Cationic rhodium(I)–diolefin complexes containing an optically active C2-symmetric bis(sulfoximine) ligand: Synthesis and catalytic activity

A series of cationic rhodium(I) complexes [Rh(diene)(N^N)][BF₄] (diene = 1,5-cyclooctadiene (cod), norbornadiene (nbd), tetrafluorobenzobarralene (tfb)), containing the optically pure bis(sulfoximine) ligand 1,2-bis(S-methyl-S-phenylsulfonimidoyl)benzene, have been synthesized and fully characterized. The structure of the R,R enantiomer of the ligand, and that of its cyclooctadiene–Rh(I) complex, were confirmed by means of single-crystal X-ray diffraction techniques. Studies on the catalytic activity of these complexes in acetophenone hydrosilylation and dimethyl itaconate hydrogenation are also reported.     

Asymmetric hydrogenation of enol phosphinates catalyzed by a chiral ferrocenylphosphine-rhodium complex. Asymmetric synthesis of optically active secondary alkyl alcohols

Catalytic asymmetric synthesis of secondary alkyl alcohols (up to 78% ee) was accomplished by asymmetric hydrogenation of enol diphenylphosphinates, derived from prochiral ketones such as acetophenone, 3-methyl-2-butanone, and 2-octanone, in the presence of a cationic rhodium complex of (R)-1-[(C)-1′,2-bis(diphenylphosphino)ferrocenyl]ethanol (BPPFOH).     

Synthesis of 3 or 3,3′-substituted BINOL ligands and their application in the asymmetric addition of diethylzinc to aromatic aldehydes

Three new substituted BINOL ligands (R)-3-[4,6-bis(dimethylamino)-1,3,5-triazin-2-yl]-1,1′-bi-2-naphthol (R)-1, (R)-3,3′-bis[4,6-bis(dimethylamino)-1,3,5-triazin-2-yl]-1,1′-bi-2-naphthol (R)-2 and 2,4-bis(2,2′-dihydroxy-1,1′-binaphthalen-3-yl)-6-(p-tolyl)-1,3,5-triazine (R,R)-3 have been obtained by directed ortho-lithiation or Suzuki cross-coupling process. Ligand (R)-1 shows improved catalytic properties for the asymmetric diethylzinc addition to aromatic aldehydes.     

Transformations of the chiral diphosphine rhodium catalyst [(1,5-COD)Rh(—)R,R-DIOP]+CF3SO3– under conditions of hydrogenation

Transformation products of the cationic rhodium complex [(1,5-COD)Rh(—)R,R-DIOP]⁺CF₃SO₃ ^– (1) (COD is cycloocta-1,5-diene and DIOP is (±)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane), which were obtained in its reactions with molecular hydrogen, base (NEt₃), and solvents in the absence of a substrate, were investigated by ¹H and ³¹P NMR spectroscopy. The solvate complexes [(Solv)₂Rh(—)R,R-DIOP]⁺CF₃SO₃ ^–, which were generated from complex 1 in its reaction with molecular hydrogen, underwent destruction of the diphosphine ligand with elimination of benzene and were subjected to oxidation by traces of moisture and oxygen to form the DIOP dioxide complex with Rh^I. In the absence of hydrogen, complex 1 in solutions produced the diphosphine dioxide rhodium(i) complex and mono- and binuclear rhodium(i) solvate complexes. The scheme of deactivation of the complex in the absence of the substrate was proposed. The catalytic activity of the solvate complexes [(ArH)Rh(—)R,R-DIOP]⁺CF₃SO₃ ^–, which contain benzene, p-xylene, and mesitylene in the coordination sphere, was studied in hydrogenation of Z--acetamidocinnamic acid.     

