Vanadium catalyzed oxidation with hydrogen peroxide

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    Vanadium catalyzed oxidation with hydrogen peroxide

    Valeria Conte *, Barbara FlorisDipartimento di Scienze e Tecnologie Chimiche, Universit di Roma Tor Vergata, Via della Ricerca Scientica, 00133 Roma IT, Italy

    a r t i c l e i n f o

    Article history:Received 15 April 2009Received in revised form 18 June 2009Accepted 29 June 2009Available online 8 July 2009

    Keywords:VanadiumOxidationHydrogen peroxideMechanismsIonic liquids

    a b s t r a c t

    Vanadium peroxides are known as very effective oxidants of different organic and inorganic substrates. Inthis short account reactivity, structural and mechanistic studies concerning the behaviour of peroxovana-dates toward a number of different substrates are collected. Homogeneous and two-phase systems arepresented, in addition, interesting synthetic results obtained with the use of ionic liquids as reactionmedia are also presented.

    2009 Elsevier B.V. All rights reserved.

    graduated in Chemistry at Padova University, Italy in 1982 and obtained in 1986 the Ph.D. degree in Chemical Sciences, University986 Ph.D. student with Prof. J.K. Stille at Colorado State University, Ft. Collins. Co. USA. 19861987 Consultant for INTEROX9871988 CNR fellowship and 19881998 CNR Researcher at Centro Meccanismi Reazioni Organiche Padova University. 1993ior Fellowship at Colorado Stato University Ft. Collins Colorado USA. 19982001 Associate Professor of Organic Chemistry, Uni-. 20012006 Associate Professor of Organic Chemistry, and since November 2006 Full Professor in Organic Chemistry at Universityrgata. Cl Societytion and

    graduat

    * Corresponding author.

    Inorganica Chimica Acta 363 (2010) 19351946

    Contents lists available at ScienceDirect

    Inorganica Chimica Acta

    journal homepage: www.elsevier .com/locate / icaE-mail address: valeria.conte@uniroma2.it (V. Conte).working at the Universities of Roma La Sapienza (19711988), Napoli Federico II (19941995), and Roma Tor Vergata (19891994 and from1996 up to-date). She spent periods of research in USA, as Visiting Professor or as Visiting Researcher (Auburn University, Virginia CommonwealthUniversity, Texas A&M University). She was also invited as Visiting Professor at the University of Katowice (Poland) and at the ComoeniusUniversity of Bratislava (then Czecoslovachia). Member of Italian Chemical Society and American Chemical Society.Valeria Conteof Padova, 1985chimica s.p.a. 1NATO-CNR Senversity of Foggiaof Roma Tor VeItalian ChemicaDioxygen Activa

    Barbara Floris0020-1693/$ - see front matter 2009 Elsevier B.V. Adoi:10.1016/j.ica.2009.06.056oordinator of COST projects in D12 D29 and D40 Actions. Italian Representative for COST action D24 and D40 Member ofand American Chemical Society. Member of international advisory boards of ADHOC International Symposium onHomogeneous Catalytic Oxidation and International Symposium on Chemistry and Biochemistry of Vanadium.

    ed in Chemistry in 1969, became Assistant Professor in 1971, Associate Professor in 1983, and Full Professor in 1994,ll rights reserved.

  • Contents

    1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2. Reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3. Structural studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4. Mechanistic studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    1. Introduction

    Among transition metals, vanadium is the second most abun-dant in the sea water (ca. 30 nM mainly in the soluble form ofNaH2VO4). Vanadium is also present in soils and fresh water bothas vanadate, V(V), and vanadyl, oxido-V(IV) [1]. This metal is there-fore available for living organisms, in particular in marine environ-ment. In fact quite a few enzymes containing vanadium in theactive site are known. Between them, vanadate dependent halo-peroxidases enzymes, isolated from red and brown algae, catalyze

    a potential as insulin-enhancing agents in the treatment of human

    The mode of action of V-dependent bromoperoxidases enzymes

    contribution in the eld dates back to the early 90s of the last

    O O

    1936 V. Conte, B. Floris / Inorganica Chimiin their active sites; this metal in the presence of hydrogen perox-4

    ide forms a peroxido vanadium derivatives which oxidizes a bro-Scheme 1. Typical oxidation reactions carried out in the presence of vanadium

    siSpcentury, when, inspired by the mode of action of such enzymessee Scheme 2, we developed a synthetic procedure for the bromin-ation of organic substrates in a two-phase system (H2O/CHCl3)(Scheme 3).

    Our proposal derived from the knowledge that the acceptedmode of action of V-BrPOs [4] involves the presence of vanadium

    XOH

    XSO2

    SO 2-

    n = 3 or 4

    OH(V-BrPO) have stimulated several research groups in searchingfor an effective functional model for their reactivity [1,4,14]. Our

    LnO

    VO R R'

    R R'the oxidation of halide ions by hydrogen peroxide [2] and areknown to play a major role particularly in the biosynthesis of bro-minated compounds. The catalytic effect is related to the forma-tion, in the active site, of the enzyme of a peroxido vanadiumspecies which is a much stronger oxidant in comparison withH2O2. These peroxido derivatives oxidize the bromide ion to forma brominating intermediate which then either brominates the or-ganic substrate or reacts with another bromide ion.

