DETERMINATION OF THE RELATIVE RATES OF FORMATION, FATES, AND STRUCTURES OF TRIPLET 1,4-BIRADICALS GENERATED IN THE PHOTOCHEMICAL CYCLOADDITION REACTIONS OF 2-CYCLOPENTENONES WITH 2-METHYLPROPENE

The structures and relative rates of formation of the isomeric triplet 1,4-biradical intermediates generated in the photocycloaddition react ions between 2-methylpropene and each of 2-cyclopentenone, 2-methyl-2- cyclopentenone, and 3-methyl-2-cyclopentenone were determined. This wa s accomplished by using hydrogen selenide as a hydrogen atom donor to trap quantitatively the 1,4-biradicals formed in each reaction, The qu antum yields of cycloadduct formation in the photocycloaddition reacti ons were measured as a function of alkene concentration. For each reac tion the relative rates of formation of the biradicals and the quantum yield data were combined to determine quantitatively how each biradic al partitions between closure (or disproportionation) to product and f ragmentation to ground state enone and alkene. It is concluded that th e regiochemistry of the enone-alkene photocycloaddition reactions stud ied is dominated by the manner in which the biradical intermediates pa rtition between products and ground state precursors and not by the re lative rates at which they are formed. It is also concluded that methy l substitution at the 3-position of cyclopentenone has little effect o n either the relative rates of formation of the various isomeric birad ical intermediates or the manner in which they partition between produ cts and starting materials. However, methyl substitution at the 2-posi tion of cyclopentenone slows formation of biradicals in which the alke ne is bonded to the enone 2-position and also inhibits closure to cycl obutane products of biradicals formed by bonding of the alkene to the 3-position of the enone. These results can be rationalized if it is as sumed that the enone triplet excited state possesses a planar carbon a t the 2-position and a pyramidalized carbon at the 3-position, and if it is also assumed that in the biradicals, radical centers at the cycl opentanone 2-position are planar and at the 3-position are pyramidaliz ed.