Reversible interconversion

It has also been argued10,40 that the second mechanism (rapid, reversible interconversion of II and IV) cannot be general. The basis for this contention is the fact that electrophilic catalysis is rare in nucleophilic aromatic substitution of non-heterocyclic substrates, an exception being the 2000-fold acceleration by thorium ion of the rate of reaction of 2,4-dinitrofluorobenzene with thiocyanate... 

Opposing reactions. The reversible interconversion of d.y-[Cr(cn)2(OH)2 + to trans is characterized by an equilibrium constant of 0.16 and a forward rate constant of 3.30 X 10 4 s"1.In an experiment starting with the pure cis isomer, how long does it take to form half the equilibrium concentration of trans Starting with pure trans, how long does it take for half the equilibrium concentration of trans to form ... 

Since its discovery two decades ago, the reversible interconversion of allylic sulfenates to sulfoxides has become one of the best known [2,3]-sigmatropic rearrangements. Certainly this is not only because of the considerable mechanistic and stereochemical interest involved, but also because of its remarkable synthetic utility as a key reaction in the stereospecific total synthesis of a variety of natural products such as steroids, prostaglandins, leukotrienes, etc. 

In some cases, the two stereoisomers can interconvert. In cis- and trans-disubstituted cyclopropanones, for example, there is reversible interconversion that favors the more stable trans isomer. This fluxional isomerization occurs via ring opening to an unseen oxyallyl valence bond isomer. ... 

A. Phosphoenolpyruvate.—The mechanisms of hydrolysis of phosphate esters of phosphoenol pyruvic acid (33) have been described in detail, and 0 studies confirm an earlier postulate that attack by water on the cyclic acyl phosphate (34) occurs at phosphorus and not at carbon. In the enolase reaction, the reversible interconversion of 2-phosphoglyceric acid(35)... 

By proper selection of the experimental conditions a reversible interconversion between the two crystal forms (Ej and E2) can be observed [119], that may be related to the structural transitions between the two major conformations of the Ca " -ATPase. 

Glucose phosphate isomerase (GPI) catalyzes the reversible interconversion of glucose-6-phosphate and fructose-6-phosphate. GPI plays an essential role in carbohydrate metabolism in all cells of the body. The substrates of this enzyme, ffuc-... 

Tautomerism, strictly defined, could be used to describe the reversible interconversion of isomers, in all cases and under all conditions. In practice, the term has increasingly been restricted to isomers that are fairly readily interconvertible, and that differ from each other only (a) in electron distribution, and (b) in the position of a relatively mobile atom or group. The mobile atom is, in the great majority of examples, hydrogen, and the phenomenon is then referred to as prototropy. Familiar examples are / -ketoesters, e.g. ethyl 2-ketobutano-ate (ethyl acetoacetate, 23), and aliphatic nitro compounds, e.g. nitro-methane (24) ... 

A key enzyme in the process of recycling lactate from muscle to liver is lactate dehydrogenase (LD) which catalyses the reversible interconversion of lactate and pyruvate. This important reaction is shown in Figure 7.10. 

Carbonate anhydrase (carbonic anhydrase, EC 4.2.1.1) catalyzes the reversible interconversion of C02 and HCO3 (see Sect. 3.7.3). The enzyme is found in erythrocytes, and in kidney and gastric juices where it contributes to the control of the acid-base balance. The esterase activity of carbonic anhydrase is probably due to the similarity between its active site and that of the zinc proteases. A possible physiological role of the esterase activity of this enzyme remains to be established. 

The ring complex shown would account for the low A factor, and further explains the formation of only cis-diene. This 1,5-hydrogen shift mechanism has also been postulated to account for the thermal isomerization of a number of 1,3-dienes (Wolinsky et al. 1962). This type of isomerization has been studied in detail as applied to the reversible interconversion of cis-2-methylpenta-l,3-diene and 4-methylpenta-l,3-diene,... 

Fig. 6.4 Reversible interconversion of amino acid and keto acid. Conjugation of the imine bond in the aldimine with the electron sink of the pyridine ring plus protonation of the pyridine nitrogen as well as the metal ion - all this results in weakening of the C-H bond of the amino acid residue. Thus, also catalyzed is a-proton exchange, racemization of a chiral center at the a-carbon atom and decarboxylation of the appropriate amino acid. ...

