| Nature efficiently does photosynthesis through photo-induced electron transfer reactions driven by sunlight. These electron transfer steps ultimately reduce carbon dioxide to carbohydrates and oxidize water to molecular oxygen. Our goal is to develop ways of similarly performing selective oxidation and reduction reactions on organic compounds where the energy required comes from visible light. Visible light provides energy of ca 80-100 kcal/Einstein which is sufficient to produce the excited state of photosensitizers. The excited state of photosensitizers could initiate electron transfer reactions which ultimately lead to photooxidation and photonreduction. Unlike ultraviolet induced photochemical reactions, visible light initiated photochemical reactions would have three major advantages: (1) offering high selectivity and clean reactions; (2) using inexpensive and safe apparatus; and (3) large scale reactions are more feasible, even relying upon sunlight. We investigated the use of visible light induced electron transfer reactions in a variety of organic transformations. First, we sought to promote photosensitized dehalogenation of halogenated environmental pollutants using visible light. Irradiation of a solution containing the organohalide, a photosensitizer dye, and an electron donor gave the dehalogenated organic compound. Complete dehalogenation was achieved by prolonged illumination. Visible light induced electron transfer reactions were also utilized in the deprotection of dithianes, oxathianes, and benzyl ethers. Irradiation of a solution containing the dithiane or oxathiane and a photosensitizer with visible light afforded the unprotected aldehyde or ketone. Similarly, irradiation of a solution containing the benzyl ether and a photosensitizer gave the corresponding alcohol. Excellent yields and selectivity were observed in most reactions. Large scale syntheses were accomplished by the use of black dyes and inorganic salts such as magnesium perchlorate. Finally, we investigated the cleavage of epoxides through an indirect route involving visible light induced electron transfer reactions. The corresponding alcohol with the hydroxyl group on the less substituted carbon was formed in high yields and regioselectivity. The successful synthetic transformations discovered in this study demonstrate the potential utility of dye-promoted photoreactions driven by visible light. |
