Generation and reactivity of 2-substituted adamantenes

Article abstract of DOI:10.1246/bcsj.72.549

The title bridgehead olefins, generated by the dehalogenation of 1,2- dihalo-2-R-adamantanes (R = Me, Et, Ph, p-CF₃C₆H₄) with excess t-BuLi, yielded 2-t-butyl-2-R-adamantanes (15), 1-t-butyl-2-R-adamantanes (16), and dimeric products. The ratios of the yields (%) of 16 to 15 were 20:3 (R = Me), 32: < 1 (R = Et), 4:60 (R = Ph), and 5:57 (R = p-CF₃C₆H₄). 2- Alkyladamantenes predominantly yielded the products t-butylated at C(1), whereas 2-aryladamantenes gave the products t-butylated at C(2). The total yields of the dimers decreased with the bulkiness of substituents at position C(2). The dimerization of 2-methyladamantene gave a head-to-head dimer, 1- (2-methyl-1-adamantyl)-2-methyleneadamantane, as well as a head-to-tail dimer, 1-(2-methyl-2-adamantyl)-2-methyleneadamantane. The [4+2] cycloaddition between two molecules of 2-phenyladamantene, in which the phenyl group participates as a part of the diene, yielded a dimer of the head-to-tail type. These results suggest that: i) a substituent at position C(2) can kinetically stabilize 2-R-adamantene and suppress the dimerization, ii) 2-alkyladamantenes might have a biradical nature, and iii) the regioselective [4+2] cycloaddition and t-BuLi addition of 2-phenyladamantene in THF can be explained by assuming a certain contribution of zwitterionic nature in the double bond that may be dependent on the media and nucleophiles.