Metal–ligand cooperative activation of nitriles by a ruthenium complex with a de-aromatized PNN pincer ligand

Abstract

The pincer complex (PNN)RuH(CO), with a de-aromatized pyridine in the ligand backbone, is shown to react with nitriles in a metal-ligand cooperative manner. This leads to the formation of a series of complexes with new Ru-N(nitrile) and C(ligand)-C(nitrile) bonds. The initial nitrile cycloaddition products, the ketimido complexes 3, have a Brønsted basic (nitrile-derived) Ru-N fragment. This is able to deprotonate a CH side-arm of the pincer ligand to give ketimine complexes (4) with a de-aromatized pyridine backbone. Alternatively, the presence of a CH group adjacent to the nitrile functionality can lead to tautomerization to an enamido complex (5). Variable-temperature NMR studies and DFT calculations provide insight in the relative stability of these compounds and highlight the importance of their facile interconversion in the context of subsequent nitrile transformations.

Secondary Phosphine Oxides: Bifunctional Ligands in Catalysis

“…the PÀ NÀ N pyridine dearomatized pincer system 36 mentioned above under hydrogenation. [90,91] In this catalyst also an anionic function of the ligand, C-based in this instance, adds onto the electrophilic C-atom of the nitrile and 37 was identified as intermediate (Scheme 17). This is the second system showing analogies between the two bifunctional ligands, a dearomatized pincer and an SPO.…”

Oxa‐Michael Addition to α,β‐Unsaturated Nitriles: An Expedient Route to γ‐Amino Alcohols and Derivatives

“…Conducting the reaction at lower temperature (−30 °C) indeed gave higher conversion (70 %) and allowed isolation of the products  5 c and 5 d in moderate yields (63 % and 40 %, respectively). As reported previously, oxa‐Michael addition to cinnamonitrile was unsuccessful,16 but testing the reactivity of the more activated p ‐CF 3 substituted cinnamonitrile derivative  2 e did form the oxa‐Michael addition product  5 e at −30 °C in 63 % yield. Similarly, 4,4,4‐trifluorobutenenitrile 2 f gave poor conversion at room temperature, but decreasing the temperature of the reaction to −30 °C allowed isolation of the benzyl alcohol addition product  5 f in 40 % yield.…”

“…[14] Conjugate additions to a,b-unsaturated substrates with an itrile as electron-withdrawing group have been studied comparatively little due to their low reactivity as Michael acceptors, [9] but Kobayashi and coworkers recently reportedC u II -catalyzed borylationo ft hese substrates. [15] We decided to test our 'metal-ligand cooperative" nitrile activation strategy [10,16] in the direct conjugatea ddition of water to unsaturated nitriles. Thus, 2-pentenenitrile (2a)w as reacted with water (20 equiv)i ntert-amyl alcohol (TAA, an alcohol that itselfi su nreactive under these conditions) in the presence of 0.5 mol %o f1 (Scheme 3).…”

Parallels between Metal‐Ligand Cooperativity and Frustrated Lewis Pairs

“…In this case the “frustration” of the Lewis acidic and basic sites is enforced by the rigid ligand framework. Otten and co‐workers have previously demonstrated the FLP‐like reactivity of a related Ru‐based PNN system with nitriles, in which the Ru/C combination added in a cooperative fashion across the CN triple bond. Milstein has also noted that the cooperative addition of CO 2 across these pincer systems bears a resemblance to FLPs , …”