Metal optically active

The origin of planar chirality is depicted in Fig. 1. For ACTCs, complexes displaying an unsymmetrical 1,2- or 1,3-disubstitution pattern (or related unsym-metrical higher substitution) are no longer superimposable with their mirror images (A vs ent-A). In case that the respective side chains are capable of exercising as donor moieties for chelation or coordination to transition metals, optically active complexes B result that have potential as asymmetric metal catalysts or catalyst precursors [9,10]. 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

The chiral BOX-metal(II) complexes can also catalyze cycloaddition reactions of other ketonic substrates [45]. The reaction of ethyl ketomalonate 37 with 1,3-conju-gated dienes, e.g. 1,3-cyclohexadiene 5c can occur with chiral BOX-copper(II) and zinc(II) complexes, Ph-BOX-Cu(OTf)2 (l )-21a, and Ph-BOX-Zn(OTf)2 (l )-39, as the catalysts (Scheme 4.29). The reaction proceeds with good yield and ee using the latter complex as the catalyst. Compared to the copper(II)-derived catalyst, which affects a much faster reaction, the use of the zinc(II)-derived catalyst is more convenient because the reaction gives 94% yield and 94% ee of the cycloaddition product 38. The cycloaddition product 38 can be transformed into the optically active CO2-... 

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. 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

A retrosynthetic analysis of fragment 152 can be completed through cleavage of the C16-C17 bond in enone 155, the projected precursor of epoxide 152. This retrosynthetic maneuver furnishes intermediates 156 and 157 as potential building blocks. In the forward sense, acylation of a vinyl metal species derived from 156 with Weinreb amide 157 could accomplish the construction of enone 155. Iodide 153, on the other hand, can be traced retrosynthetically to the commercially available, optically active building block methyl (S)-(+)-3-hydroxy-2-methyIpropionate (154). 

The synthesis of the trisubstituted cyclohexane sector 160 commences with the preparation of optically active (/ )-2-cyclohexen-l-ol (199) (see Scheme 49). To accomplish this objective, the decision was made to utilize the powerful catalytic asymmetric reduction process developed by Corey and his colleagues at Harvard.83 Treatment of 2-bromocyclohexenone (196) with BH3 SMe2 in the presence of 5 mol % of oxazaborolidine 197 provides enantiomeri-cally enriched allylic alcohol 198 (99% yield, 96% ee). Reductive cleavage of the C-Br bond in 198 with lithium metal in terf-butyl alcohol and THF then provides optically active (/ )-2-cyclo-hexen-l-ol (199). When the latter substance is treated with wCPBA, a hydroxyl-directed Henbest epoxidation84 takes place to give an epoxy alcohol which can subsequently be protected in the form of a benzyl ether (see 175) under standard conditions. 

Especially in the early steps of the synthesis of a complex molecule, there are plenty of examples in which epoxides are allowed to react with organometallic reagents. In particular, treatment of enantiomerically pure terminal epoxides with alkyl-, alkenyl-, or aryl-Grignard reagents in the presence of catalytic amounts of a copper salt, corresponding cuprates, or metal acetylides via alanate chemistry, provides a general route to optically active substituted alcohols useful as valuable building blocks in complex syntheses. 

Utilization of the Pfeiffer effect and outer-sphere complexation for the prediction of absolute configurations of optically active metal complexes. S. Kirschner and I. Bakkar, Coord. Chem. Rev., 1982,43, 325-335 (27). 

Optical activity from asymmetric transition metal atoms. H. Brunner, Angew. Chem., Int. Ed. Engl., 1971,10, 249-260 (88). 

Stereochemistry of the reactions of optically active organometallic transition metal compounds. H. Brunner, Top. Curr, Chem., 1975,56, 68-90 (74). 

The electronic spectra and optical activity of phenanthroline and dipyridyl metal complexes. S. F. Mason, Inorg. Chim. Acta, Rev., 1968, 2, 89-109 (84). 

An optically active sulfoxide may often be transformed into another optically active sulfoxide without racemization. This is often accomplished by formation of a new bond to the a-carbon atom, e.g. to the methyl carbon of methyl p-tolyl sulfoxide. To accomplish this, an a-metallated carbanion is first formed at low temperature after which this species may be treated with a large variety of electrophiles to give a structurally modified sulfoxide. Alternatively, nucleophilic reagents may be added to a homochiral vinylic sulfoxide. Structurally more complex compounds formed in these ways may be further modified in subsequent steps. Such transformations are the basis of many asymmetric syntheses and are discussed in the chapter by Posner and in earlier reviews7-11. 

Durst and coworkers were the first to report the condensation of chiral a-sulphinyl carbanions with carbonyl compounds477. They found that metallation of ( + )-(S)-benzyl methyl sulphoxide 397 followed by quenching with acetone gives a mixture of dia-stereoisomeric /i-hydroxy sulphoxides 398 in a 15 1 ratio (equation 233). The synthesis of optically active oxiranes was based on this reaction (equation 234). In this context, it is interesting to point out that condensation of benzyl phenyl sulphoxide with benzaldehyde gave a mixture of four / -sulphinyl alcohols (40% overall yield), the ratio of which after immediate work-up was 41 19 8 32478. 

Optically active polymers are potentially very useful in areas such as asymmetric catalysis, nonlinear optics, polarized photo and electroluminescence, and enantioselective separation and sensing.26 Transition metal coupling polymerization has also been applied to the synthesis of these polymers.27 For example, from the Ni(II)-catalyzed polymerization, a regioregular head-to-tail polymer 32 was obtained (Scheme 9.17).28 This polymer is optically active because of the optically active chiral side chains. 

Transition metal coupling polymerization has also been used to synthesize optically active polymers with stable main-chain chirality such as polymers 33, 34, 35, and 36 by using optically active monomers.29-31 These polymers are useful for chiral separation and asymmetric catalysis. For example, polymers 33 and 34 have been used as polymeric chiral catalysts for asymmetric catalysis. Due... 

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