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-Me2, 2-Me; L = Q-Phos, P(t-Bu)3, P(t-Bu)2(p-NMe2C6H4), P(t-Bu)2Np] have been synthesized and evaluated in the Buchwald–Hartwig aminations in detail, in addition to the preliminary studies on Suzuki coupling and α-arylation reactions. Pd(crotyl)Q-PhosCl (9) was found to be a superior catalyst to the other Q-Phos-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)2Np ligands. In α-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.