Substituent effects on chemical shifts

By using PHIP-NMR studies, various intermediates such as the previously elusive dihydrides of neutral and cationic hydrogenation catalysts, as well as hydrogenation product/catalyst complexes, have already been detected during the hydrogenation of styrene derivatives using cationic Rh catalysts. Information about the substituent effect on chemical shifts and kinetic constants has been obtained via time-resolved PASADENA NMR spectroscopy (DYPAS). 

The chemical shift <5 of a given nucleus in a molecule contains stereochemical information which, however, might often be uninterpretable because the corresponding chemical shift of another stereoisomer is not available for comparison. Thus, the evaluation of an isolated chemical shift is possible only if the contribution of the stereochemical influence can be identified unequivocally among other influences. This is, for example, the case when substituent effects on chemical shifts (SCS substituent chemical shift), instead of chemical shifts themselves, are determined, since some of these values are known to possess a well-defined stereochemical dependence. An SCS is defined as the chemical shift of a given nucleus in a substituted molecule (R—X) relative to the unsubstituted parent compound (R H) ... 

Scheme 2.4 Substituent effects on chemical shifts of bridgehead bicyclic fluorides...

Craik, D.J. and Brownlee, R.T.C. (1983) Substituent effects on chemical shifts in the sidechains of aromatic systems. Prog. Phys. Org. Chem., 14,1-73. 

Thieno[2,3-d] and [3,2-d]ieoeelenazolee. - Bromination has been effected at the 6-position and 4- and 5-positions, respectively and the 1H, 13C and 77Se N.M.R. spectra have been recorded and interpreted. The 1H N.M.R. spectrum was analysed with computer assistance. Substituent effects on chemical shifts were calculated for Me0,Me02C, and Me derivatives. ... 

Nuclear Magnetic Resonance Spectroscopy.—As noted above, conformational analysis of bicyclo[3.3.1]nonanes is still a topic of considerable interest. A variable-temperature n.m.r. analysis now provides the first case in which the boat-chair-chair-boat equilibrium is directly observed in the amines (17) and (18). In a related case, re-examination of the acetal (19) suggests that the preferred conformation involves a chair carbocyclic ring and a boat heterocyclic ring. This conclusion was made by n.m.r. analysis, using lanthanide shift reagents, by a study of nuclear Overhauser effects, and by measurement of relaxation times of protons. Details have been reported for other 3-azabicyclo[3.3.1]nonanes, and the non-additivity of substituent effects on chemical shifts in 9-thiabicyclo[3.3.1]non-2-enes has been analysed. Both and n.m.r. data have been reported for a series of 9-borabicyclo[3.3.1]non-anes and their pyridine complexes. 

Although Christ examined shifts of some substituted nitrobenzenes 35 years ago, only at the beginning of the following decade did systematic studies of the substituent effects on chemical shifts appear in the literature with the first reports of substituted anisoles, acetophenones and benzaldehides. The potential of this probe as a measure of substituent electronic eftects was demonstrated. 

Adcock, W. and Kok, G.B., Polar substituent effects on chemical shifts of aryl and vinyl fluorides a fluorine-19 nuclear magnetic resonance study of some l,l-difluoro-2-(4-substituted-bicyclo[2,2,2]oct-l-yl)ethenes, J. Org. Chem., 50, 1079, 1985. 

Two reports of n.m.r. spectra in the piperidine-piperazine and piperid-4-one series have appeared. The substituent effects on chemical shifts obtained in this work should be of use for future conformational assignments. 

The low field shift of the resonance for the C+ carbon of the 1-phenyl-1-trimethylsilylethyl cation 290 is noteworthy. This is the most deshielded shift of a benzyl cation carbon observed so far. NMR chanical shifts are dependent not only on the charge densities but also upon the substituent effect of the neighbouring groups. Consequently, the chemical shift of the C " carbon is not a good measure of the relative charge in these type of benzyl cations because the substituent effect on chemical shift is different for the substituents R = H, SiMes and CH3. The trimethylsilyl substituent has a large deshielding effect on the chemical shift of an adjacent sp -hybridized carbon. 

Table 2.4 that all these expectations are borne out in the NMR spectra of H-T PP [11] and H-H T-T PP [12], and are consistent with the a-, p-, and y-substituent effects on chemical shifts derived from paraffins. 

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