Quaternary aromatic

Figure 19 shows the normal (broad-band decoupled), APT and DEPT-135 spectra of model compound 1. Note that in the APT spectrum the solvent (CDC13) is visible, but not in the DEPT spectrum, where the two low-field quaternary aromatic carbons are also absent. 

In the case of the other correlation shown in Figure 13 as an example, the m/cH long-range magnetization transfer occurs across the anticipated three bonds from the H8 doublet to the C6 quaternary aromatic carbon. Next, the nJcc correlation, rather than occurring via two- or three bonds where n = 2 or 3, instead is via an uncanceled [ fee coupling from C6 to Cl as shown by 44. 

An example where Independent, but similar, assignments were obtained occurs for the side-chain carbonyl carbons in polymethylmethacrylate (W) and the quaternary aromatic carbons of poly- C-methylstyrene (17). Both are sp2 carbons otherwise they are in completely different environments. The assignments, as they occur from low to high field, are ... 

Already in 1946, Knox (46JBC699) presented evidence for the ability of a crude preparation of rabbit liver aldehyde oxidase to catalyze oxidation of diverse quaternary aromatic heterocyclic compounds. Later, it was demonstrated that mammalian liver contains an enzyme system that can oxidize N-methylnicotinamide 230 to both the pyridones 231 and 232 (64JBC2027 66BBA556 67JBC1271 67JBC1274). 

Solid-state 13C NMR showed significant linewidth broadening of the quaternary aromatic carbon, methine, and methylene carbons in the linear PS. This is consistent with the internal PS chains in the pores of the zeolite being more confined by their environment than in pure PS (without any zeolite). 

In the aliphatic region of the 10-oxocancentrine spectrum the signal at 20.4 ppm corresponding to C-10 in cancentrine was absent. Both C-9 and C-14 were deshielded as in the model discussed above. These results confirmed the location of the carbonyl at C-10 in oxocancentrine. The assignments made to the quaternary aromatic and carbonyl carbons were considered to be tentative by the authors. 

A combination of DEPT and IGD spectra allows more precise quantitative analysis of, for example, the number of quaternary aromatic carbons. From such results, the degree of carbon-carbon condensation on the aromatic ring can be defined as the number of quaternary carbon atoms that are neither CL nor C4 nor are methoxyl-substituted C3/C5. This kind of information is particularly useful for following condensation reactions during such processes as steam explosion treatment of aspen wood (Robert et al. 1986, Bardet 1987) and kraft cooking (Robert et al. 1984, Gellerstedt and Robert 1987). 

The complexity of the formation of poly(amic acid) was highlighted by Denisov et al. [41], who studied the structures formed by condensation of four different aromatic dianhydrides with both p-diphenylenediamine and benzidine, using solution-state NMR. The authors assigned the complex spectra of the mixtures of poly(amic acid) isomers by comparison with the spectra of model compounds, and the consideration of the well-known effects of substituents on the chemical shifts of aromatic carbons. For example, it was found that the poly(amic acid) formed by reaction of pyromellitic dianhydride with both diamines was 60% in the cis-isomer and 40% in the rra 5-isomer. More complex spectra were obtained for the poly(amic acid)s formed from the asymmetric dianhydrides. In all cases, the proportion of isomers was independent of the type of diamine and depended only on the dianhydride. A correlation was made between the effect of electron density on the relative rates of formation of the respective isomers, and the difference in chemical shifts of the quaternary aromatic carbons attached to the anhydride. 

Hypercrosslinked resins have also been the subject of an NMR study [44] (see Section 3.4). CP/MAS NMR has been used to estimate the degree of crosslinking in a series of these species. The resins were examined in a solvent swollen and a nonswollen state to determine if with swelling in a deuterated solvent it was possible to narrow the line lines further. Determination of the level of crosslinking was carried out by deconvolution of the quaternary aromatic peak at ca. 146ppm (Figure 15.2.39). 

