Highly Diastereo- and Enantioselective Access to syn-α-Amido β-Hydroxy Esters via Ruthenium-Catalyzed Dynamic Kinetic Resolution-Asymmetric Hydrogenation

Dynamic kinetic resolution (DKR) of racemic aryl α-amino β-ketoesters via Ru-diphosphine-catalyzed asymmetric hydrogenation was realized at 70 °C under 50 atm of hydrogen, affording syn α-amido β-hydroxy esters in high yields (up to 96%) with high reactivity (TON up to 940) and diastereo- and enantioselectivities (up to 99:1 dr, 98% ee). These hydrogenation products provide valuable chiral synthons in many natural products and pharmaceuticals. Gram-scale DKR asymmetric hydrogenation (DKR-AH) was also performed with retained reactivity and stereoselectivity, revealing the synthetic utility of this method.

Bridging Fluorides and Hard/Soft Mismatch in d6 and d8 Complexes: The Case of [Tl(μ-F)3Ru(PPh3)3]

[RuCl2(PPh3)3] reacts with thallium(I) fluoride to give either [Tl(mu-F)3Ru(PPh3)3] (1) or [Tl(mu3-F)(mu2-Cl)2Ru2(mu2-Cl)(mu2-F)(PPh3)4] (2) depending on the excess of TlF used. Both 1 and 2 were fully characterized, including X-ray structure determinations. Complex 1 reacts with dihydrogen to form the known ruthenium hydride complex [Ru(H)2(H2)(PPh3)3] upon hydrogenolysis of the Ru-F bond. The reaction of 1 with activated alkyl bromides (R-Br) gives the corresponding alkyl fluorides and the trinuclear complex [Tl(mu3-F)(mu2-F)(mu2-X)Ru2(mu2-Br)(mu2-F)(PPh3)4] (X=Br, F) (3), whose structure closely resembles that of 2. However, 1 is not active as catalyst for the nucleophilic fluorination of R-Br in the presence of thallium fluoride. The effect of the bridging coordination mode of fluoride on the Ru-F bond is discussed in terms of the HSAB principle, which suggests a more general model for predicting the stability of d6 and d8 complexes containing hard ligands (such as fluoro, oxo, and amido).

The first complete identification of a diastereomeric catalyst-substrate (alkoxide) species in an enantioselective ketone hydrogenation. Mechanistic investigations

The enantioselective hydrogenations of the dialkyl 3,3-dimethyloxaloacetate ketone substrates (2, 3, and 4; alkyl = Me, (i)Pr, and (t)Bu, respectively) were catalyzed by [Ru((R)-BINAP)(H)(MeCN)(n)(sol)(3-n)](BF(4)) (1, n = 0-3, sol = THF or MeOH, (R)-BINAP = (R)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) in up to 82% ee (R). Reaction of the active catalyst 1 with 1 equiv of substrate (2, 3, or 4) in THF or MeOH solution formed the diastereomeric catalyst-alkoxide complexes [Ru((R)-BINAP)(MeCN)(OCH(CO(2)R)-(C(CH(3))(2)CO(2)R))](BF(4)) (5/6 R = Me, 8/9 R = (i)Pr, and 10 R = (t)Bu, respectively) via hydride addition to the ketone carbonyl carbon and ruthenium addition to oxygen. The absolute configurations at the alkoxide groups ((R)- for the major diastereomers 5, 8, and 10) were determined via cleavage of the ruthenium-alkoxide bond with 1 equiv of HBF(4).OEt(2). The solution structures of the major diastereomer catalyst-alkoxide complexes (5, 8, and 10) were unambiguously determined by variable-temperature NMR spectroscopy. The major diastereomers (5, 8, and 10) had the same absolute configuration as the major product enantiomers from the catalytic hydrogenation of 2, 3, and 4 with 1 as catalyst. The ratio of major to minor alkoxide diastereomers was similar to the ee of the catalytic hydrogenation. The catalyst-alkoxide complexes are formed at temperatures as low as -30 degrees C with no other precursors or intermediates observed by NMR showing that ketone-hydride insertion is likely not the turnover limiting step of the catalytic hydrogenation. Results from the stoichiometric hydrogenolysis of 5/6, 8/9, or 10 indicate that their formation is rapid and only partially reversible prior to the irreversible hydrogenolysis of the ruthenium-oxygen bond. The stereoselectivities of the formation and hydrogenolysis of 5/6, 8/9, and 10 sum up to equal the stereoselectivities of the respective catalytic hydrogenations of 2, 3, and 4. The rates of the hydrogenolysis were consistent with these diastereomers being true catalytic intermediates.

3,3'-Bis(diphenylphosphino)-1,1'-disubstituted-2,2'-biindoles: easily accessible, electron-rich, chiral diphosphine ligands for homogeneous enantioselective hydrogenation of oxoesters

Racemic (+/-)-3,3'-bis(diphenylphosphinyl)-1,1'-dimethyl-2, 2'-biindole (1c) (N-Me-2-BINPO) and (+/-)-3, 3'-bis(diphenylphosphinyl)-1,1'-bis(methoxymethyl)-2,2'-biindole (1d) (N-MOM-2-BINPO) were synthesized in satisfactory yields following a three-step reaction sequence, starting from indole. Resolution of racemic 1c and 1d was achieved through fractional crystallization of their diastereomeric adducts with optically active dibenzoyl tartaric acids, followed by alkaline decomplexation of the diastereomerically pure salts. Their trichlorosilane reduction gave enantiopure phosphines (+)- and (-)-(1a) (N-Me-2-BINP) and (+)- and (-)-(1b) (N-MOM-2-BINP). The electrochemical oxidative potential of 1a and 1b was found to be 0. 52 and 0.60 V, respectively. Both the enantiomers of (1a) were tested as ligands of Ru(II) in asymmetric hydrogenation reactions of alpha- and beta-oxoesters. Reactions were found to be outstandingly fast and enantioselection quite good. Comparative kinetic experiments on the hydrogenation reaction of methyl acetoacetate carried out with 1a, 1c, BINAP, and other biheteroaromatic diphosphines as ligands of Ru(II) demonstrated that all the reactions follow a first-order kinetic. A linear relationship was found between the kinetic constant log and the electrochemical oxidative potential of the diphosphine ligand.

