NMR evidence

Estimates of spatial information can be obtained from analysis of nuclear Overhauser enhancement, in which the intensity of a resonance is compared with and without the selective saturation of a second resonance. In lysozyme a good agreement between observed and calculated effects is observed [181]. For 34 resonances, all distances were within 0.8 A of those determined by X-ray diffraction. 

The justification for including this topic under new developments is (1) the careful comparison of structures that can now be made following advances in refinement, such as, for example, in the work on insulin and cytochrome c and (2) the advances in NMR methods which have permitted detailed comparative studies on certain proteins that allow an assessment of the compatibility of the structural information obtained by the two methods, These have indicated that more work is required by the crystallographers in the description of intermolecular contacts and their possible importance. 

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Acids are poor catalysts for ring cleavage of thiirane 1,1-dioxides but are good catalysts for reactions of thiirane 1-oxides with nucleophiles. These reactions of episulfoxides are believed to proceed by protonation of the oxygen atom (but see the NMR evidence cited above for 5-protonation in fluorosulfonic acid) and will be treated in the section on nucleophilic reactions. 

S-Alkylthiiranium salts, e.g. (46), may be desulfurized by fluoride, chloride, bromide or iodide ions (Scheme 62) (78CC630). With chloride and bromide ion considerable dealkylation of (46) occurs. In salts less hindered than (46) nucleophilic attack on a ring carbon atom is common. When (46) is treated with bromide ion, only an 18% yield of alkene is obtained (compared to 100% with iodide ion), but the yield is quantitative if the methanesulfenyl bromide is removed by reaction with cyclohexene. Iodide ion has been used most generally. Sulfuranes may be intermediates, although in only one case was NMR evidence observed. Theoretical calculations favor a sulfurane structure (e.g. 17) in the gas phase, but polar solvents are likely to favor the thiiranium salt structure. 

The precedent is strong for the involvement of oxetanes as Intermediates in carbonyl additions to pyrroles. " NMR evidence has been obtained far an oxetane adduct of acetone and N-methylpyrrole. The initial photoadduct was shown to rearrange readily on workup to the 3-(hydroxyalkyl)pyrrole derivative. 

Under certain conditions surface catalytic deuterations can lead to the exchange of benzylic hydrogens. An example in the steroid field is the exchange of the benzylic hydrogens in estrone methyl ether (42) with deuterium in the presence of palladized charcoal." " According to mass spectrometric analysis, the product (43) contains three deuteriums (83 %), which have been assigned to the 6- and 9a-positions on the basis of NMR evidence." " ... 

Novel structure and bondmg such as Xe(CF3)2, with the first repotted Xe-C bond Although this compound is unstable, its stmcture is supported by ample NMR evidence [119]... 

Wang, Y X., Freedberg, D. I., Yamazaki, T, et al., 1997. Soludon NMR evidence diat the HIV-1 protease catalytic aspartyl groups have different ionization. states in the complex formed with die a.symmetric drug KNI-272. 

Tliere is ample NMR evidence for the existence of the covalent 4-amino-dinitro compounds (87a/ 87b) and (90b) with liquid ammonia (see Chapter 5). In the cases of 87c or 87d, besides the covalent 4-amino-temperature dependent (85JHC761). For example, when 87c was dissolved in liquid ammonia at -45°C the ratio of 4-amino-room temperature it changed to 40 60. However, 87d, when dissolved in liquid ammonia at -45°C, gave a mixture of 4- and 5-amino-[Pg.304]

X-ray diffraction has not located the hydrides, but NMR evidence favours a structure of the type (Figure 2.98) with one ( 72-H2) ligand. 

In another investigation (Loewenschuss et al., 1976) dediazoniation was studied in TFE and in acetonitrile in the presence of pyridine. There is UV and NMR evidence for the formation of a diazopyridinium cation in addition, -CIDNP absorption and emission signals were observed. Systems containing diazonium salts and pyridine are important in industrial chemistry, as pyridine is used as a proton acceptor in the diazo coupling reaction (see Sec. 12.8) in a considerable number of syntheses of azo dyes. At the same time pyridine has an unfavorable effect on the yield because of the competing homolytic dediazoniation. 

