Rearrangement adamantanols

Another feature of systems that are subject to B-strain is their reluctance to form strained substitution products. The cationic intermediates usually escape to elimination products in preference to capture by a nucleophile. Rearrangements are also common. 2-Methyl-2-adamantyl p-nitrobenzoate gives 82% methyleneadamantane by elimination and 18% 2-methyl-2-adamantanol by substitution in aqueous acetone. Elimination accounts for 95% of the product from 2-neopentyl-2-adaman l p-nitrobenzoate. The major product (83%) from 2-r-butyl-2-adamantyl p-nitrobenzoate is the rearranged alkene 5. 

The high yields and mild reaction conditions of each of these rearrangements to adamantane or 1-adamantanol constrast sharply with the low yields and relative severe reaction conditions required for the conversion of tetra-hydrodicyclopentadiene (2) to adamantane (l)4,4[Pg.16]

The rearrangements of several twistane derivatives to adamantyl cations under the same conditions, on the other hand, appear to involve reversible, random carbonium ion formation, at least to a limited extent. Rearrangement of 2-twistanol-2-d (38) occurs with considerable intermolecular hydrogen scrambling (Eq. (15)) 40T Similar intermolecular rearrangements are observed when a 50 50 mixture of 1-adamantanol and l-adamantanol-3,5,7-d3 in S02 is treated with SbFs 40). 

The precise mechanism of these intermolecular reactions is not known. Transient disproportionation processes, although well documented in less acidic media (see below and Section V.A.l), seem unlikely if alkyl cations alone are involved. Two possible explanations for the observed results may be considered. Small amounts of polymeric impurities may be present in the reaction mixture which could serve as a hydride source, catalyzing the intermolecular reaction as indicated in Eq. (16)4°). Alternatively, the intermolecular reactions may result from inefficient mixing during reaction initiation. In this case, unionized alcohols would serve as the hydride source. This latter alternative is consistent with the observation 4°1 that the deuterium in the 1-adamantanol obtained from the rearrangement of 38 is distributed between bridgehead and methylene positions. Unless more than one re-... 

Thus, in contrast with the results in SbF5-S02,2-methyl-2-adamantanol undergoes extensive rearrangement in concentrated sulfuric acid 63> 64 The results are summarized in Eq. (17). Similar rearrangements are observed S7> 65) during Koch-Haaf carboxylation reactions carried out in sulfuric acid 66K The intermolecular nature of these reactions is indicated by the fact that high dilution conditions suppress the rearrangements s7 ... 

Similar results are obtained with 2-adamantanol which rearranges to 1 -ada-mantanol (> 98 %) at 28°C in sulfuric acid. An equilibrium mixture containing small amounts of 2-adamantanol is rapidly achieved fromeither direction67 6 K This isomerization is one of the mechanistic bases for the preparation of ada-mantanone by the reaction of adamantane with sulfuric acid at 77°C (see Section V.A.l) 57> 67> 691. The Koch-Haaf carboxylation of 2-adamantanol similarly results in predominant 1-adamantyl carboxylic acid formation unless highly dilute reaction conditions are employed 57> 7°). 

Addition of methyl Grignard to 4-protoadamantanone gives a 2 1 mixture of 4-methyl-ejco-4-protoadamantanol and the corresponding encfo-epimer76). This mixture rearranges nearly quantitatively to l-methyl-2-adamantanol when treated with acid 76). 

Clearly, many extensions of these rr-route cyclizations should be possible for the convenient synthesis of a variety of 2,4-disubstituted adamantanes. An illustrative example is provided by the rearrangement of the alcohol 91 to 2,2-dime-thyl-4-adamantanol (Eq. (75))278>. 

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