2-Aminobenzothiazole-Containing Copper(II) Complex As Catalyst in Click Chemistry: an Experimental and Theoretical Study
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catalysts Article 2-Aminobenzothiazole-Containing Copper(II) Complex as Catalyst in Click Chemistry: An Experimental and Theoretical Study Lahoucine Bahsis 1,2,* , Meryem Hrimla 2 , Hicham Ben El Ayouchia 2 , Hafid Anane 2 , Miguel Julve 3 and Salah-Eddine Stiriba 2,3,* 1 Département de Chimie, Faculté des Sciences d’El Jadida, Université Chouaïb Doukkali, B.P.:20, El Jadida 24000, Morocco 2 Laboratoire de Chimie Analytique et Moléculaire/LCAM, Faculté Polydisciplinaire de Safi, Université Cadi Ayyad, Safi 46030, Morocco; [email protected] (M.H.); [email protected] (H.B.); ananehafi[email protected] (H.A.) 3 Instituto de Ciencia Molecular/ICMol, Universidad de Valencia, C/Catedrático José Beltrán 2, Paterna, 46980 Valencia, Spain; [email protected] * Correspondence: [email protected] (L.B.); [email protected] (S.-E.S.) Received: 21 June 2020; Accepted: 7 July 2020; Published: 11 July 2020 Abstract: The reaction of copper(II) acetate with the 2-aminobenzothiazole (abt) heterocycle affords the new copper(II) complex of formula [Cu(abt)2(OOCCH3)2](1) in a straightforward manner. Compound 1 served as a precatalyst for azide/alkyne cycloaddition reactions (CuAAC) in water, leading to 1,4-disubstituted-1,2,3-triazole derivatives in a regioselective manner and with excellent yields at room temperature. The main advantages of the coordination of such a heterocyclic ligand in 1 are its strong σ-donating ability (N-Cu), nontoxicity and biological properties. In addition, the click chemistry reaction conditions using 1 allow the formation of a great variety of 1,2,3-triazole-based heterocyclic compounds that make this protocol potentially relevant from biological and sustainable viewpoints. A molecular electron density theory (MEDT) study was performed by using density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) (LANL2DZ for Cu) level to understand the observed regioselectivity in the CuAAC reaction. The intramolecular nature of this reaction accounts for the regioselective formation of the 1,4-regioisomeric triazole derivatives. The ionic nature of the starting copper-acetylide precludes any type of covalent interaction throughout the reaction, as supported by the electron localization function (ELF) topological analysis, reaffirming the zwitterionic-type (zw-type) mechanism of the copper(I)/aminobenzothiazole-catalysed azide-alkyne cycloaddition reactions. Keywords: 2-aminobenzothiazole; click chemistry; CuAAC; MEDT; regioselectivity; ELF 1. Introduction Triazole-containing heterocycles have found many applications in several disciplines, such as medicine, biology and materials science [1]. In general, these compounds are prepared through azide-alkyne cycloaddition (AAC) reactions [2,3]. Indeed, the non-catalyzed [3 + 2] cycloaddition (32CA) reaction, also known as “Huisgen’s 1,3-dipolar cycloaddition”, between azides and alkynes is characterized by low rates and small yields at room temperature, due to the high kinetic barrier of the reaction [4,5], as well as the formation of two isomers, that is, 1,4- and 1,5-disubstituted 1,2,3-triazoles. Later, the discovery by the groups of Meldal [6] and Sharpless [7] that copper(I) is capable of accelerating the 32CA reaction of azides and alkynes with high regioselectivity, to afford the 1,4-disubstituted 1,2,3-triazole isomer under mild conditions, led to a breakthrough of such Catalysts 2020, 10, 776; doi:10.3390/catal10070776 www.mdpi.com/journal/catalysts Catalysts 2020, 10, x FOR PEER REVIEW 2 of 19 Catalysts 2020, 10, 776 2 of 17 disubstituted 1,2,3-triazole isomer under mild conditions, led to a breakthrough of such a acycloaddition cycloaddition process. process. Since Since then, then, the the copper- copper-catalyzedcatalyzed azide-alkyne azide-alkyne cycloaddition (CuAAC) reactionreaction has has become become the the most most representative representative click click reaction reaction in the in panorama the panorama of click of chemistry click chemistry due to dueits mild to its conditions mild conditions and vast and applications, vast applications, in particular in particular in life in sciences life sciences [8–11]. [8 –11However,]. However, the thethermodynamic thermodynamic instabilityinstability of copper(I), of copper(I), as well as well as its as easy its easy oxidation oxidation to copper(II), to copper(II), have have led ledto the to thesearch search for suitable for suitable stabilizing stabilizing ligands ligands for Cu(I) for Cu(I)ion. Consequently, ion. Consequently, the formation the formation of the active of the Lewis- active Lewis-acidacid copper(I) copper(I) species species is becoming is becoming crucial crucial in deve in developingloping CuAAC CuAAC reactions reactions in in terms terms of of stability stability and activity [12[12].]