Elimination and addition reactions. Part 41. Nucleophilic eliminative fission of cyclopropanes: the coiled spring effect of ring strain on nucleofugality and its evaluation

Abstract

Rates have been measured of sulphonyl-activated eliminative ring fissions of a series of six cyclopropanes in which the leaving group is stabilised by alkoxycarbonyl, cyano, or sulphonyl groups. The measurements allow assignment of ranks (nucleofugalities) to carbon leaving groups in systems in which the connection to the leaving group is strained by incorporation in a cyclopropane ring. The values obtained are compared with those obtained for a unstrained (acyclic) analogues. Rank enhancements of about 9 (log) units are obtained; these enhancements suggest that free energies of activation for leavinggroup expulsion are reduced by about 53 kJ mol-', or about 46% of the excess of enthalpy of the strained ring, notwithstanding the small degree of ring fission in the transition structure. The effect of phenyl substitution at the leaving group suggests that cleavage of the ring is very little advanced in the transition structure, although this is variable with the nature of the leaving-group stabilisation. This is the first direct determination ot the effect of strain on nucleofugality.

Elimination and addition reactions. Part 44. Eliminative fission of cycloalkanols

“…These include nucleophilic substitutive fission of oxacycloalkanes,22 nucleophilic eliminative fission of carbocycles,' homolytic eliminative fission of carbo~ycles,~~ and, most recently, eliminative fission of oxacy~loalkanes.~~ Stereochemical Effects.-There is a small difference between the rate constants for cis-and trans-isomers (4d and e). Similar small stereochemical effects have been seen in the cis-trans pairs (5a) 25 and (Sb)26 (Scheme 3). In all the data the small differences are disguised by cancellation of substantial differences in the activation parameters.…”

Syntheses and15N NMR Spectra of Iminodiaziridines – Ring‐Expansions of 1‐Aryl‐3‐iminodiaziridines to 1H‐ and 3aH‐Benzimidazoles, 2H‐Indazoles, and 5H‐Dibenzo[d,f][1,3]diazepines

“…Even at room temperature in the solid state, 6c decomposed over two weeks to furnish 13c , obviously by hydrolysis of intermediate 12c due to traces of moisture. The isomerization 6 → 12 16 may be interpreted in terms either of base‐mediated eliminative ring fission17a,17b or of [1.5] sigmatropic H‐shift 17c…”

Methylenecyclopropanes in Elimination and Addition Reactions: Quantification of the Effects of Strain

“…-7 On this basis, the methylenecyclopropylsulfone 7 reacts faster by a factor of 3.5·10. 5 Converted into an energy of activation difference (assuming as before that A factors are the same), this amounts to 32 kJmol −1 or about 60% of the energy difference between the strained and unstrained substrates.…”

“…3 We have previously quantified the effect of strain on elimination reactions. 4,5 In the systems chosen, the results showed that around 50% of the excess enthalpy difference 6 was expressed in acceleration of the reactions. Rate enhancements were of the order of 10 11 , but the results depended, because of the huge accelerations involved, on estimations of the reactivity of unstrained comparator systems.…”