Synthesis, characterization, and catalytic application of a new chiral P,N-indene ligand derived from (R)-BINOL

Lithiation of 2-dimethylaminoindene followed by quenching with [(R)-(1,1′-binaphthalene-2,2′-diyl)]chlorophosphite and treatment with triethylamine afforded the crystallographically characterized enantiopure P,N-indene 3 in 71% isolated yield. In the course of rhodium coordination chemistry studies involving 3, the formation of the isolable complex [(κ²-P,N-3)(κ¹-P,N-3)RhCl] (7) (81%) was observed, thereby confirming the propensity of this new ligand to form L_nRh(3)₂ complexes. Such coordination chemistry behavior may contribute in part to the generally poor catalytic performance exhibited by mixtures of 3 and rhodium precursor complexes in the asymmetric hydrogenation and hydrosilylation studies described herein.     

Ionic liquid-phase asymmetric catalytic hydrogenation: hydrogen concentration effects on enantioselectivity

Molecular hydrogen is almost four times more soluble in the ionic liquid 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄) than in its hexafluorophosphate (BMI·PF₆) analogue at the same pressure. The Henry coefficient solubility constant for the solution BMI·BF₄/H₂ is K=3.0×10⁻³ mol L⁻¹ atm⁻¹ and 8.8×10⁻⁴ mol L⁻¹ atm⁻¹ for BMI·PF₆/H₂, at room temperature. The asymmetric hydrogenation of (Z)-α-acetamido cinnamic acid and kinetic resolution of (±)-methyl-3-hydroxy-2-methylenebutanoate by (−)-1,2-bis((2R,5R)-2,5-diethylphospholano)benzene(cyclooctadiene)rhodium(I) trifluoromethanesulfonate and dichloro[(S)-(−)-2,2′-bis(di-p-tolylphosphino)-1,1′-binaphthyl]ruthenium(II) complexes immobilised in BMI·PF₆ and BMI·BF₄ were investigated. Remarkable effects in the conversion and enantioselectivity of these reactions were observed as a function of molecular hydrogen concentration in the ionic phase rather than pressure in the gas phase.     

Immobilization of chiral cationic diphosphine rhodium complexes in nanopores of mesoporous silica and application in asymmetric hydrogenation

Heterogeneous chiral cationic rhodium complexes bearing bidentate phosphine derived from (?)-2,2-dimethyl-4,5-bis(diphenylphosphino)methyl)-1,3-dioxolane (DIOP) were prepared by covalent immobilization onto SBA type silica. In order to introduce the tether to the surface, it was necessary to modify chemically the DIOP ligand through a reaction sequence consisting of hydrolysis and condensation with organosiloxane precursor. Two types of cationic rhodium hybrid materials based on SBA-15 and partially capped SBA-3 type silica were prepared under classical grafting procedures. The catalytic solids were fully characterized using a wide variety of molecular and solid-state techniques to determine their structural and textural properties. The performances of these latter were then evaluated in the hydrogenation of methyl (Z)-2-N-acetylaminocinnamate under various reaction conditions (pressure and temperature). Generally, the activity of supported catalysts was high as full conversions were obtained but immobilization of the system leads to significant loss of enantioselectivity. The best ee (20%) was observed in the case of the catalyst whose surface had been passivated prior to the grafting but the enantiomeric excesses were fairly below the values of the homogeneous catalysis.     

Asymmetric hydrogenation of (E)-α,β-bis(N-acylamino)acrylates catalyzed by a rhodium complex with trans-chelating chiral diphosphine ligand

The asymmetric hydrogenation of (E)-α,β-bis(N-acylamino)acrylates was promoted by a rhodium complex bearing a trans-chelating chiral diphosphine ligand (R,R)-(S,S)-PrTRAP, providing the corresponding optically active (2S,3R)-2,3-bis(N-acylamino)carboxylates with 79–82% ee. The 2,3-bis(N-acylamino)carboxylates isolated were readily hydrolyzed under acid to afford (2S,3R)-2,3-diaminocarboxylic acids in 95% yield without epimerization.     

