Transition complexes, chiral

Even more complex functionalities can be established with chiral functional units. There is a very pure tt-tt transition of 1 in the visible. As a consequence, no CD effects are observed in the visible for chiral aliphatic substituents R. This allows the construction of complex chiral stractures without interference with the optical properties of 1. 

In the ligand polarization mechanism for optical activity, the potential of the electric hexadecapole component, Hxy(x>-y>), produces a determinate correlation of the induced electric dipole moment in each ligand group which does not lie in an octahedral symmetry plane of the [Co Ng] chromophore (Fig. 8). The resultant first-order electric dipole transition moment has a non-vanishing component collinear with the zero-order magnetic moment of the dxy dxj yj transition in chiral complexes, and the scalar product of these two moments affords the z-component of the rotational strength, RJg, of the Aj -> Ti octahedral excitation. 

Circular Dichroic Intensities in the Vibronic Transitions of Chiral Metal Complexes... 

A chiral stationary phase is used comprising a silica substrate on which is fixed an amino acid. The mechanisms developed are of the "ligand exchange" type. The distinction between two enantiomers is possible due to the formation of mixed diastereoisomerical complexes (chiral solute - transition metal - amino acid fixed on the silica substrate). The transition metal is added to the mobile phase. (Fig. 4)... 

With this information in hand, it seemed reasonable to attempt to use force field methods to model the transition states of more complex, chiral systems. To that end, transition state.s for the delivery of hydrogen atom from stannanes 69 71 derived from cholic acid to the 2.2,.3-trimethy 1-3-pentyl radical 72 (which was chosen as the prototypical prochiral alkyl radical) were modeled in a similar manner to that published for intramolecular free-radical addition reactions (Beckwith-Schicsscr model) and that for intramolecular homolytic substitution at selenium [32]. The array of reacting centers in each transition state 73 75 was fixed at the geometry of the transition state determined by ah initio (MP2/DZP) molecular orbital calculations for the attack of methyl radical at trimethyltin hydride (viz. rsn-n = 1 Si A rc-H = i -69 A 6 sn-H-C = 180°) [33]. The remainder of each structure 73-75 was optimized using molecular mechanics (MM2) in the usual way. In all, three transition state conformations were considered for each mode of attack (re or ) in structures 73-75 (Scheme 14). In general, the force field method described overestimates experimentally determined enantioseleclivities (Scheme 15), and the development of a flexible model is now being considered [33]. 

The AICD (associate-induced circular dichroism) model was applied to a consideration of the d-d transitions of chiral metal complexes (Schipper, 1978). Detailed selection rules were derived, and the model was applied to a number of series of complexes. Good agreement with experimental data was obtained, and it was suggested that empirical sector rules (including the hexadecant rule) for metal complexes have little theoretical foundation. 

The theory of optical activity in the ligand-field transition of chiral transition metal complexes has been reviewed (Richardson, 1979c, and references therein). 

A number of lanthanide complexes have been shown to exhibit circularly polarized luminescence (CPL—the differential spontaneous emission of left- and right-circularly polarized light). In the absence of any externally applied fields, CPL is exhibited only by systems that have net chirality in their structures or are subject to chiral perturbations by their environment. CPL exhibited by the Af-Af transitions of chiral lanthanide systems provides a sensitive probe of coordination and structure in solution. Applications are limited to systems which possess some element of chirality, but in many cases this merely requires that > 1 ligand of interest has a chiral atom or carries a chiral label (such as a chiral substituent group). ... 

CPL investigations on Eu(III) and Tb(III) nitrate complexes with the chiral D2-symmet-ric ligand 4/, 9/, 19/ ,24/ -3,10,18,25,3l,32-hexaazapentacyclo[25.3.1.1.0.0]-dotriaconta-l-(31),2,10,12,14,16(32),17,25,27,29-decaene (7 -pydach) (Scheme 12) and its S S -pydach enantiomer have shown that the strong measured CPL, g um = —0.19 at 596 nm ( Dq Fi transition), was only due to the twisted conformation (Tsubomura et al., 1992). In the case of the complexes, [Ln(7 -pydach)] + and [Ln( S S -pydach)] +, there was no evidence of a contribution from coordinated nitrate anions on the complex chirality as previously observed. The complex structures which have been characterized by NMR and luminescence spectroscopy have suggested that the nitrate anions were not coordinated to the lanthanide (III) ion in these species and, also, that approximately three water molecules were bound to the metal center. 

