Normal and abnormal carbene complexes derived from thiazole: Preparation and a preliminary investigation of their relative catalytic performance

Abstract

Readily prepared 2-, 4-and 5-bromo-3-methyl thiazolium triflates react by oxidative substitution with M(PPh 3 ) 4 (M = Ni or Pd) to furnish five of the expected normal and abnormal cationic thiazolylidene complexes (1a, 1b, 2a, 2b, and 3b). Carbene complex formation is accompanied by a ca. 40 ppm downfield shift of the a-N carbene carbons in Pd complexes 1 and 2 in their 13 C NMR spectra but the chemical shift of C(carbene) in the abnormal 3b (d 135.7) is particularly low. Crystal and molecular structures of complexes 1a, 2b, and 3b all indicate a square planar arrangement of the ligands around the central metal atoms. The new complexes catalyse Suzuki-Miyaura aryl coupling.

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

“…350 Thiazolium triflates 437a−c containing a bromide in the 2, 4, or 5 position oxidatively add to [M(PPh 3 ) 4 ] (M = Ni, Pd) to obtain normal 2-thiazolylidene complexes 438 and 439 as well as their mesoionic 4-and 5-thiazolylidene analogues 440−442 (Scheme 182). 351 While the ca. 40 ppm downfield shift of the carbenic nucleus in the 13 C NMR spectrum suggests considerable carbene-type behavior, solid-state analysis by Xray diffraction does not support a strong carbenic character and points to a retention of the immonium character as in the precursor thiazolium salt.…”

Isomeric Palladium Complexes Bearing Imidazopyridine-Based Abnormal Carbene Ligands: Synthesis, Characterization, and Catalytic Activity in Direct C–H Arylation Reaction

“…An example in which aNHC complex was less effective than its isomeric nNHC complex in catalysis is also known. 71 Finally, the substrate scope of the catalyst precursor 3n was tested (Table 6). Entries 1−4 show that activated aryl bromides were effectively utilized as substrates to afford coupled products with good to excellent yields.…”

“…These findings imply that besides the σ donor property of carbene ligand, the effect of halide coligand is also important for an effective catalyst system in direct arylation reaction. An example in which a NHC complex was less effective than its isomeric n NHC complex in catalysis is also known …”

N, N′-Olefin functionalized Bis-Imidazolium Pd(II) chloride N-Heterocyclic carbene complex builds a supramolecular framework and shows catalytic efficacy for ‘C–C’ coupling reactions

“…39 In this regard, many M-NHC complexes have been reported. The Nfunctionalized NHCs bearing various functional groups such as pyridine, 40 pyrimidine, 41 pyrazine, 11,12,42,43,92 pyridazine 44 thiazole 45,46 and napthyl 47,93 have also attracted considerable attention among the experimental chemists.…”