Versatile precursor to ruthenium-bis(phosphine) hydrogenation catalysts

The known complex trans-RuCl2(NBD)Py2 (1, NBD is norbornadiene, Py is pyridine) reacts with either (R)-BINAP ((R)-2, 2'-bis(diphenylphosphino)-1,1'-binaphthyl), (S;S)-Chiraphos ((2S;3S-(-)-2,3-bis(diphenylphosphino)butane), (S;S)-Skewphos ((2S;4S)-(-)-2,4-bis(diphenylphosphino)pentane), (R)-(S)-Josiphos ((R)-(-)-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethyl-dicyclohexylpho sphine), (R;R)-Norphos ((2R;3R)-(-)-2, 3-bis(diphenylphosphino)bicyclo[2.2.1]hept-5-ene), or (R;R)-Me-DUPHOS ((-)-1,2-bis((2R;5R)-2, 5-dimethylphospholano)benzene) to generate in high yields the crystalline complexes trans-RuCl2(P-P*)Py2 (P-P* is the corresponding chiral bis(phosphine)). The complexes trans-RuCl2(P-P*)Py2 are active enantioselective hydrogenation catalysts for ketoesters and noncarboxylic olefins in the presence of small amounts of HBF4 (aq.). They are active for hydrogenation of carboxylic substrates in the presence of Et3N. Reaction of trans-RuCl2(P-P*)Py2 with (rac)-1,2-diphenylethylene-diamine (N-N*, either enantiomer) forms in good yields the corresponding compounds trans-RuCl2(P-P*)(N-N*). Representative hydrogenations with these catalysts are presented.

Chiral and Achiral Diphosphine Complexes of Ruthenium(II) Incorporating Labile Nitrile Ligands: Synthesis and Solution Chemistry of Mono- and Dinuclear Derivatives of Ru(2)Cl(4)(PP)(2) (PP = Chelating Diphosphine)

A family of nitrile complexes has been prepared by reaction of Ru(2)Cl(4)(PP)(2) or RuCl(2)(PP)(PPh(3)) (PP = Ph(2)P(CH(2))(4)PPh(2) (dppb), Ph(2)PCH(2)CHOCMe(2)OCHCH(2)PPh(2) (diop), 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (binap)) with MeCN or PhCN, the product formed depending strongly on the reaction conditions. At high nitrile concentrations, the principal species present is RuCl(PP)(RCN)(3)(+)X(-) (X = Cl); the cation can generally be isolated (as the PF(6) salt) by abstraction of the chloride counterion with NH(4)PF(6). Use of 2 equiv of NH(4)PF(6) generates Ru(PP)(RCN)(4)(2+)(PF(6)(-))(2) (PP = dppb). In the absence of a halide-abstracting agent, addition of hexanes or diethyl ether precipitates neutral RuCl(2)(PP)(RCN)(2) species, which undergo further loss of nitrile in the solid state (R = Me) or in solution (R = Me, Ph). Redissolution of the neutral species in chlorocarbon solvents gives Ru(2)Cl(3)(PP)(2)(RCN)(2)(+)X(-) (X = Cl) and, in benzene, Ru(2)Cl(4)(PP)(2)(RCN). The dinuclear cation (X = PF(6)) is also accessible by reaction of RuCl(PP)(RCN)(3)(+)PF(6)(-) with CH(2)Cl(2) or CDCl(3), while the mononitrile can be obtained directly by reaction of Ru(2)Cl(4)(PP)(2) or RuCl(2)(PP)(PPh(3)) with small amounts of nitrile in benzene.

New Class of Chiral Diphosphine Ligands for Highly Efficient Transition Metal-Catalyzed Stereoselective Reactions: The Bis(diphenylphosphino) Five-membered Biheteroaryls

The synthesis and application of three examples of a new class of chiral (C(2)) atropisomeric diphosphines characterized by two interconnected five-membered heteroaromatic rings, with hindered rotation around the interanular bond, are described. Optically pure (+)- and (-)-2,2'-bis(diphenylphosphino)-4,4',6,6'-tetramethyl-3,3'-bibenzo[b]thiophene (tetraMe-bitianp) (1a) and the parent unsubstituted system (+)- and (-)-bitianp (1b) were synthesized. They were found to be optically stable at 100 degrees C and were successfully employed as ligands in the Ru(II)-catalyzed hydrogenation of alpha- and beta-oxo esters to the corresponding alpha- and beta-hydroxy esters and in the hydrogenation of olefinic substrates. The optical and chemical yields were comparable with those reported for the same Ru(II)-binap-catalyzed reactions carried out under the same experimental conditions. The 2,2'-bis(diphenylphosphino)-3,3'-bibenzo[b]furan (1c), the oxygenated analogue of bitianp, was found to be configurationally unstable at room temperature. Complete structural X-ray elucidation of the Pd complexes of 1a-c is reported. The advantages of these biheteroaromatic ligands over the classical biaryl systems are discussed.