Selectivity in the Photodimerization of 6-Alkylcoumarins

Coumarin and 6-alkylcoumarins (alkyl = C1 to C16) 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 SiO2 surfaces.