Cp*Rh^III‐Catalyzed Sterically Controlled C(sp³)−H Selective Mono‐ and Diarylation of 8‐Methylquinolines with Organoborons**

Abstract

Herein, the RhIII‐catalyzed selective monoarylation and diarylation (symmetrical and unsymmetrical) of 8‐methylquinolines with organoboron reagents are disclosed. The selective monoarylation of primary C(sp3)−H bonds is achieved by using 7‐substituted 8‐methylquinolines or by changing the quantity of the aryl boronic acids. The method is also applicable for the arylation of 2‐ethylpyridines, and the heteroarylation with thiophene‐2‐ylboronic acids. Symmetrical and unsymmetrical diarylation of 8‐methylquinolines have been carried out in one‐pot and sequential manner, respectively. Late‐stage monoarylation of oxime derivatives and gram‐scale synthesis of monoarylated products has also been carried out. A mechanistic study revealed that the current reaction is first order with respect to both reactants and a five‐membered rhodacycle intermediate may be involved in the catalytic cycle.

Rhodium‐Catalyzed Selective C−H Bond Functionalization of Quinolines

“…The 7‐chloro‐8‐methylquinoline found to be more productive for selective mono‐arylation. Different concentration of arylboronic acids used for obtaining better yields …”

Ru(II)-Catalyzed Chemoselective C(sp3)–H Monoarylation of 8-Methyl Quinolines with Arylboronic Acids

“…The synthesis of C–C aryl bonds is indeed vital in organic chemistry, but these transformations are challenging for the substrates having no directing group assistance as uncontrolled regio- and chemoselectivity leads to the undesired homo- and heterocouplings and hence formation of side products. , A number of reactions have been flourished widely for the direct C­(sp 2 )–H arylation of quinolines via transition-metal catalysts owing to the availability of π-orbitals and easy formation of their metallacycle intermediates . Significantly, methodologies have also been reported for the direct functionalization and particularly for arylation ,, of more challenging C­(sp 3 )–H bonds of 8-methylquinoline (8-MQ) under expensive Pd­(II) and Rh­(III) catalysts (Scheme a). In the pioneering studies, Sanford, , Daugulis, Yu, and Larrosa groups have independently developed C−H arylation protocols through the costly Pd catalyst with a limited scope for 8-MQ.…”

RhIII-Catalyzed Direct Heteroarylation of C(sp3)–H and C(sp2)–H Bonds in Heterocycles with N-Heteroaromatic Boronates

“…In 2015, Glorius has developed the first instance of Rh III -catalyzed arylation of C­(sp 3 )–H of 2-alkylpyridine and 8-benzylquinoline derivatives with triarylboroxines (Scheme a) . Lately, Sharma disclosed the Rh III -catalyzed selective C­(sp 3 )–H monoarylation and diarylation (symmetrical and unsymmetrical) of 8-methylquinoline derivatives using organoboron reagents as the coupling partners (Scheme b) . Furthermore, You described the Rh III -catalyzed heteroarylation reaction between C­(sp 3 )–H bonds from 2-ethylpyridine and 8-methylquinoline derivatives and C­(sp 2 )–H bonds from thiophene and furan derivatives (Scheme c) …”

“…Finally, we synthesized the five-membered rhodacycle intermediate A′ which can be transformed easily into the desired products and the active catalyst for further catalysis, demonstrating the C­(sp 3 )–H bond cleavage at the beginning of the catalytic process (Scheme S1d). On the basis of the above mechanistic results and previous relevant studies, , we provided a plausible reaction mechanism in Scheme S2. Initially, the coordination of 8-methylquinoline with a Rh III catalyst facilitates the following chelation-assisted C­(sp 3 )–H bond cleavage to yield the monomeric rhodacycle intermediate A .…”

“…9 Lately, Sharma disclosed the Rh III -catalyzed selective C(sp 3 )−H monoarylation and diarylation (symmetrical and unsymmetrical) of 8-methylquinoline derivatives using organoboron reagents as the coupling partners (Scheme 1b). 10 Despite these attractive processes, Rh III -catalyzed Nheteroarylation of C(sp 3 )−H using nitrogen-containing heterocycles, including pyridine, pyrimidine, etc. as the coupling partners has not been developed, because of the inhibition of catalytic turnover resulting from the strongly coordinating nitrogen atoms in both substrates and coupling partners.…”