Second Centennial of Chemistry Award Address: Forbidden Reactions of Triplet
Oxygen
Speaker: Sir Derek Barton, Imperial College of Science and Technology Oxygen Day was climaxed by the award address by Sir Derek Barton on "Forbidden Reactions of Triplet Oxygen." Molecular oxygen exists as a triplet ground state and this fact is considered the reason for its relative unreactivity in addition reactions, for example. Addition to a system with all electrons paired would necessitate a spin inversion at some stage if product in the ground singlet state were to he produced. The electron spin characteristics of these reactions lead to the prediction of a high degree of "forbiddeness" for them. Thus, oxygen does not normally add to alkenes or dienes. However, Dr. Barton reported that his group has found molecular oxygen undergoing addition in a 1.4-manner to 1.3-cyclohexadiene and several other dienes in the presence of certain catalysts. A broad experimental survey indicated that effective catalysts in dark reactions had both an unpaired electron and high electrophilicity. A number of strong electrophiles with all spins paired catalyze a photochemical addition of 0 2 . A possible mechanism proposed for the dark reaction by Dr. Barton included an initial complexation of the odd-electron electrophile with the diene. Oxygen attack could occur after electron transfer from the attacking electrophilic catalyst. I t is noteworthy that 200 years after Priestley's discovery a report was being made of a novel and unpredicted reaction of his "dephlogisticated air." There will still be much for our students todiscover!
14
/
Journalof Chemical Education
