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

Abstract

The low-valent ß-diketiminate complex (DIPPBDI)Al is stable in benzene but addition of catalytic quantities of [(DIPPBDI)CaH]2 at 20 °C led to (DIPPBDI)Al(Ph)H (DIPPBDI=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 (DIPPBDI)Al(m-tolyl)H with I2 gave m-tolyl iodide, H2 and (DIPPBDI)AlI2 which was recycled to (DIPPBDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes (DIPPBDI)Al(H)M(DIPPBDI) (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: (DIPPBDI)Al(H)Ca(DIPPBDI). This is in equilibrium with the hydride bridged complex (DIPPBDI)Al(μ-H)Ca(DIPPBDI) 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.