Synthesis and Thermal Rearrangements of Spiro[2.3]hexadiene and Spiro[2.3]hex-4-ene Derivatives

The reaction of n-butyllithium and hexachlorocyclobutene in the presence of alkenes produced 4,5,6,6-tetrachlorospiro[2.3]hex-4-ene derivatives; with 2-butyne in place of the alkenes, the corresponding spiro[2.3]-hexadiene was obtained in 40% yield. With n-butyllithium and 3-H-pentachlorocyclobutene, reaction occurs predominately by a dechlorination and, in the presence of alkenes, leads to 4,5,6-trichlorospiro[2.3]hex-4-enes. The latter compounds were also obtained in a more efficient way, by controlled potential electroreduction of the 4,5,6,6-tetrachlorospiro[2.3]hex-4-enes and the 4,5,6,6-tetrachlorospiro[2.3]hexadiene. The formation of spirocycles in these reactions is in contrast to earlier reports where tetrachlorocyclobutadiene was postulated as intermediate in related reactions. The spirocycles are generally stable toward isomerization at ambient temperature but rearranged at elevated temperatures (265–450°C in a flow system, 60°C and above in solution). The spiro[2.3]-hexadienes produce labile products which are tentatively identified as allylidenecyclopropenes, while the spiro[2.3]-hex-4-enes undergo ring opening to allylidenecyclopropanes, which rearrange further to derivatives of 3-methylenecyclopentene. In certain cases, the latter products spontaneously lose hydrogen chloride to afford a series of fulvenes.