Inner phase reactions

Conceptually, one can divide inner phase reactions into four groups (Figure 9.5) ... 

In intermolecular inner phase reactions, the incarcerated guest reacts with a bulk phase reactant. This may require full or partial passage of the bulk phase reactant through one of the openings in the host shell. In most cases, it is difficult to discriminate between both models. Many different intermolecular inner phase reactions have been carried out with sometimes very surprising outcomes and are summarized in section 9.4. 

Light or heat triggers intramolecular inner phase reactions and causes rearrangement of the incarcerated guest or fragmentation. Such reactions have lead to the generation of incarcerated reactive intermediates and are discussed in sections 9.5-6. 

Figure 9.5 Different types of inner phase reactions...

The precise mix of free and occupied space inside the reactant filled container, the shape of the inner phase, the rigidity of the container and the electronic nature of its inner surface may all or in part contribute to the reaction dynamics and selectivity of an inner phase reaction. Reversible conformational changes of guest molecules are easily tractable spectroscopically and ideal to study the effect of confinement on transition states. Cram and co-workers studied the cis-trans isomerization of (CH3)2NCHO and (CH3)2NCOCH3 inside 5. For (CH3)2NCHO, the C—N rotational barrier deaeased in the order liquid phase > inner phase > vacuum and was 1 kcalmoT lower inside 5 than in nitrobenzene. For (CH3)2NC0CH3, the order was inner phase > liquid phase > vacuum and the barrier... 

Such steric interactions in the transition state, that serve as selectivity criteria in these inner phase reactions, also contribute to the highly structural recognition in enzyme-catalyzed reactions. The understanding of these interactions provides valuable guidelines for the rational design of novel highly selective catalysts in the future. 

In the previous section we have seen that the nucleophilicity and basicity of incarcerated lithium alcoholates strongly exceeds those of bulk phase alcoholates mainly as a result of the lack of aggregation and poor solvation by the host. As a consequence, innermolecular reaction involving lithium alcoholates are strongly accelerated. Several extrusion reactions have been studied inside container molecules and have highlighted additional ways how encapsulation inside a hemicarcerand may accelerate or decelerate inner phase reactions. 

S. Sanchez Carrera, N. Brown, J.-L. Kerdelhue, K. J. Langenwalter, R. Warmuth, Inner phase reaction dynamics the influence of hemicarcerand polarizability and shape on the potential energy surface of an inner phase reaction, Eur. J. Org. Chem., 2005, 2239-2249. 

Hemicarcerands were the first molecular containers in which chemical reactions involving encapsulated reactants have been investigated. In this section, some of the advances in inner-phase chemistry are reviewed. Inner-phase reactions may take place either entirely inside the carcerand, where they are influenced by the shape and size of the inner phase, or at the electrostatic inner surface of the hosts with its unusual high inner-phase polarizability. Typically, these reactions involve one or two encapsulated reactants, in which case the host takes over the role of the solvent cage in equivalent condensed phase reactions. Proper solvation is particularly important in reactions involving zwitterionic intermediates or ion pairs. The absence of polar solvent molecules and the hydrophobicity and reduced deformability of the inner... 

The basicity and nucleophilicity of incarcerated lithium alcoholates exceed those of bulk-phase alcoholates by several orders of magnitude, resulting in efficient inner-molecular elimination or nucleophilic transacetalization and formation of hemicarcerands with one extended portal. In these inner-phase reactions, small structural changes of the guest have a sound effect on the reaction mode. 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

Warmuth R The Inner Phase of Molecular Container Compounds As a Novel Reaction Environment J. Inclusion Phenom. Macrocyclic Chem. 2000 37 1-38 Keywords inciusion reaction, photochemistry, photoinduced eiectron transfer, fuiierenes... 

Other types of non-micro-channel, non-micro-flow micro reactors were used for catalyst development and testing [51, 52]. A computer-based micro-reactor system was described for investigating heterogeneously catalyzed gas-phase reactions [52]. The micro reactor is a Pyrex glass tube of 8 mm inner diameter and can be operated up to 500 °C and 1 bar. The reactor inner volume is 5-10 ml, the loop cycle is 0.9 ml, and the pump volume adds a further 9 ml. The reactor was used for isomerization of neopentane and n-pentane and the hydrogenolysis of isobutane, n-butane, propane, ethane, and methane at Pt with a catalyst. 

For the liquid phase kinetic studies of esterification, with a few exceptions [402,435—437] only the standard (non-porous, see Sect. 1.2.5) ion exchangers were used. The macroreticular (porous) ion exchangers with a large inner surface area are prefered for vapour phase reactions, especially in more recent studies [436—443]. The authors claimed that diffusion was not the limiting process under their conditions. This observation cannot be generalised, however, and even with vapour phase reactions and macroreticular polymers, the possibility of transport limitations through the pores or the polymer mass cannot be excluded a priori. 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destruct in the bulk phase or transform and react under atmospheric conditions. 

Surface modification of wood is a noteworthy new technique of wood improvement in which only surfaces are treated. The inner parts of the wood are unmodified and as a result retain their inherent properties. Of course, less chemical is needed to modify only the surfaces of wood compared to bulk modification. Vapor phase reactions are particularly suitable for surface treatments. This could reduce the amount of reagents required for modification and make the removal of unreacted reagent easier, thereby reducing treatment costs. 

I) and of Brown and Albright (2), who earlier studied surface reactions that occur during the pyrolysis of hydrocarbons. Such pyrolyses are used for commercial production of ethylene, other olefins, diolefins, and, to some extent, aromatics. Several important reactions occur on the inner surfaces of the high-alloy steel tubes used for pyrolyses. These surface reactions occur simultaneously and, to some extent, consecutively along with the gas-phase reactions that produce the desired products of... 

Once the gas phase Hamiltonian is parametrized as a function of the inner-sphere reaetion coordinate(s), the free energy is calculated as a function of the proton coordinate(s), the scalar solvent coordinates, and the inner-sphere reaction coordinate(s). Note that this approaeh assumes that the optimized geometries of the VB states are not significantly affected by the solvent. For proton transfer reactions, the proton donor-acceptor distance may be treated as an additional solute reaction coordinate that ean be incorporated into the molecular mechanical terms describing the diagonal matrix elements hf- and, in some cases, the off-diagonal matrix elements (/io)y. If the inner-sphere reaction coordinate represents a slow mode, it is treated in the same way as the solvent coordinates. As discussed throughout the literature, however, often the inner-sphere reaction coordinate must be treated quantum mechanically [27, 28]. In this case, the inner-sphere reaction coordinate is treated in the same way as the proton coordinate(s), and the vibrational wave functions depend explicitly on both the proton coordinate(s) and the inner-sphere reaction coordinate(s). 

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