Self-photosensitized [2 + 2] cycloaddition for synthesis of high-energy-density fuels†
Hydrocarbons with a highly strained four-membered ring are synthesized via self-photosensitized [2 + 2] cycloaddition and hydrodeoxygenation using cyclohexene and isophorone as feedstocks, which show high density and high heat values. Photoreaction can take place selectively with isophorone as a self-sensitizer and a reactant. Isophorone/cyclohexene co-cycloaddition has a much faster reaction rate than self-cycloaddition of isophorone, and both DFT calculations and photochemical kinetics show that the energy barrier for the former is much lower than that for the latter. Under optimal conditions, the selectivity of co-cycloaddition and self-cycloaddition can reach 93.3% and 94.9%, with isophorone conversion of 95.4% and 68.8%, respectively. In addition, a series of olefins can undergo self-photosensitized [2 + 2] cycloaddition with isophorone smoothly in high selectivity. After hydrodeoxygenation, hydrocarbons derived from isophorone/cyclohexene mixture and pure isophorone (with an overall yield of 82.1% and 61.3%) have high densities of 0.903 g mL−1 and 0.892 g mL−1, respectively, along with good cryogenic properties. Especially, the hydrocarbons synthesized through photosensitized cycloaddition show considerably higher density and heat values compared with those derived from a common C–C coupling route, which is attributed to the constructed strained cyclic structures. This work provides a new way for photosynthesis of high-energy-density hydrocarbons.