| Abstract
| - Access to each CC face of adamantylideneadamantane (AA) and sesquihomoadamantene (SA) ishindered by the hydrogenic canopy consisting of four β-hydrogens; otherwise, these olefins havequite normal environments. X-ray crystallography and density functional (DFT) calculations showa 0.5 Å larger annular opening in the protective cover of AA than that in SA. This contributes tothe remarkable differences in reactivity toward various reagents, not only by limiting access to theolefin site in SA but also by inhibiting reactions which force these hydrogens closer together. Thus,AA is subject to typical olefin-addition reactions with bromine, sulfuryl chloride, m-chloroperbenzoicacid, dioxygen, and so forth, albeit sometimes at attenuated rates. On the other hand, SA issingularly unreactive under identical reaction conditions, except for the notable exceptions thatinclude Brønsted (protonic) acids, a nitrosonium cation, and dichlorine. The exceptions arecharacterized as three sterically limited (electrophilic) reagents whose unique reactivity patternsare shown to be strongly influenced by steric access to the CC center. As such, the different degreesof steric encumbrance in the isomeric donors AA and SA shed considerable light on the diversenature of olefinic reactions. In particular, they evoke mechanistic features in electrophilic additionversus electron transfer, which are otherwise not readily discernible with other less hindered olefinicdonors. Transient structures of the olefinic-reaction intermediates such as the protonatedcarbocations AA−H+ and SA−H+ as well as the cation radicals AA•+ and SA•+ are probed by thecombination of X-ray crystallographic analyses and density functional theoretical computations.
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