Ionic enolate mechanism

Two free-radical chain reactions, in addition to the ionic enolate mechanism, seem reasonable for the oxidation of the sugars by oxygen. With an aldose-2-f one of the free-radical mechanisms would yield non-labeled formic acid and the next lower aldonic acid the other would yield labeled formic acid and the same aldonic acid. 

The determination of the lifetime of the ionic intermediates using the azide-clock method has been however useful in showing that electrophilic addition of Br2 can occur, even through a fully concerted mechanism, definable as SN2-like. Bromination of cyclic enol ethers (glycals) 8-10 in methanol in the presence of... 

In the proposed mechanism (Scheme 9), the rate-determining step is the reaction between aldehyde and enolate. In the absence of a solvent, a major issue with this reaction is the typical low rate and the need for a high concentration of catalyst (usually DABCO). It was reported recently that, under basic conditions, the ionic liquid [BDMIM][PF6] is inert and that the Baylis Hillman reaction in [BDMIMjPFg proceeds smoothly with better yields than in [BMIMjPFg (163). 

The plausible mechanism of the reaction is shown in Fig. 25. The reaction probably proceeds through the activation of imine (formed in situ from the o-hydroxy benzaldehyde and the aromatic amine) by the catalyst followed by the addition and subsequent cyclization of the enol ether, resulting in the formation of the fused acetal. Ionic liquids are stable enough with amines and water and also effectively activate the imines to undergo cyclization. The recovered ionic liquid can be re-used with gradual decrease in the efficiency of the method. The hydro-phobic nature of the ionic liquid also helps in recovery of the catalyst. 

Silver ion makes possible in the second reaction an isnmeri/ation-free hydrolysis of the S,0 acetal to ketone 32. Generation of the cnol triflate 33 is accomplished in the third step with the Hendrickson-Me M it rry reagent (Tf2NPh).10 Addition of an alcohol produces the potassium aikoxide, which because of its lower basicity permits isomerization to the thermodynamically favored enolate. Chemose-lective reaction to bromohydnn 9 is achieved in the last step with NBS as brominating agent in aqueous THF. NBS acts here as a source of cationic bromine in an ionic mechanism. The intermediate bromonium ion forms preferentially at a) electron-rich double bonds and h) the sterieally least hindered double bond. It also opens in such a way as to provide the most stable carbocation. 

Oxidation of arylmethyl ketoximes by phenyliodoso diacetate in glacial acetic acid was second order overall, first order each in substrate and oxidant.145 Iodine allowed the oxidative dimerization of glycine ester enolates with low to moderate diastereoselec-tivity that is consistent with kinetic control.146 Although malonic acid is not oxidized by iodate under acidic conditions, oxidation proceeds in the presence of catalytic ruthenium(III). A mechanism is put forward to account for the observed orders of reaction.147 The rate of periodate oxidation of m-toluidine in acetone-water increases with ionic strength.148... 

Stacey and Turton61 objected to Isbell s mechanism on two counts first, that he did not specify that a proton acceptor must be used to promote the reaction and second, that the orthoacetate intermediate would not be applicable in the conversion which they demonstrated (by absorption spectra data) to take place on treatment with dilute, aqueous sodium hydroxide. (The presence of the proton acceptor seems implicit in Isbell s general description of the process of enolization.) The mechanism of Stacey and Turton is shown in Formulas XXIV to XXVIII it calls for the donation of electrons by pyridine to the incipient, ionic proton at C2 and elimination of acetic acid between C2 and C3 with the formation of the partially acetylated enediol-pyridinium complex. The pyridinium ion is removed by acetic acid. Electronic readjustment results in the elimination of acetic acid from positions 4 and 5. The final step, conversion of XXVII to XXVIII, was not explained. Stacey and Turton considered that with sodium hydroxide the reaction proceeds after deacetylation by a similar mechanism except that hydroxyl groups take the place of acetyl groups. Neither mechanism requires a free hydroxyl group at Cl, a condition considered by Maurer to be essential to kojic acid formation. 

