Propene prototropic

Prototropic isomerization of the propene molecule in the presence of hydroxide ion has been studied using ab initio and DFT methods in the gas phase and in DMSO solution 152 the mechanism involves formation of an intermediate complex of the allyl anion with a water molecule. 

The reaction of the dichlorides (213, n = 3-7) with potassium t-butoxide in DMSO leads to the elimination of two molecules of HC1 and the formation of (214, n = 3-7) respectively. In each case the reaction can be explained by a sequence of elimination to a chlorocyclopropene, prototropic shifts, then a second elimination to a cyclo-propene followed by prototropic shifts. Thermolysis of (214, n = 3 or 4) leads to (215, n = 3 or 4), although the mechanism of this reaction is rather uncertain in contrast the larger ring species (214, n = 6 or 7) rearrange to a methylenecyclo-pentene, (2 1 6) 148,149). In the case of (213, n = 2), the analogous elimination product (214, n = 2) is not observed, presumably rearranging under the reaction conditions to the observed product (217)149). 

Both nitronium and nitrosonium salts are effective initiators but with quite different results. Thus, propene and nitronium fluoroborate react to produce the secondary a-nitrocarbenium ion which undergoes Ritter reaction with acetonitrile to yield amide (118). Under similar conditions, nitrosonium fluoroborate leads to heterocyclic products. Intramolecular reaction of the nitroso and nitrilium groups, followed by prototropic shifts, affords the iV-hydroxy imidazolium salt (119). This may be either neutralized to produce the iV-oxide or reduced to the imidazole (120 Scheme SS). 

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