Enolates reactivity, effect

A notable effect ol rra/i.v comdination has tilso been observed for the aniitnic pttlymeriziition of methacryloni-trile (22) initiated from a living polymer ctf methyl methacrylate (21, R = Me) with an aluminum enolate reactive end (32 ), in which the chain grttwth is promoted by the trails coordination of pyridine, to afford a narrow MWD block copolymer (42)- Ott the other hand, in the absence of axially ccatrdinating pyridine under otherwise identical conditions as described, no block copolymerization of 22 from 32 takes place. 

The effects of the counterion on the reactivity of the enolates can be important Reactivity Li+ < Na+ < K+ < ITiN+ addition of crown ethers... 

With highly reactive alkyl halides, like allylic, benzylic or phenacyl halides, the ZjA-alkylation can be a serious side-reaction. Because of a SNl-like mechanism in those cases, the effect of enolate concentration on the reaction rate is low, and the resulting monoalkylester 5 may be more acidic than the unsubstituted starting material ... 

As an example of enolate-ion reactivity, aldehydes and ketones undergo base-promoted o halogenation. Even relatively weak bases such as hydroxide ion are effective for halogenation because it s not necessary to convert the ketone completely into its enolate ion. As soon as a small amount of enolate is generated, it reacts immediately with the halogen, removing it from the reaction and driving the equilibrium for further enolate ion formation. 

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75] investigated the effects of solvent polarity on the product distribution in the reaction of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the perepoxide intermediate or the perepoxide-Uke transition state to explain their results. A recent review of the ene reactions of was based on the significant intervention of the perepoxide structure [76], which can be taken as a quasi-intermediate. 

The rate of alkylation of enolate ions is strongly dependent on the solvent in which the reaction is carried out.41 The relative rates of reaction of the sodium enolate of diethyl n-butylmalonate with n-butyl bromide are shown in Table 1.3. Dimethyl sulfoxide (DMSO) and iV,Ai-dimethylformamide (DMF) are particularly effective in enhancing the reactivity of enolate ions. Both of these are polar aprotic solvents. Other... 

The effect of HMPA on the reactivity of cyclopentanone enolate has been examined.44 This enolate is primarily a dimer, even in the presence of excess HMPA, but the reactivity increases by a factor of 7500 for a tenfold excess of HMPA at -50° C. The kinetics of the reaction with CH3I are consistent with the dimer being the active nucleophile. It should be kept in mind that the reactivity of regio- and stereoisomeric enolates may be different and the alkylation product ratio may not reflect the enolate composition. This issue was studied with 2-heptanone.45 Although kinetic deprotonation in THF favors the 1-enolate, a nearly equal mixture of C(l) and C(3) alkylation was observed. The inclusion of HMPA improved the C(l) selectivity to 11 1 and also markedly accelerated the rate of the reaction. These results are presumably due to increased reactivity and less competition from enolate isomerization in the presence of HMPA. 

Lead tetraacetate can effect oxidation of carbonyl groups, leading to formation of a-acetoxy ketones,215 but the yields are seldom high. Boron trifluoride can be used to catalyze these oxidations. It is presumed to function by catalyzing the formation of the enol, which is thought to be the reactive species.216 With unsymmetrical ketones, products from oxidation at both a-methylene groups are found.217... 

An additional example of cycloamylose-induced catalysis which can probably be attributed to a microsolvent effect is the oxidation of a-hy-droxyketones to a-diketones (Scheme VIII). The rate of this oxidation is accelerated by factors ranging from 2.1 to 8.3 as the structure of the substrate is varied. As noted by Cramer (1953), these accelerations may be attributed to a cycloamylose-induced shift of the keto-enol equilibrium to the more reactive enol form. 

Besides aldehydes and ketones, TiCl4 activates acetals effectively for reaction with enolates (Scheme 18).70,71 The reactivity of acetals is higher than that of the corresponding aldehydes in... 

Having shown that the enol silyl ethers are effective electron donors for the [D, A] complex formation with various electron acceptors, let us now examine the electron-transfer activation (thermal and photochemical) of the donor/ acceptor complexes of tetranitromethane and quinones with enol silyl ethers for nitration and oxidative addition, respectively, via ion radicals as critical reactive intermediates. 

DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

Bromination rates of aliphatic enol ethers have been included in the interactive treatment of alkenes GRIC=CR R, with G being a conjugated group most of them fit the multiparameter equation (41) satisfactorily. A more detailed analysis of reactivity-selectivity effects in the reaction of 1-ethoxyethylene [22] and its a- and / -methyl analogues [23] and [24] has been carried out,... 

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

KSIEs for the reaction of aromatic olefins, 1,1-diphenylethylene and a-methylstyrene (Table 21) are significantly smaller they can be related to transition states earlier than those in the aliphatic series. Unfortunately, for the reactions of highly reactive aromatic olefins or enol ethers, whose low sensitivity to solvent and substituent effects indicates very early transition states, there are not enough KSIE data to confirm this conclusion. 

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