Solid‐State Photocyclodimerization of 1‐Thiocoumarin (= 2H‐1‐Benzothiopyran‐2‐one)

Abstract

Dedicated to Prof. Huns-Dieter Schurf. TU Aachen, on the occasion of his 65th birthday (3.1V.95) ~ ~~Irradiation (1 > 390 nm) of 2H-1-benzothiopyran-2-one (1) in the solid state affords selectively 6act,6ba, 12bct,12ca -tetrahydrocyclobuta[1,2-c :4,3-c']bis[l]benzothiopyran-6,7-dione (2), the head-to-head ( H H ) cis-cisoid-ci~-dimer, while irradiation of 1 in the solid state using shorter wavelengths (d > 340 nm) affords a mixture of all four cis-fused tricyclic dimers 2-5. These results represent a novel wavelength effect in solid-state photochemistry.

Solvent-Free Organic Synthesis

“…Wavelength dependence effects in the solid-state photodimerization of thiocoumarin 257 have been discovered recently. 102 For example, photoirradiation of 257 at wavelengths longer than 390 nm leads to exclusive formation of the syn-head-to-head dimer 258 in 30% yield, while photoirradiation of 257 in CH 2 Cl 2 gives the anti-head-to-head dimer 259 in 20% yield. In contrast, a mixture of the dimers 258-261 was formed in a 10:11:1:3 ratio by using light of shorter wavelength (340 nm) in the solid-state irradiation of 257.…”

Organic Synthesis without Conventional Solvents

“…Therefore, the crystal modification controls the regioselectivity of the product. [63] The fact that the outcome of photodimerizations is determined by the relative geometry and orientation of the molecules within the crystal, which is controlled by electronic and steric effects as well as hydrogen bonds and π-π stacking, is used for crystal engineering and chemo-as well as enantioselective synthesis. [51,52,55,58] In this solid-state reaction, temperature control is crucial since a thermal transformation from -trans-cinnamic acid to α-trans-cinnamic acid occurs at higher temperature, resulting in a mixture of α-truxillic acid and -truxinic acid.…”

Supramolecular Photodimerization of Coumarins

“…The photodimerization of 25 in the solid state gives a complex mixture of four possible dimers, although when irradiated in CH 2 Cl 2 , (±)- 26 is obtained [ 17 ]. The enantioselective photodimerization of 25 to the optically pure anti -head-head dimer (+)- 26 in the 1:1 inclusion complex of (−) -24 with 25 was also found to proceed in a single-crystal-to-single-crystal manner [ 14 , 15 ].…”