Catalytic Alkylation of Aromatic Amines with Styrene in the Presence of Cationic Rhodium Complexes and Acid

Abstract

The first transition metal-catalyzed Friedel-Crafts alkylation of aromatic amines with styrene is reported. ortho-Alkylation of anilines occurs using catalytic amounts of [Rh(cod) 2 ]BF 4 / 4 PPh 3 and HBF 4 .There has been much effort in recent years towards the development of catalytic electrophilic aromatic substitution reactions. 1 Of special importance are methods, e.g. Friedel-Crafts reactions, which enable the introduction of carbon substituents onto aromatic rings. Although these reactions work best for electron-rich aromatic compounds, frequently the observed selectivities (ortho vs. para) are low and the reactions proceed only in the presence of stoichiometric amounts of Lewis acids. Scheme 1Unlike typical Friedel-Crafts processes of substituted benzenes, electrophilic carbon-carbon bond forming reactions of aromatic amines are more problematic. 2 This is due to the coordination of the Lewis acid to the aromatic nitrogen atom which leads to a deactivation of the aromatic ring and side reactions. Hence, few examples of alkylations of anilines with olefins under Friedel-Crafts conditions have been described. 3 On the other hand, the alkylation of aniline with ethylene has been reported using basic aluminium anilide as catalyst under drastic conditions (40-60 bar; 330°C). 4,5 In addition, the reaction of aniline with styrene, leading to the formation of ortho-Cand N-alkylation products is possible when using aluminium phenoxide or zeolites as catalysts. 6 However, these catalysts always produce a mixture of regioisomers (o-, palkylated aniline). To the best of our knowledge the only known transition metal-catalyzed alkylation of anilines was previously observed by J. J. Brunet et al. for the conversion of norbornene with aniline. 7,8 Using [(PEt 3 ) 2 RhCl] 2 / PhNHLi as catalyst the ortho-C-alkylation product (30 %) and the N-alkylation product (15 %) was obtained after 12 days at 70 °C. When applying the same catalyst to the reaction of styrene with aniline no Calkylation product was detected. 9Based on our recent investigations on the regioselective amination of aromatic olefins with aliphatic amines, 10 we became interested in the reaction of styrene with various substituted anilines in the presence of cationic rhodium complexes. Herein, we demonstrate for the first time that anilines afford alkylated anilines in the presence of catalytic amounts of HBF 4 ◊OEt 2 and a cationic rhodium species. Often high regioselectivities for the ortho-alkylated aniline 1a -5a are obtained.Recently, we described the first catalytic anti-Markovnikov hydroamination of styrene with aliphatic amines 11 using a catalyst system consisting of HBF 4 ◊OEt 2 and a cationic rhodium complex. Now, we used this catalyst system for the reaction of substituted anilines with styrene. In general, the experiments were performed in Acepressure tubes at 140 °C in the presence of 2.5 mol% [Rh(cod) 2 ]BF 4 / 4 mol% PPh 3 and 20 mol% HBF 4 ◊OEt 2 using toluene as solvent (Table 1).

A Simple Catalyst for the Efficient Benzylation of Arenes by Using Alcohols, Ethers, Styrenes, Aldehydes, or Ketones

“…[12] Compound 1 also catalysed the benzylation of anilines with styrenes, as shown in Table 4, providing results that compare well with those previously reported which made use of Brønsted acids [14,15] or metal complexes [16,17] as catalysts.…”

“…[7] Reaction of styrene with aniline derivatives: A mixture of styrene (1 mmol), aniline derivative (5 mmol), catalyst (1 mol %) and silver trif- H NMR spectroscopy and yields calculated based on the amount of arene. Products were identified according to previously reported spectroscopic data: 2-(1-phenylethyl)benzenamine and 4-(1-phenylethyl)benzenamine, N-(1-phenylethyl)aniline, [20] 2-methyl-6-(1-phenylethyl)benzenamine and 2-methyl-4-(1-phenylethyl)benzenamine, [14] a-methyl-N-(2-methylphenyl)-benzenemethanamine, [21] 4-methyl-2-(1-phenylethyl)benzenamine, amethyl-N-(4-methylphenyl)benzenemethanamine, 4-fluoro-2-(1-phenyl-A C H T U N G T R E N N U N G ethyl)benzanamine and N-(4-fluorophenyl)-a-methylbenzenemethan-A C H T U N G T R E N N U N G amine, [17] 4-chloro-2-(1-phenylethyl)benzanamine [14] and N-(4-chlorophenyl)-a-methylbenzenemethanamine, [22] 4-methoxy-2-(1-phenylethyl)-benzanamine [17] and N-(4-methoxyphenyl)-a-methylbenzenemethanamine. [21] Tandem reaction: A mixture of benzylating agent (1 mmol), anisole (10 mmol), iPrOH (1.5 mmol), catalyst (1 mol %) and silver triflate (3 mol %) was heated at 110 8C in a thick-walled glass tube fitted with a Teflon cap.…”

Amination of Aromatic Olefins with Anilines: A New Domino Synthesis of Quinolines

“…However, a completely distinct reaction pathway was observed when HBF 4´O Et 2 was added as an acid, and the Markovnikov N-alkylation and ortho-C alkylation products of aniline were isolated. [16] Nevertheless, the product yield of 1 a can be increased by simply increasing the catalyst concentration. Thus, when the catalyst concentration was raised to 10 mol % [Rh(cod) 2 ]BF 4 /4 PPh 3 , 1 a was obtained in 43 % yield.…”

Advances and Adventures in Amination Reactions of Olefins and Alkynes

“…Examples from our side include the oxidative amination of styrenes, amination reac-tions of norbornadiene, the recent oxidative amination of aldehydes 60 and the catalytic alkylation of aromatic amines with styrene. 61 In addition to the development of new catalysts and studies of the organometallic chemistry of transition metal amine complexes, we believe that unusual reaction conditions (the use of supercritical solvents, e.g. NH 3 ; microwaves, etc.)…”