Optical induction

Another example of a reaction leading to exclusive formation of one metal configuration is (+)-bis(7r-pinenyl)nickel (163), in which the Ni atom is bonded to two 7r-allyl systems. The combination of one 7r-pinenyl system with a Ni atom is a chiral entity, in which the metal is an asymmetric center. The opposite configuration at Ni would result should the 

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ICP-Optical Inductively Coupled Plasma Optical Emission... 

Optical induction in organo-transition metal compounds and asymmetric catalysis. H. Brunner, Acc. Chem. Res., 1979,12, 250-257 (85). 

The modified Sharpless reagent was also successfully applied288 for the asymmetric oxidation of a series of 1,3-dithiolanes 248 to their S-monooxides 249 (equation 134). It was observed that the optical induction on sulphur (e.e. from 68 to 83%) is not significantly affected by the substituents R1 and R2. Asymmetric oxidation of a few aryl methyl sulphides by organic hydroperoxides in the presence of a catalytic amount of the optically active Schiff base-oxovanadium(IV) complexes gave the corresponding sulphoxides with e.e. lower than 40%289. 

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

Finally, with the aim of discovering novel chiral oxomolybdenum catalysts able to perform enantioselective alkene epoxidations, Kuhn et al. have reported the exploration of the catalytic behaviour of a series of dioxomolybdenum(VI) complexes with chiral cw-8-phenylthiomenthol ligands derived from ( + )-pulegone. Therefore, the epoxidation of c -p-methylstyrene using t-butyl-hydroperoxide as the oxidant and performed in the presence of ( + )-(2i ,5i )-2-[1-methyl-l-(phenylthio)ethyl]-5-methylcyclohexanone oxime as the ligand, did not produce, however, a significant optical induction in these conditions. 

Enantioselective carbenoid cyclopropanation can be expected to occur when either an olefin bearing a chiral substituent, or such a diazo compound or a chiral catalyst is present. Only the latter alternative has been widely applied in practice. All efficient chiral catalysts which are known at present are copper or cobalt(II) chelates, whereas palladium complexes 86) proved to be uneflective. The carbenoid reactions between alkyl diazoacetates and styrene or 1,1 -diphenylethylene (Scheme 27) are usually chosen to test the efficiency of a chiral catalyst. As will be seen in the following, the extent to which optical induction is brought about by enantioselection either at a prochiral olefin or at a prochiral carbenoid center, varies widely with the chiral catalyst used. 

Even when the trifluoroaeetyl-(+)-camphor ligand is linked to a solid support (Hypersil silica 205d), if retains its activity both in terms of yield and optical induction. 

It has already been mentioned that prochirality of the olefin is not necessary for successful enantioselective cyclopropanation with an alkyl diazoacetate in the presence of catalysts 207. What happens if a prochiral olefin and a non-prochiral diazo compound are combined Only one result provides an answer to date The cyclopropane derived from styrene and dicyanodiazomethane shows only very low optical induction (4.6 % e.e. of the (25) enantiomer, catalyst 207a) 9S). Thus, it can be concluded that with the cobalt chelate catalysts 207, enantioface selectivity at the olefin is generally unimportant and that a prochiral diazo compound is needed for efficient optical induction. As the results with chiral copper 1,3-diketonates 205 and 2-diazodi-medone show, such a statement can not be generalized, of course. 

Most remarkably, the homoallylic halides 214 not only yield the thermodynamically unfavored ris-cyclopropanes 215 preferentially (see Sect. 2.2.3), but also give rise to enantioselective formation of the (1/ ) configuration, in contrast to the cyclopropanation of 1,3-butadienes with the same catalysts (see Table 15). Only in the case of olefin 214 (X = CF3, Y = Cl), may the (1 S)-trans isomer be obtained enantioselecti-vely, depending on the catalyst (Table 16, entries 8-11). In these few cases, optical induction occurs at C(3) of the cyclopropane rather than at C(l). 

