Ureas, inclusion compounds from

Fig. 17. Schematic of the nonadecane/urea-inclusion compound. From left to right single-crystal experiment by rotation around the channel axis. Critical inhomogeneous linebroadening on approaching the transition temperature from above, with a lineshape invariant by rotation around c hole burning in the last spectrum proves the inhomogeneous nature of the broadening. Schematic of rotation patterns that gives the chain orientation in the low-temperature phase.

Fig. 4 Experimental quadrupole frequencies and asymmetry parameters T for trioxane-dg in the urea inclusion compound, from [17].

Recently, Radell, Connolly and Raymond [7] prepared normal pentyl, hexyl, heptyl and octyl perchlorates from the corresponding alkyl iodide and silver perchlorate. The oily esters were purified as urea inclusion compounds. 

Urea inclusion compounds Fractionation of polyunsaturates from saturates Downstream processing of lipase-catalyzed PUFA-enriched FFA... 

The guest hydrocarbon can be recovered from the urea inclusion-compound by shaking it with water which dissolves out the urea alternatively, treatment with ether can be carried out, in which case the hydrocarbon will go into solution leaving a residue of urea. 

Both experimental evidence [63] and Monte Carlo simulation [64] have shown that the removal of guest molecules from urea inclusion compounds leads to collapse of the channel host framework to produce the tetragonal crystal structure of pure urea. Thus there is always a dense packing of guest molecules within the channel structure of urea inclusion compounds [36a]. The migration of molecules into the channel structure of urea inclusion compounds has been probed by high-resolution solid-state NMR spectroscopy [34]. 

Shown m Fig 4a is the iOkl) section of the diffraction pattern from the DBD-Urea inclusion compoimd In urea inclusion compounds, the urea molecules form a hydrogen-bonded network containing hexagonal channels that run along the c-direction The channels can accommodate... 

Fig. 2 Single-crystal x-ray diffraction oscillation photograph for an incommensurate tunnel inclusion compound (the 1,9-diiodononane/urea inclusion compound), recorded for a single crystal oscillating about its tunnel axis. The layer lines (horizontal) from the host component are indicated (h) on the left-hand side. The layer lines from the guest component are indicated (g) cn the right-hand side. In this case, the guest layer lines contain discrete scattering (sharp spots) and diffuse scattering. The fact that separate sets of layer lines are observed for the host and guest components is a consequence cf the incommensurate relationship between C h and Cg.

The empty urea tunnel stmeture is unstable, and it has been shown by experiment and computer simulation that the tunnel collapses if the guest molecules are removed from the inclusion compound, leading to the pure crystalline phase of urea, which does not contain empty tunnels. The instability of the "empty" urea tuimel stmeture clearly limits the potential for developing certain applications of urea inclusion compounds. 

Although the focus in this article is on the conventional urea inclusion compounds, we note that the inclusion compounds formed between urea and certain specific guest molecules arc commensurate tunnel stme-tures. For such commensurate systems, the host stmeture is usually distorted from the hexagonal tunnel stmeture shown in Fig. I. Examples are the inclusion compounds formed between urea and the guest molecules 1,6-dibromohexane, - " sebaconitrile and (a + l),(to - 1)-... 

In both cases, the host structure in the low-temperature phase is orthorhombic and is based on a small distortion from the orthohexagonal description of the high-temperature structure. The structural relationship between the host and guest substructures along the tuimel remains incommensurate. Other urea inclusion compounds, such as 1,10-decanedicarboxylic acid/urea exhibit more complicated structural behavior in the low-temperature phase, with the formation of large superstructures in directions perpendicular to the tuimel axis. Thus, the exact nature of the distortion of the host structure in the low-temperature phase depends critically on the type of guest molecule. 

Much early work relating to chemical reactions in urea inclusion compounds focused on studies,primarily using ESR spectroscopy, of radicals generated from the guest molecules by x-ray irradiation, and provided fundamental information about the spin density and its anisotropy in a variety of radical species. Subsequently, a number of other workers have used ESR spectroscopy to identify radicals formed by x-ray or y-ray irradiation and to study the dynamic, chemical, and electronic properties of these radicals.[ Details of these studies were reviewed elsewhere. 

A concentrate with 83% 9-cis,ll-trans isomer was obtained from gentle dehydration of ricinoleic acid from castor bean oil and subsequent purification steps (4). The use of urea inclusion compounds does not seem to be a feasible procedure to separate 9-cis,l i-trans and l0-trans,l2-cis (23). Enzymes, however, are promising tools for these separations. A 98% concentrate of 9-cis,ll-trans was reported by using lipase from Geotrichum candidum. The enzyme was capable of esterifying selectively 9-cis, -trans to monohydric alcohols from a mixture of several isomers (24). A patent has been issued on purification and characterization of iso-merases from Propionibacterium acnes and Clostridium sporogenes. The purified isomerase preparations were able to quantitatively isomerize linoleic acid into the 10-trans,12-cis isomer of CLA (25). 

Another example concerns photolysis reactions of alka-none guest molecules in urea inclusion compounds, which are found to involve Norrish Type II reactions (Figure 20). A substantial difference is observed in the relative amounts of fragmentation and cycUzation products formed in the photolysis of 5-nonanone in its urea inclusion compound and in methanol solution. Among the cycliza-tion products formed in the reactions of 5-nonanone and 6-undecanone in the urea inclusion compound, there is a marked increase in selectivity toward the cw-cyclobutanol over the -cyclobutanol products, probably because less extensive geometric rearrangement (more readily accommodated within the urea tunnel) is required to produce the cw-cyclobutanol from the Norrish Type II intermediate in this reaction (Figure 20). 

Interest in urea inclusion compounds has changed in three decades from its properties as a host to its potential as a guest, more or less directly in the hope that improvement in artificial kidneys can be achieved. There is a fine balance between the tendency of urea molecules to hydrogen bond to each other and to form external bonds, as required for encapsulation. Further, it may form uronium salts with strong acids. 

Kahlke and Richterich 1965) and plasma lipids of Refsum s (1946) case T. E. (Kahlke 1964 a). Methods and results were identical in both instances although a nuclear resonance spectrum was obtained only in the first case and a complete mass spectrometric analysis only in the second case. Phytanic acid was isolated by preparative gas-liquid chromatography from a mixture of fatty acid methyl esters. Traces of stearic acid were removed as the urea inclusion compound by treatment with a saturated methanolic solution of urea (Cason et al. 1953). After repeated crystallization from acetone at minus 70—80 C and drying under vacuum at minus 10 C, phytanic acid was obtained as a white crystalline powder with a melting point of minus 7—6 C. At room temperature phytanic acid is a colorless, odorless oil. The lack of hydrogen uptake with exhaustive... 

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