Stereochemical control catalysis

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

It is apparent that steric bulk and stereochemical control of mechanism operates in the alkaline hydrolysis of methyl 8-acyl-1-naphthoates. The proximity and favourable orientation of the carbonyl group at the 8-position facilitates intramolecular catalysis from this group. However, the formation of the tetrahedral intermediate at the 8-acyl carbonyl group has distinct... 

The surface complementarity between the quantum activated complex and the catalytic surrounding media is the main idea of the present theory. The oscillating stereochemical control of the synthesis of thermoplastic elastomeric polypropylene recently reported by Coates and Waymouth [208] can be easily interpreted in terms of catalyst changing surface complementarity. Hill and Zhang have discovered a molecular catalyst that experiences a kinetic and thermodynamic drive for its own reassembly and repair under conditions of catalysis [209]. This is basically what an enzyme does when moving from the apo-structure towards the catalytically apt conformation. 

Uncatalysed Diels-Alder reactions usually have to be carried out at relatively high temperatures (normally around 100 °C)73, often leading to undesired side reactions and retro-Diels-Alder reactions which are entropically favoured. The Diels-Alder reaction became applicable to sensitive substrates only after it was realized that Lewis acids (e.g. A Clg) are catalytically active56. As a consequence, Diels-Alder reactions can now be carried out at temperatures down to — 100°C85. The use of Lewis acid catalysts made the [4 + 2]-cycloaddition applicable to the enantioselective synthesis of many natural compounds51,86. Nowadays, Lewis acid catalysis is the most effective way to accelerate and to stereochemically control Diels-Alder reactions. Rate accelerations of ten-thousand to a million-fold were observed (Table 7, entries A and B). 

Although in the recent years the stereochemical control of aldol condensations has reached a level of efficiency which allows enantioselective syntheses of very complex compounds containing many asymmetric centres, the situation is still far from what one would consider "ideal". In the first place, the requirement of a substituent at the a-position of the enolate in order to achieve good stereoselection is a limitation which, however, can be overcome by using temporary bulky groups (such as alkylthio ethers, for instance). On the other hand, the ( )-enolates, which are necessary for the preparation of 2,3-anti aldols, are not so easily prepared as the (Z)-enolates and furthermore, they do not show selectivities as good as in the case of the (Z)-enolates. Finally, although elements other than boron -such as zirconium [30] and titanium [31]- have been also used succesfully much work remains to be done in the area of catalysis. In this context, the work of Mukaiyama and Kobayashi [32a,b,c] on asymmetric aldol reactions of silyl enol ethers with aldehydes promoted by tributyltin fluoride and a chiral diamine coordinated to tin(II) triflate... 

Asymmetric phase-transfer catalysis with (S,S)-lg can be successfully extended to the stereoselective N-terminal alkylation of Gly-Ala-Phe derivative 61 (i.e., the asymmetric synthesis of tripeptides), where (S,S)-lg turned out to be a matched catalyst in the benzylation of DL-61, leading to the almost exclusive formation of DDL-62. This tendency for stereochemical communication was consistent in the phase-transfer alkylation of DDL-63, and the corresponding protected tetrapeptide DDDL-64 was obtained in 90% yield with excellent stereochemical control (94% de) (Scheme 5.30) [31]. 

We have selected a few reactions of Co, Fe, and Cu with honourable mentions for Pt, Ir, and Cr. We could have focused on other elements—Ni, W, Ti, Zr, Mn, Ru, and Rh all have special reactions, Transition metal chemistry, particularly involving palladium catalysis, occupies a central role in modern organic synthesis because complex structures can be assembled in few steps with impressive regio- and stereochemical control. There are many books devoted entirely to this subject if you wish to take it further. 

Asymmetric dihydroxylation with 0s04 gave the syn diol 270 which was deprotected and cyclised with toluene-p-sulfonic acid (TsOH) catalysis in over 90% yield to complete this short and interesting stereochemically controlled synthesis of exo-brevicomin 256 in 65% yield from allyl bromide. 

