Ruthenium-Catalyzed Selective Hydro<i>boronolysis</i> of Ethers

Abstract

A ruthenium-catalyzed reaction of HBpin with substituted organic ethers leads to the activation of C–O bonds, resulting in the formation of alkanes and boronate esters via hydroboronolysis. A ruthenium precatalyst, [Ru (p-cymene)Cl]2Cl2 (1), is employed, and the reactions proceed under neat conditions at 135 °C and atmospheric pressure (ca. 1.5 bar at 135 °C). Unsymmetrical dibenzyl ethers undergo selective hydroboronolysis on relatively electron-poor C–O bonds. In arylbenzyl or alkylbenzyl ethers, C–O bond cleavage occurs selectively on CBn–OR bonds (Bn = benzyl); in alkylmethyl ethers, selective deconstruction of CMe–OR bonds leads to the formation of alkylboronate esters and methane. Cyclic ethers are also amenable to catalytic hydroboronolysis. Mechanistic studies indicated the immediate in situ formation of a mono-hydridobridged dinuclear ruthenium complex [{(η6-p-cymene)RuCl}2(μ–H−μ–Cl)] (2), which is highly active for hydroboronolysis of ethers. Over time, the dinuclear species decompose to produce ruthenium nanoparticles that are also active for this transformation. Using this catalytic system, hydroboronolysis could be applied effectively to a very large scope of ethers, demonstrating its great potential to cleave C–O bonds in ethers as an alternative to traditional hydrogenolysis.