Chiral Rhodium Research Articles

103Rh NMR Spectroscopy and Its Application to Rhodium Chemistry

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stereocontrolled Syntheses of the Nemorensic Acids Using 6-Diazoheptane-2,5-dione in Carbonyl Ylide Cycloadditions

Levulinic acid-derived 6-diazoheptane-2,5-dione (9) serves as a common precursor in a formal synthesis of frontalin 19, and in syntheses of cis-nemorensic acid 1, 4-hydroxy-cis-nemorensic acid 2, 3-hydroxy-cis-nemorensic acid 3, and nemorensic acid 4. The key step in these syntheses is the Rh(2)(OAc)(4)-catalyzed tandem carbonyl ylide formation-intermolecular 1,3-dipolar cycloadditions of diazodione 9 with formaldehyde, alkynes or allene, which occur with high regioselectivity. Subsequent oxidative cleavage of the ring originally derived from the cyclic carbonyl ylide intermediate provides a straightforward access to polysubstituted tetrahydrofurans, and in particular an efficient entry to the nemorensic acids. Enantioselective cycloadditions with diazodione 9, using chiral rhodium catalysts, gave cycloadducts in up to 51% ee.

Role of chiral amine cocatalysts in the helix-sense-selective polymerization of a phenylacetylene using a catalytic system

We have examined the role of the chiral amine cocatalyst in the recently discovered helix-sense-selective polymerization of a phenylacetylene using a catalytic system consisting of an achiral rhodium complex ([Rh(norbornadiene)Cl] 2) as the catalyst and a chiral amine as the cocatalyst. Several chiral amines were effective for the polymerization reaction and their effectiveness depended on their bulkiness and coordination ability to rhodium. The sense selectivity of the polymerization increased with the concentration of the chiral amines. To discuss the structure of the true active species in the catalytic system, we have synthesized a new chiral rhodium complex having two chiral amines as ligands ([Rh(norbornadiene)(chiral amine) 2] +BF 4 −). The isolated chiral complex also catalyzed the helix-sense-selective polymerization. These findings suggest that a chiral rhodium complex having two chiral amines may be the true active species when using the catalytic system consisting of [Rh(norbornadiene)Cl] 2 and a chiral amine.

Asymmetric Conjugate Reduction of α,β-Unsaturated Esters with Chiral Rhodium(bisoxazolinylphenyl) Catalysts

Chiral rhodium(bisoxazolinylphenyl) complexes reduced α,β-unsaturated esters in high enantioselectivity up to 97-98% ee in the combination of alkoxyhydrosilanes.

Asymmetric Transfer Hydrogenation of Acetophenone with 1R,2S-Aminoindanol/Pentamethylcyclopentadienylrhodium Catalyst

The chiral rhodium complex pentamethylcyclopentadienylrhodium chloride dimer combined with the ligand 1R,2S-aminoindanol provides a superior catalyst for the rapid, high-yielding asymmetric transfer hydrogenation of acetophenone with 2-propanol to produce (R)- and (S)-(1)-phenylethanol. The effects of various reaction parameters such as reaction temperature, catalyst and substrate concentration, gaseous environment, and acetone concentration on conversion and enantioselectivity were investigated. The results indicate that catalyst can be deactivated by high temperature and air atmosphere, acetone reduces the reaction rate, and enantioselectivity decreases with conversion.

Asymmetric 1,4‐Addition of Alkenylzirconium Reagents to α,β‐Unsaturated Ketones Catalyzed by Chiral Rhodium Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Chiral rhodium complexes containing bidentate ligands derived from (R,R)-1,2-diaminocyclohexane for catalytic enantioselective hydrosilylation of acetophenone

Bidentate P,P- and N,P-ligands derived from inexpensive and commercially available chiral source (R,R)-1,2-diaminocyclohexane, have been tested in the asymmetric Rh-catalyzed addition of diphenylsilane to acetophenone. Studies on the reactivity of these ligands towards the rhodium precursor [Rh(cod)Cl]2 (cod = cycloocta-1,5-diene) have shown that either monomeric, or dinuclear complexes, or both can be found in the solution. The structure of the cationic complex [Rh(cod)(3a)]PF6 (12), has been elucidated by X-ray analysis. The presence in the reaction mixture of different rhodium complexes which act as precatalysts, influences the activity and stereoselectivity of the process.

Efficient Access to Chiral β‐Arylamides via Asymmetric 1,4‐Additions of Potassium Trifluoro(organo)borates.

AbstractFor Abstract see ChemInform Abstract in Full Text.

103Rh NMR spectroscopy and its application to rhodium chemistry.

Rhodium is used for a number of large processes that rely on homogeneous rhodium-catalyzed reactions, for instance rhodium-catalyzed hydroformylation of alkenes, carbonylation of methanol to acetic acid and hydrodesulfurization of thiophene derivatives (in crude oil). Many laboratory applications in organometallic chemistry and catalysis involve organorhodium chemistry and a wealth of rhodium coordination compounds is known. For these and other areas, 103Rh NMR spectroscopy appears to be a very useful analytical tool. In this review, most of the literature concerning 103Rh NMR spectroscopy published from 1989 up to and including 2003 has been covered. After an introduction to several experimental methods for the detection of the insensitive 103Rh nucleus, a discussion of factors affecting the transition metal chemical shift is given. Computational aspects and calculations of chemical shifts are also briefly addressed. Next, the application of 103Rh NMR in coordination and organometallic chemistry is elaborated in more detail by highlighting recent developments in measurement and interpretation of 103Rh NMR data, in relation to rhodium-assisted reactions and homogeneous catalysis. The dependence of the 103Rh chemical shift on the ligands at rhodium in the first coordination sphere, on the complex geometry, oxidation state, temperature, solvent and concentration is treated. Several classes of compounds and special cases such as chiral rhodium compounds are reviewed. Finally, a section on scalar coupling to rhodium is provided.