Optical induction emanating from a chiral diazoacetamide is apparently not much higher. The 2-phenylcyclopropanecarboxylates cis-222 and trans-222, obtained in low yield from (N-diazoacetyl)oxazolidones 220,221 and styrene in the presence of Rh2(OAe)4 followed by ethanolysis, showed only small enantiomeric excesses 215). Starting with either diazo compound, the (1/ ) enantiomer was predominant in both cis- and trans-222. 

Use of optically active amino acids with Co(CN)53 gave negligible optical induction (310). 

A rhodium complex (NBD)Rh(PPh3)(DIOS)+ was active via H2Rh(PPh3)(DIOS)+, but there was no optical induction with unsaturated acid substrates, which possibly displace the chiral ligand (275). The DIOS-type ligands have not yet been resolved at the sulfur. The extent... 

Chiral catalysts remain primary targets for immobilization by using similar methods. Since the steric arrangement of bulky aromatic groups of chiral ligands is the primary source of optical induction, most approaches use the chelate backbone of ligands for functionalization in order to minimize interference with the chelate (aryl) conformation. 

E. V. Dehmlow, P. Singh, J. Heider, A Cautionary Note on Optical Inductions by Chiral P-Hydroxy-Ammonium Catalysts , J. Chem. Res. (S) 1981,292-293. 

The asymmetric reduction of C=N double bonds in prochiral oximes afforded a maximum of 18% ee [380, 384, 385]. Prochiral azomethines were reduced to the corresponding 1,2-diamines and secondary amines using 36 optically active supporting electrolytes. The dimers were optically inactive, while the monomers showed low optical inductions (<11% ee). The effect of electrolyte, substrate concentration, temperature, pH, and cathode potential on the induction was studied. It was proposed that the enantioselectivity... 

Protein ALBP-PX was the first pyridoxamine-conjugated protein to be synthesized and structurally characterized. Under single-turnover conditions, this protein demonstrated amino acid production rates of only 56% of the free cofactor. However, depending on the nature of the a-keto acid, ALBP-PX did show a range of optical inductions for the amino acid product. Notably, enantiomeric excesses in the order of 94% were observed for the production of valine. Additionally, several trends were noted. All amino acid products that showed optical induction favored the 1-enantiomer, except alanine, which favored the d-enantiomer. Furthermore, a-keto acids with branched side chains... 

To explain the observed optical induction, a substrate was incorporated into the molecular model of the protein. A substrate such as a-ketoglutarate could be included in the protein model with a geometry that allowed stereoselective protonation of the quinoid intermediate by solvent, consistent with the enantiomeric excess (ee) of the 1-stereoisomer product. Moreover, the geometry consistent with production of the d-enantiomer appeared too sterically crowded for most substrates. However, pyruvic acid, which was the only substrate to favor the d-enantiomer product, was small enough to adopt the alternative geometry and also had the potential to interact with an arginine group. 

The ability of peptides CBPOl-GBP 18 to modulate pyridoxamine-mediated transamination was determined by the conversion of pyruvic acid to alanine in both the absence and presence of copper(II) ion, which would be coordinated by the transamination intermediates [32]. In the absence of copper(II) ion,peptide CBP13 showed up to a 5.6-fold increase in alanine production relative to a pyridoxamine model compound and peptide CBP14 produced alanine with a 27% ee of the 1-enantiomer. In the presence of copper(II) ion, peptide CBP13 again showed the greatest increase in product production, with a 31.7-fold increase in alanine production relative to the pyridoxamine model compound. Peptide CBPIO showed optical induction for D-alanine with a 37% ee. 

Peptides CBPOl-GBP 18 also showed interesting trends in optical induction as a function of both reaction conditions and the identity of residues at positions 5 and 7. In the absence of copper(II) ion, L-alanine was the favored stereoisomer product for peptide CBP01-CBP18, while formation of D-alanine was favored in the presence of copper(II) ion. This suggests that the jSjSa-structure of the pep-... 

The earliest study is from 1995, when the rhodium complex of a menthyl-substituted phosphine (22) was used for the hydroformylation of styrene [99]. Although the catalytic activity was quite good (TOP up to 245 h ), regioselectivity was low (b/1 = 1.0 - 2.5) and no optical induction was observed in 2-phenylpropanal. 

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