Acetic protonation

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. 

As a measure of the stereoregularity, an index EQ-H% was defined as the precent of the equatorial acetal protons to the total acetal protons. Figure 2 illustrates the temperature dependence of EQ-H% s of the polymer obtained in toluene (A), methylene chloride (B), and 1-nitropropane (C). No significant difference is observed at... 

The difference in conformational energy between the two conformers of the trans-isomer is only 2.1 kJmol1, resulting in an equilibrium mixture of 30% of 29 and 70% of 30. Therefore, polymerization of 2 7 to a trans-1,4-tetrahydropyranoside polymer by direct displacement on the trialkyloxonium ion by monomer would be detected by -NMR as an axial acetal proton to equatorial acetal proton ratio (Hax)/(Heq) of 2.3. The most stereoregular polymer that obtained using 0.2 mol% of SiF4 at -78 °C possessed an (H /CHeq) ratio of 3.0. 

Fig. 3-19. Proton level vs. concentration curves of addic proton (Hs0 /H20) and acetic proton (HAc/Ac") unitary acetic proton level ( H (Hso /At-)= proton levels in...

The peaks corresponding to 2,3-d1-tert-butoxy-l,4-d1oxane overlap with those of the methylene protons of 2-chloroethyl dichlorophosphate, a byproduct from the reaction, but the absence of the acetal protons of the starting material is clear from the symmetry of the multiplet. 

The second half of the mechanism converts the hemiacetal to the more stable acetal. Protonation of the hydroxyl group, followed by loss of water, gives a resonance-stabilized carbocation. Attack on the carbocation by methanol, followed by loss of a proton, gives the acetal. 

The most obvious solution to this problem is to provide a better electrophile than the proton for sulfur. Mercury, Hg(II), is one solution. Another is oxidation of one sulfur to the sulfoxide, a process that would be impossible with the oxygen atoms of an ordinary acetal. Protonation can now occur on the more basic oxygen atom of the sulfoxide and the concentration of the vital intermediate is... 

Treatment with acidic ethanol simply makes the diethyl acetal from the aldehyde group Since A1 and A2 are in equilibrium, all A2 is eventually converted into B via Al. Compou.-v 1 again chiral so the ABX system reappears with further coupling of H to the acetal proton. T v xir now four triplets and quartets for the four OEt groups. 

For the polymer of 6,8-dloxablcyclo[3.2.1]octane, only one acetal proton Is observed (It Is equatorial). Indicating pure trans-... 

Another cyclization has been described using thioketene acetal protonation to initiate annula-tion33. 

H-NMR spectra of the trans-isomer consist of three signals of acetal protons, as shown in Fig. 8.4. This was attributed to the presence of different isomers (and conformers). Thus, for the a-form the following structures are possible ... 

By analogy with the results of Baggett and co-workers, who showed that in 2-phenyl-l,3-dioxolans the acetal proton is at higher field when cis to protons at C-4 and C-S than when cis to other substituents at C-4 and C-S, compound (19) was assigned the structure shown, with the... 

The mechanism of this hydrolysis reaction has been studied in great detail. The mechanism is the reverse of that for acetal formation. Acetal protonation is followed by elimination of an alcohol molecule. The resulting intermediate is a stabilized carbocation. Addition of water and a second acid-catalyzed elimination lead to the product. 

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