| dc.description | Alkali metal cation zeolites are employed in order to elucidate the role of zeolitic parameters other than acidity on the generation, absolute and bimolecular reactivity of carbocations. Two photochemical methods are used to create reactive carbocations in non-acidic zeolites: (i) rapid fragmentation of photogenerated bicumene radical cations; and (ii) photoheterolysis of 9-fluorenols. Studies of cumyl and 9-fluorenyl cations in dry zeolites (MY, M = Li+, Na+, K+, Rb + and Cs+, NaX, NaMor and Nabeta) indicate that the absolute reactivity of carbocations is strongly dependent on the alkali cation, the Si/Al ratio and the framework morphology. Investigations of the 4-methoxycumyl cation, including kinetic isotope effects and product studies, suggest that the mechanism for intrazeolite decay involves direct attack of the zeolite lattice on the carbocation leading to a framework-bound alkoxy species. The carbocation lifetime is thus correlated to zeolite nucleophilicity and can be modulated over several orders of magnitude with changes in the zeolite environment. The results establish that non-acidic zeolites support the generation of reactive carbocations, including the 9-fluorenyl cation, as transient intermediates which receive stabilization from the zeolite matrix, but are reactive towards attack by nucleophilic framework sites. Consequently, the range of reactivities for carbocations within non-acidic zeolites is reduced relative to solution. Intrazeolite addition reactions of small alcohols, alkyl enol ethers, and water to carbocations are described in terms of both static and dynamic quenching involving molecular diffusion through the heterogeneous topology and rapid intracavity coupling between the nucleophile and the carbocation. Thus, the zeolite reduces the efficiency of dynamic quenching but enhances the reactivity of the nucleophile towards intracavity addition to the carbocation. (Abstract shortened by UMI.) | en_US |
