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We report a Rh-catalyzed, enantioselective silylation of arene C-H bonds directed by a (hydrido)silyl group. (Hydrido)silyl ethers that are formed in situ by hydrosilylation of benzophenone or its derivatives undergo asymmetric C-H silylation in high yield with excellent enantioselectivity in the presence of [Rh(cod)Cl]2 and a chiral bisphosphine ligand. The stereoselectivity of this process also allows enantioenriched diarylmethanols to react with site selectivity at one aryl group over the other. Enantioenriched benzoxasiloles from the silylation process undergo a range of transformations to form C-C, C-O, C-I, or C-Br bonds.

Research has been conducted on the fluorine and secondary deuterium kinetic effects for the base-promoted HF elimination from 4-fluoro-4-(4-nitrophenyl)-(1,1,1,3,3- (super)2 H (sub)5)butan-2-one. Results indicate that these effects have been determined via the accelerator-produced short-lived radionucleotides.

Chlorine kinetic isotope effects were observed on dehalogenation catalyzed by haloalkane dehalogenase from Xanthobacter autotrophicus GJ10, in the 1.0045 plus or minus 0.0004 range for 1,2 dichloroethane and 1.0066 plus or minus 0.0004 range for 1 chlorobutane.

A study of the identity of the BINAP (1,1'-binaphthalene-2,2'-diylbis(diphenylphosphine))-ligated palladium complexes present in reactions of amines with aryl halides and rate measurements of these catalytic reactions initiated with pure precatalysts and precatalysts generated in situ from [Pd2(dba)(sub 3)] and BINAP is presented. This mechanism has helped the amine and base to react with [Pd(BINAP)(Ar)(Br)] to form an arylpalladium amido complex and reductive elimination from this amido complex to form the arylamine.

A total of 21 amino acid based ligands including hydroxy amide, thioamide, and hydroxamic acid functionalities, respectively, were combined with [Ru( p-cymene)Cl2]2 and [RhCp*Cl2]2, and used as catalysts for the asymmetric transfer hydrogenation of four different heteroaromatic ketones in 2-propanol. The reactions were performed on a Chemspeed automated high-throughput screening robotic platform. Optimal catalysts were identified for the individual heterocyclic substrate classes. Based on these results, the formal syntheses of the antidepressant drugs ( R)-fluoxetine and ( S)-duloxetine were conducted by using the found catalysts in the key reaction step, which results in high isolated yields (94 %) and excellent product enantioselectivities (>99 % ee) of the formed 1,3-amino alcohols. [ABSTRACT FROM AUTHOR]

Determines the leaving-group fluorine as well as the primary and secondary deuterium kinetic isotope effects for the base-promoted elimination of hydrogen fluoride from 4-fluoro-4(4'-nitrophenyl)-butan-2-one in aqueous solution. Kinetic isotope effects; Kinetic methods.

A very short stereoselective synthesis of enantiomerically pure (3 S , 4a S , 10a R ) - quinagolide has been developed. The key steps involved are a copper-catalyzed regioselective arylation of ( S )-epichlorohydrin with 1,6-dimethoxynaphthalene and a diastereoselective trans -reduction of a cyclic enamine intermediate. The possibility to use both enantiomers of epichlorohydrin and the diastereodivergency found in the reduction process paves the way for a general preparation also in the nonracemic form of chiral trans- fused 3-substituted octahydrobenzo[ g ]quinolines that are privileged structures in medicinal chemistry.

The stereoselective reduction of a diastereoisomeric mixture of benzo[ g ]octahydroquinolinium ion was examined in detail. A diastereoselective borohydride reduction in combination with an efficient deacylative enzymatic resolution of its β-aminoester precursor are the key steps for a stereoselective installation of the three chiral centres present in the (3 S ,4a S ,10a R )-eutomer of the medicinal drug quinagolide. The obtained data paves the way for an easy and practical attainment of chiral 3-substituted octahydrobenzo[ g ]quinolines that are privileged structures in medicinal chemistry.

