Photochemical Transformation of some 3-benzyloxy-2-(benzo[b]thiophen-2-yl)-4Hchromen-4-ones: A Remote Substituent Effect

Abstract The effect on photochemical transformations of the substituents present remotely from the reaction site in 3-benzyloxy-2-(benzo[b]thiophen-2-yl)-4H-chromen-4-ones has been determined. The structure(s) of the substrates and photoproducts were established by spectroscopic techniques (UV, IR, and NMR). The substituents had profound effects on product yield and distribution. Electron withdrawing groups (EWGs) on the benzenoid moiety of the chromenone nucleus increased the yield of the photoproducts whereas electron donating groups (EDGs) decreased the yield. These results may be attributed to “state switching” of the substituents during excitation. Graphical Abstract


Experimental 2
General 2.1 1 H NMR (300 and 400 MHz) and proton-decoupled 13 C NMR (75.4 and 100.6 MHz) spectra were taken on a Bruker spectrometer using TMS as an internal standard. The infrared (IR) spectra were recorded in KBr pellets on a MB3000 FT-IR with HORIZON MB TM FTIR software from ABB Bomen. Melting points were determined in open capillaries and are uncorrected. The photo-irradiation of the solution of substrates was carried out under nitrogen atmosphere from a 125 W (medium pressure) Hg-vapor lamp using a Pyrex filter. The columns for chromatographic separation were packed in petroleum ether with silica gel and were eluted with a mixture of pet ether and ethyl acetate (99:1). The X-ray crystallographic structure was collected on a Bruker Kappa APEX II diffractometer equipped with a CCDC detector and sealed-tube monochromated MoKα radiation using the program APEX2.

Photo-irradiation of chromenones 5(a-d) 2.5
Photolysis of compound (5a) 2.5.1 A dry methanolic solution (100.0 ml) of chromenone 5a (500 mg) was photo-irradiated with light from a 125 W Hg vapor-lamp in a Photo reactor under an inert nitrogen atmosphere for 30 min. The removal of solvent left a gummy solid, which was chromatographed to yield 6a.

Photolysis of compound (5b) 2.5.2
A dry methanolic solution of 5b (500 mg) on photolysis for 30 min furnished 6b and processed as above.

Photolysis of compound (5c) 2.5.3
A dry methanolic solution of 5c (500 mg) on photolysis for 30 min furnished 6c and processed as above.

Photolysis of compound (5d) 2.5.4
A dry methanolic solution of 5d (500 mg) on photolysis for 30 min furnished 6d and processed as above.
The targets 5(a-d) were synthesized by (i) condensing the 2-hydroxyacetophenones 1(a-d) with benzothiophene-2-carbaldehyde in the presence of NaOH/EtOH [19] followed by (ii) the cyclisation of chalcones 3(a-d) to 3-hydroxychromenones 4(a-d) under Algar-Flynn-Oyamada conditions [20][21][22] and (iii) subsequent alkylation of the latter with benzyl chloride, in the presence of dry acetone, freshly dried K 2 CO 3 and tetra n-butyl ammonium iodide (Scheme 1). The structure of the compounds 6(a-d) were found to be consistent with their spectral parameters (IR, 1 H/ 13 C NMR vide experimental). The yields of all these compounds were in the range of 74-82%.
These chromenones 5(a-d) (absorption maxima (λmax) between 352 and 364 nm in MeOH) were irradiated with Pyrex filtered UV-light. The photolysis of a dry methanolic solution of 5(a-d) with a 125 W medium pressure Hg-vapor lamp under nitrogen produced photoproducts 6(a-d), and structures of these photoproducts were established by their spectral data (IR, 1 H/ 13 C-NMR). Methanol was used as solvent because better yields of photoproducts 6(a-d) were obtained in polar protic solvents (ethanol and methanol) than in polar aprotic solvents (benzene, C 6 H 12 , DMF and CH 3 CN), a result that was in agreement with previous observations [23]. The photoproducts [15] similar to aromatic benzothiophene fused xanthenone, benzyl migrated product and substituted epoxide could not be isolated from the photolysate of these chromenones 5(a-d). These photoproducts were produced in minute quantities, as observed by TLC analysis and NMR spectra of the photolysates (data not shown) but they could not be isolated despite our best efforts.
The MM2 energy minimizations programme [24-26] (Figure 2) was applied to elucidate the stereochemical features of the dihydrogenated photoproduct 6. The various dihedral angles (Φ) and the expected J † values along with the torsional energies of configurations 6(a-d) are tabulated in Table 1. The observed 3 J values found for 6(a-d) are in accordance with the J † values for the energy minimized (MM2) configuration I as in our earlier observations [15] where protons H-5b and H-6 are in trans orientation relative to each other (as opposed to configuration II where the protons H-5b and H-6 are in cis). Such outcomes have been reported previously from our laboratory [27] as well as for the naturally occurring pterocarpans [28][29][30][31] .
A closer look on the effects of different substituents at benzenoid moiety in the present chromones suggests that an increase in electron density by the electron releasing group on the chromenone ring decreases the yield of the dihydro photoproduct (6a, R = Cl and R'= CH 3, yield 40%; 6b, R = H and R'= H, yield 38%; 6c, R = CH 3 and R'= H, yield 36%; 6d, R = -OCH 3 and R'= H, yield 28%). In our earlier studies, the benzothienyl containing chromones with electron withdrawing groups [15] (R = Cl and R'= H) resulted in a 42% yield of aromatic, 1,5-migrated and dealkoxylated photoproducts. This behavior may be attributed due to the fact that electron donating substituents may cause state switching [32][33][34] from n→π * to π→π * that may reduce the hydrogen abstraction capacity of the carbonyl group of pyrones [18].
The photo-transformations of target photochemical substrates 5(a-d) and formation of the angular pentacyclic dihydrogenated photoproduct described above, i.e. 5a→6a, 5b→6b, 5c→6c, 5d→6d, can be envisaged to occur simply through an initial H-abstraction from an -OCH 2group by the excited C=O group of the pyrone moiety to produce a 1,4-biradical [15]    A clo ser look on the effect s o f differ ent subst ituents at benzeno id mo iet y in the present chro mones suggests that an increase in electron densit y by t he   substrates. The photoproducts have been projected to be formed through bond formation between a -ĊH-radical and the C-3'-position of the thiophene ring followed by the concerted sigmatropic 1,5-H shift as reported in our earlier studies [15][16][17] yielding 6(a-d).

Conclusion 4
It may be concluded from the above studies that the electron withdrawing groups (EWG) on the benzenoid moiety of the chromenone favour the formation of dihydrophotoproducts in higher yield than do the electron releasing groups (ERG) due to "state switching". These substituents also had a profound effect on the distribution of photoproduct(s) as the aromatic 1,5-migrated and dealkoxylated photoproducts were not isolated or realised.