Hydrocarbon caged, isomerization

Recently, Rooney and co-workers (23,58,59) have questioned the view that triadsorption by loss of 3 hydrogen atoms from the alkane is the minimum requirement for bond-shift reactions. They studied the isomerization of a series of caged hydrocarbons in excess hydrogen on palladium and platinum catalysts. The compounds were chosen in order to render difficult or totally exclude a mechanism involving aory-triadsorbed species. Thus, l,7,7-trimethyl[2,2,l]-heptane interconverts with its endo- and exo-2,3,3-trimethyl isomers, bicyclo-[3,2,2] octane changes to bicyclo[3,3,l] nonane, and protoadamantane to... 

The isomerization of practically any tricyclic Cw hydrocarbons under strongly acidic conditions gives the unusually stable cage hydrocarbon adamantane (26) (Scheme 6.11). The hrst such isomerization was reported by Schleyer in 1957. The formation of adamantane from a variety of Cw precursors involves a series of hydride and alkyl shifts through the intermediacy of hypercoordinate carbocations. ... 

Sodium borohydride in excess of triflic acid at low temperature provides" a highly efficient reductive superacid system, which was found effective for the reductive superacid isomerization of unsaturated polycyclic precursers to cage hydrocarbons in high yields (equation 25). The key to the success of the ionic hydrogenation system appears to be due to the in situ formation of boron tris(triflate), a highly superacidic Lewis acid. 

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