Deuterated model compounds

Cram and Singer also noted a remarkable difference in the solvolysis of the systems 100b and 101 b. On acetolysis of the a-C deuterated model compound 101c the deuterium was found to be scrambled with 46% D recovered in the position (on hydrolysis 26% D was recovered the / position), whereas in the acetolysis and formolysis of 100c the deuterium remained totally undisturbed in the a position. 

Estimating the reactivity of various aromatic nuclei in lignin involves studies to determine the protodedeuteration rates of certain selectively deuterated model compounds. The combined effects of hydroxyl, methoxyl, and side-chain substituents are approximately additive in guaiacyl models while syringyl models show significant deviation from the predicted behavior. The reactivity of the position meta to methoxyl groups is higher while those of the ortho and para positions are lower than anticipated on the basis of combined substituent effects. 

Deuterated Model Compounds and Protodedeuteration Rate Measurements. Protodedeuteration rate studies were outlined for 2,4,6-trideuterophenol and 2,4,6-anisole, 3,5-dideutero- and 4,6-dideutero-guaiacols, 3,5-dideuteroveratrole, 5-deutero-4-hydroxy-3-methoxytoluene, and 2,6-dideutero-4-hydroxy-3-methoxytoluene (6). Using appropriate combinations of acid- and base-catalyzed deuterium exchange reactions, the following deuteroderivatives were prepared in this study 3,5-dideutero-and 4-deutero-2,6-dimethoxyphenols, 4,6-dideutero- and 5-deutero-1,2,3-trimethoxybenzenes, and 2,6-dideutero-4-hydroxy-3,5-dimethoxytoluene. 

Although anti periplanar geometry is preferred for E2 reactions, it isn t absolutely necessary. The deuterated bromo compound shown here reacts with strong base to yield an undeuterated alkene. Clearly, a svn elimination has occurred. Make a molecular model of the reactant, and explain the result. 

Fig. 15 CfH NMR spectra for end-functionalized polymer and model compounds polymer-CH2D (a), n-heptyl-CH2D (b), and 7-CH2D-pentadecane (c). The top two spectra were acquired using the APT pulse sequence to more clearly show the deuterated methyl peak...

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Using single-frequency and noise-modulated resonance and off-resonance proton decoupling, 7] relaxation time measurements, relaxation reagents like Gd (fod)3 and specifically deuterated compounds, all the carbons in retinal isomers, the model compounds a-and /i-ionone, and vitamin A and its isomers [165, 555-557] were assigned. The olefinic ring carbons (C-5 and C-6) could be identified on the assumption that the 13C relaxation times are dominated by intramolecular dipole-dipole interactions with neighboring protons and that the same rotational correlation time characterizes the interactions for both carbons. Consequently the ratio of T/s for C-5 and C-6 can be estimated from eq. (5.1)... 

Both were recorded over 5000 Hz range in deuterated dimethylsulphoxide on FT-80 A-80 MHz NMR spectrometer using tetramethylsilane as reference standard. The carbon chemical shift value are assigned on the basis of signal multiplicity, chemical shits and the comparison with the chemical shits of model compounds. 

A model of the deuterated bromo compound shows that the deuterium, bromine, and the two carbon atoms that will constitute the double bond all lie in a plane. This arrangement of atoms leads to syn elimination. Even though anti elimination is usually preferred, it doesn t occur for this compound because the bromine, hydrogen, and two carbons can t achieve the necessary geometry. 

Early experiments with bacteriorhodopsin (228) interpreted the Raman spectrum in terms of an unprotonated Schiff base, forming a charge-transfer complex with a protein functional group (210,212). This interpretation of the Raman data, essentially based on a comparison with the frequencies of model Schiff bases in solution, was criticized by Honig and Ebrey (48), who pointed out that consistency could also be obtained with a protonated Schiff base model. The latter hypothesis was subsequently confirmed by deuteration experiments similar to those described for rhodopsin (229,230), and by Raman spectra in denatured systems (231). In variance with the clear-cut similarity observed between the resonance-Raman spectra of rhodopsin and isorhodopsin, and those of the 11-cis and 9-cis model compounds, respectively,... 

Secondary isotopic effects on the chemical shift of the nucleus X, 6(X2H)-5(X H) (X = C, Sn), observed by SIMPLE NMR [20,39] for several model compounds after partial deuteration of the hydroxyl group, are displayed in Figure 27. 

Almarsson et al. [32] found that the NADH-model compound l-methyl-1,10-dihydroacridan and its deuterated form underwent oxidation by Fe(CN)( in aqueous solution by a sequence of ... 

Like most of the PLP-dependent decarboxylases, these enzymes involve retention of configuration at the a-carbon (Fig. 13). Chang and Snell first observed that histidine decarboxylase catalyzes the conversion of L-histidine to histamine in solvent with the stereospecific incorporation of one deuterium atom from H20 solvent (266). Retention of configuration was tentatively suggested on the basis of a comparison of the optical rotation properties of the deuterated histamine with those of model compounds. This conclusion was later confirmed for histidine decarboxylase from both Lactobacillus 30a and Clostridium welchii by a method that employed diamine oxidase for the configurational analysis of the (a5)-[a- H]histamine resulting from enzymic decarboxylation of (aS)-[of- H]histidine in H2O diamine oxidase catalyzes stereospecific removal of the pro- S) hydrogen at the a-methylene of histamine (267, 2 ) [Eq. (53)] ... 

The possibility that a reaction may involve movement of the double bond along the chain can usually be checked out by the use of deuterated compounds and a study of their spectra, or, perhaps less convincingly, by the use of model compounds. 

Studies both with model compounds and deuterated polymers have shown that, as with the poly butadienes, isomerization is not accompanied by double bond migration. 

Whilst in many cases the structure of these polymers may be established by infra-red, NMR and other standard methods it has been seen how the information may be supported by the use. of model compounds and of deuterated polymers. In the next chapter we shall consider the further application of these and other techniques to elucidate cross-linking mechanisms and cross-linked structures of diene polymers which are less amenable to conventional analysis. 

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