Mechanism of asymmetric hydrogenation of alpha-(acylamino)acrylic esters catalyzed by BINAP-ruthenium(II) diacetate

Desymmetrization of 1,4-Cyclohexadienyltriisopropoxysilane Using Copper Catalysis

The first catalytic desymmetrization in the field of allylsilane chemistry is presented. Desymmetrization of cyclohexadienyltriisopropoxysilane is achieved using coppper catalysis. High diastereo- and enantioselectivities are obtained, and the product dienes are highly valuable building blocks for natural product synthesis.

Recent advances in catalytic, enantioselective alpha aminations and alpha oxygenations of carbonyl compounds

The direct introduction of either a nitrogen or oxygen atom adjacent to a carbonyl group in a catalytic, enantioselective manner using both chiral Lewis acid and Lewis base catalysis has been described recently. The enantiomerically enriched products of these reactions, such as alpha-amino acids, represent fundamental building blocks for the construction of complex natural products and other important bioactive molecules. This Minireview provides a synopsis of this ever-growing field and highlights some of the challenges that still remain.

P-Phos: a family of versatile and effective atropisomeric dipyridylphosphine ligands in asymmetric catalysis

This Account outlines our efforts in the design and synthesis of a family of highly effective atropisomeric dipyridylphosphine ligands (P-Phos and its variants) and in the development of their widespread applications in transition-metal-catalyzed asymmetric reactions including hydrogenation, hydrosilylation, and C-C bond formation. Desirable attributes, such as air stability, broad substrate scope, fast rates of reaction, excellent enantioselectivities, low catalyst loading, and mild conditions, make the catalyst systems highly attractive and thus may provide excellent opportunities for practical applications.

Transition States of Binap–Rhodium(I)-Catalyzed Asymmetric Hydrogenation: Theoretical Studies on the Origin of the Enantioselectivity

By using the hybrid IMOMM(B3LYP:MM3) method, we examined the binap-Rh(I)-catalyzed oxidative-addition and insertion steps of the asymmetric hydrogenation of the enamide 2-acetylamino-3-phenylacrylic acid. We report a path that is energetically more favorable for the major enantiomer than for the minor enantiomer. This path follows the "lock-and-key" motif and leads to the major enantiomeric product via an energetically favorable binap-dihydride-Rh(III)-enamide complex. Our theoretical results are consistent with the mechanism that takes place via Rh(III) dihydride formation, that is, oxidative addition of H2 followed by enamide insertion.

Mechanism of Asymmetric Hydrogenation of Acetophenone Catalyzed by Chiral η6-Arene–N-Tosylethylenediamine–Ruthenium(II) Complexes

Chiral arene-N-tosylethylenediamine-Ru(II) complexes can be made to effect both asymmetric transfer hydrogenation and asymmetric hydrogenation of simple ketones through a slight functional modification and by switching reaction conditions. [Ru(OSO2CF3){(S,S)-TsNCH(C6H5)CH(C6H5)NH2}(eta(6)-p-cymene)] catalyzes the asymmetric hydrogenation of acetophenone in methanol to afford (S)-1-phenylethanol with 96% ee in 100% yield. Like the transfer hydrogenation catalyzed by similar Ru catalysts with basic 2-propanol or a formic acid/triethylamine mixture, this hydrogenation proceeds through a metal-ligand bifunctional mechanism. The reduction of the C=O function occurs via an intermediary 18e RuH species in its outer coordination sphere without metal-substrate interaction. The high catalytic efficiency relies on the facile ionization of the Ru triflate complex in methanol. The turnover rate is dependent on hydrogen pressure and medium acidity and basicity. The RuCl analogue can be used as a precatalyst, albeit less effectively. Unlike the well-known diphosphine-1,2-diamine-Ru(II)-catalyzed hydrogenation that proceeds in a basic alcohol, this reaction takes place under slightly acidic conditions, creating new opportunities for asymmetric hydrogenation.

Solution structures and behavior of trans-RuH(eta(1)-BH(4)) (binap)(1,2-diamine) complexes

The solution structures of a number of trans-RuH(eta(1)-BH(4))[(S)-tolbinap](1,2-diamine) precatalysts [TolBINAP = 2,2'-bis(di-4-tolylphosphino)-1,1'-binaphthyl; 1,2-diamine==(S,S)- or (R,R)-1,2-diphenylethylenediamine (DPEN), ethylenediamine (EN), and (S)-1,1-di(4-anisyl)-2-isopropylethylenediamine (DAIPEN)] have been determined using 2D NMR ((1)H--(1)H DQF-COSY, (1)H--(13)C HMQC, (1)H--(31)P HSQC, and (1)H--(15)N HSQC), and a double-pulsed field-gradient spin-echo (DPFGSE) NOE technique. All the octahedral Ru complexes adopt a trans configuration with respect to the BH(4) and hydride ligands. Amine protons of trans-RuH(eta(1)-BH(4))[(S)-tolbinap](1,2-diamine) complexes undergo H/D exchange in (CD(3))(2)CDOD. This inherent high acidity, coupled with the lability and chemical properties of the BH(4) ligand, allows for precatalyst activation without the need for an added base, in contrast to trans-RuCl(2)[(S)-tolbinap](1,2-diamine) precatalysts, which require a strong base for generation of a catalytic species. The H/BH(4) complex in a 2-propanol solution is converted to catalytically active [trans-RuH{(S)-tolbinap}{(S,S)-dpen}(ROH)](+) [(RO)(ROH)(n)](-) (R = (CH(3))(2)CH), a loosely associated ion pair of the discrete (solvated) cationic fragment and anionic species.

