Kinetics of the decomposition of some Δ¹-pyrazolines

Products from reactions of methyl (E)-2-cyano-3-(p-substituted-phenyl)acrylates with 1-phenyldiazoethane and cycloreversion of secondary pyrazolines

“…According to McGreer et al,16 the rates of decomposition of 3-acetyl-and 3-(methoxycarbonyl)-A1-pyrazolines are enhanced by the alkyl substitution at C-5. This effect is apparently attributable to the stabilization of positive charge at C-5, assisting the cleavage of diazo group polarized at the N to C-3 bond by the electron-attracting group.17 Substitution with aryl group at C-5 in this system will cause rate enhancement.…”

Uncatalysed diaryldiazo cyclopropanations on bicyclic lactams: access to annulated prolines

“…In the case of 1a, both pyrazolines are formed, only one of which immediately collapses to the products 33 and 34, and the other (30), which can easily be isolated, only does so at elevated temperature. The collapse of pyrazolines to cyclopropanes has been investigated in detail, [57][58][59] and the formation of both pyrazoline and cyclopropyl products have been observed in the reactions of diazo compounds with unsaturated maleimide systems. 60 The rates of such 1,3-dipolar cycloadditions for a series of diaryldiazo compounds were found to be largely governed by the HOMO(diazomethane)-LUMO(dipolarophile) interaction.…”

“…The collapse of 1-pyrazolines to give cyclopropane and alkene products has been the subject of a number of investigations, and the product distributions are a combination of steric and electronic effects. [57][58][59] Amongst the mechanisms for thermal collapse of pyrazoline adducts, the formation of a (90,90) trimethylene intermediate [64][65][66][67] has been proposed and is used to explain the general retention of stereochemistry of the initial pyrazoline. The stepwise or concerted formation of similar diradical species has also been investigated, 68 and Nakano et al 64 found that the thermal stability of a series of pentasubstituted 1-pyrazolines increased with variation of the diazo substitution in the order fluorenyl < diphenyl < phenylmethyl < dimethyl.…”

Asymmetric 1,3-dipolar cycloadditions of acrylamides

“…63 For example, in the addition of diazomethane to methyl methacrylate (a Type I interaction), the regioisomer 4 is predicted to form, which agrees with experimental data (Scheme 6). 64…”