Pincer-Ruthenium-Catalyzed\nDirect Formation of Fuel-Grade\nAlkanes via a Net-Decarboxylative Coupling of Alcohols

The net-decarboxylative coupling of low-molecular weight\nalcohols\nto high-molecular weight alkanes has been investigated using a series\nof NNN pincer-Ru catalysts based on bis(imino)pyridine and 2,6-bis(benzimidazole-2-yl)pyridine\nligands. Notably, a majority of the considered pincer-Ru complexes,\nincluding the Ru precursors, were either not very active or were unselective\ngiving alkene/alkane mixtures. However, in the presence of 0.5 equiv\nof NaOH in toluene at 140 °C, the complex (<sup>MeBim2</sup>NNN)RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub> based on the 2,6-bis(benzimidazole-2-yl)pyridine\nligand demonstrated very high activity giving up to 91% yield with\n100% selectivity toward the alkane (1,3-diphenyl propane) starting\nfrom 2-phenyl ethanol after 24 h of reaction. On the other hand, the\ncomplex (<sup><i>i</i>Pr2</sup>NNN)RuCl<sub>2</sub>(PPh<sub>3</sub>) based on the bis(imino)pyridine ligand was found to be the\nleast active and gave 14% 1,3-diphenyl propane at 25% selectivity.\nExperimental mechanistic studies point to the evolution of hydrogen\n(detected by GC) and formic acid (detected by <sup>1</sup>H NMR) during\nthe reaction along with the involvement of organic intermediates such\nas α,β-unsaturated aldehydes. The [(<sup>MeBim2</sup>NNN)RuCl(PPh<sub>3</sub>)<sub>2</sub>]Cl catalyzed transformation of 2-phenyl ethanol\nto 1,3-diphenyl propane demonstrated a first-order dependence of the\ninitial rate on the concentration of both the catalyst and the base.\nWhile catalyst poisoning experiments with Hg revealed the homogeneous\nand well-defined molecular nature of the catalyst, a few of these\nmolecular species, including the resting state (<sup>MeBim2</sup>NNN)RuHCl\n(experimentally trapped as its PPh<sub>3</sub> adduct), have been\nidentified by HRMS analysis and NMR studies. DFT studies complement\nthe experimental findings and indicate that in the more favorable\nhydrogenolysis path, the dehydrogenolysis step is rate-determining\n(Δ<i>G</i><sub>140</sub><sup>‡</sup> = 22.81\nkcal/mol), and it leads to the formation of 2-phenyl acetaldehyde\nalong with the resting state (<sup>MeBim2</sup>NNN)RuHCl. On the other\nhand, the corresponding cycle with the least active catalyst (<sup><i>i</i>Pr2</sup>NNN)RuCl<sub>2</sub>(PPh<sub>3</sub>)\nthat involved the insertion of 1,3-diphenyl propene into the Ru–H\nbond as the RDS had a relatively more unfavorable barrier of 27.81\nkcal/mol. This work that provides direct access to jet-fuel-grade\n1,3-diphenyl propane starting from 2-phenyl ethanol in a single-step,\none-pot strategy offers great promise to open up exciting opportunities\nin this very important field of study.