Abstract
The isolable ruthenium(II) bis(dinitrogen) complex [Ru(H)2(N2)2(PCy3)2] (1) reacts with aryl ethers (Ar–OR, R = Me and Ar) containing a ketone directing group to effect sp2C–O bond activation at temperatures below 40 °C. DFT studies support a low-energy Ru(II)/Ru(IV) pathway for C–O bond activation: oxidative addition of the C–O bond to Ru(II) occurs in an asynchronous manner with Ru–C bond formation preceding C–O bond breaking. Alternative pathways based on a Ru(0)/Ru(II) couple are competitive but less accessible due to the high energy of the Ru(0) precursors. Both experimentally and by DFT calculations, sp2C–H bond activation is shown to be more facile than sp2C–O bond activation. The kinetic preference for C–H bond activation over C–O activation is attributed to unfavorable approach of the C–O bond toward the metal in the selectivity determining step of the reaction pathway.
Highlights
The high content of elemental oxygen in the biopolymers that constitute lignocellulosic biomass has inspired chemists to develop new methods to break strong carbon−oxygen bonds.[1,2]In organometallic chemistry, a series of nickel and ruthenium precatalysts have been applied to reactions that transform carbon−oxygen bonds of ethers into carbon−hydrogen,[3,4] carbon−carbon,[5−10] or carbon−boron[11,12] bonds by hydrogenolysis, cross-coupling, or borylation, respectively.[13]
The reorganization of hydrogen atoms within the equatorial plane of ruthenium(II) complexes is well-known to proceed through low-energy, almost barrierless steps,[38,73] and related fluxional exchange process have been proposed to be facilitated by nascent H···H−H bond formation in the ground state due to donation from the σ-(M−H) orbital to the σ*-(H− H) orbital.[74,75]
In substrates where an ortho C−H bond is available there is a kinetic preference for C−H over C−O bond activation
Summary
The high content of elemental oxygen in the biopolymers that constitute lignocellulosic biomass has inspired chemists to develop new methods to break strong carbon−oxygen bonds.[1,2]. Reaction has been calculated to occur by oxidative addition of the C−O bond to a 16-electron ruthenium(0) complex, [Ru(PPh3)2(CO)(L)] (L = substrate coordinated through the directing group).[24] In related studies, Bergman and co-workers have reported ruthenium catalysts for the hydrogen-shuttling sp3C−O bond cleavage of 2-aryloxy-1-arylethanols, simple models of the β-[O]-4′ linkage of lignin. We show that [Ru(H)2(N2)2(PCy3)2] (1), originally reported as a reactive intermediate prone to decomposition,[28] can be isolated and effects both sp2C−H and sp2C−O bond activation of methyl aryl and biaryl ethers under exceptionally mild conditions (25−40 °C) provided the substrate contains a suitable ketone directing group adjacent to the C−X bond (X = H and OR). Full experimental details including the preparation of materials, conditions of C−X bond activation reactions, spectroscopic and crystallographic data, and details of the computational methods are given in the Supporting Information
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