Racemization absence

The enzyme is a single enantiomer of a chiral molecule and binds the coenzyme and substrate m such a way that hydride is transferred exclusively to the face of the carbonyl group that leads to (5) (+) lactic acid Reduction of pyruvic acid m the absence of an enzyme however say with sodium borohydride also gives lactic acid but as a racemic mixture containing equal quantities of the R and S enantiomers... 

C. By Oxidation.—This year s literature has been notable for attempts to study the details of certain phosphine oxidation reactions. In one such investigation nitric acid was found to oxidize phosphines, or phosphine sulphides, to phosphine oxides with inversion of configuration at phosphorus, whereas dinitrogen tetroxide, in the absence of acid, was found to oxidize the same compounds with predominant retention. The partial racemization observed with the latter reagent was probably due to the racemization of the oxides, since methylphenyl-n-propylphosphine oxide... 

When a reverse procedure was applied, i.e. enzymatic acetylation of racemic 3, formed in situ from the appropriate aldehydes and thiols, the reaction proceeded under the conditions of dynamic kinetic resolution and gave enantiomerically enriched acetates 2 with 65-90% yields and with ees up to 95% (Equation 2). It must be mentioned that the addition of silica proved crucial, as in its absence no racemization of the initially formed substrates 3 occurred and the reaction stopped at the 50% conversion. 

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.24 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. For these halides, the overall mechanism probably consists of two steps an oxidative addition to the metal, after which the oxidation state of the copper is +3, followed by combination of two of the groups from the copper. This process, which is very common for transition metal intermediates, is called reductive elimination. The [R 2Cu] species is linear and the oxidative addition takes place perpendicular to this moiety, generating a T-shaped structure. The reductive elimination occurs between adjacent R and R groups, accounting for the absence of R — R coupling product. 

The optical rotation of methylneophylphenyltin hydride (72) remaim unchanged if it is kept neat at —30 °C for 14 months or in benzene at 80 °C for 2 hours in the presence of hydroquinone, whereas a benzenic solution of (72) is fully racemized after 30 minutes at 80 °C in the presence of AIBN 50% racemization is observed when the same solution without AIBN is kept for 17 days at room temperature. The optical rotation of a benzenic solution of (72) remains unchanged for weeks in the presence of hydroquinone at room temperature (72) racemizes very slowly in ether or in benzene in the absence of hydroquinone and somewhat faster in benzene in the presence of AIBN at room temperature 44). 

In the presence of nucleophilic solvents, a racemization is observed too in less than one hour, in the presence or in the absence of hydroquinone, compound (12) is fully racemized in methanol44. A mechanism analogous to that proposed to rationalize the optical instability of triorganotin halides can be given here 44). 

The protodemetallation of optically active methylneophylphenyltrityltin, (—)-(<57), in diethylether yields optically inactive methyineophyltrityltin chloride (5)79). However, (5) is configurationally stable even in the presence of DMSO in pyridine (cf. Table 1) and methylneophylphenyltin chloride (12), which is already configurationally unstable in the presence of traces of pyridine (cf. Table 1) racemizes in about 10 minutes in CC14 in the absence of pyridine... 

However, when (+)-methylneophylphenyltin deuteride, (+)-(56) ([ot] s + 10.7) is kept in the dark mixed with five equivalents of diethylaluminum hydride for ten hours at room temperature in benzene, optically inactive (72) is formed 44). (In the absence of (Et2AlH)2 less than 3 % of (12) is racemized under these conditions). The four-center transition state is therefore very unlikely. 

On the other hand, it is found that only partial racemization occurs on alkaline hydrolysis of optical active 198 in aqueous methanol136) and no racemization takes place in the hydrolysis of 199 in dioxane/water137). Moreover, the latter reaction is only ca. 80 times faster at 29 °C than that of the analogous morpholide 200, for which a metaphosphorimidate mechanism is precluded a priori by the absence of an NH function and whose hydrolysis is likewise stereospecific,37). Clearly a free metaphosphorimidothioate of type 191 cannot be involved in this case. The experimental findings are compatible, however, with the hypothesis that the nucleophile water attacks a metaphosphorothioimidate/phenolate associate 201. The question of how free metaphosphates occur in solution is of a general nature it has also been considered in the previous Section. 

In cases that chiral products are formed from achiral reactants, racemic mixtures of products will be produced in the absence of chiral influence (reagent, catalyst, or solvent). 

Ag(carbene)2][AgCl2] (carbene = l,3-dimesitylimidazol-2-ylidene) Was also reported for use as a pre-catalyst for the polymerization of L-lactide with low PDI and in the absence of monomer racemization.69... 

The phenylcarbamate derivative 23j also showed chiral recognition for many racemates in CDCI3 (Figure 3.38). Figure 3.39 shows the 1H NMR spectra of ( )-2-butanol in the absence and presence of 23j. In the case of secondary alcohols, such as 2-butanol, 2-heptanol, and 2-octanol, the methyl protons at the end of the longer chain and remote from the stereogenic center were enantiomerically separated in the presence of 23j, and the methine and... 

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