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Hypervalent Heterocycles
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Hypervalent Heterocycles Viktor V. Zhdankin Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, MN, USA E-mail: [email protected]
Contents 1. Introduction 2. General Overview of Hypervalent Compounds 3. Hypervalent Heterocyclic Compounds of Group 13 3.1 Hypervalent Boron Heterocycles 3.2 Hypervalent Aluminum 4. Hypervalent Heterocyclic Compounds of Group 14 4.1 Hypervalent Silicon Heterocycles 4.2 Hypervalent Germanium, Tin, and Lead 5. Hypervalent Heterocyclic Compounds of Group 15 5.1 Hypervalent Phosphorus Heterocycles 5.2 Hypervalent Arsenic, Antimony, and Bismuth 6. Hypervalent Heterocyclic Compounds of Group 16 6.1 Hypervalent Sulfur Heterocycles 6.2 Hypervalent Selenium and Tellurium 7. Hypervalent Heterocyclic Compounds of Group 17 7.1 Hypervalent Bromine(III) Heterocycles 7.2 Hypervalent Iodine(III) Heterocycles 7.2 Hypervalent Iodine(V) Heterocycles References
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Abstract This chapter provides an overview of recent literature on heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term “hypervalent” has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used in synthetic works despite criticism from theoretical chemists. Typical hypervalent heterocycles include polycoordinated 10-electron or 12-electron centers with distorted trigonal-bipyramidal or pseudooctahedral geometry, respectively. In general, heterocyclic compounds of elements with double bonds are not classified as hypervalent molecules because of the ylidic, zwitterionic nature of such bonds resulting in the classical 8-electron species. Despite the lack of aromatic conjugation, hypervalent heterocycles often have a considerably higher thermal stability compared to their acyclic analogs, which is especially Advances in Heterocyclic Chemistry, Volume 119 ISSN 0065-2725 http://dx.doi.org/10.1016/bs.aihch.2015.11.001
© 2016 Elsevier Inc. All rights reserved.
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important in the chemistry of polyvalent bromine and iodine. This review is centered mainly on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as, boron, silicon, phosphorus, sulfur, selenium, bromine, and iodine, with emphasis on structural and synthetic aspects of their chemistry.
Keywords: Benziodoxoles; Hypervalency; Hypervalent; Hypervalent boron; Hypervalent bromine; Hypervalent heterocycles; Hypervalent iodine; Hypervalent silicon; Hypervalent sulfur; Iodine heterocycles
1. INTRODUCTION Hypervalent heterocycles are cyclic molecules with a hypervalent main-group element in the ring. The term “hypervalent” was introduced in 1969 by Jeremy I. Musher for molecules with elements of groups 15e18 bearing more than eight valence electrons (1969ACE54) and more recently this terminology has been extended to the group 13 and 14 elements (1999MI1). The chemistry of hypervalent compounds has been systematically reviewed in a book edited by K.-Y. Akiba (1999MI1). Typical hypervalent heterocycles include polycoordinated 10-electron or 12-electron heteroatoms with distorted trigonal-bipyramidal or pseudooctahedral geometry, respectively. In general, heterocyclic compounds of elements with double bonds are not classified as hypervalent molecules because of the zwitterionic nature of such bonds resulting in classical 8-electron species (2014ACE9617). Despite the lack of aromatic conjugation, hypervalent heterocycles often have a considerably higher thermal stability compared to their acyclic analogs, which is especially important in the chemistry of the generally unstable organic compounds of bromine(III), iodine(III), and iodine(V). This chapter is centered mainly on the hypervalent heterocyclic derivatives of nonmetal main-group elements, such as, boron, silicon, phosphorus, sulfur, selenium, bromine, and iodine, with emphasis on the synthetic aspects of their chemistry. Despite the widespread practical interest in heterocyclic hypervalent compounds, the chemistry of hypervalent heterocycles has never been systematically reviewed.
2. GENERAL OVERVIEW OF HYPERVALENT COMPOUNDS Hypervalent compounds of main-group elements are often classified using the Martin-Arduengo NeXeL nomenclature, where N represents
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the number of valence electrons on the hypervalent atom X, and L is the number of ligands to the central atom X (1980JA7753). Several examples of uncharged polycoordinated 10-electron or 12-electron hypervalent centers are shown in Figure 1; anionic and cationic hypervalent species are also known. In general, compounds of elements with double bonds are not classified as hypervalent molecules. According to the 1983 IUPAC recommendations (1984PAC769), the position and the valency of a hypervalent atom in a molecule is indicated by using the Greek letter l. In the lambda nomenclature, the symbol ln is used to indicate any heteroatom in a nonstandard valence state (n) in a formally neutral compound. The lambda terminology is broadly used in the modern literature to indicate the general type of a hypervalent compound and to specify the number of primary bonds at the hypervalent atom. The special structural features and high reactivity of hypervalent compounds are explained by the presence of hypervalent bonding involving a three-center four-electron bond (1994CSR111, 1999MI1, 2014ARK109). The molecular orbital description of a three-center four-electron (3ce4e) bond was independently developed by G. C. Pimentel (1951 JCP446) and R. E. Rundle (1951JA4321) in 1951. The 3ce4e bond is described as three molecular orbitals formed by the combination of a p atomic orbital on the central atom and an atomic orbital from each of the two ligands on opposite sides of the central atom. Only one of the two pairs of electrons is occupying a molecular orbital that involves bonding to the central atom, the second pair being nonbonding and occupying a molecular orbital composed of only atomic orbitals from the two ligands. A representative example of the molecular orbital description of a 3ce4e bond in hypervalent 10-I-3 species 1 is shown in Figure 2. In particular, the interaction of the filled 5p orbital of the central iodine atom and the half-filled orbitals of the two ligands Y trans to each other leads to formation of three molecular orbitals: bonding, nonbonding, and antibonding. The occupied nonbonding molecular orbital has a node at the central iodine,
Figure 1 Typical 10-electron or 12-electron uncharged hypervalent centers.
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Figure 2 Molecular orbital description of the 3ce4e bond in hypervalent 10-I-3 species.
resulting in the charge distribution of almost 0.5 on each ligand and þ1.0 on the iodine atom. The carbon substituent R is bound by a normal covalent bond and the overall geometry of molecule RIY2 is a distorted trigonal bipyramid with two heteroatom ligands Y occupying the apical positions, and the least electronegative carbon ligand R and both electron pairs residing in equatorial positions. It should be noted that the formal octet rule is not violated in this model of bonding, as can be illustrated by the resonance involving two canonical Lewis structures 1a and 1b (Figure 2). The bonding in the hypervalent iodine 12-I-5 species, RIY4, can be described as a normal covalent bond between iodine and the organic group R in an apical position, and two orthogonal, hypervalent 3ce4e bonds, accommodating four electronegative ligands Y. The molecule of RIY4 has overall a square bipyramidal (or pseudooctahedral) structure with carbon substituent R and unshared electron pair occupying the apical positions with the electronegative ligands Y residing at the equatorial positions. The structure and reactivity of hypervalent compounds can be summarized as follows (1999MI1, 2013MI2): 1. Including the nonbonding electron pairs, the geometries of pentacoordinated hypervalent compounds (10-X-5, 10-X-4, and 10-X-3) are trigonal bipyramids with the most electronegative groups occupying the apical positions. 2. The structures of hexacoordinated hypervalent compounds (12-X-6 and 12-X-5 bearing a pair of unshared electrons) are octahedral regardless of the formal charge on the central atom. 3. Hypervalent XeL bonds in general are longer than the sum of the appropriate covalent radii of atoms X and L, but shorter than purely ionic bonds. 4. Intramolecular positional isomerization (Berry pseudorotation) resulting in an exchange between the apical and the equatorial ligands is important in explaining the reaction mechanisms of hypervalent compounds.
