Catalytic Dehydrogenation of Alkanes by PCP–Pincer Iridium Complexes Using Proton and Electron Acceptors

Abstract

Dehydrogenation to give olefins offers the most broadly applicable route to the chemical transformation of alkanes. Transition-metal-based catalysts can selectively dehydrogenate alkanes using either olefinic sacrificial acceptors or a purge mechanism to remove H 2 ; both of these approaches have significant practical limitations. Here, we report the use of pincer-ligated iridium complexes to achieve alkane dehydrogenation by proton-coupled electron transfer, using pairs of oxidants and bases as proton and electron acceptors. Up to 97% yield was achieved with respect to oxidant and base, and up to 15 catalytic turnovers with respect to iridium, using t-butoxide as base coupled with various oxidants, including oxidants with very low reduction potentials. Mechanistic studies indicate that (pincer)IrH 2 complexes react with oxidants and base to give the corresponding cationic (pincer)IrH + complex, which is subsequently deprotonated by a second equivalent of base; this affords (pincer)Ir which is known to dehydrogenate alkanes and thereby regenerates (pincer)IrH 2 .

Heteroleptic Palladium(II) Complexes of Thiazolinyl‐picolinamide Derived N∩N∩N Pincer Ligand: An Efficient Catalyst for Acylative Suzuki Coupling Reactions

“…[(C 3 H 4 NS)(1,2-C 6 H 4 ) NH 2 ] (1) was prepared according to the literature method. [65] The nuclear magnetic resonance (NMR) spectra were recorded in deuterated solvents at room temperature on a Bruker Avance 300 ( 1 H and 13 C) spectrometer operating at 300 MHz for 1 H and 75 MHz for 13 C. Deuterated solvents were dried by storage over 4 Å molecular sieves. Chemical shifts (δ) are expressed in ppm using TMS as internal standard and coupling constants (J) are given in Hz.…”

“…1 H NMR (300 MHz, CDCl 3 with one drop DMSO-d 6 ) δ 9.13 (d, J = 6.0 Hz, 2H, ArÀ H), 8.62 (d, J = 9.0 Hz, 1H, ArÀ H), 8.29 (t, J = 7.5 Hz, 1H, ArÀ H), 8.22-8.12 (m, 2H, ArÀ H), 7.90 (t, J = 7.5 Hz, 2H, ArÀ H), 7.82 (d, J = 6.0 Hz, 1H, ArÀ H), 7.62-7.51 (m, 2H, ArÀ H), 7.12 (t, J = 9.0 Hz, 1H, ArÀ H), 6.97 (d, J = 6.0 Hz, 1H, ArÀ H), 3.89 (t, J = 7.5 Hz, 2H, NÀ CH 2 ), 3.37 (t, J = 7.5 Hz, 2H, SÀ CH 2 ). 13…”

“…Late transition metals stabilized through amido ligands are less explored due to the nature of metal repulsive filled p π -d π electronic repulsions which make the MÀ N bond weaker. [13,33] However, late transition metal amido complexes have been found to be potential catalysts for many organic transformations including transfer hydrogenation to an unsaturated substrate and coupling reactions (Figure 1). [34][35][36][37][38][39][40][41][42][43][44][45][46] The intrinsic structure of the pincer ligands allows the development of late transition metal amido complexes and to explore their catalytic activities.…”

“…[5,6] Pincer ligands are an exciting class of tridentate ligands bonded to a metal center meridionally via sigma and/or coordinate covalent bonds. [7][8] The new generation neutral or monoanionic pincer ligands, P \ C \ P, [9][10][11][12][13] P \ N \ P, [14][15][16][17] N \ C \ N, [18][19][20] S \ C \ S, [21][22][23] N \ N \ S, [24][25][26] S \ N \ S [27,28] and N \ N \ N, [29][30][31][32] has been shown to provide stability to the metal centers under catalytic conditions. Late transition metals stabilized through amido ligands are less explored due to the nature of metal repulsive filled p π -d π electronic repulsions which make the MÀ N bond weaker.…”

Noncovalent Interactions in Crowded Benzene Systems: How Much Strain Is Too Much? Attractions Overcome Repulsions!

“…Admittedly, crowding of the catalytic site significantly increases the intrinsic catalytic efficiency and selectivity. 1 In the synthesis of novel molecules where crowding occurs, sterically bulky ligands are often used to form cages in which unstable systems can be confined within a local minimum of the potential energy surface. The stabilization of new and ap-parently unstable molecules is a very challenging task for synthetic chemists.…”