We report the first catalytic methods for the transformation of C–H bonds of unactivated arenes into C–Al bonds.

Transition metal complexes are widely applied in the physical and biological sciences. They play pivotal roles in aspects of catalysis, synthesis, materials science, photophysics and bioinorganic chemistry. Our understanding of transition metal complexes originates from Alfred Werner's realisation that their three-dimensional shape influences their properties and reactivity. 1 The intrinsic link between shape and electronic structure is now firmly underpinned by molecular orbital theory. 2-5 Despite over a century of advances in this field, transition metal complexes remain limited to a handful of well understood geometries. Archetypal geometries for six-coordinate transition metals are octahedral and trigonal prismatic. Although deviations from ideal bond angles and lengths are common, 6 alternative parent geometries are staggeringly rare. 7 Hexagonal planar transition metals are restricted to those found in condensed metallic phases, 8 the hexagonal pores of coordination polymers, 9 or clusters containing more than one transition metal in close proximity. 10,11 Although [Ni(P t Bu)6] could be assigned as a hexagonal planar complex, 12,13 a molecular orbital analysis ultimately led to the conclusion that it is best described as a 16electron complex with a trigonal planar geometry. 14 Here we report the isolation and structural characterisation of the first simple coordination complex in which six ligands form bonds with a central transition metal in a hexagonal planar arrangement. The discovery has the potential to open up new design principles and new ways of thinking about transition metal complexes which could impact multiple fields of science.

Catalytic methods that transform C−H bonds into C−X bonds are of paramount importance in synthesis. A particular focus has been the generation of organoboranes, organosilanes and organostannanes from simple hydrocarbons (X=B, Si, Sn). Despite the importance of organozinc compounds (X=Zn), their synthesis by the catalytic functionalisation of C−H bonds remains unknown. Herein, we show that a palladium catalyst and zinc hydride reagent can be used to transform C−H bonds into C−Zn bonds. The new catalytic C−H zincation protocol has been applied to a variety of arenes—including fluoroarenes, heteroarenes, and benzene—with high chemo‐ and regioselectivity. A mechanistic study shows that heterometallic Pd–Zn complexes play a key role in catalysis. The conclusions of this work are twofold; the first is that valuable organozinc compounds are finally accessible by catalytic C−H functionalisation, the second is that heterometallic complexes are intimately involved in bond‐making and bond‐breaking steps of C−H functionalisation.

The reactions of an aluminium(I) reagent with a series of 1,2-, 1,3-and 1,5-dienes are reported. In the case of 1,3-dienes the reaction occurs by a pericyclic reaction mechanism, specifically a cheletropic cycloaddition, to form aluminocyclopentene containing products. This mechanism has been interrogated by stereochemical experiments and DFT calculations. The stereochemical experiments show that the (4+1) cycloaddition follows a suprafacial topology, while calculations support a concerted albeit asynchronous pathway in which the transition state demonstrates aromatic character. Remarkably, the substrate scope of the (4+1) cycloaddition includes dienes that are either in part, or entirely, contained within aromatic rings. In these cases, reactions occur with dearomatisation of the substrate and can be reversible. In the case of 1,2-or 1,5-dienes complementary reactivity is observed; the orthogonal nature of the C=C p-bonds (1,2-diene) and the homoconjugated system (1,5-diene) both disfavour a (4+1) cycloaddition. Rather, reaction pathways are determined by an initial (2+1) cycloaddition to form an aluminocyclopropane intermediate which can in turn undergo insertion of a further C=C p-bond leading to complex organometallic products that incorporate fused hydrocarbon rings.

In the presence of a catalytic quantity of [Pd(PCy3)2], a reagent containing a Mg–Mg bond effects the C–H functionalisation of benzene resulting in a 100% atom efficient transformation to generate an unprecedented aryl magnesium hydride.