Synthesis, thermal reactivity and kinetics of substituted [(benzoyl)(phenylcarbamoyl)methylene]triphenylphosphoranes and their thiocarbamoyl analogues

Synthesis, thermal reactivity, and kinetics of stabilized phosphorus ylides, part 2: [(Arylcarbamoyl)(cyano)methylene]triphenylphosphoranes and their thiocarbamoyl analogues

“…The relative rates of gas-phase pyrolysis of the substrates are recorded in Table III. The greater reactivity of the thiocarbamoyl ylides 3 over their carbamoyl analogues 2, a feature also noted in Table I 13 C NMR Spectra of Ylides 2 and 3, δ C (J P C ) Part 1 [6], is attributed both to C S being weaker than C O and to the greater nucleophilicity of sulfur than oxygen.…”

“…In Part 1, we showed that the early observation that compounds 6 revert to the starting components (see Scheme 2) from which they are formed upon heating in boiling toluene [25,26] applies in general to ylides of this type and their thiocarbamoyl analogues [6]. Since the mechanism of this process involves interaction between the RCO carbonyl and NH, it seemed likely that this would not happen when RCO is replaced by CN and indeed this turned out to be the case.…”

“…In these studies, no attempt was made to examine the kinetics of the processes involved and indeed until recently the thermolysis of phosphorus ylides has not previously been examined from the kinetic point of view [4]. We recently reported kinetic data for the thermolysis of a range of ␤,␦-dioxo ylides [5], and in Part 1 of this series [6] we presented a detailed kinetic study of substituted benzoyl/arylcarbamoyl ylides and their thiocarbamoyl analogues. Although this did give useful mechanistic information, the process involved turned out to be simply reversion to the starting materials from which the ylides were formed.…”

Kinetics and mechanism of gas‐phase pyrolysis of ylides. Part 3.1 Thermal reactivity of α‐carbonyl‐ and thiocarbonyl‐stabilized methylenetriphenylphosphoranes

“…It involves conformational pre‐organization of the ylide, and a four‐membered TS leading to the products of thermal elimination 15–17. In the ylide form, the α ‐ketone/thione (CX) group is arranged syn to the phosphonium moiety; and in the resonance dipolar enolate form, the oxide/sulfide moiety is consequently arranged in a conformation ideal for the subsequent extrusion of triphenylphosphine oxide (Ph 3 PO) and its sulfide analog (Ph 3 PS) 9, 15–22. The magnitude of the negative entropy of activation observed in this study and its predecessor is indicative of the extent of conformational pre‐organization and an ordered TS 17…”

“…Replacement of one of the (H) atoms particularly by an electron‐withdrawing group (EWG) leads to molecular stability, and the planar conformation at (C) provides the ideal geometry for conjugation and hyperconjugation effects to dominate the molecular reactivity of stabilized triphenylphosphonium methylides. Our pioneering studies on the kinetics of the gas‐phase pyrolysis of these ylides and their arsonium analogs have shown that the stabilized monosubstituted phosphonium ylide is less reactive than the disubstituted counterparts, and that the monosubstituted arsonium analog changes mechanism from a dipolar transition state (TS) to a pathway involving a reactive carbene intermediate 1, 16–20. The present study investigates the kinetics and mechanism of the thermal gas‐phase elimination reactions of fourteen α ‐carbonyl‐ and thiocarbonyl‐stabilized triphenylphosphonium methylides to assess the magnitude of change in reactivity with change in the structure of the stabilizing ketone/thione moiety.…”