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

The low-valent ß-diketiminate complex ((DIPP) BDI)Al is stable in benzene but addition of catalytic quantities of [((DIPP) BDI)CaH](2) at 20 °C led to ((DIPP) BDI)Al(Ph)H ((DIPP) BDI=CH[C(CH(3) )N-DIPP](2) , 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 ((DIPP) BDI)Al(m-tolyl)H with I(2) gave m-tolyl iodide, H(2) and ((DIPP) BDI)AlI(2) which was recycled to ((DIPP) BDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes ((DIPP) BDI)Al(H)M((DIPP) 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: ((DIPP) BDI)Al(H)Ca((DIPP) BDI). This is in equilibrium with the hydride bridged complex ((DIPP) BDI)Al(μ-H)Ca((DIPP) 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.