The mechanism of thermal eliminations. Part 24. Elimination from mono-, di-, and trithiocarbonates. The dependence of the transition state polarity, thione to thiol rearrangement, and ether formation via nucleophilic substitution, on compound type

Abstract

We have measured rates of thermal decompositon and Arrhenius parameters for S-alkyl O-phenyl thiocarbonates, O-alkyl O′-phenyl thiocarbonates, S-alkyl O-phenyl and O-alkyl S-phenyl dithiocarbonates (xanthates), and alkyl phenyl trithiocarbonates between 671.4 and 819.2 K. The reactivity order is: PhOCSOR > PhOCO₂R > PhSCSOR > PhSCO₂R > PhOCSSR > PhSCSSR > PhOCOSR > (PhSCOSR). Compared with the pyrolysis of acetates the rate decrease accompanying change of carbonyl to thiocarbonyl is smaller, whilst that accompanying change of OR to SR is greater, because the transition state for carbonate pyrolysis is more E1-like within the overall semi-concerted process. The accelerating effect of thion sulphur is greatest for ethyl derivatives which have the least E1-like transition state. The Prⁱ/Et rate ratios at 700 K are: 30.7(PhOCO₂R), ⩽17.4(PhOCOSR), 2.0(PhOCSSR), 1.6(PhSCSSR), and for the latter two classes of compounds attack on the β-C–H bond, rather than C_α–X bond breaking may be the most important rate-determining step. Compounds containing thione sulphur and O-alkyl groups (but not O-phenyl groups) undergo sulphur–oxygen exchange. A mechanism for this exchange is given which accounts both for these structural aspects and the much slower exchange in thionacetates. For carbonates, exchange is more severe for compounds which have the slowest competing elimination. Compounds PhSCSSPrⁱ, PhSCSSEt, PhOCSSEt, and PhOCOSEt each gave abnormally low activation energies (and low stoicheiometry) due to competing nucleophilic substitution which gives ethers. This reaction is predicted to be important also for compounds PhSCOSR.