Cyclic sulfides, protonation

The proton affinities (gas phase) of thiirane and other three-membered heterocycles have been determined azirane (902.5), thiirane (819.2), phosphirane (815.0), oxirane (793.3 kJ moF ) (80JA5151). Increasing s character in the lone electron pairs decreases proton affinities. Data derived from NMR chemical shifts in chloroform indicate the order of decreasing basicity is azirane > oxirane > thiirane (73CR(B)(276)335). The base strengths of four-, five- and six-membered cyclic sulfides are greater than that of thiirane. 

In this same vein, the synthesis of 25m and 2621) has been attempted by initial formation of the cyclic sulfides, then extrusion of sulfur by irradiation in the presence of triethylphosphite. In both cases the limited sample size precluded formation of the desired cyclophanes (25 and 26) by this photolytic procedure. Variable temperature H NMR studies have been performed on 27 and 28 27 has a barrier to conformational inversion of 12.5 kcal/mole, while 28 has a corresponding barrier of 10.4 kcal/mole. Phane 28 exhibits a shift of the aromatic protons to higher field as compared to 29 the most dramatic shift is observed for H-3, which absorbs at 7.72 ppm for 28 and at 8.37 ppm for 29. This shift has been explained in terms of an approximately syn orientation of the pyridine rings in 28. 

Cyclic ethers, acetals and cyclic amines belong to the group of hard bases while cyclic sulfides can be considered as soft bases. On the other hand proton is the hardest acid whereas alkyl cations are much softer. Thus, in good agreement with the concept of hard and soft acids and bases (the alike ones react faster giving more stable products), secondary oxonium ions react slower with monomer than their tertiary oxonium counterparts. The same is observed for cyclic amines whereas secondary sulfonium ions apparently react faster with cyclic sulfides than the corresponding tertiary sulfonium ions do l. 

Polymerization of cyclic sulfides and amines can be initiated not only by the usual cationic and cationogenic compounds like Lewis and protonic acids, carbenium and oxonium salts and esters of strong acids, but also by alkyl halides, which are active enough to induce polymerization of some azetidines 9 10). 

The positive ion FAB mass spectra of the mixtures of cyclic sulfides extracted from poly(hexamethylene sulfide), and that of the same sample doped with AgNOs or CuCl are reported in Figure 7.2. Only protonated molecular ions are present in the FAB spectrum of the undoped mixture... 

Fig. 5.2. The photosynthetic membrane of a green sulfur bacterium. The light-activated bacte-riochlorophyll molecule sends an electron through the electron-transport chain (as in respiration) creating a proton gradient and ATP synthesis. The electron eventually returns to the bacteri-ochlorophyll (cyclic photophosphorylation). If electrons are needed for C02 reduction (via reduction of NADP+), an external electron donor is required (sulfide that is oxidised to elemental sulfur). Note the use of Mg and Fe.

Depending on the linear, branched or cyclic structure of the unsaturated compound, a variety of dialkyl sulfides has been obtained in the reaction with H2S (equations 16-18). The regiochemistry depended markedly on the structure of the diene. For a mechanistic purpose, some experiments have been carried out using deuterium sulfide, D2S. The results have been interpreted in terms similar to those of Nordlander and coworkers9 (vide infra). The thiylation of 1,3-dienes was assumed to start with a regiospecific addition of a proton or a deuteron to one of the two double bonds to form two isomeric ion pairs as in equation 14 which, in the poorly dissociating solvent, collapse into products with equal probability. 

In the alcohol oxidations, the sulfonium intermediate (2, nucleophile = R.( (( 41)) loses a proton and dimethyl sulfide to give the carbonyl compound (42). The most common mechanism for the decomposition of (2) is attack by a mild base to remove a proton from one of the methyl groups. Subsequent cyclic collapse leads to the carbonyl compound and dimethyl sulfide (eq. 9) ... 

Heterocyclics containing P-atoms are usually strong nucleophiles. For example the basicity of 2-methoxy-ODP (pKa = 3.1) is much higher than that of cyclic ethers or sulfides. Therefore, a wide range of initiators, i.e., carbenium or oxonium salts, Lewis and protonic acids, and relatively nonreactive alkyl halides and organoalu-minum compounds have been used. Usually, reactions were carried out in a N2 atmosphere, although no special precautions (e.g. vacuum) were used to avoid contamination with water. 

Benzyne generated from 2-carboxybenzenediazonium chloride cleaves cyclic and acyclic sulfides, as in the conversion of thiolane 609 to 613. Apparently protonation to the sulfonium salt 612 is faster than cleavage of intermediates 610 and/or 611. 

The original Pfittner-Moffatt procedure for alcohol oxidation by activated dimethyl sulfoxide utilized dicyclohexylcarbodiimide (DCC) and a source of protons such as polyphosphoric acid or pyridinium tri-fluoroacetate. The use of strong acids such as the common mineral acids must be avoided since, although acidic conditions are initially required, the reaction must readily become basic in the later stages of the process. Mechanistically it is reasonable to suggest that the activation follows the pattern whereby initial attack of the nucleophilic sulfinyl oxygen of dimethyl sulfoxide, with the protonated carbodiimide, forms a sulfonium isourea. This is followed by displacement of dicyclohexylurea by the alcohol to form an alkoxysulfonium salt. Base treatment of this salt forms an ylide, which collapses via the proven cyclic mechanism to the carbonyl compound and dimethyl sulfide (Scheme 4). 

In the mass spectrum of the poly(m-phenylene sulfide) (polymer I, Figure 5.9a), the peaks at m z 324 + nl08 correspond to molecnlar ions (M ) of cyclic oligomers M3+-M13+. FAB mass spectra of polymers II-IV in Figure 5.9 (parts b-d, respectively), show peaks due to protonated molecular ions (MH ) of the oligomers present in mixtnres. 

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