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)2C6H3}(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=iPr or tBu, P*CHP*) and the complex [RuCl{2,6‐(Me2NCH2)2C6H3}(PPh3)], (3; Scheme 3). The crystal structures of achiral [RuCl( PCP)(PPh3)] 4c and of chiral (S,S)‐[RuCl( PCP)(PPh3)] 4a were determined by X‐ray diffraction (Fig. 3). Achiral [RuCl(PCP)(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.