Ruthenium complexes absolute configuration

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

Arene)ruthenium(II) complexes 120-124, having a planar chirality, have been obtained recently (71,72). Complexes 121-123 are formed by addition of optically active amines (L1 and L2) or phosphine (L3) to the dinuclear complex 120 (Scheme 10). They exist as a mixture of two configurationally stable diastereoisomers. The absolute configuration of one of the diastereoisomers of 123 has been determined, and complex 124 containing a chelating optically active diamine has also been isolated (71,72). 

An example of enantioselective 1,3-dipolar cycloaddition of ethyl diazopyruvate to 2,3-dihydrofuran, catalyzed by a chiral ruthenium-PyBox complex, to provide a tetrahydrofurofuran was reported (Equation 125). However, the adduct 240 was only obtained in 74% ee, and its absolute configuration not determined <2004SL2573, 2005HCA1010>. As shown in Equation (126), 2,3-dihydrofuran also participated in 1,3-dipolar cycloaddition with dipoles derived from aziridines under Sc(OTf)3-catalyzed conditions, forming rfr-fused furopyrrolidines <2001TL9089>. 

In addition to rhodium phosphane complexes, ruthenium phosphane complexes have also been successfully applied as catalysis for enantioseleetive hydrogenation of 2-acylamino-2-alkenoic acids and esters1 71,72b 3, enol acetates 18 (R = i-Pr E = COOEt X = OCOCH3 98% ee with BINAP)137, and itaeonic acid138. The absolute configuration of the products from the ruthenium-catalyzed reactions shown below is opposite to that obtained with the corresponding rhodium catalysts. 

Frauenkron and Berkessel [181], and Che et al. [171], independently reported that the ruthenium complex of the same chiral porphyrin, can be used to catalyze the cyclopropanation of styrene. The synthesis of this chiral porphyrin was previously reported by Halterman and Jan [ 176[. This reaction is particularly interesting since the enantiomeric excesses are quite high (90%). Surprisingly, changing the solvent from 1,2-dichloroethane to benzene resulted in an inversion of the absolute configuration of the major enantiomer for the ds-cyclopropane and no change for the trans-cyclopropane [181]. 

In 2001, Takahashi et al. [204] described the first Ru-catalyzed asymmetric allylic substitutions. The planar-chiral cydopentadienyl-mthenium complexes led to branched aUylation products with enantiosdectivities of up to 97% ee. Some years later, they showed that such complexes serve as effective catalysts for the kinetic resolution of racemic allyhc carbonates such as 200 in AAAs. The absolute configurations of the recovered carbonates and the alkylation products such as 201 were shown to depend on the substituent on the cyclopentadienyl group at the 4-position of the ruthenium catalyst (Scheme 12.98) [205]. 

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