Racemization reactions occurring with

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

If the product and the starting material have opposite configurations, the reaction occurred with inversion. If the product has the same configuration, the reaction went with retention of configuration. If both process occurred to the same extent, the product would be completely racemized (100% racemization) and if the two processes were not equal, partial racemization would occur. 

Scheme 65 shows a reaction occurring with opposite 2,3-syn diaste-reoselection. Hydrogenation of racemic 2-acetyl-4-butanolide, in which the 2-substituent and directing group are linked, proceeds with 98 2 yn anti selectivity to form the diastereomeric hydroxyethyl lactones. 

Carbon-Nitrogen Bonds. Several groups have studied the synthesis of optically active a-amino acids from inexpensive and readily available a-haloesters by displacement with phthalimide in the presence of chiral cinchona catalysts [1 le,24h,24i,47e,60d,77]. Early studies, with chiral, non-racemic starting material, showed that this reaction occurs with partial... 

Amino acid racemases are important for bacteria because they need D-alanine in the biosynthesis of cell walls. These enzymes require pyridoxal as the active cofactor. A racemization reaction starts with the aldimine complex between pyridoxal and an a-amino acid (Scheme 2.4). Deprotonation occurs at the a-carbon of amino acid, due to the electron-sink effect of pyridoxal. Reprotonation of the quinonoid intermediate at the opposite side provides the desired product (pathway a in Scheme 2.4). However, reprotonation may also take place at the C4 of pyridoxal (pathway b in Scheme 2.4). This kills the catalyst because one of its product, pyridoxamine, can no longer racemize an amino acid. 

We have already seen that exchange of hydrogen for deuterium, movement of double bonds into conjugation, and racemization can occur with enols or enolates as intermediates. These are chemical reactions of a sort, but it is time to look at some reactions that make significant changes to the carbonyl compound. 

Radical halogenation reactions occur with racemization at a stereogenic center. 

Problems have been observed when attempting to carry out reactions with either diacid chlorides or half-ester half-acid chlorides when the two carbonyl functions are separated by either two or three carbon atoms. Rearrangement reactions occur with those compounds and so Friedel-Crafts acylation reactions may yield mixtures of products. Optically active methyl 3-methylglutarate was shown to racemize easily Suggested explanations of these effects include the involvement of alkyl diacyloxonium and acyl-oxy-alkoxycarbenium ions. NMR studies have shown that the half methyl ester-half acid chloride from phthalic acid forms the acyloxy-alkoxycarbenium ion very easilyj and that the related ions derived from succinic and glutaric acids can also be generated under stable ion conditions. ... 

In principle, there is a possibility that the product carbanion R (or earbanionoid species) could racemize to extents that vary with conditions (solvent, metal, proton source, temperature). However, for lithium, magnesium, and calcium compounds, analogous reactions occur with com plete retention under die conditions that have been investigated 110.96.15(). following Bochc and Walborsky. we assume dial racenii/alion docs not occur once R or RM (M = Mg. C n. l.i. Na. K) is formed. 

The problem asks whether the reaction occurs with inversion of configuration or racemization. Inversion of configuration is characteristic of (bimolecular nucleo-... 

This is an 8 1 reaction because the leaving group is on a tertiary carbon, the nucleophile (methanol) is weak, and the solvent (methanol) is polar. 8 1 reactions occur with racemization, so both products are formed in this reaction. 

Experimental evidence for this pathway is diverse. For example, the reactions occur with a 2 1 stoichiometry, but the rate law is second order (proportional to [M ] and [RX]). Products are generated with racemization at carbon, and the relative reactivity of alkyl halides follows the trend tertiary > secondary > primary > Me. These relative reactivities parallel the stabilities of R instead of the reactivity as an electrophile in a nucleophilic substitution process. Furthermore the alkyl radical intermediate in this mechanism has been trapped, has undergone rearrangements, and has been detected directly by EPR. 

By a third mechanism, the reaction occurs with loss of stereochemistry at the metal-bound carbon. This stereochemistry is observed if reaction with the electrophile leads to a relatively stable carbocation. For example, the reaction of HgX in Equation 12.18 forms racemic a-methylbenzyl chloride from the optically active a-methylbenzyl complex, presumably due to the stability of the benzylic cation. 

The Hantzsch thiazole synthesis, which occurs between a-haloketones and thiourea or thioamides, generally proceeds smoothly to yield the desired thiazole. Most of the efforts that have been made to improve this reaction have focused on controlling the undesired racemization that occurs with chiral starting materials. One of the most important modifications is the Holzapfel modification, which uses neutral reaction conditions and lower temperatures to eliminate potential epimerization reactions. 

An Sn1 reaction occurs in two steps. Step 1 is a slow, rate-determining ionization of the C—X bond to form a carbocation intermediate, followed in Step 2 by its rapid reaction with a nucleophile to complete the substitution. For S vj1 reactions taking place at a stereocenter, the major reaction occurs with racemization. 

Cyclization of difunctional compounds is illustrated by the acyloin condensation of diesters (Fig. 19), conventionally performed with sodium in refluxing solvents, and improved by the presence of trimethylchlorosilane. A practical improvement was made with the use of technical-grade TMSCl and ultrasonically dispersed sodium.Thus, the reaction occurs at 0°C in 0.5 to 3 h. An experimental description is given in Ch. 9, p. 331. A chiral center at the a-position of the carbonyl does not suffer racemization. With p-halo esters, cyclizations lead to cyclopropyl derivatives in high yields, except with sterically hindered substrates. A similar reaction occurs with zinc and oxazabutadienes substituted by trifluoromethyl groups, with a fluoride ion as the leaving group (Eq. 15).ii ... 

The use of chiral substrates has allowed the importance of solvent and nucleophile-associated cation to be probed. For example, as shown in Scheme 7.10, when chiral 1-chloro-l-phenylethane is heated in ethanoic acid (acetic acid,CH3C02H]) at 50°C in the presence of potassium ethanoate (potassium acetate, CH3C02 K+), the ethanoic acid (acetic acid, CH3CO2H) ester of 1-phenylethanol is obtained. In this weakly nucleophilic system, about 15% excess inverted product is found (i.e., 85% of the product is racemic and the substitution reaction occurred with about 57.5% inversion and 42.5% retention). Presumably, the carbocation formed first and then... 

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