Palladium‐Catalyzed Direct C2‐Arylation of an N‐Heterocyclic Carbene: An Atom‐Economic Route to Mesoionic Carbene Ligands

Abstract

Blocking the C2 position of an imidazole-derived classical N-heterocyclic carbene (NHC) with an aryl group is an essential strategy to establish a route to mesoionic carbenes (MICs), which coordinate to the metal via the C4 (or C5) carbon atom. An efficient catalytic route to MIC precursors by direct arylation of an NHC is reported. Treatment of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) with an aryl iodide (RC6H4I) in the presence of 0.5 mol % of [Pd2(dba)3] (dba = dibenzylideneacetone) precatalyst affords the C2-arylated imidazolium salts {IPr(C6H4R)}I (R = H, 4-Me, 2-Me, 4-OMe, 4-COOMe) in excellent (up to 92%) yields. Treatment of {IPr(C6H5)}I with CuI and KN(SiMe3)2 exclusively affords the MIC-copper complex [(IPrPh)CuI].

Crystalline Radicals Derived from Classical N‐Heterocyclic Carbenes

“…Thed esired C2-arylated imidazoli(ni)um salts 3a,b and 4a,b (Scheme 1) are readily accessible by the direct C2-arylation of 1 and 2 under Ni or Pd catalysis. [17] Cyclic voltammograms (CVs) of 3a,b and 4a,b ( Figure 2) exhibited ar eversible one-electron reduction at E 1/2 = À1.66 V, À1.91 V, À1.42 Va nd À1.49 Vv s. Fc + /Fc,r espectively.T he reduction potential for 3a,b and 4a,b is not only significantly lower than that of the related C2-protonated imidazoli(ni)um salts of NHCs 1 and 2 (ca. À2.3 V), [13] but the redox process is also reversible ( Figure 2).…”

N‐Heterocyclic Carbene Analogues of Thiele and Chichibabin Hydrocarbons

“…Thed esired precursors [(IPr)(C 6 H 4 )(IPr)](Br) 2 (2)a nd [(IPr)(C 6 H 4 ) 2 (IPr)](Br) 2 (3)w ere prepared by the direct C2-arylation [18] of IPr (1)w ith 1,4-dibromobenzene and 4,4'-dibromobiphenyl (Scheme 1). [18a] Compounds 2 and 3 are air-stable white solids and have been characterized by NMR spectroscopy and mass spectrometry (Supporting Information).…”

Expedient Access to Normal‐ and Abnormal‐ N‐Heterocyclic Carbene (NHC) Magnesium Compounds from Imidazolium Salts

“…A similar strategy was applied to prepare aNHC‐magnesium compound (aIPr Ph )MgI 2 (OEt 2 ) ( 6 ) [aIPr Ph = 1,3‐bis(2,6‐diisopropylphenyl)‐2‐phenyl‐imidazol‐4‐ylidene] by employing a C2‐arylated imidazolium salt ( 5 ) and MeMgI (Scheme ). However, the reaction was found to be extremely slow and only 10–20 % conversion of 5 into 6 was noticed after 16 h at room temperature.…”

“…Toluene, benzene, n ‐hexane, ethyl ether, and THF were dried by refluxing over Na‐K/benzophenone‐ketyl and purified by distillation in a nitrogen atmosphere. (IPrH)Cl ( 1 ), (IPrH)I ( 2 ), and (IPrPh)I ( 5 ) were prepared by adopting literature methods. MeMgI (3 m solution in Et 2 O from Sigma‐Aldrich), MgI 2 (Sigma‐Aldrich), and KN(SiMe 3 ) 2 (Sigma‐Aldrich) were used without further purification.…”