Heck Alkynylation (Copper-Free Sonogashira Coupling) of Aryl and Heteroaryl Chlorides, Using Pd Complexes of t‑Bu2(p‑NMe2C6H4)P: Understanding the Structure–Activity Relationships and Copper Effects

L2Pd­(0) and L2Pd­(II) complexes, where L= t-Bu2(p-NMe2C6H4)­P, have been identified as efficient catalyst systems for the Heck alkynylation of a variety of aryl bromides (17 examples) and aryl/heteroaryl chlorides (31 examples) with a range of aryl- and alkyl-acetylenes in excellent yields, under relatively low Pd loadings. The single-crystal X-ray structure determination of the presumably active catalytic species, L2Pd­(0), was carried out in this study to better understand the superior activity of the current catalyst system from a structure–activity relationship point of view. The P–Pd–P bond angle indicates that the complex is bent (174.7°) in comparison to the perfectly linear (180.0°) structure of the analogous Pd­(t-Bu3P)2. Preliminary mechanistic studies on the negative copper effect and substrate effect of aryl acetylenes were conducted to better understand the cross-coupling pathway of Heck alkynylation.