Transition metal compounds resolution

This article is confined to organo-transition-metal compounds having chiral metal atoms whose optical activity has been demonstrated. Only those compounds are discussed in detail for which there is a choice with respect to the metal configuration and for which a separation or at least an enrichment of isomers with opposite metal configuration has been achieved. After the treatment of such topics as optical resolution, optical purity, optical stability, optical induction, stereochemistry of reactions, relative and absolute configurations, Table I (Section XVII) collects the information available for the compounds under consideration. 

The idea of lone pairs was originated by W. J. Pope of Cambridge in 1900 who extended the concept of the three-dimensionality of carbon and nitrogen compounds to those of sulfur. His resolution of sulfonium cations RR R"S+ with three different substituents into optically active enantiomers suggested that these species were tetrahedral with an invisible substituent. The influence of these lone pairs can hardly be detected in transition metal compounds, but the situation is different for post-transition group central atoms such as Ge(II) As(III), Se(IV), and Br(V) with 30 electrons, In(I), Sn(II), Sb(III), Te(IV), I(V), and Xe(VI) with 48 electrons, and Au( —I), T1(I), Pb(II), and Bi(III) with 80 electrons (90). 

High resolution spectroscopic investigations of rotational spectra of transition metal compounds can... 

Scheme 4 shows some optically active resolving agents used in the resolution of organometallic transition metal compounds of tetrahedral, octahedral, and square pyramidal geometry. Schemes 2 and 3 demonstrate the application of the menthoxide ion. The aminophosphine shown will be used in an example discussed later on. The pyridine imine chelate ligand has been the chiral auxiliary for the resolution of octahedral compounds [9,10], not described in detail here, and for the resolution of square pyramidal compounds to be discussed next [11]. 

The purpose of this chapter is to provide an overview of a rather wide array of experimental techniques that can tell us about the electronic structure of molecules. Some of these techniques, such as photoelectron (PE) spectroscopy, which is based on Einstein s photoelectric effect, are generally applied to gas-phase molecules. They can give high-resolution spectra, providing information about molecular vibrations and even, in a few cases, rotations. At the other end of the scale, UV/vis spectroscopy, particularly as applied to transition-metal complexes in solution, involves broad bands, and although it is an important and widely-used method, the information it gives is limited. Emission spectroscopy of transition-metal compounds has also become important. 

Unlike transition metal compounds such as Ru(phen)3, which are composed of achiral ligands but can be resolved into chiral enantiomers, no such examples of the resolution of chiral lanthanide compounds with achiral ligands have appeared in the literature. This is because lanthanide complexes are generally more labile than transition metal complexes. Just like transition metal complexes, however, it is possible in some cases to perturb the ground-state equilibrium concentration through the addition of a chiral environment compound. This is sometimes referred to as the Pfeiffer effect. An example of such an experiment is presented in Figure 4. In this... 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

The previous chapters are exclusively devoted to the measurements and interpretation of Fe spectra of various iron-containing systems. Iron is, by far, the most extensively explored element in the field of chemistry compared with all other Mdssbauer-active elements because the Mossbauer effect of Fe is very easy to observe and the spectra are, in general, well resolved and they reflect important information about bonding and structural properties. Besides iron, there are a good number of other transition metals suitable for Mossbauer spectroscopy which is, however, less extensively studied because of technical and/or spectral resolution problems. In recent years, many of these difficulties have been overcome, and we shall see in the following sections a good deal of successful Mossbauer spectroscopy that has been performed on compounds of... 

If kinetic resolution is being studied, the ratio of pseudo-e nantiomers can be measured by MS, allowing for the determination of ee-values (and/or of selectivity factors E). The same applies to the reaction of pseudo prochiral compounds. This system has been used successfully in the directed evolution of enantioselective enzymes. However, it should work equally well in the case of asymmetric transition metal catalyzed reactions. In the original version about 1,000 ee-deter-minations were possible per day (Figure 6).94 The second-generation version based on an 8-channel multiplexed spray system enables about 10,000 samples to be handled per day, the sensitivity being 2% ee.96... 

A patent procedure for formation of compounds 19 from simple tartaric acid derivatives has appeared <06USP047129> and various new routes to chiral dioxolanones include synthesis of dioxolan-2-ones either by transition metal-mediated asymmetric synthesis <06T1864> or enzyme-mediated kinetic resolution <06H(68)1329> and a new synthesis of the chiral dioxolan-4-ones 21 from lactic or mandelic acid involving initial formation of intermediates 20 with trimethyl orthoformate in cyclohexane followed by reaction with pivalaldehyde <06S3915>. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

Dynamic kinetic resolution (DKR) is an attractive protocol for the production of enantiopure compounds from racemic mixtures [45]. The concept of DKR is illustrated in Scheme 5.13. In many cases, DKRs are accomplished by the combination of enzymatic resolution and transition-metal-catalyzed racemization based on hydrogen transfer. Thus, the use of Cp Ir complexes as catalysts for racemization in DKR can be anticipated. 

Enzymatic resolution of racemic secondary alcohols by enantiomer-selective acylation gives optically pure compounds with up to 50% yield [332], When this method is coupled with the principle of dynamic kinetic resolution (see Section 1.4.1.5), the theoretical yield increases to 100%. Thus a reaction system consisting of an achiral transition-metal catalyst for racemization, a suitable enzyme, acetophenone, and an acetyl donor allows the transformation of racemic 1-phenylethanol to the R acetates with an excellent ee (Scheme 1.93) [333]. The presence of one equiv. of acetophenone is necessary to promote the alcohol racemization catalyzed by the... 

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