Sulfur deficiency

The reaction is initiated by the attack of the organolithium at sulfur to produce the pentacoordinate sulfur species. This seems to be the prime reaction of all electron-deficient sulfur compounds with organolithiums (Eq. 45). 

In the previous examples, the sulfur atom acted as a nucleophile. Electron-deficient sulfur species such as sulfenyl ion and its equivalents (e.g. disulfide/Lewis acid complexes, sulfenic acids, sulfenyl halides, sulfonium ions, sulfines, etc.), can also serve as an electrophile. Oxidative ring closure of enethiols (a-thioketocarboxylic acid) (124), which proceeds via disulfides, produces thiophenes (125) in good yields (86EUP158380, 88JHC367). 

Oxidation of alkyl phenyl sulfides by pyridinium bromochromate (PBC) is accelerated by electron-donating alkyl groups or aryl substituents, indicating an electron-deficient sulfur centre in the transition state this is accounted for in terms of rate-determining electrophilic oxygen attack from PBC to the sulfide in an. S -like process.7... 

Molybdenum(VI)-catalysed perborate oxidation of sulfides is first order with respect to the sulfide and Mo(VI) but zero order in perborate. The uncatalysed reaction is first order in each the reductant and oxidant. Trichloroacetic acid enhances the oxidation rate. Oxidation of para-substituted. S -phenylmcrcaploacclic acids yielded a Hammett p of -0.54 at 293 K, indicating an electron-deficient sulfur atom in the transition state. 

Benzene itself is not attacked by concentrated sulfuric acid, but is readily converted to benzenesulfonic acid by fuming sulfuric acid. This is a solution of sulfur trioxide in concentrated sulfuric acid, and is known as oleum. Note here that the attacking electrophile is a neutral species and that the electron-deficient sulfur atom of SO3 is the electrophilic centre (Scheme 2.7). 

Various types of electron-deficient sulfur diimides react as hetero-dienophiles. In general, these cycloadditions occur under conditions similar to those used for AT-sulfinyl compounds. However, fewer types of sulfur diimides have been utilized in this process relative to Af-sulfinyl compounds. Some examples of symmetrical sulfur diimide Diels-Alder reactions are listed in Table l-II. It should again be noted that the orientational selectivity in these cycloadditions is the same as that shown by N-sulfinyl systems (cf. Table l-I). Several examples of cycloadditions with unsymmetrical sulfur diimides are shown in Table l-III. In all cases, these reactions were totally regioselective, and as noted above, reactions occurred at the least electron-deficient nitrogen-sulfur bond. Frontier molecular orbital (FMO) theory has been used to rationalize the regio-selectivity of addition of the cationic sulfur diimide shown in entry... 

Sulfur deficiency is estimated to affect about 25 million ha of cultivated land across the region and zinc deficiency is also extensive. For example, in Bangladesh, approximately 4 million ha of cultivated land is estimated to suffer from sulfur deficiency. Sulfur and zinc deficiency is widespread in flooded rice soils (Saheed, 1992). 

Thermal cycloadditions of chiral vinylsulfoxides B with cyclopentadiene, 1,3-cyclohexadiene and 1,3-butadiene have been examined in toluene at high temperature leading to the adducts in moderate selectivities. The electron-deficient sulfur residue activates the alkene unit and serves as a temporary substituent that can be easily removed by reduction or transformed into a variety of other functionalities. An increase in reactivity and excellent selectivities are observed in the presence of additional electron-withdrawing substituents at the alkene unit as shown for the examples and in the table below. At 20°C in dichloromethane the cycloadditions of C with cyclopentadiene proceed smoodily and highly stereoselective. In reactions of menthyl-3-(3-trifluoromethyl-2-pyridylsulfinyl)acrylate A with dienes, e.g. 2-methoxyfliran or cyclopentadiene, high diastereoselectivities (>98 2) were obtained, too. 

The second is a complex of sulfenium cation and superoxide and explains the near equivalence of the spin density on the two peroxyl oxygens in thiyl peroxyls. Both valence structures display an exposed electron-deficient sulfur that makes it susceptible to nucleophilic solvents. Complexing with a lone pair donor is expected to stabilize the charge-separated state increasing its contribution to the ground state. A partial radical-cationic character of the sulfur could also make three-electron bonding with some solvents contribute to the charge separation. 

The reaction of A-(/ -chlorosulfonylphenyl) maleimide 397 with amines R R NH (two equivalents), gave the corresponding maleimidosulfonamides 400 which resulted from nucleophilic substitution by the amine at the electron deficient sulfur atom. 

Surface oxidation of sulfide minerals has been reviewed recently by Smart et al. [78]. Studies of the physical and chemical forms of oxidation products by SAM, XPS, STM. AFM, SEM and ToF-SIMS have revealed several different proces.ses of oxidation. The seminal work of Buckley et al. (e.g., [79,80)) was the first to identify the process of formation of oxyhydroxide products on underlying metal-deficient, sulfur-rich layers of similar forms to those described in Section 4.3. Other oxidation products have been observed directly, such as polysulfides, elemental sulfur, oxidized fine sulfide particles, colloidal hydroxide particles and flocculated aggregates, as well as continuous surface layers of... 

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