Conjugated hydrocarbons, aromaticity Diels-Alder reactions

The addition reactions take place at a carbon-carbon multiple bond, or carbon-hetero atom multiple bond. Because of this peculiarity, the addition reactions are not common as the first step in pyrolysis. The generation of double bonds during pyrolysis can, however, continue with addition reactions. The additions can be electrophilic, nucleophilic, involving free radicals, with a cyclic mechanism, or additions to conjugated systems such as Diels-Alder reaction. This type of reaction may explain, for example, the formation of benzene (or other aromatic hydrocarbons) following the radicalic elimination during the pyrolysis of alkanes. In these reactions, after the first step with the formation of unsaturated hydrocarbons, a Diels-Alder reaction may occur, followed by further hydrogen elimination ... 

Conjugated diynes can function as dienophiles in Diels-Alder reactions, and adducts have been obtained in which one or both of the triple bonds participates. Most of the studies have involved the use of substituted cyclopentadienones, e.g. tetraphenylcyclopentadienone ( tetracyclone ), which form adducts that undergo decarbonylation to give aromatic hydrocarbons. Butadiyne itself reacts with tetracyclone to give hexaphenylquaterphenyl (221) while diphenylbutadiyne gives the mono(222)- and di(223)-adduct . Bis(4-biphenylyl)butadiyne gives the monoadduct 224 . 

Theoretical quantitative treatments of cycloadditions mainly concern the Diels-Alder reaction. A possible approach is that of calculating the para-localisation energy, that is the variation of Tr-electron energy of the conjugated diene system, when two Tr-electrons are localised upon the atoms, in 1,4-relation to each other, which must form the new tr-bonds. This has been done by Brown , using the molecular orbital method some successful predictions of reactivity and of the positions of addition for polycyclic aromatic hydrocarbons and polyenes were made. This method has also been used by other authors - . 

The majority of cycloaddition reactions involve interactions between two (occasionally three) n systems as in the Diels-Alder reaction [equation (5.284)] or the analogous additions of allyl cations to dienes (148)-(150). Such n cycloadditions usually take place by cis addition to each conjugated system because the corresponding transition states are less strained. The reactions are then of Hiickel type (see, e.g.. Figs. 5.35a and 5.36a) and follow the same rules for aromaticity as do ordinary conjugated hydrocarbons. Some examples follow ... 

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