    More related to the classical catalytic oxidation reactions is theknowledge that vanadium peroxides are very effective oxidants ofdifferent organic and inorganic substrates like suldes, alkenes,alcohols, aromatic and aliphatic hydrocarbons, halides and sulfurdioxide, as summarized in Scheme 1 [3].

    These reactions either stoichiometric or catalytic, where theterminal oxidant is normally hydrogen peroxide or an alkylhydrop-eroxide, are carried out, on average, in mild conditions and associ-ated to good product yield and selectivity. The enantioselectiveversion of some of these processes has been also reported [3].

    RS

    R'

    RS

    R'

    O

    RS

    R'

    O O

    R R'

    R R'

    O

    OH

    +

    Br -"HOBr, Br2, Br3-"de-on peroxides complexes. L can be an organic ligand or a solvent molecule.toichiometric, with isolated species, and catalytic, with H2O2 as terminal oxidant,rocesses are known [3].diabetes of type 2 [1,5].The number and the nature of peroxidovanadates formed upon

    addition of hydrogen peroxide to vanadium precursors depend onpH, added ligands and relative concentrations of the reagents.Quite recently we showed that the association of electrospray ion-ization mass spectrometry (ESI-MS) with 51V NMR spectroscopyand theoretical calculations can be a valuable tool to obtain a de-tailed speciation of the V(V)H2O2H+-ligand systems [6,7]. Theidentication of the vanadium complexes, for a particular experi-mental condition, is clearly fundamental in order to understandthe mechanisms and products distribution of reactions in whichsuch species are involved.

    All our work carried out in the eld of solution characterizationof vanadates and peroxidovanadates is assembled, in a conciseform, in rather recent reviews [7,8] and interested readers are thusreferred there or to the original papers [6,9-12], nevertheless, someof the main gures in the eld will be presented here.

    In this short account, dedicated to our involvement in the appli-cation of vanadium catalysis in organic synthesis, we mainly illus-trate the peculiar oxidative performance of various V-peroxidoderivatives toward alkenes, alkynes and aromatics. In addition tothe classical epoxidation or hydroxylation reactions, we also callattention to oxybromination of double and triple bond. On top,we put on view our, very interesting and more recent, resultsachieved by using ionic liquids as new solvents, in place of the clas-sical chlorinated molecular ones. This aspect attracted our concern,in order to meet with our systems one of the stringent modernrequirements of Green Chemistry [13], i.e. the reduction of theamount of volatile organic compounds (VOCs) in synthetic innova-tive procedures.

    Finally, some of the mechanistic details of the reactions studiedwill be shortly reviewed.

    2. Reactivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1936

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1936

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1941

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1943

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1946

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1946

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1946

    Vanadium and vanadium peroxides, in addition, generate anumber of biological and biochemical responses [1,4] and retain

    ca Acta 363 (2010) 19351946mide ion, thus forming a bromine equivalent intermediate. Suchan intermediate may then either brominate an appropriate organicsubstrate or react with another molecule of terminal oxidant thus

  • of the enzymes. In particular the former process should take placeinto a hydrophilic portion present nearby the active site of the en-zyme, whereas the latter reaction should take place into a hydro-phobic region.

    Following this line of reasoning, we have developed a biphasicprocedure (see Scheme 3) where a vanadium precursor, H2O2 andKBr are dissolved in water. In such catalytic phase the formationof a peroxido vanadium derivative and the oxidation of Br takeplace. The intermediate thus formed is then transferred into the or-ganic phase, generally a chlorinated solvent i.e. CHCl3 or CH2Cl2,where the functionalization of the substrate occurs [16,17]. To

    Scheme 2. Simplied description of the mode of action of V(V) dependent haloperoxidases enzymes [2,4,15].

    V. Conte, B. Floris / Inorganica Chimica Acta 363 (2010) 19351946 1937causing the decomposition reaction. In spite of the fact that the

    Scheme 3. V(V) based two-phase system for bromination reactions [16].two reactions occurs concurrently, the chemoselectivity of theenzyme catalyzed reactions is very high and thus the yields of bro-minated products, based on the H2O2 consumed is also very high.This evidence suggests that the formation of the intermediateand the bromination reaction occur in two different compartments

    Table 1Vanadium(V)-catalyzed bromination of aromatic hydrocarbons under two-phase condition

    Substrate (mmol) Catalysta (mmol) Source of bromine (mmol) H2O2

    0.2 V(V) (0.2) KBr (1.0) 0.4

    0.2 V(V) (0.2) KBr (1.0) 0.4

    O

    0.2 V(V) (0.2) KBr (1.0) 0.40.4 V(V) (0.2) KBr (0.5) 0.40.4 V(V) (0.2) KBr (0.5) 0.4

    0.4 Mo(VI) (0.2) KBr (0.5) 0.40.4 Mo(VI) (0.2) KBr (0.5) 0.4

    a V(V) as NH4VO3, Mo(VI) as (NH4)2MoO42H2O.b Water adjusted at pH 1 (HClO4).c Time necessary for the complete disappearance of the substrate, 424 h dependingd 1000 r.p.m.note, the process in the aqueous phase requires acid conditions,likely to permit protonation of the peroxido moiety. Direct evi-dence on the formation of a hydroperoxido intermediate, however,have not been obtained.

