Asymmetric complexes

In an Hg22+ asymmetrically complexed by two crown-ethers of different ring size, an extremely high coupling constant /( "Hg, 9Hg) of 284 kHz between the magnetically inequivalent Hg nuclei has been observed.4 3... 

Heterobimetallic asymmetric complexes contain both Bronsted basic and Lewis acidic functionalities. These complexes have been developed by Shibasaki and coworkers and have proved to be highly efficient catalysts for many types of asymmetric reactions, including catalytic asymmetric nitro-aldol reaction (see Section 3.3) and Michael reaction. They have reported that the multifunctional catalyst (f )-LPB [LaK3tris(f )-binaphthoxide] controls the Michael addition of nitromethane to chalcones with >95% ee (Eq. 4.140).205... 

Figure 7.12 Molecular structure of the asymmetric complex [Ln2(HL)2(H2L)(N03)(py)(H20)] with the three deprotonated ligands resulting in two lanthanide sites with differing chemical...

A special case of the chain back skip polymerization mechanism and therefore an entirely different polymerization behavior was observed for differently substituted asymmetric complexes (for example catalyst 3). Although asymmetric in structure, these catalysts follow the trend observed for C2-symmetric metallocenes [20], Chien et al. [23] reported a similar behavior for rac-[l-(9-r 5-fluorenyl)-2-(2,4,7-trimethyl-l-ri5-indenyl)ethane]zirconium dichloride and attributed this difference in the stereoerror formation to the fact that both sides of the catalyst are stereoselective thus isotactic polypropylene is obtained in the same manner as in the case of C2-symmetric metallocene catalysts. 

K. Yamada, T. Arai, H. Sasai, M. Shibasaki, A Catalytic Asymmetric Synthesis of U-Deoxy-PGFla Using ALB, a Heterobimetallic Multifunctional Asymmetric Complex, J. Org. Chem 1998, 63, 3666-3672. 

H. S., and McKay, D. B. Structure and reactivity of an asymmetric complex between HslV and I-domain deleted HslU, a prokaryotic homolog of the eukaryotic proteasome. /. Mol. Biol. 2003, 330, 185-195. 

Fig. 5. NMRD profiles for an asymmetric complex calculated for different values of the transient ZFS. Reproduced with permission from Larsson, T Westlund, P.O. Kowalewski, J. Koenig, S.H. J. Chem. Phys. 1994, 101, 1116-1128. Copyright 1994 American Institute of Physics.

Dinuclear asymmetric and /u-oxo-bridged complexes in which the two technetium atoms are in intimate electronic communication have been synthesized. The reaction of solutions of [TcOCU]" or [TcClg] with neat pyridines py-R (R = H, 2-Me, 3,5-Mc2) gave asymmetric complexes of the type [Cl2(py-R)3Tc(/r-0)TcCl3(py-R)2] (378), and disymmetric complexes [Cl(py-R)4Tc-... 

The hydrocarbon 25 has been partially resolved by asymmetric complexation with Newman s reagent [TAPA ( )-a(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)prop-ionic acid] thereby establishing its chiral Z)2-structure 53). Similarity, the naphthaleno-phane 27b could be resolved by chromatography on silicagel coated with (—)-TAPA 49) and recently also by HPLC on optically active poly(triphenylmethyl methacrylate)49a) which also proved to be very useful for the optical resolution of many other axial and planarchiral aromatic compounds 49b>. 

In the case of the polymerization of methyl and butyl sorbates optical purity varies by varying the type of asymmetric complex used as catalyst. In non-systematic runs carried out with catalysts prepared from lithium-butyl and (—)-menthyl-ethyl-ether, [M]n increases... 

As the catalyst used was not asymmetric, the slight prevalence of (R) asymmetric carbon atoms bound to the CH3 group in the main chain (53% (R) and 47% (S)) could be attributed to the steric control of the propagation step by the asymmetric group present in the monomer. However, since under the polymerization conditions lithium-butyl can form a stable complex with the asymmetric monomer or polymer (34) (Scheme 7 a), the true catalyst, in this case, might be one of the above asymmetric complexes and the synthesis of the asymmetric polymer (Scheme 7b) might be of the type described above (see Section II, 1 a), in... 

Benzo[6]thiophene-2,3-quinone 2-oxime 3-thiosemicarbazone forms asymmetric complexes with nickel and palladium.220... 

While the optical yields are rather low for useful synthetic application, this represents a promising lead for future work in the area of asymmetric complexation reactions. 

Rogozhin, S.V. and Davankov, V.A. (1971) Ligand chromatography on asymmetric complex-forming sorbents as a new method for resolution of racemates, J. Chem. Soc., Chem. Commun. 192, 490. 

Heterobimetallic asymmetric complexes developed by Shibasaki et al. are known as effective catalysts for asymmetric Michael additions. They achieved a catalytic asymmetric protonation in Michael additions of thiols to a,P-unsaturated carbonyl compounds using LaNa3tris(binaphthoxide) and SmNa3tris(binaphthoxide) complexes (SmSB) 37 [42]. For instance, treatment of thioester 48 with 4-fert-butyl(thiophenol) and 0.1 equivalents of (P)-SmSB 37 (Ln=Sm) in CH2C12 at -78°C gave the adduct 49 in 93% ee and in 86% yield (Scheme 4). The high enantiomeric ratio is considered to be attributable to an... 

Asymmetrical nitrido Tc(V) complexes (simply defined as heterocomplexes) are defined as coordination compounds in which two different bidentate ligands are bound to the same Tc=N group, and are represented by the general formula [Tc(N)(L)(L )]"+/0/". The attempt to develop a high-yield synthesis of these types of complexes may first appear to be prevented by basic chemical considerations. Actually, it is reasonable to expect that the reaction of two different bidentate ligands, A and B, with the same Tc=N group would always yield a statistical mixture of symmetrical and asymmetrical complexes, namely [Tc(N)(A)2], [Tc(N)(B)2] and [Tc(N)(A)(B)]. However, the peculiar properties of mixed 7r-acceptor-7r-donor ligands offered the route to the solution of this synthetic problem. The key approach can be outlined as follows. 

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