Facile base-free in situ generation and palladation of mesoionic and normal N-heterocyclic carbenes at ambient conditions

Abstract

Synthesis of normal and mesoionic N-heterocyclic carbene Pd(II) complexes in near quantitative yields from the corresponding NHC precursor salts under base free and ambient conditions is reported. A plausible mechanism involving charge-assisted hydrogen bonded intermediates is proposed.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

“…When the reaction is performed in a 2:1 triazolium/palladium stoichiometry, the monometallic bis­(carbene) complex 74 is formed. This product diversity has been observed in related studies , and seems to be a general feature of direct palladation of triazolium salts. While transmetalation provides access to the same diversity of products, direct metalation tends to be less selective and usually produces mixtures of complexes.…”

“…Thermally induced metalation of triazolium salts with metal precursors that contain an internal base is a relatively mild method. Precursors suitable for such C−H activation have been demonstrated to be [Pd(OAc) 2 ], 58,69,159,160,162,165,167 [Pd(acac) 2 ] (acac = acetylacetonato), 169 [M(COD)(OMe)] 2 (M = Rh, Ir), 167,168 [Rh(COD)(OEt)] 2 , 137 [Rh(COD)-(OAc)] 2 , 167 Ag 2 O (see section 2.2.1), and CuO. 111 Palladation has been investigated most thoroughly.…”

“…46,52,64,165 These species can be divided in C4-linked bis(triazolylidene) species M 64,68,70,71,80,95,137,154,165,177 and N3-linked bis(triazolylidene) complexes of type N (Figure 17). 46,52,95,195 The linker between the triazolylidene heterocycles can be tailored over a wide range, including systems with no linker at all (i.e., directly connected heterocycles), 68,71,80,95,137,154,177 alkyl linkers ranging from mono-to tetramethylene units, 68,95,195 ethercontaining linkers, 68,70 sterically rigid ortho-phenylene groups, 64 biphenyl linker that produces a chiral environment after metalation, 52,165 and even a chiral binaphthyl backbone. 46 Intriguing opportunities emerge when the linker between the two triazolylidene units is functional, e.g., because it contains a coordination site.…”

“…Direct C–H bond activation of triazolium salts with a suitable metal precursor has been particularly popular as a method for the synthesis of triazolylidene complexes of Pd, ,,,,,− Ir, ,,,− Rh, ,,, and Ni. ,− In addition, a few triazolylidene complexes have been prepared by direct C–H bond activation with Cu, Pt, Mo, and Re species. This synthetic procedure can be divided in three different domains: (i) activation mediated by a metal precursor that contains a sufficiently basic ligand; (ii) activation induced by addition of an external proton scavenger; (iii) chelating groups as a driving force for C–H activation (cyclometalation).…”

“…The synthetic versatility of the copper-catalyzed click reaction has been widely exploited for the preparation of a variety of bi- and polydentate triazolylidene ligands. In particular, bis­(triazolylidene) complexes have been extensively investigated during recent years, including complexes of Re, Ir, ,,, Ru, ,, Rh, ,,, Fe, Ag, and Pd. ,,, These species can be divided in C4-linked bis­(triazolylidene) species M ,,,,,,,,, and N3-linked bis­(triazolylidene) complexes of type N (Figure ). ,,, The linker between the triazolylidene heterocycles can be tailored over a wide range, including systems with no linker at all (i.e., directly connected heterocycles), ,,,,,, alkyl linkers ranging from mono- to tetramethylene units, ,, ether-containing linkers, , sterically rigid ortho -phenylene groups, biphenyl linker that produces a chiral environment after metalation, , and even a chiral binaphthyl backbone …”