Studies On The Synthesis, Properties And Catalytic Performance For Six-membered N-heterocyclic Carbene Precursors

The organometallic chemistry of N-heterocyclic carbenes (NHCs) play veryimportant roles in the development of modern organic chemistry, especially organicsynthesis. NHCs with ring sizes greater than the more common5-memberedimidazolium based systems is called as the “expanded ring” N-heterocyclic carbenescarbenes (RE-NHCs). RE-NHCs main characteristics compared to common5-membered imidazolium based systems are: the stronger σ-electron-donating ability,the larger N-Ccarbene-N bond angles, the smaller RN-Ccarben bond angles, causingthe N-substituent to twist closer to the metal centre., which play a better stericeffection, moreover these features can greatly improve the catalytic activity andstability. But current study is mostly concentrated in the five-memberedN-heterocyclic carbenes (5-NHCs) and its metal compounds, The research reports ofRE-NHCs and its metal compounds synthesis and properties are so few. In this paper,we six-membered heterocyclic carbine-based developed the following work:1Synthesis of the precursors of asymmetric and bridged six-membered NHCsN-methyl-1,3-propanediamine or N, N’-bis (aminopropyl) ethylenediamine andN,N-dimethylformamide dimethyl acetal (DMF-DMA) as starting material,synthesized the1,4,5,6-tetrahydro-pyrimidine compounds by the condensationreaction; pyrimidine compounds with alkyl halides by quaternization reactionsynthesized a series of asymmetric and bridge linking six-membered heterocycliccarbenes precursors; the halonium salts with hexafluorophosphate through theanion-exchange reaction replaced by hexafluorophosphate six membered heterocycliccarbenes precursors. the structure of the target product and part of intermediates werecharacterized by ESI-MS,1H NMR,13C NMR, IR, et al..2The trapping of free six-membered heterocyclic carbenesChoose two of the ionic salts3d、10b as carbene precursor, The free carbenecould be generated by treatment of ionic salts with sodium bis(trimethylsilyl)amide(NaHMDS) in THF at low temperature (–40°C). Whereas free carbene appears to be viable only at low temperature, its formulation as a carbene was established by itstrapping with S8to form the thiourea.The structural of two heterocycle thioureacompounds were characterized by ESI-MS,1H NMR,13C NMR, X-ray the singlecrystal diffraction.3Application of the6-NHCs ligands for microwave-assisted palladium-catalyzedcarbon-nitrogen coupling reactionHeterocyclic aryl halide with the secondary amine compounds as the substrate,palladium acetate as the catalyst,6-NHC precursor for the ligand, Palladium-catalyzed C-N coupling reaction under microwave heating in air.The reactionconditions The conditions are initially optimized through the orthogonal experiment,including solvent, base, reaction time, temperature, the amount of catalyst et al. Andfurther optimization of reaction conditions, and filter out the preferred reactionconditions: catalyst is used in an amount of5mol%of potassium tert-butoxide as abase, DME as solvent, the reaction temperature was100°C. The reaction time is40min. and further optimize the conditions drawn relative to the preferred reactionconditions: the amount of catalyst is5mol%, tert-butoxypotassium alkoxide as base,DME as solvent, the reaction temperature is100°C, the reaction time is40min.Different carbene ligand activity in optimum conditions investigated and found thatasymmetric ligands have better catalytic activity compared to the bridged ligand; andapply the best ligand expanded amine of different structure types, and achieved abetter results.