Methyl acetate, protonated

Compare energies for the two alternative conjugate acids of methyl acetate (protonated methyl acetate and methoxy protonated methyl acetate) and dimethylacetamide (N-protonated dimethylacetamide and 0-protonated dimethylacetamide). Which acid in each pair is more stable Draw resonance contributors for the more stable conjugate acid for each system. 

LUMO for protonated methyl acetate reveals likely site of attack by water. 

Examine electrostatic potential maps for methyl acetate (X=OMe), dimethylacetamide (X=NMe2), mdacetonitrile. What is die most electron-rich site in each molecule Does this site correspond to a bonding pair of electrons or a nonbonding pair of electrons Assuming that protonation occurs onto the most electron-rich site, where would you expect each molecule to protonate ... 

Wender and coworkers conclude that cobalt-catalyzed benzyl alcohol homologation involves the intermediate formation of car-bonium ions (8). However, since the methyl cation (CH3+) is unstable and difficult to form (9), it is more likely that methanol homologation to ethanol proceeds via nucleophilic attack on a protonated methyl alcohol molecule. Protonated dimethyl ether and methyl acetate forms have been invoked also by Braca (10), along with the subsequent formation of methyl-ruthenium moieties, to describe ruthenium catalyzed homologation to ethyl acetate. 

Owing to the possibility of a proton loss from both nitrogen atoms, cyclization generally resulted in neutral compounds. iV-Substituted starting materials led to cationic species with no possibility of subsequent tautomeriza-tion. For instance, iV-methyl acetic hydrazide 194 gave the triazolopyridinium 195 upon treatment with POCI3 (Equation 20) <1996T1399>. 

In a kinetic study of the esterification of acetic acid with methanol in the presence of hydrogen iodide, iodimethane was identified as a by-product. The authors propose that this derives from iodide ion attack on protonated methanol. However, attack by iodide ion on protonated methyl acetate (10) is more likely, since acetic acid is a better leaving group than ethanol. 

Determine the frequency difference between the shifts of the protons of the methyl groups of methyl acetate in Hz at field strengths of 60 mHz, 250 mHz and 400 mHz. 

Treatment of the reduced intermediate (23-6) with butyl hthium leads to the anion from the removal of a proton on the methylene group reaction of that with methyl acetate affords the methyl ketone (24-1), which contains two of the three required side chain carbon atoms. The additional carbon atom and the basic function are incorporated by means of a Mannich condensation. Thus, reaction of (24-1) with A-methylpiperazine and formaldehyde leads to the aminoketone (24-2). The carbonyl group is then reduced with sodium borohydride and the resulting alcohol is dehydrated by reaction with phosphoms oxychloride in pyridine. In this case, too, the Z isomer is responsible for most of the activity. This is isolated from the resulting mixture of olefins to afford thiothixene (24-3) [25]. 

This last method has been used by Ramsey and Taft"5 to prepare a series of simple alkoxymethyl cations. The nmr chemical shifts, 8, of the protons of methylated methyl acetate and formate, given by Ramsey and Taft (measured in 30% S03 in H2S04) are compared below with the figures for the corresponding protonated esters, measured in FS0 tH-SbF5-S02 H. 

A study of gas-phase reactions of benzyl and methoxide anions with alkyl formate and other esters has revealed some differences in behaviour of these anions of comparable basicity.184 The delocalized benzyl anion and localized methoxide ion engage in exclusive transacylation and proton transfer, respectively, on reaction with alkyl formates. However, proton transfer is sufficiently exothermic to dominate when benzyl anion reacts with methyl acetate. Both anions react with methyl benzoate, methyl tiifluoroacetate, and methyl cyanofonnate by competing transacylation and. S n2 reactions. 

Whereas the C2—C4 alcohols are not carboxylated under the usual Koch-Haaf conditions, carboxylation can be achieved in the HF-SbF5 superacid system under extremely mild conditions.400 Moreover, Olah and co-workers401 have shown that even methyl alcohol and dimethyl ether can be carboxylated with the superacidic HF-BF3 system to form methyl acetate and acetic acid. In the carboxylation of methyl alcohol the quantity of acetic acid increased at the expense of methyl acetate with increase in reaction time and temperature. The quantity of the byproduct dimethyl ether, in turn, decreased. Dimethyl ether gave the desired products in about 90% yield at 250°C (90% conversion, catalyst/substrate ratio =1 1, 6h). On the basis of experimental observations, first methyl alcohol is dehydrated to dimethyl ether. Protonated dimethyl ether then reacts with CO to yield methyl acetate [Eq. (5.154)]. The most probable pathway suggested to explain the formation of acetic acid involves the intermediate formation of acetic anhydride through acid-catalyzed ester cleavage without the intervention of CO followed by cleavage with HF [Eq. (5.155)]. 

Cleavage of -lactones.1 The blue solution of potassium-18-crown-6 in THF cleaves (3-propiolactone at -20° to furnish a dianion (1) which on protonation gives methyl acetate. A similar reaction of the (3-lactone 2 can be used to obtain isopropyl propionate (3). 

In looking at the conjugate acid pK values listed in Figure 3.1, we realize that in order for the reactions represented in Scheme 3.1 to occur, the conjugate acid of a given base must have a pK value that is higher than the pK value associated with a proton of interest. For example, as shown in Scheme 3.2, we would not expect triethylamine to effectively depro-tonate methyl acetate because the pK of methyl acetate is 15 pKa units higher than the pK ... 

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