The preparation of optically active thiols via iso-thiuronium salts : addition and other reactions of asymmetric thiols

• Main Author: Hallgarten, Peter Alexander
• Published: University of Surrey 1956
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.751490

(-)-Octane-2-thiol has been prepared by alkaline decomposition of the thiuronium salt derived from thiourea and (+)-2-bromo-octane. Fractional crystallisation of (+)-2-octyl (+)-camphor-1C-sulphonate failed to effect resolution into the diastereoisomeric salts. (+)- and (-)-1-Methyl-2-phenylethyl toluene-p-sulphonate have been converted into the thiuronium toluene-p-sulphonates, which on decomposition gave (-)- and (+)-1-phenylpropano-2-thiol. A mechanism is proposed for the reaction of thiourea with 2-bromo-octane and with 1-methyl-2-phenylethyl toluene-p-sulphonate. Addition (peroxide- and base-catalysed) of (+)-, (+)-, and (-)-1-phenylpropane-2-thiol to w-nitrostyrene has been investigated, two diastereoisomeric sulphides (or sulphones) being obtained in each case; configurational relationships are proposed for these products. With neither catalyst is the reaction sterically unilateral. It is tentatively concluded that free radical addition proceeds in a symmetrical manner, whereas the base-catalysed addition proceeds disymmetrically. (-)-1-Methyl-2-phenylethyl 2:4-dinitrophenyl sulphone has been shown to be optically stable under acidic conditions employed for oxidation. Addition of (+)-1-phenylpropane-2-thiol to trans-1:2-dibenzoylethylene gave a solid and an oily sulphide. Oxidation of these sulphides, in glacial acetic acid with hydrogen peroxide gave a common sulphone; enolisation of the keto-sulphide, or sulphone, is thought to occur. Oxidation with the hot reagent causes elimination; a tentative mechanism is proposed. Oxidation of the benzyl thiol addition product also proceeds with (partial) elimination in hot solution. From the addition of (+)-1-phenylpropane-2-thiol to 4'-nitrochalkone a single isomer of (+)-2-(p-nitrobenzoyl)-1-phenyl-1'-benzyldiethyl sulphide has been isolated; the benzyl thiol addition product of 4'-nitrochalkone has also been prepared. Alkylation reactions of (+)-1-phenylpropane-2-thiol have been carried out with two symmetrical carbinols (yielding solid sulphides) and with four disymmetric carbinols, of which two yielded solid sulphides; a single isomer of (+)-1-methyl-2-phenylethyl p-diphenylphenylmethyl sulphide and two isomers of (+)-1-methyl-2-phenyl ethyl p-dimethylamino-diphenylmethyl sulphide have been isolated.