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5. Similarly to organometallic compounds, ligand exchange and ligand coupling (or reductive elimination) reactions are typical for hypervalent compounds. The term hypervalent and the concept of hypervalency have been sharply criticized by theoretical chemists. In particular, the concept itself has been criticized by Gillespie and Silvi who, based on the analysis of electron localization functions, wrote in 2002 that “as there is no fundamental difference between the bonds in hypervalent and nonhypervalent (Lewis octet) molecules there is no reason to continue to use the term hypervalent” (2002CCR53). Despite all the criticism, the term hypervalent has been overwhelmingly accepted by synthetic chemists, and the concept of hypervalency is currently widely used to describe special structural features and reactivity patterns of polycoordinated main-group compounds.
3. HYPERVALENT HETEROCYCLIC COMPOUNDS OF GROUP 13 ELEMENTS 3.1 Hypervalent Boron Heterocycles The anionic pentacoordinated and even hexacoordinated boron species can be stabilized in a heterocyclic system by complexation with appropriate ligands. Compounds of this type (structures 2 and 3, Figure 3) were first reported by Lee and Martin in 1984 (1984JA5745). 1H, 19F, and 13C NMR spectra of products 2 and 3 are consistent with the symmetrical structures incorporating pentacoordinated and hexacoordinated central boron atom; however, X-ray data were not available in this publication. More recently, Akiba and coworkers have synthesized pentacoordinated hypervalent boron heterocyclic species 4e6 by employing a sterically rigid anthracene skeleton (Scheme 1) (2000ACE4055, 2005JA4354). Compounds 6e8 were prepared by lithiation of bromoanthracene 4 followed by the addition of B-chlorocatecholateborane derivatives 5. Products 6e8 were
Figure 3 Pentacoordinated and hexacoordinated hypervalent boron species reported by Lee and Martin in 1984 (1984JA5745).
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Scheme 1
isolated as thermally stable solids (melting points above 200 C) and their structures were established by single crystal X-ray diffractometry. The sum of the bond angles around the central boron atom in the structures of 6-8 is 360.0 , which indicates that the central boron atom is planar with sp2 hybridization. The lone pairs on the oxygen atoms at the 1, 8 positions interact with the empty p orbital of the central boron atom, to form a three-center four-electron bond, and the overall structure can be regarded as a slightly distorted trigonal bipyramid. The two hypervalent BeO bond lengths are 2.379 and 2.441 Å in 6, 2.398 and 2.412 Å in 7, and identical (2.436 Å) in 8. The lengths are longer than covalent BeO bonds (1.39e1.40 Å) but shorter than the sum of the van der Waals radii (3.48 Å). The small difference in the hypervalent BeO bond distances observed in 6 may be a consequence of a packing effect (2000ACE4055). A similarly coordinated 10-methylacridinium-based hypervalent borone oxygen heterocyclic system 10 has been synthesized by Yamamoto and coworkers starting from compound 9 as outlined in Scheme 2 (2009 CL794). The hypervalent nature of the OeBeO has been confirmed by X-ray structural data (the average BeO bond of 2.51 Å). Another example of a hypervalent boroneoxygen heterocyclic structure is represented by the 2,6-bis(p-tolyloxymethyl)benzene-based 10eBe5
Scheme 2
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Figure 4 2,6-Bis(p-tolyloxymethyl)benzene-based 10eBe5 species.
species 11e13 (Figure 4) (2006MGC277). Compounds 11e13 were synthesized from the respective aryl bromide using the same approach as outlined in Scheme 1. X-ray crystallography revealed that the distances between the central boron and both oxygen atoms of the ligand in these structures varied from 2.50 Å to 3.16 Å. The catecholato derivative 13 was found to have the strongest BeO interactions being the closest to the ideal trigonal bipyramidal structure typical of hypervalent 10eBe5 species. Two representatives of hypervalent boronenitrogen heterocycles have been reported (2011JOC2123, 2013IC13865). Yamamoto and coworkers have prepared hypervalent 10eBe5 compound 15 (Scheme 3) utilizing the bis-(pyrimidine)benzene ligand framework (2011JOC2123). Product 15 was prepared in a low yield by lithiation of precursor 14 followed by treatment with methyl borate and then exchange of methoxy groups with ethylene glycol to afford the stable 1,3,2-dioxaborolane derivative, which was characterized by X-ray structural analysis. X-ray analysis and molecular orbital calculations suggested that compound 15 has a hypervalent pentacoordinated structure with an NeBeN hypervalent bond. In particular, the BeN distances (both equal to 2.537 Å) are substantially shorter than the sum of van der Waals radii (3.62 Å). Compound 15 has a trigonal bipyramidal structure about the central boron atom with two oxygens and carbon as the three equatorial atoms, and with two nitrogen atoms and the boron atom constituting an NeBeN three-center four-electron bond (2011JOC2123).
Scheme 3
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Scheme 4
Figure 5 Examples of hypervalent aluminum heterocycles.
Vidovic, Findlater, and coworkers reported the synthesis and characterization of terpyridine-based hypervalent 10eBe5 boronenitrogen dicationic heterocycle 16 (Scheme 4) (2013IC13865). The X-ray molecular structure of derivative 16 revealed evidence for pentacoordination at boron with four shorter BeN bonds (between 1.50 and 1.63 Å) and one longer bond of 2.94 Å.
3.2 Hypervalent Aluminum Hypervalent 10-electron structures are commonly observed in heterocyclic compounds of Group 13 metals (2001ACR201). Representative examples of such compounds can be illustrated by the alumoxane 17 (2013OM6647) and the complexes of aluminum with nitrogen ligands 18e20 (2009JCS(D) 8631) shown in Figure 5.
4. HYPERVALENT HETEROCYCLIC COMPOUNDS OF GROUP 14 ELEMENTS 4.1 Hypervalent Silicon Heterocycles Examples of stable, anionic, hypervalent 10-Si-5 siliconeoxygen heterocycles were originally reported by Martin and coworkers (1979JA1591,
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1981JOC1049). Compounds 22 and 23 were prepared by the reaction of dilithiated hexafluoroalcohol 21 with the appropriate trichlorosilane (Scheme 5) in the form of white hygroscopic solids with melting points above 355 C (1979JA1591). More recently, Goddard, Fensterbank, and coworkers have developed a new route to Martin’s spirosilanes starting from substituted o-bromobenzoic acids 24 (Scheme 6) (2015JOC3280). In particular, stable, hypervalent fluorosilicates 26 have been prepared by addition of fluoride anion to the tetracoordinated silicates 25, which can be readily synthesized from benzoic acids 24 in several steps. X-ray crystallographic study of products 26 has revealed a trigonal bipyramidal geometry of the silicon centers with both oxygen groups at the apical position, while the aromatic ring and the fluorine atom are occupying the equatorial positions. An intermediate formation of unstable heterocyclic 10-Si-5 silicone oxygen heterocycles has been proposed in cross-aldol reactions of aldehydes mediated by chlorosilanes (2001ACE4759, 2011OL1654). Structures and stabilities of three-membered rings containing hypervalent atoms of silicon or phosphorus and sulfur have been theoretically investigated by Ikeda and Inagaki (2001PCA10711).
Scheme 5
Scheme 6
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Figure 6 Examples of hypervalent heterocycles of the lower group 14 elements.
