Intermolecular hydroamination and hydroarylation reactions of alkenes in ionic liquids

Intermolecular hydroamination or hydroarylation reactions of norbornene and cyclohexadiene performed with catalytic amounts of Brönsted or Lewis acid in ionic liquids were found to provide higher selectivity and yields than those performed in classical organic solvents. The ionic liquid increases the acidity of the media and stabilizes ionic intermediates through the formation of supramolecular aggregates. 2006 Elsevier Ltd. All rights reserved. Catalytic hydroamination and hydroarylation reactions are important synthetic methodologies for forming nitrogen-containing compounds such as amines, imines and enamines. Both protocols may afford these classes of compounds in a single step from readily available unsaturated hydrocarbons. Various catalysts derived from alkali, early and late transition metals as well as lanthanides have been developed for addition of N–H bonds to unsaturated compounds, but a more general and selective process is still to be found. Additionally, the cost of these transition metal catalysts, ligands or additives is also a major drawback of these protocols. The use of acid-catalyzed reactions could represent a less expensive alternative for synthesis using these nitrogencontaining products. Indeed, it has been recently reported that the hydroamination/hydroarylation reaction of alkenes with anilines proceeds smoothly in the presence of Brönsted acids. The catalytic activity of these systems is strongly influenced by the counteranion coordination ability, that is, the efficiency of the reaction increases as the coordination ability of the counteranion decreases. The proposed reaction pathway involves proton transfer from the anilinium salt ([PhNH3] ) to the C@C bond—thus generating a carbocation—followed by nucleophilic addition of neutral aniline species that yields ionic adducts. These adducts eliminate H in a 0040-4039/$ see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2006.07.073 * Corresponding author. Tel.: +555133166321; fax: +555133167304; e-mail: [email protected] process assisted by free aniline to give the Nand/or C-arylated product. It is reasonable to assume that counteranion coordination ability not only influences the Brönsted acidity of the medium but also the stabilization/destabilization of the ionic species involved in the process. Therefore, the catalytic activity and selectivity of this acid catalysis may be modulated by controlling the solvation process of the medium. In this respect, ionic liquids (ILs) and especially those based on the imidazolium cation are known to have great ability to stabilize ionic intermediates and transition states through different types of ions pairs. These properties have been recently exploited in some reactions that involve ionic species such as Mannich-type reactions, the Mukaiyama aldol reaction, Baylis–Hillman reactions and tetrahydropyranylation. ILs are also very effective media for ‘classic’ acid catalysis and the acidity of acids normally increases when dissolved in ILs. Furthermore, ILs are less solvating than water. Herein, we describe our preliminary results concerning the performance of acid catalysis (both Lewis and Brönsted acids) in the hydroamination/hydroarylation reactions in imidazolium-based IL media as well as the investigation by electrospray ionization mass spectrometry (ESI-MS) of the possible ionic species involved in the process. The reaction conditions were optimized for the reaction of norbornene in the presence of aniline with different Brönsted and Lewis acids (5 mol %) in BMIÆNTf2 (Scheme 1 and Table 1). Table 2. Hydroamination/hydroarylation of norbornene in different ILs Entry ILs Acid (5 mol %) Yield (%) 1:2:3 1 BMIÆNTf2 HBF4 80 1.5:1:0 2 BMIÆNTf2 BF3ÆOEt2 83 1.4:1:Trace 3 BMIÆInCl4 37 2.6:1:Trace 4 BMIÆInCl4 InCl3 57 3.3:1.6:1 5 BMIÆBF4 HBF4 12 2:1:0 6 BMIÆBF4 BF3ÆOEt2 40 1.4:1:Trace 7 BMIÆPF6 HBF4 7 2.3:1:0 8 BMIÆPF6 BF3ÆOEt2 35 2:1:0 9 BMIÆOTf HBF4 9 1.3:1:0 10 BMIÆOTf BF3ÆOEt2 28 1.3:1:0 a Reaction conditions: norbornene (1.08 mmol), aniline (2.16 mmol), acid (0.05 mmol) and IL (0.2 mL); 135 C; 24 h. b Combined yield of isolated products. c Products ratios were determined by GC. Table 1. Hydroamination/hydroarylation of norbornene by aniline (X = H) with different acids as catalyst in BMIÆNTf2 a Entry Acid (5 mol %) Yield (%) 1:2:3 1 CF3SO3H 70 1.9:1:Trace 2 CF3SO3H in PhH 4 13 1:1.4:0 3 CH3SO3H 48 1.8:1:0 4 CF3CO2H 28 1:1:Trace 5 p-TolSO3H 22 1.5:1:0 6 PhB(OH)2 38 1.6:1:0 7 HBF4 80 1.5:1:0 8 BF3ÆOEt2 83 1.4:1:Trace a Reaction conditions: norbornene (1.08 mmol), aniline (2.16 mmol), acid (0.05 mmol) and BMIÆNTf2 (0.2 mL); 135 C; 24 h. b Combined yield of isolated products. c Products ratios were determined by GC. H N