. A A variety variety of homogeneousof homogeneous catalytic catalyti systemsc systems were reported were reported for the click for of the 1,4-disubstituted click of 1,4- 1,2,3-triazolesdisubstituted 1,2,3-triazoles [13–17]. In this [13–17]. regard, In several this regard, polydentate several ligandspolydentate containing ligands nitrogen containing atoms nitrogen have beenatoms used have to been thermodynamically used to thermodynamically stabilize copper stabilize ions againstcopper destabilizingions against destabilizing reactants found reactants in the reactionfound in medium. the reaction Some of thesemedium. interesting Some ligandsof these include interesting the tris-(benzyltriazolylmethyl)amine ligands include the tris- (TBTA)(benzyltriazolylmethyl)amine [18], dipicolinate [19], tris(triazolyl)methanol(TBTA) [18], dipicolinate (TBTM) [19], tris(triazolyl)methanol [20], 2-pyrrolecarbaldiminato (TBTM) [21 [20],] and 2- crowdedpyrrolecarbaldiminato tetradentate tris(2-dioctadecylaminoethyl)amine [21] and crowded tetradentate tris(2-dioctadecylaminoethyl)amine [22]. [22]. InspiredInspired byby Nature’sNature’s strategy,strategy, thethe useuse ofof ligandsligands withwith biologicalbiological activitiesactivities cancan broadenbroaden thethe CuAAC applications.applications. ManyMany heterocyclicheterocyclic nuclei, nuclei, such such as as benzothiazole, benzothiazole, have have been been recently recently reviewed reviewed as antimicrobialas antimicrobial agents agents [23 [23,24],24] and and 2-aminobenzothiazole 2-aminobenzothiazole derivatives derivatives were were reported reported with with aa varietyvariety ofof biological applicationsapplications [[25,26].25,26]. TheThe 2-aminobezothiazole2-aminobezothiazole ligand ligand (abt) (abt) contains contains sulfur sulfur and and nitrogen nitrogen in in a ring,a ring, an exocyclican exocyclic nitrogen nitrogen and π -electrons,and π-electrons, giving thegiving ligand the a highligand coordination a high coordination ability. Its coordination ability. Its tocoordination the metal center to the via metal the nitrogen center via atom the of nitrogen the ring occurs atom inof mostthe ring of the occurs reported in most crystal of structuresthe reported [27, 28crystal]. It is structures worth noting [27,28]. that It the is typeworth of noting complexes that havethe type been of widely comple studiedxes have as been they exhibitwidely interestingstudied as biologicalthey exhibit and interesting photophysical biological properties and photophysical [29–31]. properties [29–31]. Having in mindmind thethe aboveabove facts,facts, we presentpresent herehere thethe synthesissynthesis ofof aa Cu(II)-abtCu(II)-abt complexcomplex inin ethanolethanol/water/water and thethe studystudy ofof itsits catalyticcatalytic potentialpotential forfor thethe clickclick ofof 1,4-disubstituted-1,2,3-triazole1,4-disubstituted-1,2,3-triazole compoundscompounds viavia azide-alkyneazide-alkyne cycloadditioncycloaddition reactionsreactions inin waterwater atat roomroom temperature.temperature. AA theoreticaltheoretical studystudy byby the MEDT method and ELF topological analysisanalysis isis also carried out to explain the mechanism and intermediatesintermediates species that participateparticipate in thisthis chemicalchemical process.process. It deserves to be noted thatthat thethe regioselectiveregioselective clickclick ofof 1,2,3-triazoles1,2,3-triazoles is performed in absence of any reducing agent or additive bases, whilewhile thethe catalystcatalyst isis foundfound toto bebe highlyhighly activeactive atat lowerlower loadings.loadings. 2. Results and Discussion The copper(II)-abtcopper(II)-abt complexcomplex ofof formulaformula [Cu(abt)[Cu(abt)22(OOCCH(OOCCH3))22]]( (11)) was was simply prepared by thethe addition ofof an an aqueous aqueous solution solution of copper(II) of copper(II) acetate ac toetate a methanolic to a methanolic solution ofsolution abt at room of abt temperature at room (Schemetemperature1). Green (Scheme crystals 1). Green of 1 were crystals grown of 1 fromwerethe grown resulting from solution.the resulting solution. SchemeScheme 1.1. Schematic illustration of the synthesis of 11.. The optimizedoptimized structure of 1 withwith thethe atomatom numberingnumbering ofof thethe non-carbonnon-carbon atoms is shownshown inin Figure1 1.. Bond Bond distances distances and and angles angles of of the the complex complex1 1are are summarized summarized in in Table Table1. The1. The copper(II) copper(II) ion ion in 1inis 1 six-coordinateis six-coordinate with with two twotrans trans-positioned-positioned nitrogen nitrogen atoms atoms from from to to abt abt molecules molecules andand fourfour oxygenoxygen atoms ofof twotwo chelatingchelating acetateacetate ligandsligands