Upgradation of n -Butanol to Jet-Fuel-Grade Butyl-butyrate and Hydrogen Catalyzed by Pincer-Ruthenium Complexes

The production of jet-fuel-grade butyl-butyrate from butanol has been investigated using a series of NNN pincer-Ru catalysts. Notably, a majority of the considered pincer-Ru complexes, excluding the Ru precursors, demonstrated good activity toward the upgradation of n-butanol to butyl-butyrate. In particular, in the presence of 10 mol % of KOtBu at 160 °C, the complex [(MeBim2NNN)RuCl2(PPh3)2]Cl (1 mol %) based on the 2,6-bis(benzimidazole-2-yl)pyridine ligand demonstrated very high activity, giving up to 90% yield of butyl-butyrate starting from n-butanol after 72 h of reaction. The reaction proceeded with favorable green metrics (97.3% atom economy, 87.6% atom efficiency, 100% carbon efficiency, and 87% reaction mass efficiency). On the other hand, the complex ((p-OMe-C6H4)2NNN)RuCl2PPh3 based on the bis(imino)pyridine ligand exhibited the lowest activity and gave 40% yield of butyl-butyrate. Mechanistic studies reveal that the catalytic transformation of n-butanol to butyl-butyrate has a first-order dependence of the initial rate on the concentration of [(MeBim2NNN)RuCl2(PPh3)2]Cl, while a second-order dependence on the concentration of n-butanol is observed. The catalytically active species is homogeneous with a well-defined molecular nature; a few of these molecular species, including the resting-state (MeBim2NNN)RuHCl (experimentally trapped as its PPh3 adduct), have been identified by HRMS and NMR analysis. While the observed KIEs of 1.1 and 2.4 in experiments with C4H9OD and C4D9OD, respectively, are suggestive of the involvement of a rate-determining C–H bond activation, the corresponding activation Gibbs free energy is experimentally determined to be 25.3 kcal/mol by Eyring plot analysis. DFT studies further fortify the experimental observations while suggesting that the rate-determining step (RDS) involves the β-hydride elimination from hemiacetal coordinated pincer-Ru species with a barrier of 25.2 kcal/mol leading to the extrusion of butyl-butyrate along with the formation of the resting-state (RS) species (MeBim2NNN)RuHCl. In stark contrast, the catalysis with ((p-OMe-C6H4)2NNN)RuCl2PPh3, which is the least active among the considered catalysts, is kinetically unfavorable (ΔG160‡ = 34.6 kcal/mol). This study not only provides a convenient high-yield synthetic protocol to jet-fuel-grade butyl-butyrate via pincer-Ru-catalyzed dehydrogenative coupling of n-butanol but also provides key mechanistic insights that could leverage the global efforts in designing effective dehydrogenative esterification catalysts for the upgradation of alcohols.