Edited by - download.e-bookshelf.de€¦ · The Editor Prof. Dr. Pher G. Andersson Uppsala University Department of Biochemistry and Organic Chemistry Husargatan 3 75123Uppsala Sweden

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Pher G. Andersson

Innovative Catalysis in Organic Synthesis

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Edited by Pher G. Andersson

Innovative Catalysis in Organic Synthesis

Oxidation, Hydrogenation,and C–X Bond Forming Reactions

The Editor

Prof. Dr. Pher G. AnderssonUppsala UniversityDepartment of Biochemistry andOrganic ChemistryHusargatan 3751 23 UppsalaSweden

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V

Contents

Foreword XIList of Contributors XIII

Part I Oxidation Reactions 1

1 Polyoxometalates as Homogeneous Oxidation Catalysts 3Mauro Carraro, Andrea Sartorel, Masooma Ibrahim, Nadeen Nsouli,Claire Jahier, Sylvain Nlate, Ulrich Kortz, and Marcella Bonchio

1.1 Soluble Metal Oxides as Oxidation Catalysts 31.2 Homogeneous Oxidations with POMs Based Only on Mo(VI), W(VI),

V(V) Addenda Ions 61.2.1 Oxidation with Hydrogen Peroxide by

Peroxopolyoxotungstates-Dendrimers 81.2.2 Homogeneous Oxidation with Hydrogen Peroxide in the Presence of

Vacant and Hybrid POMs 101.3 Homogeneous Oxidations with TMS-POMs 121.3.1 Peroxopolyoxometalates of Hf/Zr 131.3.2 Aerobic Oxidations with Polyoxopalladates 161.3.3 TMSPs as Oxygen-Evolving Catalysts 171.4 Conclusions 19

Acknowledgments 19References 20

2 Bioinspired Oxidations Catalyzed by Nonheme Iron and ManganeseComplexes 27Isaac Garcia-Bosch, Irene Prat, Xavi Ribas, and Miquel Costas

2.1 Introduction 272.2 Catalytic Oxidation of C=C Bonds by Nonheme Iron and Manganese

Complexes 272.2.1 Epoxidation 272.2.1.1 Iron-Based Catalysts 272.2.1.2 Manganese-Based Catalysts 302.2.2 cis-Dihydroxylation 34

VI Contents

2.2.2.1 Iron-Based Catalysts 342.2.2.2 Manganese-Based Catalysts 372.3 Catalytic Oxidation of C–H Bonds by Nonheme Iron and Manganese

Complexes 382.3.1 Hydroxylation 382.3.1.1 Iron-Based Catalysts 382.3.1.2 Manganese-Based Catalysts 402.3.2 Desaturation 412.3.2.1 Iron-Based Catalysts 412.3.2.2 Manganese-Based Catalysts 42

References 43

3 The Fabulous Destiny of Sulfenic Acids 47Maria Chiara Aversa, Paola Bonaccorsi, David Madec, Guillaume Prestat,and Giovanni Poli

3.1 Introduction 473.2 Synthesis of Stable Sulfenic Acids 483.3 Generation of Transient Sulfenic Acids 523.4 Reactivity of Sulfenic Acids in the Preparation of Sulfoxides and

Unsymmetrical Disulfides 573.5 Synthesis of Stable Sulfenate Anions 623.6 Generation of Transient Sulfenate Anions Leading to Sulfoxides 653.7 Conclusions 73

References 73

4 Sustainable Catalytic Oxidations with Peroxides 77Isabel W.C.E. Arends, Valeria Conte, and Giulia Licini

4.1 Introduction 774.2 Metal-Based Selective Oxidations 784.2.1 Bromination Reactions 784.2.2 Oxidation of Nitrogen-Containing Substrates 854.2.3 Oxidation of Sulfur-Containing Substrates 854.2.4 Oxidation of Alkenes 894.3 Biocatalytic Oxidations with Hydrogen Peroxide 924.3.1 Why Enzymes and HOOH? 924.3.2 Biocatalytic Sulfoxidation 954.3.3 Biocatalytic Alkenes Epoxidation 964.3.4 Biocatalytic Alcohols Oxidation 984.4 Conclusions 99

Acknowledgments 99References 100

Contents VII

Part II Hydrogenation and Reduction Reactions 103

5 Asymmetric Hydrogenation of Dehydroamino acid Derivatives byRh-Catalysts with Chiral Monodentate P-Ligands 105Serafino Gladiali, Elisabetta Alberico, and Ilya Gridnev

5.1 Introduction 1055.2 Chiral Monodentate Phosphorus Ligands in Asymmetric

Hydrogenation 1085.3 Catalyst Precursors 1125.4 Mechanistic Insights 1175.5 Formation of the MAC Adducts 1215.6 Evolution of MAC-Adducts and Origin of Enantioselection 124

References 126

6 Recent Advances in the Synthesis and Catalytic Hydrogenation ofDehydroamino Acid Derivatives and Bicyclo[2.2.2]octenes 131Veronique Michelet, Virginie Ratovelomanana-Vidal, Vasile I. Parvulescu,and Marijan Kocevar

6.1 Introduction 1316.2 Synthesis of DDAA Derivatives and Bicyclo[2.2.2]octenes 1336.3 Ligands 1336.4 Homogeneous Hydrogenation and Hydrogenolysis Reactions with

Dehydroamino Acid Derivatives and Bicyclo[2.2.2]oct-7-enes overNanocolloids-Modified Catalysts 136

