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Hypervalent Iodine Chemistry in Synthesis Scope and New Directions.

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T. Wirth
IIII and IV Reagents
Hypervalent Iodine Chemistry in Synthesis: Scope and
New Directions
Thomas Wirth*
hypervalent compounds · iodine · oxidations ·
rearrangements · total synthesis
The impressive development of hypervalent iodine chemistry in recent
years is reflected by the number of publications in this area. Although
the synthesis of the first hypervalent iodine compound dates back more
than 100 years, the investigation of the reactivities of these compounds
and their efficient use as metal-free reagents in organic synthesis is still
ongoing. This contribution summarizes recent achievements and
highlights key findings and developments that will influence future
research and lead to novel applications of hypervalent iodine reagents
in synthesis.
1. Introduction
Organic molecules bearing hypervalent iodine moieties
have been transformed from laboratory curiosities to useful
and routinely employed reagents in organic synthesis. Several
reagents in which the oxidation state of the iodine atom is not
1 are now routinely accessible and, not surprisingly, several
aspects of their chemistry have been reviewed recently.[1] It is
therefore not the purpose of this article to summarize all
efforts in this area comprehensively; instead, a rather select
number of important developments during the last couple of
years are emphasized, and their significance for synthetic
chemistry is highlighted.
made it a quite popular reagent,[4]
despite its low solubility in most organic solvents. Reactions must usually
be carried out in DMSO at slightly
elevated temperatures. IBX (2) can be
used in DMSO at room temperature to
synthesize a-aminoaldehydes from the corresponding aminoalcohols without racemization,[5] but a survey of other
solvents and reaction conditions looks promising.[6] The
perceived disadvantage of 2 being an almost insoluble reagent
in ethyl acetate or dichloromethane is turned into an
advantage as the reagent and by-products can be completely
removed simply by filtration.
2. Oxidations
The facile oxidation of alcohols to the corresponding
carbonyl compounds is one of the prominent features of
hypervalent iodine compounds appreciated by many synthetic chemists.[2] Iodine(v) compounds such as the Dess–Martin
periodinane (DMP; 1) and ortho-iodoxybenzoic acid (IBX; 2)
are known to be explosive reagents, but the simple one-step
oxidation of 2-iodobenzoic acid to IBX with oxone[3] has
[*] Prof. Dr. T. Wirth
Cardiff School of Chemistry
Cardiff University
Park Place, Main Building, Cardiff, CF10 3AT (UK)
Fax: (+ 44) 29-2087-6968
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The use of IBX (2) in ethyl acetate led to a high-yielding
procedure for the oxidation of 1,2-dihydronaphthols to the
corresponding naphthols.[7] But other protocols and reagents
have also been developed to overcome some initial preparative limitations. More-soluble derivatives such as 3[8] and
completely insoluble polymer-bound IBX reagents 4[9] have
been prepared. Whereas first attempts focused on the synDOI: 10.1002/anie.200500115
Angew. Chem. Int. Ed. 2005, 44, 3656 –3665
Hypervalent Iodine Compounds
thesis of reagents 4 with a polymer linkage to the aromatic
core of the IBX reagent, recent developments exploit the
oxidative properties of the IBX amides 5 introduced by
Zhdankin and co-workers.[10, 11] The synthesis of polymerbound IBX amides 5 is much shorter and can lead to reagents
with similar efficiency.[12]
The replacement of DMSO as the common solvent for
oxidations with IBX (2) with ionic liquids such as [bmim]+X
(bmim = 1-butyl-3-methylimidazolium;
X = Cl ,
Br ,
PF6 ) have dramatically improved the applicability of this
reagent. Although the soluble derivative 3 and polymerbound reagents 4 and 5 seem to be limited to the oxidation of
alcohols to their carbonyl counterparts, the replacement of
DMSO with an ionic liquid was found to allow the introduction of an a,b-unsaturation in carbonyl compounds, as initially
reported by Nicolaou.[14] Although the synthesis of the
tetracoordinate 1,2-iodoxetane-1-oxide 6 is quite elaborate,
the compound has been successfully used as an oxidant for
alcohols.[15] This recent example elegantly demonstrates that
even hypervalent iodine compounds with aliphatic substituents can be prepared and that further developments along
these lines can be expected. The dehydrogenation of carbonyl
compounds can also be efficiently used for the synthesis of
quinones, and even the IBX-mediated aromatization of
polycyclic compounds 7 to form 8 proceeds in good yields
(Scheme 1).[16]
Scheme 1. Aromatization of polycyclic compounds.
