sp2 C-H bond activation in water and catalytic cross-coupling reactions

The metal-catalysed successive activation and functionalisation of sp(2) C-H bonds is the at heart of synthetic innovations for the development of C-C bond cross-coupling processes. Against expectation catalytic C-H bond transformations can be performed in water as an available, renewable, safe solvent but most importantly as a partner improving the catalyst activity. The objective of the review is to present the catalytic successes for C-H bond transformations in water, discovered mainly during the last six years and involving mostly palladium and ruthenium catalysts, often with the help of a carboxylate partner for the initial key C-H bond deprotonation. Water is beneficial for the direct catalytic arylation with (hetero)aryl halides of functional arene ortho C-H bonds with pyridine, pyrazole, oxazoline, imine, urea, amide… directing groups leading to functional biaryl derivatives, polyheterocycles and polydentate ligands. Metal-catalysed activation of the sp(2) C-H bond in water also allows the cross-coupling reaction of two different C-H bonds in the presence of an oxidant and regioselective alkenylations of arenes, heterocycles and functional alkenes are now controlled. Annulation reactions via insertion of alkynes into both activated C-H and heteroatom-hydrogen bonds in water constitute new routes to heterocycles.