Sulfur 53 : anion

D. Stalke, Polyimido Sulfur Anions [S(NR)n] Free Radicals and Coordination Behaviour, Proc. Ind. Acad. Sci. Chem. Sci., 112, 155 (2000). 

Fig. 5 Electropherogram of six sulfur anions illustrating peak fronting [thiosulfate (S2Ol-)] and peak tailing [tetrathionate (S4Ol-)] for better visibility of peak asymmetry, the perpendicular peak axis is drawn as a solid line. Separation in 20 mM chromate run buffer at pH 8.3.

Cyclic thiocarbonates, such as compound 169, react smoothly with allylmagnesium bromide. Careful control of the reaction conditions allows monoalkylation. Trapping of the intermediate sulfur anion with Mel provided the 1,3-dioxane in 78% yield (Equation 20) <2003H(59)87>. 

Included in this section are the conversions of azepin-2-ones to imidoyl chlorides and to thiones, as these processes may be viewed as nucleophilic attack by halide or sulfur anions on the modified 2-hydroxyazepine. 

For nucleophiles of different atomic numbers, nucleophilicity usually does no parallel basicity. For example, for the halogens the reactivity sequence I > Br > Cl is opposite to the sequence of basicity Cl > Br > I . Similarly, sulfur anions such as HS are better nucleophiles but weaker bases than corresponding oxyanions such as HO . 

Massoth30 proposed that the activity for thiophene HDS depended on a terminal sulfur anion vacancy (CHS) being created in flowing H2S/H2. This proposal for activity in desulfurization led us to assume a particular kind of site for the HDN of carbazole on the reduced and sulfided Mo03/A1203 catalyst.31 Carbazole was considered to be adsorbed more strongly on an O pair than on an CIS pair on a molybdenum atom. 

It seems clear therefore that more reactive cations than those for which Ritchie s N+ relationship was developed, show a distinct dependence of selectivity between nucleophiles upon the stability and reactivity of the carbocation. Richard has confirmed that for a very stable benzylic carbocation, represented by the bis-trifluoromethyl quinone methide 57, the N+ regime is restored and that a plot of log k against N+ for reactions of nucleophiles, including amines, oxygen and sulfur anions, the azide ion, and a-effect nucleophiles, shows a good correlation with N+.219... 

Metal disulfides (MS2) and metal dichalcogenides (MS4) (metal M = Cd, Mo, Zr, Ti, Nb, Ta, or Sn) are lamellar colloids with strong M—S bonds within the layers and weak van der Waals interactions between the layers. Each metal atom in these lamellae is surrounded by six sulfur anions with an approximate octahedral coordination geometry. Each sulfur anion is, in turn, coordinated to three metal atoms with a trigonal pyramidal geometry [50], Certain cationic species or polymers are adsorbed onto these surfaces via intercalation or exfoliation [51,52],... 

Although a large number of thieno[2,3-af]pyrimidines have been synthesized from pyrimidine precursors, most have evolved from pyrimidines bearing a carbonyl moiety at C-5 and an alkylated sulfur at C-6. Either 6-thiol- or 6-chloropyrimidines may be used. In the latter case, displacement of the chlorine by a sulfur anion is commonly used. In general, pyrimidines of the type (264) are the immediate precursors or are formed in situ. For most of the investigations the derivatives (264 R = R3 = Me R2 = aryl) have been used. The cyclization to the products (265) can be effected by... 

Like oxygen, sulfur is more electronegative than nitrogen. Additionally, sulfur is more polarizable. These differences stabilize the sulfur anion as reflected in the pKa value for methyl sulfide (10.4) compare to the pKa value for amines (35). Therefore, the methylsulfide anion is the better leaving group. 

Now draw the mechanism. It is easy once you have done the preparatory thinking. The sulfur anion must be the nucleophile so the C-O bond in the three-membered ring must be the electrophile. Here goes ... 

Multistep synthesis according to Eq. (110) affords sulfur anions 132a and 132b with unusual coordination via alcohol 131 (207). The spirocyclic anion 132b has been estimated to be 13 kJ/mol more stable than its open-chain isomer 132c. 

The formation of III and IV can both be understood as a series of addition-elimination reactions occurring on the central C=C bond of I, by the works of sulfur anions liberated by the interaction of o-benzodithiol (II) and trimethyl-amine. However, the direction of the addition reaction cannot be ascertained at first glance, because some confusion may occur if the relative stability of the carbanions resulting from this operation is not carefully analyzed first. For this purpose, there are the handy Baldwin s Rules, although these laws face some problems when applied to second-row elements such as sulfur. Therefore, they will be disregarded here. 

The last step is an 8 2 reaction of the nucleophilic sulfur anion with methyl iodide to give the methylated sulfur. 

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