Ruthenium(II)-Catalyzed Microwave-Promoted Multiple C–H Activation in Synthesis of Hexa(heteroaryl)benzenes in Water

Abstract

A series of hexa(heteroaryl)benzenes were synthesized by the Ru(II)-carboxylate-catalyzed multiple C-H activation of benzenes carrying pyridyl, pyrimidyl, or pyrazolyl directing groups using N-heteroaryl bromides as coupling partners. The reactions proceeded with high selectivity under microwave irradiation in water. Iterative penta-arylation could be implemented via activation of C-H bonds of generated intermediates by cascade chelation assistance of in situ installed pyridyl groups. This strategy provides multidentate ligands for selective complexation of transition metals and potential building of photoredox systems.

Synthesis and characterization of propeller-shaped mono- to hexacationic quinolinium-substituted benzenes

“…The X-ray crystal analysis of hexa(2-pyridyl)benzene, however, revealed the α,β,α,β,α,β arrangement of pyridine rings and dihedral angles between the pyridyl substituent and the benzene ring of approximately 90°. 22…”

“…Recently, the sterically congested hexa(heteroaryl)benzene (HHAB) 4 was formed as two isolable isomers which differ only in the rotation of one methyl-pyridine group. 22 In general, propeller-shaped hexaarylbenzenes (HABs) such as hexa(β-naphthyl)benzene 5 23 have numerous applications in materials sciences as they play roles as liquid crystals, 24 microporous organic solids, 25 molecular capsules, 26 supramolecular electronic materials, 27 molecular rotors, 28 nonlinear optical materials, 29 metal sensors, 30 redox materials, 31 and molecular wires. 32 We report here on quinolinium-substituted benzenes which combine the features of HHABs and polycations as they have to adopt propeller-shaped…”

Ortho C–H arylation of arenes at room temperature using visible light ruthenium C–H activation

“…High reaction temperatures are standard, however, when using [RuCl 2 (p-cymene)] 2 as catalyst. 5,6 A photo Ru arylation reaction could activate alternative mechanistic paths and offer the possibility of room temperature reaction. 7 The two Rucatalyzed processes, meta-alkylation and ortho-arylation, operate through very different frameworks in the thermal regime, with the former thought to involve discrete 2 and 3 carbon-centered radicals adding to Ru(III) metallacycles (Scheme 1A), 8 and the latter involving more typical oxidative additions of aryl halides to a Ru(II) center.…”

Conformationally Driven Ru(II)-Catalyzed Multiple ortho-C–H Bond Activation in Diphenylpyrazine Derivatives in Water: Where Is the Limit?

“…A great number of them are based on nitrogen as the metal-coordinating atom, the pyridine ring being prevalent, although other azaheterocycles, such as pyrazole, oxazoline, and quinoline have also been successfully employed in transition-metal-catalyzed ortho-C-H bond functionalization [33]. Požgan and co-workers have demonstrated that the quinazolinyl [34,35] and pyrimidinyl [36] groups efficiently allow regioselective ortho-arylations in the presence of Ru(II)-carboxylate catalyst, even in water as the solvent in some cases [37,38]. The success of Ru(II) catalysts is most likely ascribed to the easy formation of ruthenacyclic intermediate stable in water via C-H bond deprotonation with the assistance of directing groups [35,37,[39][40][41][42][43].…”

“…For these reasons, many synthetic methods for the construction of pyrazine derivatives have been developed [58][59][60][61][62][63], yet there is still great demand for the generation of novel pyrazine libraries. As a part of our ongoing interest in metal-catalyzed functionalization of (hetero)arenes via C-H bond activation [34][35][36][37][38], we reasoned that a pyrazine nucleus could direct cleavage and further functionalization of ortho-C-H bonds of the benzene ring in phenyl-substituted pyrazines. Moreover, by using 2,3-diphenylpyrazines as substrates, both nitrogen atoms could cooperate with the catalytic metal center to hypothetically promote four ortho-C-H bond activations, thus enabling the construction of highly functionalized, especially arylated or π-conjugated pyrazine derivatives.…”