Resolution Rhodium complexes

A resolution of racemic CHIRAPHOS ligand has been achieved using a chiral iridium amide complex (Scheme 8.3). The chiral iridium complex (- -)-l reacts selectively with (S.S -CHIRAPHOS to form the inactive iridium complex 2. The remaining (R,R)-CHIRAPHOS affords the catalytically active chiral rhodium complex 3. The system catalyzes asymmetric hydrogenation to give the (5)-product with 87% ee. The opposite enantiomer (—)-l gives the (R)-product with 89.5% ee, which is almost the same level of enantioselectivity obtained by using optically pure (5,5)-CHlRAPHOS. 

Minami, T., Okada, Y., Nomura, R., Hirota, S., Nagahara, Y., Mid Fukuyama, K. Synthesis and resolution of a new type of chiral bisphosphine ligand, trans-bis-l,2-(diphenylphosphino)cyclobutane. and asymmetric hydrogenation using its rhodium complex, Chem. Lett. 1986, 613-616. 

The tris(ethylenediamine) chromium (III) ion was first resolved by Werner6 by means of sodium 3-nitro-(+)-camphor. What has been said concerning the resolution of the corresponding rhodium compound holds true of the chromium compound, except that for the chromium compound the solubility difference of the diastereoisomeric chloride (+)-tartrates is so small that a resolution via these diastereoisomers has not been achieved.5,6 The method reported here is essentially the same as the one described for the rhodium complex but with minor alterations... 

Theoretically, in a simple kinetic resolution the ee value should not exceed 32 % at this specific conversion. In addition to the rhodium complex, this reaction requires acetophenone as stoichiometric hydride acceptor, phenanthroline as coligand and potassium hydroxide as base. An ee value of 98 % at 60 % conversion (theoretical value 67 %)is achieved with [Rh2(OAc)4] without an added base after 3 days. Surprisingly, the enzyme tolerates potassium hydroxide in amounts up to 20 mol% at elevated temperatures however, the enantiomeric excesses are somewhat lower than those obtained in an ordinary kinetic resolution. Unselective, base- or metal-catalyzed acylation might be the reason for the somewhat lower ee value. 

In the event, only the rhodium(I) complex features sulfur coordination, whereas the iridium(I) complex prefers a second carbene Ugand over the sulfur mediated chelate effect. The two complexes were tested for their activity in the hydrogenation of dimethyl itaco-nate. The iridium complex was inactive and the rhodium complex showed 44% conversion with a disappointingly low chiral resolution of 18% ee (R). The corresponding phosphine functionalised NHC rhodium(I) complex reacted under milder conditions, but without improvement of chiral resolution, 13% ee (S). 

The amide group is superior to the ester as a directing group (equation 4).i5i. 52 The cationic rhodium complex (15) catalyzes the hydrogenation of 3-substituted itaconic acid esters with high diastereoselectivity. When a chiral rhodium complex is employed, effective kinetic resolution occurs. 

Phenols are used as the nucleophile in the asymmetric aUylation of 7r-aUylpalladium complexes. Trost and Toste attained asymmetric phenyl ether formation in high enantiomeric excess (ee) using diphosphine ligand derived from chiral 1,2-cyclohexanediamine (equation 10). Dynamic kinetic resolution of the racemic secondary aUylic carbonate is conducted in the presence of tetrabutylammonium chloride, which increases the rate of ft—a—ft isomerization of the jr-allyl palladium intermediate (equation 11). Lautens and coworkers cleaved meio-oxabicyclic alkenes with phenol in the presence of a catalytic amount of a chiral ferrocenyldiphosphine and a rhodium complex (equation 12). ... 

Industrially, we have been using Tol-BINAP 10 instead of the prototype BINAP (Fig. 1 ). The merit is its higher crystallization properties both in the resolution and as rhodium complexes. 

The two-dimensional spin-echo experiment has found applications for static samples of compounds containing homonuclear spin pairs, where dipolar and chemical shift interactions could be separated allowing the determination of internuclear distances." " It was also shown that a considerable improvement in resolution could be obtained for samples rotated in the magic angle, thus allowing the determination of the magnitude of relatively smsdl homonuclear scalar coupling constants, e.g., P)ds in Wilkinson s-type rhodium complexes. ... 

Axially chiral biaryls are an important class of molecules for both biologically active compounds and chiral ligands (78-80). The most common approach to obtain biaryls is by aryl coupling followed by resolution of the racemic product to afford enantiopure biaryls. Even though enantioselective partial intramolecular cyclotrimerization of diyne with alkynes (81,82) or nitriles (83) were developed with various transitional metals, it was difficult to carry out complete intermolecular reaction. Using a cationic chiral rhodium complex as catalyst, a regioselective intermolecular cross-cyclotrimerization of alkynes 72 and 73 for... 

The Williams group reported the DKR of 1-phenylethanol as the first metalloenzy-matic DKR [9]. This DKR employed Pseudomonas fluorescens lipase (PPL) as the resolution enzyme, a rhodium complex as the racemization catalyst, and vinyl acetate as the acyl donor in cyclohexane, but the reaction required a stoichiometric amoxmt of acetophenone for hydrogen transfer and stopped at 60% conversion (Scheme 5.6). 

The ability of enzymes to achieve the selective esterification of one enantiomer of an alcohol over the other has been exploited by coupling this process with the in situ metal-catalysed racemisation of the unreactive enantiomer. Marr and co-workers have used the rhodium and iridium NHC complexes 44 and 45 to racemise the unreacted enantiomer of substrate 7 [17]. In combination with a lipase enzyme (Novozyme 435), excellent enantioselectivities were obtained in the acetylation of alcohol 7 to give the ester product 43 (Scheme 11.11). A related dynamic kinetic resolution has been reported by Corberdn and Peris [18]. hi their chemistry, the aldehyde 46 is readily racemised and the iridium NHC catalyst 35 catalyses the reversible reduction of aldehyde 46 to give an alcohol which is acylated by an enzyme to give the ester 47 in reasonable enantiomeric excess. 

A complex naturally occurring amino acid 5-hydroxypiperazic acid (5HyPip) 100 was prepared by a multistep procedure that included Diels-Alder addition of 2,4-pentadienoic acid to phthalazinedione 83a as a first step (Scheme 24). Adduct 97 was esterified and oxidized with mercuric acetate to 98, which on hydrogenation over rhodium on alumina and subsequent hydrolysis provided a mixture of enantiomers from which the required enantiomer 99 was obtained by resolution with quinine. Its hydrazinolysis provided 100 [71JCS(C)514 77H119],... 

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