Mechanism of Palladium-Catalyzed C-N Coupling with 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) as a Base

The mechanism of the Buchwald-Hartwig amination assisted by the base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) is explored with density fiinctional theory (DFT) calculations. Whereas the previous study indicates that the tight binding of DBU to the Pd(II) center could block the desired catalytic pathways, the recent work from Buchwald and co-workers demonstrates that the bulkier ligands, such as AlPhos, can overcome the catalyst deactivation by DBU and yield the C-N-coupled product at room temperature. The results show that the bulkier ligands sterically hinder DBU binding and enable the desired catalytic steps by improving the Pd-substrate coordination. This study confirms quantitatively and precisely how and at which step of the catalysis the steric demands engineered into the phosphine ligands improve the catalytic C-N coupling using an organic base.