Structure–Function Relationship in Ester Hydrogenation Catalyzed by Ruthenium CNN-Pincer Complexes

Abstract

A series of six pincer–ruthenium complexes has been synthesized and applied in the catalytic hydrogenation of esters. The ruthenium complexes have the formula Ru­(pincer)­HCl­(CO), where the CNN-pincer ligands feature N-heterocyclic carbene (NHC), pyridine, and dialkylamino donor groups. Through systematic variation of the steric bulk of the NHC substituent and the amine substituents, a clear structure–function relationship emerges. The most active catalysts in this series feature the bulkiest NHC substituent employed, 2,6-diisopropylphenyl. For the dialkylamino group, catalysts substituted with isopropyl or ethyl groups were the most active, while catalysts substituted with methyl groups were significantly less active. The most active catalyst discovered catalyzes the complete hydrogenation of a range of esters at loadings of 0.05–0.2 mol %.

Transition Metal Complexes Supported by N‐Heterocyclic Carbene‐Based Pincer Platforms: Synthesis, Reactivity and Applications

“…Chianese and co‐workers reported the synthesis of a series of Ru pincer complexes 87 (Scheme 29). [44] It should be particularly noted that AgCl was employed in aiming to assist the complete dissociation of PPh 3 , as AgCl is known to react with PPh 3 to form Ag 4 Cl 4 (PPh 3 ) 4 [45] . These Ru II pincer complexes, together with their analogs earlier reported by the same group, were tested for their activity in catalytic ester hydrogenation reactions (Scheme 29).…”

Unexpected CNN-to-CC Ligand Rearrangement in Pincer–Ruthenium Precatalysts Leads to a Base-Free Catalyst for Ester Hydrogenation

“…We have reported a series of CNN–pincer ruthenium catalysts for ester hydrogenation, which differs from those of Song and Sánchez in that the pyridine and NHC rings are directly linked, so that the only available site for ligand deprotonation is the CH 2 linker between the pyridine and NR 2 groups . A detailed study of the effect of ligand structure on catalyst activity showed that activity increased with increasing steric bulk on the NHC substituent . A striking effect of the amine substituent was also observed, as NEt 2 or N i Pr 2 groups gave high activity while an NMe 2 group led to greatly diminished activity.…”

“…The CNN–Ru precursors to the rearranged, CC-chelated complexes described above form catalysts for ester hydrogenation upon activation with NaO t Bu. , Our previous ester hydrogenation experiments were conducted at 105 °C, while the CNN-to-CC ligand rearrangements occur at room temperature. Although phosphine ligands were not added to catalytic reactions in our previous work, it is plausible that a similar ligand rearrangement occurs during catalysis, potentially as a pathway for activation or deactivation.…”

“…Although the addition of a strong base was required for the CNN–Ru precursors, these CC-chelated complexes are active without an additive. The highest turnover number of 990 was observed for Ru-dipp-Et-PCy 3 ; notably, this was the most effective ligand in our previous work employing the CNN–pincer form of these catalysts, with in situ activation by NaO t Bu …”

“…It is especially noteworthy that Ru-dipp-Me-PCy 3 forms a poor catalyst for this transformation, while a simple change of the NMe 2 group to NEt 2 or N i Pr 2 results in high activity. The same effect was observed in our previous work: precatalysts with an NMe 2 group were nearly inactive, while those featuring an NEt 2 or N i Pr 2 group were highly active once activated with NaO t Bu . The observation of the same subtle ligand effect on catalytic activity for both sets of complexes suggests that they may share a common catalytic intermediate ( vide infra ).…”