Gerichtete Funktionalisierung von C-H-Bindungen: jetzt auchmeta-selektiv

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

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.

Biomimetic Aerobic C-H Olefination of Cyclic Enaminones at Room Temperature: Development toward the Synthesis of 1,3,5-Trisubstituted Benzenes

A green and mild protocol for the dehydrogenative olefination of cyclic enaminones was devised via palladium catalysis at room temperature using oxygen as the terminal oxidant. The synthetic utility of the olefinated cyclic enaminones afforded a series of unique 1,3,5-trisubstituted benzenes via an unanticipated Diels-Alder tandem reaction. The broad substrate scope and good yields achieved with this new protocol provide an alternative pathway for arene functionalization.