In the investigation of efficient chemical transformations, the carbon-carbon bond-forming reactions play an outstanding role. In this context, organocatalytic processes have achieved considerable attention. 1 Beside their facile reaction course, selectivity, and environmental friendliness, new synthetic strategies are made possible. Particularly, the inversion of the classical reactivity (Umpolung) opens up new synthetic pathways. 2 In nature, the coenzyme thiamine (vitamin B1), a natural thiazolium salt, utilizes a catalytic variant of this concept in biochemical processes as nucleophilic acylations. 3 The catalytically active species is a nucleophilic carbene. 4

Organocatalysis by N-Heterocyclic Carbenes

This review gives a survey on the latest most representative developments and progress concerning ionic liquids, from their fundamental properties to their applications in catalytic processes. It also highlights their emerging use for biomass treatment and transformation.

Ionic liquids and catalysis: Recent progress from knowledge to applications

Conversion of Biomass into Chemicals over Metal Catalysts

Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes.

Trends in bioconversion of lignocellulose: Biofuels, platform chemicals & biorefinery concept

Ionic liquids are considered as an alternative to organic solvents for catalysis. The literature in this field is reviewed with focus on advantageous use of ionic liquids in biocatalysis and biotransformations. The overview reveals that the exploration and mapping of ionic liquids with respect to biocatalysis is still sketchy. It is apparent that advantages can be gained in view of activity, stability and selectivity. Furthermore, integration of reaction and separation has a high potential in the field. The review presents quantitative data on the productivities, space–time yields, as well as stability as far as they can be extracted from the literature.

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Ionic liquids in biotechnology: applications and perspectives for biotransformations

The thiamin diphosphate- and Mg2+-dependent enzyme benzoylformate decarboxylase (BFD) from Pseudomonas putida was characterized with respect to its suitability to catalyze the formation of chiral 2-hydroxy ketones in a benzoin-condensation type reaction. Carboligation constitutes a side reaction of BFD, whereas the predominant physiological task of the enzyme is the non-oxidative decarboxylation of benzoylformate. For this purpose the enzyme was obtained in sufficient purity from Pseudomonas putida cells in a one-step purification using anion-exchange chromatography. To facilitate the access to pure BFD for kinetical studies, stability investigations, and synthetical applications, the coding gene was cloned into a vector allowing the expression of a hexahistidine fusion protein. The recombinant enzyme shows distinct activity maxima for the decarboxylation and the carboligation beside a pronounced stability in a broad pH and temperature range. The enzyme accepts a wide range of donor aldehyde substrates which are ligated to acetaldehyde as an acceptor in mostly high optical purities. The enantioselectivity of the carboligation was found to be a function of the reaction temperature, the substitution pattern of the donor aldehyde and, most significantly, of the concentration of the donor aldehyde substrate. Our data are consistent with a mechanistical model based on the X-ray crystallographic data of BFD. Furthermore we present a simple way to increase the enantiomeric excess of (S)-2-hydroxy-1-phenyl-propanone from 90% to 95% by skillful choice of the reaction parameters. Enzymatic synthesis with BFD are performed best in a continuously operated enzyme membrane reactor. Thus, we have established a new enzyme tool comprising a vast applicability for stereoselective synthesis.

Benzoylformate Decarboxylase from Pseudomonas putida as Stable Catalyst for the Synthesis of Chiral 2‐Hydroxy Ketones

Substrate dependent synergetic and antagonistic interaction of ammonium halide and polyoxometalate catalysts in the synthesis of cyclic carbonates from oleochemical epoxides and CO2

The soluble hydrogenase I (H-2:NADP(+) oxidoreductase, EC 1.18.99.1) from the marine hyperthermophilic strain of the archaeon Pyrococcus furiosus was partially purified by anion-exchange chromatography. This P furiosus hydrogenase I preparation (PF H(2)ase I) has been used as biocatalyst in the enzymatic production and regeneration of beta-1,4-nicotinamide adenindinucleotide phosphate, reduced form (NADPH), utilizing cheap molecular hydrogen and forming protons as the only side-product. Any excess of dihydrogen can be removed easily. It could be demonstrated, that this hyperthermophilic hydrogenase exhibits a high stability under reaction conditions. Generation as well as regeneration of NADPH were performed in batch and repetitive batch experiments with recyclisation of the biocatalyst. In two repetitive batch-series 6.2 g l(-1) NADPH could be produced with a total turnover number (ttn: mol produced NADPH/mol consumed enzyme) of 10,000. Utilizing the thermophilic NADPH-dependent alcohol dehydrogenase from Thermoanaerobium spec. (ADH M) coupled to the PF H(2)ase I in situ NADPH-regenerating system, two prochiral model substrates, acetophenone and (2S)-hydroxy-1-phenyl-propanone (HPP), were quantitatively reduced to the corresponding (S)-alcohol and (1R,2S)-diol. An e.e. >99.5% and d.e. >98%, respectively, with total turnover numbers (ttn: mol product/mol consumed cofactor NADP(+)) of 100 and 160 could be reached. (C) 2003 Elsevier B.V. All rights reserved.

Practical applications of hydrogenase I from Pyrococcus furiosus for NADPH generation and regeneration

The application of homogeneously soluble catalysts is limited by the recovery in cases where the price of the catalyst is high. Biological catalysts, enzymes, can be efficiently recycled by means of an ultrafiltration membrane due to their high molecular weight, for example, in the continuously operated membrane reactor. In order to transfer this principle to chemical catalysis, we have attached a transfer hydrogenation catalyst, first invented by Gao and Noyori, to a polymer. The resulting homogeneously soluble, polymer-bound catalyst (chemzyme) can now be retained by ultrafiltration membranes like enzymes. On applying this catalyst in continuously operated membrane reactors, a continuous isopropoxide dosage is necessary in order to compensate deactivation caused by water residues in the feed stream. Thus, high space-time yields up to 578 g L −1 d −1 and enantioselectivities up to 94% can be achieved. These results were compared to an enzyme catalyzed system consisting of a carbonyl reductase that also utilizes 2-propanol as a hydrogen source for the cofactor regeneration of NADH.

Lasse Greiner

Papers

Ionic liquids in biotechnology: applications and perspectives for biotransformations

Benzoylformate Decarboxylase from Pseudomonas putida as Stable Catalyst for the Synthesis of Chiral 2‐Hydroxy Ketones

Substrate dependent synergetic and antagonistic interaction of ammonium halide and polyoxometalate catalysts in the synthesis of cyclic carbonates from oleochemical epoxides and CO2

Practical applications of hydrogenase I from Pyrococcus furiosus for NADPH generation and regeneration

Continuous Application of Chemzymes in a Membrane Reactor: Asymmetric Transfer Hydrogenation of Acetophenone