Effects of anion complexation on the photoreactivity of bisureido- and bisthioureido-substituted dibenzobarrelene derivatives

Abstract

Bisureido- and a bisthioureido-substituted dibenzobarrelene derivative were synthesized and the photoreactivity of two representative examples were studied. Direct irradiation of the ureido-substituted derivative induces a di-π-methane rearrangement to the corresponding dibenzosemibullvalene derivative, whereas the thioureido-substituted derivative is almost photoinert. Complexes of the latter derivative with chloride, carboxylates, or sulfonate anions, however, are efficiently transformed to the dibenzosemibullvalene product upon irradiation, presumably by suppressing the self-quenching of the thiourea units in the complex. The association of the ureido-substituted dibenzobarrelene derivative with (S)-mandelate and irradiation of this complex led to the formation of the dibenzosemibullvalene with moderate stereoselectivity (68:32 er). In contrast, the thioureido derivative showed no such effect upon complexation of chiral anions.

Keywords: (thio)urea derivatives; di-π-methane rearrangement; dibenzobarrelenes; dibenzosemibullvalenes; supramolecular photochemistry.

Scheme 1

Photorearrangements of dibenzobarrelenes 1a and 1b.

Scheme 2

Stereoselective DPM rearrangement of chiral salts in the solid-state.

Scheme 3

Synthesis of ureido- and thioureido-substituted dibenzobarrelene derivatives 1e–i.

Scheme 4

Di-π-methane rearrangements of ureido- and thioureido-substituted dibenzobarrelene derivatives 1h and 1i.

Figure 1

Photometric titration of A) tetrabutylammonium chloride (TBAC) to 1h (c_1h = 50 µM) and of B) tetrabutylammonium (10S)-camphorsulfonate (SCS) to 1i (c_1i = 30 µM). Arrows indicate the changes of the absorption with increasing concentration of the salt. Inset: Plot of the absorbance at 260 nm vs c_TBAC (A) and at 272 nm vs c_SCS (B); straight line represents the fit of the isotherm to a 1:1 stoichiometry.

Figure 2

Structures of chiral additives employed in DPM rearrangements.

Figure 3

Structure of anthracene–thiourea conjugate 4.

Figure 4

Proposed structure of the complex between 1h and mandelate SMD.