Thermodynamic data cycloaddition

Table 19 summarizes the kinetic and thermodynamic data concerning the [2 + 2]-cycloaddition of acrylonitrile and a-alkylthioacrylonitriles. The presence of the alkylthioether group lowers the activation energy by more than 10 kcal mol-. ... 

From this work, the authors concluded that the reaction of two benzocyclobutene groups with each other is exothermic to the extent of 221.7 12.5 kJ. By comparison, the reaction of one mole of benzocyclobutene moieties with one mole of double bonds in a Diels-Alder fashion is exothermic by 184.1 12.5 kJ. Thus, the thermal dimerization of benzocyclobutenes is thermodynamically favored over the cycloaddition reaction of a benzocyclobutene with a double bond by about 37-38 kJ. By contrast, the authors found that for monomers that could react either by a benzocyclobutene dimerization or by a benzocyclobutene - double bond cycioaddition (e.g. monomer 13 in Table 3) the reaction products appeared to be dominated by the cycioaddition pathway. From this data, it was concluded that the Diels-Alder reaction was kinetically... 

The C-F bonds in 1 fluoroalkenes and fluorobenzenes also are very strong (Table 17), but alkene n bond strengths vary with the level of fluonnation (Table 18) Both CHF=CF2 and CF2=CF2 have significantly weaker n bonds than CH2= CH2, CH2=CHF, and CH2=CF2, consistent with other data indicatmg that tn- and tetrafluonnation thermodynamically destabilize double bonds [75] The low jt bond energy in CF2=CF2 underlies its propensity to undergo thermal biradical [2+2] cycloadditions [703] (see p 767 )... 

Caldwell et al. have also reported a stereoselective [2 + 2] photocycloaddition where the major product was the most thermodynamically stable. Scheme 54 shows that the cyclobutane product with the two aryl groups trans to each other predominates, but it is not the exclusive product. The proposed intermediates include a 1,2-biradical, where the p-orbitals are perpendicular to each other, and a 1,4-biradical intermediate which has time to assiune the most stable conformation before closing. The 1,2-biradical intermediate is supported by rate studies and quenching data, but these studies are not conclusive [41a]. In addition, the possibility of involvement of an exciplex prior to cycloaddition cannot be ruled out based on the studies Caldwell et al. have reported. 

A bis-cyclopeptide covalent capsule 154 for selective binding of sulfate dianion has been prepared in [116] using copper(I)-catalyzed click reaction-cycloaddition of the corresponding azide to alkyne ligand syntones by Scheme 2.103 and characterized using NMR and X-ray diffraction data. Thermodynamics and kinetics of the encapsulation reaction for this guest have been studied by ITC and temperature-depended H- H NOESY NMR methods, respectively. 

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