Acetone, enolization mechanism

Halothiophenes undergo photostimulated reaction with acetone enolate ion to form substitution products (76H(5j377). This is believed to occur by the radical-chain SrnI mechanism. The propagation steps are as follows ... 

Griffiths and Gutsche (23) recently studied the interconversion of deuterated mandelaldehyde dimer and 2-hydroxyacetophenone in pyridine to obtain information concerning the glyceraldehyde-dihydroxy-acetone rearrangement. Their results support an enolization mechanism requiring a base and an acid catalyst. They found a deuterium isotope effect of ca. 1.3 for the transformation of the aldehyde to the ketone. When they corrected this for the apparently differing amounts of the aldehyde form in equilibrium with the proteo dimer and the deuterio dimer, they obtained a value of 3.9. By the Swain-Schaad equation (26) ... 

Kornblum and coworkers31a have determined the quantum yield for the ET substitution reactions of / -nitrocumyl chloride with azide ions (3.5) and quinuclidine (6000). Furthermore, by studying the wavelength dependence of the quantum yields, they have obtained evidence that photochemical initiation proceeds by means of a charge-transfer complex. Similar results have been obtained in the reaction of acetone enolate ion with Phi and PhBr in DMSO, whereas other mechanisms are in competition when Phi reacts with potassium diethyl phosphite3115. 

The photostimulated reaction of l-bromo-2,4,6-trimethylbenzene (a substrate with no o-hydrogen atoms in order to avoid the benzyne mechanism) with NH2 ions gave 1-amino-2,4,6-trimethylbenzene (70%) and the reduction product 1,3,5-trimethylbenzene (6%). This reaction did not occur in the dark221. By competition experiments of NH2 ions with Ph2P" ions toward 2,4,6-trimethylphenyl radicals, it was found that Ph2P ions are 6.4 times more reactive than NH2 ions221, whereas NH2 ions are twice more reactive than acetone enolate ions toward the same radical in liquid ammonia222. 

The second mechanism. Reaction 6.32, proposes that water acts as a base to create acetone enolate anion. This enolate next adds to acetaldehyde through a TS that is 49.1 kcal mol above reactants. The final aldol product is obtained by proton transfer. While this mechanism require less energy than the first of Houk s mechanisms, it too is unlikely to be competitive, given its large barrier for formation of the acetone enolate. 

The stereoselective enolization of 3-pentanone by LiTMP mixed aggregates with butyl-lithium was studied by Pratt and coworkers. The mixed aggregate resulted in a slightly higher stereoselectivity, which increased with decreasing amount of the lithium base. Semiempirical PM3 calculations were used in an attempt to determine the mechanism of ketone deprotonation by the mixed aggregate. Equations 20 and 21 show two alternative mechanisms for the formation of lithium acetone enolate in thf solution, involving... 

P cm 21.8 Show in detail how the enolization mechanism accounts for the following facts (a) the rate constants for acid-catalyzed hydrogen-deuterium exchange and bromination of acetone are identical (b) the rate constants for acid-catalyzed racemization and iodination of phenyl rec- tyl ketone are identical. 

The enolate anions of 2-acetylthiophene and 2-acetylfuran have been arylated under photochemical conditions in the presence of t-BuOK and good electron donors such as acetone enolate (entrainment reaction) to give the corresponding benzyl 2-thienyl and 2-furanyl ketones respectively. Use of FeBr2 as initiator in a dark reaction gives good yields of the substitution products without the need for added nucleophiles, and it is suggested that these arylation processes occur by an SrnI mechanism. 

Scheme 6.97 presents an example of a photoinduced substitution reaction of iodoben-zene (247) with a good electron donor, such as an acetone enolate (248), affording the product 249 via the Sr-nIAt mechanism in 88% chemical yield.853 The radical anion species (Phi) (250) formed in the first step is short lived and it readily releases the... 

The decarboxylation of acetoacetate is acid catalyzed (20). Metals do not catalyze the spontaneous decarboxylation of acetoacetate, presumably because the substrate and the product acetone enol are poor ligands for the metal (27). Primary amines catalyze the decarboxylation of acetoacetate by a Schiff base mechanism (Scheme VII), and this provides the best model for acetoacetate decarboxylase (94). 

Several reported kinetic studies shed light on the various steps in the Haloform reaction. Bartlett showed in 1934 that the reaction of acetone with strong alkaline solutions of hypobromite and hypoiodite involves enolization of acetone as the ratedetermining step, but the rate of the same reaction with alkaline hypochlorite involves as the slow step reaction of acetone enol with hypochlorite.15 Likewise, the mechanism of the iodoform reaction and its optimization for quantitative analysis have received much scrutiny16 including studies of acetone-1-C-l4 7 and deuterated acetone18 for isotope... 

