At sulfur

Nucleophilic reactivity of the sulfur atom has received most attention. When neutral or very acidic medium is used, the nucleophilic reactivity occurs through the exocyclic sulfur atom. Kinetic studies (110) measure this nucleophilicity- towards methyl iodide for various 3-methyl-A-4-thiazoline-2-thiones. Rate constants are 200 times greater for these compounds than for the isomeric 2-(methylthio)thiazole. Thus 3-(2-pyridyl)-A-4-thiazoline-2-thione reacts at sulfur with methyl iodide (111). Methyl substitution on the ring doubles the rate constant. This high reactivity at sulfur means that, even when an amino (112, 113) or imino group (114) occupies the 5-position of the ring, alkylation takes place on sulfiu. For the same reason, 2-acetonyi derivatives are sometimes observed as by-products in the heterocyclization reaction of dithiocarba-mates with a-haloketones (115, 116). 

A-2-Thiazoline-4-ones are usually obtained by the heterocydization method (38b-388). 2 Alkylthio-4(5)-thiazolones (162) are obtained by alkylation at sulfur of rhodanine (160) in nonpolar solvent (Scheme 85). 

Tricoordmate sulfur compounds are chiral when sulfur bears three different sub stituents The rate of pyramidal inversion at sulfur is rather slow The most common compounds m which sulfur is a chirality center are sulfoxides such as... 

The absolute configuration at sulfur is specified by fhe Cahn-Ingold-Prelog mefhod wifh fhe provision fhaf fhe unshared elecfron parr is considered fo be fhe lowesf ranking subsfifuenf... 

The name of the parent six membered sulfur containing heterocycle is thiane It is num bered beginning at sulfur Multiple incorporation of sulfur in the ring is indicated by the prefixes di tri and so on... 

Make a molecular model of dimethyl sulfide How does its bond angle at sulfur compare with the C—O—C bond angle in dimethyl ether"d... 

R. A. Belbutowski, paper at Sulfur Markets Symposium, Sulfur Institute, Washington, D.C., Mar. 24, 1994. 

Hydrolysis of dialkyl sulfites under acidic and alkaline conditions, which is followed by the use of OH2, proceeds by attack at sulfur to give S—O cleavage (72). The rate of hydrolysis is generally faster for cycHc and aryl sulfites than for dialkyl sulfites (73). Activation parameters of hydrolysis are known for some sulfites, and the increased rate for ethylene sulfite results from a reduced entropy of activation which results from a rigid ring stmcture (74). 

In the reaction of ethylene with sulfuric acid, several side reactions can lead to yield losses. These involve oxidation, hydrolysis—dehydration, and polymerization, especially at sulfuric acid concentrations >98 wt % the sulfur thoxide can oxidize by cycHc addition processes (99). 

Reactions with aldehydes and ketones. 4.11 Oxidation Attack at Sulfur... 

Isothiazoles and isothiazolium cations are attacked by carbanions at sulfur and on recyclization can give thiophenes, illustrated by (147) -> (148). 2-Alkyl-3-isothiazolinones e.g. 149) are also vulnerable to nucleophilic attack at sulfur (72AHC 14)1). 

Nucleophilic attack at sulfur is implicated in many reactions of 1,2,4-thiadiazoles generally, soft electrophiles attack at sulfur, cf. (150)— (151). -Butyllithium with 4,5-diphenyl-l,2,3-thiadiazole yields PhC = CPh, probably by initial nucleophilic attack at sulfur. 

The pattern of reactivity is similar to that discussed for the azolinones in Sections 4.02.1.1.4 and 4.02.3.7.1. A difference is the greater nucleophilicity of sulfur, and thus more reaction of the ambident anion with electrophiles occurs at sulfur. 

Benzisothiazoles also suffer N—S bond cleavage, following attack at sulfur, but 1,2-benzisothiazole 1,1-dioxides are cleaved at the C—N bond. Saccharin derivatives are attacked at the carbonyl function. In cases where N—S bond cleavage occurs, recyclization can sometimes occur, often producing thiophene compounds. 

Amines normally have no effect on the isothiazole ring, but 3-chloro-4-nitroisothiazole (49) reacts with cyclic amines such as morpholine at 0 °C to give the enamine (50) (75JOC955). The mechanism may involve attack either at sulfur or at the 5-position (Scheme 6). 

Benzisothiazole is lithiated at the 3-position, which corresponds to the 5-position in the mononuclear series (75JHC877). 4-Methylisothiazole forms the 5-lithio derivative, but the presence of by-products produced in subsequent reactions suggests the possibility of lithi-ation at the 3-position also (72AHC(14)l). 3-Substituted 1,2-benzisothiazoles suffer attack at sulfur and cleavage of the N—S bond (72AHC(14)43, 73SST(2)556). 

Scheme 2.1. Chiral Compounds with Stereogenlc Centers at Sulfur and Phosphorus...

The absolute configuration at sulfur is specified by the Cahn-Ingold-Prelog method with the provision that the unshared electron pah is considered to be the lowest ranking substituent. 

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