Gem-Dithiols

Addition of hydrogen sulfide results in formation of monomeric gem dithiols or trimeric thioketals (511,512). The initially reported thione formation (513-515), analogous to hydration of morpholinocyclohexene... 

For a review of the preparation of gem-dithiols, see Mayer Hiller Nitzschke Jentzsch Angetv. Chem. Int. Ed. Engl. 1963,2. 370-373 [Angew. Chem. 75. 1011-1014). 

However, for aliphatic low-molecular-weight members of this class, oligomerization and/or polymerization, formation of gem-dithiols and enethiolization are often observed. The purification steps can be tedious and the obtention of pure products difficult. There is still a need for progress in this field. Although much less used, another general method whose scope appears important [127] involves the reaction of ketimine... 

Heterocyclization of a-mercapto acids with (azolylimino)indolones 182 (Scheme 36) afforded the spirocyclic thiadiazepinones 183 <1989IJB639, 1994PS27>. gem-Dithiol addition to bis(methylene)-substituted diketopiper-azine 168 afforded the bridged bicyclic structure 169 featuring two dithiazepane rings (Scheme 33) <1981JHC1545>. 

It has been shown (64JHC163) that a-halo ketones readily form 1,3-dithioles with gem-dithiols. A convenient alternative pathway for the synthesis of certain l,3-dithiol-2-ylidene derivatives (295) is the reaction of a-halo ketones with dithioacids of type (294) which exist as a,(3-unsaturated gem-dithiols (65JOC732). For the synthesis of dithiafulvene derivatives via (4,5-diethoxycarbonyl-l,3-dithiolyl)tributylphosphonium tetrafluoroborate, see (79JOC930). 

There are two candidates for this dreadful smell—propane dithiol (called acetone gem-dithiol above) or 4-methyl-4 Sulfanylpentan-2-one. It is unlikely that anyone else will be brave enough to resolve the controversy. 

Long reaction times lead to the disappearance of the blue color of thione 1, related to the addition of a second molecule of hydrogen sulfide to give colorless gem-dithiols 3. Aromatic derivatives (R = Ar) are less prone to further addition of hydrogen sulfide, whereas with aliphatic derivatives (R = alkyl), gem-dithiols are generally the final reaction products and can be isolated and fully characterized.18 gem-Dithiols 3 can be converted into thioacylsilanes 1 by neutralization of the thionation solution with solid sodium hydrogen carbonate. With this procedure, enolizable acylsilanes 2 (R = R CH2) are stereoselectively transformed into Z-a-silyl enethiols 4 (vide infra). 

Hydrogen sulfide also reacts by nucleophilic substitution with alkyl halides and alcohols under appropriate conditions to form thiols (Scheme 10) (see Chapter 4, p. 42). The reaction with an alkyl halide is generally facilitated by using sodium hydrogen sulfide (NaSH) rather than hydrogen sulfide since NaSH is a much better nucleophile. Hydrogen sulfide also adds to ketones to give gem-dithiols (13) (Scheme 11). 

Thioketones (6) can be obtained by the acid-catalysed reaction of ketones with hydrogen sulfide (Scheme 2). The course of the reaction is dependent on the reaction temperature, the nature of the solvent, the concentration of the ketone and the stability of the thioketone (6), especially in relation to enolisation. This appears to be the most generally useful preparative route to thioketones, and many simple aliphatic derivatives are obtained by performing the reaction in ethanol at low temperature (-80°C to -55°C). The gem-dithiol (9) may also be converted into the thioketone (6) by heating it at approximately 200°C in the presence of a basic catalyst (Scheme 2). Reasonably stable thioketones, e.g. aromatic and heterocyclic derivatives like (10) and (11), can be prepared by heating the corresponding ketones with phosphorus pentasulfide in boiling toluene, pyridine or xylene (see Chapter 2, p. 21) (Scheme 3). 

It is known that gem-dithiols can be formed by the reaction of a ketone and HjS under pressure or under mild conditions using HCl as a catalyst. Give a mechanism for the conversion of 2-butanone to the gem-dithiol using HgS with a HCl catalyst. If this dithiol is then mixed with cyclopentanone and an acid catalyst, comment on the likelihood of forming a dithioketal. 

---