4.2 Hypervalent Germanium, Tin, and Lead Hypervalent 10-electron structures are commonly observed in the compounds of germanium, tin, and lead (2002MI3). Representative examples of such heterocyclic compounds are illustrated by germanium cyclic oxamide complexes 27 (2009JCS(D)4695) and cyclic pentaorganostannate 28 (2007JA10974) shown in Figure 6
5. HYPERVALENT HETEROCYCLIC COMPOUNDS OF GROUP 15 ELEMENTS 5.1 Hypervalent Phosphorus Heterocycles Numerous structural types of hypervalent phosphorus compounds are known (1999MI1). Representative examples of stable hypervalent phosphorus heterocycles include 10-P-3 compounds 29 (1994CR1215), anionic 10-P-4 species 30 (1983CB3301), pentacovalent 10-P-5 compounds 31 (1966CB3642), and anionic hexacoordinated 12-P-6 species 32 (1965 CB576). The first two structural types (29 and 30) are commonly termed as the low-coordinate hypervalent phosphorus compounds (1994CR1215) (Figure 7). An important example of a pentacovalent 10-P-5 compound is represented by the protonated form of proazaphosphatrane 33 (Scheme 7). Compounds 33 are strong nonionic bases (Verkade bases, also known as superbases) that serve as efficient catalysts and promoters of many reactions (2001T467). Particularly important are hypervalent 10-P-3 compounds. An example of these compounds is 3,7-di-tert-butyl-5-aza-2,8-dioxa-1-phosphabicyclo
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Figure 7 Typical structural types of hypervalent phosphorus heterocycles.
Scheme 7
[3.3.0]octa-2,4,6-triene (ADPO, 29), which shows unique chemical properties (1994CR1215). The oxidative addition of reagents such as quinones, halogens, perfluoroalkylacetylenes, hydrogen, secondary amines, or alcohols to the phosphorus center of ADPO transforms the phosphorus(III) center into a phosphorus(V) center. In a more recent work, Kornev and coworkers reported the preparation and structural studies of a series of phenylpyrazole-based hypervalent phosphorus compounds 34 and 35 (Figure 8), which represent another example of low-coordinate hypervalent phosphorus heterocycles (2015EJIC2057). According to X-ray structural data, the phosphorus atom in molecule 34 has overall trigonal bipyramidal geometry with chlorine atoms at the apical position, while the N- and C-substituents and the lone electronic pair are occupying the equatorial positions with PeN and PeC bond distances of 1.771 and 1.831 Å, respectively.
Figure 8 Phenylpyrazole-based hypervalent phosphorus heterocycles.
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Figure 9 Examples of hypervalent heterocycles of the lower group 15 elements.
5.2 Hypervalent Arsenic, Antimony, and Bismuth Hypervalent heterocyclic structures are typical of arsenic, antimony, and bismuth. The chemistry of these compounds has been overviewed by K.-Y. Akiba (2011HC207) and several specific examples are illustrated by structures 36e44 (Figure 9).
6. HYPERVALENT HETEROCYCLIC COMPOUNDS OF GROUP 16 ELEMENTS 6.1 Hypervalent Sulfur Heterocycles Several structural types of hypervalent compounds of sulfur(IV) and sulfur(VI) are known (1999MI1). Representative examples of stable hypervalent sulfur heterocycles include anionic 10-S-3 sulfuranes 45 (1978JA 7077), 10-S-4 species 46 (1981JA127) and 47 (1992CC1141), pentacoordinated 10-S-5 sulfuranes 48 (1977JA5490) and 49 (1983JA1377), and hexacoordinated 12-S-6 species 50 (also known as persulfuranes) (1982JA1683) (Figure 10). In a more recent work, Kawashima reported the preparation and structural studies of a series of four-membered 10-S-4 and 10-S-5 heterocycles 51e54 (Figure 11) (2011PSS1046). These compounds containing a tetracoordinated or pentacoordinated hypervalent sulfur atom together with other heteroatoms were isolated as thermally stable products and structurally
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Figure 10 Typical structural types of hypervalent sulfur heterocycles.
Figure 11 Four-membered heterocycles with a hypervalent sulfur atom.
characterized by a single crystal X-ray diffractometry. X-ray crystallographic analyses revealed that the hypervalent sulfur centers in these molecules have distorted trigonal bipyramidal geometry. Various sulfur heterocycles with endocyclic double bonds on sulfur are often referred to as the 10-S-3 “p-hypervalent heterocyclic systems” and shown as canonical structures with tetracovalent sulfur; for example, tetraazathiapentalenes 55 and trithiapentalenes 57 (Figure 12) (2005JHC1175, 2002JHC189). However, a more accurate representation of these compounds involves non-hypervalent betaine structures (e.g., structures 56 and 58) with eight electrons on the sulfur atom. A discussion on heterocyclic mesomeric betaines containing sulfur or other elements has been provided in a review (1985T2316) and a book (2010HHC150). These compounds have been the subject of a considerable theoretical and synthetic interest (1976TCC49, 1997JPCA4475, 1998PS(140)35).
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Figure 12 Tetraazathiapentalenes and trithiapentalenes.
Figure 13 Examples of hypervalent heterocycles of selenium and tellurium.
6.2 Hypervalent Selenium and Tellurium The chemistry of hypervalent selenium (commonly named as selenuranes) and tellurium (telluranes) in general is similar to the compounds of sulfur (1999MI1). Representative examples of stable hypervalent heterocyclic derivatives of these elements include anionic 10-Se-3 and 10-Te-3 species 59 (1995JA10153), 10-Se-4 selenuranes (1968LAC68) and 10-Te-4 telluranes 60 (1968LAC1), and hexacoordinated 12-Te-6 species 61 (pertellurane) (1984JA7529) (Figure 13). The chemistry of selenaheterocyclic compounds has been reviewed by Mlochowski and coauthors (2007ARK14).
7. HYPERVALENT HETEROCYCLIC COMPOUNDS OF GROUP 17 ELEMENTS 7.1 Hypervalent Bromine(III) Heterocycles Two examples of 10-Br-3 hypervalent bromine heterocycles (brominanes) based on the Martin ligand have been reported (1980JA7382, 1986JA3803). Compounds 63 and 64 were prepared by oxidation of aryl bromides 62 with bromine trifluoride in freon solution (Scheme 8). Both brominanes are stable for an indefinite period at room temperature (mp in
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Scheme 8
a range of 153e170 C) and are inert toward atmospheric moisture, aqueous base, aqueous hydrogen chloride, and trifluoromethanesulfonic acid. They can be sublimed at 60 C (3 torr) or passed through a neutral alumina column (ether/pentane) without decomposition. Brominanes 63 and 64 are strong oxidizing agents, and in particular can readily oxidize iodide ion, bromide ion, thiophenol, aniline, 9,10-dihydroanthracene, and tetralin (1986JA3803). X-ray structural analysis of brominane 63 revealed a distorted trigonalbipyramidal geometry around the central bromine atom. The two lone pairs of electrons are considered to occupy equatorial ligand sites. The molecule is almost planar, and the BreO bonds (1.99 and 1.97 Å) are slightly longer than the sum of the covalent radii (1.8 Å). In the solid-state structure, the oxygen atom is engaged in intermolecular interaction with the hypervalent bromine of an adjacent molecule of 63 (1986JA3803).
