Optically active complexes

Optically Active Complexes. The c.d. spectra of nineteen optically active chromium(iii) complexes have been recorded in both the spin-allowed and spin-forbidden d-d transition regions, and discussed in detail. The rotatory strengths of the d-d transitions of trigonally distorted six-co-ordinate complexes of chromium(iii) with N- and O-donor atoms have been calculated using a MO model. The results obtained suggest that neither the Piper nor the Liehr MO model provides adequate representations of the source of d-d optical activity in such systems.  

Partial photoresolution of [Cr(acac)3] in several organic solvents has been 

Anderson, F. Galsbol, S. E. Harnung, and T. Laier, Acta Chem. Scand., 1973, 27, 3973. 

Harris arid E. A. Boudraux, Chem. Phys. Letters, 1973, 23, 434. 

Cr2NbOsF M1CrM204 Nb02F + Cr203 at 900°C X. trirutile structure i 

Eu2TaCrOe LnCr03 Eu203-Ta205-Cr203 system p. X. perovskite lattice n 

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These complexes can be isolated in some cases in others they are generated in situ from appropriate precursors, of which diazo compounds are among the most important. These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, palladium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Ethyl a-diazoacetate reacts with styrene in the presence of bis(ferrocenyl) bis(imine), for example, to give ethyl 2-phenylcyclopropane-l-carboxylate. Optically active complexes have... 

Contents Introduction. - X-Ray Difraction. -Conformational Analysis. - Structure and Isomerism of Optically Active Complexes. - Electron-Density Distribution in Transition Metal Complexes. - Circular Dichroism. - References. 

The kinetics of the reduction of spinach plastocyanin PCu(II) by the optically active complexes 2,6-bis[3-(S)- or 3-(/ )-carboxyl-2-azabutyl] pyridine, here abbreviated to (S,S)- or (f ,/ )-ALAMP have been studied [107]. The latter enantiomer (A-configuration) reacts 1.6-2.0 times faster at different values of pH and temperature than the S,S form. Activation parameters have shown that the observed stereoselectivity is a consequence of the difference in activation... 

Phosphine-containing compounds are prepared by reaction of [PPh3(CO) Mn]2 with silanes and this reaction allows the synthesis of an optically active complex ... 

Electron rich reagents can displace the metal IVb hydride and in the case of optically active complexes this reaction proceeds with retention of configuration. 

Quite a few complexes with the bidentate pentasulfido ligand are also known. The first reported was the homoleptic and optically active complex [Pt(85)3] (15) (53, 64, 65, 68, 69, 176). Brick-red (NH4)2[Pt(85)3] 2H20 is formed from the reaction of K2[PtCl6] with aqueous (NH4)28 solution. Addition of concentrated HCl results in the separation of maroon (NH4)2[Pt8i7] 2H20 (54). The [Pt(85)3] ion crystallizes from the solution as a racemate, which can be resolved by forming diastereoisomers. Upon crystallization, [Pt8,7] undergoes a second-order asymmetric transformation, so that the solid contains an excess of the (—) enantiomer (54). 

The availability of ketone 26 and its effectiveness toward a wide variety of tmns-and trisubstituted olefins make the epoxidation with this ketone a useful method. Other researchers have used ketone 26 in the synthesis of optically active complex molecules. Some of these studies will be highlighted in this section. 

The electronic301 and magnetic properties of mononuclear chromium(III) complexes are quite well understood however there is a distinct tendency for octahedral symmetry to be invoked in cases where the true symmetry is much lower. Chromium(III) is a hard Lewis acid and many stable complexes are formed with oxygen donors. In particular hydroxide complexes are readily formed in aqueous solution, and this may be a problem in synthesis. Substitution at chromium(III) centres is slow302,303 and may well have some associative character in many cases. The kinetic inertness of chromium(III) has led to the resolution of many optically active complexes this work has been extensively reviewed.304... 

Fig. 12.20 Structures of the optically active complexes [Co(en)3l3+ and [Crtoxfc)3- (one enantiomer of eachl and fl stylized drawing of the two enantiomers of any iris(chelate) complex.

Another set of reactions that has received considerable attention is that ui which optically active complexes, especially tris(chelate) compounds racemize ... 

The coordination of methionine, MeSCH2CH2CH(NH2)C02H, to nickel has been investigated in solution. It has been found that the complex [Ni(D-Met)(L-Met)] (Met = monoanionic methionine) is a little more stable than the optically active complex. The ligand is supposed to act as tridentate with at least weak Ni—S (thioether) bonding. 72... 