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

The catalytic activity and stereoselectivity of Rh(I) complexes with C ₂-symmetric chiral diamines, (4S,5S)-3,4-isopropylidenedioxy-1,4-butanediamine and (4S,5S)-N,N,N',N'-tetramethyl-3,4-isopropylidenedioxy-1,4-butanediamine [skeletal analogs of 2,3-dihydroxy-2,3-O-isopropylidene-1,4-bis(diphenylphosphino)butane (DIOP)], were studied in hydrogen transfer from 2-propanol to acetophenone in the presence of KOH or t-BuOK. The product, (S)-(-)-2-phenylethanol, was thus obtained with an optical yield of 67%. Covalent chloride rhodium complexes with the above ligands give rise to the same stereoisomer, whereas the opposite stereoselectivity is observed under catalysis by cationic trifluoromethanesulfonate rhodium(I) complexes. X-Ray phase analysis showed formation of nanosize particles in the precipitate of metallic rhodium.     

New amidophosphine-phosphinites (tLANOPs) as chiral ligands for asymmetric hydrogenation reactions

The new amidophosphine-phosphinite (AMPP) ligands 4a–g (called tLANOP ligands) derived from the chiral hydroxy amide (R)- or (S)-2-hydroxy-3,3,N-trimethylbutyramide have been prepared in 48–83% yield. The crystal structures of the square planar complexes [(SP-4-3)-Pd((R)-dmphea)((S)-4a)]BF₄ and [Rh((R)-4a)(COD)]BF₄ have allowed the absolute configurations of the ligands to be assigned. In both complexes the 7-membered chelating ring of 4a has virtually the same twist-boat conformation. With this class of ligands the rhodium catalyzed asymmetric hydrogenation of 4-oxoisophorone enol acetate gave (S)-phorenol acetate in up to 71% ee. The iridium catalyzed asymmetric hydrogenation of the cyclic iminium salts 16a and 16b afforded after work-up the corresponding cyclic secondary amine (S)-17 in up to 86% ee, when bulky groups were present on the phenyl substituents on the two phosphorus atoms.     

Me-AnilaPhos: a new chiral phosphine-phosphoramidite ligand for a highly efficient Rh-catalyzed asymmetric olefin hydrogenation

A cationic rhodium(I) complex with a novel chiral phosphine-phosphoramidite ligand based on 2-diphenylphosphino-N-methylaniline and R-BINOL moieties has been synthesized. The complex provided remarkably high activity and enantioselectivity in the asymmetric hydrogenation of methyl (Z)-α-acetamidocinnamate (100% conversion after 10 min, 98% ee) and dimethyl itaconate (100% conversion after 26 min, 96% ee) under ambient conditions (1 bar hydrogen pressure, room temperature) using 1 mol % of the catalyst in dichloromethane as solvent. On the other hand, when hydrogenation was performed in methanol, both conversion and enantioselectivity were significantly diminished, due to the partial decomposition of the rhodium/phosphine-phosphoramidite complex.     

Ru-catalyzed enantioselective preparation of methyl (R)-o-chloromandelate and its application in the synthesis of (S)-Clopidogrel

The preparation of methyl (R)-o-chloromandelate via Ru-catalyzed asymmetric hydrogenation and transfer hydrogenation was investigated. With Ru-(R,R)-2,4,6-triisopropyl C₆H₂SO₂-DPEN as the catalyst and HCOOH-Et₃N azeotrope as the hydrogen donor, up to 92% ee was obtained in an optional condition. The synthesis of (S)-Clopidogrel was also studied.     

