Disambiguation of Metal and Brønsted Acid Catalyzed Pathways for Hydroarylation with Platinum(II) Catalysts

Abstract

The hydroarylation of unactivated olefins effected by Pt(II) precatalysts was found to proceed through the in situ production of protic acid followed by a Friedel−Crafts mechanism. The reaction was investigated using the hindered base 2,6-di-tert-butyl-4-methylpyridine and a variety of substrates.

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

“…22,29 Particularly germane is a recent study that reveals catalytic olefin hydroarylation using Pt(II) triflate salts likely proceeds by formation of triflic acid, which serves as the active catalyst. 30 Our groups reported a series of studies on TpRu(L)(NCMe)-(Ph) [Tp = hydridotris(pyrazolyl)borate, L = CO, PMe 3 , P(Npyrrolyl) 3 , or P(OCH 2 ) 3 CEt] catalysts for olefin hydroarylation. 18,19,23À26,31 The primary focus has been to elucidate the effect of the donor and steric properties of the ligand L on the key steps of the catalytic cycle (i.e., olefin insertion and aromatic/ olefin CÀH activation).…”

“…82 Evidence that the hydroarylation of cyclohexene using L 2 Pt(OTf) 2 (L 2 = 1,5-cyclooctadiene or t bpy, OTf = trifluoromethanesulfonate) likely proceeds via the generation of HOTf, which is the active catalyst, has been disclosed. 30 The hydrophenylation of ethylene (0.1 MPa) using 0.025 mol % HBAr 0 4 results in ∼1 TO of ethylbenzene after 16 h at 120°C. In addition, catalysis with 1 in the presence of 2,6-di-tert-butyl-4-methylpyridine (1.5 equiv relative to 1), which gave 16.8 TO of ethylbenzenes after 4 h, was not suppressed.…”

Kinetics and Mechanism of the Platinum(II)‐Catalyzed Hydroarylation of Vinyl Arenes with 1,2‐Dimethylindole

“…[11][12][13][14][15] As part of our ongoing interest in the noble metalcatalyzed hydrofunctionalization of C-C multiple bonds, [16] we have shown that PtCl 2 catalyzes the intermolecular hydroarylation of vinyl arenes, as well as ethylene and propene, with indoles. [14,[17][18][19][20][21][22][23] For example, treatment of a 3 : 1 mixture of p-chlorostyrene (1 a) with 1,2-dimethylindole (2) and a catalytic amount of PtCl 2 (5 mol %) in dioxane at 120°C led to isolation of a 1.6 : 1 mixture of the anti-Markovnikov hydroarylation product 3 a and the Markovnikov hydroarylation product 4 a along with traces (~5 %) of the Markovnikov alkenylation product 5 a in 89 % combined yield (Scheme 1). [14,24,25] The anti-Markovnikov/Markovnikov selectivity of the Pt-catalyzed hydroarylation of p-substituted vinyl arenes 1 with 2 increased from 0.5 : 1 for p-methylstyrene to 5.8 : 1 for p-nitrostyrene and the log of the Markovnikov/anti-Markovnikov ratio tracked linearly with the Hammett σparameter (ρ = À 0.98).…”

“…As part of our ongoing interest in the noble metal‐catalyzed hydrofunctionalization of C–C multiple bonds, we have shown that PtCl 2 catalyzes the intermolecular hydroarylation of vinyl arenes, as well as ethylene and propene, with indoles . For example, treatment of a 3 : 1 mixture of p ‐chlorostyrene ( 1 a ) with 1,2‐dimethylindole ( 2 ) and a catalytic amount of PtCl 2 (5 mol %) in dioxane at 120 °C led to isolation of a 1.6 : 1 mixture of the anti ‐Markovnikov hydroarylation product 3 a and the Markovnikov hydroarylation product 4 a along with traces (∼5 %) of the Markovnikov alkenylation product 5 a in 89 % combined yield (Scheme ) .…”

Oxidative Hydrophenylation of Ethylene Using a Cationic Ru(II) Catalyst: Styrene Production with Ethylene as the Oxidant

“…Transition metal catalyzed olefin hydroarylation or oxidative olefin hydroarylation that does not generate carbocation intermediates offers a strategy for the alkylation or alkenylation of arenes, respectively, that is complementary to current processes. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Along these lines, our groups have been studying a series of neutral ruthenium(II) catalysts with the general formula TpRu(L)(NCMe)Ph (Tp = hydridotris(pyrazolyl)borate; L = CO, PMe 3 , P(pyr) 3 , , P(OCH 2 ) 3 CEt; pyr = N-pyrrolyl) for catalytic ethylene hydrophenylation. [21,[27][28][29][30][31][32][33] In those studies, we demonstrated that more strongly donating ligands "L" result in an increase in the activation barrier for ethylene insertion into Ru-Ph bonds, [27,29,32] and, as a result, ethylene C-H activation, ultimately to form thermally stable and catalytically inactive h 3 -methylallyl complexes, [25,27,29,32] competes with ethylene insertion.…”