Urea/thiourea complexes

Generation and stabilization of unstable inorganic/organic anions in urea/thiourea complexes... 

Abstract After an overview of chiral urea and thiourea synthetic methods, this review describes the main applications of urea and thiourea complexes in asymmetric catalysis. Some recent examples of thioureas as catalysts are also presented. Coordination chemistry of ureas and thioureas is briefly discussed. 

Several metal complexes have been described with urea, thiourea or dimethyl derivatives [49,50]. We will focus in this section on the coordination chemistry of substituted ureas and thioureas used as neutral Ugands as well as many ureato and thioureato anions complexed to metal centres. 

Thiourea and selenourea both form canal inclusion compounds. The selenourea compounds, reported only briefly 37), appear to be isostructural with the rhombo-hedral thiourea compounds, although with larger dimensions which are more susceptible to the sizes of guest molecules. For urea, thiourea and selenourea the crystal densities of the compounds are significantly less than those of the hosts values (g cm - 3) for the pairs host/complex are 1.30/1.20 1.40/1.10 2.08/1.60-1.65 for urea, thiourea, selenourea, respectively37). 

R, Thaimattam, D. S. Reddy, F. Xue, T. C. W. Mak, A Nangia, G. R. Desiraju, Interplay of strong and weak hydrogen bonding in molecular complexes of some 4,4 -disubstituted biphenyls with urea, thiourea and water , J. Chem Soc, Perkin Trans. 2,1998,1783-1789. 

Bipyridine and 4,4 -bipyridine form molecular complexes with halo-genated nitrophenols." " The former also complexes with urea, thiourea and related compounds," benzyl alcohol, phenol," acetic... 

Complexes of maltose with urea, thiourea, imidazole, methanol, 2-oxazolidinone, N,N-dimethylformamide, and hexamethylphosphoric triamide have been described.131 These complexes were noncrystalline and hygroscopic. Such complex-forming reactions could be valuable in the preservation of food flavors during the dehydration process. Sugars have been shown to complex with ethylenediamine.133,133 The nature of the complex has been suggested to be that of a proton-transfer type, in which the carbohydrate moiety is the proton donor and the amine is the proton acceptor. 

Adequate x-ray crystallographic studies have been made of some of the structures in Table VII. The end-view cross section of channels in urea- -paraffin complexes is shown in Figure 5 53). The urea (and thiourea) molecules are hydrogen bonded to create hollow cylindrical channels whose walls are helices of linked urea or thiourea. These helices can be right or left handed in a given crystal (but not both). In the orthorhombic structure of the deoxycholic acid complex with acetic acid having a =... 

The XH NMR study of receptors 70a-c revealed [90] that these compounds complex simultaneously sodium cation (lower rim amidic cavity) and anions (upper rim urea/thiourea), which markedly improves the solubility of the cor-... 

A new method for preparing urea and thiourea complexes of Cr has been described. Thus if CrX3,6H20 (X = Cl or NO3) and urea are heated in n-propanol, [Cr(urea)6]X3 is obtained. ... 

EtNH)2C-S-Me, BF3]I has a shift of -I-18-5, slightly lower than the thiourea complex. The disproportionation of the syw-dimethyl-urea—boron trichloride complex was followed by means of the spectra, and the formation of the tetrachloroborate ion and of a cyclic compound was suggested from the shift data. 

J5] have been reported. Recent work by Thomas and Harris [36-38 and Hollingsworth [JS] have shown that the inclusion phenomenon is much more complex, varied, and interesting than had been realized all along, and the latest review on urea/thiourea/selenourea inclusion compounds was published by them in 1996 [38b. The formation, characterization, and segmental mobilities of block copolymers in crystals of their urea inclusion compounds have been reported recently [39]. 

There are more striking differences in the unit-cell dimensions of the selenourea inclusion compounds than are observed for the thiourea inclusion compounds, as adaptation of the selenourea lattice to the shape and size of the included molecules is apparently relatively easy [57-Si]. A theoretical study of the conformations and vibrational frequencies in urea, thiourea, and selenourea compounds has been reported [54]. Studies on the complexation of TT with I" and urea, thiourea, or selenourea (and their Me derivetives) using a horizontal zone electromigration method have indicated that the complexing capacities of the urea derivatives increase in the order urea < thiourea < iV,N, N -tetramethylthiourea < selenourea [55]. 

We now present results illustrating that the N-H- 0 and N-H -S hydrogen bonds play an instrumental role in the structures of a novel class of urea/thiourea inclusion compounds. The average lengths of 84 measured N-H - O hydrogen bonds in 16 urea-anion inclusion complexes and 114 measured N-H—S hydrogen bonds in 23 thiourea-anion host lattices are listed in Table 11. 

Comparison of the urea-anion inclusion compounds with the thiourea-anion inclusion compounds indicates that urea molecules more readily combine with other anionic moieties to generate composite dimers or ribbons. Composite dimers or ribbons exist in seven complexes among the nine urea-anion inclusion compounds studied here. For instance, both urea-bicarbonate complexes 1.6 and 1.7 (Figure 36 and Figure 37) contain a hydrogen-bonded [((NH2)2CO)2(HC03)2] ribbon built of alternating urea dimers and cyclic dimeric bicarbonate moieties (LB... 

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