Tandem [4 + 2]/[2 + 1] Cycloaddition for the Synthesis of High-Energy-Density Cyclopropane Fuels

Abstract

Conventional high-energy-density hydrocarbon fuels face an intrinsic trade-off between increasing ring number and decreasing net heat of combustion. To address this limitation, we propose a synthetic strategy based on tandem [4 + 2]/[2 + 1] cycloaddition. Norbornadiene and methyl dicyclopentadiene were employed as feedstocks to develop a coupled synthetic approach that combines Diels–Alder [4 + 2] cycloaddition for constructing a polycyclic framework with Simmons–Smith [2 + 1] cyclopropanation for energy enhancement. By optimizing the reaction parameters, the Diels–Alder and Simmons–Smith reactions achieved optimal conversions of 77.89% and 90.96%, respectively, with selectivities of 43.54% and 85.82%. The resulting fuel fraction exhibits a high density of 1.060 g cm–3, a volumetric net heat of combustion of 44.85 MJ L–1, and a freezing point below −60 °C. This study demonstrates an effective strategy for simultaneously achieving high density, high volumetric energy content, and excellent low-temperature performance, providing useful guidance for the molecular design and synthesis of next-generation high-energy-density fuels.