Tandem β‐Boration/Arylation of α,β‐Unsaturated Carbonyl Compounds by Using a Single Palladium Complex To Catalyse Both Steps

Diphenyl(3‐methyl‐2‐indolyl)phosphine (C₉H₈NPPh₂, 1 ) gives stable dimeric palladium(II) complexes that contain the phosphine in P,N‐bridging coordination mode. On treating 1 with [Pd(O₂CCH₃)₂], the new complexes [Pd(μ‐C₉H₇NPPh₂)(NCCH₃)]₂ ( 2 ) or [Pd(μ‐C₉H₇NPPh₂)(μ‐O₂CCH₃)]₂ ( 3 ) were isolated, depending on the solvent used, acetonitrile or toluene, respectively. Further reaction of 3 with the ammonium salt of 1 led to the substitution of one carboxylate ligand to afford [Pd(μ‐C₉H₇NPPh₂)₃(μ‐O₂CCH₃)] ( 4 ), in which the bimetallic unit is bonded by three C₉H₇NPPh₂^? moieties and one carboxylate group. Using this methodology, [Pd₂(μ‐C₆H₄PPh₂)₂(μ‐C₉H₇NPPh₂)(μ‐O₂CCX₃)] (X=H ( 7 ); X=F ( 8 )) were synthesised from the ortho‐metalated compounds [Pd(C₆H₄PPh₂)(μ‐O₂CCX₃)]₂ (X=H ( 5 ); X=F ( 6 )). Complexes 3 , 4 , 7 , and 8 have been found to be active in the catalytic β‐boration of α,β‐unsaturated esters and ketones under mild reaction conditions. Hindrance of the carbonyl moiety has an influence on the reaction rate, but quantitative conversion was achieved in many cases. More remarkably, when aryl bromides were added to the reaction media, complex 7 induced a highly successful consecutive β‐boration/cross‐coupling reaction with dimethyl acrylamide as the substrate (99 % conversion, 89 % isolated yield).