Insights into Directing Group Ability in Palladium-Catalyzed C−H Bond Functionalization

Abstract

This paper describes a detailed investigation of factors controlling the dominance of a directing group in Pd-catalyzed ligand-directed arene acetoxylation. Mechanistic studies, involving reaction kinetics, Hammett analysis, kinetic isotope effect experiments, and the kinetic order in various reagents, have been conducted for a series of different substrates. Initial rates studies of substrates bearing different directing groups showed that these transformations are accelerated by the use of electron withdrawing directing groups. However, in contrast, under conditions where two different directing groups are in competition with one another in the same reaction flask, substrates with electron donating directing groups react preferentially. These results are discussed in the context of the proposed mechanism for Pd-catalyzed arene acetoxylation.

The Scope of Ambiphilic Acetate‐Assisted Cyclometallation with Half‐Sandwich Complexes of Iridium, Rhodium and Ruthenium

“…A similar study on relative directing group ability at palladium also concluded that the C À H activation step did not correspond to the overall selectivity in some palladium-catalysed CÀH activation reactions. [31] 2 A C H T U N G T R E N N U N G (p-cymene)} 2 ] cyclometallation is less thermodynamically favourable, hence fewer ligands cyclometallate. The ligands can be divided into three categories, those that cyclometallate with all three metals (pyridine, pyrazole, alkyl imine), those that work well with Ir but fail with either Rh or Ru (aryl imine, oxazoline), and those that under these conditions only work with Ir (oxime, triazole, imidazole, amine).…”

High‐Oxidation‐State Palladium Catalysis: New Reactivity for Organic Synthesis

“…[21] With the oxidant iodosobenzene diacetate, PhI(OAc) 2 , the corresponding acetoxylation product is formed in acetonitrile, whereas selective alkoxylation reactions are possible in alcoholic solvents. The CÀH functionalization of single arenes proceeds with high selectivity [22] for the ortho position; thus, this reaction provides unprecedented access to higher functionalized aromatic compounds, such as 2-acetoxyazobenzene (Scheme 5 b). Replacement of the oxidant PhI(OAc) 2 with a combination of Oxone and acetic acid gave a system of comparable efficiency for the ortho acetoxylation of acetophenone and aniline derivatives (Scheme 5 c,d).…”

Palladium complexes of heterobidentate ligands: Active catalysts for direct acylation of aryl halides with aldehydes via C(sp 2 )-H activation

“…have been deployed for the C H activation reactions. The aldehyde C H activation by direct acylation of aryl halides with aliphatic aldehydes is an interesting prospect to get alkyl-aryl ketones; which are potential intermediates in different pharmaceutical, fragrance, dye and agrochemical industries [14][15][16][17][18][19][20][21][22]. Friedel-Crafts acylation has been a traditional way of acylating arenes, which has the strict limitation of reacting with activated benzene derivatives only [23].…”