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)(PPh 3 )]) 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‐(Me 2 NCH 2 ) 2 C 6 H 3 }(PPh 3 )], ( 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*)(PPh 3 )] 4a , b by reacting P‐stereogenic pincer‐arenes ( S,S )‐[1,3‐phenylenebis(methylene)]bis[(alkyl)(phenyl)phosphines] 2a , b (alkyl= i Pr or t Bu, P*CHP*) and the complex [RuCl{2,6‐(Me 2 NCH 2 ) 2 C 6 H 3 }(PPh 3 )], ( 3 ; Scheme 3 ). The crystal structures of achiral [RuCl(     i Pr,   i Pr PCP)(PPh 3 )] 4c and of chiral ( S,S )‐[RuCl(     t Bu,Ph PCP)(PPh 3 )] 4a were determined by X‐ray diffraction ( Fig. 3 ). Achiral [RuCl(PCP)(PPh 3 )] complexes and chiral [RuCl(P*CP*)(PPh 3 )] 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.