Singlet-Oxygen Ene-Sensitized Photo-Oxygenations: Stereochemistry and Mechanisms
I. GENERAL ASPECTS Since its discovery by Scheie and Priestly more than two centuries ago, molecu lar oxygen has moved up and down on the scale of research interest . Until that time, the theory of phlogistron (with negative weight) was developed and used to explain the oxidation of metals and the burning of organic compounds. The fall of this theory was followed by a period of exciting discovery of the role of oxygen in life processes. In the period between 1928 and 1935, Mulliken  interpreted the paramagnetic nature of molecular oxygen as the result of two outer electrons with parallel spins. Childe and Mecke  spectroscopically identified the 1S^ " higher-energy electronic state, and Herzberg  discovered another singlet ex cited state ]Ag. These two excited states ( ’S ^ , ’A^) of the molecular oxygen had an excess energy from the ground state equal to 37.5 and 22.5 kcal/mol, respectively.