Selective alkyl ether cleavage by cationic bis(phosphine)iridium complexes

Abstract

Simple cationic bis(phosphine)iridium complexes are shown to be highly selective catalysts for ether cleavage with silanes. Benzylic ethers can be cleaved under mild conditions in the presence of reductively-labile functional groups.

Light-Promoted Transfer of an Iridium Hydride in Alkyl Ether Cleavage

“…This proposal is inspired by work on related systems that operate without light, [33][34][35][36][37]43,44 as well as the reported photochemical behavior of the bipyridine-supported iridium hydride reported by Miller. 24 Complex 2 likely reacts with NaBAr F 4 and triethylsilane to form the electrophilic σ-silane complex 4 that has been previously characterized by Djukic and co-workers under relevant alcohol dehydrosilylation conditions.…”

“…Diethyl ether and diisopropyl ether show low reactivity, in line with previous non-lightpromoted iridium catalysts. 34 A selection of alkyl benzyl ethers undergo debenzylation, with a primary alkyl chloride and bromide being tolerated. The corresponding iodide undergoes hydrodehalogenation, while the fluoride halts at partial conversion.…”

“…As further evidence that olefins are not tolerated functionalities, the reduction of a citronellol-derived secondary methyl ether gives the saturated product of demethoxylation and hydrogenation (entry 9), 42 a limitation also observed in previous iridium systems that do not require light. 34,37 In comparison to dialkyl ethers, anisole derivatives proved to be relatively challenging substrates for this system. The reactivity of the selection of anisoles shown in Table 3 appears to track with the electron richness of the arene, with 4-F, 4-Br, and the parent anisole showing poor reactivity under our conditions.…”

“…35 The principal factors distinguishing 2/NaBAr F 4 from [(cod)Ir(PPh 3 ) 2 ]BAr F 4 are therefore the necessity of light and the requirement for a somewhat higher catalyst loading of 2 versus [(cod)Ir-(PPh 3 ) 2 ]BAr F 4 . 34,35 Reaction Mechanism. The similar conformational influences on selectivity observed for light-promoted hydrosilylative alkyl ether cleavage with 2/NaBAr F 4 and the thermal bis(phosphine)iridium systems strongly imply mechanistic similarities between the two systems.…”

“…35 The observation that 2/NaBAr F 4 catalyzes the hydrosilylation of carbonyl derivatives 39,40 and dehydrosilylation of alcohols 30 in the dark but does not catalyze hydrosilylative ether cleavage in the absence of light can be interpreted in the context of the relative reactivities of the plausible reaction intermediates. These three transformations share similar mechanisms, with heterolytic Si−H cleavage giving silyloxycarbenium 38,39 and mono- 30 and dialkylsilyloxonium 34,35,47 ions, respectively. Our observation that the catalyst resting state after illumination is a mixture of the neutral monohydride complex 5 and the dimeric monohydride 6 is consistent with a catalytic cycle resting at the silyloxonium/5 reactant pair (with 5 in equilibrium with 6).…”