«3‐Fluorisopren» (2‐Fluor‐3‐methyl‐1,3‐butadien) und einige heterosubstituierte Abkömmlinge

‘3‐Fluoroisoprene’ (2‐Fluoro‐3‐methyl‐1,3‐butadiene) and Some Hetero‐substituted Derivatives Thereof The readily accessible 1‐chloro‐1‐fluoro‐2‐iodomethyl‐2‐methylcyclopropane undergoes zinc‐promoted ring‐cleavage yielding 2‐fluoro‐3‐methyl‐1, 3‐butadiene (‘3‐fluoroisoprene’) in almost quantitative yield. Alternatively the iodine atom may be replaced by electron‐attracting groups such as cyano, benzenesulfonyl or triphenylphosphonio. Ring‐opening of the resulting derivatives can then be brought about by various bases and leads to substituted fluorodienes. ‐ The chemical reactivity of ‘3‐fluoroisoprene’ has been studied in some detail. For example, it was found to add bromine at temperatures below 0° under kinetic control and to afford mainly the 1,4‐adduct having the (Z)‐configuration besides smaller quantities of the 1,2‐ and 3,4‐adduct. At room temperature this mixture slowly undergoes a transformation yielding, as the sole products, both stereoisomeric 1, 4‐adducts in a 1:1 ratio.