Carbonylative Addition of Arylboronic Acids to Terminal Alkynes: A New Catalytic Access to α,β‐Unsaturated Ketones

The carbonylative addition of arylboronic acids to terminal alkynes under mild conditions affords (E)‐α,β‐unsaturated ketones with good yields. The reaction was achieved with chloro(1,5‐cyclooctadiene)rhodium(I) dimer or chlorodicarbonylrhodium(I) dimer as catalytic precursor without additional phosphine as their use inhibits the reaction. Experiments using deuterated 1‐hexyne discarded the possibility of a rhodium‐vinylidene intermediate, thus a catalytic cycle involving a 1,2‐insertion of the terminal alkyne in a rhodium‐acyl bond is proposed. This new reaction represents the first example of the hydroacylation of terminal alkynes involving rhodium‐acyl reagents generated under CO pressure and promises a wide field of interest.     

Rhodium‐Catalyzed Enantioselective Conjugate Addition of Arylboronic Acids to Dihydronitronaphthalenes

A highly enantioselective (up to 91% ee) rhodium‐catalyzed asymmetric addition of arylboronic acids has been achieved leading to the challenging dihydro‐3‐nitronaphthalenes using one equivalent of phosphoramidite ligand to rhodium catalyst. A concise formal asymmetric synthesis of the dopamine D1 agonist, dihydrexidine was accomplished using the method.     

Rhodium‐Catalyzed Addition of Boronic Acids to Vinylogous Imines Generated in situ from Sulfonylindoles

The rhodium‐catalyzed addition of arylboronic acids to vinylogous imines generated in situ from sulfonylindoles has been developed. This procedure provided a rapid approach to C‐3 sec‐alkyl‐substituted indoles.     

Ligand Control in Enantioselective Desymmetrization of Bicyclic Hydrazines: Rhodium(I)‐Catalyzed Ring‐Opening versus Hydroarylation

The efficient desymmetrization of 2,3‐bicyclic hydrazines with boronic acids through rhodium‐catalyzed ring‐opening or reductive arylation is described. Excellent levels of enantioselectivity are achieved in ring‐opening with ortho‐substituted boronic acids, using Josiphos‐type ligands. Alternatively, reductive arylation occurs selectively with electron‐poor Josiphos and Walphos ligands. A C H activation/1,4‐migration mechanism was established through deuterium transfer experiments.     

Enantioselective Rhodium‐Catalyzed Addition of Arylboronic Acids to Trifluoromethyl Ketones

A new C₂‐symmetrical, chiral bisphosphorus ligand proved to be efficient for the rhodium‐catalyzed nucleophilic addition of arylboronic acids to trifluoromethyl ketones, providing a series of chiral trifluoromethyl‐substituted tertiary alcohols in high yields (up to 93%) and excellent enantioselectivities (>99%).     

Rhodium‐Catalyzed Enantioselective Hydrogenation of (E)‐Enol Acetate Acids

The novel rhodium complex [Rh(S)‐Phanephos(cod)]‐catalyzed hydrogenation of disubstituted (E)‐enol acetate carboxylic acids is reported. The catalytic cycle works under 30 bar of hydrogen under conventional heating giving different 3‐acetoxy‐2,3‐disubstituted carboxylic acids with ee ≥90%. Hydrogenation occurred also under microwave dielectric heating without eroding the enantioselectivity but improving the overall efficiency of the process. With microwaves, hydrogen pressure and reaction time required for complete hydrogenation dropped to 5 bar and 30 min, respectively. The best performance of this catalyst under microwave irradiation was TON 100, TOF 196 h⁻¹ with ee 99% on a 6‐g scale.     

Rhodium‐Catalyzed Asymmetric 1,4‐Additions,in Water at Room Temperature,with In‐Flask Catalyst Recycling

Using the newly introduced designer surfactant polyethylene glycol ubiquinol sebacate (PQS), as the platform for micellar catalysis, non‐racemic BINAP has been covalently attached and rhodium(I) inserted to form PQS‐BINAP‐Rh. This species, the first example of a non‐racemically‐ligated transition metal catalyst‐tethered amphiphile, can be utilized for Rh‐catalyzed asymmetric conjugate addition reactions of arylboronic acids to acyclic and cyclic enones. These are performed in water at room temperature, while the catalyst can be recycled without its removal from water in the reaction vessel.     

DiPAMP’s Big Brother “i‐Pr‐SMS‐Phos” Exhibits Exceptional Features Enhancing Rhodium(I)‐Catalyzed Hydrogenation of Olefins

Switching Knowles DiPAMP’s {DiPAMP=1,2‐bis[(o‐anisyl)(phenyl)phosphino]‐ ethane} MeO groups with i‐PrO ones led to the i‐Pr‐SMS‐Phos {i‐Pr‐SMS‐Phos=1,2‐bis[(o‐isoprop‐ oxyphenyl)(phenyl)phosphino]ethane} ligand which displayed a boosted catalyst activity coupled with an enhanced enantioselectivity in the rhodium(I)‐catalyzed hydrogenation of a wide‐range of representative olefinic substrates (dehydro‐α‐amido acids, itaconates, acrylates, enamides, enol acetates, α,α‐diarylethylenes, etc). The rhodium(I)‐(i‐Pr‐SMS‐Phos) catalytic profile was investigated revealing its structural attributes and robustness, and in contrast to the usual trend, ³¹P NMR analysis revealed that its methyl (Z)‐α‐acetamidocinnamate (MAC) adduct consisted of a reversed diastereomeric ratio of 1.4:1 in favour of the most reactive diastereomer.     