The area of carbon–heteroatom bond oxidation is still
dominated by the iodine(v) reagents mentioned above, but
iodine(iii) reagents continue to increase in importance as nonexplosive and readily available reagents for such oxidations.
Several of these reagents can now be synthesized under
solvent-free conditions, and even some subsequent transformations have been performed as solid-state reactions with
high efficiency.[17] It is difficult to oxidize aliphatic alcohols
with reagents such as (diacetoxyiodo)benzene, but catalytic
Thomas Wirth received his Diplom from the
University of Bonn in 1989 and his PhD
from the Technical University of Berlin (S.
Blechert, 1992). After a postdoctoral stay
with K. Fuji at Kyoto University as a JSPS
fellow 1992/93, he started his independent
research on stereoselective oxidation reactions (University of Basel, B. Giese) and obtained his habilitation in 1999. He was a
visiting professor at the University of Toronto
(1999), Chuo University in Tokyo (2000),
and Osaka University (2004). He was
awarded the Werner Prize from the New
Swiss Chemical Society in 2000.
Angew. Chem. Int. Ed. 2005, 44, 3656 –3665
amounts of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl)[18] facilitate such reactions. Depending on the reaction
conditions, primary alcohols can be oxidized to the corresponding aldehydes[18] or to carboxylic acids[19] without
affecting neighboring stereogenic centers. The racemizationfree preparation of epoxyaldehydes or amino acid derivatives
was achieved, and this method has already found its way into
several natural product syntheses.[20]
Oxidations in the presence of metal complexes are
possible:[21] Chiral Mn(salen) complexes with (diacetoxyiodo)benzene as cooxidant have been used for efficient
racemic resolutions of secondary alcohols.[22] Several of these
reactions use water as the partial or sole solvent, and hence
the hydrolysis of (diacetoxyiodo)benzene to iodosylbenzene,
which is known to occur under alkaline conditions, cannot be
excluded. Kita and co-workers developed conditions to
oxidize alcohols in water[23] with iodosylbenzene together
with KBr, and exclusive oxidation of alcohols to carboxylic
acids was observed.[24] The mild reaction conditions and the
broad functional-group tolerance of hypervalent iodine
reagents have led to the development of sequential transformations involving an oxidation step. There is no need to
isolate and purify potentially unstable intermediates; this is
especially useful when stereogenic centers are present in the
a position to the newly generated carbonyl group.
Oxidations with iodosylbenzene and KBr in aqueous
methanol instead of water leads to the formation of the
corresponding methyl esters 9.[25] In a similar process, IBXmediated oxidations of alcohols (or aldehydes) were carried
out in the presence of N-hydroxysuccinimide and the
corresponding N-hydroxysuccinimide esters 10 were obtained
in good yields.[26] The oxidation of propargylic alcohols with
IBX (2) to the corresponding unstable aldehydes can be
performed in the presence of acetamidine, and pyrimidines
are obtained directly in a one-pot process.[27] Intramolecular
reactions are also possible, for example, oxidative cyclizations
to highly functionalized cyclopentanols 11 (Scheme 2).[28]
Hypervalent iodine reagents have also proven to be mild
oxidants for phenolic substrates. Spirocyclizations of various
compounds have been studied extensively.[29] Domino oxidation/Diels–Alder reactions are also versatile sequences,[30] as
Scheme 2. Oxidations with hypervalent iodine compounds followed by
subsequent transformations in the one-pot synthesis of esters or
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
T. Wirth
shown in studies towards the synthesis of tashironin[31] or in