The regioselectivity in the palladium-catalyzed Heck coupling reaction between an aryl halide and ethyl vinyl ether with four different phosphine ligands: PPh(n)tBu(m) (n=0-3, m=3-n) has been investigated both experimentally and computationally. A zigzag selectivity pattern was experimentally observed upon consecutive replacement of Ph by tBu in the phosphine ligand. Use of a standard DFT method (B3LYP) was shown to give a correct prediction of product preference. However, the trend in relative selectivity among the different ligands could not be correctly described. The use of a more recent DFT functional (M06) parameterized to reproduce dispersion interactions resulted in an improved description. For the sterically most demanding ligands, PtBu(3) and PPhtBu(2), unexpectedly large deviations between experimental and M06 calculated selectivities raised the question of an alternative mechanism for these ligands. In the case of PtBu(3) it was found, in agreement with literature data, that the phosphine ligand could be replaced by a second halide ligand, resulting in an anionic mechanism, with a calculated selectivity in excellent agreement with experimental data. For the PPhtBu(2) ligand, two mechanisms are suggested to operate in parallel, as demonstrated both by computational studies and experimental observation of halide-dependent selectivity. A Halpern effect is observed for all phosphine ligands investigated, that is, the least stable pre-complex results in the most abundant product., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Amino acid based thioamides, hydroxamic acids, and hydrazides have been evaluated as ligands in the rhodium-catalyzed asymmetric transfer hydrogenation of ketones in 2-propanol. Catalysts containing thioamide ligands derived from L-valine were found to selectively generate the product with an R configuration (95 % ee), whereas the corresponding L-valine-based hydroxamic acids or hydrazides facilitated the formation of the (S)-alcohols (97 and 91 % ee, respectively). The catalytic reduction was examined by performing a structure-activity correlation investigation with differently functionalized or substituted ligands and the results obtained indicate that the major difference between the thioamide and hydroxamic acid based catalysts is the coordination mode of the ligands. Kinetic experiments were performed and the rate constants for the reduction reactions were determined by using rhodium-arene catalysts derived from amino acid thioamide and hydroxamic acid ligands. The data obtained show that the thioamide-based catalyst systems demonstrate a pseudo-first-order dependence on the substrate, whereas pseudo-zero-order dependence was observed for the hydroxamic acid containing catalysts. Furthermore, the kinetic experiments revealed that the rate-limiting steps of the two catalytic systems differ. From the data obtained in the structure-activity correlation investigation and along with the kinetic investigation it was concluded that the enantioswitchable nature of the catalysts studied originates from different ligand coordination, which affects the rate-limiting step of the catalytic reduction reaction.

The combination of N-Boc-protected alpha-amino acid hydroxyamides (pseudo-dipeptides) and [{Ru(p-cymene)Cl(2)}(2)] resulted in the formation of superior catalysts for the asymmetric transfer hydrogenation (ATH) of non-activated aryl alkyl ketones in propan-2-ol. The overall kinetics of the ATH of acetophenone to form 1-phenylethanol in the presence of ruthenium pseudo-dipeptide catalysts were studied, and the individual rate constants for the processes were determined. Addition of lithium chloride to the reaction mixtures had a strong influence on the rates and selectivities of the processes. Kinetic isotope effects (KIEs) for the reduction were determined and the results clearly show that the hydride transfer is rate-determining, whereas no KIEs were detected for the proton transfer. From these observations a novel bimetallic outer-sphere-type mechanism for these ATH process is proposed, in which the bifunctional catalysts mediate the transfer of a hydride and an alkali metal ion between the hydrogen donor and the substrate. Furthermore, the use of a mixture of propan-2-ol and THF (1:1) proved to enhance the rates of the ATH reactions. A series of aryl alkyl ketones were reduced under these conditions in the presence of 0.5 mol % of catalyst, and the corresponding secondary alcohols were formed in high yields and with excellent enantioselectivities (>99% ee) in short reaction times.

The rates of oxidative addition of phenyl bromide to [Pd(BINAP)2] have been measured in the presence and absence of added amine to assess a previous hypothesis that addition to [Pd(BINAP)(amine)] is faster than addition to [Pd(BINAP)]. These data show that addition to the amine complex is not faster than addition to [Pd(BINAP)]. Instead, they are consistent with oxidative addition, even in the presence of amine, to [Pd(BINAP)] as the major pathway. These data underscore the value of studying the stoichiometric reactions of isolated complexes when assessing the mechanism of a catalytic process.

The secondary alpha-deuterium, the secondary beta-deuterium, the chlorine leaving-group, the nucleophile secondary nitrogen, the nucleophile (12)C/(13)C carbon, and the (11)C/(14)C alpha-carbon kinetic isotope effects (KIEs) and activation parameters have been measured for the S(N)2 reaction between tetrabutylammonium cyanide and ethyl chloride in DMSO at 30 degrees C. Then, thirty-nine readily available different theoretical methods, both including and excluding solvent, were used to calculate the structure of the transition state, the activation energy, and the kinetic isotope effects for the reaction. A comparison of the experimental and theoretical results by using semiempirical, ab initio, and density functional theory methods has shown that the density functional methods are most successful in calculating the experimental isotope effects. With two exceptions, including solvent in the calculation does not improve the fit with the experimental KIEs. Finally, none of the transition states and force constants obtained from the theoretical methods was able to predict all six of the KIEs found by experiment. Moreover, none of the calculated transition structures, which are all early and loose, agree with the late (product-like) transition-state structure suggested by interpreting the experimental KIEs.