Adamantyl system cation

Substituent effects do not appear to be reliable probes for hyperconjugation, however. Even in the 2-norbornyl cation, where considerable C-C bond delocalization is generally considered to be present16S), substituents fail to indicate any electron deficiency at the 6-position of the developing 2-norbornyl cation 162,166>16 ). Methyl substituent effects may also not be expected to provide a reliable test for C-C hyperconjugation in the 1-adamantyl system. 

Adamantyl and Homoadamantyl Cations. The 2-adamantyl system is exceptional among sec-alkyl substrates since it solvolyzes without nucleophilic solvent participation (Section 7.2.1). The extent of anchimeric assistance is more difficult to evaluate. Some evidence is consistent with a weakly bridged intermediate ... 

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 tertiary-secondary 1,2-H shift O itlO is not rate-determining in the interconversion of 5 and 6, but may become so in a conformationally fixed system. It has been found for the interconversion of tertiary and secondary adamantyloxocarbonium ions that <10" sec at 70°C (Hogeveen and Roobeek, 1971a) as compared with k= 1-5 x 10 sec at 20°C for the reaction 5 6. The absence of interconversion between tertiary and secondary adamantyloxocarbonium ions is due to the circumstance that 1,2-H shifts do not occur in the tertiary adamantyl ion as a result of the effect of orbital orientation (Brouwer and Hogeveen, 1970 Schleyer etal., 1970). That the secondary adamantyloxocarbonium ion can lose CO is demonstrated by the reaction with isopropyl cation in SbFs—SO2CIF solution at 0°C with formation... 

Aside from tert- mty cation, where the experimental X-ray structure is clearly suspect, and aside from semi-empirical methods applied to cations with multi-center bonding, all models perform quite well for all systems. Subtle (and not so subtle) effects such as the alternation in bond distances in benzyl and in heptamethylbenzenium cations, or the greatly elongated carbon-carbon bonds in the derivative of adamantyl cation are generally well reproduced. In fact, except for systems capable of multi-center bonding, the differences among the various methods are fairly modest. (Of course, this is due in part to "lack of precision" in the experimental data.)... 

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... 

The 8,9-dehydro-2-adamantyl cation (101) incorporates a cyclopropylcarbinyl system in the preferred bisected (97) conformation. As would be expected, then, 8,9-dehydro-2-adamantyl derivatives (e.g.102) are highly... 

Other adamantane-based dications have also been prepared. Although systems such as diadamanta-4,9-diyl dication (35) and TT-bisadamanta-3,3 -diyl dication (36) have been prepared (both 1,6-dications), experimental data suggests that these systems are more related to the monopositive adamantyl cations rather than distonic superelectrophilic systems.14 However, the adamanta-l,3-dimethyldiyl dication (38) has been prepared in superacid media (eq 16).15... 

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