Asymmetric catalysis with transition metal

I - FUNDAMENTALS AND RECENT ADVANCES n - SOME ASPECTS OF ASYMMETRIC CATALYSIS WITH TRANSITION - METAL COMPLEXES... 

Asymmetric synthesis has emerged as a major preparative method, widely used in organic chemistry and in the total synthesis of natural products, and which is also of interest for industrial chemistry. The importance of enantiomerically pure compounds is connected with the applications in pharmaceutical industries, since very often the biological activity is strongly linked to the absolute configuration. In this article the historical developments of asymmetric synthesis will be briefly presented, as well as the main methods to prepare enantiomerically enriched compounds. Then recent asymmetric synthesis of two classes of compounds will be discussed i) Sulfoxides, chiral at sulfur ii) Ferrocenes with planar chirality. The last part of the article will be devoted to asymmetric catalysis with transition-metal complexes. The cases of asymmetric oxidation of sulfides to sulfoxides and nonlinear effects in asymmetric catalysis will be mainly considered. 

Kagan H. B. and Dang, T. P. (1972) Asymmetric catalysis with transition metal complexes I. A catalytic system of rhodium (I) with a new chiral diphosihine, /. Am. Chem. Soc. 94,6429. 

Vol. I-III, Springer, Berlin, 1999 b) H. Brunner, W. Zetdmeier, Handbook of Enantioselective Catalysis with Transition Metal Compounds, Vol. I-II, VCH, Wein-heim, 1993 c) R. Noyori, Asymmetric Catalysis in Organic Synthesis, Wiley, New York, 1994 d) I. Ojima, (Ed.), Catalytic Asymmetric Synthesis, VCH, Weinheim, 1993 e) D. J. Berrisford, C. Bolm,... 

For reviews see (a) Ojima I (ed) (1993) In Catalytic asymmetric synthesis. VCH, New York (b) Noyori R (1994) In Asymmetric catalysis in organic synthesis. WUey, New York (c) Brunner H, Zettimeier W (1993) In Handbook of enantioselective catalysis with transition metal compounds. VCH, Weinheim (d) Jacobsen EN, Pfaltz A, Yamamoto H (eds) (1999) In Comprehensive asymmetric catalysis. Springer, Berlin Heidelberg New York... 

Metal Enolates. In parallel with additives, transition metals may be added to enolates to give transmetallated species which can undergo cross-coupling chemistry. Perhaps the earliest example of metal-catalyzed enolate reactions is the Reformatsky reaction. Transition metal-catalyzed enolate chemistry has been recently revived in the literature, particularly in the field of asymmetric catalysis. The transition metal-catalyzed coupling reactions of aryl halides, allyl epoxides, and allylic esters with alkyl enolates have been recently investigated. Generally the choice of base employed depends on the substrate and on the reaction performed. For enolate arylation, KHMDS seems to be the most... 

I. Ojima, Ed., Catalytic asymmetric synthesis, VCH, Weinheim 1993 H. Brunner, W. Zettl-meier. Handbook of Enantioselective Catalysis with Transition Metal Compounds, 2 Vols., VCH, Weinheim 1993. 

Catalytic, enantioselective cyclopropanation enjoys the unique distinction of being the first example of asymmetric catalysis with a transition metal complex. The landmark 1966 report by Nozaki et al. [1] of decomposition of ethyl diazoacetate 3 with a chiral copper (II) salicylamine complex 1 (Scheme 3.1) in the presence of styrene gave birth to a field of endeavor which still today represents one of the major enterprises in chemistry. In view of the enormous growth in the field of asymmetric catalysis over the past four decades, it is somewhat ironic that significant advances in cyclopropanation have only emerged in the past ten years. 

Some of the most impressive advances in the area of catalytic, enantioselective aldol addition reactions have taken place in the development of catalytic methods for enantioselective acetate aldol additions, a reaction type that has long been recalcitrant. Thus, although prior to 1992 a number of chiral-auxiliary based and catalytic methods were available for diastereo- and enantiocontrol in propionate aldol addition reactions, there was a paucity of analogous methods for effective stereocontrol in the addition of the simpler acetate-derived enol silanes. However, recent developments in this area have led to the availability of several useful catalytic processes. Thus, in contrast to the state of the art in 1992, it is possible to prepare acetate-derived aldol fragments utilizing asymmetric catalysis with a variety of transition-metal based complexes of Ti(IV), Cu(II), Sn(II), and Ag(I). 

In a similar vein, a series of papers published between 2002 and 2008 contains spectacular claims of highly enantioselective asymmetric additions of water to styrenes, unsaturated carboxylic acids, or simple terminal alkenes [34-Al]. The catalysts used are of the heterogeneous type and based on chiral biopolymers such as wool, gelatin, or chitosan as solid supports (sometimes in combination with silica or ion-exchange resins) that are doped with transition metal salts. This series of papers contains spectacular claims, insufficient experimental data, and erroneous chemical structures for the biopolymers used. As earlier work from the same group of authors on asymmetric catalysis on bio-polymeric supports is irreproducible [42], one is well advised to await independent confirmation of those results. 

Especially noteworthy is the field of asymmetric catalysis. Asymmetric catalytic reactions with transition metal complexes are used advantageously for hydrogenation, cyclization, codimerization, alkylation, epoxidation, hydroformylation, hydroesterification, hydrosilylation, hydrocyanation, and isomerization. In many cases, even higher regio- and stereoselectivities are required. Fundamental investigations of the mechanism of chirality transfer are also of interest. New chiral ligands that are suitable for catalytic processes are needed. 

This year has again emphasized the growing importance of organo-transition metal complexes in organic synthesis. In catalysed reactions the major advances have been in asymmetric catalysis with the first reports of chiral induction in catalytic epoxidation and further reports on improved catalysts for asymmetric hydrogenation and allylic alkylation. The formation of carbon-carbon bonds continues to attract attention, and several novel and potentially useful synthetic applications of organometallic complexes have been reported. 

Asymmetric Catalysis with Chiral Complexes of Transition Metals. 201... 

In 1985, Manning and N6th first reported the hydroboration of alkenes catalyzed by Wilkinson s catalystJ Since this pioneering work, the development of transition-metal-catalyzed hydroboration has been investigated extensively. Burgess and Ohlmeyer demonstrated asymmetric catalysis with the use of BINAP and Diop-derived Rh-catalysts. Hayashi et al. later reported improvement of the enantioselectivity for the hydroboration of styrenes using Rh-BINAP complexes (up to 96% ee at -78 Other catalyst systems have also been shown to be effective... 

Hydrosilanes undergo addition to carbon-carbon multiple bonds under catalysis by transition metal complexes. Nickel, rhodium, palladium, and platinum were used as catalytically active metals. By incorporating chiral ligands into the metal catalyst, the hydrosilylation can be performed analogously to other addition reactions with double bonds, for example, asymmetric hydrogenation to obtain optically active alkylsilanes. 

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