Reversible isomerization

Recently, Nesmeyanov and co-workers have published definitive evidence that dwaZ reactivity (the formation of derivatives of two different structural formulas) extends beyond tautomerism (isomers in equilibrium or reversible isomeric transformation). A single molecular species can form two series of derivatives, in one of which a transfer of the reaction center occurs in the reaction. 

At elevated temperatures, some 5-amino-l,2,3-triazoles can undergo reversible isomerization of the type 204 205 (for a review see reference 132). 

Interestingly, a reverse isomerization (1,4-dihydro 1,2-dihydro) was observed when a 1,4-dihydroquinoline derivative was obtained by the reaction of lithiated l-ethoxycarbonyl-l,2-dihydroquinoline-2-phosphonate 36 with benzyl bromide... 

Irradiation with UV light isomerized the azobenzene units from the trans to the cis form, while the reverse isomerization occurred thermally in the dark. The cis to trans conversion is catalyzed by both protons and hydroxyl ions. Hence, the catalyzed dark process for tethered azobenzene is greatly modified in comparison with that for free azobenzene. For the tethered azobenzene, beginning at pH 6, the cis to trans return rate sharply decreased with increasing pH up to 10, whereas the rate for free azobenzene rapidly increased in the same pH range owing to OH- catalysis. These observations can be explained by the electrostatic repulsion which lowers the local OH concentration on the polyion surface below that in the bulk aqueous phase. 

Isomerization and elimination reactions of alkyls and aryls Isomerizations of mono-alkyls and aryls have been widely studied [107] many ds-Pt(PR3)2ArCl undergo rapid isomerization in the presence of free phosphine, a reaction inhibited by Cl- with a mechanism believed to involve a 3-coordinate Pt(PR3)2Ar+ intermediate that is then attacked by Cl-. The cis- and trans-isomers of Pt(PEt3)2(Ph)Cl undergo reversible isomerization when irradiated at the wavelength of charge-transfer transitions (254 and 280 nm). 

This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silica-alumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/Al203 catalyst, including adsorption-desorption steps. 

The form of Equation (6.7) reveals an interesting aspect of slow binding inhibiton due to enzyme isomerization. A slow forward isomerization rate is insufficient to result in slow binding behavior. The reverse isomerization rate must also be slow, and in fact must be significantly slower that the forward isomerization rate. If this were not the case, there would be no accumulation of the E I conformation at equilibrium. As the value of k6 becomes k5, the denominator of Equation (6.7) approaches unity. Hence the value of Kf approaches Kit and one therefore does not observe any time-dependent behavior. 

This bi-exponential behavior confirms the presence of reversible isomerization steps coupled with irreversible degradation steps and accounts for the role of the di-cis isomers as reaction intermediates, according to the general reaction scheme presented in Figure 12.1. The dependence of the rate constant of each elementary step on temperature allowed the calculation of the respective activation... 

Consider a CSTR that is used to carry out a reversible isomerization reaction of the type... 

Ribulose 5-phosphate is capable of a reversible isomerization to other pentose phosphates-xylulose 5-phosphate and ribose 5-phosphate. These reactions are catalyzed by two respective enzymes, viz., pentose-phosphate epimerase and pentose-phosphate isomerase, according to the scheme below ... 

The X-ray diffraction data for this compound are presented in Section 3. Betaines containing a hydrogen atom in the a-position to the phosphonium center and capable of reversible isomerization to silylated ylides are alkylated by ethyl bromide in a different ways. This reaction resulting in a complex mixture of products is considered below in Section 5.4. 

Betaines 13 containing hydrogen atoms in the a-position to the triphenyl-phosphonium center are reversibly isomerized in a solution to form phosphoranylidenealkane dithiocarboxylic acids 68 or their salts 69 (Scheme 30).48 49... 

Similarly, oxaziridines (244) are not likely to undergo reverse isomerization to the initial nitrones (225) (Scheme 2.85) (453). 

For the exothermic, liquid-phase, reversible isomerization reaction, A B, taking place in... 

The internal hydrogen bond (N—H—N) is responsible for suppression of cis-trans isomerization in the singlet excited state of cA-l-(2-indolyl)-2-(2-pyridyl)ethene (32) of reaction 13. Clearly, the reverse isomerization is possible125. On the contrary, irradiation of c -l-(2-pyrrolyl)-2-(2-quinolyl)ethene126 (35) induces the isomerization to 34. The isomerization (cis-trans) is possible because the excitation allows the tautomerization of 35 to 36 (equation 14). 

The hydrolysis of 4-pivaloyl-L-dopa is accompanied by rapid and reversible isomerization to 3-pivaloyl-L-dopa, a reaction of O-acyl migration that... 

Chesick (1963) has studied the thermal reversible isomerization of ethyl-idenecyclopropane in the gas phase... 

As already indicated in the preceding schemes, the resonance-stabilized benzyl ion, [CvHv]", initially formed by benzylic bond cleavage reversibly isomerizes to the tropylium and tolyl ion isomers. The isomerization of [CvHv]" ions has been the subject of numerous studies, [50] revealing the tropylium ion as the thermodynamically most stable isomer. [51,52]... 

The function of piperidine was to neutralize the relatively strong acidic sites of the alumina and still leave the weak acidic sites to act catalytically. In the presence of acidic alumina a reversible isomerization of camphene to tricyclene occurs. 

The saccharinic acids formed from hexoses have been especially examined because of the relationships of the a and /8 isomers (C-2 epi-mers). Structures of saccharinic acids derived from D-glucose are glu-cometasaccharinic acid (51), glucoisosaccharinic acid (52), and glucosaccharinic acid (53). The a- and /3- isomers of metasaccharinic acid can reversibly isomerize when exposed to base because of the labile proton at C-2. 

Very early in the history of heteroeyelic chemistry it was diseovered by Dimroth (09LA183) that l-aryl-5-amino-l,2,3-triazole (1, R = H) undergoes a faeile, reversible isomerization to 5-(arylamino)-l,2,3-triazole (2, R = H) (Scheme IV.4). 

As both the first and the last step are reversible, isomerization of substrates and products can occur. The regio- and stereochemistry can thus be difficult to assess. This is especially true for methylated spiropentanes 2,3 2,5 and methylated spiropentylcarboxylic acid esters.216 More clearcut examples are collected in Table 18. 

The reverse isomerization from the Rfi,S,S to the R,Sfi,S configuration is also possible. In this process, a strongly coordinating solvent (100c) or pendent donor group induces the configurational change of the macrocycle (41). 

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