Inner-phase stabilization

Warmuth, R. Inner-phase stabilization of reactive intamediates, Eur. J. Org. Chem. 2001,423-437. 

Inner-phase stabilization of reactive intermediates (macroheterocycles as molecular containers, carceplexes) 01EJO423. 

Inner Phase Stabilization of Reactive Intermediates Concept... 

The taming of cyclobutadiene inside 15 is the first example of an inner phase stabilization of a reactive intermediate and nicely demonstrates the power of this approach (Figure 9.17). Cyclobutadiene 52 is the prototypical example to verify theory of aromaticity. ... 

The successful inner phase stabilization of cyclobutadiene suggests that this approach may allow stabilization of other molecules with highly strained multiple bonds. An interesting class of hydrocarbons with twisted C=C bonds are anti-Bredt bridgehead olefins. These bicycloalkenes have a trani-cycloalkene and are unstable if their olefinic strain (OS) is OS > 21kcalmor. Bicyclo[2.2.2]oct-l-ene 60 and (Z)-bicyclo[3.2.1]oct-l-ene... 

An important aspect of the stabilization of emulsions by adsorbed films is that of the role played by the film in resisting the coalescence of two droplets of inner phase. Such coalescence involves a local mechanical compression at the point of encounter that would be resisted (much as in the approach of two boundary lubricated surfaces discussed in Section XII-7B) and then, if coalescence is to occur, the discharge from the surface region of some of the surfactant material. 

A foam can be considered as a type of emulsion in which the inner phase is a gas, and as with emulsions, it seems necessary to have some surfactant component present to give stability. The resemblance is particularly close in the case of foams consisting of nearly spherical bubbles separated by rather thick liquid films such foams have been given the name kugelschaum by Manegold [175]. 

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). 

Qads.(max) = 5.7 molecules by unit cell). Generally speaking, Qacis.(max) is closely related to the molecular size, as it is observed for the other molecular species. Secondly, as shown on Figure 5, sorption isotherm sub-step observation could be another signature of zeolite inner surface influence. Such isotherm sub-step reflects a phase transition existence between a fluid phase and a solid phase stabilized by the inner surface sorption sites. 

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. 

Other drug-delivery systems may include double emulsions, usually W/O/W, for transporting hydrophilic dmgs such as vaccines, vitamins, enzymes, hormones [441], The multiple emulsion also allows for slow release of the delivered drug and the time-release mechanism can be varied by adjusting the emulsion stability. Conversely, in detoxification (overdose) treatments, the active substance migrates from the outside to the inner phase. 

The medium-film thickness is about 0.3-0.6 pm and generally offers the best compromise of sample capacity, retentivity, and phase stability. The phase ratio determines the capacity of the column and influences its retentivity of solutes. The phase ratio (j8) can be defined as the ratio of the inner column radius to that of the product of twice the stationary-phase film thickness or 0 = r/2df. We can now also use phase ratios to group film thicknesses and now say that thick-film columns have phase ratios of less than about 80. (In capillary SFC the typical stationary-phase film thicknesses are 0.1-0.3 pm.) The effective phase ratio can change in capillary SFC, depending on the characteristics of the stationary phase and the operating density [57]. The change in phase ratio can be attributable to a swelling of the stationary phase under certain SFC conditions. 

In a pure metal, phase stability is determined by the comparative band-forming energies for various symmetries and by the repulsive forces from closed inner shells. The various metallic phases found in the first three long periods are given in Table XVIII. 

Rasiel claims that liposphere preparation with the use of N-methylpyrrolidone can no longer be considered to be a double-emulsion formulation because there is no use of aqueous inner phase to dissolve the drug. Instead, this preparation is considered to be an o/w emulsion, which is less sensitive to stability problems [35],... 

Foam fluids are essentially two-phase fluids that consist of an inner phase and an outer phase. The inner phase is generally gaseous, usually N2 however, it can be liquid or dense vapor as is the case when C02 is used. The external phase is primarily composed of a saline—water mixture with either a surfactant or gellant depending upon the viscosity and stability requirements. Other external fluids commonly used are either hydrochloric add or alcohol—water mixtures. Diesel fuel, reformates, or other hydrocarbon-based solvents can also be used as external-phase fluids but require N2 as the energizer. Carbon dioxide and hydrocarbons produce a single-phased fluid, because C02 is very soluble in hydrocarbon liquids. 

In an extended study, Sanchez Carrera et al. explored the influence of hanicarcerand shape on the thermolysis of phenyldiazirines. All inner phase transition states were slightly accelerated. Rate constants increase in the order 9049 > 12049 > 10049 > 4049 > 13049 and vary overall by a factor of 3.2. Again, inner phase transition states where stabilized enthalpically by about 2kcalmor and moderately destabilized entropi-cally. Furthermore, faster rates were observed for hemicarceplexes whose fourth shorter bridge induces a slight kink in the host structure. This was interpreted with an induced flt model and a better ability of the latter hemicarcerands to accommodate the slightly bend transition state. 

Goto, K. Okazaki. R. Molecular bowls and capsules with an endohedral functionality The stabilization of highly reactive species in their inner phase. Liebigs Ann./Recueil... 

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