Rhodium-Catalyzed Alkenylation of Toluene Using 1-Pentene: Regioselectivity To Generate Precursors for Bicyclic Compounds

Abstract

Rhodium catalysts for arene alkenylation reported by our group (e.g., Science 2015, 348, 421; J. Am. Chem. Soc. 2017, 139, 5474; J. Am. Chem. Soc. 2018, 140, 17007) have demonstrated selectivity for 1-aryl alkenes over y-aryl alkenes (y > 1). This selectivity is notable because 1-aryl alkenes or 1-aryl alkanes cannot be generated using acid-based Friedel–Crafts arene alkylation or acidic zeolite catalysts. Herein, we report the extension of Rh arene alkenylation catalysis to generate 1-tolyl-1-pentenes, which are potential precursors for bicyclic compounds. The olefin concentration, copper­(II) oxidant identity and concentration, reaction temperature, and rhodium concentration for the alkenylation of toluene with 1-pentene have been optimized using [Rh­(η2-C2H4)2(μ-OAc)]2 as the catalyst precursor. The rhodium-based catalysis achieves up to 12(1):1 anti-Markovnikov selectivity for 1-tolyl-1-pentenes over 2-tolyl-2-pentenes and is selective for alkenylation in the meta and para positions.

Oxidative Alkenylation of Arenes Using Supported Rh Materials: Evidence that Active Catalysts are Formed by Rh Leaching

“…The quantification of various tolyl-pentane products is performed by a reported GC-MS method. [40] Briefly, for the 1-tolyl-1-pentane products, the slope and correlation coefficient of the regression lines were 1.34 and 0.998 (1-m-tolyl-1-pentane), 1.85 and 0.997 (1-p-tolyl-1-pentane) and 1.84 and 0.998 (1-o-tolyl-1-pentane), respectively. For decane, the slope and correlation coefficient were 2.08 and 0.998, respectively.…”

“…styrene or linear alkenyl arenes) per catalyst site at any given time. The quantification of various tolyl‐pentane products is performed by a reported GC‐MS method [40] . Briefly, for the 1‐tolyl‐1‐pentane products, the slope and correlation coefficient of the regression lines were 1.34 and 0.998 (1‐ m ‐tolyl‐1‐pentane), 1.85 and 0.997 (1‐ p ‐tolyl‐1‐pentane) and 1.84 and 0.998 (1‐ o ‐tolyl‐1‐pentane), respectively.…”

“…For the catalysis evaluation of supported Rh catalysts, the reactions were sampled every 4 h. The products quantification is determined after hydrogenation of the unsaturated products. The hydrogenation procedure was adopted from the literature 40 and it was performed at room temperature using 10 % Pd/C catalyst under 100 psig hydrogen pressure.…”

“…Recently, we reported Rh‐catalyzed arene alkenylation to directly synthesize styrene and 1‐aryl alkenes using molecular catalysts in homogeneous processes [13,14,36–41] . Based on the mechanistic studies, our initial proposed catalytic cycle involves (Scheme 3): a) Rh‐carboxylate assisted arene C−H activation, b) ethylene coordination and insertion into a Rh‐aryl bond, c) β‐hydride elimination, and d) alkenyl arene dissociation and oxidation of a Rh−H intermediate with CuX 2 (X=carboxylate) to regenerate the starting catalyst.…”

“…[15,16,[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] Recently, we reported Rh-catalyzed arene alkenylation to directly synthesize styrene and 1-aryl alkenes using molecular catalysts in homogeneous processes. [13,14,[36][37][38][39][40][41] Based on the mechanistic studies, our initial proposed catalytic cycle involves (Scheme 3): a) Rh-carboxylate assisted arene CÀ H activation, b) ethylene coordination and insertion into a Rh-aryl bond, c) βhydride elimination, and d) alkenyl arene dissociation and oxidation of a RhÀ H intermediate with CuX 2 (X = carboxylate) to regenerate the starting catalyst. Under some conditions, we observed an induction period, [13,36,37] which was proposed to be due to the rapid decomposition of the soluble Rh catalyst precursor to form insoluble Rh(s) followed by formation of the active catalyst through dissolution of Rh from the inactive Rh(0) species (Scheme 4).…”