Ortho-protons

The signal at 5m = 6.79 splits into a doublet of doublets. The larger eoupling (8.7 Hz) indieates a proton in the ortho position, the smaller (2.5 Hz) a further proton in a meta position, and in sueh a way that the ortho proton 5m = 7.97) does not show any additional ortho eoupling. 

The methoxy group is a +M substituent, and so shields ortho protons and C atoms in ortho positions the protons at 5 = 6.67 and 6.79 reflect this shielding. The carbonyl group as a -M substituent deshields ortho protons, and is ortho to the proton at <5// = 7.97. With the additional doublebond equivalent for a ring, 6-methoxytetralin-l-one (E) results. 

First the five protons (integral) of the //NMR spectrum (Sfj = 7.50 - 7.94) in the chemical shift range appropriate for aromatics indicate a monosubstituted benzene ring with typical coupling constants 8.0 Hz for ortho protons, 1.5 Hz for meta protons.). The chemical shift values especially for the protons which are positioned ortho to the substituent Sn = 7.94) reflect a -M effect. Using the CH COLOC plot it can be established from the correlation signal hclS = 66.AI7.94 that it is a benzoyl group A. 

Intramolecular cyclization in perfluoroaromanc systems proves useful for the synthesis of heterocyclic compounds [72] For example, the Fischer indole synthesis, which normally requires the presence of an ortho proton, occurs satisfactonly with an ortho fluonne in theperfluoronaphthalene senes [73] (equation 37)... 

Racemic amino acids have been resolved via stereoselective coordination to the square planar chiral matrix complex (178).584 The bisamidobispyridyl ligand (179) forms a square planar Ni11 complex with considerable tetrahedral twist due to repulsion of the ortho protons of the pyridyl rings.585... 

In compound 1, all interproton distances lie in a range which would be expected to give rise to an NOE, as the experiment confirmed. In 3, although the structural formula is very similar, only the distance between the CH proton and the neighbouring ortho protons lies clearly in the NOE range . The others are close to or above 5 A, so that only very small NOEs or none at all could be expected. 

A third useful example is provided by aromatic residues, where the coupling constant between ortho protons is large (xx-xx Hz), while that between meta protons is much smaller (1-3 Hz). The coupling between para protons is often of the same magnitude as the Hnewidth. 

Oxidative additions involving C-H bond breaking have recently been the topic of an extensive study, usually referred to as C-H activation the idea is that the M-H and M-hydrocarbyl bonds formed will be much more prone to functionalization than the unreactive C-H bond. Intramolecular oxidative additions of C-H bonds have been known for quite some time see Figure 2.15. This process is named orthometallation or cyclometallation. It occurs frequently in metal complexes, and is not restricted to "ortho" protons. It is referred to as cyclometallation and is often followed by elimination of HX, while the metal returns to its initial (lower) oxidation state. 

According to isotope studies the rate-determining step of this sequence is the reductive elimination, and all other reactions (C-H activation, insertion of alkene) are reversible. The first indication of this behaviour was the H/D exchange of the ortho proton of acetophenone. Secondly, and perhaps useful for many other systems, was the kinetic isotope effect observed for 13C natural... 

Yamamoto and coworkers used two-dimensional, nuclear Over-hauser effect experiments (NOESY) to determine the proximity of particular protons situated on an included p-nitrophenolate ion to particular protons of a host alpha cyclodextrin molecule. The experiments showed cross-peaks connecting the H-3 resonance of alpha cyclodextrin to both meta and ortho proton resonances of the p-nitrophenolate ion, whereas H-5 of the alpha cyclodextrin gave a cross-peak only with the resonance of the meta proton thereof. As a consequence, it was unequivocally confirmed that the p-nitrophenolate ion is, in solution, preferentially included with its nitro group oriented to the narrow end of the alpha cyclodextrin... 

The anion-radicals from aromatic nitro compounds preserve the second-order axis of symmetry. The analysis of superfine structure of the ESR spectrum of the nitrobenzene anion-radical reveals equivalency of the ortho and meta protons (Ludwig et al. 1964, Levy and Myers 1965). With the anion-radical of nitrosobenzene, the situation is quite different. This was evidenced from the ESR data (Levy and Myers 1965, Geels et al. 1965). Following electron transfer, the bent nitroso group fixes in the plane of the benzene ring to a certain extent. This produces five different types of protons, since both meta and ortho protons become nonequivalent. The nonequivalence of the ortho and meta protons has also been established for the anion-radicals of acetophenone (Dehl and Fraenkel 1963) and 5-methylthiobenzoate (Debacher et al. 1982 Scheme 6.17). 

During transformation from the neutral molecules to the corresponding anion-radicals, the rate of the fragment rotation, relative to one another, decreases. This also results in the nonequivalence of the meta and ortho protons. Thus, 3-acetylpyridine gives two different anion-radicals as conformational isomers (Cottrell and Rieger 1967). In the case of 3-benzoylpyridine anion-radical, the phenyl group rotates freely about the carbonyl center, whereas the rotation of the pyridyl group is limited. The ESR spectrum shows that the spin density in the phenyl ortho positions is half of that... 

Besides Pt and Pd orf/i<9-metalated species, related Rh complexes were also prepared which are shown in Fig. 27. Thus, the reaction of four equivalents of C (PPh3)2 with [ClRh(cod)]2 produces the complex 43 as with the Pt compounds the ortho proton of the phenyl group is removed with formation of the salt (HC PPh3 2)Cl. Addition of two equivalents of PMes to 43 causes a second orf/jo-metallation with loss of cod and H transfer to the metal to give the Rh(lll)... 

Acyltriphenylphosphonium methylides (98) react with 86 in an 8 + 2 cycloaddition to afford intermediate 99. This then loses triphenylphos-phane oxide to yield 100. H-NMR was utilized to elucidate structure 100 Ortho protons in 100 were deshielded R = Ph was taken as support. 

In order to use only a stoichiometric amount of alkyllithiums, a modified sulfonylhy-drazone leaving group containing no acidic ortho protons was required, such as 2,4,6-triisopropylbenzenesulfonyl (Trisyl 94) (equation 33). 

Fluorine acidifies an ortho proton rather more than chlorine or bromine, whose weak directing effects are more or less comparable . Deprotonation of fluorobenzene is feasible in THF, and at temperatures below — 50°C the lithiated species 162 is stable and does not collapse to a benzyne (Scheme 81) ... 

Figure 1.16 H, H NOESY spectrum of 80. The cross peaks indicated by the arrows arise from selective interligand NOE effects, i.e., the ortho protons of the bipyridyl recognize the p-pinene...

Daley and Bergens [124] have used this approach to characterize the 3-D structure of the intermediate [Ru(ii)(alkoxide)(CH3CN)(Binap)]BF4, 84, in their study of an enantioselective catalytic hydrogenation of a ketone. The ortho-protons of the equatorial and axial P-phenyl rings provide the reporter protons. 

Observations on O-phenyl compounds have been mentioned previously (Section III.B.7(f)) in these compounds the phosphorus coupling to the para protons is as great as that to the ortho protons (1966> 75 i967,ii7) which leads to similar conclusions regarding the geometry of such derivatives and the coupling mechanism. 

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