High‐Performance Jet Fuels Derived from Bio‐Based Alkenes by Iron‐Catalyzed [2+2] Cycloaddition

A series of high-performance cycloparaffinic fuels have been generated by [2+2] cycloaddition of the bio-derived alkenes 1-hexene, isoprene, and 1-pentene, catalyzed by a low-valent iron pyridine(diimine) complex [(Me PDI)Fe(N2 )2 (μ-N2 )] [Me PDI=N,N'-(2,6-pyridinediyldiethylidyne)bis(2,6-dimethylbenzenamine)]. Reactions with 1-pentene and 1-hexene resulted in 85 % selectivity to 1,2-cyclobutanes, and 12 % selectivity to acyclic alkenes generated by β-hydride elimination. Self-dimerization of isoprene was sluggish and generated heavier oligomer products, but cross-dimerization of isoprene with 1-hexene afforded primarily a 1,3-cyclobutane product, along with isomers of acyclic C11 mixed dimers. Hydrocarbon mixtures were hydrogenated and fractionally distilled to yield finished fuel mixtures in overall yields of 83-93 % at the multigram scale. The fuels exhibited densities ranging between 0.767 and 0.783 g mL-1 , and net heats of combustion (NHOC) of up to 120.6 kBtu gal-1 (43.8 MJ kg-1 ). These values are higher than conventional synthetic paraffinic kerosenes owing to the higher density and ring strain afforded by the cyclobutane rings. The fuel mixtures also exhibited extremely low viscosities ranging from 2.38 to 4.78 mm2 s-1 at -20 °C, due in part to the presence of the acyclic dimers. The excellent fuel properties of the product mixtures, selectivity for dimer products, high yields, and the ability to use simple bio-derived alkenes as substrates, make the [Fe]-catalyzed [2+2] cycloaddition of unactivated alkenes a compelling route to the synthesis of sustainable high-performance fuels.