Catalytic Transformation of Levulinic Acid to 2‑Methyltetrahydrofuran Using Ruthenium–<i>N</i>‑Triphos Complexes

A series of pre- or in situ-formed ruthenium complexes were assessed for the stepwise catalytic hydrogenation of levulinic acid (LA) to 2-methyltetrahydrofuran (2-MTHF) via γ-valerolactone (γVL) and 1,4-pentanediol (1,4-PDO). Two different catalytic systems based on the branched triphosphine ligands Triphos (CH₃C­(CH₂PPh₂)₃) and <i>N</i>-triphos (N­(CH₂PPh₂)₃) were investigated. The most active catalyst was the preformed ruthenium species [RuH₂(PPh₃)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>5</b>), which gave near quantitative conversion of LA to 1,4-PDO when no acidic additives were present, and 87% 2-MTHF when used in conjunction with HN­(Tf)₂. Various acidic additives were assessed to promote the final transformation of 1,4-PDO to 2-MTHF; however, only HN­(Tf)₂ was found to be effective, and NH₄PF₆ and <i>para</i>-toluenesulfonic acid (<i>p</i>-TsOH) were found to be detrimental. Mechanistic investigations were carried out to explain the observed catalytic trends and importantly showed that PPh₃ dissociation from <b>5</b> resulted in its improved catalytic reactivity. The presence of acidic additives removes catalytically necessary hydride ligands and may also compete with the substrate for binding to the catalytic metal center, explaining why only an acid with a noncoordinating conjugate base was effective. Crystals suitable for X-ray diffraction experiments were grown for two complexes: [Ru­(NCMe)₃{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>14</b>) and [Ru₂(μ-Cl)₃{N­(CH₂PPh₂)₃-κ³<i>P</i>}₂]­[BPh₄] (<b>16</b>)

Synthesis, Characterization, and Reactivity of Ruthenium Hydride Complexes of N‑Centered Triphosphine Ligands

The reactivity of the novel tridentate phosphine ligand N­(CH₂PCyp₂)₃ (N-triphos^Cyp, <b>2</b>; Cyp = cyclopentyl) with various ruthenium complexes was investigated and compared that of to the less sterically bulky and less electron donating phenyl derivative N­(CH₂PPh₂)₃ (N-triphos^Ph, <b>1</b>). One of these complexes was subsequently investigated for reactivity toward levulinic acid, a potentially important biorenewable feedstock. Reaction of ligands <b>1</b> and <b>2</b> with the precursors [Ru­(COD)­(methylallyl)₂] (COD = 1,5-cycloocatadiene) and [RuH₂(PPh₃)₄] gave the tridentate coordination complexes [Ru­(tmm)­{N­(CH₂PR₂)₃-κ³<i>P</i>}] (R = Ph (<b>3</b>), Cyp (<b>4</b>); tmm = trimethylenemethane) and [RuH₂(PPh₃)­{N­(CH₂PR₂)₃-κ³<i>P</i>}] (R = Ph (<b>5</b>), Cyp (<b>6</b>)), respectively. Ligands <b>1</b> and <b>2</b> displayed different reactivities with [Ru₃(CO)₁₂]. Ligand <b>1</b> gave the tridentate dicarbonyl complex [Ru­(CO)₂{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>7</b>), while <b>2</b> gave the bidentate, tricarbonyl [Ru­(CO)₃{N­(CH₂PCyp₂)₃-κ²<i>P</i>}] (<b>8</b>). This was attributed to the greater electron-donating characteristics of <b>2</b>, requiring further stabilization on coordination to the electron-rich Ru(0) center by more CO ligands. Complex <b>7</b> was activated via oxidation using AgOTf and O₂, giving the Ru­(II) complexes [Ru­(CO)₂(OTf)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}]­(OTf) (<b>9</b>) and [Ru­(CO₃)­(CO)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>11</b>), respectively. Hydrogenation of these complexes under hydrogen pressures of 3–15 bar gave the monohydride and dihydride complexes [RuH­(CO)₂{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>10</b>) and [RuH₂(CO)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}] (<b>12</b>), respectively. Complex <b>12</b> was found to be unreactive toward levulinic acid (LA) unless activated by reaction with NH₄PF₆ in acetonitrile, forming [RuH­(CO)­(MeCN)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}]­(PF₆) (<b>13</b>), which reacted cleanly with LA to form [Ru­(CO)­{N­(CH₂PPh₂)₃-κ³<i>P</i>}­{CH₃CO­(CH₂)₂CO₂H-κ²<i>O</i>}]­(PF₆) (<b>14</b>). Complexes <b>3</b>, <b>5</b>, <b>7</b>, <b>8</b>, <b>11</b>, and <b>12</b> were characterized by single-crystal X-ray crystallography