Iodide-enhanced palladium catalysis via formation of iodide-bridged binuclear palladium complex

Abstract

The prevalence of metalloenzymes with multinuclear metal complexes in their active sites inspires chemists' interest in the development of multinuclear catalysts. Studies in this area commonly focus on binuclear catalysts containing either metal-metal bond or electronically discrete, conformationally advantageous metal centres connected by multidentate ligands, while in many multinuclear metalloenzymes the metal centres are bridged through μ2-ligands without a metal-metal bond. We report herein a μ2-iodide-bridged binuclear palladium catalyst which accelerates the C-H nitrosation/annulation reaction and significantly enhances its yield compared with palladium acetate catalyst. The superior activity of this binuclear palladium catalyst is attributed to the trans effect-relay through the iodide bridge from one palladium sphere to the other palladium sphere, which facilitates dissociation of the stable six-membered chelating ring in palladium intermediate and accelerates the catalytic cycle. Such a trans effect-relay represents a bimetallic cooperation mode and may open an avenue to design and develop multinuclear catalysts.

Fig. 1. Palladium-catalysed C–H nitrosylation/annulation reaction of azobenzene with [NO][BF₄].

a The equimolar combination of (n-Bu₄N)I with Pd(OAc)₂ leads to enhancement of palladium catalysis via formation of the iodide-bridged binuclear palladium catalyst that is able to destabilize six-membered chelating ring of reaction intermediate due to trans effect-relay through iodide bridge, and therefore facilitate the reaction. The trans effect-relay through iodide bridge represents a bimetallic cooperation mode for catalysis. b The reaction catalyzed by 10 mol% Pd(OAc)₂ alone affords lower yield than the reaction with combination of 1 mol% (n-Bu₄N)I with 1 mol% Pd(OAc)₂. The difficult in decomplexation of six-membered chelating ring of reaction intermediate retards the reaction catalysed by Pd(OAc)₂ alone and results in incomplete reaction.

Fig. 2. Detection, preparation and reactivity of iodide-bridged binuclear palladium complex bearing cyclopalladated azobenzene ligand.

a Isolation of iodide-bridged binuclear palladium complex bearing cyclopalladated azobenzene ligand (4a) from the reaction under conditions mimicking catalysis process. b Preparation of iodide-bridged binuclear palladium complex bearing cyclopalladated 4,4′-di-n-butyl-azobenzene ligand 4b from self-assembly of 4,4′-di-n-butyl-azobenzene ligand and Pd(OAc)₂. c Stoichiometric reaction of 4b with [NO][BF₄] at room temperature to generate 2H-benzotriazole N-oxide. d 4b-catalysed reaction of 4,4′-di-substituted azobenzenes with [NO][BF₄]. e Comparison of activity of catalyst systems by determining the initial rates on the basis of plot of product 3c concentration (M) versus time (min). The reaction of 4,4′-dimethyl-azobenzene with [NO][BF₄] was used a model, which was analysed by HPLC with 3,4-dichlorotoluene as the internal standard. Catalyst system A is 3 mol% Pd(OAc)₂ and 6 mol% TsOH; catalyst system B is 3 mol% Pd(OAc)₂, 3 mol% TBAI and 6 mol% TsOH; catalyst system C is 1.5 mol% 4b; catalyst system D is 1.5 mol% 5b and 6 mol% TsOH. f A first-order dependence of the initial rate Δ[3c]/Δt (M/minute) on the 4b concentration [4b] (mol%) catalyst, using the reaction of 4,4′-dimethyl-azobenzene with [NO][BF₄] as a model.

Fig. 3. Reactivity of the palladium complexes related to 4b and their conversions to 4b.

a The reaction of acetate-bridged binuclear palladium complex bearing cyclopalladated 4,4′-di-n-butyl-azobenzene ligand (5b) with 2 equivalents of TBAI to generate 4b. b Stoichiometric reaction of 4b with 3 equivalents of [NO][BF₄] and 6 equivalents of 4,4′-di-n-butyl-azobenzene. The reaction formed 3b in 133% yield relative to 4b with 21% of 4b recovered. c Catalytic reaction of azobenzene with [NO][BF₄] using 1.5 mol% [Pd₂I₆](n-Bu₄N)₂ as a catalyst. d Stoichiometric reaction of [Pd₂I₆](n-Bu₄N)₂ with 7 equivalents of [NO][BF₄] and 10 equivalents of 4,4′-di-n-butyl-azobenzene. The reaction formed 3b and 4b. e The reaction of acetate-bridged binuclear palladium complex bearing cyclopalladated azobenzene ligand 5a with 3 equivalents of [NO][BF₄] with or without fully deuterated azobenzene (1a-D₁₀).

Fig. 4. Computational investigation on the reaction mechanism.

a Free energy profiles for reaction catalyzed by 4a (blue line) and 5a (red line). b Optimized structures (bond length in Å) of iodide-bridged (LM2′) and acetate-bridged (LM2) binuclear palladium intermediates bearing six-membered chelating rings, and activation barriers for the dechelation/N–N bond formation steps from LM2′ via TSA′ transition state and LM2 via TSA transition state in the absence of azobenzene. c Dissociation curves along Pd1–N1 bond for LM2 (red line) and LM2′ (blue line).

Fig. 5. Substrate scope of Pd-catalyzed C–H nitrosylation/annulation reaction of azobenzenes with [NO][BF₄].

a Substrate scope with respect to symmetrical azoarenes. b Substrate scope with respect to symmetrical azoarenes. Reaction conditions: azobenzene (0.2 mmol), [NO][BF₄] (3 equiv), Pd(OAc)₂ (3 mol%), TBAI (3 mol%), TsOH (6 mol%), DCB (1.5 mL), 90 °C, 48 h. Yields are isolated yields. ⁱThe reaction was run at 100 °C for 24 h. ⁱⁱThe ratio of product isomers was determined by NMR. ⁱⁱⁱThe reaction was run in nitrobenzene (1.5 mL) at 100 °C for 24 h. ^iv0.6 mmol scale reaction was run at 80 °C for 24 h, with [NO][BF₄] (2 equiv), Pd(OAc)₂ (0.5 mol%), TBAI (0.5 mol%). ^vThe reaction was run in DCB (1.0 mL) and nitrobenzene (0.5 mL) at 90 °C for 48 h.