Zirconium porphyrin dicarboxy lates have been shown to catalyze the stereo- and regioselective ethylalumination of alkynes. Although little is known about likely reaction intermediates, NMR evidence forethy Izirconium complexes was observed for the products of the reaction of Zr(TPP)(02C-t-Bu)2 with AIEt. attributed to Zr(TPP)(02C-r-Bu)Et and [Zr(TPP)(/<-02C-/-Bu)(/t-Et) AIMe2." ... 

Blanco FJ, Jimenez MA, Herranz J et al (1993) Nmr evidence of a short linear peptide that folds into a beta-hairpin in aqueous-solution. J Am Chem Soc 115 5887-5888... 

Risbud, S. H., Kirkpatrick, R. J., Taglialavore, A. P. Montez, B. (1987). Solid-state NMR evidence of 4-, 5- and 6-fold aluminium sites in roller-quenched SiOj-Alj O3 glasses. Journal of the American Ceramic Society, 70, ClO-12. 

Fig. 43. H NMR evidence that the reversible reaction between [TpBu Me]ZnOH and C02 is rapid on the NMR time scale ( = internal standard). Reprinted with permission from Ref. (151). Copyright 1993 American Chemical Society.

Fig. 44. 170 NMR evidence for the [TpBut Me]ZnOH-catalyzed exchange of oxygen atoms between C02 and H2l70 in benzene, (a) 170 NMR spectrum of H2170 in benzene, (b) 170 NMR spectrum of solution in (a) to which [TpB"t,Me]ZnOH has been added. Two resonances are observed, one due to H2170 and one due to [TpBut Me]Zn17OH produced by an exchange process, (c) 170 NMR spectrum of solution in (b) to which excess C02 has been added. The 170 label has been completely transferred from H2170 to C02 within a period of minutes at room temperature. Reprinted with permission from Ref. (151). Copyright 1993 American Chemical Society.

Ralph, J. Lapierre, C. Lu, F. Marita, J. M. Pilate, G. Van Doorsselaere, J. Boerjan, W. Jouanin, L. NMR evidence for benzodioxane structures resulting from incorporation of 5-hydroxyconiferyl alcohol into lignins of O-methyl-transferase-deficient poplars. j. Agric. Food Chem. 2001, 49, 86-91. 

A nickel-promoted C—S bond cleavage has been reported,860 which occurs when solutions of the Ni1 complex of (330) are electrogenerated. The product was identified by cyclic voltammetry and spectroscopy as [Ni(C6H4S2)2]2. EPR and NMR evidence suggests a one-electron mechanism, involving reduction to a 19-electron Ni1 species, electron transfer, and concomitant C—S bond cleavage, extrusion of ethylene followed by a further one-electron reduction and extrusion of ethylene sulfide. 

The reaction of 9 (generated thermally from 7 in toluene) with tetraphenyl-cyclopentadienone is more complex. Both the [6 + 2]-cycloadduct 3416), for which an X-ray structure analysis is available, and the [12 + 2]-cycloadduct 3516), whose constitution has been assigned primarily on the basis of H-NMR evidence, are obtained. The two cycloadducts presumably have a common intermediate which, in accord with the general reactivity of 9, should possess betaine character (.31 - 32) it is caused by nucleophilic attack by the carbonyl oxygen atom on the phosphorus of the heterocumulene. Ring closure of the carbanionic carbon atom... 

Every species ranging from small ones with three Si to larger species with 31 Si are observed. The distribution of the silicate species is shown in Figure 2. There are mostly species up to 12 Si with a majority of pentamers and tetramers, whereas the signal intensities for the larger species are relatively weak. This is consistent with the NMR evidence for silicate speciation of oligomers with Si number up to 12 atoms [6],... 

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