New chiral diphosphoramidite rhodium(I) complexes for asymmetric hydrogenation

Chiral 1,5‐cyclooctadiene rhodium(I) cationic complexes with C₂‐symmetric chelate diphosphoramidite ligands containing (R,R)‐1,2‐diaminocyclohexane as the backbone and two atropoisomeric biaryl units were easily synthesized and fully characterized by multinuclear one‐ and two‐dimensional NMR spectroscopy and elemental analysis. These complexes were used as catalysts in the asymmetric hydrogenation of dimethyl itaconate, methyl 2‐acetamidoacrylate and (Z)‐methyl‐2‐acetamido‐3‐phenylacrylate. The rhodium complexes derived from diphosphoramidite ligands that contain two (R) or (S) BINOL (2,2′‐dihydroxy‐1,1′‐binaphthyl) units proved to be efficient catalysts, giving complete conversion and very good enantioselectivity (up to 88% ee). An uncommon positive H₂ pressure effect on the enantioselectivity was observed in the hydrogenation of dimethyl itaconate catalyzed by Rh‐complex with diphosphoramidite ligand that contains two (S)‐binaphthol moieties. Copyright © 2014 John Wiley & Sons, Ltd.     

Novel phosphine-phosphite and phosphine-phosphinite ligands for highly enantioselective asymmetric hydrogenation

Two novel phosphine-phosphite (S,R)-o-BINAPHOS and phosphine-phosphinite (S)-o-BIPNITE ligands based on ortho phenyl substituted (S)-BINOL have been synthesized. Extremely high enantioselectivity (over 99% ee in most cases) has been achieved for the Rh-catalyzed asymmetric hydrogenation of α-dehydroamino acid derivatives.     

Synthesis of (−)-(4R,5R)-4,5-bis[di-3′-(2′,6′-dimethoxypyridyl)phosphinomethyl]-2,2-dimethyl-1,3-dioxolane and its application in the Rh-catalyzed asymmetric hydrogenation reactions

The chiral ligand (−)-(4R,5R)-4,5-bis[di-3′-(2′,6′-dimethoxypyridyl)phosphinomethyl]-2,2-dimethyl-1,3-dioxolane 3 [(R,R)-Py*-DIOP] was synthesized via a key intermediate bis[3-(2,6-dimethoxypyridyl)]phosphine-borane 9. The asymmetric hydrogenation of prochiral olefins was investigated using a rhodium catalyst containing 3. For the hydrogenation of amidoacrylic acids, enols and itaconic acid, while the enantioselectivity of [Rh-(R,R)-Py*-DIOP] was similar to that of [Rh-(R,R)-DIOP] the absolute configurations of the products from the two catalyst systems were found to be opposite.     

Counter-ligand and solvent dependent oxygen–metal interactions of hemilabile coordinating hydroxy groups in chiral diphosphine rhodium(I) hydrogenation catalysts

In cationic Rh[(R,R)-1,4-bis(diphenylphosphino)butane-2,3-diol] complexes the interaction of the hemilabile coordinating hydroxy groups with the metal is strongly dependent upon the nature of the counter-ligand and the solvent. As a result of the `arm-off, arm-on' mechanism the conformation of the chirality inducing backbone is changed. Spectroscopic and catalytic investigations demonstrate that the η³-coordination mode of the tetradentate ligand is responsible for the deceleration of the asymmetric hydrogenation. By the assistance of the second HO-group the Rh–O interaction can be suspended.     

DPAMPP in catalytic asymmetric reactions: enantioselective synthesis of l-homophenylalanine

l-Homophenylalanine 1, a key intermediate of most commercially important ACE inhibitors, was prepared via catalytic asymmetric hydrogenation of (Z)-2-acetamido-4-phenylcrotonate 3a with a cationic rhodium complex of (1R,2S)-DPAMPP, followed by acid-hydrolysis of the protecting groups in good yield with >99.9% enantioselectivity.     

