Arene C−H Activation at Aluminium(I): meta Selectivity Driven by the Electronics of S(N)Ar Chemistry

The reactivity of the electron‐rich anionic Al(I) aluminyl compound K(2)[(NON)Al](2) (NON=4,5‐bis(2,6‐diisopropylanilido)‐2,7‐di‐tert‐butyl‐9,9‐dimethylxanthene) towards mono‐ and disubstituted arenes is reported. C−H activation chemistry with n‐butylbenzene gives exclusively the product of activation at the arene meta position. Mechanistically, this transformation proceeds in a single step via a concerted Meisenheimer‐type transition state. Selectivity is therefore based on similar electronic factors to classical S(N)Ar chemistry, which implies the destabilisation of transition states featuring electron‐donating groups in either ortho or para positions. In the cases of toluene and the three isomers of xylene, benzylic C−H activation is also possible, with the product(s) formed reflecting the feasibility (or otherwise) of competing arene C−H activation at a site which is neither ortho nor para to a methyl substituent.