The photostimulated reactions of thiolate anions with 2-halo-2 -nitropropane derivatives yield both oc-nitrosulphides via an S l pathway and disulphides (equation 71a)282 284. In contrast with the case of the oxidative dimerisation products of the mono-enolates, the disulphides are formed via an ionic mechanism nucleophilic attack by the thiolate anion on the a-halogen and subsequent reaction of a second thiolate with the sulphenyl halide. As expected for such a process, disulphide formation is favoured (and thus a-nitrosulphide formation is disfavoured) the more nucleophilic the thiolate (i.e. derived from a less acidic thiol) and the easier the abstraction of the halo-substituent (i.e. I > Br > Cl). Use of the protic solvent methanol instead of the usual dipolar aprotic solvents for the reaction of equation 71a is detrimental to the yield of the S l substitution products exclusively disulphides are formed285 (equation 71b). Methanol solvation probably retards the dissociation of the radical anion intermediate in the SRN reaction, into radical and anion, and hence retards the chain reaction relative to the ionic reaction. The non-nucleophilic methylsulphinate ion gives only an S l reaction product with 2-bromo-2-nitropropane286. 

Reports of nucleophilic vinylic photosubstitution reactions, which occur via the S l mechanism, are conspicuously scarce. One such example is the cobalt carbonyl catalysed photostimulated carbonylation of vinylic halides339. By this method 1-bromo and 1-chlorocyclohexene are converted into 1-cyclohexenecarboxylic acid in 98 and 97% yield, respectively. In a prototype vinylic S l reaction, of / -bromostyrene with the enolate anion CH2COCMe3, an ionic elimination-addition route seems to be followed along with the S l route340. 

N-Iodosuccinimide reacts with enol acetates derived from ketones to give a-iodoketones, and the reaction has found application in the steroid field/ The iodination of the enol acetates seems to proceed by an ionic mechanism, and preliminary work indicates that N-iodosuccinimide is not capable of at least some of the radical-chain iodinations analogous to radical-chain brominations brought about by N-bromosuccinimide. ... 

Thus, treatment of 2-chloroquinoxaline (1) with ketone enolate 2 in liquid ammonia affords l-(quinoxalin-2-yl)-3.3-dimethylbutan-2-one (3), via a thermal Sr 1 mechanism, and 2-/f r(-butylfuro[2,3-Z)]quinoxaline (4), via a competing ionic addition-substitution process. ... 

In the case of mercurated tiglic acid adducts (12), the erythro product is obtained with aqueous Na2S, and an ionic mechanism has been proposed with an intermediate carboxylate enolate (25). Sodium tri-thiocarbonate and, especially, 1,3-propanedithiol exhibit a preference for retention of configuration leading to the erythro isomer. Such sulfur-containing reagents are probably capable of intramolecular hydrogen atom delivery as illustrated schematically in (26), and they also promote demercuration by an ionic mechanism. 

The aromatization can happen by an ionic mechanism. If the extended enol is protonated a remote end it can then lose a proton from the ring junction and form the phenol. 

Figure 6.9 Possible mechanism for the field-driven ionic binding to thiobis(ethyl aceto-acetate) moieties arranged in a compact monolayer on gold. It includes a field assisted enolization in acid solution followed immediately by a binding of a metal ion. At negative E the first step may include penetration of the ion into the monolayer to form a weak diketone complex. ...

A possible mechanism has been suggested, based on the initial deprotonation of the parent ionic liquid and on the following nucleophilic addition of N-heterocyclic carbene to diphenylketene to generate the enolate intermediate (Fig. 16.2) [95, 96]. 

Work by Ashby et al. has established the intermediacy of radicals in a variety of processes previously thought to be purely ionic. The Claisen condensation of ethyl p-nitrobenzoate with the lithium enolate of pinacolone gave an EPR-active species whose rate of formation and decay indicated that it was on the pathway to the product. The postulated mechanism is shown in Scheme 3. However, this was the only example of SET observed by this group for this reaction and the intermediacy of the radical anion of the ester in this case is plausible but not proved."... 

Tokunaga and coworkers reported the enantioselective hydrolysis of enol esters (111) in the presence of catalyst 8b under phase-transfer conditions with aqueous KOH. The proposed mechanism of this reaction has the protonation of the ammonium-enolate ionic complex as the enantioselective step. Their achievement of the first nonbiomimetic asymmetric hydrolysis of esters catalysed by organocatalysts with high catalytic efficiency in buffer-free conditions has considerable potential to replace enzymatic resolutions in industrial processes (Scheme 16.41). ... 

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