Clathrates photochemical reactions

That the situation is different for photochemical reactions is indicated by a particularly interesting recent study of some dialkylketones (239). In solution, 5-nonanone, 152, reacts photochemically to yield the cyclobutanol 153 and its isomer 154 in comparable amounts. Within the urea clathrate, however, 153 is the dominant product, with only traces of 154 being formed. The cyclobutanols analogous to 153, that is, having methyl and hydroxyl cis, also predominate in the urea-clathrate-mediated photocyclization of 2-hexanone and 2-undecanone. It might be expected that the bulky cyclobutane derivatives, which almost certainly cannot be crystallized in a urea clathrate, would also not be formed in such a clathrate. There are decomposition pathways (cleavage reaction 0 of the diradical intermediate that occur both in the clathrate and in solution. Nevertheless, the ring closure is a major pathway of reaction even in the clathrate. 

As a natural development of the successful molecular inclusion concept, which involved electrostatic as well as van der Waals forces between the interacting host and guest entities, an increasing interest has been shown in the systematic study of lattice-inclusion type systems. A considerable effort has been devoted to the design of new hosts for the formation of stable crystaUine clathrates and the improvement of selective complexations with potential guests. Suitable examples of clathrates studied in recent years include hosts such as Dianin s compounds , perhydrotriphenylene , cyclotriveratrylene triphenylmethane hexakis-(arylthio) and -(arylthiomethyl)benzenes tri-o-thymotide (TOT) and choleic acids (cf. Fig. 1 in Ch. 1 of Vol. 140). Selected series of such clathrate inclusion systems have particularly been useful in research of photochemical reactions in the solid state and of selective molecular complexation that is central to biological phenomena... 

These and other experiments imply that even reactions that proceed via bulky transition states can take place in the clathrate if the initiation is photochemical, perhaps as a result of local deformation of the host lattice, and presumably as a result of the large energy dissipation involved. However, even here there is some lattice control of the reaction pathway. 

The most extensive study in the field of host-guest reactions in clathrates has been that of Lahav, Leiserowitz, and co-workers (56,241,243) on the choleic acids. The results of these combined chemical and crystallographic investigations are of possible importance for stereoselective steroid functionalization. In these studies potentially reactive guests were activated thermally or photochemically to produce species that attacked the walls of the channel at specific sites determined by the proximity, orientation, and reactivity of the host molecules at the wall relative to the activated guest species. 

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