Chemical preceding

When the current is anodic, component Red is consumed and the equilibrium in the electrolyte close to the surface is disturbed reaction (13.37) will start to proceed from left to right, producing additional amounts of species Red. In this case the chemical precedes the electrochemical reaction. However, when the current is cathodic, substance Red is produced and the chemical reaction (13.37), now as a subsequent reaction, will occur from right to left. When component Ox rather than component Red is involved in the chemical reaction, this reaction will be the preceding reaction for cathodic currents, but otherwise all the results to be reported below remain valid. 

ANSWER There is no chemical precedent for that kind of transformation. 

This lesson is brought home in the following example taken from a recent experiment. Ample chemical precedent suggested that the treatment of E with methyl amine should give F ... 

The sequence of reactions by which dinitrogen is reduced on a metal is probably that outlined in Scheme 25. This sequence is based on the accumulated data of the chemical systems and is similar to that of Scheme 1, with the important distinction that there is chemical precedence for all the steps shown and others in addition. It seems likely that the mechanism of function of nitrogenase will possess the general features of Schemes 23, 24, and 25 outlined in this article. 

Sirovatka, J. M., and Finke, R. G., 1997, Coenzyme B,2 chemical precedent studies Probing the role of the imidazole base-on motif found in B12-dependent methylmalonyl-CoA mutase, J. Am. Chem. Soc. 119 305703067. 

Finke RG. Coenzyme Bi2-based chemical precedent for Co-C bond homolysis and other key elementary steps. In Vitamin B12 and Bi2 Proteins. Krautler B, Arigoni D, Golding BT, eds. 1998. Wiley VCH, Weinheim, Germany. 

Bidentate ligands containing OS donors, particularly in conjunction with carboxylate moieties, form strong complexes at pH values near the p/y, value for the thiol.584,602,687-689 The reduction of Viv to V111 by cysteine methyl ester in water has been found to be enhanced in the presence of amino polycarboxylates.690 The ligand 2-mercapto-4-methylphenol forms a square-pyramidal five-coordinate complex with the vanadyl cation (150) the reported crystal structure is of the cis isomer.691 The bidentate ligand 2-aminocyclopent-l-ene-l-carbodithioic acid forms non-oxo-Viv complexes based on spectroscopic studies and chemical precedence, an eight-coordinate vanadium atom is inferred.692... 

Chemical precedent for the nonenzymic elimination of base and inorganic pyrophosphate may be found in the attempted synthesis of 3 -ketothymidine by oxidation of TMP (54), which led instead to the formation of Pj, thymine, and unidentified sugars. Furthermore, Hershfield reported that 2 -deoxyadenosine inactivated S-adenosylhomocysteine hydrolase and that this inactivation was accompanied by adenine release (55). 

A working hypothesis for this inactivation mechanism has been proposed (63) and is presented in Scheme 11 (compounds 16-20). Consistent with the mechanisms of substrate and inhibitor utilization by RDPR, the initial step is proposed to be abstraction of the 3 -hydrogen by the tyrosyl radical X to generate a 3 -nucleotide radical, 16. There are no known good chemical precedents for decomposition of )3-azidoalkyl radicals such as 16. The best analogy is an inter-molecular reaction of 1-hydroxy-1-methylene radical with primary alkyl azides... 

This persuasive evidence for homolytic chemistry occurring in the PFL reaction has led Kozarich and co-workers to propose a working hypothesis for the overall enzymic reaction (787). A chemical precedent exists in the H202/Fe(II)-mediated homolytic cleavage of ethyl pyruvate to ethyl formate radical and acetate. This is the Minisci reaction, which has been employed in the selective carboxylation of heteroaromatic bases (Scheme 40) (796-798). The initial step in this process is the formation of a hydroperoxy hemiketal with ethyl pyruvate and H2O2, followed by Fe(II) reduction of the hemiketal to the alkoxy radical. 

One of the keys to defining an enzyme catalytic reaction pathway is the identification of enzyme reaction intermediates. The criteria (Scheme 1) to establish an enzymatic reaction pathway with a postulated intermediate may be defined by addressing the following questions (1) Can the intermediate be isolated and its structure determined directly or if it is unstable can analysis of breakdown products support the postulated structure. (2) Is the chemical rationale of the reaction intermediate based upon chemical precedent and reasonable thermodynamics. and (3) Is the intermediate kinetically competent , in other words, is it formed and broken down at the enzyme active site on a timescale that is consistent with the disappearance of substrate and the formation of product. ... 

Pulsed radiolysis studies indicate that the reaction proceeds via two sequential steps, both first order in 02 , in which O2 is alternately oxidized to O2 or reduced to H2O2 by the Fe ion. Thus, the Fe cycles between the III and II states. Fe SOD takes up a proton upon reduction throughout the pH range of activity, and chemical precedent as well as NMR studies (Miller et indicate that the proton is taken up by coordinated OH in the oxidized state to... 

Finke has established a chemical precedent for the proposed mechanism for thiyl radical formation in the Bi2-dependent ribonucleotide reductase. Thermolysis of AdoCbl with excess /3-mercaptoethanol under anaerobic conditions yielded 90% Co—C homolysis and 10% heterolysis, as determined by product characterization. The homolysis products were 5 -deoxyadenosine, cob(II)alamin, and the disulfide 2,2 -dithiodiethanol. Kinetic studies established a zero-order dependence on thiol at high [RSH], consistent with rate-limiting Co—C homolysis and formation of a discrete Ado- that subsequently abstracts an H atom from the thiol. Consequently, the... 

In an alternative scheme (Fkure 6B), the species Cu -NO+ undergoes linkage isomerization to form a Cu" -A-NO species. By analogy with reactions of model systems this would facilitate catalysis via the putative Cu -NO intermediate and NO formation would occur by the decomposition of the unstable Cu -NO species formed by internal electron transfer. A proposal involving a Cu -ON species appears less likely since it is without chemical precedent. Further studies are required to resolve the mechanism, and the controversial issue of the activity of T2DNiR. 

Chemical precedent (Walsh, 1979 Jencks, 1987) Behavior of model chemistry in solution for the reaction of interest provides a transition state benchmark for comparison with the transition state structure imposed by the enzyme. This information leads to a design to trap transition state or to synthesize transition state inhibitors. 

This postulate is based on numerous chemical precedents, among which is the following which shows the electrophilic character of the oxaziridine system ... 

It is not surprising that there seems to be no appropriate chemical precedent for the a n -Markovnikov cyclization to a seven-membered ring. On the contrary, the ring contraction of secondary carbonium ion (64) to the tertiary bisabolyl ion should be very rapid. However, the Prins-like conversion of the cycloheptenone (66) to the carotol ether (67) in the presence of stannic chloride mimics, to some degree, the second cyclization in the biogenesis of carotol 119). 

---