6.4.1 Nanometal Colloids-Modified Catalysts 1366.4.2 Nanooxide Colloids-Modified Catalysts 1406.5 Heterogeneous Catalysts for Hydrogenolysis of

Bicyclo[2.2.2]oct-7-enes 1426.5.1 Heterogeneized Ligand-Modified Nanoclusters 1426.6 Layered-Double Hydroxides as a Support for Rh(TPPTS)3 and

Rh-(m-TPPTC)3 Homogeneous Catalysts 1446.7 Conclusions 147

Acknowledgments 147References 148

7 Ir-Catalyzed Hydrogenation of Minimally Functionalized Olefins UsingPhosphite–Nitrogen Ligands 153Montserrat Dieguez, Pher G. Andersson, and Oscar Pamies

7.1 Introduction 1537.2 Application of Phosphite–Nitrogen Ligands 1557.3 Conclusions 161

Acknowledgments 163References 163

VIII Contents

8 Modeling in Homogeneous Catalysis: a Tutorial 167Eric Clot and Per-Ola Norrby

8.1 Introduction 1678.2 Molecular Modeling 1678.3 Wave Function Theory, WFT 1688.4 Density Functional Theory, DFT 1698.5 Orbitals 1708.6 Basis Sets 1728.7 Solvation 1748.8 Analyzing the Reaction Energies 1758.9 Analyzing the Electronic Structure 1778.9.1 The NBO Method 1788.9.1.1 How Does It Work? 1788.9.1.2 Departure from the Lewis Structure 1808.9.1.3 NBO and Transition Metal Complexes 1838.9.2 The AIM Method 1878.9.2.1 How Does It Work? 1878.9.2.2 Nature of the Bonded Interaction 189

References 190

Part III C–C and C–Hetero Bond-Forming Reactions 193

9 Golden Times for Allenes 195Norbert Krause

9.1 Introduction 1959.2 Cyclization of Hydroxyallenes 1969.3 Cyclization of Aminoallenes 2039.4 Cyclization of Thioallenes 2069.5 Conclusion 206

References 207

10 Copper Catalysis in Arene and Heteroarene Functionalization throughC–H Bond Activation 211Sandro Cacchi, Giancarlo Fabrizi, and Antonella Goggiamani

10.1 Introduction 21110.2 C–C Bond-Forming Reactions 21210.2.1 Via (Hetero)aryl-H/R-X Coupling 21210.2.1.1 R–X = (Hetero)aryl Halides 21210.2.1.2 R–X = Alkenyl Bromides 21510.2.1.3 R–X = BrCH2Ar 21610.2.2 Via (Hetero)aryl-H/Ar2I+X− Coupling 21710.2.2.1 Direct (Hetero)arylation of Heteroarenes 21710.2.2.2 Direct Arylation of Arenes 21810.2.3 Via (Hetero)aryl-H/C–H Coupling 21910.2.3.1 Dimerization of (Hetero)arenes 219

Contents IX

10.2.3.2 Cyclization of Anilides 22010.2.3.3 Cyclization of N-aryl β-Enaminones 22110.2.4 Via Aryl-H Addition to Terminal Alkynes 22310.3 C–N Bond-Forming Reactions 22310.4 C–O Bond-Forming Reactions 22710.5 C–Halogen Bond-Forming Reactions 229

References 230

11 Ligated Organocuprates: an A–Z Routemap of Mechanism andApplication 233Simon Woodward and Darren Willcox

11.1 Introduction 23311.2 Accepted Mechanistic Proposals 23311.2.1 Kinetic and NMR Studies 23511.2.2 Computational Studies 24211.2.3 Nonlinear Effects 24311.2.4 Challenges 24511.3 Selective Applications in Privileged Copper(I) Catalysis 24511.3.1 Conjugate Addition 24511.3.2 Additions to Allylic Halides 250

References 252

12 Rh-, Ag-, and Cu-Catalyzed C–N Bond Formation 257Philippe Dauban, Camille Lescot, M. Mar Diaz-Requejo, andPedro J. Perez

12.1 Introduction 25712.2 Historical Background 25812.3 Copper- and Silver-Catalyzed C–N Bond Formation 26012.4 Rhodium-Catalyzed C–N Bond Formation 26512.5 Conclusions 273

References 274

13 Development of the Asymmetric Nozaki–Hiyama–Kishi Reaction 279Grainne C. Hargaden and Patrick J. Guiry

13.1 Introduction 27913.2 Development of a Catalytic Nozaki–Hiyama–Kishi Reaction 27913.3 Catalytic Enantioselective Nozaki–Hiyama–Kishi Reaction 28113.4 Application of Salen-Derived Ligands in the Enantioselective

Nozaki–Hiyama–Kishi Reaction 28313.5 Application of Oxazoline-Containing Ligands in the Catalytic

Enantioselective Nozaki–Hiyama–Kishi Reaction 28613.6 Application of Tethered Bis(8-quinolinato) Chromium Complexes in

the Catalytic Enantioselective Nozaki–Hiyama–Kishi 29913.7 Application of Chiral Spirocyclic Borate Ligands to the Catalytic

Enantioselective Nozaki–Hiyama–Kishi Allylation 303

X Contents

13.8 Applications of Catalytic Nozaki–Hiyama–Kishi Reaction in TotalSynthesis 303

13.9 Conclusions 305References 306

14 Chiral Imidate Ligands: Synthesis and Applications in AsymmetricCatalysis 309Timothy Noel, Katrien Bert, Pieter Janssens, and Johan Van der Eycken

