Chiral furanoside phosphite–phosphoroamidites: new ligands for asymmetric catalytic hydroformylation

We have designed a new series of phosphite–phosphoroamidites ligands 1–4 based on a furanoside backbone. These ligands were screened in the Rh-catalyzed asymmetric hydroformylation of styrene, inducing high regioselectivities with 2-phenylpropanal and moderate enantioselectivities (up to 65% e.e.). The results showed that the configuration of the stereogenic carbon atom C(3) at the ligand backbone had remarkable effects on the activity and enantioselectivity. Replacing the tert-butyl substituents with methoxy substituents at the para positions of the biphenyl moieties improved the enantioselectivities. We have also studied the solution structures of HRh(PP)(CO)₂ complexes.     

Selective hydroformylation of various olefins using diphosphinite ligands

Novel diphosphinite ligands are synthesized by the reaction of various derivatives of 1,3‐diols with chlorodiphenylphosphine. The synthesized ligands exhibited considerable impact on hydroformylation of various olefins with excellent regioselectivity toward branched aldehyde. The effect of solvent, temperature, pressure and catalyst loading on the hydroformylation reaction is also described. The synthesized diphosphinite ligands with rhodium precursor works under milder reaction conditions as compared to traditional phosphine and phosphite‐based ligands. Copyright © 2013 John Wiley & Sons, Ltd.     

Sugar–phosphite–oxazoline and phosphite–phosphoroamidite ligand libraries for Cu-catalyzed asymmetric 1,4-addition reactions

Sugar–phosphite–oxazoline L1–L5a–g and phosphite–phosphoroamidite L6a–c ligand libraries were tested in the asymmetric Cu-catalyzed 1,4-conjugate addition reactions of cyclic and acyclic enones. Systematically varying the electronic and steric properties of the oxazoline and biaryl phosphite substituents and the functional groups attached to the basic sugar-backbone had a strong effect on the catalytic performance. In general, good activities and enantioselectivities were obtained. The enantioselectivity (up to 80%) was optimized with catalyst precursors containing the phosphite–oxazoline ligands L1c and L1f that contain encumbered biaryl phosphite moieties and a phenyl oxazoline group.     

TARTROL-derived chiral phosphine–phosphite ligands and their performance in enantioselective Cu-catalyzed 1,4-addition reactions

By using (R,R,R,R)-2,3-dimethoxy-2,3-dimethyl-1,4-dioxane-5,6-bis-diphenylmethanol (TARTROL) as a chiral building block, a set of six modular phosphine–phosphite ligands (with a 1,2-phenylene backbone) were synthesized and evaluated in the Cu-catalyzed asymmetric 1,4-addition of Grignard reagents to cyclohexenone. Ligands with bulky substituents at the ortho- and para-positions to the chiral phosphite moiety were found to be the most selective affording the 1,4-addition products with enantioselectivities of up to 84% ee.     

Encapsulation of transition metal catalysts by ligand-template directed assembly

Encapsulated transition metal catalysts are presented that are formed by templated self-assembly processes of simple building blocks such as porphyrins and pyridylphosphine and phosphite ligands, using selective metal-ligand interactions. These ligand assemblies coordinate to transition metals, leading to a new class of transition metal catalysts. The assembled catalyst systems were characterized using NMR and UV-vis spectroscopy and were identified under catalytic conditions using high-pressure infrared spectroscopy. Tris-3-pyridylphosphine binds three mesophenyl zinc(II) porphyrin units and consequently forms an assembly with the phosphorus donor atom completely encapsulated. The encapsulated phosphines lead exclusively to monoligated transition metal complexes, and in the rhodium-catalyzed hydroformylation of 1-octene the encapsulation of the catalysts resulted in a 10-fold increase in activity. In addition, the branched aldehyde was formed preferentially (l/b = 0.6), a selectivity that is highly unusual for this substrate, which is attributed to the encapsulation of the transition metal catalysts. An encapsulated rhodium catalyst based on ruthenium(II) porphyrins and tris-meta-pyridyl phosphine resulted in an even larger selectivity for the branched product (l/b = 0.4). These encapsulated catalysts can be prepared easily, and various template ligands and porphyrins, such as tris-3-pyridyl phosphite and ruthenium(II) porphyrins, have been explored, leading to catalysts with different properties.     

