Alkylthio group, electronic effects

Neighboring group participation effects appear to play a crucial role in the nucleophilic substitution of chlorine in Michael adducts of 1-R, 2-R, 3-X. Thus, this substitution proceeds very easily in any of the adducts formed with an electron rich nitrogen, sulfur and oxygen Michael donor. For the adducts of nitrogen nucleophiles, the facile substitution of the chlorine has been suggested to occur via formation of intermediate aziridinium ions 103 [8] (Scheme 32), and this postulate was later supported by isolation of azaspiropentane derivatives under appropriate conditions in several reactions (see Sect. 3.2.2) [11b, 53,56]. It is most likely that alkylthio substituents in adducts of type 85 participate in the same way to first form spirocyclopropane-annelated thiiranium ion intermediates which are subsequently opened by attack of the incoming nucleophile. 

The EPR spectra of the radical cations derived from pyrrole solutions can all be simulated by electronic structures in which the unpaired electron is located in an orbital with the nodal plane on the nitrogen and (2) showing large coupling constant values at the 2- and 5-positions and small values at the 3- and 4-positions <2000J(P2)905>. The EPR parameters of the radical cations from 2,5-dimethyl-l-phenylpyrroles and 3,4-bis(alkylthio)-2,5-dimethyl-l-phenylpyrroles (Table 30) denote a marked stabilization of the radical cations by the sulfanyl groups through mesomeric effects. 

A different picture was observed for SETA, which contains a /3-positioned acetyl group. The main pathway involves the formation of hydroxysulfuranyl radicals SETA(>S -OH) (A, = 340 nm), and highly stabilized through captodative effect a-(alkylthio)alkyl radicals H3C-CH2-S- CH-C(=0)-CH3 (A , = 380 nm). At low pH, SETA (>S -OH) radicals undergo efficient conversion to intermolecularly three-electron-bonded dimeric radical cations SETA(S.. S) ... 

Sheppard WA (1963) The effect of fluorine substitution on the electronic properties of alkoxy, alkylthio and alkylsulfonyl groups. J Am Chem Soc 85 1314-1318. doi 10.1021/ja00892a021... 

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