Aromatic protons, exchange

The pyridine-catalyzed aromatic proton exchange with deuterium provides a simple indication of the ability of a phenol to participate in chromene formation. Only those phenols which undergo exchange react with the unsaturated carbonyl compound, the attack occurring at the positions of deuteration (64JA2084). 

Sometimes the observation of general acid catalysis may be sufficient to identify the mechanism with considerable confidence. For example, in aromatic proton exchange catalyzed by ammonium salts, the only structurally attractive A-2 mechanism, shown in equations (4) and (5), violates the principles of microscopic reversibility. Thus the elimination of the A-l mechanism leaves only the A-SE2 mechanism. In a more... 

Kresge et al. (1967) have recently chosen to emphasize those features of AS and zlH for aromatic proton exchange which behave as would be predicted from equilibrium theory. However these systems show the quantitative anomalies referred to in a fairly typical way. The AS values for H+ are more negative by 5-10 cal mole""1 deg-1 than can be readily accommodated and the catalytic coefficient for HSOj" is as large or larger than that for H+ (Kresge et al., 1965a). 

Einsla ML, Kim YS, Hawley M et al (2008) Toward improved conductivity of sulhniated aromatic proton exchange mtanbranes at low relative humidity. Chem Mater 20 5636-5642... 

Comparison of localization energies has frequently been applied to prediction of the relative positional reactivity in polycyclic aromatic hydrocarbons. Simple HMO calculations have only marginal success. CNDO/2 and SCF calculations give results which show good correlation with experimental data on the rate of proton exchange. ... 

The exchange of aromatic protons can be effected in the absence of any -OH or —NH2 activating group during the course of a Clemmensen reduction in deuteriochloric and deuterioacetic acid mixture (see section Ill-D). This reaction has been carried out with various tricyclic diterpenes and is best illustrated by the conversion of dehydroabietic acid into its 12,14-d2-labeled analog (40 -+ 41).Amalgamated zinc is reportedly necessary for the exchange reaction since the results are less satisfactory when a zinc chloride-mercuric chloride mixture is used. 

A solution of estradiol (38, 15 mg) in methanol-OD (4 ml) and one drop of 10% deuteriosulfuric acid in deuterium oxide is heated under reflux for 5 days. After cooling the reaction mixture is diluted with ether, washed with dilute sodium bicarbonate solution and water, then dried over anhydrous sodium sulfate. Evaporation of the ether gives crystalline 2,4-d2-estradiol (39, 15 mg, 99%), mp 173-175° (ether-hexane), exhibiting 82% isotopic purity and only one aromatic hydrogen by NMR. (For an experimental procedure describing the exchange of aromatic protons under Clemmensen conditions, see section III-D.)... 

After completion of the reaction, the mixture is diluted with water, extracted with ether and the residue from the ether phase purified by chromatography and/or recrystallization. If the substrate contains aromatic protons, the reduction procedure is repeated in protic medium to back exchange deuteriums incorporated into the aromatic ring. 

The H-NMR spectra of compound 71a in DMSO-de showed the presence of a signal at 12.5 ppm corresponding to the exchangeable NH proton, the ethylenic proton as a singlet at S 5.6 ppm, and the aromatic protons appear between 7.27 and 7.80 ppm. The elemental and spectral analysis was in agreement with the structures of these compounds. 

Note that deuterium exchange of the -OH leads to incorporation of deuterium alpha to the carbonyl in the ketone form. This may happen, even if there is no evidence of any enol signals in the spectrum initially, i.e., it can occur even when the equilibrium is heavily in favour of the ketone. Aromatic protons of rings which bear two or more -OH groups are also prone to undergo slow exchange, as are nitrovinyl protons. 

Rusanov, A.L., Likhatchev, D., Kostoglodov, P.V., Mullen, K. and Klapper, M. Proton-Exchanging Electrolyte Membranes Based on Aromatic Condensation Polymers. Vol. 179, pp. 83-134. 

This involves rate-determining proton transfer, equation (37) in principle it should show general acid catalysis, but in practice this usually cannot be seen as the catalyzing acid is simply H30+ . A typical example would be aromatic hydrogen exchange, such as the detritiation of tritiated benzene shown in equation (38) 147... 

The NMR spectrum shown in Figure 5 was obtained by dissolving hydralazine hydrochloride in deuterium oxide containing 3-(trimethylsilyl)-1-propane-sulfonic acid sodium salt (DSS). The series of peaks at 0, 0.6, 1.8, and 3 ppm are all due to the DSS. The peak at 4.8 ppm is due to HDO which forms on exchange with the solvent and the peaks at 8.01 and 8.61 ppm are due to the aromatic protons. The NMR spectrum of the base (Figure 6) was obtained in a 1 1 mixture of dimethylsulfoxide-d,- deuterochloroform. 

LA represents Lewis acid in the catalyst, and M represents Bren sled base. In Scheme 8-49, Bronsted base functionality in the hetero-bimetalic chiral catalyst I can deprotonate a ketone to produce the corresponding enolate II, while at the same time the Lewis acid functionality activates an aldehyde to give intermediate III. Intramolecular aldol reaction then proceeds in a chelation-controlled manner to give //-keto metal alkoxide IV. Proton exchange between the metal alkoxide moiety and an aromatic hydroxy proton or an a-proton of a ketone leads to the production of an optically active aldol product and the regeneration of the catalyst I, thus finishing the catalytic cycle. 

A fundamentally different approach to the synthesis of 3-pyrrolines is evidenced in the annulation in Eq. 13.50 [58]. Ethyl 2,3-butadienoate 150 reacts with N-sulfony-limine 151 in the presence of triphenylphosphine under very mild conditions to give JV-protected 3-pyrroline 152 in 90% yield. The mechanism that has been postulated is related to that of the Baylis-Hillman reaction. Michael addition of triphenylphosphine to the allenyl ester generates a zwitterion that combines with the imine to give 153 in a non-concerted process. This is followed by ring closure, proton exchange and expulsion of triphenylphosphine to give 152. This annulation is successful only for aromatic or cinnamyl imines [59]. 

Fig. 6.6 Schematics of hydrogen bonds between purine and pyrimidine bases with indicated trans-hydrogen bond scalar interactions and related coupling constants, which can be measured using NMR. In addition to correlations between exchangeable protons and nitrogens, also a relayed transfer to nonexchangeable aromatic protons, shown by a dashed arrow, can be employed. The...

N.M.R. Spectra and exchange processes The fact that aromatic substances are relatively weak bases always requires the use of strong acids. However, proton exchange processes take place in such media, and these can interfere to an extraordinary extent with this particular method of measurement. 

Shobha, H. K., Sankarapandian, M., Glass, T. E. and McGrath, J. E. 2000. Sulfonated aromatic diamines as precursors for polyimides for proton exchange membranes. Abstracts of Papers of the Americttn Chemical Society 220 11278-11278. 

Wholly aromatic polymers are thought to be one of the more promising routes to high performance PEMs because of their availability, processability, wide variety of chemical compositions, and anticipated stability in the fuel cell environment. Specifically, poly(arylene ether) materials such as poly-(arylene ether ether ketone) (PEEK), poly(arylene ether sulfone), and their derivatives are the focus of many investigations, and the synthesis of these materials has been widely reported.This family of copolymers is attractive for use in PEMs because of their well-known oxidative and hydrolytic stability under harsh conditions and because many different chemical structures, including partially fluorinated materials, are possible, as shown in Figure 8. Introduction of active proton exchange sites to poly-(arylene ether) s has been accomplished by both a polymer postmodification approach and direct co-... 

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