Direct addition of VI to chlorocarbonylbis(phosphine)rhodium complexes containing more basic phosphine ligands to yield optically active complexes of the type III did not occur. An optically active benzylrho-dium complex (X) was obtained, however, by adding (S)a-trifluoro-methylbenzylchlorosulfite (IX) to chlorocarbonylbis (diethylphenylphos-... 

A few complexes with the bidentate pentasulfido ligand are known. The first one reported was the homoleptic and optically active complex [Pt(S5)3]2 (45).133147 The MS5 moiety has a chair conformation in all known mononuclear complexes. It is worth noting that the bite varies strongly and that the largest one is found in [(S3)Fe(MoS4)]2-.132... 

The synthesis of threonine can be made stereospecific using optically active complexes of the type L-[Co(en)2Gly]2+ but with low asymmetric yield.442 In the case of dipeptide complexes only... 

The optically active Mo complexes 13a and 13b are configurationally stable28,29 Their optical rotations in solution remain constant over long periods of time. If, however, a trace of free R—(—)-a-phenyl ethyl isonitrile30,31 is added to solutions of 13a at room temperature, its optical rotation decreases within about one hour to values around 0 28,29). We explain this behavior by a backside attack of the free isonitrile on the optically active complex 13a according to Eq. (11). 

Another important feature is the planar chirality of complexes of ortho and meta disubstituted arenes, and asymmetric synthesis is possible using optically active complexes. 

Although the optical yield was unsatisfactory, stereoselective monocoupling of the o-dichlorobenzene complex 298 with phenylboronic acid catalysed by an optically active Pd catalyst gave rise to the optically active complex 299 [70]. 

The general structure of the crystallised optically active complexes was [Ca(L)2]DBTA (11 in the formula L stands for 8, 9 or 10, separately) and they contained two moles of the corresponding (R)-alcohols (L) in all cases (Table 2). [25]... 

Conversion of the separated diastereoisomers 10a and 10b into the enantiomers +9 and —9 was achieved by treatment with HC1 in benzene solution. In the examples given earlier, to accomplish conversion of diastereoisomers into enantiomers, only those bonds were broken that did not involve the chiral metal atom this was to avoid loss of optical purity through possible change in configuration of the metal atom. In this work, Ti—O bond cleavage had to be used to convert the diastereoisomers (10) to the enantiomers (9). However, HCI cleavage of the Ti—OR bond in compounds of type 10 was shown to be stereospecific with respect to the chiral Ti atom, and to occur with retention of configuration (44-48). Optically active complexes with a chiral Ti atom could also be obtained by asymmetric decomposition (49, 50). 

A number of stereospecific non-enzyme catalysts have been developed that convert achiral substrates into chiral products. These catalysts are usually either complex organic (Figure 10.8(a)) or organometallic compounds (Figure 10.8(b)). The organometallic catalysts are usually optically active complexes whose structures usually contain one or more chiral ligands. An exception is the Sharpless-Katsuki epoxidation, which uses a mixture of an achiral titanium complex and an enantiomer of diethyl tartrate (Figure 10.8(c)). 

Another important argument is provided by the stereochemistry of substitution reactions at silicon. In the optically active complex 6 nucleophilic substitution by H, OH-, or OMe- occurs with inversion of configuration at silicon, while retention was observed with the related... 

The sex attractant pheromone of the gypsy moth [Lymantrla dispar (L.)] is disparlure (cis-7,8-epoxy-2-methyloctadecane). The natural attractant is the (+) enantiomer it is a powerful attractant for male moths and is used throughout the United States as a bait in traps to detect infestations. A convenient and economic synthesis, recently reported, involves oxidation of an inactive unsaturated precursor with an optically active complex to yield an epoxide of high enantiomeric purity. 

Asymmetric induction has been observed in the hydrosilation of prochiral olefins with optically active complexes as catalysis. See I. Ojima, K. Yamamoto, and M. Kumada, Aspects Homogen. Catal. 3, 185 (1977). 

Azumaya et al. reported an interesting example of retention of the molecular chirality when the chiral crystal of l,2-bis(Af-benzoyl-Af-methylamino)benzene 65 was dissolved in a cold solution (Fig. 6) [37]. Furthermore, Tissot et al. reported a fine example of the formation of optically active complex 67 ( 100%, ee in 93% yield) using axially chiral ligand 66 (Scheme 32) prepared by chira ... 

Scheme 32 Optically active complex formation in solution.

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