Mesomeric Vinyl Cations. Part II. Solvolysis of 2‐bromo‐1, 3‐butadienes

In contrast to 2‐bromo‐1, 3‐butadiene ( 5 ), the di‐, tri‐ and tetramethyl derivatives 6 – 11 show remarkable solvolytic reactivity. First order rate constants and products of these bromodienes in 80% ethanol have been determined. Rates are very sensitive to the ionising power of the solvent but are not affected by triethylamine. Three types of products are formed, i.e. alkenynes, α,β‐unsaturated ketones and 4‐ethoxyallenes. One methyl group on C‐4 of 5 increases the reaction rate by a factor of ca. 10 2 ; two methyl groups increase the rate by ca. 10 4 . Methyl substituents at C‐1 have only a slight influence, apparently as a result of a rate‐decreasing polar effect and a compensating rate‐increasing steric effect. The results of this study are consistent with a unimolecular ionization mechanism involving a mesomeric vinyl cation 12a ↔ 12b as an intermediate.