Ureas coordination chemistry

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. 

The marrying of the fields of coordination chemistry, chemotherapy and enzymology was finally spurred on by the advent of modern instrumental and synthetic techniques, and not least by the dramatic developments in organic synthesis, which was born as a discipline in itself in 1828 with Friedrich Wohler s synthesis of urea from ammonium cyanate. In the course of the development of supramolecular chemistry, enormous progress has been made on quantifying the details of receptors with an affinity for guests which fit inside them. The lock and key image especially has suffered... 

The novel /V-hcicrocyclic carbene (NHC) l,3-dicyclohexyl-l,3-diazepan-2-ylidene 80 and its 5,6-dioxolane derivative 81 were synthesised and their coordination chemistry with Rh(I), Ir(I), and Pt(0) explored. The coordinated carbene ligands display extremely large NCN bond angles in crystal structures <0704800>. The cyclic urea 82 was synthesised and fluorescent properties studied in a search for new DNA/RNA bioprobes <07JOC102>. 

In the evolution of anion coordination chemistry, it was discovered that neutral molecules can also operate as effective receptors for anions provided that they contain polarised N-H fragments (e.g. of amides [53], ureas [54], thioureas [55] or pyrroles [56]), which act as H-bond donors for anions. These receptors cannot compete for hydrogen bonding with water and alcohols and must be studied in aprotic solvents of varying polarity, e.g. CHCI3, MeCN, DMSO. In this vein,... 

The Tp ligands have been successfirUy used to prepare the stable copper carbonyl complexes as [Cu(Tp)(CO)]. Recently a solution of CuTp has been employed to improve the solubihty of CO that upon release from the complex has been used in low-pressure, one-pot palladium-mediated carbonylation reactions for the production of C-radiolabeled amides and ureas. The coordination chemistry oftris(3-R-5-R -pyrazolyl)borate ligands bearing different substituents (Tp , Tp ", "ppPhos.Me ... 

The reactivity of dioxygen with nitrogen-coordinated copper(I) complexes has received extensive attention over the past two decades [53,54]. To date, analogous reactivity has not been realized for NHC-coordinated Cu(I). Ster-ically unhindered bis-carbene complexes of Cu(I) undergo rapid conversion to the corresponding ureas upon exposure to air in CH2CI2 solution (Eq. 4) [55]. This result suggests NHCs may not be universally applicable to metal-mediated oxidation chemistry. 

In methanol solution, the terminal water ligands in the phtalazine-based dinickel complex 32 (see Section 11.B.2) can be replaced by O-bound urea to give 75 (Fig. 15). When acetonitrile is used as the solvent instead of methanol, one of the substrate molecules shifts to an unusual single atom O-bridging position in 76 (the only other known example in dinickel chemistry being 56). Solid-state (IR) spectra reveal a shift of the C=0 stretching frequency of urea from 1690 to 1663 cm in 75 or 1661 cm in 76upon coordination to the dinickel sites. Due to... 

The urea pentaammine complexes of Co" , Rh " and Cr" do not provide any evidence for detectable production of [(NH3)sM—OOCNH2] required for reactions to model the chemistry of urease. This is not altogether surprising as there is little apparent activation upon coordination. The pXa of the free ligand (ca. 14) is not lowered substantially on Co" (13.19), Rh" (13.67) or... 

The most widely documented aspect of phosgene chemistry is the interaction of the reagent with nitrogen nucleophiles (for example, R3N). The intermediate complex 129 formed is unstable and usually collapses, with elimination of RCl, to give a stable carbamoyl chloride derivative 130. The carbamoyl chloride thus formed can eliminate RCl to create a new site of unsaturation (isocyanate, 131) and/or react further with another molecule of starting material to afford urea, 132, a tri-coordinated carbonic acid derivative (see Scheme 4.1). 

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