Directed ortho C–H borylation catalyzed using Cp*Rh(iii)–NHC complexes

Abstract

Cp*Rh(NHC) complexes with bulky chiral bidentate NHC-carboxylate ligands were efficiently synthesized and fully characterized including solid-state structures. These unprecedented rhodium(iii) complexes demonstrated high selectivity in pyridine-directed ortho-C-H borylation of arenes under mild conditions.

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

“…Research areas where metal mediated organic molecule transformations were conducted using bidentate NHC ligands to a lesser degree include reduction of carbonyl compounds using hydrosilylation process [ 209 , 214 , 215 , 216 , 217 , 218 , 219 ], isomerization of allylic alcohols [ 220 ], cross metathesis [ 221 ], arene borylation [ 222 , 223 ], amination [ 224 ], reduction of sulfoxides [ 225 ], alcohol oxidation [ 132 , 226 , 227 ], CO 2 reduction [ 228 , 229 , 230 , 231 ], norbornene polymerization [ 232 , 233 , 234 ], alkylation of amines [ 235 ], C-H borylation [ 236 ], CO 2 activation [ 237 ], hydroamination [ 238 ], functionalization of propane [ 239 , 240 ], ylide cycloaddition [ 74 ], three component coupling [ 241 ], hydroxylation of benzene [ 242 ], hydroaminoethlyation [ 243 ], water oxidation [ 244 ], amine alkylation [ 245 ], homopolymerization of ethylene [ 246 ], destruction of chemical warfare [ 247 ], Friedel-Crafts alkylation [ 248 ], C–H arylation [ 249 ], C–H oxidation [ 250 ], SABRE catalyst [ 251 ], and alcohol amidation reactions [ 211 ]. S...…”

Visible Light Induced Rhodium(I)‐Catalyzed C−H Borylation

“…[17] We recently developed an ew class of bidentate carboxylate NHC ligands, [18] which proved to be beneficial for Cp*Rh III -based catalysts in aromatic and aliphatic CÀHb ond borylations under thermal conditions. [19] These bidentate NHC-carboxylate ligands have also been recently used in the preparation of particularly robust luminescent octahedral Ir III complexes,w hich were shown to display some interesting photophysical properties useful in the context of visible light induced catalysis. [20] Therefore, with the objective to achieve borylation under the mildest reaction conditions possible,w eh ypothesized that aR h I complex having as trong electron-donating bidentate NHC- carboxylate ligand in association with ar elatively labile 1,5cyclooctadiene (cod) ligand should afford simple visible light absorbing complexes that fit the requirements to achieve the desired photocatalytic activity.T othis end, aseries of aminoacid-based NHC ligand precursors (1)w ere converted into their corresponding silver intermediates and then transmetalated using [Rh(cod)Cl] 2 to afford the desired Rh I complexes in good yields ( Figure 2).…”

“…to examine steric and electronic influences on the reaction efficiency.T he yield of the ortho-borylated product 4 was determined by 1 HNMR spectroscopy and the crude reaction mixture was then treated with Oxone to allow the efficient isolation of the corresponding hydroxylated product 5. [19] Aryl groups bearing asubstituent at para-(3c,3d), meta-(3e), and ortho-positions (3f)w ere well tolerated, affording the functionalized products (5c-f)i ng ood yields upon isolation. When compared with the result obtained for the electron-rich 3g,aslight decrease of reactivity was observed with the electron-deficient 3h and 3i.2 -phenylpyridine bearing at rifluoromethyl (3j)and aphenyl (3k)substituent on the pyridyl group were also amenable substrates.B orylation of the 1naphthyl fragment of the sterically congested substrate 3l was highly efficient and allowed the isolation of 5l in racemic form.…”

Design of Chiral NHC‐Carboxylates as Potential Ligands for Pd‐Catalyzed Enantioselective C−H Activation

“…Amino acid-derived chiral NHC-carboxylates (4), wherein the stereogenic center and carboxylate group are located on the same side of the imidazole ring, were previously reported by Baslé, Mauduit and coworkers. These NHCs were employed in enantioselective Cu-catalyzed reactions [27,28] as well as in Rhcatalyzed CÀ H borylation, [29,30] hence demonstrating the potential of bifunctional NHC-carboxylate ligands in catalysis. [31 -33] We surmised that ligand 3, wherein the chiral substituent and the carboxylate group are located on opposite sides of the imidazole ring, would be well suited to the enantiodetermining CÀ H activation step (see the transition-state model in Figure 1), with the rigid oxazoline ring providing enantioselectivity and the carboxylate arm providing both reactivity and sufficient flexibility to adapt to the substrate.…”