oxidations followed by radical cyclization reactions to generate bicyclic compounds.[32]
In all these oxidations stoichiometric amounts of an
iodoarene are generated as a by-product. To facilitate the
purification of the product and to achieve an easy separation
from the iodoarene, polystyrene-bound versions[33] of (diacetoxy)iodobenzene have been prepared and used with similar
efficiency in the reactions mentioned above.[24, 25, 34] The
synthesis of fluorous hypervalent iodine compounds such as
12 also allows easy recovery and recycling of the reduced
The synthesis of diazo compounds by oxidation of
hydrazone derivatives 20 was reported recently.[37, 40] Silylprotected hydrazones 20 are oxidized with (difluoroiodo)benzene in situ to the corresponding diazo compounds 21,
which then react with carboxylic acids to form the corresponding esters 22.[40] The main advantage of this elaborate
esterification is the wide applicability to a range of labile
substrates. The oxidation of the nitrogen–nitrogen bonds in 2pyridylhydrazones led to the formation of the corresponding
triazolopyridines, compounds with remarkably high antibacterial activities.[41] The oxidation of hydrazones to diazo
compounds of type 21 in the presence of thioketones results in
the high-yielding formation of thioepoxides, which are
precursors for sterically hindered tetrasubstituted alkenes.[42]
3. Formation of Carbon–Carbon Bonds
reagent through extraction into a fluorous organic phase.[35]
Similar ease of separation was reported in reactions with the
adamantane derivative 13, in which case the reduced compound can be easily recovered and reoxidized.[36]
Analogously to the oxidation of a carbon–oxygen single
bond, secondary amines can be oxidized with IBX (2). A
series of amines 14 were oxidized to the corresponding imines
15 efficiently.[37] Other hypervalent iodine reagents such as
the alkylperoxy-l3-iodane 16 are also suitable for this transformation.[38] Mechanistic investigations indicate that both 2
and 16 serve as single-electron-transfer reagents in this
reaction. Under certain reaction conditions, compounds
containing a carbon–nitrogen double bond (e.g. aldoximes
17) have been oxidized to the corresponding nitrile oxides 18,
which are trapped in a 1,3-dipolar cycloaddition to form
isoxazolines 19 (Scheme 3).[39]
Reactions leading to the formation of carbon–carbon
bonds are important applications of hypervalent iodine
reagents. These reactions proceed either through reactive
intermediates such as radicals or carbocations, or they are
ligand-coupling reactions mediated by trivalent iodine derivatives. The oxidative coupling of appropriately substituted
phenol derivatives has been developed into a powerful tool
for the synthesis of polycyclic compounds. Kita and coworkers recently showed that this reaction can be used
successfully as a key step in the synthesis of complex natural
products. They found that intermediate 23 could only be
cyclized with R = CH2OTBS (but not with R = CO2Me) in the
presence of [bis(trifluoroacetoxy)iodo]benzene and MK10 as
a solid acid additive. After optimization of the diastereoselectivity of the spirocyclization to 24 by a variation of the
protecting groups, this reaction was key to the first diastereoselective synthesis of discorhabdin A (Scheme 4).[43]
A combination of [bis(trifluoroacetoxy)iodo]benzene and
heteropoly acids (e.g. H3[PW12O40]) were recently used as
efficient reagent combinations for intramolecular oxidative
coupling reactions in the synthesis of spirodienone alkaloids
such as morphinandienone and neospirinedienone.[44] The
Scheme 3. Oxidation of carbon–heteroatom and heteroatom–heteroatom bonds with hypervalent iodine reagents. Acc = acceptor
substituent, TBS = tert-butyldimethylsilyl.
