Highly Productive CNN Pincer Ruthenium Catalysts for the Asymmetric Reduction of Alkyl Aryl Ketones

Abstract

Chiral pincer ruthenium complexes of formula [RuCl(CNN)(Josiphos)] (2-7; Josiphos = 1-[1-(dicyclohexylphosphano)ethyl]-2-(diarylphosphano)ferrocene) have been prepared by treating [RuCl(2)(PPh(3))(3)] with (S,R)-Josiphos diphosphanes and 1-substituted-1-(6-arylpyridin-2-yl)methanamines (HCNN; substituent = H (1 a), Me (1 b), and tBu (1 c)) with NEt(3). By using 1 b and 1 c as a racemic mixture, complexes 4-7 were obtained through a diastereoselective synthesis promoted by acetic acid. These pincer complexes, which display correctly matched chiral PP and CNN ligands, are remarkably active catalysts for the asymmetric reduction of alkyl aryl ketones in basic alcohol media by both transfer hydrogenation (TH) and hydrogenation (HY), achieving enantioselectivities of up to 99 %. In 2-propanol, the enantioselective TH of ketones was accomplished by using a catalyst loading as low as 0.002 mol % and afforded a turnover frequency (TOF) of 10(5)-10(6) h(-1) (60 and 82 degrees C). In methanol/ethanol mixtures, the CNN pincer complexes catalyzed the asymmetric HY of ketones with H(2) (5 atm) at 0.01 mol % relative to the complex with a TOF of approximately 10(4) h(-1) at 40 degrees C.

Ruthenium and osmium complexes containing 2-(aminomethyl)pyridine (Ampy)-based ligands in catalysis

“…The Ru-complexes 40, 41 and 43 (Scheme 26) were found to efficiently catalyze the AHY of ketones in ethanol or methanol/ethanol under 5 atm of dihydrogen in the presence of KOt-Bu (6 mol%) (Scheme 62) [35]. However, it was noted that the alcohol solvent plays an important role in the HY vs TH.…”

“…Conversely, with 43 the addition of MeOH led to an increase in both activity and enantioselectivity. Thus, in methanol the reduction had to be regarded as pure HY, affording (S)-1-phenylethanol with 89% ee (78% conversion in 3 h), whereas in 2-propanol the S alcohol was formed by TH with a negligible contribution from HY (91% ee and 75% conversion) [35]. By using a MeOH/EtOH = 1/1 mixture, the complex 43 rapidly catalyzes the reduction of acetophenone by HY to give (S)-1-phenylethanol with 90% ee (99% within 30 min).…”

“…Thus, trans-cinnamaldehyde was quickly and quantitatively converted into cinnamyl alcohol in 30 s, while further reduction of the C C double bond required hours. We later reported a straightforward approach for the preparation of highly enantioselective catalysts containing both chiral phosphine and pincer ligands, obtained without the need to use the optically active pincer ligand [35].…”

“…The in situ generated pincer complexes RuCl(CNN)(PP), prepared from RuCl 2 (PPh 3 ) 3 , (R,S)-Josiphos* and these ligands efficiently Ar = 4-MeO-3.5-Me 2 C 6 H 2 , R 1 = H, R 2 = Me 42: Ar = Ph, R 1 = Me, R 2 = H; 43: Ar = 4-MeO-3.5-Me 2 C 6 H 2 , R 1 = Me, R 2 = H 44: Ar = Ph, R 1 = t-Bu, R 2 = H; 45: Ar = 4-MeO-3.5-Me 2 C 6 H 2 , R 1 = t-Bu, Synthesis of the Ru-complexes 40-45[35].…”

Reactions of benzene based half sandwich ruthenium(II) complex with 2,6-bis((phenylseleno)methyl)pyridine: Preferential substitution of ring resulting in a catalyst of high activity for oxidation of alcohols

“…Ruthenium(II) complexes of pincer ligands of (N, N, N), (P, N, N), (P, N, P), (C, N, N) and (P, C, P) types have been investigated in the recent past [1][2][3][4][5][6][7][8][9] due to their potential catalytic applications. Recently ruthenium(II) complexes with (N, N, N) pincer ligand 2,6-bis (pyrazolyl)pyridine have been used as effective catalysts for hydrogen transfer reaction of ketones [1,[10][11][12].…”