Thion groups

The thiones are readily desulfurized with Raney nickel to give the corresponding unsubstituted compounds in bicyclic systems in the 2-, 4- and 7-positions, and in tricyclic systems such as (95). The 2-methylthio derivatives may be similarly desulfurized. Thione groups in the 4-position, but not the 2-position, in pyrido-[2,3- f ]- and -[3,2- f]-pyrimidines may be replaced directly with ammonia or amines. 

This process of aging is believed to be critical in the development of delayed neuropathy, after NTE has been phosphorylated by an OP (see Chapter 10, Section 10.2.4). It is believed that most, if not all, of the B-esterases are sensitive to inhibition by OPs because they, too, have reactive serine at their active sites. It is important to emphasize that the interaction shown in Fignre 2.11 occurs with OPs that contain an oxon group. Phosphorothionates, which contain instead a thion group, do not readily interact in this way. Many OP insecticides are phosphorothionates, but these need to be converted to phosphate (oxon) forms by oxidative desulfuration before inhibition of acetylcholinesterase can proceed to any significant extent (see Section 2.3.2.2). 

Phosphorothionates OP compounds containing thion groups (cf. organophos-phates that contain oxon groups). 

Tautomeric equilibria. The presence of a hydrogen at the a carbon near the thione group induces the thione-enethiol tautomerism, Scheme 1, where C=S turns into the more stable C S single bond.1,2,3 Calculated enolization enthalpies4,5 show that C=0 is stronger than C=S. 

Electron acceptor C=S. The thione group in 13 reacts with hydrazine hydrate (and other /V-nucleophiles),200 forming 14 (Scheme 15). 

The l,3-dioxol-2-one ring in these compounds is relatively stable, thereby allowing acetylation, reduction, and the replacement of the keto group by a thione group giving photochromes 243-245 (02IZV1588, 05IZV1299). 

In some additions the thione group alone can also be a reaction site as with PhS02NCO which adds with its NC bond (Scheme 24) <92JHC69> and PhCNO (Scheme 25) <90JPR181>. 

Thioureas (211) combine with hexafluorothioacetone (212) to give 1,2,4-dithiazoline derivatives (213). The proposed mechanism involves nucleophilic attack by the nitrogen atom of thiourea on the carbon atom of the thione group in (212) (Equation (24)) <90CB69l). 

The thione group of dithiazolethiones is a very reactive heterodipolarophile. In Scheme 18 are given cycloadditions with nitrile oxides <67BSF2239>, diphenylnitrilimine, and ethyl azidoformate <85JCS(P1)1205>. The primary adducts are spiro derivatives, but only compound (131), which is obtained from nitrile oxides is isolable at low temperature. All are decomposed to give respectively compounds (132)-(134) and occasionally nitriles and sulfur. Compound (134) reacts further with nitrilimine affording compound (135) which is also isolated. 

Some reactions concerning the thione group of dithiazole thiones (126) have been discussed (see Section 4.14.5.7). As expected, the thione group is methylated with Me2S04 affording, after treatment... 

Photocycloaddition of an alkene to the thione group of 13 gave the thietane 14 which was stable at room temperature, but on refluxing 14 in toluene an iminothietane 15 and/or a 2-substituted benzoxazole were obtained by rearrangement processes <99JCS(P1)1151>. 

Anodic oxidation of heterocyclic thiones leads generally to disulfides. Thus cyclic voltammetric data at a pyrolytic graphite anode of purine-2,6-dithione show three peaks. The first and second correspond to a disulfide formation from the 6- and 2-thione groups, respectively, whereas the third is due to an oxidation to purine-2,6-disulfonic acid.432 Similarly, the electrochemical oxidation of benzthiazole-2-thione and benzimidazole-2-thione in CH3CN-NaC104 at a platinum electrode afforded the corresponding disulfides in good yield.433... 

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