Oxidative Addition of Water, Alcohols, and Amines in Palladium Catalysis

Abstract

The homolytic cleavage of O−H and N−H or weak C−H bonds is a key elementary step in redox catalysis, but is thought to be unfeasible for palladium. In stark contrast, reported here is the room temperature and reversible oxidative addition of water, isopropanol, hexafluoroisopropanol, phenol, and aniline to a palladium(0) complex with a cyclic (alkyl)(amino)carbene (CAAC) and a labile pyridino ligand, as is also the case in popular N‐heterocyclic carbene (NHC) palladium(II) precatalysts. The oxidative addition of protic solvents or adventitious water switches the chemoselectivity in catalysis with alkynes through activation of the terminal C−H bond. Most salient, the homolytic activation of alcohols and amines allows atom‐efficient, additive‐free cross‐coupling and transfer hydrogenation under mild reaction conditions with usually unreactive, yet desirable reagents, including esters and bis(pinacolato)diboron.

Cyclic (Alkyl)(amino)carbene Ligand-Promoted Nitro Deoxygenative Hydroboration with Chromium Catalysis: Scope, Mechanism, and Applications

“…Interestingly, a relaxed potential energy surface scan for consecutively shortening the distance of O(1) and H(1) atoms from IN6 offered evidence of the noninnocent ligand behavior of CAAC, in which the carbene carbon (C(1)) acts as an H-shuttle. 53,54 This offered an indication of a carbene-assisted H migration, which leads to polarity reversal of the hydride. Indeed, H(1) can undergo a facile migration to C(1) via TS4 (−101.1 kcal mol −1 ), and this process thus gives rise to an umpolung of the H(1) atom.…”

“…Attempts to locate a transition state that corresponds to an O(1)–H(1) bond formation from IN6 were unsuccessful, presumably because of the unmatched charge distribution between the H(1) (−0.18 au) and O(1) (−0.99 au) atoms, indicated by natural population analysis. Interestingly, a relaxed potential energy surface scan for consecutively shortening the distance of O(1) and H(1) atoms from IN6 offered evidence of the noninnocent ligand behavior of CAAC, in which the carbene carbon (C(1)) acts as an H-shuttle. , This offered an indication of a carbene-assisted H migration, which leads to polarity reversal of the hydride. Indeed, H(1) can undergo a facile migration to C(1) via TS4 (−101.1 kcal mol –1 ), and this process thus gives rise to an umpolung of the H(1) atom.…”

“…It has been demonstrated that the non-negligible π-acceptor abilities of singlet carbenes as ligands can influence the outcome of transition-metal catalysis, primarily because of the metal–ligand π-back-donation that tunes the electronic properties of metals . However, on the basis of the H-shuttle behavior observed for CAAC, such carbene ligands may not only be spectator Lewis bases, but also involved in key steps in the Cr-catalyzed deoxygenative hydroboration. ,, Inspection of the intrinsic bond orbitals (IBOs), which has been shown to give an exact representation of any Kohn–Sham DFT wave function, reveals that a d-type unpaired electron at Cr highly delocalizes into the formal vacant p-orbital of C(1) in IN6 (Figure b). This is indicative of the partial radical character of C(1), which may facilitate the H-transfer to IN7 .…”

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

“…It has also been demonstrated that higher steric bulk in cAACs, than in NHCs and phosphine, also increases the catalytic activity of cAACs complexes, owing to sp 3 ‐hybridization of the α‐carbon [15] . According to the report by Grunwald et al., cAAC stabilized palladium complexes can undergo oxidative addition with water, alcohols, and amines and can be a useful catalyst for O−H and N−H bond activation [32] . Gold(I)‐complexes bearing cAACs as ligands are probably the most thoroughly studied complexes with a broad range of applications in catalysis.…”

“… [15] According to the report by Grunwald et al., cAAC stabilized palladium complexes can undergo oxidative addition with water, alcohols, and amines and can be a useful catalyst for O−H and N−H bond activation. [32] Gold(I)‐complexes bearing cAACs as ligands are probably the most thoroughly studied complexes with a broad range of applications in catalysis. In 2007, Bertrand's group demonstrated that cationic cAACs−Au (I) complexes can promote the coupling of terminal alkynes with enamines to produce allenes, [33] while with other catalysts this coupling gives propargyl amines ( A2 ).…”