Scheme 4. Hypervalent iodine mediated spirocyclization in the total
synthesis of discorhabdin A.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Hypervalent Iodine Compounds
established combination of [bis(trifluoroacetoxy)iodo]benzene and boron trifluoro etherate proved to be efficient in the
synthesis of aporphine alkaloids such as glaucine.[45] These
reactions are believed to be initiated by a SET (singleelectron transfer) to the electron-rich aromatic moiety. This
concept has now been extended to couple 3-alkylthiophenes
to give the corresponding 2,2’-bithiophene derivatives in
moderate yields.[46] Whereas the oxidative coupling reactions
initiated by a SET lead to radical cations as intermediates,
mechanistic investigations revealed that radicals are not
involved in aryl transfer in ligand-coupling reactions of
trivalent iodine derivatives.[47]
Although the generation of iodonium ylides as carbene
precursors in cyclopropanation reactions is known, moreefficient routes for the cyclopropanation of alkenes involve
carbene precursors with two acceptor substituents. Malonic
acid derivatives can be employed to synthesize cyclopropanes
25,[48] and a-nitro carbonyl compounds have been successfully
added under rhodium catalysis to generate compounds 26 as
versatile precursors for cyclopropane amino acids.[49] Iodonium ylides derived from bis(phenylsulfonyl)methane can
also be used for insertion reactions into the CH bonds of
alkenes. This reaction has been used to functionalize pyrroles[50] and flavones.[51] Reactions of unfunctionalized alkenes can lead to subsequent extrusion of sulfur dioxide and
to the formation of functionalized indanes such as 27
(Scheme 5).[52]
The generation of benzynes by using silylated hypervalent
iodine compounds such as 28 a is known and was recently
optimized by the introduction of more-soluble compounds
28 b (Scheme 6), which led to a better performance in
subsequent Diels–Alder reactions.[53] Similar precursors such
as 28 c were recently used for the generation of cyclohexyne
intermediates, and the elimination/addition mechanism was
established by kinetic deuterium isotope studies.[54]
4. Formation of Carbon–Heteroatom Bonds and
Heteroatom–Heteroatom Bonds
The use of hypervalent iodine reagents for the formation
of carbon–heteroatom and heteroatom–heteroatom bonds is
well established, and a wide range of substrates have been
used in these transformations. The functionalization of
carbonyl compounds at the a position is still under intensive
investigation. Different types of halogenations have been
reported with (difluoroiodo)arene and (dichloroiodo)arene
derivatives as very powerful reagents. Advances in the
preparation of organofluorine compounds by using hypervalent iodine reagents were extensively reviewed recently.[55]
The selective introduction of fluorine atoms is possible under
mild and neutral conditions in the presence of p-(difluoroiodo)toluene (30) as a stable and safe reagent. Various
improved methods for the synthesis of 30 were reported
recently,[56, 57] but new hypervalent difluoroiodo derivatives
have also been synthesized.[58] The fluorination of sulfursubstituted esters or amides 29 (E = S) has been investigated
intensively, and the reaction proceeds through a sulfurPummerer-reaction to yield 31.[59] Depending on the reaction
conditions, difluorination or oxidation to the sulfoxides
occurs, whereas the corresponding selenium substrates 29
(E = Se) only yield monofluorinated products 31
(Scheme 7).[57]
Scheme 7. Fluorination of sulfur- or selenium-substituted esters or
Scheme 5. Synthesis of cyclopropane and indane derivatives with
hypervalent iodine reagents. Ts = p-toluenesulfonyl.
Scheme 6. Generation of benzyne and cyclohexyne from suitable
hypervalent iodine precursors. Piv = pivaloyl.
Angew. Chem. Int. Ed. 2005, 44, 3656 –3665
p-(Difluoroiodo)toluene (30) can also be used to fluorinate b-dicarbonyl compounds. Hara et al. showed that the
addition of a hazardous HF–amine complex, as described
earlier,[60] is not necessary.[61] The combination of pyridinium
polyhydrogen fluoride (Olah reagent) with either (diacetoxyiodo)benzene or [bis(trifluoroacetoxy)iodo]benzene is, however, necessary for an efficient ipso-fluorination of parasubstituted phenols.[62] A similar reagent combination is
necessary for the fluorinative ring expansion of cyclic iodo
ethers (69!70; see Scheme 19 in Section 5).[63]
Catalytic asymmetric halogenation reactions are still rare.
p-(Dichloroiodo)toluene (32) was recently used in combination with a titanium–taddol catalyst 33 for stereoselective
chlorinations of 1,3-diketone derivatives and led to products
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
T. Wirth
such as 34 with up to 70 % ee Scheme 8.[64] This enantioselectivity is slightly lower than that attained when using Nchlorosuccinimide as the chlorinating agent in similar reactions.[65] The recently reported biphenyl derivative 35 might
Scheme 10. Novel polymer-supported hypervalent iodine reagents in
Scheme 8. Catalytic approach to stereoselective chlorination with
hypervalent dichloroiodo derivatives. Naphth = naphthyl.
