Understanding the Reactivity of Pd0/PR3‑Catalyzed Intermolecular C(sp3)–H Bond Arylation

• Published in: Journal of the American Chemical Society 2013-09, Vol.135 (38), p.14206-14214
• Main Authors: Figg, Travis M, Wasa, Masayuki, Yu, Jin-Quan, Musaev, Djamaladdin G
• Format: Article
• Language: eng ; jpn
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja4053416

Mechanistic details pertaining to the Pd0/PCy3-catalyzed intermolecular arylation of a terminal β-C(sp3)–H bond aryl amide substrate (SM = EtCONH-Ar, where Ar = C6H5, C6F5 and CONH-Ar is a directing group (DG)) in the presence of CsF base were elucidated. Key mechanistic features of this reaction are (1) oxidative addition of the aryl halide PhI to Pd0/PCy3, (2) deprotonation of SM by CsF to form DG′ = [EtCON-Ar]Cs+ for subsequent coordination to intermediate I–PdII(PCy3)Ph (the substantially lower pK a of the EtCONHC6F5 in comparison to EtCONHC6H5 is instrumental for the presence of a larger population of the reactive deprotonated amides for Ar = C6F5), (3) “Cs2–I–F” cluster formation upon external (the second) CsF molecule approach to the active site of the I–PdII(PCy3)Ph(DG′) intermediate, (4) “Cs2–I–F cluster” assisted β-C(sp3)–H bond activation via a concerted metalation–deprotonation (CMD) mechanism, and (5) reprotonation of the amide directing group to facilitate the C(sp3)–Ph reductive elimination. The energy barriers, ΔG ⧧ (ΔG ⧧ disp), associated with the “Cs2–I–F cluster” mediated β-C(sp3)–H bond activation transition state are 6.5 (8.7) and 10.2 (12.9) kcal/mol when DG = CONHC6H5, CONHC6F5, respectively. It was shown that (a) the PCy3 ligand only semidissociates upon β-C(sp3)–H bond cleavage and (b) the I-to-F substitution in I–[PdII](Ph)(PCy3)(DG′) is a facile process that makes the “direct-halide” assisted β-C(sp3)–H bond activation relatively less energy demanding and opens the possibility for a competing Ph–F bond formation reaction. It was shown that the “direct-I” assisted C–H bond activation TS, which associates with a relatively large energy barrier, is an H-atom insertion transition state into the Pd–I bond, while the “direct-F” assisted C–H bond activation TS, which occurs with a relatively low energy barrier (but still is much larger than that required for the “Cs2–I–F cluster” assisted pathway), is a direct proton abstraction transition state.