Azo-heterocycles Synthesis Through Transition Metal Promoted B/C-H Bond Activation

Transition-metal promoted B/C-H bond activation provides a green and atom-economical approach for chemical synthesis.Furthermore,they own many other advantages such as mild reaction conditions,fewer reaction steps and high yields.By reviewing the development on transition metal promoted B-H activation and azo-heterocycles synthesis based on C-H bond activation,the importance and efficiency of the bond activation strategy in bio-active and functional compounds synthesis was presented and stressed,therefore,we focused our efforts on transition metal promoted B/C-H activation.All the reactions can be performed at mild conditions without protection from oxygen.1.Cobalt promoted B-H bond activationBy introducing different diynes into the reaction between CpCo-1,2-S2C2B10H10,and diazo esters,we successfully established a three component reaction,from which,a novel type of pyrrolylmethyl group decorated carborane derivatives was prepared.Furthermore,a stable intermediate which contains a Co-B bond was isolated.The Co-B bond is very active that it can be inserted by alkynes.The pyrrolylmethyl carborane derivatives were produced in the presence of oxygen.The reactivity varies from the types of diynes,and all of the tried ones can lead to B(3)-functionalized carborane derivatives.2.A novel type of nonclassical interaction:B-H…? hydrogen bondingThe nonclassical B-H…? interaction was observed in the stable 18e complexes Cp*IrS2C2B10H10[PR3](R=Ph,4-OMe-Ph,4-F-Ph).Their single-crystal structures reveal that one of the B-H bonds points to one of the arenes and the distances are between 2.40-2.76 A.Furthermore,1H NMR also confirms the existence of the weak interaction in solution.The signal of the B-H…? interaction appears at high-field(?0.0 ppm)at room temperature,which is distinct from the other BH signals appearing in the range of 1.4-3.0 ppm.This is in line with a shielding of the proton due to the ring current of the nearby phenyl ring.However,for complexes Cp*IrS2C2B10H10[PPhMeR](R=Me or Ph),the B-H…? interaction wasn't observed in solid state,which is probably due to the involvement of the phenyl rings in intermolecular weak interaction when packing.In solution,the distinguished signal wasn't observed at room temperature either,which is probably due to the fast rotation of the phosphine ligand,thus the signal showed at very low temperature(<70?).Herein,we can conclude that the formation of B-H…? interaction needs conditions:stable sources of arenes,suitable steric hindrance or low temperature.3.Copper(?)catalyzed Csp-H interactionCopper(?)acetate has been testified to be efficient in catalyzing Csp-H activation to form homo-coupling products of electron deficient alkynes,and the products can react with anilines through hydroamination to generate C=N bond and then construct quinoline framework by intramolecular Friedel-Crafts reaction.All the transformations can be realized in one pot as a cascade reaction at room temperature,and the reaction is compatible with air and moisture.Both of the reactants are widely available and the reaction has a broad substrate scope of anilines.4.Rh(?)-catalyzed Csp2-H coupling with allyl reagentsCsp2-H bond activation by Rh(?)catalysis was successfully realized under the direction of amides at room temperature.(1)When the amides were prepared from anilines,allyl products can be produced in high yields at room temperature with the usage of allyl bromide,which suffered from either low yield or severe isomerization of products before.The reaction is compatible with air and moisture,thus normal solvents can be used without purification.The reaction can be applied to both arenes and alkenes.By combining the successful allylation reaction with aminobromination,a domino reaction was designed to prepare dihydroisoquinolones.(2)When the amides were prepared from N-methylhydroxylamine,tremendous solvent effect was observed in the reaction between the amides and allyl acetate.1)In THF,cinnamyl derivatives through ?-H elimination were generated.Because of the existence of inner oxidant NHOMe,no extra oxidant was needed.2)In MeOH,the allylation products through ?-OAc elimination were afforded.Both routes shared the advantages such as broad substrate scope,mild condition and valuable products in organic synthesis and biological activity.Furthermore,the domino synthesis of dihydroisoquinolones through allyl products using NBS,NCS,NIS or related compounds were accomplished,and isoindolines were also prepared through the cinnamyl derivatives.