Catalytic Transformation of Levulinic Acid to 2‑Methyltetrahydrofuran
Using Ruthenium–<i>N</i>‑Triphos Complexes
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Abstract
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 (CH<sub>3</sub>C(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>) and <i>N</i>-triphos
(N(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>){N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<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
HN(Tf)<sub>2</sub>. Various acidic additives were assessed to promote
the final transformation of 1,4-PDO to 2-MTHF; however, only HN(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: [Ru(NCMe)<sub>3</sub>{N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}] (<b>14</b>) and [Ru<sub>2</sub>(μ-Cl)<sub>3</sub>{N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}<sub>2</sub>][BPh<sub>4</sub>] (<b>16</b>)
