Synthesis Of Novel N-Heterocyclic Carbene Metal Complexes And Their Studies As Reaction Intermediates In The Cyclization Of Alkynes

N-heteocyclic carbenes（NHCs）have emerged as one of the most important ligands for the preparation of transition-metal complexes due to their diverse structures,good modifiable properties and powerful coordination ability.NHC itself and its transition-metal complexes have been widely used as excellent catalysts in modern organic synthetic chemistry.In addition,ditopic NHC containing two carbene coordination centers in the same nitrogen heterocyclic ring have attracted extensive attention,due to their capability of linking two same or different transition metals.To date,most of studies on ditopic NHC are focus on five-membered structure of NHC.Six-membered ditopic NHC are supposed to have a more diverse two-coordination mode.However,the study on six-membered ditopic NHC and its metal complexes still remain rare.Recently,the transition metal promoted intramolecular cyclization of nitrogen-containing conjugated ene-yne compounds have been developed as a novel method for directly synthesizing NHC metal complexes,becoming one of the hottest research areas in NHC-based organometallic chemistry.Under certain circumstances,these transition-metal carbene species are also the active intermediates of such catalytic cyclization reactions.Thus,the isolation of these metal carbene species and investigation of their reactivities can better understand the mechanism of such cyclization reactions.This thesis mainly focuses on two areas including metal promoted intramolecular cyclization of conjugated ene-yne and the development of a novel method for the direct and efficient synthesis of NHC precursor salts.Thus,we have developed new methods for efficiently synthesizing some novel NHC metal complexes,and studied the related reaction mechanisms,followed by the development of some unknown organic catalytic reactions.This thesis mainly includes four parts as follows:（1）Gold-promoted cyclization/cross-coupling reaction of terminal alkyne:the isolation of the six-membered a NHC-Au intermediate and the synthesis and characterization of related ditopic NHC metal complexesIn the presence of base,a novel six-membered a NHC-Au complex could be synthesized by using Au（I）-promoted the intramolecular cyclization/cross-coupling reaction between terminal propiolic formamidine and aryl boronic acid.Moreover,through further deprotonation/metal complexation,two novel six-membered1,3-ditopic NHC digold and 1,3-heterodinuclear Au/Ag complexes could be prepared from the a NHC-Au product.The structures of these NHC complexes were characterized by X-ray diffraction and NMR analysis.（2）Studies on the mechanism of alkynyl transmetalation triggered by nucleophilic attack and the isolation,characterization and reactivity of related heterodinuclear Ni/Au and Pt/Au complexesTwo novel six-membered 1,2-ditopic NHC heterodinuclear Ni/Au and Pt/Au complexes were synthesized from Au（I）-promoted 6-endo-dig cyclization of nickel and platinum acetylides at 50 ℃,respectively.Such cyclization undergoes a key alkynyl transmetalation process triggered by nucleophilic attack.In addition,lowering the reaction temperature to-20 ℃,two five-membered vinylidene heterodinuclear Ni/Au and Pt/Au complexes were also isolated via the 5-exo-dig cyclization of metal acetylides,which could transform into their corresponding six-membered endo-isomers at 50 ℃.（3）Synthesis of 3-methyleneisoindolin-1-ones and isoquinolinium salts via exo and endo selective cyclization of 2-（1-alkynyl）benzaldiminesIn the presence of oxidant and under the catalysis of Au（I）,2-（1-alkynyl）benzaldimines underwent 5-exo-dig cyclization/oxidation tandem reaction could produce several N-aryl substituted 3-methyleneisoindolin-1-one compounds.By contrast,in the presence of base and under the activation of Ag（I）,2-（1-alkynyl）benzaldimines underwent 6-endo-dig cyclizations,giving six-membered N-aryl and N-alkyl substituted isoquinolinium salt products.Two key carbene gold and pyridine-based NHC-Ag reaction intermediates have been isolated and characterized.Further studies of their reactivities confirmed the plausible mechanisms.Additionally,derivatization reactions of these 3-methyleneisoindolin-1-one and isoquinolinium salt products have also been investigated,respectively.（4）Multicomponent one-pot synthesis of unsymmetrical backbonedisubstituted imidazolium salts as N?Heterocyclic carbene precursors and their applications in Pd-catalyzed cross-coupling reactionsIn this part,by using commercially available sterically congested arylamines,alkylamines,2,3-butanedione,formaldehyde and acetic acid as the starting materials,a multicomponent one-pot reaction was developed for the straightforward synthesis of various novel（a）chiral 1-aryl-3-alkyl-4,5-dimethyl-imidazolium salts.In addition,by utilizing α-keto-imine compound as the starting materials,several 4-phenyl-5-methyl substituted 1-aryl-3-alkyl unsymmetrical imidazole salts were also prepared through similar multicomponent reaction.Compared with the classic synthetic method,the synthetic methodology features easily available starting materials,short synthetic steps,and mild reaction conditions.Through deprotonation/metal complexation from the imidazolium salts,two NHC-Pd complexes bearing a large aliphatic ring N-substituent in NHC ligand have been prepared.The backbone dimethyl substituted NHC-Pd complex with a 15-membered-ring moiety proved to be more efficient than its backbone-unsubstituted NHC-Pd counterpart in both classical Suzuki-Miyaura and Negishi cross-coupling reactions.