Aryl protons

Hydrogens that are directly attached to double bonds (vinylic protons) or to aro matic rings (aryl protons) are especially deshielded... 

FIGURE 13 8 The induced magnetic field of the tt elec trons of (a) an alkene and (b) an arene reinforces the applied field in the regions where vinyl and aryl protons are located... 

The formation of these compounds has been rationalized according to Scheme 6. The reaction of Os (E )-CH=C 11 Ph C1 (C())( P Pr3)2 with n-BuLi involves replacement of the chloride anion by a butyl group to afford the intermediate Os (/i> CH=CHPh ( -Bu)(CO)(P Pr3)2, which by subsequent hydrogen (3 elimination gives OsH ( >CI I=CHPh (CO)( P Pr3)2. The intramolecular reductive elimination of styrene from this compound followed by the C—H activation of the o-aryl proton leads to the hydride-aryl species via the styrene-osmium(O) intermediate Os r 2-CH2=CHPh (CO)(P Pr3)2. In spite of the fact that the hydride-aryl complex is the only species detected in solution, the formation of OsH ( )-CH=CHPh L(CO)(P Pr3)2 and 0s ( )-CH=CHPh (K2-02CH)(C0)(P,Pr3)2 suggests that in solution the hydride-aryl complex is in equilibrium with undetectable concentrations of OsH ( )-CH=CHPh (CO)(P,Pr3)2. This implies that the olehn-osmium(O) intermediate is easily accessible and can give rise to activation reactions at both the olefinic and the ortho phenyl C—H bonds of the... 

The differences in the pmr spectrum previously reported in aqueous media30 are attributed to the solvent used. The proton-proton couplings of the aryl protons are virtually identical, however. 

Johnson and Schubert (56) reported that the H-NMR spectrum of phenacylidene triphenylarsorane showed 20 aryl protons at 8 7.2-8.1 and a single methine proton at 8 4.75. The latter signal was quite broad unless the sample and solvent were perfectly dried, probably because of proton exchange. The I3C-NMR spectra of some arsoranes were also studied (26, 102). The data are listed in Table 1. 

Dimethylbenzene has two types of protons those attached directly to the benzene ring and those of the methyl groups. Aryl protons are significandy less shielded than alkyl protons. As shown in text Table 13.1 they are expected to give signals in the chemical shift range 8 6.5-8.5 ppm. Thus, the... 

A comparison of the spectrum of thalidoxine [65] with that of thalidoxine acetate [69] shows that one of the aryl protons in thalidoxine absorbing at <5 either 6-50 or 6-57 is shifted to lower frequency to 3 6 40 in thalidoxine acetate as a result of the presence of the acetoxyl function. This has led to the suggestion of structure [70] for thalidoxine... 

Figure 3.84 NOE peak intensities upon excitation of the meta-aryl protons of the calixarene cavity...

Fig. 11 VT proton spectra showing dynamic exchange between homo- and heterochiral dimers in the aryl proton region. In a, A shows observed and B calculated spectra b is the Arrhenius plot of the data. This gives a half-life for one dimer of ca. 14 s at 20 °C...

A dimeric capsule can also be formed by two different tetra-urea molecules, and in aprotic solvents a mixture of two tetra-urea derivatives usually contains not only the two homodimers but also the heterodimer in a more or less statistical ratio. This formation of heterodimers is an additional proof of the dimerization [39], which is also valid when other indications (e.g. m-coupled doublets for aryl protons) fail. As an example, sections of the spectra of tetra-tolylurea (1), tetra-hexylurea (3), and of a mixture of the two are shown in Figure 5.3a-c [37a],... 

The experimental KIEs were determined for the aliphatic Claisen rearrangement in p-cymene at 120°C and for the aromatic Claisen rearrangement either neat at 170°C or in diphenyl ether at 220°C. Changes in 2H, 13C or 170 composition were determined for unreacted substrates. For carbon analysis of allyl vinyl ether the C5 carbon was used as an internal standard. The C4 atom and rneta aryl protons were used as references in analysis of allyl phenyl ether. The 170 analysis was based on a new methodology. The results are summarized in Table 1, along with predicted isotope effects calculated for experimental temperatures by means of different computational methods. The absolute values of predicted isotope effects for C4 and C5 atoms varied with theoretical level and all isotope effects were rescaled to get reference effects equal to 1.000. 

The structure of 2 -norfuniferine (19) was established similarly, aided by selective decoupling of the aryl proton resonances and a complete NOE analysis that allowed a phenol function to be placed at C-12 and a methoxyl at C-12, as in... 

The H-NMR spectra of the S,S and S,R 11-11 biphenyl and 8-7 aryl ether-linked dimers show subtle differences that allow both stereoisomeric series to be differentiated. Most obvious is the higher, broader range of chemical shifts of the aryl protons (6.4-7.6 ppm) observed in the spectra of the S,S bases as compared with the corresponding range (6.3-7.3 ppm) found for the S,R substances. The CD spectra of these alkaloids are complex, but a positive extremum can always be observed near 220 nm for the S,S dimers and a negative one for their S,R counterparts. A more readily accessible criterion is provided by the magnitude of the specific rotation of these compounds in chloroform, which is around 40° for the S,S and about 190° for the S,R alkaloids. Application of these rules to a number of other bases of this type allowed their absolute configurations to be established (31). 

Substituent Sydnone proton Chemical shifts (5, p.p.m.) 3-Alkyl 4-Alkyl protons protons Aryl protons... 

Oxidation of the porphyrin 7r-system of iron(IH) (45) and iron(IV) tet-raarylporphyrins (8) leads to large chemical shifts for the meso-aryl protons. This is a result of spin density on the raeso-carbon, which is delocalized into the aryl rings via a 7r-spin delocalization mechanism (50). Accordingly, the chemical shifts of ortho-, meta-, and para-protons have alternating signs. Coupling of the porphyrin spin to the metal spin in both a ferro- and antiferromagnetic manner has been demonstrated (45). 

The large paramagnetic contribution to the chemical shifts for the meso-aryl protons is consistent with the presence of a porphyrin 7r-radical species (45). The shift pattern for these signals (o-H upfield, m-H and p-CH3 downfield) is similar to that observed for (TPP )Fein(Cl04)2 (o-H, p-H upfield, m-H downfield), which exhibits ferromagnetic coupling between the porphyrin 7r-radical spin and the metal spin (45). The small... 

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