Selectivity in the Photodimerization of 6-Alkylcoumarins

Abstract

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.

New Insight into Photoalignment of Liquid Crystals on Coumarin-Containing Polymer Films

“…The regioisomerism of the resultant photodimers was positively identified on the basis of proton-NMR signals associated with the cyclobutane ring. 37 The UV-vis absorption spectra of ∼100 nm thick PMMA films containing monomer I, monomer II, and dimers I and II at mole fractions of 0.10, 0.20, and 0.33, respectively, without encountering phase separation detected as crystalline domains of the dopants, are shown in Figure 1 in terms of the extinction coefficient, ) A/lC, where A, l, and C are absorbance, film thickness, and the chromophore concentration in mole fraction, respectively. The mole fraction was calculated on the basis of the unreacted or reacted coumarin moieties and the methacrylate repeat units.…”

Chlorination of aromatic compounds in micellar media: Regioselectivity

“…Hence, it is anticipated that the attack of a strong electrophile such as chlorine at para position of phenol would be suppressed by the non-polar micellar core, while the polar aqueous phase would provide a more favorable locale for the attack of chlorine at the ortho position. Similar effects of orientation on selectivity improvement [24][25][26][27][28][29][30][31][32] and rate enhancement [33][34][35][36][37][38][39][40] in micellar and microemulsion media were also reported.…”

Hydrogen bonded foldamer-bridged biscoumarins: A UV-Vis absorption and fluorescent study of the solvent effect

“…A solution of compound 5 [14] (5.72 g, 21.8 mmol) in acetic acid (15 mL) and concentrated nitric acid (15 mL) was stirred for 3 h and then poured into ice water (100 mL). The formed precipitate was filtered, washed with cold water thoroughly and dried in vacuo and then recrystallized from ethanol to give compound 6 as a yellow solid (5.85 g, 76%).…”