14.1 Introduction 30914.2 Cyclic Imidates 31114.3 Synthesis of Imidates 31214.4 Synthesis of Imidate Ligands 31314.5 Synthesis of Imidate–Copper (I) Complexes 31314.6 Application of Chiral Imidate Ligands in Enantioselective

Catalysis 31514.6.1 Copper (I)-Catalyzed Asymmetric Aziridination 31514.6.2 Asymmetric Diethylzinc Addition 31614.6.3 Asymmetric Palladium(0)-Catalyzed Allylic Alkylations 31614.6.4 Asymmetric Iridium (I)-Catalyzed Hydrogenations 31814.7 Novel Synthetic Applications of Cyclic Imidates 32014.7.1 One-Step Synthesis of Chiral Oxazoline–Alcohol Ligands 32014.7.2 Synthesis of Chiral spiro-2-Alkoxy-Imidazolidines 32114.8 Conclusions 322

References 324

15 Catalyzed Organic Reactions in Ball Mills 327Achim Stolle, Bernd Ondruschka, Anke Krebs, and Carsten Bolm

15.1 Introduction 32715.2 Acid- or Base-Catalyzed Reactions 32815.3 Organocatalytic Methods 33315.3.1 Asymmetric Aldol Reactions 33315.3.2 Cycloaddition and Related Reactions 33515.4 Metal-Catalyzed Reactions 33815.4.1 Suzuki–Miyaura Reaction 33815.4.2 Mizoroki–Heck Reaction 34015.4.3 Sonogashira Reaction 34115.4.4 Cu-Catalyzed Reactions 34315.4.5 Miscellaneous Metal-Catalyzed Reactions 34515.5 Conclusion and Perspective 347

References 348

Index 351

XI

Foreword

This book had its genesis at a meeting on European Cooperation in Science andTechnology (COST) in Ankara, Turkey, in 2010. The Actions of COST not onlypromote the development of new and exciting science but they are also a marvellousmechanism for bringing together new alliances and friendships between disparatecommunities of scientists where the sum is most definitely worth more than theseparate paths. When Pher Andersson volunteered to coordinate a book describingsome of the highlights of the endeavours of our own Action (D40) – ‘‘InnovativeCatalysis: New Processes and Selectivities’’ I and others were wildly supportive.Not only did it seem an appropriate way to mark the end of five years of previouscollaboration between laboratories in 23 separate countries, but the time is ripeto define what is new and exciting in the, now mature, field of selective catalysis.The experts of D40 have come together to give their own personal take on whatthey consider to be ‘‘innovative’’ approaches to catalysis in this first decade of thetwenty-first century. I am most grateful to them all for freely volunteering their timeand especially to Pher Andersson for bringing this mission to a speedy conclusion.I am sure that you will find something to pique your imagination for your ownresearch in the next decade within – enjoy!

Simon WoodwardChair, COST Action D40 (2007–2011)

ESF provides the COST Office through an EC contract

COST is supported by the EU RTD Framework programme

XII Foreword

COST – the acronym for European Cooperation in Science and Technology –is the oldest and widest European intergovernmental network for cooperation inresearch. Established by the Ministerial Conference in November 1971, COST ispresently used by the scientific communities of 36 European countries to cooperatein common research projects supported by national funds.

The funds provided by COST – less than 1% of the total value of the projects –support the COST cooperation networks (COST Actions) through which, with EUR30 million per year, more than 30 000 European scientists are involved in researchhaving a total value that exceeds EUR 2 billion per year. This is the financial worthof the European added value, which COST achieves.

A ‘‘bottom-up approach’’ (the initiative of launching a COST Action comesfrom the European scientists themselves), ‘‘a la carte participation’’ (only countriesinterested in the Action participate), ‘‘equality of access’’ (participation is openalso to the scientific communities of countries not belonging to the EuropeanUnion) and ‘‘flexible structure’’ (easy implementation and light management ofthe research initiatives) are the main characteristics of COST. As precursor ofadvanced multidisciplinary research COST has a very important role for the reali-sation of the European Research Area (ERA) anticipating and complementing theactivities of the Framework Programmes, constituting a ‘‘bridge’’ toward the sci-entific communities of emerging countries, increasing the mobility of researchersacross Europe and fostering the establishment of ‘‘Networks of Excellence’’ inmany key scientific domains such as Biomedicine and Molecular Biosciences;Food and Agriculture; Forests, their Products and Services; Materials, Physical andNanosciences; Chemistry and Molecular Sciences and Technologies; Earth SystemScience and Environmental Management; Information and Communication Tech-nologies; Transport and Urban Development; and Individuals, Societies, Cultures,and Health. It covers basic and more applied research and also addresses issues ofpre-normative nature or societal importance.

