Ethyl oxygen exchange with

If the carbonyl oxygen of methyl and ethyl acetate exchanges with the solvent under the conditions of acid-catalyzed hydrolysis in the same way as ethyl benzoate, and if it is assumed that exchange and hydrolysis go by the same intermediate, then the mechanism is the slow bimolecular addition to the ester protonated on the carbonyl oxygen. Asknes and Prue (1959) have presented other evidence suggestive of carbonyl-oxygen protonation. 

Ethyl phenylacetate, reaction with (2-bromoethyl)benzene, 47, 72 Ethyl 2-phenylhexanoate, 47, 74 Ethyl 2-phenylpropionate, 47, 74 Exchange of oxygen by sulfur in preparation of chloromethylphospho-nothioic dichlonde, 46, 21 Extractor, stirred in isolation of 3-hydroxyglutaromtnle, 46, 49... 

The behavior of chiral phenyl /-butyl sulfoxide 219 and a-phenyl-ethyl phenyl sulfoxide 220 is completely different in strongly acidic media and in the presence of halide ions. Two reactions were found (266) to occur in parallel. One results in the loss of optical activity, and the second leads to the decomposition of the sulfoxide. It was observed that the racemization process is not accompanied by [ 0] oxygen exchange. In the case of sulfoxide 220 the complete loss of optical activity at chiral sulfur is accompanied by partial racemization at the chiral carbon center. These results are consistent with a sulfenic acid-ion-pair mechanism formulated by Modena and co-workers (266) as follows (it is obvious that the formation of achiral sulfenic acid is responsible for racemization). 

Carbonyl oxygen exchange was found during the cupric ion-catalyzed hydrolysis of DL-phenylalanine ethyl ester-carbonyl-O18 at pH 7.3 (11). This indicates that an additional intermediate is formed in this reaction. A mechanism (II) consistent with both the kinetic evidence and the oxygen-exchange evidence is given below. 

Bender104 found that when ethyl benzoate, labelled with excess, sO in the carbonyl group, is hydrolyzed at 99°C in isotopically normal aqueous 1 M acid, oxygen exchange between the unreacted ester and the solvent takes place, and the enrichment of the remaining ester decreases steadily as hydrolysis proceeds. This is precisely the result expected if hydrolysis involves a full intermediate and the addition elimination mechanism receives further support from the observation that hydrolysis and exchange proceed at similar rates, with a constant ratio, Arhyd/)tex of 5.2 for ethylbenzoate. The reaction can thus be written as... 

Clearly with ratios of khyJkt.yeh in the region observed for ethyl benzoate under these conditions, the rates and activation parameters are largely determined by the addition step. And in this case at least the small rate decrease due to the breakdown step is entirely an entropy effect. In several instances, notably the hydrolyses of methyl and substituted benzyl benzoates, phthalide and y-butyrolactone. kh>rt > kcsch. since no concurrent oxygen exchange could be detected on alkaline hydrolysis212-3". ... 

Transalkylation reactions are observed in Y zeolites partially exchanged with ethyl-, diethyl-, and triethylammonium cations (EA, DEA, and TEA, respectively) heated above 150° C in air or under vacuum, in the presence of residual water molecules. The main reactions may be depicted schematically as follows (EA) Y — (DEA)Y > (NHt+)Y, (DEA)Y - (EA)Y > (TEA)Y, and (TEA)Y - (DEA)Y > (EA)Y, iAc main constituent in the gas phase being CJh. They are similar to those observed in montmorillonite in the presence of a water vapor pressure of a few torr. It is proposed that in both cases the transalkylation processes are acid catalyzed, the residual water molecules and the surface oxygen being the active spots recycling the protons in montmorillonite and zeolite, respectively. 

Triethylborane in combination with oxygen provides an efficient and useful system for iodine atom abstraction from alkyl iodide, and thus is a good initiator for iodine atom transfer reactions [13,33,34]. Indeed, the ethyl radical, issued from the reaction of triethylborane with molecular oxygen, can abstract an iodine atom from the radical precursor to produce a radical R that enters into the chain process (Scheme 13). The iodine exchange is fast and efficient when R is more stable than the ethyl radical. 

In the present work, the Jacobsen s catalyst was immobilized inside highly dealuminated zeolites X and Y, containing mesopores completely surrounded by micropores, and in Al-MCM-41 via ion exchange. Moreover, the complex was immobilized on modified silica MCM-41 via the metal center and through the salen ligand, respectively. cis-Ethyl cinnamate, (-)-a-pinene, styrene, and 1,2-dihydronaphtalene were used as test molecules for asymmetric epoxidation with NaOCl, m-CPBA (m-chloroperoxybenzoic acid), and dimethyldioxirane (DMD) generated in situ as the oxygen sources. 

Bender et al.136 have measured the rate of incorporation of, 80 from enriched water into several substituted benzoic acids. The catalyst was 0.07 M HC1, and the solvent 33% dioxan-water. The rate coefficients for exchange at 80°C are given in Table 14, which also contains a comparison of these rate coefficients with those for the hydrolysis of the corresponding ethyl esters, measured by Timm and Hinshelwood128 in 60% acetone and 60% ethanol. As noted earlier by Roberts and Urey, the absolute rates are very similar for the two reactions. Also, as expected, the exchange rate of benzoic acid, with two equivalent oxygen atoms, is almost exactly twice as fast as that of ethyl benzoate, with only one ( h.vdM< xch is 5.2 for the ester). 

---