Recent advances in transition-metal catalyzed nitrene transfer reactions with carbamates

Nitrene transfer reactions are powerful tools in synthetic organic chemistry.In recent years,transitionmetal catalyzed nitrene transfer reactions with carbamates as the nitrene precursors have been widely pursued.Such species undergoes facile C-H amination,aziridination,and bifunctionalization of alkenes under the catalysis of different transition metals including Rh,Fe,Ru and others,enabling the efficient construction of various nitrogen-containing molecules.In this review,the recent developments in nitrene transfer reactions with carbamates via N-O bond cleavage were introduced based on different types of reaction,and the key mechanistic information and synthetic applications of the methodologies were discussed.

Catalytic enantioselective synthesis of β-amino alcohols by nitrene insertion

Chiral β-amino alcohols are important building blocks for the synthesis of drugs, natural products, chiral auxiliaries, chiral ligands and chiral organocatalysts. The catalytic asymmetric β-amination of alcohols offers a direct strategy to access this class of molecules. Herein, we report a general intramolecular C(sp^(3))–H nitrene insertion method for the synthesis of chiral oxazolidin-2-ones as precursors of chiral β-amino alcohols. Specifically, the ring-closing C(sp^(3))–H amination of N-benzoyloxycarbamates with 2 mol% of a chiral ruthenium catalyst provides cyclic carbamates in up to 99% yield and with up to 99% ee.The method is applicable to benzylic, allylic, and propargylic C–H bonds and can even be applied to completely non-activated C(sp^(3))–H bonds, although with somewhat reduced yields and stereoselectivities. The obtained cyclic carbamates can subsequently be hydrolyzed to obtain chiral β-amino alcohols. The method is very practical as the catalyst can be easily synthesized on a gram scale and can be recycled after the reaction for further use. The synthetic value of the new method is demonstrated with the asymmetric synthesis of a chiral oxazolidin-2-one as intermediate for the synthesis of the natural product aurantioclavine and chiral β-amino alcohols that are intermediates for the synthesis of chiral amino acids, indane-derived chiral Box-ligands, and the natural products dihydrohamacanthin A and dragmacidin A.

Theoretical Studies of the Isomerizations of Singlet and Triplet Silyl Nitrene

The isomerizations of singlet and triplet silyl nitrene are studied using ab initio method ofquantum chemistry.The singlet transition state has been optimized.The mechanism proposed here is thatthe triplet species transforms first to singlet one through intersystem crossing and then the isomerization re-action takes place.The difference of energies between the triplet and singlet species has been obtained.Theactive barrier and the reaction energy have been calculated.The relative structure data of reactants,transi-tion state and products are given.

Theoretical Study of Electronic Structure, Formation Mechanism and Intramolecular Sulfoxide Imidation Reactivity of Iron Phthalocyanine Nitrene Complex

Density functional theory(DFT) calculations are performed to investigate recent experimentally studied ring-closing sulfoxide imidation catalyzed by Fe(Ⅱ)-phthalocyanine(FeⅡPc). Our results reveal that the ground state of iron phthalocyanine nitrene intermediate(PcFeNR, R =(CH_(2))_(3)(SO)Ph), which is believed to mediate the intramolecular imitation, is triplet state featuring a diradical structure. The formation of Pc Fe NR is the result of a denitrification process with a calculated high-barrier of 23.4 kcal/mol which is in good agreement with the experimentally observed high reaction temperature of 100 ℃. The generated Pc Fe NR undergoes a low-barrier intramolecular nucleophilic attack by proximal nitrogen atom on the sulfur accomplishing the cyclization of sulfoxide. This study provides theoretical insights into the mechanism-based design of useful catalysts for nitrene transfer reactions.

Skeletal Editing of Cyclic Molecules Using Nitrenes

Nitrenes,as neutral monovalent nitrogen-centered molecular species,can insert into various bond or remove nitrogen atoms from amines.Nitrene assisted single-atom skeletal editing,discovered decades ago,provides an efficient approach for the precise alteration of cyclic skeletons.In this review,we briefly summarize early studies on skeletal editing of cyclic frameworks involving nitrene species,and introduce several recent important advances systematically.

