Chiral ureas

Further upgrading by crystallization is possible. Presumably a highly diastereomerically enriched titanium intermediate is formed by chelation with the chiral urea moiety. 

Recently, enantioselective organo-catalytic procedures for the aza-Henry reaction have been disclosed. The presence of either an acidic or a basic function appears to be a requisite of the catalyst. In fact, the condensation of ni-tromethane with M-phosphinoyl arylimines 72 is catalyzed by the chiral urea 85 derived from (R,R)-l,2-diaminocyclohexane and gives the product (R)-74 with good yield and moderate enantioselectivity (Scheme 15) [50]. The N-phosphinoyl substituent is determinant, as the addition of nitromethane to the N-phenyl benzaldimine failed and the reaction of the N-tosyl ben-zaldimine gave the expected adduct with quantitative yield but almost no... 

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. 

Chiral amines and diamines are readily available substrates for the synthesis of ligands for transition metal-catalysed reactions since they can easily be transformed into chiral ureas and thioureas. Therefore, several groups have prepared chiral symmetrical ureas and thioureas, dissymmetrical ureas and thioureas, amino-urea and thiourea derivatives. Finally polyureas and non-soluble polythioureas were also prepared and tested as ligands for asymmetric catalysis. 

Finally, nitrone cyanation were performed with non-chiral urea and thiourea derivatives, the latter being more efficient for this process. No chiral compound has been described yet (Scheme 41) [ 159]. 

Asymmetric deprotonation of prochiral cychc ketones (Scheme 50) was performed with chiral ureas in the presence of butylhthium. Yields were good (85-88%) with high enantioselectivities (83-87%). Moderate enantioselectiv-ity is obtained with the cyclopentyl-containing urea (Scheme 50 37% ee with R = Ph 7% ee with R = Me) [ 168,169]. 

Breuzard JAJ, Christ-Tommasino ML, Lemaire M (2005) Chiral Ureas and Thiroureas in Asymmetric Catalysis. 15 231-270 Briiggemann M, see Hoppe D (2003) 5 61-138... 

TABLE 12. The asymmetric reduction of prochiral ketones under catalysis of chiral urea derivative 8173 (in all reactions 5% catalyst was used)... 

Jacobsen (1998), Corey (1999) chiral ureas/thioureas and guanidine Enamine Cataiysis... 

Since Curran and Kuo and Schreiner and coworkers reported that urea and thiourea derivatives act like Lewis acid catalysts, several chiral urea and thiourea catalysts have been designed by Jacobsen et al. and Takemoto et al. ... 

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