Aryl Ether Cleavage by Group 9 and 10 Transition Metals: Stoichiometric Studies of Selectivity and Mechanism

The reactivity of terphenyl diphosphines bearing aryl–methyl ether or aryl–aryl ether moieties with M0 (M = Ni, Pd, Pt), M′I (M′ = Co, Rh, Ir), or MII centers was investigated to gain mechanistic insight into intramolecular aryl–ether bond cleavage in structurally related metal complexes. RhI converts the aryl–methyl ether moiety to an aryl C–H bond. This is similar to reactivity previously observed at Ni0 that involves C–O oxidative addition, β-H elimination liberating CH2O, reductive elimination of an aryl C–H bond, and decarbonylation of CH2O. IrI leads to unselective aryl and alkyl C–O bond activation. In the presence of excess CO, RhI and IrI display a shift in selectivity and reactivity and cleave the alkyl C–O bond. CoI does not perform C–O cleavage. Alkyl C–O bond activation was observed with MII–halide complexes with loss of MeCl via a Lewis acid–base mechanism. Pd0 and Pt0 cleave selectively the O–Me bond via oxidative addition. With a diaryl ether moiety, Pd0 and Pt0 are found to be capable of performing aryl C–O bond activation. Various levels of interactions between the central arene and the metal center were observed, and these were correlated with trends in bond activation. Overall, selective cleavage of the stronger aryl ether C–O bond was observed only with Ni0 and RhI. Pd0 and Pt0 can perform aryl ether C–O cleavage, but if available, they will cleave the weaker O–Me bond. This study provides insight into the relative reactivity of group 9 and 10 metal centers with aryl ether bonds and suggests future directions for designing systems for metal-catalyzed cleavage of ether C–O bonds in synthetic methodology as well as lignin deoxygenation.

本研究针对带有芳基甲基醚（aryl–methyl ether）或芳基芳基醚（aryl–aryl ether）结构的三联苯二膦配体（terphenyl diphosphines），分别与零价金属中心（M0，M=镍、钯、铂）、一价金属中心（M'I，M'=钴、铑、铱）以及二价金属中心（MII）的反应性展开探究，旨在从机理层面解析结构相近金属配合物中分子内芳基醚键断裂（intramolecular aryl–ether bond cleavage）的过程。一价铑可将芳基甲基醚结构转化为芳基C-H键，该反应路径与此前在零价镍体系中观测到的过程高度相似，具体包含C-O氧化加成（C–O oxidative addition）、β-氢消除（β-H elimination）释放甲醛（CH₂O）、芳基C-H键的还原消除（reductive elimination）以及甲醛的脱羰基反应（decarbonylation）。一价铱会引发非选择性芳基与烷基C-O键活化（unselective aryl and alkyl C–O bond activation）。当体系中存在过量一氧化碳（excess CO）时，一价铑与一价铱的反应选择性与反应活性发生转变，转而断裂烷基C-O键。一价钴则无法实现C-O键的断裂。二价金属卤化物配合物可通过路易斯酸碱机理（Lewis acid–base mechanism）实现烷基C-O键活化，反应过程中伴随甲基氯（MeCl）的脱除。零价钯与零价铂可通过氧化加成选择性断裂O-Me键。对于带有二芳基醚（diaryl ether）结构的底物，零价钯与零价铂能够实现芳基C-O键活化。研究观测到中心芳烃环（central arene）与金属中心之间存在不同程度的相互作用，且该相互作用与键活化的趋势存在相关性。总体而言，仅在零价镍与一价铑体系中实现了较强的芳基醚C-O键的选择性断裂。零价钯与零价铂虽可实现芳基醚C-O键的断裂，但当底物存在更易断裂的O-Me键时，它们会优先断裂该弱键。本研究明确了第9族与第10族金属中心（group 9 and 10 metal centers）与芳基醚键的相对反应活性，为设计可应用于合成方法学以及木质素脱氧（lignin deoxygenation）领域的金属催化醚C-O键断裂体系指明了未来研究方向。