There has been intense interest in the control of stereochemistry of free-radical addition reactions by use of chiral auxiliaries [6] and, even more recently, using enantioselec-tive catalysis [7-12]. As a result, the current understanding of stereochemical control has clearly progressed to the point where this obstacle can be overcome. There have also been reports of strategies that simplify telomer distribution in free-radical oligomeriza-... 

The development of novel chiral metal complexes and chiral ligands is crucial for both progress and development of asymmetric catalytic synthesis [1-3]. Within this area, the appearance of planar-chiral ferrocenes as ligands in asymmetric catalysis has been an important advancement [4-7]. While most of these complexes bear side chains or atom groups with stereogenic centres, it is often the 1,2-disubstitution pattern of the n-complexed ring that creates an inherent planar chirality [8] and exercises efficient stereochemical control. 

Among very rare examples of catalytic transformations involving polyene and polyenyl ligands is Pd-catalyzed oxidation of diene derivatives by the use of acetic acid and quinone with unique stereochemical control being achieved by judicious choice of the reaction condition (Scheme 8.65) [121]. Thus, the oxidation carried out with high Cl concentration afforded cis diacetate product, while trans adduct was obtained in the absence of Cl ion. The initial step of the catalytic cycle would be the exo attack of OAc at the Pd-bound diene, giving rise to r -allyl intermediate with OAc and Pd positioned trans to each other. This then underwent either exo or endo attack of the second OAc in the presence or absence of Pd-bound Cl ligand, respectively. The endo attack may have proceeded in a manner similar to Scheme 8.50. The hnal step of the catalysis would be oxidation of Pd(0), formed by the OAc attack at the r -allyl terminus, with benzoquinone as an oxidant. 

Yamazaki et al. employed the Evans oxazolidinone enolate in diastereoselective Michael additions to /I-CF3 acrylates to afford intermediate allyl silyl ketene acetals [8]. The products were isolated as ca. 2 1 mixtures of pentenoic acids and Michael addition adducts (Scheme 4.59). The rearrangement of the silyl ketene acetal was catalyzed by PdCl2(CH3CN)2. The rearrangement apparently occurred via the Z-silyl ketene acetal and exhibited high 1,2-asymmetric induction. Aspects of stereochemical control and Pd catalysis have been discussed previously (cf Scheme 4.25). 

It has now been demonstrated beyond any doubt that it is possible to use self-assembly processes to construct molecular assemblies and supramolecular arrays, which are of nanometre-scale, with a high degree of control and precision from molecular components comprised of simple and inexpensive building blocks. Furthermore, it is often easier to make molecular assemblies and supramolecular arrays than it is to make some of the components on their own. The molecular components are "intelligent in that they hold all of the information necessary for the construction of the precisely assembled structures and superstructures without the need for external reagent or catalysis. It is becoming increasingly obvious that self-assembly occurs under very precise constitutional and stereochemical control. The viability of self-assembly as a concept for the synthesis of novel molecular architectures on the nanometre-scale is surely vindicated by now. 

Aldol Reactions Addition to Aldehydes and Imines. Since its discovery, the Mukaiyama aldol reaction. has attracted considerable attention and several improvements in reaction conditions have heen reported. Most useful catalysts for this reaction appear to he recently reported lanthanide triflates (eq 5), bis(cyclopentadienyl)titanium bis(trifluoromethanesulfonate), or Cp2Zr(OTf)2 THF. The metallocene salt also catalyzes additions to ketones (eq 6). This reaction can also be carried out under essentially neutral conditions by warming (70 °C) a stoichiometric mixture of the aldehyde and the KSA in acetonitrile (eq 7). When an optically active aldehyde is used, a slightly better stereochemical control is noticed under catalysis of zinc iodide. ... 

---