Carboxamidate ligands

Chiral dirhodium(II) catalysts with carboxylate or carboxamidate ligands have recently been developed to take advantage of their versatility in metal carbene transformation, and these have now become the catalysts of choice for cyclopropanation. Chiral carboxylate ligands 195,103 196,104 and 197105 have been used for tetrasubstitution around a dirhodium(II) core. However, the enantioselectivity in intermolecular reactions with simple ketenes is marginal. 

The carboxylate attachment to the carboxamidate ligand is essential for high levels of enantiocontrol. Replacement of the COOMe moiety by benzyl (25a), isopropyl (25b), or phenyl (25c) group, which is of similar size, almost always results in significantly lower selectivities [68,71]. 

Fig. 20.4.31(a)], In the Ag /ify-Cg) motif, one carbon atom is embraced by four silver atoms in a butterfly arrangement and the other bonds to two silver atoms. Its existence can be rationalized by the fact that it is stabilized by four surrounding pyrazine-2-carboxamide ligands so that steric overcrowding obstructs the aggregation of silver(I) into a closed cage [Fig. 20.4.31(b)],... 

Enantioeontrol in cyclopropenation reactions is obviously highly dependent on the carboxylate substituent of the dirhodium(II) carboxamide ligand and on the carboxylate substituent of the intermediate carbene. High enantioseleetivity is achieved with the use of Rh2(MEPY)4 catalysts and menthyl diazoacetates in reactions with 1-alkynes, and further enhancement in % ee can be anticipated. 

The basicity constants of a series of diaminoamides in aqueous solution were determined potentiometrically.48 Among the sulfur carboxamide ligands, a series of benzoyl-protected dimercaptodiamides like (38) should be mentioned.49 This amide thiol ligand in the tetra-anionic form (R = H) spans the basal position of an expected square-pyramidal structure, increasing the kinetic stability of the low-spin TcO core which can be used in diagnostic nuclear medicine procedures. 

In rhodium(II)-catalyzed intermolecular cyclopropanation reactions, chiral dirhodium(II) carb-oximidates provide only limited enantiocontrol. " Tetrakis(5-methoxycarbonyl-2-pyrrolidonato)dirhodium [18, Rh2(MEPY)J, in both enantiomeric forms of the carboxamide ligands, produces the highest enantioselectivities. As can be seen for the cyclopropanation of styrene with diazoacetates, a high level of double diastereoselectivity results from the combination of this chiral catalyst with /- or d-menthyl diazoacetate, but not with diazoacetates bearing other chiral residues.In terms of trans/cis selectivity and enantioselectivity for styrene giving 19 this catalyst is comparable to the Aratani catalysts, but they cannot match the high enantiocontrol of the chiral copper catalysts developed by Pfaltz, Masamune, and Evans vide supra). 

Numerous subsequent studies showed that a high degree of control of both reactivity and selectivity could be achieved by varying the nature of the carboxy-late or carboxamide ligands on the metal core. This approach to the control of selectivity has proved to be a very effective guide to catalyst development. 

As already mentioned (Section 2.5.1.2.1.3), rhodium(III) borohydride complexes with chiral carboxamide ligands have also been used as enantioselective catalysts for the hydrogenation of a,/ -unsaturated carboxylic esters89. In one case, with methyl 3-phenyl-2-butenoate as substrate, optical yields in the range of 60% have been achieved. 

Afshar RK, Patra AK et al (2004) Syntheses, structures, and reactivities of (Fe-NO)s nitrosyls derived from polypyridine-carboxamide ligands photoactive NO-donors and reagents for S-nitrosylation of alkyl thiols. Inorg Chem 43 5736-5743... 

Rh catalysts have achieved impressive transformations that are without precedent with other catalysts, in particular with regard to stereoselective C—C and C—N bond formations. Du Bois and coworkers have developed a series of Rh" complexes bearing carboxylate and carboxamidate ligands. The resulting catalysts show remarkably high reactivities and selectivities in intramolecular (30) and intermolecular C—H aminations (31). " ... 

Structurally similar, dimeric Rh" catalysts with four carboxylate or carboxamidate ligands efficiently catalyze carbene insertions into C—H bonds. In this field, highly enantioselective catalysts have been developed by Davies and coworkers, together with mechanistic models that help predict selectivities in these transformations. The resulting protocols have been widely applied, in particular for the synthesis of complex molecules such as R-(-)-rolipram (34). 5... 

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