Effect of Reaction Media on Photosensitized [2+2]-Cycloaddition of Cinnamates

Abstract

The outcome of photosensitized [2+2]-cycloaddition reactions of various cinnamates has been compared in different reaction media, including homogeneous organic solutions under inert conditions, degassed water, and aerated physical gels. The reactions were performed under LED blue light (λ_max=455 nm) irradiation and [Ir{dF(CF₃)ppy}₂(dtb-bpy)]PF₆ (1.0 mol%) as photocatalyst. The processes were optimized taking into consideration solvent, gelator, and substrate. Comparative kinetics analyses, as well as the effect of the reaction media on the diastereoselectivity of the process, were evaluated during this investigation. In a number of cases, carrying out the reaction in a less polar solvent, like toluene or highly polar solvent, like water had a tremendous impact on the diastereoselectivity of the process, pointing towards an effect on the stabilization of the putative diradical intermediate in this medium. Moreover, while for reactions run in homogeneous solution oxygen needs to be excluded, no erosion in yield is observed when the photoadditions were run in aerated gel media.

Keywords: [2+2]-cycloadditions; diastereoselectivity; diradicals; photocatalysis; photosensitizers; supramolecular gels.

Figure 1

General scheme of visible light mediated [2+2]‐cycloaddition reaction in different reaction media studied in this work. Ir‐catalyst=[Ir{dF(CF₃)ppy}₂(dtb‐bpy)]PF₆.

Figure 2

LMW gelators selected for this study: N,N′‐bis(octadecyl)‐L‐Boc‐glutamic diamide (G1), N‐((1R,2R)‐2‐undecanamidocyclohexyl)undecanamide (G2), (+)‐(R,R)‐dodecyl‐3‐[2‐(3‐dodecyl‐ureido)cyclohexyl]urea (G3), 1,3 : 2,4‐dibenzylidene‐D‐sorbitol (G4), 4‐(dodecyloxy)‐N‐((4‐(dodecyloxy)phenyl)carbamothioyl)benzamide (G5).

Figure 3

Second order kinetics of the [2+2]‐cycloaddition model reaction in different reaction media. Reaction conditions: Cinnamate 1 a (0.5 mmol), [Ir{dF(CF₃)ppy}₂(dtb‐bpy)]PF₆ (1.0 mol%), solvent (DMF, toluene, CH₃CN or H₂O) (1 mL), LED₄₅₅, RT. Reactions in solution were carried with dry solvents out under strict nitrogen atmosphere, while those performed in gel were performed under aerobic conditions. Rate constants, k, are given in (×10⁻²) M⁻¹ h⁻¹; half‐lives, t_1/2, are given in h. Additional kinetics parameters are available from the authors upon request.

Figure 4

Diastereomeric excess (d.e.) obtained for substrates 1 a–1 f in different reaction media as described in Table 7.