Phenol-derived chiral phosphine–phosphite ligands in the rhodium-catalyzed enantioselective hydrogenation of functionalized olefins

A set of 15 chiral Taddol- and Binol-based phosphine–phosphite ligands were tested in the Rh-catalyzed asymmetric hydrogenation of three olefins, methyl 2-hydroxymethyl-acrylate, 1-phenylvinyl acetate, and α-methyl cinnamic acid. The best enantioselectivities (up to 92% ee) were observed in the hydrogenation of methyl 2-hydroxymethyl-acrylate using Binol-based ligands. As previously observed in other applications of this class of modular chiral ligand in enantioselective catalysis, the stereochemical outcome of the reactions greatly depended on the substituents at the ligand aryl backbone in the ortho-position to the chiral phosphite unit.     

The effects of structures and compositions of polymer-metal complexes on the hydroformylation of olefins

A number of linear and crosslinked polymer ligands containing P or N or S as coordinative atoms were prepared. The hydroformylation of olefins was carried out at mild conditions in the presence of complexes as catalysts which were made by complexation reactions of these ligands with various homogeneous complexes or metal salts. The obtained results were explained in terms of the influences of the polymer matrix, or the structures and compositions of the polymer chain, and the properties of polymer-metal complexes on activity and selectivity. The most important factor for enhancing the selectivity towards linear aldehyde lies in the properties of the polymer-metal complex. High selectivity up to 95% was obtained by using polymer phosphine ligand-Pt-Sn complexes as catalyst in the hydroformylation of olefins. The hydroformylation mechanisms were discussed.     

Rhodium/bisphosphite catalytic system for hydroformylation of styrene and its derivatives

Different kinds of mono‐ and bidentate phosphite ligands were used in Rh‐catalyzed hydroformylation of styrene to illustrate the influence of steric and electronic properties of ligands on catalytic performance. High activity (99.9%) and good regioselectivity (85.4%) to the linear aldehyde were achieved under optimum conditions in the presence of Rh/bisphosphite complex (bisphosphite: 2,2′‐bis(dipyrrolylphosphinooxy)‐1,1′‐(±)‐binaphthyl). This system makes it possible to prepare functionalized terminal aldehydes from readily available styrene or its derivatives through hydroformylation with high linear selectivity. Copyright © 2013 John Wiley & Sons, Ltd.     

Modular Synthesis of Phosphite and Phosphoramidite Ligands for Rh-Catalyzed Hydroformylation

Rhodium-catalyzed hydroformylation of olefins is an important method for synthesizing aldehydes, and it is worth noting that regioselectivity and enantioselectivity of the product controlled by ligand are the most commonly used strategies. The modular synthesis of phosphite and phosphoramidite ligands have significant advantages of simple synthesis and high catalytic activity, which was why these ligands have been widely used in hydroformylation reactions. Herein, we focus on the synthesis methods and design ideas of such ligands, as well as their application effects in hydroformylation reactions. This review aims to provide a reference for the design and synthesis of ligands in hydroformylation subsequently.     

Furanoside phosphite–phosphoroamidite and diphosphoroamidite ligands for Cu-catalyzed asymmetric 1,4-addition reactions

A sugar-based phosphite–phosphoroamidite and diphosphoroamidite ligand library L1–L5a–g was tested in the asymmetric Cu-catalyzed 1,4-conjugate addition reactions of β-substituted and β,β′-disubstituted enones. Our results indicated that the selectivity was strongly dependent on the ligand parameters and on the substrate structure. Moderate-to-good enantioselectivities (ees up to 84%) were obtained in the 1,4-addition of several types of β-substituted cyclic and linear substrates. Of particular note is the high enantioselectivity (ees up to 90%) obtained for the more challenging β,β′-disubstituted 3-methyl-cyclohexenone.     