Solid-state MAS NMR spectroscopy has had much success in examining amorphous insoluble polymers [59]. In recent years, however, there has been some debate on the reliability of quantitative data derived from CP experiments [60] and work on fossil fuels in particular has highlighted the problem [61,62]. Undoubtedly, the issues arise in the analysis of polymers as well [63-66]. While CP results in signal-to-noise enhancement and hence reduced accumulation times, carbon atoms present with no proximal protons tend to have their peak intensities reduced relative to other signals. Quaternary aromatic carbons are likely to suffer badly in this respect. The modulation of the dipolar interactions by the motion of some moieties can also introduce quantitative errors [67]. The rotation of the methyl group about its 3-fold axis of symmetry is a good example of this. Single pulse excitation (SPE) [60] however overcomes the problems that are associated with CP,... 

Synthesis of azabicycles containing a quaternary aromatic bridgehead nitrogen, by condensation of a-methylcycloimmonium salts and 1,2-diketones. 

A one-step synthesis of the protoberberine alkaloid karachine (18), employing a presumed Mannich reaction of 4-methyl-2-trimethylsilyloxy-2,4-pentadiene with berberine, has been detailed by Stevens and Pruitt.154 It is plausible that the initial step of the one-flask reaction sequence leading to karachine is, in fact, a [4+ + 2] cycloaddition of the quaternary aromatic salt, berberine chloride, with the terminal electron-rich olefin of the silyloxydiene [Eq. (70)]. 

The great number of quaternary aromatic carbons made it difficult to assign all of them, but careful examination of the H and 13C spectra using HMQC and HMBC with different pulse lengths has led to an unambiguous assignment of all signals (Tables 4 and 5). 

The l3C, DEPT-135, and DEPT-90 experiments showed a nonconjugated ketone at 5 209.8, flanked by two quaternary bridgehead carbons (5 59.5 and 68.9), an enolized 1,3-diketone [5 117.9 and 194.5 (2x)], a methylene carbon at 5 42.7, and a methine carbon at 5 47.0 which, together with a quaternary carbon at 5 50.4, established the bicyclo[3.3.1]nonane system in xanthochymol (138). The DEPT-135 showed five sp2 methines and six aliphatic methylenes. The number of methine carbons, five sp2 and two aliphatic, was confirmed by a DEPT-90 experiment. The carbon and DEPT experiments further indirectly confirmed the AMX 3-spin system with l3C quaternary aromatic resonances at 5 128.7, 145.3, and 151.4, and the number of prenyl groups as four (two isopent-2-enyl and two isopent-3-enyl groups). 

Frequency Decoupling) 13C NMR spectra, recorded in the same conditions, allowed chemical shift identification of the protonated carbons, while the quaternary aromatic carbons were identified through NOE-Time Dependence measurements. These assignments were further supported by the 13C NMR data of the a-dihydrolycorine and the corresponding lactame, which were consistent with those reported in literature [145] for other structurally related naturally occurring compounds [144]. 

Urdahl et al. studied crude oils and silica-absorbed compounds (asphaltenes and resins) using C-NMR techniques (65). It was found that the asphaltenes and resins were enriched in condensed aromatics compared to the whole crude oils. There were strong indications of a long straight-chain aliphatic compound containing a heteroatom substituent which is abundant in paraf-finic oils. There was also reason to believe that this compound was active in the formation of stable water-in-crude oil emulsions. The range from 130 to 210 ppm in the NMR spectra was particularly of interest. This region represents quaternary aromatic and heteroatom-bonded carbons. 

One problem inherent to the HMBC experiment is attaining sufficient digital resolution when very wide Fi spectral widths must be employed to include quaternary aromatic or carbonyl carbons. The same problem occurs with aralkyl alkaloids. Frequently, there may be a number of aromatic carbons separated by a wide region containing few or no responses from a congested aliphatic region. 

In the carbon-13 nmr spectrum all of the individual resonances were revealed and the APT spectrum permitted the observation of fourteen quaternary aromatic carbons, in addition to the protonated carbons anticipated from the H-nmr spectrum. Two carbonyl carbons were observed at 6 185.78 and 180.44, and this information, together with an analysis of the carbon framework requirements, suggested that the central nucleus was a naphthoquinone, substituted by a methoxyl, a methyl, a 4-hydroxy-2(or 3)-methoxyphenyl and a 4-hydroxy-2(or 3)-methoxyphenylmethyl group. Crucial in defining the relative locations of the functional groups was the powerful combination of the nOe difference, CSCM ID and the selective INEPT techniques. 

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