The Mechanism of Alkene Elimination from the Oxonium Ions (CH3CH2)2C=OH+, CH3CH2CH2(CH3)C=OH+ and (CH3CH2CHM2)2C=OH+

The reactions of the metastable oxonium ions (CH3CH2)2C=OH+,CH3CH2CH2(CH3)C=OH+ and (CH3CH2CH2)2C=OH+ have been studied by 13C-labelling experiments. The mechanism of alkene elimination from these oxonium ions is discussed in the light of earlier studies on the behaviour of their lower homologues, (CH3)2C=OH+ and CH3CH2CH=OH+, which eliminate ethylene. Propene loss from (CH3CH2)2C=OH+ must entail skeletal rearrangement leading to CH3CH2CH2(CH3)C=OH+ or related structures. These isomerisation sequences may be formulated in three plausible ways. The first two possibilities involve 1,2-H shifts in conjunction with ring-closures to either protonated oxiranes or oxetanes, followed by ring-opening in the opposite sense, thus breaking the original C–O bond and allowing the hydroxy function to migrate along the carbon chain. Alternatively, a combination of 1,2-H and 1,2-alkyl shifts permits the carbon skeleton to be isomerised via the isomeric oxonium ion, CH3CH2(CH3)CHCH=OH+, without disrupting the C–O bond. Both (CH3CH2)213C=OH+ and CH3CH2CH2(CH3)13C=OH+ lose C3H6 with closely similar high selectivities (88 and 89%, respectively). This observation shows that the first two routes compete very poorly with the third pathway in which rearrangement of the carbon skeleton occurs without migration of the oxygen function. Extension of the mechanistic investigation to include propene and butene expulsion from metastable (CH3CH2CH2)2C=OH+ shows that this preference for retaining the initial C–O connection is general: (CH3CH2CH2)213C=OH+ eliminates C3H6 and C4H8 with extremely high selectivities (∼99%).