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

Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes.

Transition metal catalysis is a powerful means of effecting organic reactions, but it has some inherent drawbacks, such as the cost of the catalyst and the toxicity of the metals. Organocatalysis represents an attractive alternative and, in some cases, offers transformations unparalleled in metal catalysis. Unique transformations are a particular hallmark of N-heterocyclic carbene (NHC) organocatalysis, a versatile method for which a number of modes of action are known. The NHC-catalyzed umpolung (that is, the inversion of polarity) of electrophilic aldehydes, through formation of the nucleophilic Breslow intermediate, is probably the most important mode of action. In this Account, we discuss the reaction of Breslow intermediates with unconventional reaction partners.In two traditional umpolung reactions, the benzoin condensation and the Stetter reaction, some selectivity issues represent significant challenges, especially in intermolecular variants of these reactions. In intermolecular cross-benzoin reac...

Extending NHC-Catalysis: Coupling Aldehydes with Unconventional Reaction Partners

Organocatalyzed conjugate umpolung of α,β-unsaturated aldehydes for the synthesis of γ-butyrolactones

The efficiency, selectivity and sustainability benefits offered by enzymes are enticing chemists to consider biocatalytic transformations to complement or even supplant more traditional synthetic routes. Increasing demands for efficient and versatile synthetic methods, combined with powerful new discovery and engineering tools, has prompted innovations in biocatalysis, especially the development of new enzymes for precise transformations or ‘molecular editing’. As a result, the past decade has witnessed an impressive expansion of the catalytic repertoire of enzymes to include new and useful transformations not known (or relevant) in the biological world. In this Review we illustrate various ways in which researchers have approached using the catalytic machineries of enzymes for new-to-nature transformations. These efforts have identified genetically encoded catalysts that can be tuned and diversified by engineering the protein sequence, particularly by directed evolution. Discovery and improvement of these new enzyme activities is opening a floodgate that connects the chemistry of the biological world to that invented by humans over the past 100 years. Advances in enzyme performance and capabilities are making them increasingly attractive to synthetic chemists. In this Review Chen and Arnold outline the ways that enzymes have been engineered to achieve reactivities well beyond their original functions.

Engineering new catalytic activities in enzymes

Pyruvate decarboxylase from Zymomonas mobilis (PDC) and benzoylformate decarboxylase from Pseudomonas putida (BFD) are thiamine diphosphate-dependent enzymes that decarboxylate 2-keto acids. Although they share a common homotetrameric structure they have relatively low sequence similarity and different substrate spectra. PDC prefers short aliphatic substrates whereas BFD favours aromatic 2-keto acids. These preferences are also reflected in their carboligation reactions. PDC catalyses the conversion of benzaldehyde and acetaldehyde to (R)-phenylacetylcarbinol and predominantly (S)-acetoin, whereas (R)-benzoin and mainly (S)-2-hydroxypropiophenone are the products of BFD catalysis. Comparison of the X-ray structures of both enzymes identified two residues in each that were likely to be involved in determining substrate specificity. Site-directed mutagenesis was used to interchange these residues in both BFD and PDC. The substrate range and kinetic parameters for the decarboxylation reaction were studied for each variant. The most successful variants, PDCI472A and BFDA460I, catalysed the decarboxylation of benzoylformate and pyruvate, respectively, although both variants now preferred the long-chain aliphatic substrates, 2-ketopentanoic and 2-ketohexanoic acid. With respect to the carboligase activity, PDCI472A proved to be a real chimera between PDC and BFD whereas BFDA460I/F464I provided the most interesting result with an almost complete reversal of the stereochemistry of its 2-hydroxypropiophenone product.

/pdf/exchanging-the-substrate-specificities-of-pyruvate-39cs9aj7ri.pdf

Exchanging the substrate specificities of pyruvate decarboxylase from Zymomonas mobilis and benzoylformate decarboxylase from Pseudomonas putida.

Highly enantioenriched mixed benzoins are obtained selectively through a biocatalytical cross-coupling reaction of aromatic aldehydes using ThDP-dependent enzymes.

/pdf/development-of-a-donor-acceptor-concept-for-enzymatic-cross-27j3su05as.pdf

Development of a donor-acceptor concept for enzymatic cross-coupling reactions of aldehydes: the first asymmetric cross-benzoin condensation.

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Exchanging the substrate specificities of pyruvate decarboxylase from Zymomonas mobilis and benzoylformate decarboxylase from Pseudomonas putida.

Development of a donor-acceptor concept for enzymatic cross-coupling reactions of aldehydes: the first asymmetric cross-benzoin condensation.