Intermolecular ring current

Figure 8. (a) A representative rotor-synchronized 1H DQ MAS spectrum, (b) A schematic representation showing the positions of the six possible DQ peaks the observed AB and CC peaks (filled circles) imply the proton—proton proximities indicated in (c). As discussed in section VIIB, in this particular case, the inequivalence of the aromatic protons A, B, and C is a consequence of intermolecular ring current effects. (Reproduced with permission from ref 24. Copyright 2000 Elsevier.)... 

Shifts in frequency can occur as a result of intermolecular ring current field eflFects present in the dimer but not in the monomer (8, 9, 10, 11). 

Fig. 8. The intermolecular shielding values (AS) due to the ring current in the N(1)H and N(9)H tautomers of purine (in a plane 3.4 A distant from the molecular surface).

Very recently, an even more directly useful presentation of the results has been produced by a direct evaluation of the intermolecular nuclear shielding values for protons of purines. Figure 8 represents such values (in ppm) due to the ring current in the N(9)H and N(1)H tautomers of purine in a plane 3.4 A distant from the molecular surface of the purine (which is the mean intermolecular distance in complexes involving purines with other conjugated rings). The diagrams for the N(7)H and N(3)H tautomers are very similar to the ones for N(9)H and N(1)H. 

Figure 11-29 Stereo plot of the membrane bound conformation of a SK F96079 10 and b SK F 96464 11, consistent with the intermolecular NOEs and ring current perturbation of chemical shifts, c a hypothetical complex between a DMPC molecule and SK F 96464 11, consistent with fatty acid spin probe data. The structure is according to the authors only intended to be representative and not unique (reproduced from [104] with permission from Elsevier Science, Amsterdam, The Netherlands).

The 2D spectra in solid state direcdy probe intermolecular or intramolecular interactions through correlations between specific sites, mainly produced by homonuclear and heteronuclear dipolar coupfings. Then through these 2D spectra, it is possible to provide a better assignment in complex spectra, obtain high-resolution spectra and retrieve direct information on distance, proximity, bonding, n-n interactions, ring currents, etc. 

Haigh and Mallion used CCI4 as a solvent and measured either several samples of increasing dilution or - where this was not possible - very dilute solutions. Thus solvent effects and intermolecular interactions of solute molecules could be nearly eliminated. Normally these effects are the reason for a somewhat less satisfactory agreement between theory (which deals with isolated molecules) and experiment for hydrogen chemical shifts. Since the chemical shifts can be calculated up to such a precision the calculated chemical shift tensors, especially their out-of-plane components, can be used further to analyze some of the empirical ring current models in more detail (see also Ref. 56 for comparable studies). 

Carbenoid sources other than those derived from diazo precursors for catalytic cyclopropanation reactions are currently limited. Inter- and intramolecular catalytic cyclopropanation using iodonium ylide have been reported. Simple olefins react with iodonium ylides of the type shown in equations 83 and 84, catalysed by copper catalysts, to give cyclopropane adducts in moderate yield127 128. In contrast to the intermolecular cyclopropanation, intramolecular cyclopropanation using iodonium ylides affords high yields of products (equations 85 and 86). The key intermediate 88 for the 3,5-cyclovitamin D ring A synthon 89 was prepared in 80% yield as a diastereomeric mixture (70 30) via intramolecular cyclopropanation from iodonium ylide 87 (equation 87)1 0. 

Recent development of a Ni catalysed alkylzincation of alkynes62 has further expanded the scope of both intramolecular and intermolecular carbozinca-tion. It should be noted, however, that carbozincation may be followed by cross-coupling under the reaction conditions. The current scope of the intramolecular process appears to be limited to five-membered ring formation, and that of the intermolecular version has been limited to phenylethyne derivatives (Scheme 11.18). Further exploration of the scope of this reaction is highly desirable. 

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