Web: http://www.cost.eu

XIII

List of Contributors

Elisabetta AlbericoIstituto di Chimica BiomolecolareConsiglio Nazionale delleRicerchetrav. La Crucca n. 3, Li Punti07040 SassariItaly

Pher G. AnderssonUppsala UniversityDepartment of Biochemistry andOrganic ChemistryBOX 576751 23 UppsalaSweden

and

University of KwaZulu-NatalSchool of ChemistryWestville Campus4000 DurbanSouth Africa

Isabel W.C.E. ArendsDelft University of TechnologyBiocatalysis and OrganicChemistryDepartment of BiotechnologyJulianalaan 1362628 DelftThe Netherlands

Maria Chiara AversaUniversita degli Studi di MessinaDipartimento di Chimicaorganica e biologicaViale F. Stagno d’Alcontres 31(vill. S. Agata)98166 MessinaItaly

Katrien BertGhent University, Laboratory forOrganic and Bioorganic SynthesisDepartment of OrganicChemistryKrijgslaan 281 (S.4)9000 GhentBelgium

Carsten BolmRWTH Aachen UniversityInstitute for Organic ChemistryLandoltweg 152056 AachenGermany

Paola BonaccorsiUniversita degli Studi di MessinaDipartimento di Chimicaorganica e biologicaViale F. Stagno d’Alcontres 31(vill. S. Agata)98166 MessinaItaly

XIV List of Contributors

Marcella BonchioUniversity of PadovaITM-CNR and Department ofChemical Sciencesvia Marzolo 135131 PadovaItaly

Eric ClotUniversite Montpellier 2Institut Charles Gerhardt34000 MontpellierFrance

Sandro CacchiSapienza Universita di RomaDipartimento di Studi di Chimicae Tecnologie del FarmacoP.le A. Moro 500185 RomeItaly

Mauro CarraroUniversity of PadovaITM-CNR and Department ofChemical Sciencesvia Marzolo 135131 PadovaItaly

Valeria ConteRoma Tor Vergata UniversityDepartment of Chemical Sciencesand TechnologiesVia della Ricerca Scientifica snc00133 RomaItaly

Miquel CostasUniversitat de GironaFacultat de CienciesQbis Group, Department ofChemistryCampus de Montilivi17071 GironaCataloniaSpain

Philippe DaubanInstitut de Chimie desSubstances NaturellesUPR 2301 CNRSAvenue de la TerrasseGif-sur-Yvette91198 CedexFrance

M. Mar Diaz-RequejoUniversidad de HuelvaCentro de Investigacion enQuımica Sostenible (CIQSO)Departamento de Quimica yCiencia de Materiales21007 HuelvaSpain

Montserrat DieguezUniversitat Rovira i VirgiliDepartament de Quamica Fasica iInorganicaC/Marcel·li Domingo s/n43007 TarragonaSpain

Giancarlo FabriziSapienza Universita di RomaDipartimento di Studi di Chimicae Tecnologie del FarmacoP.le A. Moro 500185 RomeItaly

List of Contributors XV

Isaac Garcia-BoschUniversitat de GironaFacultat de CienciesQbis Group, Department ofChemistryCampus de Montilivi17071 GironaCataloniaSpain

Serafino GladialiUniversita di SassariDipartimento di Chimicavia Vienna 207100 SassariItaly

Antonella GoggiamaniSapienza Universita di RomaDipartimento di Studi di Chimicae Tecnologie del FarmacoP.le A. Moro 500185 RomeItaly

Ilya GridnevGraduate School of Science andEngineeringTokyo Institute of TechnologyDepartment of Applied ChemistryOokayama, Meguro-kuTokyo 152-8552Japan

Patrick J. GuiryUniversity College DublinCentre for Synthesis andChemical BiologySchool of Chemistry andChemical BiologyBelfieldDublin 4Ireland

Grainne C. HargadenDublin Institute of TechnologyFOCAS Institute and School ofChemical and PharmaceuticalSciencesKevin StreetDublin 8Ireland

Masooma IbrahimJacobs UniversitySchool of Engineering andScienceP.O. Box 750 56128725 BremenGermany

Claire JahierUniversity Bordeaux 1IECB-CBMN UMR 5248 CNRS2 Rue Robert Escarpit33607 Pessac CedexFrance

Pieter JanssensGhent UniversityLaboratory for Organic andBioorganic SynthesisDepartment of OrganicChemistry9000 GhentBelgium

Marijan KocevarUniversity of LjubljanaFaculty of Chemistry andChemical TechnologyDepartment of Chemistry andBiochemistryChair of Organic ChemistryAskerceva 51000 LjubljanaSlovenia

XVI List of Contributors

Ulrich KortzJacobs UniversitySchool of Engineering andScienceP.O. Box 750 56128725 BremenGermany

Norbert KrauseDortmund University ofTechnologyOrganic ChemistryOtto-Hahn-Strasse 644227 DortmundGermany

Anke KrebsRWTH Aachen UniversityInstitute for Organic ChemistryLandoltweg 152056 AachenGermany

Camille LescotInstitut de Chimie desSubstances NaturellesUPR 2301 CNRAvenue de la TerrasseGif-sur-Yvette91198 CedexFrance

Giulia LiciniPadova UniversityDepartment of Chemical SciencesVia Marzolo 135131 PadovaItaly

David MadecUniversite de Toulouse118 route de Narbonne31062 ToulouseFrance

Veronique MicheletEcole Nationale Superieure deChimie de Paris ChimieParisTechLaboratoire Charles Friedel11 rue Pierre et Marie Curie75231 Paris Cedex 5France

Sylvain NlateUniversity Bordeaux 1European Institute of Chemistryand BiologyIECB-CBMN UMR 5248 CNRS2 Rue Robert Escarpit33607 Pessac CedexFrance

Timothy NoelMassachusetts Institute ofTechnologyDepartment of Chemistry77 Massachusetts AvenueCambridgeMA 02139USA

Nadeen NsouliJacobs UniversitySchool of Engineering andScienceP.O. Box 750 561Campus Ring 128725 BremenGermany

