Chiral catalysis/catalysts

This example illustrates a subtle control of a chemical reaction by a delicate manipulation of tire stereochemical environment around a metal centre dictated by tire selection of tire ligands. This example hints at tire subtlety of nature s catalysts, tire enzymes, which are also typically stereochemically selective. Chiral catalysis is important in biology and in tire manufacture of chemicals to regulate biological functions, i.e., phannaceuticals. 

The most frequently encountered, and most useful, cycloaddition reactions of silyl enol ethers are Diels-Alder reactions involving silyloxybutadicncs (Chapter 18). Danishefsky (30) has reviewed his pioneering work in this area, and has extended his studies to include heterodienophiles, particularly aldehydes. Lewis acid catalysis is required in such cases, and substantial asymmetric induction can be achieved using either a chiral lanthanide catalyst or an a-chiral aldehyde. 

Corey, E.J. Helal, C.J. (1998) Reduction of Carbonyl Compounds with Chiral Oxazaborolidine Catalysts A New Paradigm for Enantioselective Catalysis and a Powerful New Synthetic Method. Angewandte Chemie International Edition, 37, 1986-2012. 

Synthesis of optically pure compounds via transition metal mediated chiral catalysis is very useful from an industrial point of view. We can produce large amounts of chiral compounds with the use of very small quantities of a chiral source. The advantage of transition metal catalysed asymmetric transformation is that there is a possibility of improving the catalyst by modification of the ligands. Recently, olefinic compounds have been transformed into the corresponding optically active alcohols (ee 94-97%) by a catalytic hydroxylation-oxidation procedure. 

The production of enantiomerically pure products is of great importance in chemical industry. The most desirable way to obtain these products is by chiral catalysis. Homogeneous complexes can often be used as chiral catalysts however, because of their difficult regenerability, the development of heterogeneous chiral catalysts by immobilization of these complexes is difficult but highly desired. 

For applications in ring-opening catalysis of chiral lactides, pyrazolylborates have been suitably modified to yield chiral zinc complexes. An example of such a chiral pre-catalyst is the highly sterically encumbered tris(indazolyl)bo-rate methylzinc complex 107, introduced by Chisholm et al., whose C -symmetric structure is shown in Figure 53.165... 

There are, then, three critical requirements of any catalyst if it is to be exploited on a commercial scale these are activity, selectivity and stability. It has been widely demonstrated and generally accepted that homogeneous catalysts are superior to their heterogeneous counterparts in terms of both activity (certainly under mild reaction conditions) and selectivity (the classical example is chiral catalysis). 

Perhaps the most compelling research objective in this area will involve the development of a chiral metathesis catalyst that effects C-C bond formation efficiently and with excellent levels of enantioselectivity [41 ]. In such a case, all the reactions discussed herein, in addition to those expertly developed in other laboratories [40], will become subject to asymmetric catalysis. Such a development should prove to have an enormous impact on the field of inorganic, organome-tallic and synthetic organic chemistry. 

The self-assembly of a chiral Ti catalyst can be achieved by using the achiral precursor Ti(OPr )4 and two different chiral diol components, (R)-BINOL and (R,R)-TADDOL, in a molar ratio of 1 1 1. The components of less basic (R)-BINOL and the relatively more basic (R,R)-TADDOL assemble with Ti(OPr )4 in a molar ratio of 1 1 1, yielding chiral titanium catalyst 118 in the reaction system. In the asymmetric catalysis of the carbonyl-ene reaction, 118 is not only the most enantioselective catalyst but also the most stable and the exclusively formed species in the reaction system. 

Bianchini, C. Barbaro, P. (2002) Recent aspects of asymmetric catalysis by immobilized chiral metal catalysts.. Topics in Catalysis, 19 17-32. 

MacMillan DWG, Lelais G (2007) History and perspective of chiral organic catalysts. In Mikami K, Lautens M (eds) New frontiers in asymmetric catalysis. WUey, Hoboken, NJ, p 313... 

The initial work on the asymmetric [4-1-2] cycloaddition reactions of A -sulfinyl compounds and dienes was performed with chiral titanium catalysts, but low ee s were observed <2002TA2407, 2001TA2937, 2000TL3743>. A great improvement in the enantioselectivity for the reaction of AT-sulfinyl dienophiles 249 or 250 and acyclic diene 251 or 1,3-cyclohexadiene 252 was observed in the processes involving catalysis with Cu(ll) and Zn(ii) complexes of Evans bis(oxazolidinone) (BOX) ligands 253 and 254 <2004JOC7198> (Scheme 34). While the preparation of enantio-merically enriched hetero-Diels-Alder adduct 255 requires a stoichometric amount of chiral Lewis acid complex, a catalytic asymmetric synthesis of 44 is achieved upon the addition of TMSOTf. 

Asymmetric synthesis starts with a prochiral compound. This is a compound which is not chiral, but can be converted into a chiral compound by a chiral (bio) catalyst. Subsequently, two types of prochiral compounds can be distinguished The first one has a stereoheterotopic face (which usually is a double bond) to which an addition reaction takes place. An example is the conversion of the prochiral compound propene into 1,2-epoxypropane (which has two enantiomers, of which one may be preferentially formed using an enantioselective catalyst). The second type of prochiral compound has two so-called enantiotopic atoms or groups. If one of these is converted, the compound becomes chiral. Meso-compounds belong to this class. Figure 10.5 and 10.6 show some examples of the different types of asymmetric catalysis with prochiral compounds. 

The first reported attempts of what was then called "absolute or total asymmetric synthesis" with chiral solid catalysts used nature (naturally ) both as a model and as a challenge. Hypotheses of the origin of chirality on earth and early ideas on the nature of enzymes strongly influenced this period [15]. Two directions were tried First, chiral solids such as quartz and natural fibres were used as supports for metallic catalysts and second, existing heterogeneous catalysts were modified by the addition of naturally occuring chiral molecules. Both approaches were successful and even if the optical yields were, with few exceptions, very low or not even determined quantitatively the basic feasibility of heterogeneous enantioselective catalysis was established. 

From a synthetic point of view, there are a few reaction types catalyzed by chiral heterogeneous catalysts which are useful for preparative chemists. But it is also evident that the scope of most catalytic systems is rather narrow and very high substrate specificity is observed. Compared to homogeneous or bio-catalysis, enantioselectivities are usually lower but there are exceptions. 

Finn, F M, Hofmann, K 1976, in Neurath, H, Hill, R L (eds), The Proteins, 3rd edn, Vol II, chapter 2(p 106—237), Academic Press New York London Finn, M G, Sharpless, fC. B 1985, On the Mechanism of Asymmetric Epoxidation with Titanium-Tartrate Catalysts, in Momson, J D (ed), Asymmetric Synthesis, Vol 5 Chiral Catalysis chapter 8, p 247, Academic Press New York Fischer, E 1914, Chem Ber 47,196 Fischer, H, Slangier G 1927, Liebigs, Ann Chem 459, 53 Fischer, H Neber, M 1932. Liebigs Ann Chem 496,1... 

Keywords Asymmetric synthesis, Chiral catalysis, Diversity-based approaches, Supported chiral catalysts, Solid-phase chemistry... 

Corey EJ, Helal CJ (1998) Reduction of carbonyl compounds with chiral oxazaborolidine catalysts a new paradigm for enantioselective catalysis and a powerful new synthetic method. Angew Chem Int Ed 37 1986-2012... 

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