7.2 Hypervalent Iodine(III) Heterocycles Hypervalent iodine heterocycles represent a particularly important class of compounds because of their wide application as reagents for organic synthesis. Preparation, structure, and chemistry of hypervalent iodine compounds have been summarized in numerous books (2013MI2, 2014MI3, 2003MI4, 1992MI5) and reviews (2015SL1785, 2015CR650, 2015ACE8876, 2015 ACE5290, 2015AJC699, 2015RET49, 2015S587, 2014CCR54, 2014 CH419, 2014OBC4278, 2014CAJ950, 2014CAJ972, 2013SL424, 2012 COS247, 2011S517, 2011NPR1722, 2011ARK370, 2009ARK1, 2008CR 5299), and most recently in the chapter on “Iodine Heterocycles” in Volume 115 of AHC (2015AHC1). Typical structural types of trivalent iodine heterocycles are represented by five-membered cyclic compounds 65e77, which incorporate iodine, oxygen, nitrogen, and some other elements, in the heterocyclic ring (Figure 14). The general name “benziodoxoles” is used for the heterocycles 65e70 with iodine and oxygen atoms in a five-membered ring and various
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Figure 14 Examples of hypervalent iodine(III) five-membered heterocycles.
substituents attached to iodine. The most important representative of benziodoxoles, 1-hydroxy-1,2-benziodoxol-3-(1H)-one 65, commonly known as 2-iodosobenzoic acid (IBA), was first prepared over 100 years ago by oxidation of 2-iodobenzoic acid (1892B2632). A. R. Katritzky and coworkers have significantly contributed to the study of structure and reactivity of IBA and other benziodoxoles and benziodoxole oxides in a series of works published in 1989e1990 (1990JCS(P2)1657, 1990JCS(P2)1515, 1989MRC1007, 1989OPPI157). According to X-ray structural data (1990JCS(P2)1657), the five-membered ring in benziodoxoles is highly distorted with almost linear alignment of the two electronegative ligands. The endocyclic IeO bond length in iodine-substituted benziodoxolones varies in a wide range from 2.11 Å in carboxylates to 2.48 Å in 1-phenyl benziodoxolone (1986IC1415), which is indicative of considerable changes in the ionic character of this bond. The CeIeO bond angle in benziodoxole ring is typically around 80 , which is a significant deviation from
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the expected angle of 90 for the normal T-shaped geometry of hypervalent iodine. In the mid-1980s, IBA and other hydroxybenziodoxoles attracted a significant research activity due to their excellent catalytic activity in the cleavage of toxic phosphates and reactive esters (2002CR2497). Compared to benziodoxoles, the analogous five-membered iodinenitrogen heterocycles, benziodazoles 71, have received much less attention. The first representative of benziodoxoles, acetoxybenziodazole was synthesized in 1965 by the peracetic acid oxidation of 2-iodobenzamide (1965CC449). The structural parameters of benziodazoles 71 are, in general, similar to those of benziodoxoles (1979JOC1447, 1998JOC6590, 2003OL1583, 1997JA7408). Besides benziodoxoles and benziodazoles, the other iodine(III) heterocyclic systems are represented by the following compounds shown in Figure 14: fused benziodazoles 72 (2001JA4095), benziodoxazoles 73 (1965JOC617), benziodoxaboroles 74 (2011IC11263), benziodoxathioles 75 (2006EJOC 4791), benziodathiazoles 76 (1975JOC797), and cyclic phosphonate 77 (1978JOC4538). Five-membered heterocyclic iodine(III) compounds in general are more stable compared to their acyclic analogs, which has made possible the preparation and isolation of otherwise unstable trivalent iodine derivatives with azido, trifluoromethyl, cyano, and alkynyl substituents (e.g., structures 66e69). Various benziodoxole derivatives have found synthetic application as oxidants and reagents for “atom-transfer reactions” (2011 CC102, 2005COS121, 1997RHC133). For example, it has been demonstrated in a recent work that a complex of 2-iodosobenzoic acid 65 with triflic acid (IBA-OTf) is a powerful oxidant toward a variety of organic substrates, such as sulfides, phenols, alkenes, and aldoximes (2015CC7835). Azidobenziodoxole 66 is a particularly useful reagent for the iron(II)-catalyzed azidation of tertiary CeH bonds suitable for late-stage functionalization of complex organic molecules (2015N600, 2015ACE5290). Reagent 66 has also been used for direct conversion of aldehydes to acyl azides (2015 OL5212). Trifluoromethylbenziodoxole 67 is a useful reagent for electrophilic trifluoromethylation of various organic substrates (2015CR650). Cyanobenziodoxole 68 is an excellent reagent for direct electrophilic a-cyanation of b-keto esters and amides (2015OBC365). Triisopropylsilylethynylbenziodoxole 69 is an efficient acetylene transfer reagent, both under metal-free and metal-catalyzed reaction conditions (2015ACE8876, 2015 ACE5438, 2015CEJ8745, 2015ACE11200, 2015CC14497, 2015OL3054, 2015OL1938, 2015OL736).
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7.2 Hypervalent Iodine(V) Heterocycles Organoiodine(V) heterocycles are represented by several typical classes 78e86 shown in Figure 15. The most important representative of these compounds, 2-iodoxybenzoic acid (IBX, 78), was originally reported in 1893 (1893CB1727). The cyclic structure of IBX derivatives was confirmed by an X-ray crystallographic study in the work of A. R. Katritzky and coworkers (1990JCS(P2)1657). In the 1980se1990s, J. C. Martin and coworkers reported the synthesis, structure, and properties of several cyclic l5-iodanes (79e82) (1983JOC 4155, 1991JOC6565, 1993JA2488, 1991JA7277). A particularly important compound is triacetoxybenziodoxolone 79, prepared by heating IBX with acetic anhydride (1983JOC4155). The triacetate 79, known as DesseMartin periodinane (DMP), has emerged as the reagent of choice for the oxidation of alcohols to the respective carbonyl compounds. Iodine(V) heterocycles have found broad practical application as mild and selective reagents for the oxidation of alcohols and some other useful
Figure 15 Examples of hypervalent iodine(V) heterocycles.
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oxidative transformations. Numerous reviews on the chemistry and synthetic applications of IBX and related iodine(V) heterocycles have been published (2011JOC1185, 2006ARK26, 2010T7659, 2011ACE1524, 2001ACE2812). Several recent examples of other iodine(V) heterocycles are represented by structures 83e86 (Figure 15). Moorthy and coworkers have prepared tetramethyl-IBX (TetMe-IBX, 83) (2011JOC9593, 2014JOC11431), a highly reactive oxidant that can oxidize alcohols in common organic solvents at room temperature due to the “hypervalent twisting”-promoted rate enhancement (2005JA14146). Fluorous IBX 84 is a recyclable oxidant that can also be used as a catalyst for oxidation of alcohols to the corresponding carbonyl compounds (2011CC1875). Tosylate derivative of 2-iodoxybenzoic acid (IBX-tosylate, 85), one of the most powerful hypervalent iodine(V) oxidants, can be prepared by the reaction of IBX with p-toluenesulfonic acid in acetic anhydride (2013CC11269). 2-Iodoxybenzenesulfonic acid, which is a thia-analog of IBX and a powerful oxidizing reagent, exists in a cyclic tautomeric form of 1-hydroxy-1H-1,2,3-benziodoxothiole 1,3,3trioxide 86 (2006EJOC4791). Ishihara and coworkers have demonstrated that thia-IBX 86 is the most powerful catalyst in the iodine(V)-catalyzed oxidation of alcohols using oxone as a terminal oxidant (2009JA251).
