Thermally forbidden conversions

Figure 6.18a gives a schematic representation of the orbital correlation diagram fur the thermally forbidden conversion of one alternant hydrocarbon into another one. The following configuration correlations are easily verified from this diagram ... 

Figure 6.18. Excited-state barriers a) orbital correlation diagram for a thermally forbidden conversion of an alternant hydrocarbon b) the corresponding configuration and state correlation diagram for the case that Ihe HOMO— LUMO excitation does not represent the longesl-wavelength absorption.

The mechanisms of these reactions are not completely understood, although relief of strain undoubtedly supplies the driving force. The reactions are thermally forbidden by the orbital-symmetry rules, and the role of the catalyst is to provide low-energy pathways so that the reactions can take place. The type 1 reactions are the reverse of the catalyzed 2 + 2 ring closures discussed at 5-49, The following mechanism, in which Ag attacks one of the edge bonds, has been suggested for the conversion of 141 to 142.587... 

Valence. Several examples of these thermally forbidden processes, exemplified by the conversion of quadricyclene to norbornadiene (Reaction 15)... 

Photolysis (> 290 nm) induced quantitative conversion of (20c and d) to the crystalline 3-azaquadricyclanes (22c and d) that isomerize in solution at 20° to the azepines (23c and d) at measurable rates. The azepines rapidly dimerize. The thermally forbidden 47 reversal of 22 to 20 was not observed.80 Prinzbach and Eberbach81 have carried out numerous cycloadditions and acid-catalyzed rearrangements of 22c and d and of 23c and d. The 3-azaquadricyclanes 22 have diene properties but, as valence isomerization to azepines is so easy (above), cycloadditions were achieved only with the very reactive dienophiles DMAD and EP. Reactions with 22c and d at -15° to - 30° over 24 to 48 hours using a large excess of ester gave 24 (R = H or E). If equimolecular proportions of the quadricyclane and ester are used, the products (24) compete as dienophile for example, 24d (R = H) added to 20d to give 25.81... 

Disrotatory opening of Dewar benzene, which would produce benzene, is thermally forbidden. Therefore, even though the conversion of Dewar benzene to benzene is quite exothermic, it might indeed prove possible to isolate Dewar benzene because there is no low-energy pathway for its conversion to benzene. 

A more detailed consideration of the Woodward-Hofimann postu-ulates for olefinic systems in the presence of a transition metal indicates that the thermally forbidden dimerization of two ethylene molecules to cyclobutane becomes allowed if the orbitals of the olefins can interact symmetrically with the dxt and dyz orbitals of the transition metal catalyst (53). One would consequently also expect transition metal complexes to catalyze the conversion of quadricyclene (IV) back to norbornadiene. This has been reported to be the case (54). The reactions leading to the formation of VI, XXX, and XXXI are examples of processes in which thermally allowed sigmatropic reactions become subject to catalysis by transition metal complexes. The catalysts thus display the dual role of removing symmetry restrictions and of generally lowering activation energies. 

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

Dicyano-l,2-dithiete is an intermediate in the oxidation of the dianion of cis-2,3-dicyano-2,3-dimercaptoethylene 530 and related compounds.1,2-Dithietes are believed to be in equilibrium with 1,2-dithiocarbonyl compounds, as exemplified by 531. Several of these unstable 1,2-dithietes are observed by their electronic spectra at 77°K. Theoretical considerations indicate that the photochemical conversion of an a-dithione into a 1,2-dithiete is allowed the thermal process is forbidden. ... 

Tike the thermal 1,3-hydrogen shift, a 1,7-hydrogen shift is allowed when antarafacial but forbidden when suprafacial. Because a tt system involving seven carbon atoms is more flexible than one involving only three carbon atoms, the geometrical restrictions on the antarafacial TS are not as severe as in the 1,3-case. For the conversion of Z,Z-l,3,5-octatriene to Z,Z, -2,4,6-octatriene, the E is 20.2 kcal/mol. 

Application of this method to pericyclic reactions led to the generalization that thermal reactions take place via aromatic or stable transition states whereas photochemical reactions proceed via antiaromatic or unstable transition states. This is the case because a controlling factor in photochemical processes is conversion of excited state reactants into ground state products. Thus, the photochemical reactions convert the reactants into the antiaromatic transition states that correspond to forbidden thermal pericyclic reactions and so lead to corresponding products. 

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