Phosphine-based catalysts, asymmetric

Asymmetric Catalysis with Immobilized Phosphine-Based Catalysts. 64... 

Chiral phosphine based transition metal complexes are nsed as a powerful tool for asymmetric synthesis (3). A fundamental mechanistic nnderstanding is required for rhodium and mthenium catalyzed reactions. The starting point of those investigations was the clear and detailed stractnral description of the isolated pre catalyst system. 

Since its discovery by Roelen in 1938 [l],the hydroformylation process was exclusively based on cobalt as catalyst metal, until the development of rhodium-phosphine complexes in the late 1960s [2]. Industrial efforts have been focused on the preparation of norraaZ-aldehydes (linear aldehydes) from 1-alkenes. In contrast, asymmetric hydroformylation, which requires iso-aldehydes (branched aldehydes) to be formed from 1 -alkenes, was first examined in the early 1970s by four groups independently, using Rh(I) complexes of chiral phosphines as catalysts [3,4,5,6]. Since then, a number of chiral ligands have been developed for... 

The present interest in asymmetric catalysis was demonstrated by awarding Nobel prizes to three winners W. S. Knowles (USA) for elaboration of rhodium complex catalysts effective in asymmetric synthesis of anti-Parkinson medicine, R. Noyori (Japan) for elaboration of a new catalytic system based on chiral ruthenium-phosphine complex catalysts that are very effective in hydrogenation reactions, and B. Sharpless (USA) for elaboration of epoxidation and other reactions under the action of chiral titanium complexes. 

While the visual appearance of the products was sufficient to identify the active catalysts, the authors employed a parallel UV plate reader to quantify the results. The rhodium catalysts identified provided encouragement that metals other than palladium could be developed for this transformation, and in a subsequent experiment, the colorimetric technique was used to discover the first non-phosphine-based iridium catalyst for allylic alkylations. A high-throughput approach to the discovery of catalysts for asymmetric allylic alkylations is also described in Section 1.13.3.6. 

In contrast to chiral amines, phosphine-based chiral catalysts were less developed for asymmetric MBH transformations. In the first asymmetric cycloisomerization reaction, the cyclopentenol derivative 233 was prepared from 232 in the presence of (-)-CAMP (Scheme 2.114). ° ° The low asymmetric control (14% ee) was attributed to the reversibility of the cyclization. Notably, this reaction is not suitable for the preparation of six-membered rings. 

A new catalyst incorporating chiral thiourea and nucleophilic Lewis base showed efficiency in the asymmetric BH reactions. The use of a binaphthyl-based amino-thiourea catalyst 63 synthesized by Wang et al. [ 114] resulted in good yields and enantioselectivities in the reaction of cyclohexenone and aldehydes. Another amino-thiourea 12 was demonstrated as an efficient bifunctional catalyst for the enantio-selective aza-BH reaction of (3-methyl-nitrostyrene and iV-tosyl-aldimines, affording P-nitro-y-enamines in modest to excellent enantioselectivities and diastereoselec-tivities (Scheme 9.32). It was found that no reaction occurred in the absence of the methyl group of nitroalkene [115]. A similar phophine-thiourea catalyst 64 was reported in 2008 by Wu and co-workers [116] and turned out to be efficient in the asymmetric BH reaction of MVK and aldehydes, providing fast reaction rate, good yields, and excellent enantioselectivities (87-94% ee). More recently, aL-threonine-derived phosphine-thiourea catalyst 65 was readily synthesized by Lu and coworkers [117] and applied in the enantioselective BH reaction of aryl aldehyde with methyl acrylate. 

Successful representative catalysts for asymmetric acylations include phosphine catalysts, chiral pyridine derivatives, other N-heterocycles " and peptide-based catalysts. Miller and coworkers designed peptide catalysts with -turns, a common feature within proteins and enzymes, in order to obtain secondary structure in which the catalytically active residues can be incorporated. During their initial work on asymmetric acylation reactions peptides were employed containing the catalytic histidine moiety to serve as nucleophile in a series of j0-tum type small peptides for the kinetic resolution of trans-1,2 acetamidocyclohexanol (- -/-)146 (Figure 53a). 

Asymmetric hydrosilylation can be accomplished (i) by [RhO(PPh3)3] if the organic substrate is optically active, e.g. (-)-menthane or ( + )-camphor, or (ii) if chiral phosphine-rhodium catalysts are used. In the particular case where the catalyst is a (diop)rhodium(i) derivative molecular models of intermediates based on oxidative addition of the silane to Rh, e.g. (61), can be used to predict the chirality of products... 

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