Selective, Base-Free Hydrogenation of Aldehydes Catalyzed by Ir Complexes Based on Proton-Responsive Lutidine-Derived CNP Ligands

Abstract

Metal catalysts based on ligands containing protonresponsive sites have found widespread applications in the hydrogenation of polar unsaturated substrates. In this contribution, Ir complexes incorporating lutidine-derived CNP (C = N-heterocyclic carbene, NHC; P = phosphine) pincer ligands with two nonequivalent Brønsted acid/base sites have been examined in the hydrogenation of aldehydes. To this end, Ir(CNP)H 2 Cl complexes were synthesized in two steps from the CNP ligand precursors and Ir(acac)(COD). These derivatives react with an excess of NaH to yield the trihydride derivatives Ir(CNP)H 3 , which were assessed as catalyst precursors in the hydrogenation of a series of aldehydes. The catalytic reactions were performed using commercial-grade substrates under neutral, mild conditions (0.1 mol % Ir-CNP; 4 bar H 2 , room temperature) with high conversions and selectivities for the reduction of the carbonyl function in the presence of other readily reducible groups such as CC, nitro, and halogens. Reaction of an Ir(CNP)H 2 Cl complex with base in the presence of an aromatic aldehyde produces the reversible formation of alkoxide Ir complexes in which the aldehyde is bound to the deprotonated pincer framework (CNP*) through the CH-NHC arm of the ligand. These species, along with a carboxylate complex resulting from the Ir mediated oxidation of the aldehyde by water, is observed in the reaction of Ir(CNP)H 3 with benzaldehyde. Finally, investigation of the mechanism of the hydrogenation of aldehydes has been carried out by means of DFT calculations considering the involvement of each arm of the Ir-CNP/CNP* derivatives. Calculations support a mechanism in which the catalyst switches its metal−ligand cooperation sites to follow the lowest energy pathway for each step of the catalytic cycle.

N-Methylation of Amines with Methanol Catalyzed by Iridium(I) Complexes Bearing an N,O-Functionalized NHC Ligand

“…Metal complexes based on lutidine-derived CNC, CNP, and CNN pincer ligands (C stands for an NHC moiety) have also received considerable attention in recent years . Although deprotonation of asymmetric lutidine-derived pincer ligands can take place in both methylene arms, selective deprotonation of one of the arms has generally been observed …”

Catalytic Formal Conjugate Addition: Direct Synthesis of δ-Hydroxynitriles from Nitriles and Allylic Alcohols

“…The reaction of phenyl acetonitrile and trans -cinnamaldehyde with base (KO t Bu, 2 mol %) alone or under the standard condition with catalyst 1a failed to provide the 1,4-conjugate addition product 96 (Scheme i). Perhaps this failure can be attributed to the strong binding of the aldehyde functionality to the metal center and ligand backbone, leading to the deactivation of the active catalytic intermediate. , However, the reaction of 4-phenylbut-3-en-2-one with benzyl nitrile under standard reaction conditions afforded the product 97 in 81% isolated yield. A similar reaction performed without catalyst resulted in no product formation (Scheme ii), and this observation confirms the role of the catalyst in the C–C bond formation.…”

Ammonia–Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNNH Complexes

“…Following previously reported procedures for the synthesis of Ir complexes based on related NHC-containing lutidine-derived ligands, , the diolefin derivatives 2a and 2b were prepared by the reactions of Ir­(acac)­(COD) and the N-heterocyclic carbene ligand precursors 1a and 1b , respectively (Scheme ). These complexes were isolated as yellow solids in good yields (99% and 77%, respectively), and spectroscopical and analytically characterized.…”

“…Therefore, to gain insight into the species formed upon treatment of complexes 4 with a base, derivative 4a was initially reacted with t BuOK (2.5 equiv) in THF- d 8 (Scheme ). Reaction of the Ir-CNN H derivatives with a strong base could induce deprotonation of the pincer ligand at its methylene carbon in the side arm − and/or at the secondary amino group. ,, Analysis of the deuterated solution through NMR spectroscopy showed the formation of a major species (ca. 90%), which was characterized as the highly air-sensitive amido iridate­(III) 5 , confirming the deprotonation of both the NH and the NHC–methylene arm of 4a .…”

“…Moreover, development of homogeneous catalytic systems based on lutidine-derived ligands has been mainly focused on phosphine-containing PNX (X = phosphine or N-donor) pincers, albeit substitution of the flanking PR 2 groups by another strong σ-donor such as N-heterocyclic carbenes (NHCs) has also been briefly addressed. Upon deprotonation of the methylene CH 2 –NHC arms, these organometallics are catalytically active in ligand-assisted processes. − …”