Chapter Three - Hypervalent Heterocycles
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 15–18 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.
Section snippets
General Overview of Hypervalent Compounds
Hypervalent compounds of main-group elements are often classified using the Martin-Arduengo N–X–L nomenclature, where N represents 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
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
Hypervalent Silicon Heterocycles
Examples of stable, anionic, hypervalent 10-Si-5 silicon–oxygen heterocycles were originally reported by Martin and coworkers (1979JA1591, 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
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 (1965CB576). The first two structural types (29 and 30) are commonly termed as the low-coordinate hypervalent phosphorus compounds (1994CR1215) (Figure 7).
An important
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 (1978JA7077), 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
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 a range of 153–170 °C) and are inert toward atmospheric moisture, aqueous base, aqueous hydrogen chloride, and trifluoromethanesulfonic acid. They can be
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