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Hypervalent Heterocycles Viktor V. Zhdankin Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, MN, USA E-mail: [email protected]
Contents 1. Introduction 2. General Overview of Hypervalent Compounds 3. Hypervalent Heterocyclic Compounds of Group 13 3.1 Hypervalent Boron Heterocycles 3.2 Hypervalent Aluminum 4. Hypervalent Heterocyclic Compounds of Group 14 4.1 Hypervalent Silicon Heterocycles 4.2 Hypervalent Germanium, Tin, and Lead 5. Hypervalent Heterocyclic Compounds of Group 15 5.1 Hypervalent Phosphorus Heterocycles 5.2 Hypervalent Arsenic, Antimony, and Bismuth 6. Hypervalent Heterocyclic Compounds of Group 16 6.1 Hypervalent Sulfur Heterocycles 6.2 Hypervalent Selenium and Tellurium 7. Hypervalent Heterocyclic Compounds of Group 17 7.1 Hypervalent Bromine(III) Heterocycles 7.2 Hypervalent Iodine(III) Heterocycles 7.2 Hypervalent Iodine(V) Heterocycles References
Elements
Elements
Elements
Elements
Elements
2 2 5 5 8 8 8 10 10 10 12 12 12 14 14 14 15 18 19
Abstract This chapter provides an overview of recent literature on heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term “hypervalent” has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used in synthetic works despite criticism from theoretical chemists. Typical hypervalent heterocycles include polycoordinated 10-electron or 12-electron centers with distorted trigonal-bipyramidal or pseudooctahedral geometry, respectively. In general, heterocyclic compounds of elements with double bonds are not classified as hypervalent molecules because of the ylidic, zwitterionic nature of such bonds resulting in the classical 8-electron species. Despite the lack of aromatic conjugation, hypervalent heterocycles often have a considerably higher thermal stability compared to their acyclic analogs, which is especially Advances in Heterocyclic Chemistry, Volume 119 ISSN 0065-2725 http://dx.doi.org/10.1016/bs.aihch.2015.11.001
© 2016 Elsevier Inc. All rights reserved.
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important in the chemistry of polyvalent bromine and iodine. This review is centered mainly on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as, boron, silicon, phosphorus, sulfur, selenium, bromine, and iodine, with emphasis on structural and synthetic aspects of their chemistry.
Keywords: Benziodoxoles; Hypervalency; Hypervalent; Hypervalent boron; Hypervalent bromine; Hypervalent heterocycles; Hypervalent iodine; Hypervalent silicon; Hypervalent sulfur; Iodine heterocycles
1. INTRODUCTION Hypervalent heterocycles are cyclic molecules with a hypervalent main-group element in the ring. The term “hypervalent” was introduced in 1969 by Jeremy I. Musher for molecules with elements of groups 15e18 bearing more than eight valence electrons (1969ACE54) and more recently this terminology has been extended to the group 13 and 14 elements (1999MI1). The chemistry of hypervalent compounds has been systematically reviewed in a book edited by K.-Y. Akiba (1999MI1). Typical hypervalent heterocycles include polycoordinated 10-electron or 12-electron heteroatoms with distorted trigonal-bipyramidal or pseudooctahedral geometry, respectively. In general, heterocyclic compounds of elements with double bonds are not classified as hypervalent molecules because of the zwitterionic nature of such bonds resulting in classical 8-electron species (2014ACE9617). Despite the lack of aromatic conjugation, hypervalent heterocycles often have a considerably higher thermal stability compared to their acyclic analogs, which is especially important in the chemistry of the generally unstable organic compounds of bromine(III), iodine(III), and iodine(V). This chapter is centered mainly on the hypervalent heterocyclic derivatives of nonmetal main-group elements, such as, boron, silicon, phosphorus, sulfur, selenium, bromine, and iodine, with emphasis on the synthetic aspects of their chemistry. Despite the widespread practical interest in heterocyclic hypervalent compounds, the chemistry of hypervalent heterocycles has never been systematically reviewed.
2. GENERAL OVERVIEW OF HYPERVALENT COMPOUNDS Hypervalent compounds of main-group elements are often classified using the Martin-Arduengo NeXeL nomenclature, where N represents
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the number of valence electrons on the hypervalent atom X, and L is the number of ligands to the central atom X (1980JA7753). Several examples of uncharged polycoordinated 10-electron or 12-electron hypervalent centers are shown in Figure 1; anionic and cationic hypervalent species are also known. In general, compounds of elements with double bonds are not classified as hypervalent molecules. According to the 1983 IUPAC recommendations (1984PAC769), the position and the valency of a hypervalent atom in a molecule is indicated by using the Greek letter l. In the lambda nomenclature, the symbol ln is used to indicate any heteroatom in a nonstandard valence state (n) in a formally neutral compound. The lambda terminology is broadly used in the modern literature to indicate the general type of a hypervalent compound and to specify the number of primary bonds at the hypervalent atom. The special structural features and high reactivity of hypervalent compounds are explained by the presence of hypervalent bonding involving a three-center four-electron bond (1994CSR111, 1999MI1, 2014ARK109). The molecular orbital description of a three-center four-electron (3ce4e) bond was independently developed by G. C. Pimentel (1951 JCP446) and R. E. Rundle (1951JA4321) in 1951. The 3ce4e bond is described as three molecular orbitals formed by the combination of a p atomic orbital on the central atom and an atomic orbital from each of the two ligands on opposite sides of the central atom. Only one of the two pairs of electrons is occupying a molecular orbital that involves bonding to the central atom, the second pair being nonbonding and occupying a molecular orbital composed of only atomic orbitals from the two ligands. A representative example of the molecular orbital description of a 3ce4e bond in hypervalent 10-I-3 species 1 is shown in Figure 2. In particular, the interaction of the filled 5p orbital of the central iodine atom and the half-filled orbitals of the two ligands Y trans to each other leads to formation of three molecular orbitals: bonding, nonbonding, and antibonding. The occupied nonbonding molecular orbital has a node at the central iodine,
Figure 1 Typical 10-electron or 12-electron uncharged hypervalent centers.
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Figure 2 Molecular orbital description of the 3ce4e bond in hypervalent 10-I-3 species.
resulting in the charge distribution of almost 0.5 on each ligand and þ1.0 on the iodine atom. The carbon substituent R is bound by a normal covalent bond and the overall geometry of molecule RIY2 is a distorted trigonal bipyramid with two heteroatom ligands Y occupying the apical positions, and the least electronegative carbon ligand R and both electron pairs residing in equatorial positions. It should be noted that the formal octet rule is not violated in this model of bonding, as can be illustrated by the resonance involving two canonical Lewis structures 1a and 1b (Figure 2). The bonding in the hypervalent iodine 12-I-5 species, RIY4, can be described as a normal covalent bond between iodine and the organic group R in an apical position, and two orthogonal, hypervalent 3ce4e bonds, accommodating four electronegative ligands Y. The molecule of RIY4 has overall a square bipyramidal (or pseudooctahedral) structure with carbon substituent R and unshared electron pair occupying the apical positions with the electronegative ligands Y residing at the equatorial positions. The structure and reactivity of hypervalent compounds can be summarized as follows (1999MI1, 2013MI2): 1. Including the nonbonding electron pairs, the geometries of pentacoordinated hypervalent compounds (10-X-5, 10-X-4, and 10-X-3) are trigonal bipyramids with the most electronegative groups occupying the apical positions. 2. The structures of hexacoordinated hypervalent compounds (12-X-6 and 12-X-5 bearing a pair of unshared electrons) are octahedral regardless of the formal charge on the central atom. 3. Hypervalent XeL bonds in general are longer than the sum of the appropriate covalent radii of atoms X and L, but shorter than purely ionic bonds. 4. Intramolecular positional isomerization (Berry pseudorotation) resulting in an exchange between the apical and the equatorial ligands is important in explaining the reaction mechanisms of hypervalent compounds.
