A New Dimeric Copper(II) Complex of Hexyl Bis(pyrazolyl)acetate Ligand as an Efficient Catalyst for Allylic Oxidations.

Luca Bagnarelli,Roberto Giacomantonio,Maura Pellei,Carlo Santini,Alessandro Dolmella,Serena Gabrielli,Riccardo Vallesi

doi:10.3390/molecules26206271

Luca Bagnarelli, Roberto Giacomantonio + Show 5 more

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https://doi.org/10.3390/molecules26206271

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Abstract

A new dimeric copper(II) bromide complex, [Cu(LOHex)Br(μ-Br)]2 (1), was prepared by a reaction of CuBr2 with the hexyl bis(pyrazol-1-yl)acetate ligand (LOHex) in acetonitrile solution and fully characterized in the solid state and in solution. The crystal structure of 1 was also determined: the complex is interlinked by two bridging bromide ligands and possesses terminal bromide ligands on each copper atom. The two pyrazolyl ligands in 1 coordinate with the nitrogen atoms to complete the Cu coordination sphere, resulting in a five-coordinated geometry—away from idealized trigonal bipyramidal and square pyramidal geometries—which can better be described as distorted square pyramidal, as measured by the τ and χ structural parameters. The pendant hexyloxy chain is disordered over two arrangements, with final site occupancies refined to 0.705 and 0.295. The newly synthesized complex was evaluated as a catalyst in copper-catalyzed C–H oxidation for allylic functionalization through a Kharasch–Sosnovsky reaction without any external reducing agent. Using 0.5 mol% of this catalyst, and tert-butyl peroxybenzoate (Luperox) as an oxidant, allylic benzoates were obtained with up to 90% yield. The general reaction time was only slightly decreased to 24 h but a very significant decrease in the alkene:Luperox ratio to 3:1 was achieved. These factors show relevant improvements with respect to classical Kharasch–Sosnovsky reactions in terms of rate and amount of reagents. The present study highlights the potential of copper(II) complexes containing functionalized bis(pyrazol-1-yl)acetate ligands as efficient catalysts for allylic oxidations.

Highlights

Nowadays, the focus on innovative and proper chemical transformations using catalytic metal complexes to devise functionalized intermediates remains indispensable in many chemical areas
The crystal structure investigation revealed that, in the solid state, the compound exists as a dimer of formula [Cu(LOHex)Br(μ-Br)]2, with the Br(1) ions binding two symmetry-related units to each other
The X-ray crystal structure investigation revealed that, in the solid state, the compound exists as a dimer of formula [Cu(LOHex)Br(μBr)]2, with the Br(1) ions binding two symmetry-related units to each other, in which the copper ion shows a distorted square pyramidal arrangement

Summary

Introduction

The focus on innovative and proper chemical transformations using catalytic metal complexes to devise functionalized intermediates remains indispensable in many chemical areas. The allylic oxidation of alkenes [1,2,3,4] allows access to highly functionalized compounds with a high added value, such as alcohols, aldehydes, ketones, epoxides, and carboxylic acids, which are suitable for further manipulations in synthetic and industrial applications [5,6] These chemical derivatives can be generally obtained throughout an epoxidation and a dihydroxylation reaction, but, in contrast, allylic oxidation with copper catalysts and peroxy ester oxidant, often referred to as the Kharasch–Sosnovsky reaction, generates products with the olefin left intact [7,8,9,10,11,12,13] (Scheme 1). The search for aimlyprsotvsed[C8u,-1b0as,e2d0c,a3ta0ly–st3s3to].prHomootwe aellvyliecro,xidinatiotnhaendKwhidaenrtahsecrehla–teSdofiselndoofvsky cathpoepmcliocpaptpileoern-xscaehtaasslybrzeeeednsauallnlytliimsc poioxnridtaanbtitoetnotptoeifcroilneafithnnesdsiscitemhnetiokfiecryecsotmerpmeipunnrtihoteyds[y2u2n]ct.hiIebnsilpseaorftyimcuiaelnalyrd, s com wnathuriaclhpr,odinuctas adndd ipthiaormna,ceruetqicaulsi[r2e3–2t9h].eThueseaeseooff heanxdtleinrgnaandl trhee dstaubciliitny gof agen rcthoeepapKcehtra(iIroIa)snscahl–htsSaomssnakobevestkheyenmreavccaotliuonantb,rltehoeavnudesersousfiitCaabull,(eI)accanotamdlypsldtesxi[ef8sf,1ree0rs,2ue0l,nt3s0ti–n3ab3l]e.ttteHerronawnaedvtmeirv,oirenes hav yreepraordsuci[b3le4y–ie3ld9s]c.omTphariesdctoaCnu(bII)espeexcieps,lwahiniche,din abdydithioon,wrequciorepthpe eusre ocfhexetmer- istry dnalateleprrendeautnicvidensgihnaaggveenotbsen.eTnhipetsrmeseoecnxhtaeinddisoamvteiorofttnhheesyreetaaacrttsieo[3n,4h–ta3hs9i]b.seTemhnicseocntatanrolbvecereasxinpall,apainnoeddwdbifyeferhrfeounwtlly ca oconppee-r achnemdi/sotrry itswincore-deilbelycdtirvoernse,maned,cdheapennidsinmg osn. its oxidation state, this metal can powerfully catalyze reactions involving one- and/or two-electron mechanisms

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