Racemization reactions

The kinetics of racemization of A-[Cr(ox)2(phen)] ion has been investigated in the presence of ions at / = 2.0 mol dm. A rate law of the type rate = ( 1 + k2 [Cu(II)]/) complex was observed where [Cu(II)]/ is the uncomplexed copper(II) concentration. Perchlorate, nitrate, chloride, and sulfate copper(Il) 

Substitution Reactions of Inert Metal Complexes—6 and Above 

The trans-cis isomerization of [Cr(mal)2(H20)2] ion (mal = malonate ) has been investigated in mixed aqueous-organic solvents (MeOH, EtOH, Me2CO, and dioxane). The rates increase with a decrease in the water concentration, but are independent of the organic diluent used. This contrasts with the behavior of the corresponding oxalato complexes, and the rate-determining step is traced to the loss of 

Complexes of the type [Cr(NH3)L] containing the 0-bonded ligands OCHNH2, 0C(NH2)2, and OC(NHMe)2 have been synthesized from the [Cr(NH3)50S02CF3] ion which was discussed previously (Reference 13). Base-catalyzed linkage isomerism of the [Cr(NH3)5L] ions occurs to the deprotonated N-bonded isomers, and without competitive hydrolysis. A 

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Isothermal and dynamic studies [1286] of the thermal deamination and racemization reactions of (+)5 9-[Cr(en)3](NCS)3 gave activation energy values 113 and 100 kJ mole"1, respectively. The mechanisms of these and... 

Some chemical reactions also obey first-order kinetics. Isomerization and racemization reactions are normally first-order. Note that whereas nuclear... 

PreUminary studies on the racemic reaction of protected imines with ni-tromethane showed that the thiourea and the amine mutually weakened their reactivities. However, the bifunctional amino-thiourea led to good results. Enantioselectivity of the adduct depended on the protecting group, P(0)Ph2 affording the best results (76% ee). Then, other aromatic imines substrates were successfully phosphorylated with good to high enantioselec-tivities (63-76% ee). 

For successful DKR two reactions an in situ racemization (krac) and kinetic resolution [k(R) k(S)] must be carefully chosen. The detailed description of all parameters can be found in the literature [26], but in all cases, the racemization reaction must be much faster than the kinetic resolution. It is also important to note that both reactions must proceed under identical conditions. This methodology is highly attractive because the enantiomeric excess of the product is often higher than in the original kinetic resolution. Moreover, the work-up of the reaction is simpler since in an ideal case only the desired enantiomeric product is present in the reaction mixture. This concept is used for preparation of many important classes of organic compounds like natural and nonnatural a-amino acids, a-substituted nitriles and esters, cyanohydrins, 5-alkyl hydantoins, and thiazoUn-5-ones. 

Most racemization reactions are catalyzed by acid and/or by base. A notable exception is the spontaneous racemization of the diuretic and antihypertensive agent, chlorthalidone, which undergoes... 

O Brien, P. (1983). Polyhedron 2, 223. An excellent review of racemization reactions of coordination compounds in the solid state. 

Another detectable example of internal return is afforded by the solvolysis of xo-norborriylbromobenzenesulfonate (XL) it racemizes from 40 to 240% faster than it produces bromobenzenesulfonic acid, yet both reactions are strictly first order. The racemization reaction is kinetically independent of bulk bromobenzenesulfonate concentration but its rate is dependent on the nature of the solvent, pyridine being less effective than alcohol, acetic acid, or aqueous acetone.238 The behavior of 3-phenyl-2-butyl- -toluenesulfonate is similar.250... 

Bada, J. L., Schroeder, R. A., Amino Acid Racemization Reactions and Their Geochemical Implications, Die Naturwessen-schaften, 1975, 62, 71-79. 

Hydrogen transfer reactions are reversible, and recently this has been exploited extensively in racemization reactions in combination with kinetic resolutions of racemic alcohols. This resulted in dynamic kinetic resolutions, kinetic resolutions of 100% yield of the desired enantiopure compound [30]. The kinetic resolution is typically performed with an enzyme that converts one of the enantiomers of the racemic substrate and a hydrogen transfer catalyst that racemizes the remaining substrate (see also Scheme 20.31). Some 80 years after the first reports on transfer hydrogenations, these processes are well established in synthesis and are employed in ever-new fields of chemistry. 

Bada, J.L. and Protsch, R. (1973). Racemization reaction of aspartic acid and its use in dating fossil bones. Proceedings of the National Academy of Sciences of the USA 70 1331-1334. 

In this reaction, an equilibrium is set up. The nucleophile, iodide, is the same as the leaving group. Therefore, inversion of configuration merely converts the (-l-)-isomer into the (—)-isomer. As a result, the optical activity gradually disappears and ultimately becomes zero as the mixture becomes the racemic ( )-form. We are never going to get complete conversion of the (-1-)- into (—)-enantiomer because the reverse reaction will also occur. This is mechanistically identical to the forward reaction, so either (-1-)- or (—)-2-iodobutane as starting material would give racemic product, i.e. it is a racemization reaction. 