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5. Similarly to organometallic compounds, ligand exchange and ligand coupling (or reductive elimination) reactions are typical for hypervalent compounds. The term hypervalent and the concept of hypervalency have been sharply criticized by theoretical chemists. In particular, the concept itself has been criticized by Gillespie and Silvi who, based on the analysis of electron localization functions, wrote in 2002 that “as there is no fundamental difference between the bonds in hypervalent and nonhypervalent (Lewis octet) molecules there is no reason to continue to use the term hypervalent” (2002CCR53). Despite all the criticism, the term hypervalent has been overwhelmingly accepted by synthetic chemists, and the concept of hypervalency is currently widely used to describe special structural features and reactivity patterns of polycoordinated main-group compounds.
3. HYPERVALENT HETEROCYCLIC COMPOUNDS OF GROUP 13 ELEMENTS 3.1 Hypervalent Boron Heterocycles The anionic pentacoordinated and even hexacoordinated boron species can be stabilized in a heterocyclic system by complexation with appropriate ligands. Compounds of this type (structures 2 and 3, Figure 3) were first reported by Lee and Martin in 1984 (1984JA5745). 1H, 19F, and 13C NMR spectra of products 2 and 3 are consistent with the symmetrical structures incorporating pentacoordinated and hexacoordinated central boron atom; however, X-ray data were not available in this publication. More recently, Akiba and coworkers have synthesized pentacoordinated hypervalent boron heterocyclic species 4e6 by employing a sterically rigid anthracene skeleton (Scheme 1) (2000ACE4055, 2005JA4354). Compounds 6e8 were prepared by lithiation of bromoanthracene 4 followed by the addition of B-chlorocatecholateborane derivatives 5. Products 6e8 were
Figure 3 Pentacoordinated and hexacoordinated hypervalent boron species reported by Lee and Martin in 1984 (1984JA5745).
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Scheme 1
isolated as thermally stable solids (melting points above 200 C) and their structures were established by single crystal X-ray diffractometry. The sum of the bond angles around the central boron atom in the structures of 6-8 is 360.0 , which indicates that the central boron atom is planar with sp2 hybridization. The lone pairs on the oxygen atoms at the 1, 8 positions interact with the empty p orbital of the central boron atom, to form a three-center four-electron bond, and the overall structure can be regarded as a slightly distorted trigonal bipyramid. The two hypervalent BeO bond lengths are 2.379 and 2.441 Å in 6, 2.398 and 2.412 Å in 7, and identical (2.436 Å) in 8. The lengths are longer than covalent BeO bonds (1.39e1.40 Å) but shorter than the sum of the van der Waals radii (3.48 Å). The small difference in the hypervalent BeO bond distances observed in 6 may be a consequence of a packing effect (2000ACE4055). A similarly coordinated 10-methylacridinium-based hypervalent borone oxygen heterocyclic system 10 has been synthesized by Yamamoto and coworkers starting from compound 9 as outlined in Scheme 2 (2009 CL794). The hypervalent nature of the OeBeO has been confirmed by X-ray structural data (the average BeO bond of 2.51 Å). Another example of a hypervalent boroneoxygen heterocyclic structure is represented by the 2,6-bis(p-tolyloxymethyl)benzene-based 10eBe5
Scheme 2
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Figure 4 2,6-Bis(p-tolyloxymethyl)benzene-based 10eBe5 species.
species 11e13 (Figure 4) (2006MGC277). Compounds 11e13 were synthesized from the respective aryl bromide using the same approach as outlined in Scheme 1. X-ray crystallography revealed that the distances between the central boron and both oxygen atoms of the ligand in these structures varied from 2.50 Å to 3.16 Å. The catecholato derivative 13 was found to have the strongest BeO interactions being the closest to the ideal trigonal bipyramidal structure typical of hypervalent 10eBe5 species. Two representatives of hypervalent boronenitrogen heterocycles have been reported (2011JOC2123, 2013IC13865). Yamamoto and coworkers have prepared hypervalent 10eBe5 compound 15 (Scheme 3) utilizing the bis-(pyrimidine)benzene ligand framework (2011JOC2123). Product 15 was prepared in a low yield by lithiation of precursor 14 followed by treatment with methyl borate and then exchange of methoxy groups with ethylene glycol to afford the stable 1,3,2-dioxaborolane derivative, which was characterized by X-ray structural analysis. X-ray analysis and molecular orbital calculations suggested that compound 15 has a hypervalent pentacoordinated structure with an NeBeN hypervalent bond. In particular, the BeN distances (both equal to 2.537 Å) are substantially shorter than the sum of van der Waals radii (3.62 Å). Compound 15 has a trigonal bipyramidal structure about the central boron atom with two oxygens and carbon as the three equatorial atoms, and with two nitrogen atoms and the boron atom constituting an NeBeN three-center four-electron bond (2011JOC2123).
Scheme 3
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Scheme 4
Figure 5 Examples of hypervalent aluminum heterocycles.
Vidovic, Findlater, and coworkers reported the synthesis and characterization of terpyridine-based hypervalent 10eBe5 boronenitrogen dicationic heterocycle 16 (Scheme 4) (2013IC13865). The X-ray molecular structure of derivative 16 revealed evidence for pentacoordination at boron with four shorter BeN bonds (between 1.50 and 1.63 Å) and one longer bond of 2.94 Å.
3.2 Hypervalent Aluminum Hypervalent 10-electron structures are commonly observed in heterocyclic compounds of Group 13 metals (2001ACR201). Representative examples of such compounds can be illustrated by the alumoxane 17 (2013OM6647) and the complexes of aluminum with nitrogen ligands 18e20 (2009JCS(D) 8631) shown in Figure 5.
4. HYPERVALENT HETEROCYCLIC COMPOUNDS OF GROUP 14 ELEMENTS 4.1 Hypervalent Silicon Heterocycles Examples of stable, anionic, hypervalent 10-Si-5 siliconeoxygen heterocycles were originally reported by Martin and coworkers (1979JA1591,
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1981JOC1049). Compounds 22 and 23 were prepared by the reaction of dilithiated hexafluoroalcohol 21 with the appropriate trichlorosilane (Scheme 5) in the form of white hygroscopic solids with melting points above 355 C (1979JA1591). More recently, Goddard, Fensterbank, and coworkers have developed a new route to Martin’s spirosilanes starting from substituted o-bromobenzoic acids 24 (Scheme 6) (2015JOC3280). In particular, stable, hypervalent fluorosilicates 26 have been prepared by addition of fluoride anion to the tetracoordinated silicates 25, which can be readily synthesized from benzoic acids 24 in several steps. X-ray crystallographic study of products 26 has revealed a trigonal bipyramidal geometry of the silicon centers with both oxygen groups at the apical position, while the aromatic ring and the fluorine atom are occupying the equatorial positions. An intermediate formation of unstable heterocyclic 10-Si-5 silicone oxygen heterocycles has been proposed in cross-aldol reactions of aldehydes mediated by chlorosilanes (2001ACE4759, 2011OL1654). Structures and stabilities of three-membered rings containing hypervalent atoms of silicon or phosphorus and sulfur have been theoretically investigated by Ikeda and Inagaki (2001PCA10711).
