C-H magnesiation of unactivated arenes by a bulky β-diketiminato magnesium n-butyl complex.

The bulky monomeric n-butyl magnesium complex [(AdBDI)Mg n Bu] (1Ad; AdBDI = HC(C(Ad)Ndipp)2, dipp = 2,6-diisopropylphenyl; Ad = 1-adamantyl) deprotonates benzene to provide direct access to the corresponding phenylmagnesium derivative [(AdBDI)MgPh] (3Ad) which, in turn, can be utilised in palladium-catalysed Kumada biaryl cross-coupling. By contrast, when the less sterically demanding methyl-substituted ligand, MeBDI (HC(C(Me)Ndipp)2) is used, the deprotonation reaction is slower and competitive with β-hydride elimination of 1-butene. The enhanced steric demand of 1Ad results in the deprotonation of benzene, even in the presence of more acidic 1,2,3,4-tetramethylcyclopentadiene (Cp4MH). In contrast, Cp4MH is deprotonated by [(MeBDI)Mg(µ- n Bu)]2 ([1Me]2) in benzene to yield [(MeBDI)Mg(η5-Cp4M)] (4). Compound 1Ad also mediates the kinetically undiscriminating magnesiation of the C(sp3)-H and meta- and para-C(sp2)-H bonds of toluene, ortho-xylene, and meta-xylene at 120 °C, albeit para-xylene is deprotonated exclusively in the C(sp3)-H position while 1,3-di-tertbutylbenzene is selectively deprotonated at the 5-position of the aromatic ring. Both 1Ad and [1Me]2 mediate the directed ortho-magnesiation of anisole. The mechanism of benzene- and toluene deprotonation and a possible origin for the contrasting reactivity of 1Ad and [1Me]2 has been investigated computationally.