Mechanistic organic photochemistry

Some chemical reactions take place by the action of light. These are called, 'photochemical reactions', or 'photolysis'. Mechanistic organic photochemistry is the aspect of organic photochemistry which seeks to explain the mechanisms of organic photochemical reactions. The absorption of ultraviolet light by organic molecules often leads to reactions. In the earliest days, sunlight was employed, while in more modern times ultraviolet lamps are employed. Organic photochemistry has proven to be a very useful synthetic tool. Complex organic products can be obtained simply. Over the last century and earlier an immense number of photochemical reactions have been uncovered. In modern times the field is quite well understood and is used in organic synthesis and industrially. The utility of organic photochemistry has arisen only by virtue of the available mechanistic treatment; reactions which appear unlikely in ground-state understanding become understandable and accessible in terms of electronic excited-state consideration. Some chemical reactions take place by the action of light. These are called, 'photochemical reactions', or 'photolysis'. Mechanistic organic photochemistry is the aspect of organic photochemistry which seeks to explain the mechanisms of organic photochemical reactions. The absorption of ultraviolet light by organic molecules often leads to reactions. In the earliest days, sunlight was employed, while in more modern times ultraviolet lamps are employed. Organic photochemistry has proven to be a very useful synthetic tool. Complex organic products can be obtained simply. Over the last century and earlier an immense number of photochemical reactions have been uncovered. In modern times the field is quite well understood and is used in organic synthesis and industrially. The utility of organic photochemistry has arisen only by virtue of the available mechanistic treatment; reactions which appear unlikely in ground-state understanding become understandable and accessible in terms of electronic excited-state consideration. One of the earliest photochemical studies dealt with the natural product santonin. In the 19th century it had been observed by Ciamician that in Italian sunlight santonin gave several photoproducts. The structure of santonin was first correctly described by Clemo and Hayworth in 1929. The initial photoproduct obtained from santonin is lumisantonin. As depicted in Eqn. 1, the photoreaction involves a rearrangement. Using steroid numbering, we note that the C-3 carbonyl group has moved to C-2, the C-4 methyl has moved to C-1, and the C-10 carbon has been inverted. A comparatively bizarre example was uncovered by Egbert Havinga in 1956. The curious result was activation on photolysis by a meta nitro group in contrast to the usual activation by ortho and para groups. Over the decades, many interesting but puzzling organic photochemical reactions were discovered that did not proceed by ordinary organic ground state processes. Rather, they arose from the excited states of electrons in the compounds. The real problem was that, at the time, organic chemists were not versed in quantum mechanics and physical chemists were not versed in organic chemistry. Real mechanistic treatments were not possible. Starting in 1961 it was found that one could understand organic photochemical reactions in the context of the relevant excited states.One example is the n-pi* excitation of mono-carbonyl compounds, the simplest being that of formaldehyde. The structure was first described by Mulliken. The three-dimensional representation (top drawing) is simplified in the second line using a two-dimensional representation, which facilitates arrow pushing. In this early research simple Hückel computations were used to get excited state electron densities and bond-orders. The stereochemistry in Scheme 1 is shown three-dimensionally. The Hückel computations revealed that the beta-carbons (i.e. C2 and C5) of the cyclohexadienone ring had a large bond-order. As seen in the scheme a beta-beta bond is formed. Subsequent to this, radiationless decay leads to a zwitterion ground state. The final rearrangement leads to lumisantonin as can be discerned by comparing the three-dimensional drawing with the earlier two-dimensional representation.