Per-Ola NorrbyUniversity of GothenburgDepartment of Chemistry andMolecular BiologyKemigarden 441296 GoteborgSweden

List of Contributors XVII

Bernd OndruschkaFriedrich-Schiller University JenaInstitute for Technical Chemistryand Environmental ChemistryLessingstr. 1207743 JenaGermany

Vasile I. ParvulescuUniversity of BucharestDepartment of OrganicChemistryBiochemistry and CatalysisB-dul Regina Elisabeta 4-12030016 BucharestRomania

Oscar PamiesUniversitat Rovira i VirgiliDepartament de Quımica Fısica iInorganicaC/Marcel· li Domingo s/n43007 TarragonaSpain

Pedro J. PerezUniversidad de HuelvaCentro de Investigacion enQuımica Sostenible (CIQSO)Departamento de Quimica yCiencia de Materiales21007 HuelvaSpain

Giovanni PoliUniversite Pierre etMarie Curie – UPMCInstitut Parisien de ChimieMoleculaire4, Place Jussieu, boıte 18375252 Paris Cedex 5France

Irene PratUniversitat de GironaFacultat de CienciesQbis Group, Department ofChemistryCampus de Montilivi17071 GironaCataloniaSpain

Guillaume PrestatUniversite Pierre etMarie Curie – UPMCInstitut Parisien de ChimieMoleculaire4, Place Jussieu, boıte 18375252 Paris Cedex 5France

Michelet Ratovelomanana-VidalEcole Nationale Superieure deChimie de Paris ChimieParisTechLaboratoire Charles Friedel11 rue Pierre et Marie Curie75231 Paris Cedex 5France

Xavi RibasUniversitat de GironaFacultat de CienciesQbis Group, Department ofChemistryCampus de Montilivi17071 GironaCataloniaSpain

XVIII List of Contributors

Andrea SartorelUniversity of PadovaITM-CNR and Department ofChemical Sciencesvia Marzolo 135131 PadovaItaly

Achim StolleFriedrich-Schiller University JenaInstitute for Technical Chemistryand Environmental ChemistryLessingstr. 1207743 JenaGermany

Johan Van der EyckenGhent University, Laboratory forOrganic and Bioorganic SynthesisDepartment of OrganicChemistry9000 GhentBelgium

Darren WillcoxUniversity of NottinghamSchool of ChemistryUniversity ParkNottingham NG7 2RGUnited Kingdom

Simon WoodwardThe University of NottinghamSchool of ChemistryUniversity ParkNottingham NG7 2RGUnited Kingdom

1

Part IOxidation Reactions

Innovative Catalysis in Organic Synthesis: Oxidation, Hydrogenation, and C–X Bond Forming Reactions,First Edition. Edited by Pher G. Andersson.© 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

3

1Polyoxometalates as Homogeneous Oxidation CatalystsMauro Carraro, Andrea Sartorel, Masooma Ibrahim, Nadeen Nsouli, Claire Jahier,Sylvain Nlate, Ulrich Kortz, and Marcella Bonchio

1.1Soluble Metal Oxides as Oxidation Catalysts

Polyoxometalates (POMs) are discrete multitransition metal oxides characterizedby a formidable structural variety, resulting in different dimensions, shape, chargedensity, surface reactivity, and in a rich redox chemistry [1–7]. A first classificationof POMs is based on the chemical composition of these species, essentiallyrepresented by two types of general formula [8]:

1) [MmOy]p− (isopolyanions)2) [XxMmOy]q− (heteropolyanions)

where M is the main transition metal constituent of the POM, O is the oxygenatom, and X can be a nonmetal of the p block or a different transition metal.

Most of POMs are based on edge- and corner-shared MO6 octahedra, where M isan early transition metal in its highest oxidation state (V(V), Nb(V), Ta(V), Mo(VI),or W(VI)). Such metal ions exhibit dimensions (cationic radius) compatible withan octahedral coordination and possess empty d orbitals that allow the formationof terminal metal–oxygen double bonds, required to avoid the assembly of theoctahedra into an extended material (as for most common metal oxides) [1, 2, 9,10]. A recently discovered subclass of POMs based on noble metal ions comprisessquare-planar MO4 addenda units (M = Pd(II) and Au(III)) [11–15].

Owing to their particular composition and electronic structure, POMs can beconsidered as discrete models of extended metal oxides. As for the latters, thedoping process is a winning strategy to improve their catalytic behavior. Evenif there are several examples concerning electrostatic interaction with differenttransition metal cations, the most stable coordination mode is on incorporation ofthe transition metal in the POM structure with the formation of transition metalssubstituted polyoxometalates (TMSPs).

TMSPs can be obtained by using vacant or ‘‘lacunary’’ polyanions, derived fromthe corresponding saturated POMs, through the formal loss of one or more MO6

octahedral units, resulting from the hydrolytic cleavage of M–O bonds under

Innovative Catalysis in Organic Synthesis: Oxidation, Hydrogenation, and C–X Bond Forming Reactions,First Edition. Edited by Pher G. Andersson.© 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

4 1 Polyoxometalates as Homogeneous Oxidation Catalysts

M MM M

‘‘In-pocket’’ coordination ‘‘Out-of-pocket’’ coordination

Figure 1.1 ‘‘In-pocket’’ and ‘‘out-of-pocket’’ structural motif involving the divacant[SiW10O36]8− unit and two transition metal ions M.

alkaline conditions [16]. Such polyanions feature reactive terminal, coordinativelyunsaturated, oxygen atoms that surround the defect and form a ‘‘polydentate’’site, able to coordinate to one or more redox-active transition metals M’ as iron,manganese, cobalt, and ruthenium or different d0 metal ions (zirconium, hafnium,and titanium). In some cases, one-pot synthesis of TMSPs can be achieved by usingacidic/buffered solutions of suitable mononuclear precursors of both the transitionmetal and the POM itself.

