| The past two decades have witnessed the flourishing of transition-metal carbene chemistry,which are widely applied in X-H insertion(X=C,N,O,etc.),cyclopropanation,cycloaddition,cross-coupling,and others.Diazo molecules are commonly used to generate transition-metal carbenes in the presence of appropriate catalysts.However,the drawbacks of diazo compounds,such as instability,toxicity and explosiveness,limit their broad applications.Hence,the developments of new approaches for the formation of transitionmetal carbenes from non-diazo compounds have attracted substantial research interests recently.In this context,non-diazo carbene precursors,such as hydrazones,cyclopropenes,allenyl ketones,1,2,3-triazoles,and others,have been demonstrated to be capable of generating transition-metal carbenoids in the presence of appropriate catalysts.Nevertheless,there is still a lack of direct experimental characterization data to support such carbene transformation.Some carbene precursors,such as allenyl ketones and cyclopropenes are very reactive,which may not undergo the desired transformation to yield metal carbenes.Therefore,potential alternative non-carbene mechanistic pathways should not be overlooked.Currently,less research attention is paid to the transformation of supposed carbene precursors via non-carbene pathways.In this thesis,density functional theory(DFT)method was used to study the mechanisms of transition-metal-catalyzed some new carbene precursors related organic reactions,involving allenyl ketones,1,2,3-thiadiazoles,dienynes and cyclopropenes.The computational study revealed the mechanistic pathways of transiton-metal catalyzed transformation of carbene precursors,including both carbene and non-carbene routes.The factors responsible for the chemo-,stereo-and regio-selectivity in the conversions of these non-diazo carbene precursors are investigated.In addition,we also evaluated the potential differences in activating various non-diazo carbene precursors promoted by transition metal catalysts.The research details were summarized as follows:(1)The Pd(0)-catalyzed coupling cyclization of 1,2-allenyl ketones with aryl halides to construct multi-substituted furan derivatives has been developed.Nevertheless,the detailed mechanism of this reaction,proceeding via either the Pd(Ⅱ)-carbenoid or the π-allyl-Pd(Ⅱ)intermediate,still remains in debating.Herein,computational studies were performed to provide mechanistic insights into the reactions and substituent-dependent mechanistic pathways were revealed.Computational results suggest that the substituents,R1-R3 attached to 1,2-allenyl ketones and R4 attached to the aryl moiety of aryl halides,could play significant roles on the variation of the mechanistic pathway.It would be favorable to form the Pd(Ⅱ)-carbenoid intermediate when R1 is an aryl/alkyl group or steric hindrance is present between R2 and R3.For the substrate of aryl halide,the aryl moiety bearing electrondonating group(R4)at the para position is more ready to undergo the migratory insertion to form the π-allyl-Pd(Ⅱ)intermediate than that with electron-withdrawing group.The factors responsible for the formation of both Pd(Ⅱ)-carbenoid and π-allyl-Pd(Ⅱ)intermediates are discussed.(2)DFT calculations were performed to gain an in-depth mechanistic understanding of the Rh(Ⅰ)-catalyzed transannulation of 1,2,3-thiadiazoles with alkenes,alkynes,and nitriles.Computational results indicate that the denitrogenation of 1,2,3-thiadiazoles promoted by the Rh(Ⅰ)catalyst may not afford the commonly proposed α-thiavinyl Rh-carbenoid intermediate.Instead,the four-membered cyclometalated Rh(Ⅲ)complex is suggested to be the key intermediate,which could be formed via the cleavage of S-N bond of 1,2,3thiadiazoles to generate a six-membered cyclometalated Rh(Ⅲ)complex followed by N2 extrusion.The easy chelation of S atom with Rh is mainly responsible for the favorable formation of the four-membered cyclometalated Rh(Ⅲ)intermediate.Next,the substrates of alkenes,alkynes,and nitriles could undergo migratory insertion with the four-membered rhodacycle followed by reductive elimination to furnish the corresponding products.The origins of divergent regioselectivities for the Rh(Ⅰ)-catalyzed transannulation of 1,2,3thiadiazoles with alkenes,alkynes,and nitriles are discussed,respectively,which are not only determined by the feasible migratory insertion pathway but also the feasibility of subsequent reductive elimination.(3)DFT calculations were performed to gain an in-depth mechanistic understanding of Ni(0)-and Au(I)-catalyzed diastereoselective[4+2+1]cycloadditions between dienynes and diazo compounds.The computational results indicate that,for both Ni(0)-and Au(I)catalysis,transition-metal catalysts are more readily available to activate diazo compounds to form transition-metal carbene intermediates prior to the activation of the dienynes.Subsequently,the insertion of the alkynyl moiety of the dienynes with the generated transition-metal carbene intermediates can afford the metallacyclobutene intermediates.Afterwards,the transition-metal-dependent transformation of the obtained metallacycle intermediate was revealed.For the Ni(0)-catalyzed reaction,the subsequent intramolecular migratory insertion,followed by cyclization,is the favored route for the formation of the divinylcyclopropane intermediate.However,for the Au(I)-catalyzed reaction,the subsequent ring-opening of the obtained metallacycle occurs,affording the vinyl Au-carbene intermediate,from which an analogous divinylcyclopropane can be obtained via an intramolecular cyclopropanation.The generated divinylcyclopropane intermediates can furnish the corresponding products via a[3,3]-sigmatropic rearrangement for both reactions.The stereoselectivity of the Ni(0)-catalyzed reaction is mainly controlled by minimizing the steric hindrance in the denitrogenation,migratory insertion,and[3,3]-sigmatropic rearrangement steps.Meanwhile,for the Au(I)-catalyzed reaction,the C-H...O H-bonding interactions in the ring-opening of the metallacyclobutene intermediate and the steric hindrance effects in the subsequent cyclopropanization and[3,3]-sigmatropic rearrangement steps collectively determine the experimentally observed stereoselectivity.(4)Computational studies were carried out to investigate the mechanisms of Pd-catalyzed ring-opening reactions of cyclopropenes with pronucleophiles(such as 2methylmalononitrile)and aryl iodide electrophiles,respectively.The computational results indicate that the activation order by Pd(0)for these three substrates is aryl iodides>cyclopropenes>2-methylmalononitrile.The mechanistic path for the reaction of cyclopropenes with 2-methylmalononitrile mainly involves ring-opening of cyclopropenes,isomerization,proton transfer,and nucleophilic attack.For the Pd-catalyzed arylation of cyclopropenes with aryl iodides,the mechanisms and chemo-selectivity are found to be substituent-dependent on the C(sp3)atom of cyclopropenes.When a β-H atom is present in the groups attached to the C(sp3)of cyclopropenes,the reaction could proceed successively through two cycles.The first cycle mainly includes oxidative addition of aryl iodides,migration insertion of cyclopropenes,and base-assisted β-H elimination,leading to an aryl cyclopropene derivative.In the second cycle,after the substrate of aryl iodides is completely transformed,the Pd(0)catalyst would further activate the generated aryl cyclopropene to form a metallacyclobutane intermediate.Subsequently,the formed metallacyclobutane intermediate could sequentially undergo isomerization,proton transfer,and base-assisted βH elimination to furnish the 1,3-butadiene product.While,if such β-H atom is absent,the second cycle cannot be performed.As a result,the final product is the aryl cyclopropene generated in the first cycle.Based on the studies mentioned above,we proposed that the significance of this theis is not only to gain more insights on the mechanisms of transition-metal-catalyzed transformation of new carbene precursors and the origins of chemo-,stereo-and regio-selectivity,but also has some instructive impact on the design or development of new transition-metal carbene involved reactions. |
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