Current Organic Chemistry - Volume 7, Issue 17, 2003

It is indeed a pleasure for me to begin the third term as a guest editor for Current Organic Chemistry: Asymmetric Synthesis. Volume 7 of the series provides four contributions concerned with stereoselective and enantioselective synthesis. Two of the chapters address synthetic methodology, while the other two focus on catalysis. Professor Rainer Mahrwald of the Institute for Chemistry at the Humboldt University of Berlin contributed the first chapter of this volume. The aldol-Tishchenko reaction, which involves the aldol reaction of two aldehydes and the subsequent reduction to a monoester of a 1,3-diol, is reviewed with an eye on applications in stereodefined synthesis. It is related, through mechanistic overlap, to the Canizzarro reaction. The chapter flows well from the introduction and description of the classic Tishchenko reaction - formation of an ester from two aldehydes - to more complex applications. In the closing part of the chapter the aldol-Tishchenko reaction of ketones and aldehydes is reviewed in the context of diastereo- and enantioselective synthesis. It is this methodology that has the potential of creating three contiguous and well-differentiated chiral centers. The chapter provides a detailed discussion of mechanisms and gives examples of concise preparation of synthons of the type that are contained in various polyketide-derived natural products. This is an excellent contribution especially in view of the fact that the Tishchenko reaction and its analogues are likely not in the daily repertoire of most synthetic chemists. The review contains 83 references to the primary literature in this field. The second chapter was written by Professor P.J. Persichini III, of Allegheny College, Pennsylvania, and provides an overview of the last ∼40 years in the area of boron mediated transfer reactions as means of C-C bond formation. The chapter is organized according to the distance of the boron complex from the reactive center, i.e., 1,2-transfer, 1,3-transfers, etc., through 1,6-transfers of the functional group. Each section is further organized according to the hybridization of the recipient atom. Mechanistic rationale and the synthetic utility are emphasized by providing examples and applications from natural product syntheses. The chapter is a welcome addition to the literature covering synthetically useful methodologies derived from boron chemistry. The literature coverage contains 82 references, some of which contain multiple citations. Following the two reviews describing modern aspects of synthetic methodology are two reviews dealing with aspects of catalysis. The third chapter is provided by Dr. Andrei V. Malkov and Professor Pavel Kocovsky of the University of Glasgow. It systematically reviews the synthesis and applications of chiral bipyridines to catalytic enantioselective processes. Structure and chirality of such compounds are discussed first according to the symmetry groups and the type of chirality. This part of the review is eminently useful to those chemists not familiar in detail with the field. Next, methods of synthesis are provided for a variety of bipyridine-containing ligands. In the application section cyclopropanation, allylic oxidation, allylic substitution, reduction of ketones, and allylation are discussed along with numerous synthetically useful examples. The review is equally useful for an organometallic expert as it is for a novice to asymmetric catalysis. It includes 106 multiple citation references. The last contribution to this volume is provided by Professor T.V. RajanBabu, Yuan-Yong Yan, and Seunghoon Shin of The Ohio State University. It discusses catalytic reactions performed in water with hydroxyphosphines and derivatives. The prospects and problems of environmentally benign synthesis performed in water are discussed with a focus on hydroxylated phosphines derived from trehalose, salicin, and mannitol. Several types of reactions that depend on phosphine-type catalysts (hydrogenation, hydrocyanation, hydroformylation, allylation, and hydrovinylation) are normally preformed in organic solvent. The authors provide a chronological account of catalyst development in order to allow these reactions to be performed in water. Comparisons of enantiomeric excess are provided for various solvents and catalyst types. It is clear that significant developments in this area have been achieved and the chapter also offers insights on further optimization and substrate specificity of the water-soluble catalysts. This review contains 29 references, several with multiple citations.

The classic Tishchenko reaction embodies dimerization of aldehydes to their corresponding esters via a formal oxidation-reduction sequence, whereas the classic aldol-Tishchenko reaction involves the trimerization of enolizable aldehydes. In an aldol-Tishchenko reaction, two aldehyde molecules undergo addition to form an aldol adduct which is subsequently reduced by a third aldehyde to yield 1,3-diol monoesters, with the occurrence of an intramolecular hydride shift. This step - the intramolecular reduction - has been utilized for highly diastereoselective reduction of ß-hydroxyketones. Many literature examples of this transformation are found in natural product syntheses. The utility of this transformation becomes clear upon reaction of aldehydes with ketones. By the use of these substrates in the aldol-Tishchenko reaction, a facile approach to stereotriades is provided. Three adjacent stereogenic carbon centers, with defined configuration, can be synthesized in a single step. In this review, examples of diastereoselective and enantioselective reactions utilizing the aldol-Tishchenko reaction, as well as several related reactions, are presented and the proposed mechanisms of reaction are discussed.