Fundamental modes of binding include ‘‘in-pocket’’ coordination, where someaddenda atoms are replaced by the same number of different transition metals,and ‘‘out-of-pocket’’ structural motif, with transition metals occupying a vacancywithout being completely embedded (Figure 1.1). This latter coordination modegives access to ‘‘sandwichlike’’ dimeric structures, where one or more transitionmetals bridge two vacant POM subunits, and to other bigger molecular architectureswhere clusters of four or more transition metals are stabilized by two or morevacant subunits (Figure 1.2) [17].

The nucleophilicity of vacant sites on the polyanions can be exploited to promotereactions with electrophilic organic moieties to give organic–inorganic hybridcomplexes [22–25]. The covalent functionalization of a vacant polyoxoanion mayimpart a stabilization of the inorganic domain and generates tailored catalysts tobe used in different media or for the preparation of hybrid materials [26–29].

Finally, the choice of a suitable counterion for such complexes allows theirsolubilization in a wide range of solvents: apolar organic solvents, by using highlylipophilic ammonium/phosphonium cations; polar organic solvents (acetonitrile,dimethylsulfoxide (DMSO)), with the tetrabutylammonium (TBA) cation; and waterwith alkali metals or protons as counterions.

1.1 Soluble Metal Oxides as Oxidation Catalysts 5

[M′3(a-XW9O33)2]n− [M′4(b-XW9O33)2]n−

[{Co4(OH)3PO4}4(A-a-PW9O34)4]28− [Cu20Z(OH)24(H2O)12(P8W48O184)]25−

Z = Cl, Br, I

(a) (b)

(c) (d)

Figure 1.2 Some structural types ofTMSPs, namely species with isolatedtransition metal ions, represented bythe polyanions [Pd3(α-SbW9O33)2]12− (a)and [Fe4(H2O)10(β-AsW9O33)2]6− (b)

[18, 19], and species with bridged transitionmetal ions, represented by the polyanions[{Co4(OH)3PO4}4(A-α-PW9O34)4]28− (c) and[Cu20Cl(OH)24(H2O)12(P8W48O184)]25− (d)[20, 21].

In the field of oxidation catalysis, the adoption of a totally inorganic ligand systemderived from POMs represents a distinct advantage over coordination complexesdisplaying a set of organic ligands or organometallic moieties, because of theirrelevant stability under harsh oxidative conditions [30–32]. Oxidation reactions aretraditionally performed using stoichiometric amounts of inorganic oxidants [33],whereas the design of robust inorganic catalysts featuring well-defined multinuclearactive sites contributes to the development of sustainable and efficient oxidativeprocesses with environmentally benign O2 and H2O2. The occurrence of differenttransition metals within the POM structure generates the basis of their catalyticactivity in oxidation reactions [34–36], since diverse oxidation mechanisms areaccessible, including the formation of metal-peroxo and metal-oxene intermediatesas well as thermal/photochemical activation (Figure 1.3).

6 1 Polyoxometalates as Homogeneous Oxidation Catalysts

O O

O

O

MnMn*

Mn−1

O

Mn+2

O O

Mn+

O

O O

OO O

O

OM

M

M

M

M

MO

O O

O

O

O O

OO O

O

OM

M

M

M

M

MO

O

O

O

O O

O OO

O

O

M

M

M

M

M

MO O

O

O

O

O OO

O

M

M

M

M

M

MO O

O

O

O2

or H2O2

Figure 1.3 Activation modes occurring during oxidations with O2 and H2O2.

In the next sections, some relevant and recent examples about the use of POMsin different oxidation processes are presented.

1.2Homogeneous Oxidations with POMs Based Only on Mo(VI), W(VI), V(V)Addenda Ions

Dioxygen is the most attractive terminal oxidant, since it is cheap and abundant inthe atmosphere. Moreover, it presents the highest active oxygen content with noharmful by-products. Several POMs have, thus, been used as catalysts to activatedioxygen through different approaches.

Keggin-type mixed-addenda heteropolyacids (HPAs) such as H3+n[PMo12−n

VnO40] · nH2O (HPA-n, with n = 1, 2, 3, etc.) act as electron-transfer oxidants [34,37–39], and they have been used for the cleavage of vic-diols [40], α-hydroxyketones,and ketones [41]. As an example, regioselective cleavage of 2-hydroxycyclohexanoneby HPA-2 gave adipic acid or its dimethyl ester as the major products (yields80–90%) at 65 ◦C in aqueous acetic acid (in 3.5 h) or methanol (in 7 h), respectively[42].

HPAs have also been applied for the liquid-phase direct catalytic oxidation ofbenzene to phenol at room temperature. When using HPA-1 in glacial acetic acid,phenol was obtained with 26% yield and 91% selectivity in 100 min [43].