When aryl halides (ArX) react with nucleophiles (Y ) in the presence of solvated electrons or under UV irradiation, a radical chain mechanism occurs (Eq. 17). In reactions with an acetone enolate salt, both 2-chloro- and... 

Rossi and Bunnett64 studied the chemical reductive cleavage of diphenyl sulfoxide, diphenyl sulfone and methyl phenyl sulfone under the action of potassium metal in liquid ammonia in the presence of acetone. The enolate ion is used to trap phenyl radicals formed eventually during the process, in order to determine whether one or two electrons are required for the mechanism of cleavage (Scheme 7). In all the runs, phenyl anion is... 

In the following scheme, an oxidation pathway for propane and propene is proposed. This mechanism, that could be generalized to different hansition metal oxide catalysts, implies that propene oxidation can follow the allylic oxidation way, or alternatively, the oxidation way at C2, through acetone. The latter easily gives rise to combustion, because it can give rise to enolization and C-C bond oxidative breaking. This is believed to be the main combustion way for propene over some catalysts, while for other catalysts acrolein overoxidation could... 

In this reaction, the enol form of acetone reacts with the Pt(III) dimer complex, and therefore the reaction mechanism seems to be the same with those of the olefin reactions 90). 

Figure 2 shows illustrated mechanism for acetone self-condensation over calcined hydrotalcites, where the enolate ion is formed in a first step followed by two possible kinetic pathways 1) In the first case the subtracted proton is attracted by the basic sites and transferred to the oxygen of the enolate ion to form an enol in equilibrium. 2) In the second case the enolate ion reacts with an acetone molecule in the carbonyl group, to produce the aldol (diacetone alcohol). Finally, the p carbon is deprotonated to form a ternary carbon and then loses an OH group to obtain the final products. 

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... 

Erlenmeyer was first to consider ends as hypothetical primary intermediates in a paper published in 1880 on the dehydration of glycols.1 Ketones are inert towards electrophilic reagents, in contrast to their highly reactive end tautomers. However, the equilibrium concentrations of simple ends are generally quite low. That of 2-propenol, for example, amounts to only a few parts per billion in aqueous solutions of acetone. Nevertheless, many important reactions of ketones proceed via the more reactive ends, and enolization is then generally rate-determining. Such a mechanism was put forth in 1905 by Lapworth,2 who showed that the bromination rate of acetone in aqueous acid was independent of bromine concentration and concluded that the reaction is initiated by acid-catalyzed enolization, followed by fast trapping of the end by bromine (Scheme 1). This was the first time that a mechanistic hypothesis was put forth on the basis of an observed rate law. More recent work... 

Many reactions have also been carried out in water. The mechanisms of the reactions of acetone and 1,3-dihydroxyacetone using zinc-proline and related catalysts have been probed kinetically.98 The former exhibits an enamine route, whereas the latter involves rate-limiting deprotonation of the -carbon and formation of an enolate. An umbelliferyl ether of dihydroxyacetone (37) has been used as a fluorogenic probe for enolization, which may prove useful in screening of aldolases in water. 

The role of the metal ion may be purely conformational, acting to place the reactants in the correct spatial arrangement for cyclisation to occur, or it may play a more active role in stabilising the enol, enolate, imine or enamine intermediates. The prototypical example of such a reaction is shown in Fig. 6-18. The nickel(n) complex of a tetradentate macrocyclic ligand is the unexpected product of the reaction of [Ni(en)3]2+ with acetone. There are numerous possible mechanisms for the formation of the tetradentate macro-cyclic ligand and the exact mechanism is not known with any certainty. 

Hetero-Diels-Alder reaction of 44 with enol ether 13 as the dienophile gives cycloadduct 45, which is not isolable but reacts with the water formed in the condensation step with loss of acetone and C02 to lactone 15. A suggested mechanism for the formation of lactone 15 is a retro Diels-Alder reaction which leads to the ketene intermediate 46. Ketene 46 adds to the water formed in the previous condensation step, yielding /3-keto-carboxylic acid 47, which then undergoes decarboxylation to 48. 

The disconnection looks tricky but it is all right if you simply reverse the rearrangement, by drawing the mechanism for the imaginary reverse reaction. There may well be two possible starting materials. Thus cyclopentene 40, needed for a photochemical experiment, could be disconnected as 40a or 40b. There is no obvious way to continue from 41 but 42 has an enone that could be made from aldehyde 43 and some reagent for the enolate of acetone 44. 

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