Alkenes from cyclobutane reversions

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

Although not strictly correct from a mechanistic viewpoint, the course of alkene metathesis can be easily understood and the products predicted by considering the reversible formation and cleavage of the hypothetical cyclobutane 16 from alkene 6 to give the alkenes 11 and 15. The whole process involves the cleavage of double bonds and the formation of new double bonds. Such a process cannot be realized by any other means. The cyclobutane 17 is nonproductive. 

The mechanism of the enone-aikene [2+2] photocycloaddition presumably follows the scheme below. Upon irradiation 1) a triplet exciplex is irreversibly formed from the triplet enone and ground state alkene 2) the triplet exciplex collapses to one or more 1,4-biradicals. 3) the biradicals either cyclize to the cyclobutane or revert to starting materials and 4) the biradical reversion decreases the overall efficiency of the process. 

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