Chiral metal complexes optical induction

Arylation, olefins, 187, 190 Arylketimines, iridium hydrogenation, 83 Arylpropanoic acid, Grignard coupling, 190 Aspartame, 8, 27 Asymmetric catalysis characteristics, 11 chiral metal complexes, 122 covalently bound intermediates, 323 electrochemistry, 342 hydrogen-bonded associates, 328 industrial applications, 8, 357 optically active compounds, 2 phase-transfer reactions, 333 photochemistry, 341 polymerization, 174, 332 purely organic compounds, 323 see also specific complexes Asymmetric induction, 71, 155 Attractive interaction, 196, 216 Autoinduction, 330 Axial chirality, 18 Aza-Diels-Alder reaction, 220 Azetidinone, 44, 80 Aziridination, olefins, 207... 

A chiral bisphosphine such as 2,2 -h -(diphenylphosphino)-l,F-binaphthyl (BINAP) has been extensively used as a chiral chelator in asymmetric catalysis. When Stang et al. reacted the chiral metal complex 42 with 40, they synthesized a square box (Figure 25) and asymmetric induction was observed [79,82] with the formation of an excess of one of the preferred diastereoisomers as measured by NMR spectroscopy. The same reaction has been carried out with 42 and 6is-4-(4 -pyridyl)phenyl)iodonium triflate, but in this case the diaza ligands of the iodonium species possess rotational symmetry about their linkages. Consequently, the optical activity of the molecular squares obtained is due exclusively to the chiral transition metal auxiliary BINAP. 

The reasons for the increasing acceptance of enzymes as reagents rest on the advantages gained from utilizing them in organic synthesis Isolated or wholecell enzymes are efficient catalysts under mild conditions. Since enzymes are chiral materials, optically active molecules may be produced from prochiral or racemic substrates by catalytic asymmetric induction or kinetic resolution. Moreover, these biocatalysts may perform transformations, which are difficult to emulate by transition-metal catalysts, and they are environmentally more acceptable than metal complexes. 

When my interest returned and we began researching the analytical applications of CD in the 70 s, I felt I had a head start. But there was so much that was new. A great deal had happened to CD over the years as it matured and expanded to include the far-UV the study of optical activity in excited state emissions, and in vibrational and Raman spectroscopy and the evolution of new empirical models applicable to the interpretation of the structural properties of macromolecules. Most important of all, perhaps, was the arrival of high tech electronics and materials which had brought CD instrumentation out of the dark ages. And now, ironically, almost 35 years after my introduction to CD, my special interest is the exploitation of chiral transition metal complexes as chirality induction reagents in chemical analysis. 

My own introduction to this field came during the course of my graduate studies when my mentor decided that I needed to learn the Jones calculus for treating optical phenomena. The department also possessed a Cary 60 spectrometer system, and in the time between my final oral exam and the beginning of my postdoctoral work I investigated the induction of circular dichroism in several metal complexes of acetylacetone by various chiral agents. Little did I know at the time that this particular work would subconsciously prepare me for the writing of one of the chapters in this book. 

Most commonly used chiral Lewis acids have been derived from main group and early transition series elements. An initial attempt at utilizing optically active catalysts of late transition metal complexes for the enantioselective addition of allyltributylstannane to aldehydes was made by Nuss and Rennels [30]. Employment of Rh(COD)[(-)-DIOP]BF4 (11) as a catalyst, however, resulted in only a small degree of asymmetric induction (17% ee). 

Stereoselective oxycarborative addition is also achieved in cycloaddition and cyclooligomeriza-tion reactions. Thus, hetero-Diels-Alder reactions of dienes and aldehydes are not only catalyzed by main group Lewis acids, but also by transition metal complexes 10°. Tris[3-(heptafluoropropyl-hydroxymethylene)-( + )-camphorato]europium [( + )-Eu(hfc)3] and similar vanadium complexes have been used as the chiral catalyst in [4 + 2] cycloadditions of various achiral and chiral dienes to aldehydes63 67-101. With achiral silyloxydienes only moderate asymmetric inductions are observed, however, with chirally modified dienes, high double diastereoselectivities are achieved. Thus, the reaction of benzaldehyde with 3-terf-butyldimethylsilyloxy-l-(/-8-phenvl-menthoxy)-l.3-butadiene (1) gives (2/ .6/ )-4-wf-bntyldimethylsilyloxy-5,6-dihydro-6-phenyl-2-[(17 ,3/ ,45 )-8-phenylmenthoxy]-2f/-pyran (2) in 95% yield with a diasteieoselectivity of 25 1 ss. After crystallization and hydrolysis with trifluoroacetic acid, optically pure (2/ )-2,3-di-hydro-2-phenyl-4-(4//)-pyranone (3) is obtained in 87% yield. 

Chiral bis(l, 2-dioximato)cobalt(II) complexes synthesized from ( + )-camphor (A-C overleaf)67 68 are useful catalysts in the additions of diazo esters to phenylethene derivatives. Interestingly, the two catalysts (A and B) which induce remarkably high optical yields produce opposite chirality at carbon-1 of the resulting cyclopropanes, Apparently it is the geometry around the metal center which is crucial for optical induction and which might be quasi-enan-tiomeric for complexes A and B68. The alternative complex C. which incorporates ( , -configurated bisoxime ligands, is a much less effective cobalt catalyst. 

Asymmetric induction by polymer-immobilized complexes is an important reaction in oxidation processes (this has already been demonstrated for the hydrogenation transformations described in Section 12.2.9). There are three different methods of synthesis of optically active compounds from optically inactive racemic mixtures spontaneous, biochemical and chemical. The chemical method is the most common. Immobilized metal complexes are the best models of asymmetric induction by enzymes. They produce large quantities of enantiomeric products from small quantities of chiral compounds. (Ascorbate oxidase is a copper-containing enzyme catalyzing aerobic oxidation of vitamin C. Its... 

This gives control over the stereochemistry of the product, because 8.14 can be resolved, thanks to the presence of the optically active group (R ) on the Cp ring, in which case carrying out the addition with one enantiomer of the metal complex means that the new asymmetric center on the ligand is formed with very high asymmetric induction. This reaction therefore constitutes a chiral synthesis of the alkenes shown. 

Fe(CO)3-diene aldehydes have been formed using Fe2(CO)9 in work that culminated in an efficient asymmetric induction using chiral allylborane reagents to give access to metal complexes in high optical purity. ... 

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