THE MECHANISM OF ALKENE ELIMINATION FROM THE OXONIUM IONS (CH3CH2)(2)C=OH+, CH3CH2CH2(CH3)C=OH+ AND (CH3CH2CH2)(2)C=OH+

The reactions of the metastable oxonium ions (CH3CH2)(2)C=OH+,CH3CH2CH 2(CH3)C=OH+ and (CH3CH2CH2)(2)C=OH+ have been studied by C-13-labellin g 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)(2)C=OH+ and CH3CH2CH=OH+, which eliminat e ethylene. Propene loss from (CH3CH2)(2)C=OH+ must entail skeletal re arrangement leading to CH3CH2CH2(CH3)C=OH+ or related structures, Thes e isomerisation sequences may be formulated in three plausible ways, T he first two possibilities involve 1,2-H shifts in conjunction with ri ng-closures to either protonated oxiranes or oxetanes, followed by rin g-opening in the opposite sense, thus breaking the original C-O bond a nd allowing the hydroxy function to migrate along the carbon chain, Al ternatively, a combination of 1,2-H and 1,2-alkyl shifts permits the c arbon skeleton to be isomerised via the isomeric oxonium ion, CH3CH2(C H3)CHCH=OH+, without disrupting the C-O bond, Both (CH3CH2)(2)C-13=OH and CH3CH2CH2(CH3)C-13=OH+ lose C3H6 With closely similar high select ivities (88 and 89%, respectively). This observation shows that the fi rst two routes compete very poorly with the third pathway in which rea rrangement of the carbon skeleton occurs without migration of the oxyg en function, Extension of the mechanistic investigation to include pro pene and butene expulsion from metastable (CH3CH2CH2)(2)C=OH+ shows th at this preference for retaining the initial C-O connection is general :(CH3CH2CH2)(2)C-13=OH+ eliminates C3H6 and C4H8 With extremely high s electivities (similar to 99%).