Asymmetric chemistry

Palladium-catalyzed allylic substitution reactions are popular in the chemical community and the number of applications of the reaction, perhaps in particular for asymmetric procedures, continues to grow [53]. The efficiency of asymmetric chemistry is best described in terms of the enantiomeric excess (ee) of the reaction, and it... 

A polymer-supported oxazolidine aldehyde 226 was developed for asymmetric chemistry <02JOC6646> as well as a soluble polymer-bound Evan s chiral auxiliary <02TA333> and 3,5-disubstituted oxazolidin-2-one 227 anchored on a solid phase <02TL8327>. 

This chapter deals with one possible determinate mechanism, i.e., the role spin-polarized electrons might play in inducing an ee in chiral compounds. Of all the determinate mechanisms, probably the most discussed is the application of circularly polarized light (CPL) to induce asymmetry in a photolytic reaction. For a detailed discussion of this area the interested reader is recommended to several reviews in this area [13-17] and, in particular, the recent book by Meierhenrich [18]. For comparative purposes we will briefly go over the salient points of asymmetric chemistry induced by CPL. 

One big caveat should be kept in mind in some cases, the results in asymmetric chemistry have not proved to be reproducible, and the same holds for asymmetric photochemistry as well, especially when small effects are observed, and even more so if ORD is used as an analytical tool at long wavelengths. In this case, every optically active species in the sample contributes to ORD. Therefore CD should be preferred where the spectrum can be used to identify the species. 

Kunz RK, MacMillan DWC (2005) Enantioselective organocatalytic cyclopro-panations. The identification of a new class of iminium catalyst based upon directed electrostatic activation. J Am Chem Soc 127 3240-3241 Lacour J, Hebbe-Viton V (2003) Recent developments in chiral anion mediated asymmetric chemistry. Chem Soc Rev 32 373-382 Li X, List B (2007) Catalytic asymmetric hydrogenation of aldehydes. Chem Commun 17 1739-1741... 

New synthetic pathways for the preparation of chiral cyclic oxaza- and diaza-phosphoramidates suitable for use in asymmetric chemistry were studied with respect to the imide-amide rearrangement of cyclic phosphorimidates (Scheme 23). New types of oligomeric organophosphorus compounds (112), formed by a novel ring opening polymerisation, have been identified. These compounds are stable intermediates in the imide-amide rearrangement. 

Another possible approach for stereospecific synthesis is the use of zeolites with appropriate transition metal fi amework ions to effect asymmetric chemistry. One basis of the prerequisite asymmetry may be the disruption of the local symmetry introduced by the size and the electronic structure of the chosen transition-metal ion impurity contained in the ciystal fi amework. additional chemical manipulations, such as the replacement of the oxide ligand or the addition of activating ligands, may result in desired catalytic activity and selectivity and the maintenance of uniformity among active sites. 

We have witnessed a veritable Golden Age of asymmetric chemistry over the last 25-30 years, yet we need to be mindful of the fact that the scientific discipline is actually over a century old. For several decades fhe development was rather sluggish, but this has now been amply compensated for as new results and achievements are reported at high speed and in quantities that virtually make it impossible to stay abreast wifh aU aspects of the field. Nonetheless, the need for technologies that can convert a substrate molecule into a desired product in high chemical and optical yield is very pronounced - both from academia and in industry - and therefore the large investments made can easily be justified. To operate reactions under catalytic conditions is in demand (not forgetting fhat about 80% of all che-... 

For an overview, see J. Lacour and V. Hebbe-Viton (2003) Chemical Society Reviews, vol. 32, p. 373 - Recent developments in chiral anion mediated asymmetric chemistry . 

Lacour J, Hebbe-Viton V (2003) Recent Developments in Chiral Anion Mediated Asymmetric Chemistry. Chem Soc Rev 32 373... 

Investigation of the production of planar chiral ferrocene derivatives of use in catalytic asymmetric chemistry has led to the lithiation of N,AI-dimethylferrocenylmethylamine in the presence of non-stoichiometric levels of the chiral diamine tmcda (20% wrt ferrocenyl ligand). Data suggest the ability of this ligand to promote catalytic stereoselective ortho lithiation through the facile ability of Et20 to replace the chiral auxiliary as the lithiate converts from tmcda-complexed monomer to Et20-complexed dimer with retention of stereochemistry. ... 

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