Ortho rule

More complete interpretations of Diels-Alder regioselectivity have been developed. MO results can be analyzed from an electrostatic perspective by calculating potentials at the various atoms in the diene and dienophile. These results give a more quantitatively accurate estimate of the substituent effects. Diels-Alder regioselectivity can also be accounted for in terms of HSAB theory (see Section 1.2.3). The expectation would be that the most polarizable (softest) atoms would lead to bond formation and that regioselectivity would reflect the best mateh between the diene and dienophile termini. These ideas have been applied using 3-2IG computations. The results are in agreement with the ortho rule for normal-electron-demand Diels-Alder reactions. ... 

The diastereoselectivity inherent to the Diels-Alder reaction can be seen in most of the examples in preceding reactions. The reaction is not, however, enantioselective since there is no facial control for intermolecular reactions (some facial control is available for intramolecular reactions). The ortho rule, the endo rule (secondary orbital interactions), and steric interactions provide some orientational control but facial control is also required for enantioselectivity. When ethyl acrylate reacts with 2-methyl-1,3-pentadiene, it can approach from the bottom as in 247A or from the top as in 247B. Clearly, the two products (248A and 248B) are mirror images and enantiomers. This lack of facial selectivity leads to racemic mixtures in all Diels-Alder cyclizations discussed to this point. 

The regioselective metalation of arenes at a position neighboring a heterosubstituent has been mentioned above. Deviations from the ortho rule do exist but are scarce. The metal can nevertheless be systematically directed to meta or para positions if one resorts to the deployment of protective groups or similar stratagems. Organometallics enable regiocontrol also in the alkenyl series. The addition of bromine onto the double bond of co-chlorostyrene and subsequent base-mediated... 

These results show that the measured values of H° are close to those calculated from the thiazole value and the increments. That compounds substituted ortho-ortho to nitrogen (2,4-dimethyI and 2,4,5-trimethyl-thiazole) also obey this rule shows that the methyl groups do not interact sterically (150). The same conclusion had been reached in the case of the picolines and lutidines (151). 

The entrance of a third or fourth substituent can be predicted by Beilstein s rule. If a substituent Z- enters into a compound C H XY, both X and Y exert an influence, but the group with the predominant influence directs Z- to the position it will occupy. Since all meta-directing groups are deactivating, it follows that ortho—para activating groups predominate when one of them is present on the benzene ring. 

The term aromatic is used for historical reasons to refer to the class of compounds related structurally to benzene. Aromatic compounds are systematically named according to TUPAC rules, but many common names are also used. Disubstituted benzenes are named as ortho (1,2 disubstituted), meta (1,3 disub-stituted), or para (1,4 disubstituted) derivatives. The C6H5- unit itself is referred to as a phenyl group, and the Cb f5CH2— unit is a benzyl group. 

Exceptions to the ortho para rule have been observed, so the prediction of the regiochemistry is still a stimulating challenge. 

The ortho-para rule is explained by FMO theory on the basis of the orbital coefficients of the atoms forming the cr-bonds. The regiochemistry is determined by the overlap of the orbitals that have larger coefficients (larger lobes in Scheme 1.15). The greater the difference between the orbital coefficients of the two end atoms of diene and two atoms of dienophile, which form the two cr-bonds, the more regioselective the cycloaddition. 

The DD site can be ensured by the chemical shift changes of the BPA signals. The chemical shift differences of the ring proton signals of BPA on the delivery from water to bilayer phases are —0.04 and —0.11 ppm for the ortho and meta sites, respectively. Negative values mean upfield shifts recall the HCS rule. It is concluded that both benzene rings of BPA are trapped in the bilayer from the water phase and the meta site penetrates more deeply into the hydrophobic interior. 

The startling observation made in this work was that meta derivatives were more reactive toward photosubstitution than ortho and para isomers, as shown below for the photoreaction of dimethoxynitrobenzene and methyl amine, in contradistinction to the rules of classical ground state nucleophilic substitution ... 

The ortho/para orientation rule of ground state chemistry appears to be followed in the photosubstitution reactions of nitrobenzene derivatives in liquid ammoniaa40) ... 

Carbanions of a-chloroalkyl phenyl sulfones react with nitrobenzenes to effect direct nucleophilic replacement of hydrogens located ortho and para to the nitro group (Eq. 9.24).38 A very important feature is that VNS of hydrogen usually proceeds faster than conventional SNAr of halogen located in equally activated positions (Eq. 9.25).38 The rule that VNS of... 

Photoexcited aromatic compounds undergo substitution reactions with (non-excited) nucleophiles. The rules governing these reactions are characteristically different and often opposite to those prevailing in aromatic ground state chemistry 501a,b>, in contrast to the well known ortho/para activation in thermal aromatic substitutions, nitro groups activate the meta position in the photochemical substitution, as shown in (5.1) 502). 

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