Overview of Thermal and Photochemical -Cycloaddition Reactions

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). 

SOMO = Singie occupied molecuiar orbitai LUMO= Lowest unoccupied molecuiar orbital HOMO= Highest occupied molecular orbital 

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