Organoboron chemistry continues to make an ever-increasing impact on general methodology in synthetic organic chemistry. Over the recent years advances in the reduction, hydroboration, and coupling protocols have served to broaden the synthetic chemists' arsenal with methods illustrating the utility of boron. If one were to include boron enolate chemistry in conjunction with the previously mentioned areas this would encompass most of the literature related to organoboron chemistry. What remains paramount to the synthetic organic chemist are those synthetic methods that result in the formation of carbon-carbon bonds. With respect to organoboron chemistry, the most widely used methods (reduction and hydroboration) do not result in carbon-carbon bond formation. Although boron enolate chemistry and Suzuki based coupling reactions result in the formation of carbon-carbon bonds, it could be argued that boron is not directly involved in the formation of these bonds. However, there exists an area of organoboron chemistry that has been sporadically visited but has yet to be categorized. What is presented here is a review of the carbon-carbon bond forming reactions that are mediated through direct transfer of an appendage from boron to result in a new carbon-carbon bond. This review of boron mediated transfer reactions is not meant to be exhaustive, but it does span ∼40 years in an attempt to assemble a number of examples that best illustrate this category, and to provide a template for substantial expansion of this area.

The current review presents a systematic outline of the types of chiral bipyridines known to-date, in particular the methods of their preparation and their application in selected catalytic enantioselective processes. The first part deals with the main structural types of chiral bipyridines, based on the topological origin of chirality of these systems, namely central, axial, planar, and helical, or any combination of these. The sp2 nitrogens can be oxidised to the corresponding N-oxides, and that not only alters the electronic and steric properties of the system but also allows different modes of coordination. The synthetic approaches are illustrated for each structural type. Several applications of chiral bipyridine derivatives in enantioselective catalysis are reviewed. The most successful transformations where chiral bipyridine-type compounds are employed as the ligands, include copper-catalysed cyclopropanation and allylic oxidation, and palladium- catalysed allylic substitution. Enantioselective nucleophilic catalysis is a new emerging area where chiral bypiridine N-oxides have recently made a considerable impact; a substantial progress has been made in the enantioselective allylation of aromatic aldehydes with allyltrichlorosilanes.

The efficiency and selectivity of Rh(I)-catalyzed asymmetric hydrogenation of dehydroaminoacids have been investigated using water-soluble organometallic catalysts prepared from three readily available carbohydrates, trehalose, D-salicin [2-(hydroxymethyl)phenyl β-D-glucopyranoside] and D-mannitol. Bisdiarylphosphinite- Rh(I) complexes prepared from trehalose give moderate selectivity in hydrogenations in aqueous media. However, investigations of partition coefficients of these complexes between water and organic solvents show that they have significant solubility in organic solvents, suggesting that increasing the number of hydroxyl groups alone (in this case 6) may not assure adequate water solubility to enable catalyst recycling. A series of water-soluble chelating bis-diarylphosphinite ligands with an additional tetraalkylammonium group have been prepared from D-salicin. These are excellent ligands for hydrogenation in organic solvents where the reaction is fast. However, reactions in neat aqueous or biphasic media are considerably slower, and under these conditions, only poor enantioselectivities are observed. Anecdotal evidence suggests that the hydrophobic diarylphosphinite moiety might be responsible for the low solubility and the attendant low reactivity. Competitive hydrolytic degradation of the P-O bond might be another significant problem. An attractive solution to the dual problems of instability in water and hydrophobicity is the use of polyhydroxyphospholanes bearing robust P-C bonds (vis-a-vis P-O bonds of the phosphinites), but with a reduced number of aryl groups on phosphorus. Cationic Rh-complexes of these ligands have been found to be excellent catalysts for organic and aqueous phase hydrogenation of dehydroaminoacids. The viability of catalyst recovery has been demonstrated in three different systems, including two cases where >99% ee can be achieved under recycling conditions.