Dioxygen-Promoted Regioselective Oxidative Heck Arylations of Electron-Rich Olefins with Arylboronic Acids

Abstract

Arylations of electron-rich heteroatom-substituted olefins were performed with arylboronic acids. This appears to constitute the first example of palladium(II)-catalyzed internal Heck arylations. The novel protocol exploits oxygen gas for environmentally benign reoxidation and a stable 1,10-phenanthroline bidentate ligand to promote the palladium(II) regeneration and to control the regioselectivity. Internal arylation is strongly favored with electron-rich arylboronic acids. DFT calculations support a charge-driven selectivity rationale, where phenyls substituted with electron-donating groups prefer the electron-poor alpha-carbon of the olefin. Experiments, verified by calculations, confirm the cationic nature of the catalytic route. This Heck methodology provides a facile and mild access to functionalized enamides. Controlled microwave heating and increased oxygen pressure were used to further reduce the reaction time to 1 h.

Microwave‐Promoted Palladium(II)‐Catalyzed CP Bond Formation by Using Arylboronic Acids or Aryltrifluoroborates

“…The use of electron-poor arylboronic acids 1 j and 1 l resulted in good isolated yields of the corresponding phosphonate products 2 h and 2 j ( Table 2, entries 11 and 13), an outcome that is not always realized in Pd II transformations with electron-poor arylboronic acids. [11] Owing to competing debromination, a significantly lower yield of product 2 i was obtained by using m-bromo aryl trifluoroborate 1 k as compared with the p-bromo substituted arylboronic acid counterpart 1 j ( Table 2, entries 12 and 11). Excellent chemoselectivity was observed in the coupling of both 4-bromophenylboronic acid 1 j and 3-bromophenyltrifluoroborate 1 k, without any trace of the Suzuki product ( Table 2, entries 11 and 12).…”

“…[8] As a consequence of this advancement, a number of MW-assisted protocols for Pd 0 -catalyzed C À P bond formation can be found in the literature. [6,9] During the last few years we have focused [10][11][12][13] on developing Pd II -catalyzed oxidative Heck reactions, [14] and we now aim to extend the useful concept of Pd II -catalysis to other coupling reactions. To the best of our knowledge, the use of Pd II catalysis and arylboronic acids or aryl trifluoroborates as starting materials remains totally unexplored in the field of C À P bond formation.…”

Sinteza I Primjena Diiminskih Liganada U Oksidativnoj Heckovoj Reakciji

“…Oni imaju sposobnost efikasne reoksidacije paladijuma (0) u paladijum (II) korištenjem molekularnog kiseonika. Takođe, njihova jeftina cijena kao i otpornost i velika stabilnost u prisustvu kiseonika je velika prednost u poređenju sa fosfatnim ligandima (4)(5)(6)(7)(8). Stabiliziranje katalizatora ligandima je omogućilo smanjenje korištenja katalizatora sve do 1 % u poređenju sa 10 ili 20 % koliko je korišteno u prethodnim slučajevima.…”

Recent Developments in Microwave‐Assisted, Transition‐Metal‐Catalysed C–C and C–N Bond‐Forming Reactions

“…[49] The reaction was performed on a multigram scale in a multimode autoclave reactor applying a continuous flow of oxygen (Scheme 36). An excellent α/β selectivity of 97:3 was obtained and a yield of 66 %.…”