be advantageous in such reactions.[66] Although the stereoselectivities are only moderate, the transfer of ligands X from
the great number of known compounds of the type ArIX2 to
enolizable substrates by using a stereoselective catalytic
protocol is very promising. The main challenge in the
development of such transformations will be the efficient
suppression of the uncatalyzed background reaction between
the hypervalent iodine reagent and the substrate. A two-step
approach for the halogenation of enolizable substrates has
been reported as well. Microwave-induced a tosylation with
[hydroxy(tosyloxy)iodo]benzene followed by treatment with
magnesium halides led to an efficient synthesis of the
corresponding a-halocarbonyl compounds.[67]
The direct reaction of hypervalent iodine reagents with
enolizable substrates has been the focus of a series of other
publications. The introduction of oxygen functionalities a to
carbonyl groups is well established and is being used
advantageously in natural product synthesis.[68] The activation
of the 2-position of tetrahydrofurans with hypervalent iodine
compounds has been investigated, and processes for the
protection of alcohols as tetrahydrofuranyl ethers 36 have
been developed by using peroxyiodane 16 in combination
with carbon tetrachloride in a radical process[69] or with
(diacetoxyiodo)benzene through an ionic mechanism
Scheme 9.[70] A similar reaction has also been observed with
alkenyl or alkynyl iodanes to oxidize tetrahydrofuran.[71] Also
tetrahydrothiophene derivatives can be employed in a similar
process for the synthesis of thionucleosides.[72]
Scheme 9. Protection of alcohols as THF acetals with hypervalent
iodine compounds.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The synthesis of polymer-bound hypervalent iodine
reagents[33c] such as 37 is already well established, and many
reports indicate an efficiency almost similar to that of their
soluble counterparts. The reactions of the novel polymerbound compounds 38 with enolizable substrates yield polymer-bound products 39 (Scheme 10), which can be used in
various subsequent reactions, from simple substitutions to the
elaborate syntheses of heterocyclic compounds.[73] For example, reactions of 39 with thioamides led to the formation of
thiazoles in good yields.[74]
Polymer-bound reagents 40 were developed by Kirschning
and co-workers (Scheme 11).[78, 79] The diazide 40 a is a safe
Scheme 11. Azidations and related reactions with polymer-supported
hypervalent iodine reagents 40 a–c.
reagent for reactions with azide anions or azidyl radicals. The
syn diazidation of a double bond with 40 a was used as a key
step in the preparation of 41 in the synthesis of ( )dibromophakellstatin.[75] The reaction of 40 a with aldehydes
allowed the synthesis of carbamoyl azides 42 via the
corresponding acyl azides in good yields.[76] The polymersupported azide 40 a have also been used for the azidonation
of benzyl ethers and are a safe alternative to IN3. Furthermore, the diazide based on 37 (X = N3) was used successfully
in the synthesis of organotellurophosphanes.[77] Reagent 40 b
was used for the activation of thioglycosides in the synthesis
Angew. Chem. Int. Ed. 2005, 44, 3656 –3665
Hypervalent Iodine Compounds
of 2-desoxyglycoconjugates 43,[78] and 40 c can be employed in
iodoacylation reactions.[79]
The introduction of nitrogen functions into organic
molecules with hypervalent iodine compounds is carried out
with iminoiodanes 44 rather than with azide-based reagents.
Many processes such as metal-catalyzed aziridinations of
alkenes rely on the capability of these reagents to serve as
useful nitrene precursors.[80] Unfortunately, these reagents 44
are sometimes difficult to prepare owing to their thermal
sensitivity, and some have even been claimed to be explosive.[81] Recently, one-pot procedures were developed starting
from easily accessible hypervalent iodine compounds and
appropriate nitrogen sources. A mixture of (diacetoxyiodo)benzene and sulfonamides were used in manganese or
ruthenium porphyrin catalyzed aziridations of alkenes or
amidations of the benzylic position of hydrocarbons such as
indane (45) as shown in the reaction to give 46 Scheme 12.[82]
Scheme 12. Activation of CH bonds in hypervalent iodine mediated
iminoiodane followed by rhodium-catalyzed CH insertion
or aziridination, which can be followed by nucleophilic ringopening. Recent stereochemical results indicate that the
addition of the metallonitrene might also proceed in a
stepwise fashion without the formation of an aziridine
intermediate, and in some cases the reaction even proceeds
without the rhodium catalyst.[89]
Under different reaction conditions, the aziridinations of
alkenes with a combination of (diacetoxyiodo)benzene and
N-aminophthalimide can also proceed without metal catalysts.[90] The direct formation of carbon–heteroatom bonds by
replacing CH bonds of aromatic compounds has been used
for the synthesis of various target molecules. The reactions of
electron-rich arenes might proceed through a SET mechanism, but others have been shown to involve an ionic pathway.