Scheme 5
Scheme 6
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Figure 6 Examples of hypervalent heterocycles of the lower group 14 elements.
4.2 Hypervalent Germanium, Tin, and Lead Hypervalent 10-electron structures are commonly observed in the compounds of germanium, tin, and lead (2002MI3). Representative examples of such heterocyclic compounds are illustrated by germanium cyclic oxamide complexes 27 (2009JCS(D)4695) and cyclic pentaorganostannate 28 (2007JA10974) shown in Figure 6
5. HYPERVALENT HETEROCYCLIC COMPOUNDS OF GROUP 15 ELEMENTS 5.1 Hypervalent Phosphorus Heterocycles Numerous structural types of hypervalent phosphorus compounds are known (1999MI1). Representative examples of stable hypervalent phosphorus heterocycles include 10-P-3 compounds 29 (1994CR1215), anionic 10-P-4 species 30 (1983CB3301), pentacovalent 10-P-5 compounds 31 (1966CB3642), and anionic hexacoordinated 12-P-6 species 32 (1965 CB576). The first two structural types (29 and 30) are commonly termed as the low-coordinate hypervalent phosphorus compounds (1994CR1215) (Figure 7). An important example of a pentacovalent 10-P-5 compound is represented by the protonated form of proazaphosphatrane 33 (Scheme 7). Compounds 33 are strong nonionic bases (Verkade bases, also known as superbases) that serve as efficient catalysts and promoters of many reactions (2001T467). Particularly important are hypervalent 10-P-3 compounds. An example of these compounds is 3,7-di-tert-butyl-5-aza-2,8-dioxa-1-phosphabicyclo
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Figure 7 Typical structural types of hypervalent phosphorus heterocycles.
Scheme 7
[3.3.0]octa-2,4,6-triene (ADPO, 29), which shows unique chemical properties (1994CR1215). The oxidative addition of reagents such as quinones, halogens, perfluoroalkylacetylenes, hydrogen, secondary amines, or alcohols to the phosphorus center of ADPO transforms the phosphorus(III) center into a phosphorus(V) center. In a more recent work, Kornev and coworkers reported the preparation and structural studies of a series of phenylpyrazole-based hypervalent phosphorus compounds 34 and 35 (Figure 8), which represent another example of low-coordinate hypervalent phosphorus heterocycles (2015EJIC2057). According to X-ray structural data, the phosphorus atom in molecule 34 has overall trigonal bipyramidal geometry with chlorine atoms at the apical position, while the N- and C-substituents and the lone electronic pair are occupying the equatorial positions with PeN and PeC bond distances of 1.771 and 1.831 Å, respectively.
Figure 8 Phenylpyrazole-based hypervalent phosphorus heterocycles.
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Figure 9 Examples of hypervalent heterocycles of the lower group 15 elements.
5.2 Hypervalent Arsenic, Antimony, and Bismuth Hypervalent heterocyclic structures are typical of arsenic, antimony, and bismuth. The chemistry of these compounds has been overviewed by K.-Y. Akiba (2011HC207) and several specific examples are illustrated by structures 36e44 (Figure 9).
6. HYPERVALENT HETEROCYCLIC COMPOUNDS OF GROUP 16 ELEMENTS 6.1 Hypervalent Sulfur Heterocycles Several structural types of hypervalent compounds of sulfur(IV) and sulfur(VI) are known (1999MI1). Representative examples of stable hypervalent sulfur heterocycles include anionic 10-S-3 sulfuranes 45 (1978JA 7077), 10-S-4 species 46 (1981JA127) and 47 (1992CC1141), pentacoordinated 10-S-5 sulfuranes 48 (1977JA5490) and 49 (1983JA1377), and hexacoordinated 12-S-6 species 50 (also known as persulfuranes) (1982JA1683) (Figure 10). In a more recent work, Kawashima reported the preparation and structural studies of a series of four-membered 10-S-4 and 10-S-5 heterocycles 51e54 (Figure 11) (2011PSS1046). These compounds containing a tetracoordinated or pentacoordinated hypervalent sulfur atom together with other heteroatoms were isolated as thermally stable products and structurally
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Figure 10 Typical structural types of hypervalent sulfur heterocycles.
Figure 11 Four-membered heterocycles with a hypervalent sulfur atom.
characterized by a single crystal X-ray diffractometry. X-ray crystallographic analyses revealed that the hypervalent sulfur centers in these molecules have distorted trigonal bipyramidal geometry. Various sulfur heterocycles with endocyclic double bonds on sulfur are often referred to as the 10-S-3 “p-hypervalent heterocyclic systems” and shown as canonical structures with tetracovalent sulfur; for example, tetraazathiapentalenes 55 and trithiapentalenes 57 (Figure 12) (2005JHC1175, 2002JHC189). However, a more accurate representation of these compounds involves non-hypervalent betaine structures (e.g., structures 56 and 58) with eight electrons on the sulfur atom. A discussion on heterocyclic mesomeric betaines containing sulfur or other elements has been provided in a review (1985T2316) and a book (2010HHC150). These compounds have been the subject of a considerable theoretical and synthetic interest (1976TCC49, 1997JPCA4475, 1998PS(140)35).
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Figure 12 Tetraazathiapentalenes and trithiapentalenes.
Figure 13 Examples of hypervalent heterocycles of selenium and tellurium.
6.2 Hypervalent Selenium and Tellurium The chemistry of hypervalent selenium (commonly named as selenuranes) and tellurium (telluranes) in general is similar to the compounds of sulfur (1999MI1). Representative examples of stable hypervalent heterocyclic derivatives of these elements include anionic 10-Se-3 and 10-Te-3 species 59 (1995JA10153), 10-Se-4 selenuranes (1968LAC68) and 10-Te-4 telluranes 60 (1968LAC1), and hexacoordinated 12-Te-6 species 61 (pertellurane) (1984JA7529) (Figure 13). The chemistry of selenaheterocyclic compounds has been reviewed by Mlochowski and coauthors (2007ARK14).
7. HYPERVALENT HETEROCYCLIC COMPOUNDS OF GROUP 17 ELEMENTS 7.1 Hypervalent Bromine(III) Heterocycles Two examples of 10-Br-3 hypervalent bromine heterocycles (brominanes) based on the Martin ligand have been reported (1980JA7382, 1986JA3803). Compounds 63 and 64 were prepared by oxidation of aryl bromides 62 with bromine trifluoride in freon solution (Scheme 8). Both brominanes are stable for an indefinite period at room temperature (mp in
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Scheme 8
a range of 153e170 C) and are inert toward atmospheric moisture, aqueous base, aqueous hydrogen chloride, and trifluoromethanesulfonic acid. They can be sublimed at 60 C (3 torr) or passed through a neutral alumina column (ether/pentane) without decomposition. Brominanes 63 and 64 are strong oxidizing agents, and in particular can readily oxidize iodide ion, bromide ion, thiophenol, aniline, 9,10-dihydroanthracene, and tetralin (1986JA3803). X-ray structural analysis of brominane 63 revealed a distorted trigonalbipyramidal geometry around the central bromine atom. The two lone pairs of electrons are considered to occupy equatorial ligand sites. The molecule is almost planar, and the BreO bonds (1.99 and 1.97 Å) are slightly longer than the sum of the covalent radii (1.8 Å). In the solid-state structure, the oxygen atom is engaged in intermolecular interaction with the hypervalent bromine of an adjacent molecule of 63 (1986JA3803).