Functionalization of carbon nanotubes and other nanocarbons by azide chemistry

Following the conventional carbon allotropes of diamond and graphite,fullerene,carbon nanotubes(CNTs) and graphene as 0D,1D and 2D graphitic macromolecules have been discovered recently in succession,declaring the unlimited potential of carbon-based nanomaterials and nanotechnology.Although CNTs exhibit significant potential applications in advanced materials and other fields due to their extraordinary mechanical strength and electrical/thermal conductivity properties,their low solubility,poor wettability and bad dispersibility in common solvents and solid matrices have limited their processing and applications.Thus,the attempt to achieve wettable/processable CNTs by functionalization has attracted increasing attention in both scientific and industrial communities.In recent years,azide chemistry has been demonstrated as a powerful means to covalently modify CNTs.It consists of two major approaches:click chemistry and nitrene chemistry,which both involve the usage of various azide compounds.The former one is based on highly reactive and stereospecifical Cu(I) catalyzed azide-alkyne cycloaddition reaction;the latter one is based on the electrophilic attack to unsaturated bonds of CNTs with nitrenes as reactive intermediates formed from thermolysis or photolysis of azides.In this mini-review paper,the azide chemistry to functionalize CNTs is highlighted and the corresponding functionalization routes to build CNT-based complex structures are also discussed.Besides,covalent functionalizations of other graphitic nanomaterials such as fullerence and graphene,via azide chemistry,are commented briefly.

Copper-catalyzed enantioselective desymmetrizing C(sp^(2))-H functionalization of azide-ynamides via α-imino copper carbenes

α-Imino metal carbenes are versatile intermediates in organic synthesis,and have broad applications in the assembly of divergent N-heterocycles.However,the catalytic enantioselective desymmetrization based onα-imino metal carbenes has not been developed to date.Herein,we disclose an enantioselective desymmetrizing C(sp^(2))-H functionalization of azide-ynamides viaα-imino copper carbenes,leading to the efficient assembly of divergent chiral indoloazepines in generally moderate to excellent yields with high enantioselectivities.Notably,this reaction represents the first enantioselective desymmetrization based onα-imino metal carbenes.Further synthetic transformations and biological tests show the potential utility of this method.Moreover,computational studies are employed to elucidate the reaction mechanism and the origin of enantioselectivity.

Enantioselective and Enantioconvergent Iron-Catalyzed C(sp^(3))-H Aminations to Chiral 2-Imidazolidinones

Comprehensive Summary Enantioselective or enantioconvergent iron-catalyzed ring-closing C(sp^(3))-H aminations of N-aroyloxyurea through intermediate iron nitrene species provide chiral 2-imidazolidinones in up to 99%yield and with up to 95%ee(40 examples).This is a rare example in which sustainable iron catalysis is combined with C(sp^(3))-H amination and asymmetric catalysis.

Iron-Catalyzed Intramolecular C-H Amidation of N-Benzoyloxyureas

Main observation and conclusion A redox-neutral Fe-catalyzed intramolecular C-H amidation of N-benzoyloxyureas is described.This methodology employs a simple iron complex in situ generated from Fe(OTf)2 and bipyridine as the catalyst and N-benzoyloxyureas as the nitrene precursors without using exogenous oxidants.An array of cyclic ureas were synthesized via aliphatic C(sp^(3))-H amidation in excellent yields.In addition,this catalytic system is also amenable to aryl C(sp^(2))-H nitrene insertion to provide benzimidazolones in moderate yields.

Catalyst-Free Imidation of Allyl Sulfides with Chloramine-T and Subsequent [2,3]-Sigmatropic Rearrangement

A facile synthesis of various allyl sulfonamides based on imidation of allyl sulfides with chloramine-T and sub- sequent [2,3]-sigmatropic rearrangement has been achieved without metal catalysts. The reaction completes smoothly within 10 min, providing excellent yields in environment friendly solvent of alcohol. Functional groups such as bromine, hydroxyl, protected amido and aldehyde are tolerant under this condition.

Palladium（o）-Catalyzed Si-Si Bond Insertion by the Terminal Nitrogen of Diazo Compoundst

The palladium（o）-catalyzed nitrogen insertion into cyclic Si-Si bonds has been realized by using N-tosylhydrazones/diazo compounds as the nitrogen source. The palladium（Ⅱ） nitrene formation and subsequent migratory insertion process are proposed as the key steps for this reaction.