Reversible Insertion of Ir into Arene Ring C–C Bonds with Improved Regioselectivity at a Higher Reaction Temperature

Abstract

Regioselective metal insertion into aromatic C−C bonds is a long-standing problem critical for development of new arene functionalizations and cleaner conversion of fossil fuel into value-added chemicals. We report reversible insertion of iridium into the aromatic C−C bonds of η 4 -bound methyl arenes to give eight-membered diiridium metallacycles with yields up to 99%. While at 50−100 °C the reaction yields a mixture of isomers corresponding to iridium insertion in both unsubstituted and Me-substituted ring C−C bonds, at 150 °C a single isomer dominates. Kinetic and DFT studies suggest that at 150 °C insertion of iridium is reversible, allowing equilibration of the metallacycle products via a diiridium arene sandwich complex. The selectivity of metal insertion is determined by the relative stabilities of isomeric metallacycles governed by steric repulsion between methyl groups of the hydrocarbon chain of the cleaved arene and the Cp* ligands.

Rationalizing the AlI‐Promoted Oxidative Addition of C−C Versus C−H Bonds in Arenes

“…[9] Compared to the activation of CÀX( X = halogen, heteroatom, hydrogen) bonds, the CÀCb ond cleavage in unstrained arenes, and particularly in benzene, has been much more underdeveloped. [10] This is mainly due not only to the loss of aromaticity during the process but also to the sterically protected and directional nature of the CÀCb ond. Despite that, Aldridge and co-workers very recently reported that the aluminum(I) species 1,i solatedf rom its dimericp otassium aluminyl counterpart, [11] can be used to oxidatively activate the CÀCb ond in benzene, at room temperature and in ar eversible manner,t o produce the aluminum(III)-cycloheptatriene 2 (Scheme 1a).…”

Reversible, Room-Temperature C—C Bond Activation of Benzene by an Isolable Metal Complex

“…[7][8][9] Not unexpectedly, examples of metal-centred reactivity which lead to C-C bond cleavage in unstrained arenes are very rare, reflecting not only thermodynamic factors relating to aromatic stabilization, but also kinetic limitations due to the sterically protected and highly directional nature of the C-C bond. [10][11][12][13][14][15][16][17][18][19] Oxidative activation of the C-C bond in benzene is therefore known only in conjunction with transient, highly reactive species which are generated in situ (such as carbenes or triplet silylenes). [20][21][22][23] A well-known example is the Buchner ring expansion reaction, which classically involves the reaction of an unactivated arene with a carbene derived from a diazo-acetic ester by a thermal, photolytic or transition-metal catalysed protocol.…”

Transition‐Metal‐Mediated Cleavage of C−C Bonds in Aromatic Rings

“…The scope of the methyl arenes undergoing this C−C cleavage was further expanded to all xylenes (Scheme ) . While regioselectivity of the insertion into xylenes and toluene was modest to good at 50–100 °C, it increased with temperature, exceeding 93 % selectivity for most substrates at 150 °C (Scheme a).…”

“…The scope of the methyl arenes undergoing this CÀCc leavage was further expanded to all xylenes (Scheme 6). [22] While regioselectivity of the insertioni nto xylenes and toluenew as modest to good at 50-100 8C, it increased with temperature, exceeding 93 %s electivity for most substrates at 150 8C (Scheme 6a). Moreover,t he regioselectivity of Ir insertion into m-xylene in complex 6 was completely switched by changing the reaction temperature from 50 to 150 8Ct og ive predominantly regioisomer 7 over 8 (Scheme 6b).…”