Adamantane reactions with carbonium ions

Cycloheptatriene and derivatives thereof donate hydride readily to a variety of carbonium ion acceptors. The position of the end equilibrium depends on the thermodynamics of the exchange. " These reactions are prototypes of a broad area of carbonium ion chemistry wherein carbonium ions equilibrate via intra- and inter-molecular hydride shifts between a donor C—H bond, usually jp hybridized, and a carbonium ion acceptor. This chemistry is often achieved with heterogeneous catalysts and is of great industrial significance it lies outside the emphasis of this review, however. Excellent treatises are available, and a review has appeared on the use of carriers like adamantane to promote hydride transfer in hydrocarbons under strongly acidic conditions. ... 

The skeletal isomerization of tetrabydrodicyclopentadiene into adamantane is an example of a very complex rearrangement diat is commercially carried out over strong Lewis acids with a hydride transfer initiator. The reaction can be catalyzed by rare earth (La, Ce, Y, Nd, Yb) exchanged faujasites (Scheme 1) in a Hj/HCl atmosphere at 25(yX3. Selectivities to adamantane of up to 50% have been reported, when a metal fimction, such as Pt, capable of catalyzing hydrogenation is added [54]. Initially acid catalyzed endo- to exo- isomerization of tetrahydro-dicyclopentadiene takes place and then a series of 1,2 alkyl shifts involving secondary and tertiary carbonium ions leads eventually to adamantane[55]. The possible mechanistic pathways of adamantane formation from tetrahydro-dicyclopentadiene are discussed in detail in ref [56]. 

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