Calcium-Catalyzed Arene C-H Bond Activation by Low-Valent AlI

The low-valent ss-diketiminate complex ((BDI)-B-DIPP)Al is stable in benzene but addition of catalytic quantities of [((BDI)-B-DIPP)CaH](2) at 20 degrees C led to ((BDI)-B-DIPP)Al(Ph)H ((BDI)-B-DIPP=CH[C(CH3)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 ((BDI)-B-DIPP)Al(m-tolyl)H with I-2 gave m-tolyl iodide, H-2 and ((BDI)-B-DIPP)AlI2 which was recycled to ((BDI)-B-DIPP)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes ((BDI)-B-DIPP)Al(H)M((BDI)-B-DIPP) (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)-B-DIPP)Al(H)Ca((BDI)-B-DIPP). This is in equilibrium with the hydride bridged complex ((BDI)-B-DIPP)Al(mu-H)Ca((BDI)-B-DIPP) 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.