C-H Activation

Metal catalysed C-H activation followed by C-C bond formation is potentially extremely efficient in terms of atom economy and chemoselectivity and is thus a very desirable transformation from a Green Chemistry point of view. Currently the most successful strategy involves cyclometallation reactions. A few years ago we discovered an acetate-assisted C-H activation which provides a very mild (room temperature) route to cyclometallated half-sandwich complexes.

In collaboration with a computational chemistry group (Prof S. A. Macgregor, Heriot Watt University) we examined the mechanism. Our intial results on cyclometallation with palladium acetate showed that the reaction goes by an electrophilic like activation, however, this occurs via an agostic intermediate rather than the traditionally assumed Wheland intermediate, with considerable intramolecular hydrogen bonding to a coordinated acetate involving a 6-membered transition state. We have subsequently shown that a similar mechanism operates for formation of Cp*Ir cyclometallated complexes (see fig below for calculated transition states).

Asymmetric Catalysis
Asymmetric Catalysis

This is the first electrophilic C-H activation for iridium and the first which doesn’t proceed via oxidative addition for a Cp*iridium species. These cyclometallations are examples of a new mechanism for C-H activation involving simultaneous activation by a Lewis acidic metal (agostic interaction) and a basic ligand (hydrogen bond) which operate in a synergistic manner to provide a low energy pathway to C-H activation. We have termed this Ambiphilic Metal Ligand Activation. We have now shown that this process can also be used to achieve sp3 CH activation. We have also surveyed the types of directing groups that can be used.

AMLA CH activation can be incorporated into a catalytic cycle to achieve catalytic CH functionalisation. We have studied the mechanisms of these processes and have identified intermedaites from CH activation followed by alkyne insertion.

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Department of Chemistry
University of Leicester
Leicester, LE1 7RH, UK

Email: chemistry@le.ac.uk

Tel: [+44] (0)116 252 2100

Fax: [+44] (0)116 252 3789

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