Homogeneous asymmetric catalysis reactions

After the first successful asymmetric hydroformylation, although in low optical yields, the reaction was further investigated by different groups all over the world. The results have been rather disappointing from a synthetic point of view, as in a few cases only, optical yields as high as 30 to 50% have been achieved. However, some interesting information has been obtained, both on the mechanism of hydroformylation and on the basic aspects of homogeneous asymmetric catalysis by transition metal complexes. 

The discovery of the stereoregular polymerization of alkenes by Ziegler-Natta catalysis opened a possible route to optically active polymers by a suitable modification of the catalyst. Indeed, Natta, in 1961, succeeded in polymerizing ben-zofurane 2 (Scheme 2) under the influence of a catalyst obtained by a combining of AICI3 and phenylalanine. Optical activity was detected for the polymer [15]. This reaction seems the first example of homogeneous asymmetric catalysis by a metal complex, however it is difficult to estimate the efficiency of the process from the specific rotation of the polymer. The asymmetric polymerization of... 

Combinatorial Approach for Homogeneous Asymmetric Catalysis 1157 Table 5.2 Solvent-free asymmetric HDA reaction of aldehydes 1 with Danishefsky s diene 2. ... 

The publication (70) in 1976 of the preparation of optically active epoxyketones via asymmetric catalysis marked the start of an increasingly popular field of study. When chalcones were treated with 30% hydrogen peroxide under (basic) phase-transfer conditions and the benzylammonium salt of quinine was used as the phase-transfer catalyst, the epoxyketones were produced with e.e. s up to 55%. Up to that time no optically active chalcone epoxides were known, while the importance of epoxides (arene oxides) in metabolic processes had just been discovered (71). The nonasymmetric reaction itself, known as the Weitz-Scheffer reaction under homogeneous conditions, has been reviewed by Berti (70). 

The detailed design of chiral reaction spaces for asymmetric catalysis on surfaces is still a serious challenge [1, 26, 27]. The chemistry of homogeneous metal complexes in solution can not straightforwardly be transferred to chemistry on solid surfaces, and both catalytic activity and enantioselectivity in homogeneous systems often decrease upon simple immobilization of the homogeneous meal complexes on surfaces. 

BINOL and its derivatives have been utilized as versatile chiral sources for asymmetric catalysis, and efficient catalysts for their syntheses are, ultimately, required in many chemical fields [39-42]. The oxidative coupling of 2-naphthols is a direct synthesis of BINOL derivatives [43, 44], and some transition metals such as copper [45, 46], iron [46, 47] and manganese [48] are known as active metals for the reaction. However, few studies on homogeneous metal complexes have been reported for the asymmetric coupling of 2-naphthols [49-56]. The chiral self-dimerized V dimers on Si02 is the first heterogeneous catalyst for the asymmetric oxidative coupling of 2-naphthol. 

Nozaki, H., Takaya, H., Moriuti, S. and Noyori, R. (1968) Homogeneous catalysis in the decomposition of diazo compounds by copper chelates. Asymmetric carbenoid reactions. Tetrahedron, 24, 3655. 

The hydrocyanation reaction is important not only because it is practiced industrially on a large scale, but also because it clearly illustrates some of the fundamental postulates of homogeneous catalysis. The potential of the hydrocyanation reaction in asymmetric catalysis has also been explored and appears to be promising (see Chapter 9). 

In recent years the synthesis of chiral and achiral tripodal phosphines and their application in homogeneous catalysis has been studied in more detail [2]. Enantiomerically pure tripodal ligands were synthesized from the corresponding trichloro compounds and chiral, cyclic lithio-phosphanes, e.g. 17, (Scheme 6) [21,22], Using a rhodium(I) complex of ligand 18, an enantiomeric excess of 89 % was obtained in the asymmetric hydrogenation reaction of methyl acetami-docinnamate (19). 

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