Protonation studies on epimeric homoallylic adamantylideneadamantyl alcohols, 4-methyleneadamantylideneadamantane, adamantylideneadamantane (AdAd) and sesquihomoadamantene, and reaction of AdAd and sesquihomoadamantene with NO2+BF4− and PhI(OH)OTs: a stable-ion NMR and theoretical (GIAO-NMR) study

Abstract

Under stable-ion conditions, the “pseudo-axial” homoallylic alcohol 1-ax is protonated at the AdAd double-bond and the resulting carbocation undergoes rapid cyclization to produce the oxonium ion 9H⁺ as the only observable species, which on quenching furnishes the cyclic ether 9. In contrast, protonation of the “pseudo-equatorial” homoallylic alcohol 1-eq gave a persistent homoallylic cation 1a⁺. Protonation of the methylene-substituted derivative 8 gave a 5 ∶ 1 mixture of two carbocations, in which the minor ion is homoallylic 8aH⁺. Adamantylideneadamantane AdAd 5 reacts with FSO₃H–CH₂Cl₂ to give the previously observed 5H⁺ (as a rapidly equilibrating pair of carbocations), together with the oxonium ion 6H⁺. The outcome of the reaction of AdAd with NO₂BF₄, NOBF₄ and with PhI(OH)OTs is the same, in all cases producing the spirocyclic ketone 6. Sesquihomoadamantene 7 is protonated in FSO₃H–SO₂ClF and the initially formed carbocation (not observed) rapidly rearranges to 5H⁺. Compound 7 also reacts with NO₂BF₄, and TFA–CH₂Cl₂ to produce the spirocyclic ketone 6 and 5, after quenching. With PhI(OH)OTs the recovered material is 7 itself plus PhI. For comparison, the homoallylic carbocations 1a⁺, 8aH⁺, their skeletally rearranged allylic cations 1b⁺, 8cH⁺, and the oxonium ions 9H⁺ and 6H⁺ were studied computationally by GIAO-NMR at the BLYP/6-31G(d)//BLYP/6-31G(d) level.