Calcium-Catalyzed Arene C-H Bond Activation by Low-Valent Al.

The low-valent ß-diketiminate complex ( BDI)Al is stable in benzene but addition of catalytic quantities of [( BDI)CaH] at 20 °C led to ( BDI)Al(Ph)H ( BDI=CH[C(CH )N-DIPP] , DIPP=2,6-diisopropylphenyl). Similar Ca-catalyzed C-H bond activation is demonstrated for toluene or p-xylene. For toluene a remarkable selectivity for meta-functionalization has been observed. Reaction of ( BDI)Al(m-tolyl)H with I gave m-tolyl iodide, H and ( BDI)AlI which was recycled to ( BDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes ( BDI)Al(H)M( BDI) (M=Mg, Zn) were formed. DFT calculations on the Ca hydride catalyzed arene alumination suggest that a similar but more loosely bound complex is formed: ( BDI)Al(H)Ca( BDI). This is in equilibrium with the hydride bridged complex ( BDI)Al(μ-H)Ca( BDI) which shows strongly increased electron density at Al. The combination of Ca-arene bonding and a highly nucleophilic Al center are key to facile C-H bond activation.