Carboxylate hgands

In approaching the design of a chiral catalyst, the first question was whether or not our computational approach would allow prediction of the conformation of the ligands around the Rh-Rh core. Although several hundred tetrakiscarboxylato metal-metal dimers were known [20], there had been no report of a dicarboxylic acid that would bridge two positions on such a dimer [21]. We reasoned that the best chance for success would be with a dicarboxyhc acid that was specifically designed to fit across the 5.4 A gap between the carboxylate Hgands. 

Here, L Fe stands for the (H20)(NH2)Fe-fragment. This table does not include the portion of spin densities located on the bridging carboxylate hgands, each of which may have about... 

Ethyl 6-methyl-4-phenyl-2-(phenylthio)-l,4-dihydropyrimidine-5-carboxyla-te was easily synthesized from ethyl 6-methyl-4-phenyl-2-thioxo- 1,2,3,4-tetra-hydropyrimidine-5-carboxylate and phenylboronic acid via microwave-assisted Cu-mediated S-phenylation (Scheme 108) [107]. The reaction involves the use of a stoichiometric amount of Cu(OAc)2 and a two-fold excess of 1,10-phenanthroline as a hgand. 

Rhodium(II) forms a dimeric complex with a lantern structure composed of four bridging hgands and two axial binding sites. Traditionally rhodium catalysts faU into three main categories the carboxylates, the perfluorinated carboxylates, and the carboxamides. Of these, the two main bridging frameworks are the carboxylate 10 and carboxamide 11 structures. Despite the similarity in the bridging moiety, the reactivity of the perfluorinated carboxylates is demonstrably different from that of the alkyl or even aryl carboxylates. Sohd-phase crystal structures usually have the axial positions of the catalyst occupied by an electron donor, such as an alcohol, ether, amine, or sulfoxide. By far the most widely used rhodium] 11) catalyst is rhodium(II) acetate [Rh2(OAc)4], but almost every variety of rhodium] 11) catalyst is commercially available. 

Sun, S. et al. Chiral hgand exchange chromatography for separation of three stereoisomers of octahydroindole-2-carboxylic acid. Chromatographia 2006, 63, 331-335. 

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