Thiols dehalogenation

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

It has been shown that thiols can carry out dehalogenation both in the presence and absence of quinone mediators ... 

Organochalcogen(II) compounds, 100-102 Organochalcogen(IV) compounds, 100-102 Organoselenium compounds dehalogenation reactions, 96 electrochemical reduction, 113-117 haloperoxidase-like activity, 108-113 with odd number of ligands, 100-102 one-electron oxidation, 117-118 oxidation of thiols, 102-106 redox reactions, 79-80 thioperoxidase-like activity, 108-113 Organotellurium compounds... 

Lewis, T. A., M. J. Morra, and P. D. Brown, Comparative product analysis of carbon tetrachloride dehalogenation catalyzed by cobalt corrins in the presence of thiol or titanium(III) reducing agents , Environ. Sci. Technol., 30, 292-300 (1996). 

DEHALOGENATION Aluminum chloride-E thane thiol. Sodium sulfide. Zine borohydride. 

Although many nucleophiles dehalogenate a-halocarbonyl compounds, the most generally used have been halides, especially iodide ion, thiols and other nucleophilic sulfur species, and phosphines. All of these are effective by themselves, but all benefit by addition of an electrophile to the medium to assist cleavage by coordination to the carbonyl (equation 11). 

Thiols efficiently dehalogenate a-halo and a,a-dihalo ketones. Reduction is typically carried out in ethanol containing a weak base. Although this system will also reduce a-thio ketones, it is believed that the halo ketone is reduced, and that reaction proceeds as in equation (11), with a proton from the solvent serving as Lewis acid to assist dehalogenation by thiol (or thiolate). The reactions are of course accelerated by inclusion of a metal salt in the medium. ... 

Reversible addition of ) -thiolethanol to the 5,6-bond in l,3-dimethyl-5-nitrouracil shows 76% of the 6-adduct in the equilibrium in solution. In 5-halo-uracils and -uridines, adduct formation with thiols, including cysteine, may be accompanied by subsequent dehalogenation, as was also the case for bisulfite adducts . 

Examples of application of the iert-dodecanethiol/silane couple include typical reductive dehalogenations, but also Barton-McCombie deoxygenations of alcohols, through conversion of the latter into xanthate esters (eq 6), hydrosilylations of alkenes (eq 7), and preparation of silanethiols. The thiol can also be used as a polarity reversal catalyst in conjunction with tris(trimethylsilyl)silane and hexabutyldistannane/malonic acid. ... 

---