Cycloaddition reactions symmetry-forbidden thermal addition

According to the Woodward-Hoffmann rules, thermal supra-suprafacial [2 + 2] cycloadditions are symmetry forbidden. A number of these forbidden processes as well as the allowed reactions of the excited states have been investigated theoretically. In addition, the symmetry-allowed (Jr -Jrj] have been studied. The only example of these reactions to occur thermally under standard reaction conditions is the addition of ketenes to double bonds, because the steric repulsion between the hydrogen atoms prevents the [n -n ] dimerization of two ethylene molecules. ... 

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

On orbital symmetry grounds, the addition of ethylene to ethylene with ring closure (cycloaddition) should be thermally forbidden. If one compares this reaction with the reaction of trimethylene with approaching ethylene and butadiene (Fig.4), it is readily seen that, the A level being below the S level in trimethylene, the behaviour with respect to cycloaddition to olefins is reversed, that is, trimethylene is essentially an anti-ethylene structure. This principle can be generalized for instance (16) ... 

Thermal [2+2]-cycloaddition reactions are less common, but photochemical [2+2]-cycloaddition reactions are very common. This fact can be explained by analyzing these cycloaddition reactions using Woodward-Hofifmann selection rules. In frontier orbital approach, the thermal reaction of two ethene molecules (one is HOMO and other is LUMO) is orbital symmetry forbidden process for its suprafacial-suprafacial [7t s+7t s]-cycloaddition, but a suprafacial-antarafacial [jt s+jt a]-cycloaddilion reaction is symmetry allowed process (Fig. 3.1). It signifies that the cycloaddilion of one two-7t electron system with another two-ji electron system will be a thermally allowed process when one set of orbitals is reacting in a suprafacial mode and other set in an antarafacial mode ( s means suprafacial and a means antarafacial). Thermal [7t s+Ji a]-reactions usually occur in the additions of alkenes to ketenes, when alkene is in the ground state and ketene in the excited state [1] (Fig. 3.2). 

Although thermal [2 + 2] cycloadditions are forbidden as concerted reactions by the orbital symmetry conservation rules the same structural features which promote intermolecular cy-cioadditions will also promote intramolecular reactions. In addition, the proximity between two alkene moieties dictated by the tether length and rigidity would make these processes entropically favorable. A few reports have documented thermal intramolecular cycloadditions to cyclopropenes and activated alkenes. The thermal Cope rearrangement of allylcyclopropenes apparently proceeds by a two-step mechanism in which intramolecular [2 + 2] adducts have been observed.72-73... 

The scheme considered is fully valid also in the case of cation-radical [2 + l]-cycloadditions. These reactions, like the corresponding thermal reactions of the [2 + 2]-cycloaddition of neutral molecules, are forbidden by the orbital symmetry conservation rules. The same calculations [95] have shown that the addition of the cation-radical of ethylene to a neutral ethylene molecule proceeds in an unconcerted and nonsynchronous fashion. Unlike the [2 + 2]-cyclodimerization of ethylene (Sect. 10.1.1), the [2 + l]-cycloaddition involves the formation of an intermediate LIII with the energy barrier calculated for this highly exothermal step being extremely low (1.3 kcal/mol by the MNDO method). A barrier lower still (1.0 kcal/mol) is expected for the step of transformation of LIII into the cation-radical of cyclobutane LIV in which the... 

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