Stereoselectivity of Photocycloadditions and Photocyclizations
In Volumes I and 3 of the series "Molecular and Supramolecular Photochemistry" from 1997 and 1999, respectively, two chapters were published that describe the regio-and stereosclectivity of photocycloadditions, photocyclizations, and other photochemical reactions f I]. The reader might wonder about the motivation to write another at1icle with stronger emphasis on the stereoselectivity of photocyclizations and photocycloadditions. We feel that the fascination of organic photochemistry mainly originates from the combination of well-known ground-state effects (such as solvent or substituent effects, steric, electronic, and stereoelectronic control) and additional effects that originate from the excited-state nature of the substrates and unusual spin multiplicities of reagents or intermediates. This makes the analysis of many photochemical processes more difficult. the design of new reactions tricky, and the predictability of efficiency and selectivity problematic. On the other hand, new mechanistic aspects might lead to an important new dimension in control of stereosclectivity in photochemistry and, as recently stated by Peter Wagner. "stereochemistry is the essence of organic chemistry" [2). Recent investigations of temperature, spin, and substituent effects on the stereochemistry of photochemical processes have shown that at least two con-
cepts have to be taken seriously into account: reactive triplet biradical conformations and the complex kinetic interplay between product formation and deactivation to restore ground·state molecules from reactive intermediates.