Enantiometric excess

Partially successful attempts towards chiral electrochemical synthesis have involved chiral supporting electrolytes chiral solvents and chiral adsorbates, mostly alkaloids With the latter method enantiometric excess values >40% have... 

Table 4). Chemical and optical yields were highly dependent on the mode of fabrication of the polymer layers Enantiometric excess values, chemical... 

TABLE 6D.1. Relationship of Ligand Concentration to Enantiometric Excess... 

The intramolecular 2 + 2-photo-cycloadditions of optically active allenesilanes (5) with enones and enoates produce silyl-substituted exo-methylenecyclobutanes (6) in high enantiometric excess. Photo-desilation leads to the parent unsaturated exo-methylenecyclobutanes (7) (Scheme 3).19 The cycloaddition of naphthoquinone to allyltrimethylsilane in the presence of Me2 A1C1 yields the expected 2 + 2-cycloadduct that slowly rearranges to the 2 + 3-adduct.20 hi the presence of bases, Cephalosporin triflates (8) undergo 2 + 2- and 4 + 2-cycloaddition with alkenes, alkynes, and dienes via an intermediate six-membered cyclic allene (9) (Scheme 4).21... 

The catalytic asymmetric Henry reaction has been reviewed.42 Mild and efficient enantioselective nitroaldol reactions of nitromethane with various aldehydes have been catalysed by chiral copper Schiff-base complexes yielding the corresponding adducts with high yields and good enantiometric excess.43,44... 

The use of asymmetric catalysts in chiral syntheses is taking on increasing importance. Asymmetric ligands or asymmetric metal complexes used in these transformations are quite expensive and need to be efficiently separated from reaction mixtures and recycled. Scheme 16 shows the preparation of a polymer-anchored dibenzophosphole-DIOP platinum-tin catalysts system. The asymmetric ligand places the Pt-SnClj system in a chiral environment. This catalyst has given the highest enantiometric excesses ever observed in catalytic hydroformylation. The initially achieved 70-83% e.e. values were improved to >95% by the use of triethylorthoformate (TEOF) as the solvent. ... 

A proposed mechanism, shown in Scheme 1, involves a six-member cyclic transition state between the aryl ketone and the active form of the catalyst, 2 [6]. The stable catalyst precursor 1 is transformed to the active catalyst, 2, through the loss of HCl. Treatment with 2-propanol forms ruthenium hydride 3 as a single diastereomer. Complexation of an aryl ketone precedes the hydride transfer step, which results in the reduced product. The mild reaction conditions make this catalyst an excellent candidate for incorporation in an imprinted network. The reported enantiometric excesses (ee s, +90%) serve as a useful benchmark to evaluate the influence of the imprinted polymer on the reduction. To the extent that the ruthenium center is situated in an imprinted cavity, the MIP can influence the approach of the ketone to the metal ion or better accommodate a specific reduction product. 

The first enantioselective synthesis of pyrrolyl-substituted triarylmethanes 57 was reported using a novel organocatalyst 58.The scope of the reaction was broad as pyrrole 56 was allowed to react with a variety of differentially substituted aryl—indolyl 55 coupling partners to afford products of the form 57 in good to excellent yields with high enantiometric excess (ee) (14OL1096). 

A number of pyridines can be alkylated by alkenes with a cationic halfsandwich scandium complex in an enantioselective manner (Scheme 48). The alkylation occurs at C-2 with high yields and high enantiometric excess (ee).The pyridines can be substituted with alkyl, aryls, and halides with no effect of the yields or selectivity. The reaction also occurs on isoquinolines. A number of alkenes will react with 2-picoline with high selectivity and yields (14JA12209). 

For an asymmetric version of this reaction which leads to low enantiometric excesses, Tillack, A. Selke, R. Fischer, C Bilda, D. Kortus, K. J. Organomet. Chem. 1996,518, 79. 

In 2012, Itami and Yamaguchi discovered the bisoxazoHne-Pd catalysts such as 173 and 174 that enable the synthesis of hindered heterobiaryls by direct C-H coupling. Moreover, they demonstrated the first enantioselective C-H biaryl cross-coupling (Scheme 17.49) [227]. For example, when an n-PrOH solution of thiophene 175 and arylboronic acid 176 was treated with a Pd(OAc)2/biox 174 catalyst and TEMPO at 70 °C under air, (S)-177 was obtained in 41% ee (63% yield). When a more hindered arylboronic add 178 was used, the enantiometric excess of product (S)-179 was increased to 72% at the expense of lower yield. 

All complications listed earlier lead to the conclusion that computational studies of asymmetric catalytic reactions with low enantiometric excess (ee s) (less than 90% ee at the very least) are not expected to provide any data that could be reasonably rationalized. A low optical yield may just mean that several pathways are competing, and not necessarily those differing only in the handedness of the product. 

D.2. GENERAL FEATURES OF OSMIUM-CATALYZED ASYMMETRIC DIHYDROXYLAnON 371 TABLE 6D.1. Relationship of Ligand Concentration to Enantiometric Excess... 

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