Benzothiazole Nickelation: An Obstacle to the Catalytic Arylation of Azoles by Cyclopentadienyl Nickel N-Heterocyclic Carbene Complexes

Abstract

NiCp † L(NHC)] (+) complexes (Cp † = Cp (η 5 -C 5 H 5 ), Cp* (η 5 -C 5 Me 5 ); NHC = N-heterocyclic carbene; L = Cl − or NCMe) have been tested as pre-catalysts for the direct arylation of benzothiazole in the presence of an alkoxide. Only the pentamethylcyclopentadienyl derivative, [NiCp*Cl(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenylimidazol-2-ylidene), enabled low conversion to the desired coupling product with phenyl iodide as the electrophilic coupling partner. In contrast, all cyclopentadienyl complexes proved to be inactive. 1 H NMR studies of the "catalytic" reaction mixtures demonstrate that they cleanly convert to an unreactive C(2)-benzothiazolyl derivative, whose identity has been confirmed by an independent synthesis and characterization. The latter constitutes a potential energy well that quenches all further reactivity, and provides a rare example of C(2)-metallated azolyl complex.

Optimal Balance in the Catalyst Dynamics Enables C(2)−H Arylation of (Benz)imidazoles and (Benz)oxazoles by an In Situ‐Generated Ni/NHC System

“…Fast darkening of catalytic systems containing Cp due to the formation of Ni‐containing precipitates was also observed in the present work. An additional reason for the decreased activity of 1 d may be related to the possibility of the formation of inactive and quite stable (NHC)NiCp(heteroaryl) complexes from (NHC)NiCpX precatalysts and heteroaromatic substrates in the presence of strong bases [16b] …”

Synthesis of Imidazolium Cations Linked to Para-t-Butylcalix[4]arene Frameworks and Their Use as Synthons for Nickel-NHC Complexes Tethered to Calix[4]arenes

“…Our research group has been interested in the chemistry and homogeneous catalysis potential of N-heterocyclic carbene complexes of nickel for the past 15 years, and we have published many papers in this area [31][32][33][34][35][36][37][38][39]. It was of interest to us to graft nickel NHC complexes onto a calixarene backbone to see whether the catalytically active organometallic fragment close to the calixarene cavity would lead to any enhanced catalytic activity.…”

“…We also recently reported on the syntheses and anion-complexing ability of a series of calix [6]arenes that were functionalized with imidazolium ligands [29]. We are still interested in combining the relatively undeveloped organometallic chemistry of calixarenes [30] with our current research on the chemistry of N-heterocyclic carbene complexes of nickel [31][32][33][34][35][36][37][38][39]. This manuscript describes some steps in this direction via the synthesis of a series of calix [4]arene-linked imidazolium cations.…”