Pincer-Ruthenium-Catalyzed Direct Formation of Fuel-Grade Alkanes via a Net-Decarboxylative Coupling of Alcohols

Abstract

The net-decarboxylative coupling of low-molecular weight alcohols to high-molecular weight alkanes has been investigated using a series of NNN pincer-Ru catalysts based on bis(imino)pyridine and 2,6-bis(benzimidazole-2-yl)pyridine ligands. Notably, a majority of the considered pincer-Ru complexes, including the Ru precursors, were either not very active or were unselective giving alkene/alkane mixtures. However, in the presence of 0.5 equiv of NaOH in toluene at 140 °C, the complex (^MeBim2NNN)RuCl₂(PPh₃)₂ based on the 2,6-bis(benzimidazole-2-yl)pyridine ligand demonstrated very high activity giving up to 91% yield with 100% selectivity toward the alkane (1,3-diphenyl propane) starting from 2-phenyl ethanol after 24 h of reaction. On the other hand, the complex (^iPr2NNN)RuCl₂(PPh₃) based on the bis(imino)pyridine ligand was found to be the least active and gave 14% 1,3-diphenyl propane at 25% selectivity. Experimental mechanistic studies point to the evolution of hydrogen (detected by GC) and formic acid (detected by ¹H NMR) during the reaction along with the involvement of organic intermediates such as α,β-unsaturated aldehydes. The [(^MeBim2NNN)RuCl(PPh₃)₂]Cl catalyzed transformation of 2-phenyl ethanol to 1,3-diphenyl propane demonstrated a first-order dependence of the initial rate on the concentration of both the catalyst and the base. While catalyst poisoning experiments with Hg revealed the homogeneous and well-defined molecular nature of the catalyst, a few of these molecular species, including the resting state (^MeBim2NNN)RuHCl (experimentally trapped as its PPh₃ adduct), have been identified by HRMS analysis and NMR studies. DFT studies complement the experimental findings and indicate that in the more favorable hydrogenolysis path, the dehydrogenolysis step is rate-determining (ΔG₁₄₀^‡ = 22.81 kcal/mol), and it leads to the formation of 2-phenyl acetaldehyde along with the resting state (^MeBim2NNN)RuHCl. On the other hand, the corresponding cycle with the least active catalyst (^iPr2NNN)RuCl₂(PPh₃) that involved the insertion of 1,3-diphenyl propene into the Ru–H bond as the RDS had a relatively more unfavorable barrier of 27.81 kcal/mol. This work that provides direct access to jet-fuel-grade 1,3-diphenyl propane starting from 2-phenyl ethanol in a single-step, one-pot strategy offers great promise to open up exciting opportunities in this very important field of study.