A bulky carboxylic acid bearing three cyclohexylmethyl substituents at the α-position, namely, tri(cyclohexylmethyl)acetic acid, is demonstrated to act as an efficient ligand source in Pd-catalyzed intramolecular C(sp )-H and C(sp )-H arylation reactions. The reactions proceed smoothly under mild reaction conditions, even at room temperature due to the steric bulk of the carboxylate ligands, which accelerates the rate-determining C-H bond activation step in the catalytic cycle.

A bulky carboxylic acid bearing one 1-adamantylmethyl and two methyl substituents at the α-position is demonstrated to work as an efficient carboxylate ligand source in Pd-catalyzed intermolecular C(sp2)–H bond arylation reactions.

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Ab ulky carboxylic acid bearing three cyclohexylmethyl substituents at the a-position, namely,t ri(cyclohexylmethyl)acetic acid, is demonstrated to act as an efficient ligand source in Pd-catalyzed intramolecular C(sp 2 )ÀHa nd C(sp 3 )À Ha rylation reactions.T he reactions proceed smoothly under mild reaction conditions,even at room temperature due to the steric bulk of the carboxylate ligands,w hich accelerates the rate-determining C À Hb ond activation step in the catalytic cycle.Carboxylate ligands have proven to be invaluable components in transition-metal-catalyzed carbon-hydrogen (CÀH) bond activation reactions. [1] Pioneering studies reported by Fagnou and co-workers showed that in Pd-catalyzed C À H arylation reactions,the pivalate ligand is particularly efficient compared to the conventional acetate ligand. [2] Thea ddition of pivalic acid allows the use of milder reaction conditions in various intramolecular CÀHarylation reactions [3] as well as in the intermolecular C À Ha rylation of electron-deficient arenes [4] and electron-rich heteroarenes. [5] Pivalate is aslightly stronger base than acetate (the pK a values [6] of pivalic acid and acetic acid are 5.03 and 4.76, respectively), which facilitates the intramolecular deprotonation step. [7] In contrast, the steric effect of the carboxylate ligand could be crucial. However,o nly af ew studies have elucidated the specific effect of the steric bulk of carboxylate ligands on catalytic C À Hb ond activation reactions. [3e,5g, 8] In fact, only bulky carboxylic acids such as pivalic and 1-adamantanecarboxylic acids are currently employed as efficient carboxylate ligand sources. [2][3][4][5] Recently,T hompson et al. extensively studied the steric effect of carboxylate ligands using 24 different carboxylic acids as additives in Pd-catalyzed direct arylation polymerization (DArP) at 160 8 8C. [9] Ther esults indicated that the yields of the polymers were not affected by the steric bulk of added carboxylic acids,whereas the smallest one,namely,cyclopropanecarboxylic acid, provided apolymer with very high molecular weight.We have previously developed various sterically bulky ligands,such as pyridines, [10a] amines, [10b] phosphines, [10c-e] and N-heterocyclic carbenes, [10f] that enable high catalytic activ-

Paddle‐Wheel dirhodium complexes bearing bulky carboxylate ligands were synthesized and characterized. The steric bulk of carboxylate ligands could affect the reaction selectivity in Rh‐catalyzed intramolecular competitive carbene insertions: Rh catalysts with bulky carboxylates could provide five‐membered ring products preferentially via the insertion into a carbon–hydrogen bond while conventional Rh catalysts gave six‐membered ring products via the insertion of a carbon‐carbon double bond. More information can be found in the Research Article by Tetsuaki Fujihara et al.