Transfer Hydrogenation of Alkenes Using Ethanol Catalyzed by a NCP Pincer Iridium Complex: Scope and Mechanism

Abstract

The first general catalytic approach to effecting transfer hydrogenation (TH) of unactivated alkenes using ethanol as the hydrogen source is described. A new NCP-type pincer iridium complex (-NCP)IrHCl containing a rigid benzoquinoline backbone has been developed for efficient, mild TH of unactivated C-C multiple bonds with ethanol, forming ethyl acetate as the sole byproduct. A wide variety of alkenes, including multisubstituted alkyl alkenes, aryl alkenes, and heteroatom-substituted alkenes, as well as O- or N-containing heteroarenes and internal alkynes, are suitable substrates. Importantly, the (-NCP)Ir/EtOH system exhibits high chemoselectivity for alkene hydrogenation in the presence of reactive functional groups, such as ketones and carboxylic acids. Furthermore, the reaction with CDOD provides a convenient route to deuterium-labeled compounds. Detailed kinetic and mechanistic studies have revealed that monosubstituted alkenes (e.g., 1-octene, styrene) and multisubstituted alkenes (e.g., cyclooctene (COE)) exhibit fundamental mechanistic difference. The OH group of ethanol displays a normal kinetic isotope effect (KIE) in the reaction of styrene, but a substantial inverse KIE in the case of COE. The catalysis of styrene or 1-octene with relatively strong binding affinity to the Ir(I) center has (-NCP)Ir(alkene) adduct as an off-cycle catalyst resting state, and the rate law shows a positive order in EtOH, inverse first-order in styrene, and first-order in the catalyst. In contrast, the catalysis of COE has an off-cycle catalyst resting state of (-NCP)Ir(H)[O(Et)···HO(Et)···HOEt] that features a six-membered iridacycle consisting of two hydrogen-bonds between one EtO ligand and two EtOH molecules, one of which is coordinated to the Ir(III) center. The rate law shows a negative order in EtOH, zeroth-order in COE, and first-order in the catalyst. The observed inverse KIE corresponds to an inverse equilibrium isotope effect for the pre-equilibrium formation of (-NCP)Ir(H)(OEt) from the catalyst resting state via ethanol dissociation. Regardless of the substrate, ethanol dehydrogenation is the slow segment of the catalytic cycle, while alkene hydrogenation occurs readily following the rate-determining step, that is, β-hydride elimination of (-NCP)Ir(H)(OEt) to form (-NCP)Ir(H) and acetaldehyde. The latter is effectively converted to innocent ethyl acetate under the catalytic conditions, thus avoiding the catalyst poisoning via iridium-mediated decarbonylation of acetaldehyde.

Applying Le Chatelier's Principle for a Highly Efficient Catalytic Transfer Hydrogenation with Ethanol as the Hydrogen Source

“…published a very nice example of an iridium catalyst using ethanol for the transfer hydrogenation alkenes and of heterocycles. Here again ethyl acetate is found as the product of alcohol oxidation . It should not be overlooked here, that ethanol has already been applied as hydrogen source in the palladium catalyzed (stereoselective) transfer hydrogenation of other substrates such as ynamides and enones…”

n -Alkanes to n -alcohols: Formal primary C─H bond hydroxymethylation via quadruple relay catalysis

“…5) (40). The automatic deactivation of cat-1a by exposure to syngas alleviates the concerns of compatibility between the catalysts and eliminates potential side reactions caused by the Ir catalyst, such as olefin direct hydrogenation (41). The control experiment (Eq.…”

Controlled partial transfer hydrogenation of quinolines by cobalt-amido cooperative catalysis

“…Significant progress has been made in hydrogenation of quinolines to tetrahydroquinolines (THQs) [26][27][28][29] , and transfer hydrogenation reactions with formates 30 , alcohols 31,32 or Hantzsch esters [33][34][35] (Fig. 1b), but the related catalysis to access DHQs has not been revealed to date.…”