Studies on bis(1′-ortho-carboranyl)benzenes and bis(1′-ortho-carboranyl)biphenyls

Reactions between the C,C′-dicopper(I) derivative of ortho-carborane and ortho-, meta- and para-diiodobenzene are reported. The reaction with 1,2-C₆H₄I₂ unexpectedly afforded 2,2′-bis(1′-ortho-carboranyl)biphenyl, [HCB₁₀H₁₀CC₆H₄]₂2, whereas reactions with 1,3- or 1,4-C₆H₄I₂ provided alternative routes to 1,3-bis(1′-ortho-carboranyl)benzene 3 and 1,4-bis(1′-ortho-carboranyl)benzene 4, respectively. The crystal structure of the biphenyl derivative 2 revealed significant distortions in the biphenylene framework attributable to the proximity of the two bulky carborane cages. UV absorption spectra and electrochemical data on 2 and 3 showed little electronic communication between the two carborane cages in either, and negligible π-conjugation between the two ortho-phenylene rings in 2. However, substantial evidence was found of electronic communication between the carborane cages via the para-phenylene bridge in 4. B3LYP/6-31G^∗ computations have been carried out on compounds 2 and 4, on 4,4′-bis(ortho-carboranyl)biphenyl 6 and on 1,2-bis(1′-ortho-carboranyl)benzene 7. Those on 2, 4 and 6 show the computed geometries to be in very good agreement with the experimental geometries: those on 7 allowed the reported molecular geometry of this compound to be revised and revealed a long cage C–C bond of 1.725(3) Å.     

Asymmetric hydrogen-transfer hydrogenation on rhodium(I) complexes with new optically active salen ligands derived from (4<Emphasis Type="Italic">S</Emphasis>,5<Emphasis Type="Italic">S</Emphasis>)-4,5-bis(aminomethyl)-2,2-dimethyl-1,3-dioxolane

New optically active C ₂-symmetric salen-type ligands were synthesized on the basis of (4S,5S)-4,5-bis(aminomethyl)-2,2-dimethyl-1,3-dioxolane. These ligands were used to obtain cationic (trifluoromethanesulfonate) and neutral (chloride) rhodium(I) complexes with [(4S,5S)-2,2-dimethyl-5-{[(E)-pyridin-2-ylmethylidene]aminomethyl}-1,3-dioxolan-4-yl]-N-[(E)-pyridin-2-ylmethylidene]methanamine and [2,2-dimethyl-5-{[(E)-quinolin-2-ylmethylidene]aminomethyl}-1,3-dioxolan-4-yl]-N-[(E)-quinolin-2-ylmethylidene] methanamine. The latter complex ensured preparation of (S)-2-phenylethanol with an optical yield of 34.8% by transfer hydrogenation of acetophenone.     

Enantioselective hydrogenation of functionalized olefins catalyzed by Rh-chiral bidentatephosphite complexes

A novel catalytic system for the hydrogenation of dimethyl itaconate has been developed by using rhodium–diphosphite complexes. These chiral diphosphite ligands were derived from glucopyranoside, d-mannitol derivatives, and binaphthyl or H₈-binaphthyl phosphochloridites. The ligands based on the methyl 3,6-anhydro-α-d-glucopyranoside backbone and (R)- and (S)-binaphthol and/or (R)- and (S)-2,2′-dihydroxy-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthol gave almost complete conversion of the dimethyl itaconate and both enantiomers of dimethyl 2-methylsuccinate with excellent enantioselectivities. The stereochemically matched combination of methyl 3,6-anhydro-α-d-glucopyranoside and H₈-(S)-binaphthyl in ligand 2,4-bis{[(S)-1,1′-H₈-binaphthyl-2,2′-diyl]-phosphite} methyl 3,6-anhydro-α-d-glucopyranoside was essential to afford dimethyl 2-methylsuccinate with up to 98% ee. The sense of the enantioselectivity of products was predominantly determined by the configuration of the biaryl moieties of the ligands. An initial screening of [Rh(cod)₂]BF₄ with these ligands in the hydrogenation of (E)-2-(3-butoxy-4-methoxybenzylidene)-3-methylbutanoic acid was carried out. Good enantioselectivity (75% ee) and low yield for (R)-2-(3-butoxy-4-methoxybenzyl)-3-methylbutanoic acid were obtained.