Adamantane rearrangements

All Cio polycylie hydrocarbons under strongly acidic conditions undergo a unique isomerization to give adamantane.32-34 The adamantane rearrangement was discovered by Schleyer in connection with the catalytic interconversion of endo- and evo-trimethylenenorbomane 35-37... 

The interconversion of possible CioH16 isomers to adamantane, the thermodynamically most stable isomer, is a very complex system. It involves practically thousands of carbocationic intermediates via 1,2-hydride, alkyde shifts. The interconversion possibilities were summarized in a complex map of rearrangement graph ( adamantaneland ).45,46 The adamantane rearrangement is now recognized to be general for polycyclic hydrocarbons with the formula C4 +6H4 +12 (n = 1,2,3). 

Thermodynamic equilibria are independent of the route by which they are established (provided such routes are available). The rates with which such equilibria are achieved depend on the energies of the transition states involved. The strain energies of the intermediates in the adamantane rearrangement will be reflected in the transition states for their formation. If one step in the rearrangement involves the formation of an excessively strained intermediate not only will the overall rearrangement be retarded but also undesirable side reactions including fragmentation and catalyst deactivation, will be able to compete more effectively. 

Brossi, M., and C. Ganter. 1988. The adamantane rearrangement of syn- and anti-tricyclo[4.2.1.12,5]decane. Part II. Rearrangements initiated by regioselective formation of carbocations at C(3) and C(9). Helv. Chim. Acta 71 848. 

The Lewis-acid-catalysed rearrangement of polycyclic hydrocarbons C H2 4 c (x = 1,2,3,...) into their thermodynamically more stable diamond-like isomers is usually referred to as the adamantane rearrangement . The archetype of this unique isomerization is the AlCl3-catalyzed transformation of en /o-tricyclo[5.2.1.0 ]decane (11) (tetrahydrodicyclopentadiene) into adamantane (Ad-H) (Scheme 4), which was first discovered by Schleyer " more than 30 years ago. This process is thermodynamically controlled and it involves carbenium ion intermediates which rearrange via successive 1,2-C bond and 1,3 hydride shifts. Numerous theoretical and experimental investigations have established the most direct thermodynamically feasible pathway (exo-11 -> 12 13 -> 14 -> 15 Ad-H) for the isomerization of endo-11 to adamantane. 

SCHEME 4. The most direct pathways in the adamantane rearrangement of tricyclo[5.2.1.0 ]decane (11)... 

Reviews of general interest this year have concerned force-field calculations (by Altona), Bredt s rule, adamantane rearrangements," triptycene chemistry, the relation between mass-spectral behaviour and thermolytic and photolytic reactivity, the removal of orbital symmetry restrictions to organic reactions, catalysis by transition metals of symmetry-disallowed reactions, and [2,2]paracyclophanes, many of whose reactions lead to bridged or cage molecules. 

Treatment of (736) with bromine at room temperature surprisingly yields (737) in a markedly exothermic reaction. A mechanism is discussed in which formation of a tertiary cation at C-5 followed by adamantane-protoadamantane-adamantane rearrangement is invoked for the initial stages of the reaction. In a projected synthesis of (738) it was found that Wolff-Kishner reduction of (739) gave only pyrazolone (a common reaction in such reduction of P-keto esters) and more vigorous conditions (NaOMe-MeOH-NHjNHj, sealed autoclave at 220 C) gave 1,3-adamantanedicar-boxylic acid as well as the desired material (738). The mechanism for the conversion of (739) into (738) is not clear, but reaction is dependent on both methoxide and... 

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