Proton donors thiourea

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. 

Squaramides offer yet another variant of the bis-NH proton donor with different geometry compared to thioureas and their congeners. Thus, the quinine-derived squaramide (238b) can catalyse Michael addition of -dicarbonyls to ArCH=CHN02. Here, brine has been found to provide remarkable rate acceleration and a higher level of stereoselectivity (>98% ee) over organic solvents, presumably because of the hydrophobic hydration effect. ... 

Soon afterward, various types of carbon [40-44], oxygen [45], and phosphorous [46] Michael donors were successfully employed in the thiourea-catalyzed addition to nitroalkenes. In the presence of the bifunctional epi-9-amino-9-deoxy cinchonine-based thiourea catalyst 79a, the 5-aryl-l,3-dioxolan-4-ones 138 bearing an acidic a-proton derived from mandelic acid derivatives and hexafluoroacetone were identified by Dixon and coworkers as effective pronucleophiles in diastereo- and enantioselective Michael addition reactions to nitrostyrenes 124 [40]. While the diastereoselectivity obtained exceeded 98%, the enantiomeric excess recorded... 

As described above, cinchona-based (thio)ureas have proven to be highly efficient H-bond donor catalysts. In 2008, Rawal and coworkers developed a highly promising new family of cinchona-based H-bond donor catalysts such as 157 by replacing the thiourea moiety of cinchona-based thiourea catalysts with the squaramide unit [47]. The squaramide moiety of 157 is able to form two H-bonds to a reactant due to the more accessible reaction site and fixed syn-orientation of the NH-protons. Using only 0.5 mol% of the cinchonine-derived squaramide catalyst 157, various Michael donors 158 and nitroalkenes 130 were smoothly converted to the desired adducts 159 in excellent yield and ee values (up to 99% ee) (Scheme 9.54). 

A number of these complexes have been used as anion receptors (492,1470,1471). These include cychc (350) and acyclic (351) dithiocarba-mate-based anion receptors (Fig. 190) incorporating thiourea and amide hydrogen-bond donor groups (1470). The NMR titrations with 351 in DMSO-tf resulted in significant downshield shifts, particularly for the amide protons, while UV-vis titrations with both 350 and 351 in DMSO show that 350... 

A further important advance was also achieved by Rouden et al. [17] who developed the use of thiourea cinchona alkaloids 33-34 in the enantioselective decarboxylative protonation of a-aminomalonates (Scheme 3.12). The basic idea in using these bifunctional catalysts was to take advantage of the good hydrogen-bond donor properties of the thiourea moiety to promote further interactions between the chiral proton source and the prochiral enolate intermediate. Bifunctional catalyst 33 in quinidine series turned out to be especially efficient with a large range of substrates... 

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