Suggested Mechanisms

Mixtures of chlorine and hydrogen reaa only slowly in the dark but the reaction proceeds with explosive violence in light. A suggested mechanism for the photochemical chain reaction is ... 

Possible role of the induced acidity and basicity in catalysis and environmental chemistry is discussed. The suggested mechanism explains the earlier reported promotive effect of some gases in the reactions catalyzed by Bronsted acid sites. Interaction between the weakly adsorbed air pollutants could lead to the enhancement of their uptake by aerosol particles as compared with separate adsoi ption, thus favoring air purification. 

Figure 5 A suggested mechanism for the enolization of acetyl-CoA by the enzyme citrate synthase (CS). The keto, enolate, and enol forms of the substrate are shown.

Suggest mechanisms for the following reactions. Classify the orbital symmetry-controlled process as clearly as you can with respect to type. 

Irradiation of l,l,3,3-tetraphenyl-5-methyl-l,4-hexadiene gives the two products shown below. When the reaction is carried out by photosensitization, B is not formed. Suggest mechanisms for the formation of A and B. What other products might have been expected Can you rationalize their absence ... 

Interesting results were also obtained on treatment of 2-amino-4,6,6-trimethyl-dihydropyrimidine 50 and 2,4,6,6-tetramethyldihydropyrimidine 51 with CD3OD in the absence of a base (91TL2057). It was shown that, under these conditions, the 4-methyl protons of 50, the 2,4-dimethyl protons of 51, and H(5) in 50 and 51 undergo H-D exchange. The suggested mechanism involves annular (1,4-dihydro 4,5-dihydro) as well as substituent tautomeric equilibria, as shown in Scheme 20 for H-D exchange in 50. 

One suggested mechanism is that the reaction may take place by a conjugate hydride-transfer mechanism, analogous to what occurs during alcohol oxidations with NAD+. Electrons on the enolate ion might expel a (3 hydride ion, which could add to the doubly bonded NS nitrogen on FAD. Protonation of the intermediate at N1 would give the product. 

McCapra, F., and Hysert, D. W. (1973). Bacterial bioluminescence — identification of fatty acid as product, its quantum yield and a suggested mechanism. Biochem. Biophys. Res. Commun. 52 298-304. 

The suggested mechanism involves breaking of a platinum-ligand bond, again forming a platinum(IV) hydride that can then eliminate the alkane. 

This agreement between the various methods of determinations contrasts with the case of non-catalyzed reactions where the results differ widely. However, in spite of this concordance the various authors suggest mechanisms which involve either ion pairs or free ions. 

Several authors suggested mechanisms for esterifications catalyzed by titanium tetraalk-oxides. Bolotina et al.16,221,2221 who studied the polyesterification of 2-ethylhexyl phtha-late with 2-ethylhexanol found the same reaction order with respect to catalyst, acid and alcohol, namely 1 they suggested the following rate-determining step ... 

To verify that a proposed reaction mechanism agrees with experimental data, we construct the overall rate law implied by the mechanism and check to see whether it is consistent with the experimentally determined rate law. However, although the constructed rate law and the experimental rate law may be the same, the proposed mechanism may still he incorrect because some other mechanism may also lead to the same rate law. Kinetic information can only support a proposed mechanism it can never prove that a mechanism is correct. The acceptance of a suggested mechanism is more like the process of proof in an ideal court of law than a proof in mathematics, with evidence being assembled to give a convincing, consistent picture. 

Fig. 3 Suggested mechanisms for the reaction of cellulose with tosyl chloride in the presence of pyridine (Py) and/or triethylamine (EtsN) base catalyst. Redrawn from [147]...

One should note overall, that while some of these suggested mechanisms may in the future prove to have a role in the control of smooth muscle contraction, in chemically skinned preparations maximum force development follows activation by the MLCK active subunit in extremely low Ca " ion concentrations. The conclusion can hardly be avoided that phosphorylation alone is sufficient for activation, and if another mechanism is involved, it is not necessary for the initial genesis of force. If such mechanisms are operative, then they might be expected to run in parallel or consequent to myosin phosphorylation. A possible example of this category of effect is that a GTP-dependent process (G-protein) shifts the force vs. Ca ion concentration relationship to lower Ca ion concentrations. This kind of mechanism calls attention to the divergence of signals along the intracellular control pathways. 

A general synthesis of pyrrolo[3,4-mediated reductive cyclisation of the pyrrolidinones 16 with amidines. The suggested mechanism is that shown in Scheme 2 < 95JOC7687 >. 

Treatment of dialkylacetylenes with 1 equivalent of TTN in aqueous acidic glyme resulted in formation of acyloins in high yield [Eq. (30)] the suggested mechanism of this transformation, shown in Scheme 24, is... 

The recently reported (757) conversion of 5-pyrazolones directly to a,j8-acetylenic esters by treatment with TTN in methanol appears to be an example of thallation of a heterocyclic enamine the suggested mechanism involves initial electrophilic thallation of the 3-pyrazolin-5-one tautomer of the 5-pyrazolone to give an intermediate organothallium compound which undergoes a subsequent oxidation by a second equivalent of TTN to give a diazacyclopentadienone. Solvolysis by methanol, with concomitant elimination of nitrogen and thallium(I), yields the a,)S-acetylenic ester in excellent (78-95%) yield (Scheme 35). Since 5-pyrazolones may be prepared in quantitative yield by the reaction of /3-keto esters with hydrazine (168), this conversion represents in a formal sense the dehydration of /3-keto esters. In fact, the direct conversion of /3-keto esters to a,jS-acetylenic esters without isolation of the intermediate 5-pyrazolones can be achieved by treatment in methanol solution first with hydrazine and then with TTN. 

B. By Hydrolysis Reactions.—Details have appeared of the synthesis of dibenzophosphorin oxides (15) from 5-alkyldibenzophospholes, by reaction with methyl propiolate in the presence of water, and of confirmatory syntheses from phosphinic acid chlorides, as shown below. Evidence for the suggested mechanism of the ring-expansion reaction is presented. The hydrolysis of enamine phosphine oxides is an efficient, although somewhat indirect, method for the preparation of j8-ketoalkylphosphine oxides (16) [see Section 3(iii), for the preparation of enamine oxides]. Reasonable yields (48—66%) of trialkylphosphine oxides (17) have been obtained by the alkaline hydrolysis of the products from the pyrolysis at 220 °C of red phosphorus with alkyl halides, in the presence of iodine. 

A useful new method of preparing arylphosphonates (123) involves the reaction of trialkyl phosphites with aryl halides in the presence of a nickel catalyst.The suggested mechanism is via the nickel complex (124), and is non-radical. 

Suggest mechanisms by which the following transformations occur. 

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