Air-Stable Pd(R-allyl)LCl (L = Q-Phos, P(t-Bu)3, etc.) Systems for C-C/N Couplings: Insight into the Structure Activity Relationship and Catalyst Activation Pathway

A series of Pd(R-allyl)LCl complexes [R = H, 1-Me, 1-Ph, 1-gem-Me-2, 2-Me; L = Q-Phos, P(t-Bu)(3), P(t-Bu)(2)-(p-NMe2C6H4), P(t-Bu)(2)Np] have been synthesized and evaluated in the Buchwald Hartwig aminations in detail, in addition to the preliminary studies on Suzuki coupling and a-arylation reactions. Pd(crotyl)Q:PhosCl (9) was found to be a superior catalyst to the other QPhos-based catalysts, and the reported in situ systems, in model coupling reactions involving 4-bromoanisole substrate with either N-methylaniline or 4-tert-butylbenzeneboronic acid. Precatalyst 9 also performed better than the catalysts bearing P(t-Bu)(2)(p-NMe2C6H4) ligand; however, it is comparable to the new crotyl catalysts bearing P(t-Bu)(3) or P(t-Bu)(2)Np ligands. In alpha-arylation of a biologically important model substrate, 1-tetralone, Pd(allyl)P(t-Bu)(2)(p-NMe2C6H4)Cl (15) was found to be the best catalyst. The reason for the relatively higher activity of the crotyl complexes in comparison to the allyl derivatives in C-N bond formation reactions was investigated using X-ray crystallography in conjunction with NMR spectroscopic studies.