Asymmetric conjugate reductions with samarium diiodide: asymmetric synthesis of (2S,3R)- and (2S,3S)-[2-2H,3-2H]-leucine-(S)-phenylalanine dipeptides and (2S,3R)-[2-2H,3-2H]-phenylalanine methyl ester

The highly diastereoselective samarium diiodide and D(2)O-promoted conjugate reduction of homochiral (E)- and (Z)-benzylidene and isobutylidene diketopiperazines (E)-5,7 and (Z)-6,8 has been demonstrated. This methodology allows the asymmetric synthesis of methyl (2S,3R)-dideuteriophenylalanine 27 in > or = 95% de and >98% ee, and (2S,3R)- or (2S,3S)-dideuterioleucine-(S)-phenylalanine dipeptides 37 and 38 in moderate de, 66% and 74% respectively. A mechanism is proposed to account for this process.

The Mechanism of Aluminum-Catalyzed Meerwein−Schmidt−Ponndorf−Verley Reduction of Carbonyls to Alcohols

The mechanistic details of the Meerwein-Schmidt-Ponndorf-Verley (MSPV) reduction of ketones to the corresponding alcohols were investigated both experimentally and computationally. Density functional theory (DFT) was used to assess the energetics of several proposed pathways (direct hydrogen transfer, hydridic, and radical). Our results demonstrate that a direct hydrogen transfer mechanism involving a concerted six-membered ring transition state is the most favorable pathway for all calculated systems starting from a small model system and concluding with the experimentally investigated BINOLate/Al/(i)PrOH/MePhC=O system. Experimental values for the activation parameters of acetophenone reduction using the BINOLate/Al/(i)PrOH system (DeltaG# = 21.8 kcal/mol, DeltaH# = 18.5 kcal/mol, DeltaS# = -11.7 au) were determined on the basis of kinetic investigation of the reaction and are in good agreement with the computational findings for this system. Calculated and experimental kinetic isotope effects support the concerted mechanism.

Toward efficient asymmetric hydrogenation: architectural and functional engineering of chiral molecular catalysts

Asymmetric hydrogenation uses inexpensive, clean hydrogen gas and a very small amount of a chiral molecular catalyst, providing the most powerful way to produce a wide array of enantio-enriched compounds in a large quantity without forming any waste. The recent revolutionary advances in this field have entirely changed the synthetic approach to producing performance chemicals that require a high degree of structural precision. The means of developing efficient asymmetric hydrogenations is discussed from a mechanistic point of view.

Catalytic, Highly Enantio, and Diastereoselective Nitroso Diels−Alder Reaction

This communication presents studies that nitroso Diels-Alder adduct has been furnished in uniformly high yield and high enantioselectivity using nitrosopyridine as a substrate and copper as a catalyst. The obtained Diels-Alder adduct was easily transformed to corresponding chiral amino alcohols without loss of enantioselectivity.

Diastereoselective conjugate reduction with samarium diiodide: asymmetric synthesis of methyl (2S,3R)-N-acetyl-2-amino-2,3-dideuterio-3-phenylpropionate

A highly diastereoselective conjugate reduction using SmI2 and D2O has been demonstrated on a homochiral benzylidene diketopiperazine template, giving methyl (2S,3R)-N-acetyl-2-amino-2,3-dideuterio-3-phenylpropionate in 93% de and 90% ee after deprotection, hydrolysis and N-acetylation.

Asymmetric catalytic hydrogenation. Design of new Ru catalysts and chiral ligands: from laboratory to industrial applications

This Account covers the design of Ru catalysts and ligands. Two classes of chiral phosphine ligands are prepared: the electron-rich trans-2,4-substituted phosphetanes, readily available from optically pure 1,3-diol cyclic sulfates, and atropoisomeric ligands (SYNPHOS, MeO-NAPhePHOS, bearing heterotopic biaryl moieties, and a chiral water-soluble diguanidinium binaphthyl diphosphine, Digm-BINAP). Applications of these ligands to rhodium- and ruthenium-mediated hydrogenation of ketones and olefins have been reported with high enantioselectivities. The recognition abilities of Ru-SYNPHOS for a wide range of ketones is superior to those observed with BINAP, MeO-NAPhePHOS, and MeO-BIPHEP. Several biologically active compounds have been prepared through dynamic kinetic resolution. This work gives access to a number of highly active catalysts of the type [Ru(biphosphane)(H)(eta(6)-cot)]BF(4). These catalysts have demonstrated their utility in the enantioselective hydrogenation of the tetrasubstituted cyclopentenone "dehydrodione", which leads to the commercially important perfume component Paradisone (Firmenich).

Catalytic Leuckart-Wallach-type reductive amination of ketones

A CpRh(III) complex catalyzes reductive amination of ketones using HCOONH(4) at 50-70 degrees C to give the corresponding primary amines in high yields. The reaction is clean and operationally simple and proceeds at a lower temperature and with higher chemoselectivity than the original Leuckart-Wallach reaction. The new method has been applied to the synthesis of alpha-amino acids directly from alpha-keto acids.