Reactivity of 1-iodoadamantane with carbanions by the SRN1 mechanism

Abstract

The photostimulated reaction of 1-iodoadamantane (1) with carbanions in DMSO has been studied. Acetone enolate ion (2a) reacted with 1 to give the substitution product (4a, 20 %) and adamantane (3, 17 %); acetophenone enolate ion (2b) gave the substitution product (4b, 65 %) in the presence of 18-crown-6. In the reaction with propiophenone enolate ion (2c), the yield was lower (27 %). Anthrone anion (5) gave good yields (75 %) of the substitution product 6. The anion of nitromethane (7) did not react with 1 under photostimulation, but in the presence of 2a or b, 87 % of 1-adamantylnitromethane (8) was obtained (entrainment reactions). Low amounts of substitution products were found with the fluorene anion and no substitution was observed with other carbanions. It is suggested that these reactions occurred by the S_RN1 mechanism.

By competition experiments, the following reactivity order toward 1-adamantyl radicals was determined: 2a(1.0) < 2b(11) < 7(32) < 5(80). The reactivity observed is explained on the basis of the pK_a of the conjugate acid of the anions and on the changes in π energy (HOMO carbanion – SOMO radical anion intermediate) that takes place in the coupling reaction.

By competition experiments of 1 and Phl toward 2b, a k₁:K_Phl ratio of ca. 16 was found while 1-bromoadamantane and PhBr gave a k_1–BrAd:K_phBr ratio of ca. 0.13. This difference can be attributed to the capacity of phenyl radicals to abstract iodine atoms from 1 giving Phl and 1-adamantyl radicals, which ultimately results in an apparent greater reactivity of 1vs. Phl.