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Diboron Catalyzed by Cu3(BTC)2 Using Cesium Carbonate As a Base

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Diboron Catalyzed by Cu3(BTC)2 Using Cesium Carbonate As a Base nanomaterials Communication α,β-Enone Borylation by Bis(Pinacolato)Diboron Catalyzed by Cu3(BTC)2 Using Cesium Carbonate as a Base Amarajothi Dhakshinamoorthy 1,*, Mercedes Alvaro 2, Abdullah M. Asiri 3 and Hermenegildo Garcia 2,3,4,* 1 School of Chemistry, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India 2 Departamento de Quimica, Universitat Politecnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain; [email protected] 3 Center of Excellence in Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia; [email protected] 4 Instituto Universitario de Tecnologia Quimica (CSIC-UPV), Universitat Politecnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain * Correspondence: [email protected] or [email protected] (A.D.); [email protected] (H.G.) Abstract: Cu3(BTC)2 (BTC: 1,3,5-benzenetricarboxylate) as a heterogeneous catalyst in the pres- ence of cesium carbonate as a base is reported for the borylation of α,β-conjugated enones by bis(pinacolato)diboron (B2pin2). According to the hot-filtration test, Cu3(BTC)2 is acting as a hetero- geneous catalyst. Further, Cu3(BTC)2 exhibits a wide substrate scope and can be reused in consecutive runs, maintaining a crystal structure as evidenced by powder X-ray diffraction (XRD). A suitable mechanism is also proposed for this transformation using Cu3(BTC)2 as catalyst. Keywords: borylation; 2-cyclohexenone; heterogeneous catalysis; metal organic frameworks Citation: Dhakshinamoorthy, A.; Alvaro, M.; Asiri, A.M.; Garcia, H. α,β-Enone Borylation by Bis(Pinacolato)Diboron Catalyzed by 1. Introduction Cu3(BTC)2 Using Cesium Carbonate as a Base. Nanomaterials 2021, 11, 1396. Organoboron compounds are important synthetic intermediates for a large variety of https://doi.org/10.3390/ transition metal-catalyzed C–C and C–X (X: O, N, S) bond-forming reactions [1,2]. These nano11061396 types of reactions generally exhibit high yields and are compatible with a large variety of functional groups, allowing the selective formation of new C–C bonds in highly functional- Academic Editors: Francisco Alonso ized substrates. More specifically, cross coupling reactions are compatible with carbonyl and Nikolaos Dimitratos groups that are generally unreactive under the conditions required for the coupling of boronate moieties. One of the easiest ways to obtain organoboronates is the use of diborane Received: 22 April 2021 as a reagent in the presence of suitable catalysts [3,4]. Depending on the reaction conditions Accepted: 20 May 2021 and catalysts, diboranes can provide electrophilic and nucleophilic boron species that are Published: 25 May 2021 able to form C–B bonds [5]. The conjugate addition of B2pin2 reagent to α,β-unsaturated carbonyl compounds Publisher’s Note: MDPI stays neutral is a convenient strategy to prepare functionalized organoboron compounds through the with regard to jurisdictional claims in incorporation of a Bpin unit at the β-position of the carbonyl group [6]. This organic published maps and institutional affil- transformation was reported with a series of homogeneous catalysts consisting of transition iations. metals such as Pt [7,8], Rh [9], Cu [10–13], Ni [14], N-heterocyclic carbenes [15,16], and Brönsted bases [17]. In this regard, it has been recently reported that 2-cyclohexenone can react with B2pin2 in the presence of metallic Cu, Cu(I) or Cu(II) salts or complexes to obtain β-ketopinacolboronates [18–23]. Although homogeneous copper salts or complexes have Copyright: © 2021 by the authors. been reported for this reaction [23], examples on the use of heterogeneous copper-based Licensee MDPI, Basel, Switzerland. catalysts are still limited. This article is an open access article Cu3(BTC)2 (BTC: 1,3,5-benzenetricarboxylate) is a type of metal organic framework distributed under the terms and (MOF) whose metal nodes are constituted by dimeric Cu2+-ions with octahedral coordina- conditions of the Creative Commons tion positions around each Cu2+ ion that are satisfied by four carboxylate groups of different Attribution (CC BY) license (https:// BTC linkers, a Cu-Cu bond and a solvent molecule, typically N,N’-dimethylformamide creativecommons.org/licenses/by/ (DMF), employed during the synthesis. This solvent molecule can be easily removed 4.0/). Nanomaterials 2021, 11, 1396. https://doi.org/10.3390/nano11061396 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, x FOR PEER REVIEW 2 of 11 Nanomaterials 2021, 11, 1396 2 of 11 N,N’-dimethylformamide (DMF), employed during the synthesis. This solvent molecule can be easily removed by thermal treatment under vacuum without damaging the crystal structure,by thermal generating treatment coordinative