Whereas racemization is the complete loss of optical activity with time, epimerization is the reversible interconversion of diastereoisomers to an equilibrium mixture which is not necessarily optically inactive. Diastereoisomers arise from the combination of the two chiral centers in 9, namely the metal centered, R and S, and the resolved (S) optically active ligand center. The diastereoisomers (RS) and (SS) differ in their properties. 

This enzyme [EC 5.1.1.15], also referred to as 2-amino-hexano-6-lactam racemase, catalyzes the reversible interconversion of the L- and D-stereoisomers of 2-amino-hexano-6-lactam. The enzyme, which utilizes pyridoxal phosphate, will also catalyze the interconversion of 2-aminopentano-5-lactam and 2-amino-3-mercaptohex-ano-6-lactam. The enzyme exhibits a minor aminotransferase activity with certain a-amino acids. 

Figure 4-6. Mechanism of facilitated diffusion mediated by a glucose transporter. This is an example of uniport. The reversible interconversion between conformations of the transporter in which the glucosebinding site is alternately exposed to the exterior and interior of the cell is called a ping-pong mechanism.

I do want to indicate that in considering the kinetics, although a high concentration of the diaquo species is not formed, their formation is presumably the path which leads to what looks like a first-order isomerization reaction. In other words, there is a reversible interconversion of the cis and trans isomers which has to be expressed kinetically. 

Wright and Brandner84 have shown the reversible interconversion of aqueous solutions of these three dianhydrides in the presence of a nickel-on-kieselguhr catalyst under hydrogenating conditions. At the steady state, the product contains 57% of l,4 3,6-dianhydro-L-... 

It is interesting to note that the CD spectrum of 11 can be tuned by changing the oxidation states of the TTF units in the side-chains of 11. Polymer 11 shows reversible interconversion between three univalent and two very broad mixed-valence redox states that have different chiroptical properties. 

Scheme 15.4 The reversible interconversion between the ring-open and ring-closed forms of...

The additional reaction to be included with the above subscheme is the reversible interconversion of A to a third intermediate C ... 

We start with a simple reversible redox reaction for which we can directly measure the free energy of reaction, Ar<7, with a galvanic cell. This example helps us introduce the concept of using (standard) reduction potentials for evaluating the energetics (i.e., the free energies) of redox processes. Let us consider the reversible interconversion of 1,4-benzoquinone (BQ) and hydroquinone (HQ) (reaction 14-5 in Table 14.1). We perform this reaction at the surface of an inert electrode (e.g.,... 

Many reactions that are promoted by enzymes can also be catalyzed by acids or bases or by both. An example is mutarotation, the reversible interconversion of the a- and (i-anomeric forms of sugars (Eqs. 4-1 and 9-86). This reaction is catalyzed by a specific mutarotase and also by inorganic acids and bases. 

The thermal rearrangement of vinylcyclopropane to cyclopentene was uncovered in I96090 91. That vinylcyclopropanes, like other cyclopropanes, may undergo cis, trans iso-merizations was inferred in 1964 when trans-l-vinyl-2-methylcyclopropane was thermally converted to mostly (4Z)-1,4-hexadiene, a product formed at much lower temperatures from cw-1-vinyl-2-methylcyclopropane92. The reversible interconversion of the cis and trans isomers of l-vinyl-2-d-cyclopropane (equation 2) was reported soon thereafter, in 196793"96. Additional examples, including cases showing both geometrical isomerization and enantiomerization processes, soon followed. 

In homogeneous systems, a- and > 3-alkylallyl complexes, as well as alkylallyl carbanions, have been prepared. Furthermore, a reversible interconversion of the /3-allyl complex to the corresponding olefin complex was demonstrated by using NMR spectroscopy. Equation (9) shows an equilibration between the /3-allylhydridotrifluorophosphinenickel complex and n-bonded propene complex (20). Brennemann affirmed also that the >/3-allyldeuterio nickel complex changes exclusively to propene-l-d, or propene-3-rf, by the reversible operation of Eq. (9). 

Measurement of Reversible Interconversion. A process that is not often observed in matrices and glasses is the reversible interconversion of species that differ only in their positions or orientations. Because of the high resolution of the IR spectra of C02, we have been able to document several cases of reversible interconversion of intermediates. While some of these have been observed by EPR, spectral complexity precluded its use in observing others. 

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