Self-photosensitized [2+2] cycloaddition for synthesis of high-energy-density fuels
Hydrocarbons with 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 value. The photoreaction can take place selectively with isophorone as the self-sensitizer and reactant. The isophorone/cyclohexene co-cycloaddition has a much faster reaction rate than self-cycloaddition of isophorone, and both DFT calculation and photochemical kinetics show energy barrier for the former is much lower than the latter. Under optimal conditions, the selectivity of co-cycloaddition and self-cycloaddition can reach 93.3% and 94.9%, with isophorone conversion as 95.4% and 68.8%, respectively. In addition, a series of olefins can undergo the 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 density of 0.903 g mL-1 and 0.892 g mL-1 respectively, along with good cryogenic properties. Especially, the hydrocarbons synthesized through the photosensitized cycloaddition show considerably higher density and heat value compared with that derived from common C-C coupling route, attributed to the constructed strained cycles. This work provides a new way for photosynthesis of high-energy-density hydrocarbons.