Novel P-stereogenic PCP pincer-aryl ruthenium(II) complexes and their use in the asymmetric hydrogen transfer reaction of acetophenone

Achiral P-donor pincer-aryl ruthenium complexes ([RuCl(PCP)(PPh3)]) 4c,d were synthesized via transcyclometalation reactions by mixing equivalent amounts of [1,3-phenylenebis(methylene)]bis[diisopropylphosphine] (2c) or [1,3-phenylenebis(methylene)]bis[diphenylphosphine] (2d) and the N-donor pincer-aryl complex [RuCl{2,6-(Me2NCH2)(2)C6H3}(PPh3)], (3; Scheme 2). The same synthetic procedure was successfully applied for the preparation of novel chiral P-donor pincer-aryl ruthenium complexes [RuCl(P*CP*)(PPh3)] 4a,b by reacting P-stereogenic pincer-arenes (S, S)-[1,3-phenylenebis(methylene)]bis[ (alkyl)(phenyl)phosphines] 2a,b (alkyl = Pr-i or Bu-t, P*CHP*) and the complex [RuCl{2,6-(Me2NCH2)(2)C6H3}(PPh3)], (3; Scheme 3). The crystal structures of achiral [RuCl((PCP)-P-iPr,iPr)(PPh3)] 4c and of chiral (S,S)-[RuCl-((PCP)-P-tBu,Ph)(PPh3)] 4a were determined by X-ray diffraction (Fig. 3). Achiral [RuCl(P*CP*)(PPh3)] complexes and chiral [RuCl(P*CP*)(PPh3)] complexes were tested as catalyst in the H-transfer reduction of acetophenone with propan-2-ol. With the chiral complexes, a modest enantioselectivity was obtained.