CN-palladacycles offer an opportunity to realize this process under extremely mild conditions. Thus, model reaction of para-tolyl bromide with phenyl boronic acid may be conducted at –18 oC, if cyclopalladated N,N-dimethylaminomethylferrocene is used as catalyst. Further we have performed screening a broad set of chiral CPC's in the reaction of 2-methoxynaphthyl-1 iodide with 1-naphthaleneboronic acid affording atropoisomeric 1,1'-binaphthyl, with main attention focused on the catalysts of planar chirality. As the result, we have achieved maximum enantioselectivity (up to 53% ee) using non-metallocenic cyclopalladated imine dimer (Rpl)-1a as catalyst at low temperature. To note, this optical yield exceeds the known values of 26-40 %ee obtained previously for the same reaction with pincer CPC's The optimal optical yield was obtained in toluene in air, the reaction occurs without any induction period and any indications on the palladium black formation. Cyclopalladated catalyst was recovered in nearly quantitative yield (96 %) in the form of its myu-iodide analogue (Rpl)-1b. The structure of the latter was unambiguously confirmed by spectral and X-ray diffraction study of its mononuclear PPh3 derivative (Rpl)-1c. Results of several mechanistic experiments may be considered as an indirect evidence against the operation of the classical catalytic cycle, based on the PdII/Pd0 redox processes; the intact state of the palladacycle seems to be a rather probable.