Calcium‐Catalyzed Arene C−H Bond Activation by Low‐Valent Al I

Abstract

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.