Classical carbonium ions. Part 13. Rearrangements from secondary to primary alkyl groups during reactions involving carbonium ions
The rearrangement of cyclohexylamine to cyclopentylmethyl derivatives, earlier reported in brief, is confirmed. A similar reaction with 4-trans-t-butylcyclohexylamine is shown to give trans-3-t-butylcyclopentylmethyl derivatives, defining the conformational requirements and stereochemical course of the rearrangement. In similar reactions 2-butylamine and 3-pentylamine give, in small yield, derivatives of primary alcohols. Cyclohexyl toluenesulphonate probably gives a very small yield of cyclopentylmethyl acetate. All these reactions involve the formation of products formally derived from carbonium ions much less stable than those initially generated, and yields, though small, are much larger than can be accounted for by classical descriptions. It is proposed that corner-protonated cyclopropanes (‘non-classical carbonium ions’) are ‘intermediates’ of very short lifetime in these reactions; the extent to which it is possible to regard species of very short lifetime as intermediates is discussed.