Diphosphite ligands based on ribose backbone as suitable ligands in the hydrogenation and hydroformylation of prochiral olefins

Rh(I) and Ir(I) cationic complexes [M(cod)(PP)]BF₄ have been synthesised from diphosphite ligands 4–6 derived from ribofuranose. They have been used in the rhodium and iridium catalysed asymmetric hydrogenation of acrylic acid derivatives. Ribose derivative ligands 4–6 have also been used as auxiliaries in the Rh-catalysed hydroformylation of styrene. Hydroformylation results have been explained on the basis of the species formed under hydroformylation conditions. Comparative experiments with the related epimer d-(+)-xylose derivatives showed that the configuration of the product is controlled by the absolute configuration of the stereogenic carbon atom C-3.     

Synthesis and Application of Modular Phosphine–Phosphoramidite Ligands in Asymmetric Hydroformylation: Structure–Selectivity Relationship

A series of hybrid phosphine–phosphoramidite ligands has been designed and synthesized in moderate yields from chiral BINOL (1,1′‐bi‐2‐naphthol) or NOBIN (2‐amino‐2′‐hydroxy‐1,1′‐binaphthyl). They have achieved highly regio‐ and enantioselectivities in Rh‐catalyzed asymmetric hydroformylations of styrene derivatives (branched/linear ratio up to 56.6, ee up to 99 %), vinyl acetate derivatives (up to 98 % ee), and allyl cyanide (up to 96 % ee). Systematic variation of ligand structure showed that the steric factor on the phsophoramidite moiety determined the performance of the ligand. With the increased hindrance, the branched/linear ratio rose, while the ee value dropped in the hydroformylation of styrene. However, the N‐substituents did not influence the selectivities much.     

Chiral diphosphites derived from d-glucose in the copper-catalyzed conjugate addition of diethylzinc to cyclohexenone

A series of diphosphite ligands 1–3 derived from readily available d-(+)-glucose and bisphenol or binaphthol derivatives have been applied as ligands in the Cu-catalyzed 1,4-addition of diethylzinc to cyclohexenone. Excellent reaction rates (TOF>1200 h⁻¹) and good enantioselectivities (e.e. of up to 84%) were achieved. The modular nature of these ligands allows easy systematic variation in the configuration of the stereocenters (C(3), C(5)) at the ligand backbone and in the biaryl substituents, so the optimum configuration for maximum enantioselectivity in the asymmetric 1,4-addition can be determined. The results obtained show that the enantioselectivity induced by the ligand depends strongly on the absolute configuration of the C(3) stereogenic centre, while the sense of the enantiodiscrimination is predominantly controlled by the configuration of the biaryl groups of the phosphite moieties.     

Asymmetric hydroformylation of styrene catalyzed by furanoside phosphine–phosphite–Rh(I) complexes

A series of phosphine–phosphite ligands, derived from inexpensive -(+)-xylose, were tested in the Rh-catalyzed asymmetric hydroformylation of styrene. Systematic variation of the phosphite moiety revealed a remarkable effect on the selectivity of the hydroformylation catalysts. High regioselectivities for 2-phenylpropanal (up to 95%) and moderate enantioselectivity were found under mild reaction conditions (25–40°C, 25 bar of syn gas). The hydroformylation results are explained by the solution structures of the intermediate species formed under hydroformylation conditions.     

Styrene Hydroformylation with In Situ Hydrogen: Regioselectivity Control by Coupling with the Low-Temperature Water–Gas Shift Reaction

The hydroformylation of olefins is one of the most important homogeneously catalyzed industrial reactions for aldehyde synthesis. Various ligands can be used to obtain the desired linear aldehydes in the hydroformylation of aliphatic olefins. However, in the hydroformylation of aromatic substrates, branched aldehydes are formed preferentially with common ligands. In this study, a novel approach to selectively obtain linear aldehydes in the hydroformylation of styrene and its derivatives was developed by coupling with a water–gas shift reaction on a Rh single-atom catalyst without the use of ligands. Detailed studies revealed that the hydrogen generated in situ from the water–gas shift is critical for the highly regioselective formation of linear products. The coupling of a traditional homogeneous catalytic process with a heterogeneous catalytic reaction to tune product selectivity may provide a new avenue for the heterogenization of homogenous catalytic processes.     