under vacuumly unsaturated without damaging positions the around crystal Cu structure,2+ that are generat- able to activateing coordinatively substrates unsaturatedand reagents positions [24]. The around structure Cu 2+of thatCu3(BTC) are able2 is to shown activate in substratesScheme 1 [25].and reagentsThese metal [24 ].nodes The and structure linkers of define Cu3(BTC) large2 cavitiesis shown in ina highly Scheme open1[ 25 structure]. These with metal a highnodes surface and linkers area (~1300 define m2 large/g) and cavities porosity in a(1.6 highly nm). openAmong structure the various with MOFs a high that surface have beenarea (~1300tested min2/g) heterogeneousand porosity (1.6catalysis nm). Amongto promote the various organic MOFs tran thatsformations have been [26,27], tested Cuin heterogeneous3(BTC)2 is one of catalysis the MOFs to promote frequently organic used transformationsfor a broad range [26 of,27 reactions], Cu3(BTC) due2 isto onethe presenceof the MOFs of coordinatively frequently used unsaturated for a broad metal range sites of around reactions Cu due2+ ions. to theThese presence Cu2+ ions of coor-with adinatively coordination unsaturated free position metal behave sites around as Lewis Cu acid2+ ions. sites Thesein a heterogeneous Cu2+ ions with fashion, a coordina- thus providingtion free position opportunities behave to as replace Lewis acidhomogeneous sites in a heterogeneous Lewis acid catalysts fashion, for thus organic providing reac- tions.opportunities Therefore, to replaceCu3(BTC) homogeneous2 has been widely Lewis used acid catalystsas a catalyst for organicto promote reactions. a variety There- of Lewisfore, Cu acid-catalyzed3(BTC)2 has beenorganic widely reactions used including as a catalyst condensation to promote [28], a variety the ring of Lewisopening acid- of epoxidecatalyzed [29], organic and reactionscyclizations including [30–32], condensation among others [28 [33–35].], the ring Furthermore, opening of epoxideCu2+ ions [29 in], 2+ Cuand3(BTC)cyclizations2 MOF [with30–32 unsaturated], among others positions [33–35 ].have Furthermore, also been Cu reportedions in in Cu many3(BTC) organic2 MOF transformationswith unsaturated like positions the oxidation have also of beenstyrene reported [36], aerobic in many epoxidation organic transformations of olefin [37], likeCO oxidationthe oxidation [38], of oxidative styrene [ 36synthesis], aerobic of epoxidation quinazolinones of olefin [39], [37 synthesis], CO oxidation of tetrazoles [38], oxida- [40], arenetive synthesis borylation of quinazolinones[41], Friedel–Crafts [39], alkylation synthesis of of indoles tetrazoles with [40 nitroalkenes], arene borylation [42], dehy- [41], drogenativeFriedel–Crafts coupling alkylation of dimethylphenylsilane of indoles with nitroalkenes with phenol [42], dehydrogenative [43], acetalization coupling of benzal- of dehydedimethylphenylsilane [44], aldol synthesis with phenol of pyrimidine-c [43], acetalizationhalcone ofhybrids benzaldehyde [45], and [44 hydrogenation], aldol synthesis of acetophenoneof pyrimidine-chalcone by silanes hybrids [46]. [45], and hydrogenation of acetophenone by silanes [46]. Scheme 1. Structure of Cu3(BTC)2. Reproduced with permission from [25]. Copyright 2015 Elsevier. Scheme 1. Structure of Cu3(BTC)2. Reproduced with permission from [25]. Copyright 2015 Elsevier. 3 2 Considering the broad scope ofof CuCu3(BTC)(BTC)2 inin heterogeneous heterogeneous catalysis catalysis [47–49], [47–49], one general tendency in in catalysis is is to develop solid solid and and recoverable catalysts catalysts for for those those pro- pro- cesses thatthat areare carriedcarried out out using using soluble soluble homogeneous homogeneous catalysts. catalysts. Herein, Herein, it is it reported is reported that thatCu3 (BTC)Cu3(BTC)2, a commercially2, a commercially available available MOF, MOF, is a suitable is a suitable and stable and solid stable catalyst solid tocatalyst promote to promotethe formation the formation of β-keto of organoboranes. β-keto organoboranes. We wish We to makewish to use make of the use active of the Cu active2+ ions Cu in2+ ionsCu3(BTC) in Cu23(BTC)as catalytically2 as catalytically active sitesactive for sites this for transformation. this transformation. The observed The observed results results in this inwork this are work highly are highly promising promising due to theduefact to the that fact the that catalyst the catalyst is readily is readily available available as well as wellthe fact as the that fact it can that be it easilycan be reused easily inreused consecutive in consecutive cycles. cycles. Nanomaterials 2021, 11, 1396 3 of 11 2. Materials and Methods 2.1. Materials Cu3(BTC)2 was purchased from Sigma Aldrich with the commercial trade name Baso- lite C300. Similarly, other related MOFs like Al(OH)(BDC)
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