Various functional groups in combination with various hypervalent iodine reagents have been used to effect aromatic
substitution. Depending on the reaction conditions and on the
substitution pattern of the aromatic moiety, spiro compounds
can be obtained by ipso cyclization, especially with 4-methoxy
or 4-halo substitutents on the aromatic moiety.[91] This
principle has also been applied to the synthesis of a
muscarinic M1 receptor antagonist.[92] Products of such initial
spirocyclizations are likely to be intermediates in the reaction
to products 50, 52, and 54 (Scheme 14). Sulfonamides 49
Similar combinations of iodosylbenzene (PhI=O) and sulfonamides or sulfonimidamides can be used for the generation of
iminoiodanes in situ and have been applied to coppercatalyzed aziridinations of alkenes.[48b, 83] Other nucleophiles
such as sulfur trioxide can be used together with iodosylbenzene for the direct formation of cyclic sulfates from alkenes.[84]
Although rhodium catalysts are known to be efficient in
CH insertion reactions, they have rarely been used together
with hypervalent iodine compounds. Du Bois and co-workers
have developed procedures for aminations through selective
CH bond oxidation. Carbamates[85] and sulfamate esters
47[86] can be efficiently cyclized to products 48 under rhodium
catalysis (Scheme 13). Because (diacetoxyiodo)benzene is
Scheme 14. Cyclizations of aromatic compounds in heterocycle
synthesis. NPhth = phthalimidoyl.
Scheme 13. Activation of CH bonds in rhodium-catalyzed cyclization
used as oxidant, a base such as magnesium oxide is necessary
to scavenge the generated acetic acid. New catalysts[87] are
now allowing intermolecular CH insertions, and combinations with subsequent transformations[88] have expanded the
scope of this transformation.
Similar reaction conditions have been employed in
aziridinations of alkenes in inter- and intramolecular reactions. The proposed mechanism involves the formation of an
Angew. Chem. Int. Ed. 2005, 44, 3656 –3665
undergo cyclization to form the corresponding 2,1-benzothiazine compounds 50 and the highest yields are attained with
[hydroxy(tosyloxy)iodo]benzene as reagent.[93] However,
polymer-supported reagents have been investigated as well.[94]
Alcohols 51 are direct precursors for the chroman derivatives
52,[95] whereas the acylimino phthalimides 53 can be cyclized
to give a variety of different substituted lactams 54.[96]
The chemistry of alkynyl(aryl)iodonium compounds is
manifold, and their preparation usually involves a ligandexchange reaction of the corresponding alkynyl stannane.
Several new approaches for their synthesis have been
developed, including ligand exchange of silylated alkynes[97]
or from alkynyl boronates[98] as precursors; polymer-supported reagents[99] have also been synthesized. They are powerful
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
T. Wirth
and versatile carbene precursors in natural product synthesis[100] which was recently demonstrated in the preparation
of 55 as a key intermediate in the synthesis of halichlorine
Scheme 15.[101] Alkynyl(aryl)iodonium compounds are frequently used for insertion reactions into heteroatom–hydrogen bonds for heterocycle synthesis.[102] These reagents were
recently used in the alkynylation of metal complexes. The
reaction with diorganoplatinum(ii) complexes provides a
general route for the synthesis of alkynylplatinum(iv) complexes 56[103] as well as iridium(iii)–s-alkynyl complexes in
high yields (Scheme 15).[104]
and was reviewed recently.[110] The oxidative fragmentation of
hemiacetals has been applied to the synthesis of functionalized medium-sized ring systems by using a combination of
(diacetoxyiodo)benzene and iodine. This reaction is believed
to proceed through a radical pathway by b scission of the
corresponding alkoxy radicals. Medium-sized lactones 62[111]
or, with photochemical assistance, cycloheptane or cyclooctane derivatives 63 have been obtained (Scheme 16).[112]
Scheme 16. Oxidative fragmentations in the synthesis of medium-sized
ring systems.
