Chivers T. - A Guide to Chalcogen-Nitrogen Chemistry (2005)(en)

in 15.18 is remarkably resistant to oxidation. Whereas the other sulfur(II) atom is oxidized to a sulfone, an excess of 4-chloroperbenzoic acid reacts preferentially with the C=N double bond giving the epoxide 15.19.18 By contrast, the two sulfur atoms in the diazene 15.11c are oxidized readily to sulfur(VI) with simultaneous reduction of the N=N double bond and formation of a thiatriazole ring to yield 15.20 (Ar = 4-CH3C6H4).19

The enzyme protein tyrosine phosphatase 1B (PTB1B) is a potential therapeutic agent for treating diabetes. X-Ray crystallographic studies reveal that the sulfur atom of the cysteine in the active site of this enzyme is covalently bonded to a nitrogen atom in the backbone of a neighbouring residue.24 PTB1B uses its active-site cysteine to remove a phosphate group from a tyrosine on the insulin receptor. Its activity is turned off by H2O2-mediated oxidation of the cysteine to cysteine sulfenic acid, which rapidly converts to a sulfenyl amide species. It has been suggested that this sulfur•••nitrogen interaction protects cysteine from being further oxidized before it can return to the active thiol state.24

In some reactions intramolecular chalcogen•••nitrogen interactions may lead to stereochemical control. For example, selenenyl bromides react with C=C double bonds, providing a convenient method of introducing various functional groups. The reaction proceeds readily, but affords a racemic mixture. The modified reagent 15.22 contains a chiral amine in close interaction with the selenium atom. It reacts with olefins affording up to 97% ee of isomer A (Scheme 15.2).25

15.6 Stabilization of Reactive Functional Groups

Intramolecular chalcogen interactions may also stabilize reactive functional groups enabling the isolation of otherwise unstable species or their use as transient intermediates, especially in the case of selenium and tellurium. For example, tellurium(II) compounds of the type ArTeCl are unstable with respect to disproportionation in the absence of such interactions. The diazene derivative 15.23 is stabilized by a Te•••N

Presumably, intramolecular coordination hinders the disproportionation process. Other derivatives of the type RTeX that are stabilized by a Te•••N interaction include 8-(dimethylamino)-1- (naphthyl)tellurium bromide,26 2-(bromotelluro)-N-(p-tolyl)benzylamine,27 and 2-[(dimethylamino)methyl]phenyltellurium iodide.28 Intramolecular donation from a nitrogen donor can also be used to stabilize the Se–I functionality in related compounds.4,29

+H2O2 / CH2Cl2

Scheme 15.2 Stereochemical control under the influence of an Se•••N interaction

Intramolecular heteroatom coordination may also influence the stabilities or structures of catenated tellurium compounds. For example, a rare example of a tritelluride, bis[2-(2-pyridyl)phenyl]tritelluride, is stabilized by a Te•••N contact of 2.55 Å.30 The ditelluride (2- MeOC6H4COTe)2 has an unusual planar structure. Although the C=O•••Te interaction is longer (3.11 Å) than the Me•••O contact (2.76 Å), ab initio molecular orbital calculations indicate that the planarity results predominantly from the former intramolecular connection.31

Transannular Te•••N interactions have also been employed to stabilize compounds of the type 15.24 with terminal Te=E (E = S, Se) bonds.32 The Te=Se bond length in 15.24b is 2.44 Å (cf. 2.54 Å for a Te– S single bond) and d(Te•••N) = 2.62 Å. Intramolecular coordination was also employed in the isolation of the first aryl-selenenium and – tellurenium cations 15.25a,b as [PF6]- salts.33

NMe2

E

The Se•••N interaction has been utilized in the stabilization of a transient selenenic acid ArSeOH.34 Through such a reactive intermediate the diselenide 15.26 catalyzes the oxidation of alkenes to allylic esters or ethers in the presence of sodium persulfate.35 Compound 15.26 also catalyzes the oxidation of thiols to disulfides by hydrogen peroxide serving as a model to study the role of the amino nitrogens located at the active centre of glutathione peroxidase.11,36 Characterization of the intermediate steps by 77Se NMR spectroscopy and kinetic studies indicate that the model behaves in the same way as the enzyme, although the latter possesses two nitrogens in proximity to the selenium of a selenocysteine. The proximal nitrogen is thought to play an additional role in activating the selenol into selenolate.

Although bulky aryl groups, e.g., mesityl, are not effective in stabilizing arylselenium (II) azides, the use of intramolecular coordination in 2-Me2NCH2C6H4SeN3 has enabled the first structural characterization of this reactive functionality.37 The Se–N3 (azide) bond length is 2.11 Å, while the intramolecular Se•••N distance is 2.20 Å, cf. 2.14 Å in the arylselenium bromide 15.5, and 2.13 Å and 2.17 Å, respectively, in the corresponding chloride and iodide.37 This

arylselenium (II) azide is thermally unstable at 25°C; it decomposes with loss of N2 to give the corresponding diselenide.

Metal selenolates of the type M(SeAr)2 (M = Zn, Cd, Hg) are usually insoluble, polymeric compounds. Intramolecular Se•••N coordination has been employed to stabilize monomeric mercury selenolates, e.g., 15.27, but this approach was not successful for the zinc and cadmium derivatives.38

Chiral organoselenenyl halides may also be stabilized by intramolecular Se•••N interactions; 77Se NMR chemical shifts indicate that these interactions are maintained in solution.29b

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Subject Index

Anion, [SSNO]- (see perthionitrite)

309