Modification of rhodium carbonyl catalysts for hydroformylation of 2-butenes by organophosphorus ligands

The influence of the nature of the organophosphorus ligand and the P/Rh molar ratio on the catalytic properties of rhodium carbonyl complexes in hydroformylation of 2-butenes was studied. The difference between phosphine and phosphite ligands during the formation of highly selective catalytic complexes was found. It was supposed that a decrease in the selectivity with respect to 2-methylbutanal is due to the isomerization of olefins under conditions of a decrease in the competitive capability of CO for coordination sites (the high P/Rh ratio, a decrease in the total pressure of the synthesis-gas). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 702–704, April, 1999.     

Styrene Hydroformylation with In Situ Hydrogen: Regioselectivity Control by Coupling with the Low‐Temperature Water–Gas Shift Reaction

The hydroformylation of olefins is one of the most important homogeneously catalyzed industrial reactions for aldehyde synthesis. Various ligands can be used to obtain the desired linear aldehydes in the hydroformylation of aliphatic olefins. However, in the hydroformylation of aromatic substrates, branched aldehydes are formed preferentially with common ligands. In this study, a novel approach to selectively obtain linear aldehydes in the hydroformylation of styrene and its derivatives was developed by coupling with a water–gas shift reaction on a Rh single‐atom catalyst without the use of ligands. Detailed studies revealed that the hydrogen generated in situ from the water–gas shift is critical for the highly regioselective formation of linear products. The coupling of a traditional homogeneous catalytic process with a heterogeneous catalytic reaction to tune product selectivity may provide a new avenue for the heterogenization of homogenous catalytic processes.     

α-Fluorinated cyclic amidophosphite ligands. Their synthesis, Rh complexes and catalytic activity in the hydroformylation of styrene

The synthetic approaches to cyclic phosphite and amido(diamido)phosphite ligands bearing the residues of electron withdrawing perfluorinated tails at the β-position to the phosphorus atom have been elaborated. Catalytic systems based on rhodium complexes of these ligands formed in situ using Rh(CO)₂(acac) as a catalytic precursor demonstrate high activity in the hydroformylation of styrene along with good selectivity in respect to branched aldehyde. Quantum-chemical calculations proved that both the rate of the formation of branched alkyl complex, as well as its reactivity are influenced by the steric and electronic parameters in the same manner.     

Pyranoside phosphite–phosphoroamidite ligands for Pd-catalyzed asymmetric allylic alkylation reactions

We have designed and synthesized a new family of readily available phosphite–phosphoroamidite ligands for Pd-catalyzed allylic substitution reactions of several substrates with different steric and electronic properties. These ligands are derived from d-glucosamine and contain several substituents in the biphenyl moieties, with different steric and electronic properties. Systematic variation of the ligand parameters indicates that enantioselectivities are mainly affected by the substituents at the para-positions of the biphenyl moieties. Enantiomeric excesses of up to 89% with high activities were obtained for rac-1,3-diphenyl-3-acetoxyprop-1-ene S1, rac-(E)-ethyl-2,5-dimethyl-3-hex-4-enylcarbonate S2 and rac-3-acetoxycycloheptene S5.     

Rhodium/tris-binaphthyl chiral monophosphite complexes: Efficient catalysts for the hydroformylation of disubstituted aryl olefins

A family of threefold symmetry phosphite ligands, P(O–BIN–OR)₃ (BIN = 2,2′-binaphthyl; R = Me, Bn, CHPh₂, 1-adamantyl), derived from enantiomerically pure (R)-BINOL, was developed. Cone angles within the range 240–270° were calculated for the phosphite ligands, using the computational PM6 Hamiltonian. Their rhodium complexes formed in situ showed remarkable catalytic activity in the hydroformylation of hindered phenylpropenes, under relatively mild reaction conditions, with full chemoselectivity for aldehydes, high regioselectivity, however with low enantioselectivity. The ether substituents at the ligand affected considerably the catalytic activity on the hydroformylation of 1,1- and 1,2-disubstituted aryl olefins. The kinetics of the hydroformylation of trans-1-phenyl-1-propene, using tris[(R)-2′-benzyloxy-1,1′-binaphthyl-2-yl]phosphite as model ligand, was investigated. A first order dependence in the hydroformylation initial rate with respect to substrate and catalyst concentrations was found, as well as a positive order with respect to the partial pressure of H₂, and a slightly negative order with respect to phosphite concentration and CO partial pressure.