Scheme 15. Alkynyl iodo compounds in cyclizations via carbene
intermediates (top) and in the synthesis of metal complexes
(bottom). Tf = trifluoromethanesulfonyl.
The stabilization of hypervalent iodine moieties by intramolecular coordination has led to the development of chiral
reagents such as 57;[105] recently IBX esters 58 were reported
as reagents for the selective oxidation of sulfides to sulfoxides.[106] Several sulfamide derivatives 59 have also been
prepared and evaluated in oxidation reactions.[107] Recent
work by Ochiai et al. has shown that intermolecular stabilization of the hypervalent iodine moiety using [18]crown-6 can
be used to form complexes 60[108] or 61,[109] which show
interesting reactivities as they can be used for reactions in
aqueous solvents.
The property of hypervalent iodine compounds to react as
electrophilic reagents followed by the exploitation of their
excellent leaving-group character has been used in various
rearrangements for the synthesis of functionalized molecules.
Their use in Hofmann-type rearrangements is well established, and the intermediate isocyanate has been used for the
synthesis of heterocycles.[113] This reaction has now been
extended to peptide chemistry in water, thus allowing the
efficient rearrangement of N-protected glutamine to diaminobutyric acid derivatives, which have been used successfully
for the synthesis of polymyxin B heptapeptide.[114] Treatment
of aryl-substituted alkenes with hypervalent iodine compounds can lead to the formation of intermediates that can be
stabilized by the aryl substituent via phenonium ions such as
64; a rearrangement might occur. Internal nucleophiles lead
to cyclized products 65,[115] and external nucleophiles such as
methanol/water result in the corresponding ketones 66
(Scheme 17).[116]
Similar intermediates might be involved in the rearrangement of chalcones. This reaction was used recently for the
efficient synthesis of isoflavones 67 in a one-pot reaction
(Scheme 18).[117]
5. Fragmentations and Rearrangements
The use of hypervalent iodine reagents in the fragmentation of tertiary cyclopropanol systems is already established
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 17. Rearrangements of aryl alkenes via phenonium ion
Angew. Chem. Int. Ed. 2005, 44, 3656 –3665
Hypervalent Iodine Compounds
University in autumn 2004 where parts of this review were
written, and Dr. B. Linclau, Southampton University, for a
Received: January 12, 2005
Published online: April 12, 2005
Scheme 18. Rearrangement–cyclization sequence for isoflavone
The generation of a hypervalent iodine moiety as a
leaving group has been explored with various substrates.
Iodolactone 68[118] can be oxidized to the diacetoxyiodo
derivative, which then rearranges via phenonium ion 64 to
compound 65.[115] Iodoethers 69 are readily obtained from the
corresponding unsaturated alcohols by an iodocyclization
reaction.[119] Treatment of 69 with (difluoroiodo)toluene (30)
leads to difluorination of the iodine atom and finally to
rearranged ring-expanded cyclic ethers 70 in good yield
(Scheme 19).[120]
Scheme 19. Oxidative ring expansion with fluorination.
An oxidative rearrangement of allylic tertiary alcohols 71
has only precedence in organochromium chemistry. Iwabuchi
and co-workers showed that IBX (2) in DMSO can be used
for efficient rearrangement to the a,b-unsaturated carbonyl
compounds 72 (Scheme 20); this process is compatible with a
variety of protecting groups.[121]
Scheme 20. IBX-controlled oxidative rearrangement of allylic alcohols.
6. Summary and Outlook
The great advantages of hypervalent iodine compounds,
for example, their low toxicity compared with heavy-metal
reagents, mild reaction conditions, fast accessibility of a large
variety of reagents, and easy handling have led to their
increased use in synthesis. This Mini-review will hopefully
stimulate the application of hypervalent iodine reagents in
synthesis, and the discovery of new transformations with these
compounds will surely be the basis for different and improved
synthetic strategies and concepts.
I thank my co-workers for their excellent contributions,
Professor H. Sasai for his hospitality during my stay at Osaka
Angew. Chem. Int. Ed. 2005, 44, 3656 –3665
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