    The results obtained with model aromatic substrates are col-lated in Table 1. Interestingly, from the synthetic point of view,even more attractive results were obtained by using Mo-catalysis.

    First of all, a clean and cheap oxidant, H2O2, and a sustainablesource of positive bromine, such as KBr, are used in very mild con-ditions. Moreover the products are easily isolated from the reactionmixture. In fact they are obtained almost pure following separation

    s at 25 C.

    (mmol) Solventb Time (h) Conversion (%) Reference

    Br

    H2O/CHCl3 c 40 [16]Br

    H2O/CHCl3 c 50 [16]

    O

    Br

    H2O/CHCl3 c >98 [16]H2O/CHCl3d 4 35 [17]H2O/CHCl3d 72 48 [17]

    [17]H2O/CHCl3d 4 56 [17]H2O/CHCl3d 24 68 [17]

    on the substrate.

  • cond

    l)

    imiTable 2Vanadium(V)-catalyzed oxybromination vs. bromination of alkenes under two-phase

    Substrate (mmol) Catalyst a (mmol) Source of bromine (mmol) H2O2 (mmo

    0.2 BrO3, Br, H+

    0.2 NBS0.2 HOBr0.2 V(V) (0.2) KBr (1.0) 0.20.4 V(V) (0.2) KBr (0.5) 0.40.1 V(V) (0.2) KBr (0.5) 0.10.2 V(V) (0.2) KBr (0.5) 0.20.2 V(V) (0.2) KBr (0.5) 0.20.2 V(V) (0.1) KBr (1.0) 0.20.2 V(V) (0.1) KBr (1.0) 0.20.2 V(V) (0.1) KBr (1.0) 0.2

    0.2 Mo(VI) (0.2) KBr (0.5) 0.4

    1938 V. Conte, B. Floris / Inorganica Chof the two-phases and elimination, after drying with MgSO4, of theorganic one. On top of this, it has been demonstrated that, uponaddition of more H2O2 and substrate, the reaction continues untilcomplete disappearance of bromide ions.

    It is to be said that looking at the system after 15 years, the newrequirements of sustainable chemistry are calling for the substitu-tion of the chlorinated phase with a less polluting one or, whenpossible, to perform the reaction by using the substrate as solventitself.

    A more mechanistic driven study concerning bromination ofdouble bonds [18], clearly indicated that an intermediate wasformed in the course of the reaction. To note, the reaction is typi-cally characterized by a good degree of selectivity toward forma-tion of the bromohydrin [12]. In that respect, we observed thatthe products yields strongly depend on the rate of stirring. In sim-ilar experimental conditions other classical brominating systemsproduce selectively dibromide. There is evidence for substratecoordination, and more nucleophilic substrates favour the forma-tion of bromohydrin. Hammett correlations indicate two parallelprocesses. On this basis we believe that two intermediates are in-volved in the process: the rst one, a vanadium bound hypobro-mite ion, responsible for the formation of bromohydrin and the

    0.2 Mo(VI) (0.2) KBr (1.0) 0.20.2 Mo(VI) (0.2) KBr (1.0) 0.2

    0.2 V(V) (0.2) KBr (1.0) 0.40.2 V(V) (0.1) KBr (1.0) 0.20.2 V(V) (0.2) KBr (1.0) 0.20.2 V(V) (0.2) KBr (1.0) 0.2

    O

    >0.2 V(V) (0.2) KBr (1.0) 0.40.2 V(V) (0.2) KBr (1.0) 0.20.2 V(V) (0.2) KBr (1.0) 0.20.2 V(V) (0.2) KBr (1.0) 0.20.2 V(V) (0.1) KBr (1.0) 0.2

    a V(V) as NH4VO3, Mo(VI) as (NH4)2MoO42H2O.b Water adjusted at pH 1 (HClO4) with V catalysis.c 424 h depending on the substrate.itions at 25 C.

    Solventa Time (h) Conversion (%) Relative ratio Reference

    (OH, Br) (Br, Br)

    OH

    Br

    Br

    Br+

    H2O/DCM b 99 63 37 [18]H2O/DCM b 72 10 90 [18]H2O/DCM b 99 25 75 [18]H2O/DCM b 79 20 80 [18]H2O/DCM 66 72 94 6 [18]H2O/DCM 24 65 78 22 [19]bmimBF4 2 27 58 42 [19]bmimOTf 2 68 79 21 [19]H2O/bmimPF6 4 96 98 2 [19]H2O/bmimNTf2 6 92 97 3...

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