Ureido complexes

The formamide is the major product with relatively small alkyl groups (R = Pi ) and urea predominates when bulky alkyl groups (R = Bu , 1-adamantyl) are attached to the nitro function. This is an apparent indication of the importance of steric effects. An ureido complex serves as a precursor to urea (eq. 23) ... 

In the presence of excess CO, the isocyanate complex (Scheme 20) initially gives isocyanate displacement [218]. Several pieces of evidence indicate that, as the concentration of free isocyanate raises in solution, head-to-tail insertion of isocyanate into the Rh-C bond of the isocyanate complex competes for elimination. The product of this insertion immediately loses one CO to afford an ureido complex ... 

FIG U RE 2.25 Stereo views of the complexes between (a) TAG and iV-acetyl-D-phenylgly cine or (c) iV-acetyl-L-phenylglycine, generated by computer modeling, (b) Exploded view of the complex between TAG and N-acetyl-phenylglycine (unspecified stereochemistry at the ligand). The primary amino group of TAG is converted to a ureido function to model the TAG CSP. 

Fig. 11. The structure of eNOS complexed with SEITU. The ureido function H-bonds with Glu363 while the ethyl group interacts with both Val338 and Phe355.

The crystal structure of the eNOS—SEITU complex is shown in Fig. 11. As expected, the ureido group H-bonds with Glu363 similar to the way L-Arg interacts with Glu363. Two water molecules form H-bonding bridges between the inhibitor and heme propionates. The inhibitor sulfur is A from the heme iron but appears not to coordinate with iron,... 

The macrocyclic receptors 32 [43] and 33 [44] combining two calix[4]arene motifs within the molecule were designed for anion recognition. While 32 creates 1 1 complexes with several anions (halides, HSOj, H2POj), compound 33 is too rigid to efficiently complex halides or benzoate. By contrast, similar cage molecule 34 with the ureido bridges showed complexation ability towards chloride or benzoate. 

Two ureido functions introduced into the upper rim of calix[4]arene [52] or the corresponding calix[4]diquinone derivative 41 exhibit good complexa-tion abilities for various anions. Even in highly competitive solvents such as DMSO-d6, quinone 41 shows [53] very strong complexation for H2POj (K4l= 13,900 M-1) or for benzoate (K41=2,430 M-1). The complexation process can be observed using electrochemical methods (cyclic voltammetry) where the addition of anions generates substantial cathodic shifts. 

Winstein and coworkers have summarized work compaiing the anchi-meric ability of some nitrogen-containing, complex, neighboring groups, and have noted that, under neutral conditions, the order is benzamido > ureido > urethano > acetoxy. On this basis, the ureido and urethano groups would both be expected to be very effective in participation reactions the few examples of such reactions reported in the sugar field bear out this expectation. 

Rh complex 170 and the one derived from [(cod)2Rh]BF4 and diphosphine 171 are active in promoting reduction of a-ureido-a,p-unsaturated esters and enamides, respectively. 

Very high association constants (>10 M ) for the formation of the capsular 1 1 complex in CD2CI2/CD3OD (2 1) solution were estimated for alkyl chains of C12-C16. The ureido functionality of the bridge at the wider rim of host 11-24 supports the compl-exation in nonprotic solvents by binding with anions such as chloride, picrate, or car-boxylate (see Figure 12.4). Shorter alkyl chains are encapsulated by only a single cavity. 

The reactivity of biotin with several reagents can be exploited in some chemical assays like its reaction with diazo derivatives, or the reaction of the ureido ring with p-dimethylaminocinnamaldehyde in an acidic medium, but the sensitivity of these assays is relatively poor. Several methods described for biotin assay involve a competitive complex formation between biotin and avidin bound with a chromophore/ fluorophore probe. In these assays, biotin, which has a higher affinity for avidin, quantitatively displaces the probe from the complex. 

The monomeric unit used in the studies in shown in Fig. 45.3. It is based on poly(s-caprolactone) with a terpyridine ligand (for metal complexation) at one end and an ureido-pyrimidone group (for hydrogen bonding) on the other end. The polymeric spacer is used to control the solubility in various solvents, in such a way that both hydrogen-bonding and metal-complexation can take place simultaneously. This unit shows a good solubility in chloroform. 

---