In transamination and racemization reactions, we have seen loss of a proton from the aldimine, i.e. breaking of bond a. Let us now consider the two... 

C-14 from the a face. It should be noted that except for the methyl group at C-13, 20 is quite flat it is not unreasonable to assiime that adsorption to the catalyst will take place at the face opposite that substituent, thus leading to the observed stereochemistry. The product is, of course, racemic. Reaction of 20 with lithixun acetylide completes the synthesis of estrazinol (22). It is of note that, in contrast to 15, this compound shows activity as an estrogen. 

In a subsequent paper, the authors developed another type of silica-supported dendritic chiral catalyst that was anticipated to suppress the background racemic reaction caused by the surface silanol groups, and to diminish the multiple interactions between chiral groups at the periphery of the dendrimer 91). The silica-supported chiral dendrimers were synthesized in four steps (1) grafting of an epoxide linker on a silica support, (2) immobilization of the nth generation PAMAM dendrimer, (3) introduction of a long alkyl spacer, and (4) introduction of chiral auxiliaries at the periphery of the dendrimer with (IR, 2R)-( + )-l-phenyl-propene oxide. Two families of dendritic chiral catalysts with different spacer lengths were prepared (nG-104 and nG-105). 

Alibrandi, G., Coppolino, S., D AUberti, S., Ficarra, P., MicaU, N., and Villari, A. (2002), Temperature-rate proUles by polarimetric variable-temperature kinetic experiments to study racemization reactions, I. Pharm. Biomed. Anal., 29,1025-1029. 

Bacteria synthesize D-amino acids from L-amino acids in racemization reactions requiring pyridoxal phosphate. 

Carbonyldiimidazole does certainly stand out as one of the most remarkable reagents for peptide coupling. Carboxylic acids react readily with this reagent to yield acyl imidazoles which in turn couple with the amino group of a peptide to produce a new acyl-nitrogen linkage with very little racemization. Reaction of the acylimidazolide with other nucleophiles can lead to a variety of carbonyl derivatives (62AG(E)35l). 

The best preventive measure against racemization in critical synthetic steps (e.g. fragment condensation, see p. 239) is to use glycine (which is achiral) or proline (no azlactone) as the activated carboxylic acid component. The next best choice is an aliphatic monoamino monocarboxylic acid, especially with large alkyl substituents (valine, leucine). Aromatic amino acids (phenylalanine, tyrosine, tryptophan) and those having electronegative substituents in the /7-position (serine, threonine, cysteine) are, on the other hand, most prone to racemization. Reaction conditions that inhibit azlactone formation and racemization are non-polar solvents, a minimum amount of base, and low temperature. If all precautions are taken, one still has to reckon with an average inversion of 1 % per condensation reaction. This means, for example, that a synthetic hectapeptide contains only 0.99100 x 100% = 37% of the fully correct diastereomer (see p. 233 f.). 

The optical antipodes have been obtained recently by resolution using the (+)D-(ethylenediamine)bis(oxalato)cobalt(III) anion, and have been isolated as the iodide salt.2 The exchange reaction with carbonate and the racemization reaction in aqueous solution have been investigated kinetically.3... 

The hydantoinase process, consisting of a racemization reaction and hydrolyses of the hydantoin and the carbamoylic acid (Figure 7.14), has been enjoying much industrial success (> 1000 tpy) for almost 30 years in the production of D-amino acids such as D-phenylglycine and p-OH-phenylglycine which serve as side chains for /3-lactam antibiotics ampicillin and amoxicilin (Cecere, 1976). 

A non-planar conformation for 3,4-5,6-dibenzophenanthrene derivatives is supported by optical activity studies. The work of Bell and Waring on substituted dibenzophenanthrenes has been discussed in Section III, A, 1. Optically active 9,10-dihydro-3,4-5,6-dibenzophen-anthrene (93) has been prepared by Hall and Turner (1953) the activation energy for the racemization reaction is 31 kcal mole-1 (Hall, 1956). The preparation of optically active 9,10-dihydrodinaphtho(2, 3 -3,4)-... 

Fig. 2. Effect of p-substituents Xin the complexes CsHsMntCO-p-CglLiXHNOlPtp-CglfyXJa 18a-32a on the half-lives r,p of the first-order racemization reaction in toluene at 20 °C (complex concentration 2 mg/ml)...

Racemization reactions due to successive exchange of ligand roles and successive Walden inversions were discussed in Sections 3.1 and 3.2. The decarbonylation reaction seems to be stereospecific, yet, as mentioned in Section 3.3, subsequent to CO elimination there is a racemization of the decarbonylation products. 

Numerous kinetic studies of racemization reactions have been performed with tris(oxalato) metal complexes, 17, M(OX)3 3 in aqueous solution where... 

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