A practical synthesis of optically active aromatic epoxides via asymmetric transfer hydrogenation of α-chlorinated ketones with chiral rhodium–diamine catalyst

A practical method for the synthesis of optically active aromatic epoxides has been developed via the formation of optically active α-chlorinated alcohols and intramolecular etherification. Optically active alcohols with up to 99% ee can be obtained from the asymmetric reduction of aromatic ketones with a substrate/catalyst ratio of 1000–5000 using a formic acid/triethylamine mixture containing a well-defined chiral Rh complex, Cp*RhCl[( R, R)-Tsdpen]. The asymmetric reduction of α-chlorinated aromatic ketones with a chiral Rh catalyst is characterized by a rapid and carbonyl group-selective transformation because of the coordinatively saturated nature of diamine-based Cp*Rh(III) hydride complexes. The outcome of the reduction is significantly influenced by the structures of the ketonic substrates as well as the hydrogen source such as formic acid or 2-propanol. Commercially available reagents and solvents can be used in this reaction without special purification. This epoxide synthetic process in either a one- or two-pot procedure is practical and particularly useful for the large-scale production of optically active styrene oxides from α-chlorinated ketones.

New Insights on the Asymmetric Hydroboration of Perfluoroalkenes.

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Efficient access to chiral $beta;-arylamides via asymmetric 1,4-additions of potassium trifluoro(organo)borates

This paper describes an efficient enantioselective conjugate addition of highly stable potassium trifluoro(organo)borates to α,β-unsaturated amides. This reaction, catalyzed by chiral rhodium(I) complexes affords Michael adducts with high yields and enantiomeric excesses up to 95%.

Asymmetric 1,4-addition of alkenylzirconium reagents to α,β-unsaturated ketones catalyzed by chiral rhodium complexes

Highly enantioselective 1,4-addition of alkenylzirconocene chlorides to α,β-enones was found to be catalyzed by a chiral rhodium complex generated from [Rh(cod)(MeCN) 2]BF 4 and ( S)-BINAP. The reaction can be applied to either cyclic or acyclic enones and the optical yield was up to 99% ee. The reaction mechanism would involve the transmetalation between the alkenylzirconocene chloride and the rhodium complex to give the alkenylrhodium species as a key intermediate.

Efficient access to chiral β-arylamides via asymmetric 1,4-additions of potassium trifluoro(organo)borates

This paper describes an efficient enantioselective conjugate addition of highly stable potassium trifluoro(organo)borates to α,β-unsaturated amides. This reaction, catalyzed by chiral rhodium(I) complexes affords Michael adducts with high yields and enantiomeric excesses up to 95%.

Enantioselective Hydrogenation of β‐Acylamino Acrylates Catalyzed by Rhodium(I)‐Monophosphite Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Asymmetric 1,4-addition of alkenylzirconium reagents to $alpha;,$beta;-unsaturated ketones catalyzed by chiral rhodium complexes

Highly enantioselective 1,4-addition of alkenylzirconocene chlorides to α,β-enones was found to be catalyzed by a chiral rhodium complex generated from [Rh(cod)(MeCN)2]BF4 and (S)-BINAP. The reaction can be applied to either cyclic or acyclic enones and the optical yield was up to 99% ee. The reaction mechanism would involve the transmetalation between the alkenylzirconocene chloride and the rhodium complex to give the alkenylrhodium species as a key intermediate.

Catalytic Hydrogenation of Acetophenone with Hydrogen Transfer over Chiral Diamine Rhodium(I) Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalytic enantioselective intermolecular cycloadditions of 2-diazo-3,6-diketoester-derived carbonyl ylides with alkene dipolarophiles.

Catalyzed cascade reactions that generate molecular complexity rapidly and in an enantioselective manner are attractive methods for asymmetric synthesis. In the present article, chiral rhodium catalysts are shown to effect such a transformation by using a range of 2-diazo-3,6-diketoesters with bicyclo[2.2.1]alkenes and styrenes as reaction partners. The reactions are likely to proceed by formation of a catalyst-complexed carbonyl ylide from the diazo compound, followed by intermolecular cycloaddition with the alkene dipolarophile. It was possible to obtain high levels of asymmetric induction [up to 89% enantiomeric excess (ee) and 92% ee for the two chiral catalysts investigated]. Enantioselectivity is not highly sensitive to substituent variation at the ketone that forms the ylide; however, branching does improve ee. Observations of dipolarophile-dependent enantiofacial selectivity in the cycloadditions indicate that the dipolarophile can be intimately involved in the enantiodiscrimination process.

Tandem 1,4‐Addition/Enantioselective Protonation Catalyzed by Rhodium Complexes: Efficient Access to α‐Amino Acids

Conjugate addition of potassium trifluoro(organo)borates 2 (R=aryl, alkenyl) to 2-acetamidoacrylate 1 mediated by a chiral rhodium catalyst and with in situ enantioselective protonation afforded a variety of α-amino acids 3 with high yields and ee values up to 90 % (see scheme, cod=cycloocta-1,5-diene, binap=2,2′-bis(diphenylphosphanyl)-1,1′-binaphthyl). Among the tested chiral ligands and proton sources, binap in combination with 2-methoxyphenol (guaiacol), an inexpensive and nontoxic phenol, afforded the highest asymmetric inductions.

Rh(I)‐Catalyzed Asymmetric Intramolecular Pauson—Khand Reaction in Aqueous Media.

AbstractFor Abstract see ChemInform Abstract in Full Text.