Reaction cavity types

Several interesting examples of solid inclusion complexes with ketone guests which undergo the Norrish II reactions have been examined. They illustrate the breadth of reaction cavity types and resultant selectivities that can be expected in such systems. 

This distinction is, however, only practical. Several reaction types are not easily encompassed in the above description. For instance, Scheffer (see within this book) has provided ample examples of stereocontrolled solid-state reactions [11], while solid-state isomerizations have been studied by Coville and Levendis [12]. These processes can be explained with the reaction cavity concept, i.e. reactivity takes place in a constrained environment generated by the surrounding molecules. Relevant contributions to the field have also derived from the studies of Eckhardt [13] and those of Ohashi and collaborators [14]. 

It is very important to note that the exact size and shape of a reaction cavity (initial, effective, and final) that control the excited state behavior of guest reactants will depend on the particular reaction as well as on the guest and intermediate s) themselves. Whether the information regarding the space explored (effective reaction cavity) by the excited molecule will be registered in the distribution or stereochemistry of the products will depend on the nature of the mechanism involved in the product formation. In some cases, explorations over a larger space by excited state species and their intermediates may not be germane to the distribution and types of products formed. In certain cases, especially those that involve the probability of encounters, all of the space excited molecules and their intermediates explore before they yield final products may be important. In cases for which the distribution of specific product types is being probed, only the site in which... 

Thus the boundaries of the enclosures in organized media may be of two types they may be stiff (i.e, none of the guest molecules can diffuse out and the walls do not bend), as in the case of crystals and some inclusion complexes, or flexible (i.e., some of the guest molecules may exit the cavity and the walls of the cavity are sufficiently mobile to allow considerable internal motion of the enclosed molecules), as in the case of micelles and liquid crystals. In these two extremes, free volume needed for a reaction is intrinsic (built into the reaction cavity) and latent (can be provided on demand). 

Scheme 3 summarizes this problem with a minimum number of sites and competing processes. In this scheme, two sites, square-well type (X) and spherical-well type (Y), are available for the residence of reactant molecules (A). For the sake of convenience, molecules residing at sites X and Y are labeled Ax and AY. Excitation of these molecules gives rise to A and A. Photoreactivity of molecules excited in each site will be identical if they equilibrate between X and Y before becoming photoproducts. In media with time-independent structures, such as crystals, equilibration requires diffusion of molecules of A in media with time-dependent structures, such as micelles and liquid crystals, equilibration can be accomplished via fluctuations in the microstructure of the reaction cavities as well as translational motion of A (Scheme 4). An additional mechanism for site selective reactions or equilibration of A and A molecules can be achieved via energy migration (e.g., energy hopping, exciton migration, or Forster energy transfer).

We noted earlier (Section III.D) that there can be more than one type of reaction cavity in an organized medium. If the interconversion between molecules experiencing different environments of sites of an organized medium is slow on the timescale of excited state processes, then the excited state behavior of reactant molecules must be considered in terms of several reaction cavities accessed. Studies from several laboratories have shown that site inhomogeneity in organized media is more common than site homogeneity. We highlight this point with a few illustrative examples below. 

All reactions involving catalyst preparations were carried out under an atmosphere of prepurified nitrogen or argon in previously dried equipment. IR spectra were obtained on a Perkin-Elmer model 621 or model 257 spectrometer. The IR cells were of the sealed cavity type (<—- 100//,) with Teflon stoppers. Four types of cell window materials were used As2S3, KBr, Irtran 2, and Ge metal. The spacers were Teflon except for the KBr cell in which lead was used. Analyses were performed by Phillips Research Division. 

When the axial base ligand was replaced with the other amines or phosphines, several types of racemization and different reaction rates were observed [15]. In order to explain the different types and reaction rates, we defined the reaction cavity for the reactive group as shown in Fig. 1 [14,16]. The reaction cavity is represented by the concave space limited by the envelope surface of the spheres, whose centers are positions of intra- and intermolecular atoms in the neighborhood of the reactive 1-cyanoethyl group, the radius of each sphere being... 

Abstract There are two types of solid-state reactions keeping the single crystal form single crystal-to-single crystal (SCSC) transformations and crystaUine-state reactions. In the former reactions, the crystal structures before and after the reaction are very similar to each other, but the crystallinity is not kept during the reaction. In the latter reactions, the crystallinity is kept in a whole process of the reaction. The reaction cavity was defined to estimate the void space around the reactive group. For the crystalline-state reaction, it was easy to understand the way how the void space is effectively utilized in the process of the reaction, comparing the void space before and after the reaction. 

The Norrish type II reaction of various phenyl alkanones in zeohtes has been studied by Ramamurthy and his collaborators. " The size and shape of zeohte reaction cavities control the behavior of triplet ketone as well as the 1,4-biradical intermediate that is consequently monitored as a difference in photoproduct distribution and triplet lifetimes. Norrish type I and type II reactions of 2-pentanone included within cavities of various types of zeohtes and the alkah metal cation-exchanged ZSM-5 zeolite have been investigated by experimental and theoretical approaches. " Ion-exchanged cations had significant effects not only on the adsorption state but also on the photochemical reactions of the ketone. The... 

An example of a cyclophane-type cavity is the azacyclophane CP66 supra-molecular system which provides a lipophilic cavity with an internal width of approximately 6.5 A, as well as positive charges which accelerate and increase the selectivity of the process. The Diels-Alder reaction of cyclopentadiene with diethylfumarate at 20 °C in 10% and 50 To dioxane-water is accelerated by the presence of CP66 by 2.9 and 1.5 times, respectively [65c] (Equation 4.12). 

The photochemical behaviour of 7 OEt is the first example in which the reaction of achiral molecules in an achiral crystal packing does not occur at random but stereospecifically, resulting in a syndiotactic structure. As no external chiral catalyst exists in the reaction, the above result is a unique type of topochemical induction , which is initiated by chance in the formation of the first cyclobutane ring, but followed by syndiotactic cyclobutane formation due to steric repulsions in the crystal cavity. That is, the syndiotactic structure is evolved under moderate control of the reacting crystal lattice. 

Ad(ii) On catalysts with pores and cavities of molecular dimensions, exemplified by mordenite and ZSM-5, shape selectivity provides constraints of the transition state on the S 2 path in either preventing axial attack as that of methyl oxonium by isobutanol in mordenite that has to "turn the comer" when switching the direction of fli t through the main channel to the perpendicular attack of methyl oxonium in the side-pocket, or singling out a selective approach from several possible ones as in the chiral inversion in ethanol/2-pentanol coupling in HZSM-5 (14). Both of these types of spatial constraints result in superior selectivities to similar reactions in solutions. 

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