Аннотация: Two factors has stimulated our research: (i) high thermal, oxidative and hydrolytic stability of cyclopalladated compounds (CPCs) provides their excellent performance in achiral homogenic catalysis [1]; (ii) the most impressive successes in enantioselective catalysis were achieved with planar chiral CPCs [2]. As consequence, we undertook the preparation of two new planar chiral palladacycles, viz., CP-dimer with ferrocene backbone (1), and non-metallocenic CN-dimer (2) based on [2.2]paracyclophane-derived ligand.
Enantiopure CP-dimer (SCRpl)-1 was prepared by double asymmetric synthesis including (i) stereospecific substitution of Me2N group in the known amine (SC)-FcCH(Me)NMe2 with But2P group, following (ii) direct diastereoselective cyclopalladation of the tertiary phosphine thus formed. For preparation of the enantiopure CN-dimer (Spl)-2 we used standard approach based on the corresponding racemic dimer optical resolution with (SC)-prolinate ligand as chiral derivatizing agent. The absolute configuration of the new palladacycles was established by an X-ray diffraction study of their (SC)-prolinate precursors.
The new dimers 1, 2 along with their mononuclear derivatives were tested as catalysts in Suzuki cross-coupling of bulky naphthyl reagents and Michael reactions of 2-cyclohexen-1-one with aryl boronic acids. Two known planar chiral CPCs (3, 4) were also used in the same reactions for comparison purpose. Previously we have shown a very high catalytic activity of the CN-dimer rac-4 in the model Suzuki reaction, allowing to perform it under mild conditions [3].
Asknowledgements: The work was supported by the RFBR (Project 09-03-01097)
References:
[1] J. Dupont, C.S. Consorti, J. Spencer, Chem. Rev. 2005, 105, 2527.
[2] V. Dunina, O. Gorunova, P. Zykov, K. Kochetkov, Russ. Chem. Rev. 2011, 80, 53.
[3] O.N. Gorunova, P.A. Zykov, M.V. Livantsov, K.A. Kochetkov, Yu.K. Grishin, V.V. Dunina, Russ. Chem. Bull. 2010, 59, 1840.