Protonated ester

The non-equivalence of the ester protons in the monomethyl- and monophenyl-phosphinic ester function, as in (44, Ch = chalkogen), has been studied. Compounds of type (45) have some interesting stereochemistry. They are prepared from racemic secondary butyl alcohol, and the presence of three signals in the P n.m.r. spectrum confirms that the phosphorus atom is the centre of pseudo-asymmetry. A 1 2 1 triplet is observed which is attributed to the presence of equal amounts of two mesa forms, (45) and (46), which have different values of Sp (outer peaks), and two racemic forms, (47) and (48), which have identical values of 8p (central peak). 

Applications Albert et al. [455] have shown continuous-flow SFC-NMR spectra of five plasticisers (DEP, DNPP, DPP, BBP, DNBP). On-flow and stopped-flow pSFC-NMR of synthetic mixtures of phthalates were reported [457]. The feasibility of SFC-NMR coupling has been demonstrated with real-life applications [458]. Figure 7.20 shows a reconstruction of an extraction profile from a PVC tube [152]. The profiles of the integral aromatic proton signals between 7.2 and 8.2 ppm and the ester protons at 4.42 ppm display the relative concentration of the extracted phthalate as a function of the proceeding extraction. The structure of the extracted phthalate could be assigned to DEHP (Figure 7.21). 

The ethyl ester protons are worthy of note in this molecule. Though there is no chiral centre present, these are non-equivalent by virtue of being diastereotopic (remember the Z test ). 

FIGURE 3.8 Deprotection of carboxyl groups by acid-catalyzed hydrolysis (A) of amides and (B) of esters. Protonation generates a relatively stable carbenium ion that usually requires heat to fragment it. 

The H NMR spectrum of simple iV-methyl-substituted oxicam 48 has been determined (Figure 15) <1997JME980>. The A -methyl group occurs at 2.95 ppm, while the methyl ester protons are observed at 3.96 ppm. The aromatic protons (C-3 to C-6) occur as an unresolved multiplet in the range of 7.71-8.05 ppm. 

Under acidic conditions, the mechanism of amide hydrolysis resembles the acid-catalyzed hydrolysis of an ester. Protonation of the carbonyl group activates it toward nucleophilic attack by water to give a tetrahedral intermediate. Protonation of the amino group enables it to leave as the amine. A fast exothermic proton transfer gives the acid and die protonated amine. 

Reactions of Phosphines. - 1.2.1 Nucleophilic Attack at Carbon. Interest has continued in developing the synthetic applications of the 1 1 adducts of tertiary phosphines with dialkyl acetylenedicarboxylate esters. Protonation of the initial adduct from triphenylphosphine by phthalimide, followed by nucleophilic addition of the nitrogen of the resulting imido anion to the intermediate vinylphosphonium salt, has given the stabilised ylide system (189). ° Similar reactions with isatin and 3-chlorotetrahydrofuran-2,4-dione have given the yl-... 

Figure 4.11. The application of selective proton decoupling in the measurement of heteronuclear long-range proton-carbon coupling constants. Lower traces are from the fully proton-coupled carbon-13 spectrum and the upper traces from that in which the methyl ester protons of 4.3 were selectively decoupled to reveal the three-bond coupling of the carbonyl carbon across the alkene.

TaUe t. Chemical shift value of methyl ester protons in various tetrathiacuppedophanes... 

If we assume possible transition states corresponding to those in Table 7, then the only ones consistent with the foregoing discussion are those formed by a bimolecular substitution of the ester protonated on the ether oxygen,... 

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. 

Fig. 20. (Top) SPIROE-TOCSY pulse sequence comprised of proton saturation, adabatic CHIRP transfer, and proton magnetization buildup during and TOCSY using the DIPSI-2 mixing sequence. (Bottom) 2D SPIROE-TOCSY performed in deuterated 1,1,2,2-tetra-chloroethane. Broken line indicates the diagonal. The inset shows the region of the camp-hanic ester protons. The total experiment time is only 4.5min. (Courtesy of Herve Desvaux. Reprinted from ref. 313 with permission. Copyright 2004, Elsevier SAS.)...

In poly(methyl methacrylate), -[-CH2—C(CH3)(COOCH3)4" signals can be expected from the methylene protons CH2, from the a-methyl protons CH3, and from the methyl ester protons COOCH3. The assignment of the signals from the three types of protons is made possible by comparison with the spectrum of methyl pivalate (CH3)3C—COOCH3. In both poly(methyl methacrylate) and methyl pivalate, the a-methyl protons and the methyl ester protons appear at the same position in the nmr spectrum. Information about the tacticity of the polymer can be obtained as described below. 

Figure 3-9. Section from the proton resonance spectra of isotactic (it), syndiotactic (st), and atactic (at) poly(methyl methacrylates). The signals of the methyl ester protons are not shown. TMS = reference signal of tetramethyl silane. (According to P. Goeldi and H.-G. Elias.)...

The methyl ester protons give rise to a single signal, which is independent of tacticity (not shown in Figure 3-9). Therefore, the methyl ester proton signal cannot be used for tacticity determinations. The chemical shift is not affected by the configuration of the main chain since they are too far removed from an asymmetric center. 

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