7.2 Hypervalent Iodine(III) Heterocycles Hypervalent iodine heterocycles represent a particularly important class of compounds because of their wide application as reagents for organic synthesis. Preparation, structure, and chemistry of hypervalent iodine compounds have been summarized in numerous books (2013MI2, 2014MI3, 2003MI4, 1992MI5) and reviews (2015SL1785, 2015CR650, 2015ACE8876, 2015 ACE5290, 2015AJC699, 2015RET49, 2015S587, 2014CCR54, 2014 CH419, 2014OBC4278, 2014CAJ950, 2014CAJ972, 2013SL424, 2012 COS247, 2011S517, 2011NPR1722, 2011ARK370, 2009ARK1, 2008CR 5299), and most recently in the chapter on “Iodine Heterocycles” in Volume 115 of AHC (2015AHC1). Typical structural types of trivalent iodine heterocycles are represented by five-membered cyclic compounds 65e77, which incorporate iodine, oxygen, nitrogen, and some other elements, in the heterocyclic ring (Figure 14). The general name “benziodoxoles” is used for the heterocycles 65e70 with iodine and oxygen atoms in a five-membered ring and various
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Figure 14 Examples of hypervalent iodine(III) five-membered heterocycles.
substituents attached to iodine. The most important representative of benziodoxoles, 1-hydroxy-1,2-benziodoxol-3-(1H)-one 65, commonly known as 2-iodosobenzoic acid (IBA), was first prepared over 100 years ago by oxidation of 2-iodobenzoic acid (1892B2632). A. R. Katritzky and coworkers have significantly contributed to the study of structure and reactivity of IBA and other benziodoxoles and benziodoxole oxides in a series of works published in 1989e1990 (1990JCS(P2)1657, 1990JCS(P2)1515, 1989MRC1007, 1989OPPI157). According to X-ray structural data (1990JCS(P2)1657), the five-membered ring in benziodoxoles is highly distorted with almost linear alignment of the two electronegative ligands. The endocyclic IeO bond length in iodine-substituted benziodoxolones varies in a wide range from 2.11 Å in carboxylates to 2.48 Å in 1-phenyl benziodoxolone (1986IC1415), which is indicative of considerable changes in the ionic character of this bond. The CeIeO bond angle in benziodoxole ring is typically around 80 , which is a significant deviation from
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the expected angle of 90 for the normal T-shaped geometry of hypervalent iodine. In the mid-1980s, IBA and other hydroxybenziodoxoles attracted a significant research activity due to their excellent catalytic activity in the cleavage of toxic phosphates and reactive esters (2002CR2497). Compared to benziodoxoles, the analogous five-membered iodinenitrogen heterocycles, benziodazoles 71, have received much less attention. The first representative of benziodoxoles, acetoxybenziodazole was synthesized in 1965 by the peracetic acid oxidation of 2-iodobenzamide (1965CC449). The structural parameters of benziodazoles 71 are, in general, similar to those of benziodoxoles (1979JOC1447, 1998JOC6590, 2003OL1583, 1997JA7408). Besides benziodoxoles and benziodazoles, the other iodine(III) heterocyclic systems are represented by the following compounds shown in Figure 14: fused benziodazoles 72 (2001JA4095), benziodoxazoles 73 (1965JOC617), benziodoxaboroles 74 (2011IC11263), benziodoxathioles 75 (2006EJOC 4791), benziodathiazoles 76 (1975JOC797), and cyclic phosphonate 77 (1978JOC4538). Five-membered heterocyclic iodine(III) compounds in general are more stable compared to their acyclic analogs, which has made possible the preparation and isolation of otherwise unstable trivalent iodine derivatives with azido, trifluoromethyl, cyano, and alkynyl substituents (e.g., structures 66e69). Various benziodoxole derivatives have found synthetic application as oxidants and reagents for “atom-transfer reactions” (2011 CC102, 2005COS121, 1997RHC133). For example, it has been demonstrated in a recent work that a complex of 2-iodosobenzoic acid 65 with triflic acid (IBA-OTf) is a powerful oxidant toward a variety of organic substrates, such as sulfides, phenols, alkenes, and aldoximes (2015CC7835). Azidobenziodoxole 66 is a particularly useful reagent for the iron(II)-catalyzed azidation of tertiary CeH bonds suitable for late-stage functionalization of complex organic molecules (2015N600, 2015ACE5290). Reagent 66 has also been used for direct conversion of aldehydes to acyl azides (2015 OL5212). Trifluoromethylbenziodoxole 67 is a useful reagent for electrophilic trifluoromethylation of various organic substrates (2015CR650). Cyanobenziodoxole 68 is an excellent reagent for direct electrophilic a-cyanation of b-keto esters and amides (2015OBC365). Triisopropylsilylethynylbenziodoxole 69 is an efficient acetylene transfer reagent, both under metal-free and metal-catalyzed reaction conditions (2015ACE8876, 2015 ACE5438, 2015CEJ8745, 2015ACE11200, 2015CC14497, 2015OL3054, 2015OL1938, 2015OL736).
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7.2 Hypervalent Iodine(V) Heterocycles Organoiodine(V) heterocycles are represented by several typical classes 78e86 shown in Figure 15. The most important representative of these compounds, 2-iodoxybenzoic acid (IBX, 78), was originally reported in 1893 (1893CB1727). The cyclic structure of IBX derivatives was confirmed by an X-ray crystallographic study in the work of A. R. Katritzky and coworkers (1990JCS(P2)1657). In the 1980se1990s, J. C. Martin and coworkers reported the synthesis, structure, and properties of several cyclic l5-iodanes (79e82) (1983JOC 4155, 1991JOC6565, 1993JA2488, 1991JA7277). A particularly important compound is triacetoxybenziodoxolone 79, prepared by heating IBX with acetic anhydride (1983JOC4155). The triacetate 79, known as DesseMartin periodinane (DMP), has emerged as the reagent of choice for the oxidation of alcohols to the respective carbonyl compounds. Iodine(V) heterocycles have found broad practical application as mild and selective reagents for the oxidation of alcohols and some other useful
Figure 15 Examples of hypervalent iodine(V) heterocycles.
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oxidative transformations. Numerous reviews on the chemistry and synthetic applications of IBX and related iodine(V) heterocycles have been published (2011JOC1185, 2006ARK26, 2010T7659, 2011ACE1524, 2001ACE2812). Several recent examples of other iodine(V) heterocycles are represented by structures 83e86 (Figure 15). Moorthy and coworkers have prepared tetramethyl-IBX (TetMe-IBX, 83) (2011JOC9593, 2014JOC11431), a highly reactive oxidant that can oxidize alcohols in common organic solvents at room temperature due to the “hypervalent twisting”-promoted rate enhancement (2005JA14146). Fluorous IBX 84 is a recyclable oxidant that can also be used as a catalyst for oxidation of alcohols to the corresponding carbonyl compounds (2011CC1875). Tosylate derivative of 2-iodoxybenzoic acid (IBX-tosylate, 85), one of the most powerful hypervalent iodine(V) oxidants, can be prepared by the reaction of IBX with p-toluenesulfonic acid in acetic anhydride (2013CC11269). 2-Iodoxybenzenesulfonic acid, which is a thia-analog of IBX and a powerful oxidizing reagent, exists in a cyclic tautomeric form of 1-hydroxy-1H-1,2,3-benziodoxothiole 1,3,3trioxide 86 (2006EJOC4791). Ishihara and coworkers have demonstrated that thia-IBX 86 is the most powerful catalyst in the iodine(V)-catalyzed oxidation of alcohols using oxone as a terminal oxidant (2009JA251).
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