Samarium iodide promotes the pinacol homo-coupling of aldehydes to give 1,2-diols, but typically with little difference in the yields of the syn- and anti-isomers. Addition of Lewis acids, however, improves the selectivity, apparently by complex-ing both reactants in the transition state. Chiral aldehydes such 2-phenylpropanal can give syn/anti ratios >50, even without additives, and in some cases they give exclusively one product with appropriate choice of conditions. 

The relay catalysis combining a mechanistically distinct transition metal catalyst and an organocatalyst for one cascade has stimulated intensive interest in recent years, as it could potentially enable highly efficient and/or unprecedented transformations in a one-pot operation. Indeed, excellent transformations have been established by this approach. One elegant example is relay catalysis using a rhodium complex/chiral Brpnsted acid binary system (Scheme 5.87) [88]. It is believed that the first catalytic... 

The reaction of [2+2+2] cycloaddition of acetylenes to form benzene has been known since the mid-nineteenth century. The first transition metal (nickel) complex used as an intermediate in the [2+2+2] cycloaddition reaction of alkynes was published by Reppe [1]. Pioneering work by Yamazaki considered the use of cobalt complexes to initiate the trimer-ization of diphenylacetylene to produce hexasubstituted benzenes [54]. Vollhardt used cobalt complexes to catalyze the reactions of [2+2+2] cycloaddition for obtaining natural products [55]. Since then, a variety of transition complexes of 8-10 elements like rhodium, nickel, and palladium have been found to be efficient catalysts for this reaction. However, enantioselective cycloaddition is restricted to a few examples. Mori has published data on the use of a chiral nickel catalyst for the intermolecular reaction of triynes with acetylene leading to the generation of an asymmetric carbon atom [56]. Star has published data on a chiral cobalt complex catalyzing the intramolecular cycloaddition of triynes to generate a product with helical chirality [57]. 

Benedetti M, Biscarini P and Brillante A, The effect of pressure on circular dichroism spectra of chiral transition metal complexes Physica B 265 1... 

Metal Complex. Complexation gas chromatography was first introduced by V. Schurig in 1980 (118) and employs transition metals (eg, nickel, cobalt, manganese or rhodium) complexed with chiral terpenoid ketoenolate ligands such as 3-ttifluoroacetyl-lR-camphorate (6),... 

Technetium-99m coordination compounds are used very widely as noniavasive imaging tools (35) (see Imaging technology Radioactive tracers). Different coordination species concentrate ia different organs. Several of the [Tc O(chelate)2] types have been used. In fact, the large majority of nuclear medicine scans ia the United States are of technetium-99m complexes. Moreover, chiral transition-metal complexes have been used to probe nucleic acid stmcture (see Nucleic acids). For example, the two chiral isomers of tris(1,10-phenanthroline)mthenium (IT) [24162-09-2] (14) iateract differentiy with DNA. These compounds are enantioselective and provide an addition tool for DNA stmctural iaterpretation (36). 

Preparation of enantiomerically enriched materials by use of chiral catalysts is also based on differences in transition-state energies. While the reactant is part of a complex or intermediate containing a chiral catalyst, it is in a chiral environment. The intermediates and complexes containing each enantiomeric reactant and a homochiral catalyst are diastereomeric and differ in energy. This energy difference can then control selection between the stereoisomeric products of the reaction. If the reaction creates a new stereogenic center in the reactant molecule, there can be a preference for formation of one enantiomer over the other. 

In 1990, Jacobsen and subsequently Katsuki independently communicated that chiral Mn(III)salen complexes are effective catalysts for the enantioselective epoxidation of unfunctionalized olefins. For the first time, high enantioselectivities were attainable for the epoxidation of unfunctionalized olefins using a readily available and inexpensive chiral catalyst. In addition, the reaction was one of the first transition metal-catalyzed... 

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