Nitrones as directing groups in transition metal-catalysed C-H activation

During about the last 15 years, nitrones have emerged as versatile directing groups for transition metal-catalysed C-H activation reactions. Apart from simple functionalization of the aryl ring attached to the nitrone moiety, the nitrone functional group can play the role of a transient directing group or an internal oxidant, allowing for the design of redox-neutral reactions initiated by a C-H activation step. Moreover, the dipolar character of nitrones has been exploited in cascade reactions involving C-H activation followed by 1,3-dipolar cycloaddition, resulting in the incorporation of the nitrone moiety into the structure of polycyclic, nitrogen-containing scaffolds found in biologically active compounds like aminoalcohols, indoles, carbazoles, quinolines, etc. Reactions of all of the above types, as well as the C(sp²)-H functionalization of the aldonitrone moiety itself, are the focus of this review. Apart from the already well-developed Rh, Ru and Pd-catalysed reactions, emerging applications of cobalt catalysis for nitrone functionalisation are discussed as well. Catalytic reactions of isoelectronic azomethine imines are also mentioned in connection with analogous reactions of nitrones.