The possibility to undergo photoinduced multielectron transfers without chang-ing their structure makes POMs very attractive for the photocatalytic oxidation oforganic substrates in the presence of O2 [44–51]. The general reaction mechanisminvolves (i) irradiation with wavelength <350–400 nm to promote a ligand-to-metalcharge transfer (LMCT) transition, (ii) the oxidation of the organic substratethrough hydrogen atom abstraction (or electron abstraction) to generate radicalspecies, and (iii) the reaction between the reduced POM, (the heteropolyblue com-plex) [52–54] and dioxygen, to restore the initial form of the catalyst, while forming

1.2 Homogeneous Oxidations with POMs Based Only on Mo(VI), W(VI), V(V) Addenda Ions 7

reduced oxygenated species (superoxide radical and hydroxyl radical), which arealso able to react with the substrate and the reaction intermediates. [W10O32]4− isthe most investigated photoactive polyoxotungstate and it can be used in water, assodium salt, to perform the oxidation of alcohols to their carbonyl compounds. Insuch conditions, however, an extensive overoxidation occurs at higher conversions[55, 56], owing to the production of highly reactive hydroxyl radicals OH · arisingfrom solvent oxidation [57, 58]. In some cases, this strong oxidative capability hasbeen applied for the mineralization of hydrosoluble pollutants [49]. Cation exchangeallows its use in organic solvents: the tetrabutylammonium salt of [W10O32]4− hasbeen mainly used in CH3CN to oxidize aliphatic secondary alcohols and hydro-carbons [59, 60]. Cyclohexane has been oxidized to cyclic alcohols and ketones,depending on the O2 pressure, whereas cyclohexene has been converted into thecorresponding secondary hydroperoxide and α,β-unsaturated cycloketone [61]. In-terestingly, the photocatalytic behavior of [W10O32]4− has been used to promote theformation of C-C bonds and alkane dehydrogenations through radical reactions inanaerobic conditions [62].

The decatungstate has been associated with different materials, so as to tune itsselectivity toward different organic substrates: hydrophilic silica has been used totrap the decatungstate by a sol–gel procedure and oxidize alcohol to ketones andaldehydes [63], while hydrophobic mesoporous silica SBA-15 has been used as asupport for the photocatalyst to oxidize aromatic and aliphatic hydrocarbons to thecorresponding ketones in a CH3CN/H2O mixture [64].

The entrapping of decatungstate in polymeric membranes has also offeredinteresting perspectives to improve yield and selectivity of the reactions[56, 65–67].

Fluorinated media have conveniently been exploited in POM photocatalysis:besides the outstanding thermal and oxidative resistance, a preferential perme-ability of dioxygen can be obtained in fluorinated solvents [68]. The fluorophilic{[CF3(CF2)7CH2CH2CH2]3NCH3}4[W10O32] is soluble in fluorinated solvents as1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), and it has been used for ethylbenzeneoxygenation at the benzylic carbon atom to obtain the corresponding hydroperox-ide, alcohol, and ketone (56 : 23 : 21 product distribution), with turnover number(TON) = 580 [66].

Among oxygenation processes with hydrogen peroxide, those catalyzed by highvalent d0 transition metals are the most studied and selective [69]. The decompo-sition of hydrogen peroxide to form molecular oxygen is usually negligible, thusallowing a safely handling. In this respect, POMs are suitable candidates for theactivation of hydrogen peroxide aimed at performing heterolytic oxidations.

The interaction between POMs and hydrogen peroxide has been investigatedby many authors [70–77]. In some cases, alkylhydroperoxides have also beenused [78].

Active peroxometal species have been prepared by employing polyoxoanions inphase-transfer conditions to perform selective epoxidations of olefins and ben-zyl alcohol oxidation [79–81]. Cetylpyridinium salts of polynuclear peroxometalshave been used in biphasic conditions using chloroform as the solvent to obtain

8 1 Polyoxometalates as Homogeneous Oxidation Catalysts

the epoxidation of terminal and internal olefins and enols or the ketonizationof secondary alcohols. The oxidative cleavage of vic-diols and carbon–carbondouble bonds to carboxylic acids with H2O2 has been obtained under homoge-neous conditions, using tert-butyl alcohol as the solvent [81]. In such conditions,the oxidant species in solution are dimeric peroxotungstate complexes such as{PO4[WO(O2)2]4}3− [82–85].

1.2.1Oxidation with Hydrogen Peroxide by Peroxopolyoxotungstates-Dendrimers

Dendrimers and metallodendrimers are generating much attention for theirpotential applications in various areas [86–89]. The increasing use of thesemacromolecules in homogeneous catalysis is an emerging field, as they mayallow the easy recovery of the catalysts after use, an essential feature for reac-tion efficiency, economy, and environmental concerns [90–93]. However, dendriticcatalysts for oxidation reactions are relatively underrepresented [94–99], and onlya few are based on POMs (DENDRIPOMs, dendritic polyoxometalates). The fix-ation of POM catalysts onto dendrimers is a very promising route, allowing tocouple mechanistic knowledge and catalytic efficiency of the molecular POM unit,with the indispensable means of recovery and recycling. Two strategies can beused to incorporate POMs into the dendritic structure: covalent bonding or elec-trostatic interaction. In the first successful attempt to attach a POM anion tosimple dendrimers, [H4P2V3W15O62]5− was covalently bonded at the peripheryof dendritic tetra-armed structure, and the DENDRIPOM hybrids were used asrecoverable catalysts in the oxidation of tetrahydrothiophene to its sulfoxide byt-BuOOH or H2O2 [100]. On the basis of a similar approach, the synthesis ofmannose- and ethoxyethanol-functionalized poly(amido)amine dendrimers boundto the [H4P2V3W15O62]5− has also been reported [101].

