The 4-protoadamantyl ? 2-adamantyl rearrangement; chirality of the 2-adamantyl cation

Reactivity of Cations and Zwitterions Formed in Photochemical and Acid-Catalyzed Reactions from m-Hydroxycycloalkyl-Substituted Phenol Derivatives

“…Recently, we reported an example of solvent-assisted ESPT coupled with dehydration in hydroxyadamantylphenol 1 that gives rise to zwitterionic m -quinone methide 2 , which rearranges via 1,3-H shift and subsequent addition of water to alcohol 3 (eq ). This type of rearrangement has not been documented in the adamantane series, whereas rearrangements of the protoadamantane skeleton in thermal solvolysis reactions via nonclassical carbocations are well-known. − …”

“…The solvolysis of different polycyclic alcohols and the corresponding tosylates has been used in the study of rearrangement of nonclassical carbocations. − Accordingly, we performed acid-catalyzed solvolysis of 4 – 6 to investigate the reaction selectivity of the corresponding benzyl cations formed in the ground state. Acid-catalyzed solvolysis of 4 carried out in CH 3 OH–H 2 O (3:1) gave elimination product 10 (12%), along with the rearranged products 11 (33%) and 12 (24%), which were all isolated and characterized by NMR (eq ).…”

Sterically Congested Adamantylnaphthalene Quinone Methides

“…In addition, protonation of the carbonyl moiety of QM11 gives carbocation that undergoes rearrangement to less sterically hindered protoadamantyl cation 11’ + that reacts with phenol in the intramolecular reaction giving 14 (Scheme ). Rearrangement of the adamantyl cation to the protoadamantyl skeleton has been intensively studied, as well as reactions of protonated QMs. , However, to the best of our knowledge the lack of reactivity of protonated QMs with H 2 O has not yet been reported. The unusual stability of QM10 and QM11 , compared to the previously studied QMs may be rationalized by loss of aromaticity in only one benzene ring of the naphthalene moiety, high-resonance stabilization of the carbocations obtained by protonation, and steric effect of the adamantane hindering the attack of nucleophiles to QMs or the carbocations.…”

Synthesis of 1,2-Disubstituted Adamantane Derivatives by Construction of the Adamantane Framework

“…The peculiar nature of the protoadamantane-adamantane rearrangement is further pronounced by the chirality of the bridged carbocation intermediate. Delocalized electrons occupy one side of the vacant 2-adamantyl p-orbital, forcing a nucleophilic attack to occur from the opposite side with the complete retention of enantiopurity (Scheme 29) [83]. Not only is the rate of solvolysis accelerated by the methyl substituent of the exo-isomer 116b but the transition state is a fully bridged carbonium ion (117) as the strained but now third-degree 4-protoadamantyl cation and the less strained second-degree 2-adamantyl cation both have an equal contribution to its stability (Scheme 28).…”

“…The peculiar nature of the protoadamantane-adamantane rearrangement is further pronounced by the chirality of the bridged carbocation intermediate. Delocalized electrons occupy one side of the vacant 2-adamantyl p-orbital, forcing a nucleophilic attack to occur from the opposite side with the complete retention of enantiopurity (Scheme 29) [83]. The first efforts to utilize protoadamantan-4-one to synthesize 1,2-disubstituted adamantane derivatives are shown in Scheme 30.…”