Rh^I‐Catalyzed Hydration of Organonitriles under Ambient Conditions

Abstract

Dedicated to Professor Ryoji Noyori on the occasion of his 70th birthdayThe hydration of organonitriles is a reaction of great synthetic significance for the preparation of organoamides (e.g., acrylamide and nicotinamide) in view of its broad industrial and pharmacological applications.[1] For example, hydration of acrylonitrile is used to produce more than 2 10 5 tons of acrylamide per year.[2] Classically the reaction proceeds in a sequence of distinct steps upon treatment with strong inorganic acid or base, but these methods are frequently unable to control overhydrolysis.[1f] Although several pioneering precedents involving molecular [3] and heterogeneous [4] catalysts have been reported, in many cases, drastic conditions including high temperatures (80-180 8C) or high pressure (e.g., 80 psi) are required. As an exception, Co III [3a,d] and Pt II [3e,o] complexes mediate hydration under milder conditions; however, the substrate range applicable under standard or ambient conditions remains unclarified. We report here a notable advance towards expanding the substrate scope, by demonstrating an easier to conduct and milder hydration of organonitriles using a low-valent Rh I -(OMe) species as the molecular catalyst (Scheme 1).The Rh I catalyst was prepared by treatment of commercially available [{Rh(OMe)(cod)} 2 ] (0.01 equiv) with PCy 3 (0.04 equiv) in anhydrous THF at 25 8C for 15 min under argon (cod = cyclooctadiene, Cy = cyclohexyl). After the solvent THF and residual cod had been removed by evaporation in vacuo, oxygen-free, Ar-saturated iPrOH was added. The iPrOH solution of the Rh I OMe/2 PCy 3 catalyst (Rh: 0.04 m) was treated sequentially with benzonitrile (1 a) (1 equiv) and H 2 O (5 equiv) at 25 8C under argon, and the reaction mixture was stirred at 25 8C for 17 h. Subsequent purification by column chromatography on silica gel provided benzamide (2 a) in 90 % yield. The reaction mixture was not contaminated by further hydration and/or alcoholysis products; in contrast, the formation of benzoic acid and/or the ester was frequently the side reaction in several methods previously described.[1f] Use of [{Rh(OH)(cod)} 2 ] in place of [{Rh(OMe)(cod)} 2 ] resulted in a slightly lower yield (74 %). Scant reactivity was observed with other Rh I complexes including [{RhCl(cod)} 2 ] and [Rh(acac)(cod)] (acac = acetylacetonate) under otherwise identical conditions, suggesting that the OR (R = H, Me) component is the critical functional group in facilitating the reaction. When we screened the solvents, we found that protic solvents (MeOH, EtOH, iPrOH, and tBuOH) led to rate enhancement (2 a, 60-99 %: 25 8C, 24 h), whereas aprotic solvents (dimethylacetamide (DMA), dimethyl sulfoxide, and N-methylpyrrolidinone (NMP)) resulted in low conversion (< 10 %). Additional experiments revealed that the yield of benzamide (2 a) depends least on the concentration of the Rh catalyst when the hydration is carried out in iPrOH, so that we chose this solvent for further screening. Two equivalents of PCy 3 per equivalent of Rh ...

Copper‐Catalyzed Site‐Selective Oxidative C−C Bond Cleavage of Simple Ketones for the Synthesis of Anilides and Paracetamol

“…Recent data gathered by medicinal chemists disclosed that in modern pharmaceuticals, the most frequently used methods for N-acylation rely on the reaction between amines and activated carboxylic acids. [12] Additionally, their preparation based on acid amine coupling, [13] cross-coupling reactions, [14] Beck-mann rearrangement, [15] rearrangement of keto azides, [16] reaction of amines with aldehydes or alcohols, [17] using nitrile substrates [18] and using amino carbonylation methods. [19] However, these protocols suffer from significant limitations such as the use of sensitive substrates and reagents, expensive catalysts, and hazardous conditions.…”

Direct Synthesis of Organic Carbonates from CO2and Alcohols Using Heterogeneous Oxide Catalysts

“…The reaction rate of benzonitrile hydration was higher than that of acetonitrile hydration, especially in the case of low CO 2 pressure. The higher activity and selectivity of benzonitrile hydration over acetonitrile hydration have been reported for other catalyst systems, such as [Cp′ 2 Mo(OH)(OH 2 )] + [49], [(dippe)Ni(η 2 -NCR)] [50], and [{Rh(OMe)(cod)} 2 ]PCy 3 [51]. The higher selectivity for benzonitrile hydration was interpreted in terms of the higher stability to alcoholysis of benzamide than acetamide.…”

Palladium‐Catalyzed, Ligand‐Controlled Chemoselective Oxidative Coupling Reactions of Benzene Derivatives with Acrylamides under an Oxygen Atmosphere

“…Recently, Tsuji and co‐workers reported an improved synthetic method for the preparation of cinnamamides, and it involves the Pd‐catalyzed addition of formamide to an alkyne 4a. Saito and co‐workers achieved the Rh‐catalyzed hydration of cinnamonitrile to cinnamamide 4b…”