Sulfonium ions structure

Prolonation of thiepins leads to //-thiepinium ions, for which a homoaromatic character has been discussed.24 The 5//-1 -benzothiepinium ion can be regarded as a benzohomothiopyrylium ion, whereas the 3//-l-benzothiepinium ion is postulated as having a localized sulfonium ion structure.25... 

C-NMR spectra were in accord with the formation of only one silicon-containing species having only one type of isopropyl group. According to the authors, the isomeric sulfonium ion structure and the dimer structure can be eliminated with a high degree of confidence. (4) Treatment of the product with diisobutylaluminum hydride led only to the recovered silane precursor. 

Both 5H-l-benzothiepinium ion (23) and 3H-l-benzothiepinium ion (24) were generated in FSO3H/SO2/CD2CI2 and characterized by H and C NMR spectra <84CL919>. The former ion can be regarded as benzohomothiopyrylium ion whereas the latter is postulated to have a localized sulfonium ion structure. 

Interestingly, the sulfanes H2S are both proton acceptors and donors. In the first case sulfonium ions H3S are formed, in the second case hydrogen polysulfide anions HS are the result. While the latter have never been isolated in salts, several salts with sulfonium cations derived from the sulfanes with n = 1, 2, and 4 have been published. However, none of these salts has been structurally characterized by a diffraction technique. Therefore, the structures of the HsSn cations and HS anions are known from theoretical calculations only. 

In an extensive ab initio MO study the structures, energies and vibrational spectra of the sulfonium ions H3S with n=l-4 were studied at the MP2/6-311(2df,2pd) level of theory [70]. It was confirmed that HsS is of Csv symmetry with dsH= 134.6 pm and bond angles of 94.2° This cation had previously been isolated as a component of the salt [H3S][SbF6] [71] and had been observed spectroscopically [72]. The experimental gas phase re geometry is dsH= 135.02 pm and 0 hsh=94.189° [72] which agrees well with the ab initio calculated values by Botschwina et ah dsH=135.0 pm, aHsn=94.2° [73]. 

The numerous straightforward examples of internal displacement reactions leading to isolable cyclic products will not be discussed here, but only, for the most part, those ionization reactions in which a cyclic intermediate or transition state is deduced from the rearranged structure of the product. A well-known example is mustard gas and other alkyl chlorides with sulfur on the /3-carbon atom. Although mustard gas is a primary and saturated alkyl chloride, its behavior is like that of a typical tertiary alkyl chloride. It reacts so fast by a first order ionization that the rate of the usual second order displacement reaction of primary alkyl halides is not measureable. Only the ultimate product, not the rate, is determined by the added reagent.228 Since the effect of the sulfur is too large to be explicable in terms of a carbon sulfur dipole or similar explanation, a cyclic sulfonium ion has been proposed as an... 

Aliphatic onium ions such as immonium, oxonium, and sulfonium ions have been introduced as even-electron ionic products of the a-cleavage occurring from molecular ions of amines, alcohols, and ethers or thiols and thioethers, respectively (Chap. 6.2.5). All these and analogous onium ions are capable of further fragmentation reactions, the majority of which are alkene losses [141] yielding fragments of high relevance for structure elucidation. 

Sulfonium ions are possible important reactive intermediates in stereoselective gly-cosylations, although their role has recently been disputed.191A number of ions including 74,19275,193 and 76194 have been observed by low-temperature NMR spectroscopy, whereas the structure of 77 was determined by X-ray crystallography.195... 

Nucleophilic substitution of fluorine in perfluorinated sulfonium ions yields the corresponding aminosulfonium ions when reacted with Me3SiNMe2 [Eq. (4.66)]. Crystal structure analysis of CF3S(NMe2)2+, (CF3)2SNMe2+, and F2SNMe2+ hexa-fluoroarsenates gives similar S—N bond distances (1.605, 1.578, and 1.535 A, respectively) 215,244... 

In both cases, intramolecular displacement of the chloride leaving group by the sulfur atom—or, as we should call it, participation by sulfur (see Chapter 37)—gives a three-membered cyclic sulfonium ion intermediate (an episulfonium or thiiranium ion). Nucleophilic attack on this electrophilic sulfonium ion, either by water or by the structural proteins of the skin, is very fast. Of course, mustard gas can react twice in this way. You will see several more examples of reactions in which a sulfonium ion intermediate acts as an electrophile in the next section. 

Sulfonium ions differ substantially in structure. The inversion barrier is much higher (AH = 20 + 30 kcal mole ) and in son cases optically active isomers having a high stability were separated (e.g. Me, Et, n-Bu—The bond angles are much smaller and much closer to 90°. With increasing size of the electronic cloud around the heteroatom, the hybridization of the free electron pair with those involved in chemical bonding is of less importance and bond angles approach 90° e.g. ... 

The structure of the proposed dication, [ArSR] , is not clear. In the case of the anodic oxidation of dialkyl sulfides [67], the dicationic transient intermediate suggested involves an S-S bond, but with dimer [ArSR] its transformation into the sulfonium ion implies in a rather unusual although not demonstrated mechanistic path, as in Eq. (51). 

Inspection of Table XII suggests a tentative correlation between the structure of the methyl group donor and net reaction stereochemistry. In cases where the donor species has been clearly identified as an active methylating agent, owing to the presence of a sulfonium ion, inversion of configuration is observed. This is consistent with a one-step Sn2 mechanism of the same type that is normally observed with S-adenosylmethionine in nonenzymic displacement reactions [Eq. (65)1 ... 

Halogeno-l,2,4-thiadiazol-5-yl sulfenyl chlorides (455) react rapidly with olefins at — 40°C to yield adducts (e.g., 456 from butene). The stereospecific mechanism of the exclusive trans addition probably involves epi-sulfonium ions as intermediates. Thus, erythro adducts arise from trans-2-buteneand threo adducts from ds-2-butene terminal olefins (e.g., isobutylene, 3,3-dimethylbut-l-ene) produce single products, the assigned structures of which (Markovnikov or otherwise) are consistent with their H NMR spectra.209... 

Other elements can produce chiral centers besides carbon, although their importance in industry and in health studies is not as great. The sulfur atom can produce chiral centers for example the sulfoxides, sulfoximides, sulphonates and the sulfonium ion. An example of a chiral sulfur atom is given by the following sulfoxide structure. 

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