S, bonding

The Cyc conformer represents the structure adopted by the linear peptide prior to disulfide bond formation, while the two /3-turns are representative stable structures of linear DPDPE. The free energy differences of 4.0 kcal/mol between pc and Cyc, and 6.3 kcal/mol between pE and Cyc, reflect the cost of pre-organizing the linear peptide into a conformation conducive for disulfide bond formation. Such a conformational change is a pre-requisite for the chemical reaction of S-S bond formation to proceed. 

Analysis We have an obvious Diels-Alder disconnection, some C-S bonds, and a 1,6-dicarbonyl relationship. The only one that gives any rapid simplification is the D-A, so we ll start with that ... 

Both C-S bonds are now P to carbonyl groups and so can be discomiected in turn by reverse Michael reactions. 

Another widely used route to cyclopropanes involves the addition of sulfoniutn ylides to a,/3-unsaturated carbonyl compounds (S.R. Landor, 1967 R. Sowada, 1971 C.R. Johnson, I973B, 1979 B.M. Trost, 1975 A). Non-activated double bonds are not attacked. Sterical hindrance is of little importance in these reactions because the C—S bond is extraordinarily long... 

S—Cg is perpendicular to the amide plane of the / -lactam and therefore weakened. The S—bond, on the other hand, is not affected by electronic interactions with the benzamide plane. It was now thought, that a bridging of the thiazolidine moiety would bring the —S bond into a more orthogonal position with respect to the amide plane of the new lactam and make this bond more fragile. The tricyclic thiazolidine was synthesized as described above and fulfilled the predictions (J.E. Baldwin, 1978). 

In the presence of [PtC ] and a base. 2-aminothiazole undergoes ring cleavage of the C-S bond to give PtLCF (L=HSCH=CHNHCN) (699). The Pd and Pt complexes of 2-aminothiazoles show biological activity (1596i. 

The C-S bond in purine derivatives undergoes cleavage under mild conditions by nucleophilic agents such as benzylmercaptan or glutathione in dimethylformamide with a phosphate buffer of pH 6.5 (277). The salt (110) of dithiazolylsulfide heated at 190 C yields the A-4-thiazo-line-2-thione (112) and 2-chlorothiazole (111) (Scheme 56) (278-280). 

In Table 1-9 we have collected only the 7r-bond orders calculated by allvalence-electrons methods and compared their values with those deduced from experimental bond lengths. Both data are indicative of an aromatic molecule with a large dienic character. The 2-3 and 4-5 bonds especially present a large double-bond character, whereas both C-S bonds are relatively simple. 

F. 1-26. (a) ir-Bond order of the C-S bonds in the ground state, (fc) ir-Bond order of the C-S bonds in the first excited state, (c) Free-valence number of the intermediate diradicaf. (Most probable bicyclic intermediate resulting from the ring closure of the diradicai. 

The three dimensional shapes of many proteins are governed and stabilized by S—S bonds connecting what would ordinarily be remote segments of the molecule We 11 have more to say about these disulfide bridges m Chapter 27... 

Disulfide bridge (Section 27 7) An S—S bond between the sulfur atoms of two cysteine residues in a peptide or pro tein... 

This mechanism not only accounts for the substitution of the more labile chlorine atom on the polymer chain, it also results in the elimination of a new potential initiation site by moving the double bond out of conjugation with any adjacent chlorine atoms. The newly formed C—O or C—S bonds, with AH > 484 kJ/mol (100 kcal/mol), are significantly more thermally stable than even the normal C—Cl bonds in PVC at about 411 kj/mol (85 kcal/mol) (11). 

There are two manganese(II) sulfides, MnS and MnS2. Manganese(II) disulfide contains a S—S bond and has a pyrite stmcture. When a solution of a manganous salt is treated with ammonium sulfide, a flesh-colored hydrated precipitate is formed which is comprised of MnS and Mn(II)S2. This mixture very slowly changes to the mote stable green-black MnS. 

However, conventional systems ia natural mbber do provide better flex life than EV cures, and this is one of the limitations of EV curiag. The short monosulftde bonds are less able to rearrange to reheve localized stresses which build duriag flexing, whereas the longer S bonds can. This abiUty for stress rehef is thought to be the mechanism for the superior flex life of conventional cures. 

Two of the more recendy developed polysulftde polymers are the mercaptan-terminated polyoxypropylene urethane polymer and the polythioether polymer. The urethane-backbone-based polymer is used in many sealant formulations for insulating glass appHcations. The thioether backbone contains sulfur, but no S—S bonds, which are the weakest part of the conventional polysulftde polymer. This polymer improves the thermal stabiHty and reduces the gas—Hquid permeabiHty. 

Most of the reactions of thiophosgene involve the expected chemistry of an acid chloride, in which the chlorine atoms are replaceable by various nucleophiles. A reaction involving the C=S bond is the Diels-Alder addition ... 

Manufacture. Trichloromethanesulfenyl chloride is made commercially by chlorination of carbon disulfide with the careful exclusion of iron or other metals, which cataly2e the chlorinolysis of the C—S bond to produce carbon tetrachloride. Various catalysts, notably iodine and activated carbon, are effective. The product is purified by fractional distillation to a minimum purity of 95%. Continuous processes have been described wherein carbon disulfide chlorination takes place on a granular charcoal column (59,60). A series of patents describes means for yield improvement by chlorination in the presence of dihinctional carbonyl compounds, phosphonates, phosphonites, phosphites, phosphates, or lead acetate (61). 

SuIfona.tlon, Sulfonation is a common reaction with dialkyl sulfates, either by slow decomposition on heating with the release of SO or by attack at the sulfur end of the O—S bond (63). Reaction products are usually the dimethyl ether, methanol, sulfonic acid, and methyl sulfonates, corresponding to both routes. Reactive aromatics are commonly those with higher reactivity to electrophilic substitution at temperatures > 100° C. Tn phenylamine, diphenylmethylamine, anisole, and diphenyl ether exhibit ring sulfonation at 150—160°C, 140°C, 155—160°C, and 180—190°C, respectively, but diphenyl ketone and benzyl methyl ether do not react up to 190°C. Diphenyl amine methylates and then sulfonates. Catalysis of sulfonation of anthraquinone by dimethyl sulfate occurs with thaHium(III) oxide or mercury(II) oxide at 170°C. Alkyl interchange also gives sulfation. 

Chemisorption of alkanethiols as well as of di- -alkyl disulfides on clean gold gives indistinguishable monolayers (251) probably forming the Au(l) thiolate species. A simple oxidative addition of the S—S bond to the gold surface is possibly the mechanism in the formation of SAMs from disulfides ... 

The low value of the S—S force constant compared to that of S—O is consistent with the ease of cleavage of the S—S bond. Spectral data indicate that the stmcture of the thiosulfate ion in soHd thiosulfates is the same as that of the ion in solution. 

X-ray crystallographic analysis of the sodium thiosulfate pentahydrate [10102-17-7] crystal indicates a tetrahedral stmcture for the thiosulfate ion. The S—S bond distance is 197 pm the S—O bond distance is 148 pm (5). Neutron diffraction of a barium thiosulfate monohydrate [7787-40-8] crystal confirms the tetrahedral stmcture and bond distances for the thiosulfate ion (6). 

In the presence of a large excess of acid, sulfones such as diphenyl sulfone [127-63-9] (C H )2S02, can be formed (see Sulfolanes and sulfones). Sulfamation forms a —C—N—S— bond as in sodium cyclohexylsulfamate [139-03-9], C H NHSO Na, (see Sulfamic acid and sulfamates). Reviews of chlorosulfuric acid reactions are available (21,22). 

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