Ortho ring carbons

Chemical shifts of the methyl protons (t= 7.41-7.64) are in the sequence 3-methyl 6-methyl < 5-methyl 4-methyl and those of the methoxy group (t = 5.82-6.07) are in the sequence of 3-methoxy < 6-methoxy <4-methoxy. The spin-spin coupling constants are J= 5.3-6, J35 = 2.0-3.7, J3a = 0.5-1.0, J45 = 8.0-8.8, 46 = 0.7-1.0, and Jga = 5.8-7.0. Long-range coupling constants between methyl protons and protons attached to the ortho ring carbon are smaller than 1 cps. 

Benzyne also reacts with compounds containing nucleophilic carbon atoms such as enolates and aryl anions. Intramolecular nucleophilic addition to an aryne by the ortho ring carbon atom of another benzene ring substituted... 

The ease with which an orf/to-qninodimethane can be formed is related to the stability of the aromatic heterocycle from which it is derived and to the degree of donble-bond character between the ortho ring carbons. The first of these aspects can be nicely illnstrated by comparing the thiophene 2,3-qninodimethane with its furan counterpart " - the latter is more stable than the former - the thiophene-derived species has mnch more to lose in its formation from an aromatic thiophene (and much more to gain by reacting to regain that aromaticity) than does the latter. 

Similarly, with phenoxide ions, oxygen attack is kineticaUy favored, while attack by the para- or ortho-ring carbons gives thermodynamically more stable adducts [130]. It is worth noting that earlier work was hampered by the competition with phenoxide of attack by hydroxide ions formed from small amounts of water in the solvent. Ambident attack by pyrrolide anions may involve reaction at nitrogen or carbon centers [132]. DMSO usually acts as an inert solvent for these measurements, but attack by its anion, the dimsyl ion, may involve successive reaction at oxygen, sulfur, and carbon centers [133]. 

A methyl group is an electron releasing substituent and activates all of the ring carbons of toluene toward electrophilic attack The ortho and para positions are activated more than the meta positions The relative rates of attack at the various positions m toluene compared with a single position m benzene are as follows (for nitration at 25°C)... 

The unpaired electron in benzyl radical is shared by the benzylic carbon and by the ring carbons that are ortho and para to it as shown by the spin density surface in Figure 11.9. Delocalization of the unpaired electron from the benzylic car bon to the ortho and para positions can be explained on the basis of resonance contributions from the following structures ... 

The pyridopyrimidines discussed in this review are derived by the ortho fusion of the pyridine and pyrimidine rings through ring carbon atoms. There are four such compounds for which the nomenclature and numbering of Chemical Abstracts (1-4) will be used. Alternative names used in the literature are 1,3,8-triazanaphthalene (1), 1,3,5-tri-azanaphthalene (2), 1,3,7-triazanaphthalene or copazoline (3), and 1,3,6-triazanaphthalene (4). There has been no previous review of the... 

It is into the LUMO, the energetically most accessible unfilled molecular orbital, that any further electrons will go. Hence, it may be thought of as demarking the location of positive charge in a molecule. The LUMO in planar benzyl cation is delocalized away from the formal cation center and onto the ortho and para ring carbons, in accord with classical resonance structures. On the other hand, the LUMO in perpendicular benzyl cation remains primarily localized on the benzy lie carbon. Resonance theory suggests that delocalization of the positive charge leads to stabilization. Thus, planar benzyl cation is more stable than perpendicular benzyl cation. 

The latter reveals heavy concentration of positive charge (blue color) on the benzylic carbon and perpendicular to the plane of the ring. This is consistent both with the notion that only a single resonance structure may be drawn, as well as with the fact that the LUMO is localized almost entirely on the benzylic carbon (see discussion earlier in this chapter). On the other hand, planar benzyl cation shows no such buildup of positive charge on the benzylic carbon, but rather delocalization onto ortho and para ring carbons, exactly as suggested... 

---