N-Heterocyclic Carbenes with a N-2,4-Dinitrophenyl Substituent: Comparison with PPh3 and IPr

Synthesis and characterization of N-heterocyclic carbenes (NHCs) bearing a 2,4-dinitrophenyl (DNP) substituent on an NHC framework were performed. The treatment of 1-(2,4-dinitrophenyl)-1H-imidazole (1) with 1-bromo-2,4-dinitrobenzene or methyl triflate afforded imidazolium salts 2a center dot HBr or 2b center dot HOTf, respectively, which were corresponding precursors of NHC ligands 1,3-bis(2,4-dinitrophenyl)-1H-imidazol-2-ylidene (2a) and 1-(2,4-dinitrophenyl)-3-methyl-1H-imidazol-2-ylidene (2b). Rh and Au complexes-RhCl(2a)(cod) [3a (cod = 1,5-cyclooctadiene)], RhCl(2b)(cod) (3b), RhCl(2a)(CO)(2) (4a), RhCl(2b)(CO)(2) (4b), and AuCl(2a) (5a)-were synthesized using 2a.HBr and 2b.HOTf. IR, NMR, and crystallographic analysis of the Rh complexes demonstrated that the DNP substituent remarkably decreased the sigma-donating ability of the carbenic carbon and increased the pi-accepting ability. In addition, IR spectroscopy of Rh dicarbonyl complexes revealed that the average CO stretching frequency of 2a was equal to that of PPh3. Au complex Sa exhibited a significantly higher catalytic activity than AuCl(PPh3) 5d in the Au(I)-catalyzed hydroalkoxylation of cyclohexene. The computational analysis of the Au(I) complexes supported the experimental data, and the results suggested that the Au -C-carbene pi-back-bonding interaction energy of 5a was 17-20% larger than that of 5d.