Iridium-catalysed direct C-C coupling of methanol and allenes.

Methanol is an abundant (35 million metric tons per year), renewable chemical feedstock, yet its use as a one-carbon building block in fine chemical synthesis is highly underdeveloped. Using a homogeneous iridium catalyst developed in our laboratory, methanol engages in a direct C-C coupling with allenes to furnish higher alcohols that incorporate all-carbon quaternary centres, free of stoichiometric by-products. A catalytic mechanism that involves turnover-limiting methanol oxidation, a consequence of the high energetic demand of methanol dehydrogenation, is corroborated through a series of competition kinetics experiments. This process represents the first catalytic C-C coupling of methanol to provide discrete products of hydroxymethylation.

Trinuclear rhodium complexes and their relevance for asymmetric hydrogenation.

Various trinuclear rhodium complexes of the type [Rh(3)(PP)(3)(mu(3)-OH)(x)(mu(3)-OMe)(2-x)]BF(4) (where PP = Me-DuPhos, dipamp, dppp, dppe; different ligands and mu-bridging anions) are presented, which are formed upon addition of bases such as NEt(3) to solvate complexes [Rh(PP)(solvent)(2)]BF(4). They were extensively characterized by X-ray diffraction and NMR spectroscopy ((103)Rh, (31)P, (13)C, (1)H). Their in situ formation resulting from basic additives (NEt(3)) or basic prochiral olefins (without addition of another base) can cause deactivation of the asymmetric hydrogenation. This effect can be reversed by means of acidic additives.

The major/minor concept: dependence of the selectivity of homogeneously catalyzed reactions on reactivity ratio and concentration ratio of the intermediates.

The homogeneously catalyzed asymmetric hydrogenation of prochiral olefins with cationic Rh(I) complexes is one of the best-understood selection processes. For some of the catalyst/substrate complexes, experimental proof points out the validation of the major/minor principle; the concentration-deficient minor substrate complex, which has very high reactivity, yields the excess enantiomer. As exemplified by the reaction system of [Rh(dipamp)(MeOH)2]+/methyl (Z)-alpha-acetamidocinnamate (dipamp=1,2-bis((o-methoxyphenyl)phenylphosphino)ethane), all six of the characteristic reaction rate constants have been previously identified. Recently, it was found that the major substrate complex can also yield the major enantiomer (lock-and-key principle). The differential equation system that results from the reaction sequence can be solved numerically for different hydrogen partial pressures by including the known equilibrium constants. The result displays the concentration-time dependence of all species that exist in the catalytic cycle. On the basis of the known constants as well as further experimental evidence, this work focuses on the examination of all principal possibilities resulting from the reaction sequence and leading to different results for the stereochemical outcome. From the simulation, the following conclusions can be drawn: 1) When an intermediate has extreme reactivity, its stationary concentration can become so small that it can no longer be the source of product selectivity; 2) in principle, the major/minor and lock-and-key principles can coexist depending on the applied pressure; 3) thermodynamically determined intermediate ratios can be completely converted under reaction conditions for a selection process; and 4) the increase in enantioselectivity with increasing hydrogen partial pressure, a phenomenon that is experimentally proven but theoretically far from being well-understood, can be explained by applying both the lock-and-key as well as the major/minor principle.

Rhodium-complex-catalyzed asymmetric hydrogenation: transformation of precatalysts into active species.

The use of diolefin-containing rhodium precatalysts leads to induction periods in asymmetric hydrogenation of prochiral olefins. Consequently, the reaction rate increases in the beginning. The induction period is caused by the fact that some of the catalyst is blocked by the diolefin and thus not available for hydrogenation of the prochiral olefin. Therefore, the maximum reaction rate cannot be reached initially. Due to the relatively slow hydrogenation of cyclooctadiene (cod) the share of active catalysts increases at first, and this leads to typical induction periods. The aim of this work is to quantify the hydrogenation of the diolefins cyclooctadiene (cod) and norborna-2,5-diene (nbd) for cationic complexes of the type [Rh(ligand)(diolefin)]BF(4) for the ligands Binap (1,1'-binaphthalene-2,2'-diylbis(phenylphosphine)), Me-Duphos (1,2-bis(2,5-dimethylphospholano)benzene, and Catasium in the solvents methanol, THF, and propylene carbonate. Furthermore, an approach is presented to determine the desired rate constant and the resulting respective pre-hydrogenation time from stoichiometric hydrogenations of the diolefin complexes via UV/Vis spectroscopy. This method is especially useful for very slow diolefin hydrogenations (e.g., cod hydrogenation with the ligands Me-Duphos, Et-Duphos (1,2-bis(2,5-diethylphospholano)benzene), and dppe (1,2-bis(diphenylphosphino)ethane).