A different class of DENDRIPOMs has been built by exploiting electrostaticinteractions between dendrons and the heteropolytungstate [(Mn(H2O)3)(SbW9O33)2]12− [102].

A peroxophosphonatotungstate mixture has been prepared by in situ assemblyof dendritic phosphonates with diperoxotungstates, efficiently exploited for theepoxidation of alkenes with hydrogen peroxide [103, 104].

Various dendritic ammonium structures with different sizes and end groupshave been used as countercations for the trianionic [PO4{WO(O2)2}4]3−, and theireffects on the solubility, stability, and catalytic properties of the POM unit havebeen investigated. The heteropolyacid H3PW12O40 decomposes in the presence ofexcess H2O2 to form the dinuclear peroxotungstate [{WO(O2)2(H2O)}2O]2− and thetrianionic peroxophosphotungstate [PO4{WO(O2)2}4]3− [105–107]. The latter reactsselectively with the dendritic cations in a biphasic mixture of water and methylenechloride to give the DENDRIPOM hybrid, which contains the [PO4{WO(O2)2}4]3−

core, in a very good yield. It has been demonstrated that the stability, catalyticefficiency, and recyclability of DENDRIPOMs largely depend on the location ofthe active species within the dendritic architecture that is used. Thus, three

1.2 Homogeneous Oxidations with POMs Based Only on Mo(VI), W(VI), V(V) Addenda Ions 9

POM unit

(a) (b) (c)

POM unit POM unit

Figure 1.4 Schematic representation of different families of DENDRIPOMs (a) peripheral,(b) central, and (c) encapsulated catalytic domains.

families of dendritic POM hybrids built on the trianionic [PO4{WO(O2)2}4]3− havebeen studied [108–112]. In the first one, POM units are located at the dendrimerperiphery (Figure 1.4a), whereas the POM unit represents the core of the dendrimerin the second family (Figure 1.4b) or it is encapsulated in the dendrimer cavities inthe third example (Figure 1.4c).

Core-type DENDRIPOMs (Figure 1.4b) and encapsulated DENDRIPOMs(Figure 1.4c) were found to be more stable and more efficient catalysts inoxidation reactions than their homologs with POM units located at the periphery(Figure 1.4a).

Even if a negative dendritic effect, probably caused by the increased bulk aroundthe catalytic center, has generally been observed on the reaction kinetics withDENDRIPOMs, the dendritic structure increases the stability of the POM unit,allowing oxidation of more challenging substrates, and facilitates the recovery andreutilization of the catalyst. The efficiency of DENDRIPOMs based on peroxophos-photungstate has been demonstrated in the oxidation of alkenes, sulfides, andalcohols to the corresponding epoxides, sulfoxides and/or sulfones, and ketones, inthe presence of hydrogen peroxide, at 30 ◦C (Figure 1.5). These systems are easilyrecovered by precipitation, after each catalytic cycle, without any discernible loss ofactivity.

Recently, optically active DENDRIPOMs have been prepared by assemblingchiral dendritic amines with achiral peroxophosphotungstate [PO4{WO(O2)2}4]3−

in acidic medium [113, 114]. These compounds selectively oxidize sulfides tothe corresponding sulfoxides with up to 14% enantiomeric excess (ee). Despitethe modest ee, this work demonstrates chirality transfer to the POM unit in anasymmetric transformation. The chemo- and enantioselectivity of the POM unitare highly sensitive to the structure of the dendrimer. Interestingly, these chiralDENDRIPOMs can be recovered and reused. The appropriate design of chiraldendrimers from enantiopure amines with greater chiral induction potential,

10 1 Polyoxometalates as Homogeneous Oxidation Catalysts

S

O

O

CH3

CH3CH3

Dendripomcatalyst

CH2

S

S

S

S

O

O

O

S

S

OO

OH

OH

OH

threo erythro

OH

O

O O

O

O

O

O

CH3

4

4

4 4

+ CH3S

O

O

O

+

Figure 1.5 Reaction scope for the oxidation with DENDRIPOMs.

along with concomitant electrostatic and charge transfer interactions, will enablethe increase of DENDRIPOMs enantioselectivity.

These findings highlight that some key parameters, important in catalysis, can betuned by modifying the local environment of the polyanion. Hence, DENDRIPOMsrepresent a promising and elegant approach with respect to catalytic activity,selectivity (including enantioselectivity), and catalyst recovery.

1.2.2Homogeneous Oxidation with Hydrogen Peroxide in the Presence of Vacant andHybrid POMs

Promising catalysts for selective oxidations with hydrogen peroxide belong to thevacant polyoxotungstates family [115–118].

The divacant heteropolytungstate (TBA)4[γ-SiW10O34(H2O)2] has been employedin CH3CN to catalyze the oxidation of various substrates including olefins, allylicalcohols, and sulfides in the presence of 30% aqueous H2O2. Terminal olefins havebeen epoxidized with 90% yield in 10 h, while cis- and cyclic internal olefins havebeen oxidized in <6 h with yields up to 99% [119–121]. A high regioselectivity hasbeen observed for the epoxidation of the less hindered, even if less nucleophilic,double bonds. This stable POM is a tetraprotonated species with two aquo ligandsW–(OH2), a key feature for the formation of terminal peroxotungstate groups asactive species [122–126].