title: Expanded-ring N-heterocyclic carbenes efficiently stabilize gold(I) cations, leading to high activity in π-acid-catalyzed cyclizations.
abstract: A series of six- and seven-membered expanded-ring N-heterocyclic carbene (er-NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er-NHC)AuX] (X(-) = BF4(-), NTf2(-), OTf(-)) were generated in solution. According to their (13)C NMR spectra, the ionic character of the complexes increases in the order X(-) = Cl(-) < NTf2(-) < OTf(-) < BF4(-). Additional factors for stabilization of the cationic complexes are expansion of the NHC ring and the attachment of bulky substituents at the nitrogen atoms. These er-NHCs are bulkier ligands and stronger electron donors than conventional NHCs as well as phosphines and sulfides and provide more stabilization of [(L)Au(+)] cations. A comparative study has been carried out of the catalytic activities of five-, six-, and seven-membered carbene complexes [(NHC)AuX], [(Ph3 P)AuX], [(Me2S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD-Dipp)Au]BF4 (1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydrodiazepin-2-ylidene gold(I) tetrafluoroborate) catalyst bearing seven-membered-ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1 mol % catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations.
authors.affiliation: A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky Prosp., 29, Moscow, 119991 (Russian Federation).
authors.forename: Oleg S
authors.initials: OS
authors.lastname: Morozov
authors.forename: Andrey V
authors.initials: AV
authors.lastname: Lunchev
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Bush
authors.forename: Aleksandr A
authors.initials: AA
authors.lastname: Tukov
authors.forename: Andrey F
authors.initials: AF
authors.lastname: Asachenko
authors.forename: Victor N
authors.initials: VN
authors.lastname: Khrustalev
authors.forename: Sergey S
authors.initials: SS
authors.lastname: Zalesskiy
authors.forename: Valentine P
authors.initials: VP
authors.lastname: Ananikov
authors.forename: Mikhail S
authors.initials: MS
authors.lastname: Nechaev
mesh_terms: D000143:Acids / Q000737:chemistry; D000480:Alkynes / Q000737:chemistry; D001572:Benzofurans / Q000138:chemical synthesis; D002384:Catalysis; D002412:Cations / Q000737:chemistry; D003500:Cyclization; D006046:Gold / Q000737:chemistry*; D006571:Heterocyclic Compounds / Q000737:chemistry*; D007211:Indoles / Q000138:chemical synthesis; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: D000143:Acids; D000480:Alkynes; D001572:Benzofurans; D002412:Cations; D006571:Heterocyclic Compounds; D007211:Indoles; C030011:carbene; D006046:Gold; C030374:indole; D008697:Methane
keywords: N-heterocyclic carbenes; alkynes; density functional theory; gold; hydroamination