Synthesis and Reactions of 5-Methylenebicyclohex-2-ene Derivatives from Hexamethyl(Dewar benzene)

Article abstract of DOI:10.1021/ja00188a034

Treatment of hexamethyl(Dewar benzene) (HMDB) with tert-butyl hypochlorite provides a rearranged chlorinated derivative, exo-6-chloro-1,2,3,4,6-pentamethyl-5-methylenebicyclo<2.2.0>hex-2-ene (1).Thermal rearrangement of 1 gives pentamethylbenzyl chloride (4); the activation energy decreases in more polar solvents, suggesting an ionic intermediate during the chloride migration and/or ring opening.The intermediate is postulated to be a delocalized carbocation that can be intercepted by nucleophiles to give substitution products.Treatment of 1 with NaOCH3 in methanol gives two isomeric methoxide substitution products, 2 and 3, in 60:40 ratio.The structure of 2 involves the same skeletal structure and retention of stereochemistry relative to 1; the structure of 3 indicates neighboring group participation of the transannular ? bond.The rate law for the formation of 2 and 3 is first order in 1 and independent of NaOCH3 concentration.A common ion rate depression is observed, added chloride ion causing a decreased rate of formation of both 2 and 3 equally, indicating reversible ionization to a common delocalized carbocation.Thermolysis of 2 gives hexamethylbenzene plus formaldehyde; NMR spectra provide evidence for a methylenecyclohexadiene intermediate, indicating that the ring opening precedes loss of formaldehyde.Inclusion of basic alumina in the thermolysis of 2 diverts the reaction to formation of pentamethylbenzyl methyl ether (5).Thermolysis of 3 gives a complex mixture of products, including 5.