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Hypervalent Iodine Reagents in High Valent Transition Metal Chemistry

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molecules Review Hypervalent Iodine Reagents in High Valent Transition Metal Chemistry Felipe Cesar Sousa e Silva, Anthony F. Tierno and Sarah E. Wengryniuk * Department of Chemistry, Temple University, 1901 N. 13th St., Philadelphia, PA 19122, USA; [email protected] (F.C.S.S.); [email protected] (A.F.T.) * Correspondence: [email protected]; Tel.: +1-215-204-0360 Academic Editor: Margaret A. Brimble Received: 29 March 2017; Accepted: 8 May 2017; Published: 12 May 2017 Abstract: Over the last 20 years, high valent metal complexes have evolved from mere curiosities to being at the forefront of modern catalytic method development. This approach has enabled transformations complimentary to those possible via traditional manifolds, most prominently carbon-heteroatom bond formation. Key to the advancement of this chemistry has been the identification of oxidants that are capable of accessing these high oxidation state complexes. The oxidant has to be both powerful enough to achieve the desired oxidation as well as provide heteroatom ligands for transfer to the metal center; these heteroatoms are often subsequently transferred to the substrate via reductive elimination. Herein we will review the central role that hypervalent iodine reagents have played in this aspect, providing an ideal balance of versatile reactivity, heteroatom ligands, and mild reaction conditions. Furthermore, these reagents are environmentally benign, non-toxic, and relatively inexpensive compared to other inorganic oxidants. We will cover advancements in both catalysis and high valent complex isolation with a key focus on the subtle effects that oxidant choice can have on reaction outcome, as well as limitations of current reagents. Keywords: hypervalent iodine; oxidation; oxidant; redox; high valent; high oxidation state; catalysis 1. Introduction Over the last 20 years, high valent metal complexes have transitioned from mere curiosities to being at the forefront of modern catalytic method development. This approach has enabled transformations complimentary to those possible via traditional manifolds, most prominently carbon-heteroatom bond formation. Key to the advancement of this chemistry has been the identification of oxidants that are capable of accessing these high oxidation state complexes. The oxidant has to be both powerful enough to achieve the desired oxidation as well as provide heteroatom ligands for transfer to the metal center; these heteroatoms are often subsequently transferred to the substrate via reductive elimination. The choice of heteroatom can be critical depending on the application. For example, chloride ligands can aide in the stabilization and isolation of high valent complexes whereas acetate ligands are often more successful in catalytic manifolds. Hypervalent iodine reagents have seen wide application in this field as they are environmentally benign, non-toxic, and relatively inexpensive compared to other inorganic oxidants. Furthermore, they provide an excellent balance of versatile reactivity, heteroatom ligands, and mild reaction conditions. We will cover advancements in the use of hypervalent iodine reagents for both catalysis and high valent complex isolation with a key focus on the subtle effects that oxidant choice can have on reaction outcome, as well as limitations of current reagents. Many of these areas have been covered in the context of more broad reviews and in those cases the discussion will not be comprehensive but focus on the key aspects and most relevant elements for this review. The discussion is organized broadly Molecules 2017, 22, 780; doi:10.3390/molecules22050780 www.mdpi.com/journal/molecules Molecules 2017, 22, 780 2 of 54 Molecules 2017, 22, 780 2 of 54 by the metal center being oxidized, including palladium, platinum, gold, nickel, copper, and finally oxidized, including palladium, platinum, gold, nickel, copper, and finally isolated examples with isolated examples with other transition metals. Below a summary graphic has been provided that other transition metals. Below a summary graphic has been provided that includes oxidants common includes oxidants common to high valent transition metal chemistry that will be discussed in this to high valent transition metal chemistry that will be discussed in this review as well as common review as well as common nomenclature and how they will be presented in the text (Figure1). nomenclature and how they will be presented in the text (Figure 1). Hypervalent Iodine Reagents - Featured in blue throughout text Other Oxidants Commonly Utilized General reactivity towards transition metals General reactivity towards transition metals - 2-e– inner sphere oxidants (1-e– is possible but rare) - 1e – or 