Selective ortho-C-H Activation in Arenes without Functional Groups.

Aromatic C-H activation in alkylarenes is a key step for the synthesis of functionalized organic molecules from simple hydrocarbon precursors. Known examples of such C-H activations often yield mixtures of products resulting from activation of the least hindered C-H bonds. Here we report highly selective ortho-C-H activation in alkylarenes by simple iridium complexes. We demonstrate that the capacity of the alkyl substituent to override the typical preference of metal-mediated C-H activation for the least hindered aromatic C-H bonds results from transient insertion of iridium into the benzylic C-H bond. This enables fast iridium insertion into the ortho-C-H bond, followed by regeneration of the benzylic C-H bond by reductive elimination. Bulkier alkyl substituents increase the ortho selectivity. The described chemistry represents a conceptually new alternative to existing approaches for aromatic C-H bond activation.

Selective, radical-free activation of benzylic C-H bonds in methylarenes.

We report rare examples of exclusive benzylic C-H oxidative addition in industrially important methylarenes using simple η4-arene iridium complexes. Mechanistic studies showed that coordinatively unsaturated η2-arene intermediates are responsible for the selective activation of benzylic, not aromatic C-H bonds and formation of stable benzyl complexes after trapping with a phosphine ligand.

Selective cleavage of unactivated arene ring C-C bonds by iridium: key roles of benzylic C-H activation and metal-metal cooperativity.

The cleavage of aromatic C-C bonds is central for conversion of fossil fuels into industrial chemicals and designing novel arene functionalisations through ring opening, expansion and contraction. However, the current progress is hampered by both the lack of experimental examples of selective oxidative addition of aromatic C-C bonds and limited understanding of the factors that favour insertion into the C-C rather than the C-H bonds. Here, we describe the comprehensive mechanism of the only reported chemo- and regioselective insertion of a transition metal into a range of substituted arene rings in simple iridium(i) complexes. The experimental and computational data reveal that this ring cleavage requires both reversible scission of a benzylic C-H bond and cooperativity of two Ir centres sandwiching the arene in the product-determining intermediate. The mechanism explains the chemoselectivity and scope of this unique C-C activation in industrially important methylarenes and provides a general insight into the role of metal-metal cooperativity in the cleavage of unsaturated C-C bonds.

A family of rhodium(i) NHC chelates featuring O-containing tethers for catalytic tandem alkene isomerization-hydrosilylation.

The rhodium complex Rh(HL)(COD)Cl, 1, L being a functionalized N-heterocyclic carbene (NHC) ligand with an oxygen-containing pendant arm, has been used as the entry point to synthesize a series of neutral and cationic Rh(i) O,C chelates. While the Rh-carbene interaction is similar in all these 16-electron complexes, structural analysis reveals that the strength of the Rh-O bond is greatly affected by the nature of the O-donor: R-O- > R-OH > R-OBF3. These subtle changes in the nature of the O-containing tether are found to be responsible for large differences in the alkene hydrosilylation catalytic activity of these compounds: the stronger the Rh-O interaction, the better the catalytic performances. The most active catalyst, [Rh(L)(COD)], 2, demonstrated good catalytic activity under mild reaction conditions for the hydrosilylation of a range of alkene substrates with the industrially relevant non-activated tertiary silane, 1,1,1,3,5,5,5-heptamethyltrisiloxane (MDHM). Furthermore, this complex is an effective catalyst for the selective remote functionalization of internal olefins at room temperature via tandem alkene isomerization-hydrosilylation.

Developing a Highly Active Catalytic System Based on Cobalt Nanoparticles for Terminal and Internal Alkene Hydrosilylation.

This work describes the development of easy-to-prepare cobalt nanoparticles (NPs) in solution as promising alternative catalysts for alkene hydrosilylation with the industrially relevant tertiary silane 1,1,1,3,5,5,5-heptamethyltrisiloxane (MDHM). The Co NPs demonstrated high activity when used at 30 °C for 3.5-7 h in toluene, with catalyst loadings 0.05-0.2 mol %, without additives. Under these mild conditions, a set of terminal alkenes were found to react with MDHM, yielding exclusively the anti-Markovnikov product in up to 99% yields. Additionally, we demonstrated the possibility of using UV irradiation to further activate these cobalt NPs not only to enhance their catalytic performances but also to promote tandem isomerization-hydrosilylation reactions using internal alkenes, among them unsaturated fatty ester (methyl oleate), to produce linear products in up to quantitative yields.

Transition-Metal-Mediated Cleavage of C-C Bonds in Aromatic Rings.

Metal-mediated cleavage of aromatic C-C bonds has a range of potential synthetic applications: from direct coal liquefaction to synthesis of natural products. However, in contrast to the activation of aromatic C-H bonds, which has already been widely studied and exploited in diverse set of functionalization reactions, cleavage of aromatic C-C bonds remains Terra incognita. This Minireview summarizes the recent progress in this field and outlines key challenges to be overcome to develop synthetic methods based on this fundamental organometallic transformation.

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

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 η4-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.

Selective Arene Cleavage by Direct Insertion of Iridium into the Aromatic Ring.

We report an unprecedented selective cleavage of aromatic C-C bonds through the insertion of well-defined iridium complexes into the aromatic ring of simple alkylarenes. The insertion occurs at 50-100 °C without the activation of weaker C-H and C-C bonds and gives unique metallacycles in high yields. Key to the success of this approach is metal-induced deformation of the arene ring, which creates temporary ring strain and promotes direct and selective insertion of the metal into the otherwise inert arene ring C-C bonds.