Yu Xiuling, Scheller Dieter, Rademacher Otto, Wolff Thomas
Institute of Physical Chemistry, Institute of Organic Chemistry, Institute of Inorganic Chemistry, Technische Universität Dresden, Mommsenstr. 13, D-01062 Dresden, Germany.
J Org Chem. 2003 Sep 19;68(19):7386-99. doi: 10.1021/jo034627m.
Coumarin and 6-alkylcoumarins (alkyl = C(1) to C(16)) were photodimerized in homogeneous solvents differing in polarity and in aqueous micellar solutions. The four possible photodimers, syn head-to-head (hh), anti head-to-head, syn head-to-tail (ht), and anti head-to-tail, were identified through a combination of X-ray analysis and NMR spectroscopy. In 6-methylcoumarin the concentration-corrected dimerization (quantum) yield increases with decreasing concentration of the educt; anti-hh was formed exclusively in nonpolar solvents and upon triplet sensitization and was the main product under all conditions except for ionic micellar systems, which direct to preferred syn-hh dimerization. Long alkyl substituents, however, lead to anti-hh in polar solvents and in micelles, too. Predominating ht dimer formation was observed for nonsubstituted coumarin in polar solvents only. Thus, syn/anti and hh/ht selectivity can be steered by varying the 6-alkyl substituent. Syn-hh photodimers of 6-methylcoumarin can be photochemically split into the monomers; they partly proved thermally unstable against acids, bases, methanol, and on SiO(2) surfaces.
香豆素和6-烷基香豆素（烷基 = C(1)至C(16)）在极性不同的均相溶剂和水性胶束溶液中发生光二聚反应。通过X射线分析和核磁共振光谱相结合的方法，鉴定出了四种可能的光二聚体，即同向头对头（hh）、反向头对头、同向头对尾（ht）和反向头对尾。在6-甲基香豆素中，浓度校正后的二聚化（量子）产率随着反应物浓度的降低而增加；反向hh仅在非极性溶剂中以及三重态敏化时形成，并且是除离子胶束体系外所有条件下的主要产物，离子胶束体系会导致优先形成同向hh二聚体。然而，长烷基取代基在极性溶剂和胶束中也会导致形成反向hh。仅在极性溶剂中观察到未取代香豆素主要形成ht二聚体。因此，可以通过改变6-烷基取代基来控制同向/反向和hh/ht的选择性。6-甲基香豆素的同向hh光二聚体可以通过光化学方法分解为单体；它们部分被证明在酸、碱、甲醇以及在SiO(2)表面上热不稳定。