2-e– inner sphere oxidants - Transfer of oxidant ligands to metal center (carbon or heteroatom) - Generation of electrophilic metal center - C–C or C–X reductive elimination pathways accessible - Can transfer functional groups to metal center (not always) - Re-establishment of active catalysts Advantages Advantages - Powerful oxidants - Powerful oxidants - Inexpensive - Non-toxic - Broad ligand scope for transference to metal center - Environmentally benign - Rapid synthesis - Transfer of ligands resistant to reductive elimination - Facile modulation of electronics and sterics Disadvantage Disadvantages - Expensive - Low atom economy - Harsh oxidantion conditions - Generate organic byproducts (ex- phenyl iodine, iodobenzoic acid) - Minimal modulation of electronic/steric effects - Limited by ligands available for transfer Neutral, Acyclic λ3-Iodanes, Iodine(I) Reagents One-Electron Oxidants O O H R = Me; PhI(OAc) R = CF ; PhI(OTFA) , PhI(OCOCF ) O CuCl O O R 2 3 2 3 2 2 2 phenyliodine(III)diacetate phenyliodine(III) bis(trifluoroacetate) I t-butylhydrogenperoxide O Copper dichloride Oxygen R = Ph; PhI(O2CPh)2; PhI(OBz)2 R = tBu; PhI(O2CtBu)2, PhI(OPiv)2 tBuOOH, TBHP [bis-(benzoyloxy)iodo]benzene phenyliodine(III)dipivalate R O O Cl OH I I O N O O N O Cl OTs Cl Br O Me OAc N-Chlorosuccinimide N-Bromosuccinimide Benzoquinone, I phenyliodine(III)dichloride Hydroxo-phenyliodine(III)tosylate NCS NBS BQ OAc PhI(Cl)2 PhI(OH)OTs, Koser's reagent Me Me Two-Electron Oxiants I NTs Iodomesitylene diacetate CH3 OTf MesI(OAc)2 IOAc Iodine monoacetate OTf [N-(phenylsulfonyl)imino]phenyliodinane S H C N CH PhINTs 3 3 CF F 3 Neutral, Cyclic, λ3-Iodanes 1-Fluoro-2,4,6- S - (trifluoromethyl)- trimethylpyridinium triflate dibenzothiophenium triflate NFTPT TDTT F3C I O TIPS I O Me Me O 2BF SiH3 Cl 4 3,3 - Dimethyl-1-(trifluoromethyl)-1,2- N 1-[(triisopropylsilyl)ethynyl]-1,2- SiH benziodoxole; Togni's reagent benziodoxol-3(1H)-one;TIPS-EBX N 3 F 1,2 disilylbenzene (Poly)cationic, Acyclic λ3-Iodanes 1-Chloromethyl-4-fluoro-1,4- diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate), R = H [(Py) IPh]2OTf– 2OTf 2 F-TEDA, SelectfluorR R R N,N dipyridinium phenyliodine(III) triflate NN I – R = CN, [(4CNPy)2IPh]2OTf PhO2S N,N [4- cyano]dipyridinium phenyliodine(III) triflate N F H3CI PhO2S R = NMe , [(DMAP) IPh]2OTf– 2 2 N-fluorobis(phenylsulfonyl)imide Methyliodide, N,N [4- dimethylamino]dipyridiniumphenyliodine(III) triflate NFSI MeI Iodonium salts . 2- 2KHSO5 KHSO4 K2SO4 PtCl6 OxoneR Hexachloroplatinum(IV) R BF R BF I+ 4 I+ C CR 4 R O N O O N O Diaryliodonium tetrafluoroborate Alkynylliodonium tetrafluoroborate Br [Ph I]BF [PhICCR]BF Cl 2 4 4 N-Chlorosuccinimide N-Bromosuccinimide NCS NBS Figure 1. Common oxidants utilized in high valent metal catalysis. Figure 1. Common oxidants utilized in high valent metal catalysis. Molecules 2017, 22, 780 3 of 54 2. Palladium Molecules 2017, 22, 780 3 of 54 2.1. Introduction 2. Palladium Palladium has a long and storied history in transition metal catalysis, facilitating such iconic cross2.1. coupling Introduction reactions as the Heck, Suzuki-Miyara, Negishi, Buchwald-Hartwig, Sonagashira, and others.Palladium Its ubiquity has a long stems and storied not only history from in itstransi excellenttion metal reactivity, catalysis, butfacilitating also from such iconic a detailed understandingcross coupling of reactions its underlying as the Heck, mechanisms Suzuki-Miyar anda, Negishi, predictable Buchwald-Hartwig, reactivity, which Sonagashira, facilitates and novel reactionothers. development. Its ubiquity stems For manynot only years, from palladiumits excellent reactivity, catalysis reliedbut also on from the a Pd(0)/Pd(II)detailed understanding redox couple, however,of its workunderlying in the 21stmechanisms century hasand shownpredictable the powerreactivity, and which promise facilitates of the highnovel oxidation reaction state Pd(II)/Pd(IV)development. manifold, For many as years, well palladium as the potential catalysis role relied of Pd(III)on the Pd(0)/Pd(II) species in redox catalysis couple, (Scheme however,1). This chemistrywork in has the enabled 21st century transformations has shown the previouslypower and promise inaccessible of the high via traditionaloxidation state catalytic Pd(II)/Pd(IV) manifolds, manifold, as well as the potential role of Pd(III) species in catalysis (Scheme 1). This chemistry has most notably carbon-heteroatom bond forming reductive eliminations, the microscopic reverse of the enabled transformations previously inaccessible via traditional catalytic manifolds, most notably oxidative addition pathways commonly encountered in low valent palladium catalysis. In this context, carbon-heteroatom bond forming reductive eliminations, the microscopic reverse of the oxidative hypervalentaddition iodinepathways reagents commonly
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