Para rule

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 presence of two substituents at C-4 also strongly influences the regios-electivity as shown in the cycloaddition of dienone 13 with isoprene (2) (Equation 3.1). In violation of the para-rule for Diels-Alder reaction, only metfl-adduct was obtained [19,20]. 

The two exceptions to the ortho-para rule are the reactions between an X-substituted diene and an X-substituted dienophile. The frontier orbitals in Fig. 6.27, taken from Fig. 6.22, indicate that the preferred combination, whichever pair of frontier orbitals and whichever diene, 1-substituted or 2-substituted, is taken, will lead to the meta adduct. Neither frontier orbital interaction is between orbitals close in energy (> 11 eV in all combinations), with the result that the reaction can be expected to be very slow, and is in practice exceedingly rare. One example is the combination of the (vinylogous) 2-X-substituted diene 6.158, derived from the benzcyclobutene by electrocyclic ring-opening, and ethyl vinyl ether 6.159. Although not usefully regioselective, the reaction does take place, assisted by the aromatisation of the diene, and the meta adduct is the major product. 

In fact, because most Diels-Alder reactions proceed by the reaction of nucleophilic dienes with electrophilic dienophiles, the following rule can be formulated as follows Diels-Alder reactions proceed to put the most electron donating substituent on the diene and the most electron withdrawing substituent on the dienophile either ortho or para to one another. This ortho-para rule misuses the terms ortho and para, which really apply only to benzene rings, but it allows one to remember the regioselectivity of these reactions fairly easily. 

Sometimes, the ortho-para rule doesn t work well, especially when two substituents on the diene have competing directing abilities. For example, a PhS substituent at Cldiene s more strongly directing than a MeO substituent at C2diene-By resonance arguments, the opposite should be the case, as MeO is a better resonance donor than PhS. 

There are 18 possible combinations of C-, Z- and X-substituted dienes and C-, Z- and X-substituted dienophiles. The ortho adduct is predicted (and found) to be the major product for eight of the nine possible combinations with 1-substituted dienes, and the para adduct predicted (and found) for eight of the nine possible combinations of 2-substituted dienes. The reactions of Z-substituted dienophiles with the 1-X-substituted diene 6.194 (see p. 303) and with the 1-Z-substituted diene 6.208 (see p. 304) illustrate two of these combinations, and here are examples of most of the rest which obey the ortho-para rule. 

The two exceptions to the ortho-para rule are the reactions between an X-substituted diene, with the X-substituent at C-l or C-2, and an X-substituted dienophile. The frontier orbitals in Fig. 6.32, taken from Fig. 6.25, indicate that the preferred combination, whichever pair of frontier orbitals and whichever diene, 1-substituted or 2-substituted, is taken, will lead to the meta adduct. 

Experimental results and theoretical analyses have enabled rational predictions for the regiochemical preferences in Diels-Alder reactions. Known as the ortho, para rule, the strongest electron donating group on the diene ends up either ortho (1,2) or para (1,4) to the strongest electron-withdrawing group on the dienophile in the final cyclohexene product. For example, combination of isoprene (7) with methyl acrylate (8) yields mostly para product 9, while reaction of 8 with 1-methylbutadiene 11 furnishes ortho cyclohexene 12 as the major product. As shown in these cases, formation of mixtures is common under thermal conditions. ... 

The neat resin preparation for PPS is quite compHcated, despite the fact that the overall polymerization reaction appears to be simple. Several commercial PPS polymerization processes that feature some steps in common have been described (1,2). At least three different mechanisms have been pubUshed in an attempt to describe the basic reaction of a sodium sulfide equivalent and -dichlorobenzene these are S Ar (13,16,19), radical cation (20,21), and Buimett s (22) Sj l radical anion (23—25) mechanisms. The benzyne mechanism was ruled out (16) based on the observation that the para-substitution pattern of the monomer, -dichlorobenzene, is retained in the repeating unit of the polymer. Demonstration that the step-growth polymerization of sodium sulfide and /)-dichlorohenzene proceeds via the S Ar mechanism is fairly recent (1991) (26). Eurther complexity in the polymerization is the incorporation of comonomers that alter the polymer stmcture, thereby modifying the properties of the polymer. Additionally, post-polymerization treatments can be utilized, which modify the properties of the polymer. Preparation of the neat resin is an area of significant latitude and extreme importance for the end user. 

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. 

Carbanions of ct-chloroalkyl phenyl sulfones react with nitrobenzenes to effect tlirect nucleophilic replacement of hydrogens located on o and para to the nitro group fEq. 9.24. A very important feature is that VNS of hydrogen usually proceeds faster than conventional SnAt of halogen located In equally activated positions fEq. 9.25. The rule that VNS of... 

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. 

The combination of carbon-centered radicals usually involves head-to-head (a,a ) coupling. Exceptions to this general rule occur where the free spin can be delocalized into a n-system. The classic example involves the triphenylmethyl radical (13) which combines to give exclusively the a-para coupling product (26), Scheme I.8).27 This chemistry is also seen in cross reactions of 13 with other tertiary radicals.146... 

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