Terpyridine Diphosphine Ruthenium Complexes as Efficient Photocatalysts for the Transfer Hydrogenation of Carbonyl Compounds
Maurizio Ballico, Dario Alessi, Christian Jandl, Denise Lovison, Walter Baratta
Abstract
Abstract The cationic achiral and chiral terpyridine diphosphine ruthenium complexes [RuCl(PP)(tpy)]Cl (PP=dppp ( 1 ), ( R , R )‐Skewphos ( 2 ) and ( S , S )‐Skewphos ( 3 )) are easily obtained in 85–88 % yield through a one‐pot synthesis from [RuCl 2 (PPh 3 ) 3 ], the diphosphine and 2,2′:6′,2′′‐terpyridine (tpy) in 1‐butanol. Treatment of 1 – 3 with NaPF 6 in methanol at RT affords quantitatively the corresponding derivatives [RuCl(PP)(tpy)]PF 6 (PP=dppp ( 1 a ), ( R , R )‐Skewphos ( 2 a ) and ( S , S )‐Skewphos ( 3 a )). Reaction of [RuCl 2 (PPh 3 ) 3 ] with ( S , R )‐Josiphos or ( R )‐BINAP in toluene, followed by treatment with tpy in 1‐butanol and finally with NaPF 6 in MeOH gives [RuCl(PP)(tpy)]PF 6 (PP=( S , R )‐Josiphos ( 4 a ), ( R )‐BINAP ( 5 a )) isolated in 78 % and 86 % yield, respectively. The chiral derivatives have been isolated as single stereoisomers and 3 a , 4 a have been characterized by single crystal X‐ray diffraction studies. The tpy complexes with NaO i Pr display high photocatalytic activity in the transfer hydrogenation (TH) of carbonyl compounds using 2‐propanol as the only hydrogen donor and visible light at 30 °C, at remarkably high S/C (up to 5000) and TOF values up to 264 h −1 . The chiral enantiomers 2 , 2 a and 3 , 3 a induce the asymmetric photocatalytic TH of acetophenone, affording ( S )‐ and ( R )‐1‐phenylethanol with 51 and 52 % ee , respectively, in a MeOH/2‐propanol mixture.