Urea inclusion

Eig. 12. Hydrogen-bonded network of an urea inclusion channel (--hydrogen bonds (90). 

Thermal Changes and Structural Modifications of Urea Inclusion... 

Canals in the urea inclusion compounds have a diameter of approximately 5.25 A. 

Linear guest molecules are included along these canals in an extended planar zig-zag conformation. Branched molecules are generally excluded unless branching occurs near the end of a long linear chain, but aromatic derivatives can be included if they have a long alkyl chain 38). The review article by Takemoto and Sonoda 21) contains an excellent survey of the types of molecules known to form urea inclusion compounds and of the means used to study their detailed conformations and thermal motion. 

Yet another type of canal structure has been reported for the urea inclusion compound of 1,4-dichlorobutane 51). Even though the canals are pseudo-hexagonal in dimension, there is a significant difference in their symmetry. The host lattice is orthorhombic, space group Pbcn. The difference lies in the directions of the six pseudo-3j helices of host molecules around the walls of each canal the sequence is cyc/o-RRRLLL, as opposed to cyc/o-RRRRRR in the hexagonal inclusion compounds and cvc/o-RLRLRL in the rhombohedral. This orthorhombic host structure probably occurs also in the urea inclusion compounds with 1,5-dichloropentane and 1,6-dibromohexane 51). 

Well-developed applications 19-21) of urea inclusion compounds have been in the areas of detergents, dewaxing and petroleum chemistry. Some recent novel advances include the use of the peroxydodecanoic acid-urea inclusion compound in a laundry bleach product60 and in the purification of insect pheromones61 62. ... 

Observation of the Norrish Type II reaction presents some difficulty in that generation of the biradical intermediate 12 requires a six-membered transition state and this is in conflict with the linear guest arrangement normally expected in the channel. However, as noted earlier, accommodation of planar six-membered rings in urea inclusion complexes has been observed 38. It appears that in this case the necessary six-membered transition state can be produced in the channel without destruction of the crystal structure. 

Nurex A process for extracting C8 - C30 linear hydrocarbons from petroleum fractions, using their ability to form urea inclusion complexes. Branched-chain hydrocarbons do not form such complexes. Developed by the Nippon Mining Company, Japan, and operated until 1979. 

Cyclobutanol formation is not usually an efficient process for simple aliphatic ketones. It has, however, been shown12 that irradiation of the urea inclusion complex of 5-nonanone is more effective, providing l-butyl-2-methylcyclobutanol in 40% yield, with the balance of the ketone undergoing photochemical fragmentation. The cyclobutanol product is a 97 3 cisjtrans mixture. In the absence of urea, photolysis proceeds to give the cyclobutanol in 24% yield, as a 60 40 cisjtrans mixture. Photocyclization has also been improved by inclusion in zeolites13. 

Nonanone is added to a concentrated solution of urea in methanol, which leads to crystal formation of the urea inclusion complex. After irradiation (313 nm line from a mercury lamp, isolated with an interference filter similar results are obtained with a Rayonet RPR-3000 lamp), the residue is taken up in methanol, then purified by VPC yield 40% d.r. (cisjtrans) 97 3. 

Irradiating nonan-5-one as a urea inclusion compound affords only one cyclobutanol isomer in a much higher yield (as compared to both diastereomers in solution). Apparently the cyclobutanol also remains an included molecule.7... 

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. 

Single crystal of urea inclusion compound (originally with I.S-dibromooctane guest ... 

One of the main interests in urea inclusion compounds is their potential application in the separation of linear and branched hydrocarbons in the petroleum industry. Because the channel diameter is only slightly larger than the van der Waals diameter of a linear hydrocarbon, only a small amount of branching can be tolerated. Whether a particular urea inclusion compound will form or not may be assessed simply by comparison of the channel size with the diameter of the guest. For example, benzene is... 

Figure 7.8 Host and guest repeat distances in urea inclusion compounds are not necessarily the same.

Brown, M. E., Hollingsworth, M. D., Stress-induced domain reorientation in urea inclusion-compounds. Nature 1995, 376, 323-327. 

Bombykol was for the first time prepared by Butenandt using the Wittig reaction of 2-hexynylidene-triphenylphosphorane with 9-formylnonanoic ester. The resulting (Z)- resp. ( )-10-hexadecen-12-ynoic acid esters were resolved by urea, the ( )-isomer hydrogenized with Lindlar catalyst and reduced with lithium aluminium hydride. Further purification via the urea inclusion compound and low-temperature crystallization gave (10F,12Z)-10,12-hexadecadien- l-ol 186,187). 

The method in Figure 17 often gives a product which contains traces of the corresponding (E,E)-diene. This isomer can be selectively removed from the (E., Z)-isomer, as described above, by formation of its Diels-Alder adduct with excess tetracyano-ethylene in tetrahydrofuran followed by chromatography on silica gel (cf. 17,18). Alternatively, the (E.,E)-isomer can be removed in many cases by the selective formation of its crystalline urea inclusion complex in methanol (cf. 13). 

Recently, it has been shown by Hollingsworth et al. that urea inclusion compounds of formula X(CH2)6CN/urea (X = Cl, Br) undergo reversible crystal-... 

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