2-Cyclopropyl-2-adamantyl cation

The 2-methyl, 2-cyclopropyl and 2-phenyl substituted 8,9-dehydro-2-adamantyl cations 25 were prepared from their respective alcohols using fluorosulfuric acid in sulfuryl chloride fluoride at low temperatures (equation 27). The relative extent of charge delocalization in these cations was estimated by comparing their NMR spectra. The ions are nonequilibrating static cations, as shown by their proton NMR spectra56. 

The stability and symmetry of the 8,9-dehydro-2-adamantyl cation have been investigated and comparison has been made with the cyclopropyl car-binyl cation itself, 3S). For a complete discussion see Section V.B.l. 

That maximum acceleration occurs when the vacant p orbital is parallel to the plane of the cyclopropyl ring can be seen from the solvolysis of spiro[cyclo-propane-l,2 -adamantyl] chloride (71). The carbocation formed by departure of Cl is unable to adopt the geometry of the bisected cyclopropylcarbinyl cation, but can orient its empty p orbital properly to form the bicyclobutonium ion. This compound solvolyzes 103 times more slowly than 1-adamantyl chloride.82 On the other hand, 72 solvolyzes 10s times faster than 73. The cation from 72 does have its p orbital parallel to the plane of the ring as in the bisected cyclopropylcarbinyl cation.83... 

The rigid geometry of the adamantyl nucleus has also been utilized to quantitatively evaluate the stereoelectronic requirements of cyclopropylcarbinyl cations. The bisected conformation of the cyclopropyl cation, 97, is known to be preferred strongly relative to the perpendicular conformation, 98 280 The adamantyl derivate 99 incorporates a cyclopropyl carbinyl system locked in the... 

There is very good evidence that if the cyclopropyl group of a cyclo-propylcarbinyl derivative can interact with the developing cationic center in a bisected conformation a very large rate enhancement results, whereas if this conformation is prevented sterically, there is no rate enhancement. Thus on the basis of the rate of solvolysis of 8,9-dehydro-2-adamantyl 3,5-dinitrobenzoate in aqueous acetone it was estimated that the corresponding toluene-p-sulfonate (43) would undergo acetolysis about 8 powers of 10 faster than 2-adamantyl toluene-p-sulfonate (44). It was also esti-... 

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