2 research outputs found
Catalytic Transformation of Levulinic Acid to 2âMethyltetrahydrofuran Using Rutheniumâ<i>N</i>âTriphos 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 Îł-valerolactone (ÎłVL)
and 1,4-pentanediol (1,4-PDO). Two different catalytic systems based
on the branched triphosphine ligands Triphos (CH<sub>3</sub>CÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>) and <i>N</i>-triphos
(NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>) were investigated.
The most active catalyst was the preformed ruthenium species [RuH<sub>2</sub>(PPh<sub>3</sub>)Â{NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><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
HNÂ(Tf)<sub>2</sub>. Various acidic additives were assessed to promote
the final transformation of 1,4-PDO to 2-MTHF; however, only HNÂ(Tf)<sub>2</sub> was found to be effective, and NH<sub>4</sub>PF<sub>6</sub> 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 PPh<sub>3</sub> 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: [RuÂ(NCMe)<sub>3</sub>{NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><i>P</i>}] (<b>14</b>) and [Ru<sub>2</sub>(ÎŒ-Cl)<sub>3</sub>{NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><i>P</i>}<sub>2</sub>]Â[BPh<sub>4</sub>] (<b>16</b>)
Synthesis, Characterization, and Reactivity of Ruthenium Hydride Complexes of NâCentered Triphosphine Ligands
The
reactivity of the novel tridentate phosphine ligand NÂ(CH<sub>2</sub>PCyp<sub>2</sub>)<sub>3</sub> (N-triphos<sup>Cyp</sup>, <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 NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub> (N-triphos<sup>Ph</sup>, <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 [RuÂ(COD)Â(methylallyl)<sub>2</sub>] (COD = 1,5-cycloocatadiene) and [RuH<sub>2</sub>(PPh<sub>3</sub>)<sub>4</sub>] gave the tridentate coordination complexes
[RuÂ(tmm)Â{NÂ(CH<sub>2</sub>PR<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><i>P</i>}] (R = Ph (<b>3</b>), Cyp (<b>4</b>); tmm = trimethylenemethane) and [RuH<sub>2</sub>(PPh<sub>3</sub>)Â{NÂ(CH<sub>2</sub>PR<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><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 [Ru<sub>3</sub>(CO)<sub>12</sub>]. Ligand <b>1</b> gave the tridentate dicarbonyl complex [RuÂ(CO)<sub>2</sub>{NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><i>P</i>}] (<b>7</b>), while <b>2</b> gave
the bidentate, tricarbonyl [RuÂ(CO)<sub>3</sub>{NÂ(CH<sub>2</sub>PCyp<sub>2</sub>)<sub>3</sub>-Îș<sup>2</sup><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 O<sub>2</sub>, giving the RuÂ(II) complexes [RuÂ(CO)<sub>2</sub>(OTf)Â{NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><i>P</i>}]Â(OTf) (<b>9</b>) and [RuÂ(CO<sub>3</sub>)Â(CO)Â{NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><i>P</i>}] (<b>11</b>), respectively. Hydrogenation of these complexes
under hydrogen pressures of 3â15 bar gave the monohydride and
dihydride complexes [RuHÂ(CO)<sub>2</sub>{NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><i>P</i>}] (<b>10</b>) and [RuH<sub>2</sub>(CO)Â{NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><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 NH<sub>4</sub>PF<sub>6</sub> in acetonitrile, forming [RuHÂ(CO)Â(MeCN)Â{NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><i>P</i>}]Â(PF<sub>6</sub>) (<b>13</b>), which reacted cleanly with LA to form
[RuÂ(CO)Â{NÂ(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-Îș<sup>3</sup><i>P</i>}Â{CH<sub>3</sub>COÂ(CH<sub>2</sub>)<sub>2</sub>CO<sub>2</sub>H-Îș<sup>2</sup><i>O</i>}]Â(PF<sub>6</sub>)
(<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