In the gas phase, both N -t -butylthioacetamide and N-acetylthioacetamide undergo unimolecular firstorder elimination reactions for which log A = 11.58 s-' and 10.64 s-', and €, = 16.45 kJ mol-' and 117.1 5 kJ mol-', respectively. The results are in accord with a reaction pathway involving a cyclic six-membered transition state, and show each compound to be more reactive than its oxygencontaining analogue. At 600 K, the statistically corrected reactivity ratios: t-butyl thioacetate (1 ) / t-butyl acetate (2); N-acetylthioacetamide (3)/diacetamide (4); and N -t -butylthioacetamide (5)/N-t-butylacetamide (6) are 83, 173, and 1 404, respectively. The above rate factors are consistent with the tenet that as C,-X bond fission becomes less rate-contributing in these electrocyclic reactions, so attack by the C=Y bond upon the P-hydrogen atoms becomes more important. Thus, whereas t-butyl acetate at 600 K is some 68 700 times more reactive than N-t-butylacetamide, t-butyl thioacetate is only 4 060 times more reactive than N -t -butylthioacetamide.The gas-phase thermal elimination of esters, amides, and anhydrides containing P-hydrogen atoms are thought to involve pathways in which the transition states approximate to cyclic six-membered structures (Scheme 1).

Nine substituted sulfonyl-stabilized phosphorus ylides were prepared by treating their intermediate ylide analogues with phenylmethanesulfonyl fluoride. The stoichiometric ratio of the reactants for each preparation needed to be adjusted according to the basicity of each ylide intermediate. The nine ylide compounds were then subjected to conventional (sealed-tube) gas-phase pyrolysis at 470–545 K. The pyrolytic reactions were homogeneous and obeyed a first-order rate equation. The values of the Arrhenius log A (s–1) and Ea (kJ mol–1) obtained for these reactions averaged 11.12 ± 2.00 and 131.8 ± 24.4, respectively. Analysis of the pyrolysates from conventional pyrolysis and from flash vacuum pyrolysis at 600 K showed the products to be complex mixtures of triphenylphosphine, triphenylphosphine oxide, triphenylphosphine sulfide, and symmetric and unsymmetric alkenes. Conventional pyrolysis also gave novel mixed sulfones and, for the p-methoxyaryl substituent, p-anisaldehyde. The products of the reactions under study are explained on the basis of a mechanism involving a sulfonyl carbene intermediate, and the reaction mechanism is used to rationalize the kinetic results and molecular reactivities.Key words: ylides, synthesis, pyrolysis, kinetics, mechanism.

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Fourteen ketone/thione-stabilized triphenylphosphonium methylides were subjected to conventional gas-phase and flash vacuum pyrolysis (FVP). The kinetics of the first-order thermal gas-phase reactions of all these compounds were investigated over 360-653 K temperature range. The values of the Arrhenius log A and energy of activation of these ylides averaged 11.52 W 0.34 s S1 and 133.20 W 3.14 kJ mol S1 , respectively. The products of sealed-tube (static) and FVP were analyzed and compared. A mechanism is proposed to account for the products of reaction. The rate constants [k (s S1 )] of the substrates at 500 K were calculated and used to substantiate the proposed mechanism of pyrolysis, and to rationalize the thermal gas-phase reactivities of the ylides under study.