In the previous posts, I explored reactions which can be flipped between two potential (stereochemical) outcomes. This triggered a memory from Alex, who pointed out this article from 1999[cite]10.1070/MC1999v009n02ABEH000995[/cite] in which the nitrosonium cation as an electrophile can have two outcomes A or B when interacting with the electron-rich 2,3-dimethyl-2-butene.

A reader asked me about the mechanism of the reaction of 2-picoline N-oxide with acetic anhydride to give 2-acetoxymethylpyridine (the Boekelheide Rearrangement[cite]10.1002/ejoc.201000936[/cite]). He wrote “ I don’t understand why the system should prefer to go via fragmentation-recombination (… the evidence being that oxygen labelling shows scrambling) when there is an easy concerted pathway available (… a

In a previous post on the topic, I remarked how the regiospecific ethanolysis of propene epoxide[cite]10.1021/ja01208a047[/cite] could be quickly and simply rationalised by inspecting the localized NBO orbital calculated for either the neutral or the protonated epoxide. This is an application of Hammond’s postulate[[cite]10.1021/ja01607a027[/cite] in extrapolating the properties of a reactant to its reaction transition state.