Rhodium‐Catalyzed Asymmetric Hydroformylation of 1,1‐Disubstituted Allylphthalimides: A Catalytic Route to β3‐Amino Acids

The high enantioselective rhodium‐catalyzed hydroformylation of 1,1‐disubstituted allylphthalimides has been developed. By employing chiral ligand 1,2‐bis[(2S,5S)‐2,5‐diphenylphospholano]ethane [(S,S)‐Ph‐BPE], a series of β³‐aminoaldehydes can be prepared with up to 95% enantioselectivity. This asymmetric procedure provides an efficient alternative route to prepare chiral β³‐amino acids and alcohols.     

Rhodium/MonoPhos‐Catalysed Asymmetric Hydrogenation of Enamides

The monodentate phosphoramidite MonoPhos has been used in the rhodium‐catalysed asymmetric hydrogenation of N‐acetyl‐α‐arylenamides. This ligand is readily available via a one‐step procedure and is air stable. Its Rh(I) complex, which is an effective catalyst precursor for the hydrogenation of dehydroamino acids, also gives high enantioselectivities for this class of substrates. Because of the facile synthesis and stability of MonoPhos, its complex provides a general solution in preparing chiral amine derivatives through asymmetric hydrogenation.     

Rhodium‐Cobalt Bimetallic Nanoparticles: A Catalyst for Selective Hydrogenation of Unsaturated Carbon‐Carbon Bonds with Hydrous Hydrazine

Our studies on the effect of metal compositions on the catalytic activity of the rhodium‐based bimetallic nanocatalysts revealed that the nanoparticles with a 4:1 ratio of rhodium to cobalt, were more active than the rhodium monometallic nanoparticles in the selective hydrogenation of unsaturated carbon‐carbon bonds with hydrous hydrazine as a hydrogen source. The nanocatalysts effected this hydrogenation process in good‐to‐excellent yields with high functional group tolerance, and could be reused 10 times without the loss of catalytic activity. The catalyst characterization by X‐ray photoelectron spectroscopy, transmission electron microscopy and line‐scanning analysis suggested that the coexistence of metallic rhodium and cobalt plays an important role in the enhancement of catalytic activity.     

Rhodium‐Catalyzed Asymmetric Cycloisomerization of 1,6‐Ene‐ynamides

The rhodium‐catalyzed asymmetric cycloisomerization of heteroatom‐bridged 1,6‐ene‐ynamides proceeded to give high yields of functionalized 3‐aza‐ and oxabicyclo[4.1.0]heptene derivatives with high enantioselectivity, which was achieved by use of a rhodium/chiral diene catalyst. The 1,6‐ene‐ynamides substituted with 2‐oxazolidinone and 2‐azetidinone moieties at the alkyne terminus were found to display high reactivity towards the rhodium/chiral diene catalyst, where the chelate coordination of the alkyne moiety and the carbonyl oxygen of the ene‐ynamides might be responsible for the high catalytic activity.     

An Ion‐Pair Immobilization Strategy in Rhodium‐Catalyzed Asymmetric Transfer Hydrogenation of Aromatic Ketones

A chiral diamine‐based homogeneous cationic rhodium catalyst was developed and two heterogeneous cationic rhodium catalysts were obtained via the encapsulation of the homogeneous cationic rhodium catalyst within Me‐SBA‐15 and Me‐SBA‐16. All these catalysts presented excellent catalytic activities and high enantioselectivities in ultrasound‐promoted asymmetric transfer hydrogenation of aromatic ketones and represent a successful use of the ion‐pair immobilization strategy. More importantly, the encapsulation of the cationic rhodium functionality within Me‐SBA‐16 had an obvious high recyclability, in which the recycled catalyst could be reused nine times without significantly affecting its enantioselectivity, showing good potential in industrial application.     

Chelating Bis(1,2,3‐triazol‐5‐ylidene) Rhodium Complexes: Versatile Catalysts for Hydrosilylation Reactions

NHC‐rhodium complexes (NHC=N‐heterocyclic carbenes) have been widely used as efficient catalysts for hydrosilylation reactions. However, the substrates were mostly limited to reactive carbonyl compounds (aldehydes and ketones) or carbon‐carbon multiple bonds. Here, we describe the application of newly‐developed chelating bis(tzNHC)‐rhodium complexes (tz=1,2,3‐triazol‐5‐ylidene) for several reductive transformations. With these catalysts, the formal reductive methylation of amines using carbon dioxide, the hydrosilylation of amides and carboxylic acids, and the reductive alkylation of amines using carboxylic acids have been achieved under mild reaction conditions.

     

Rhodium‐Catalysed Alkoxycarbonylative Cyclisation Reactions of 1,6‐Enynes

The rhodium‐catalysed carbonylation of 1,6‐enynes possessing an electron‐deficient alkenyl moiety in an alcohol reagent in the presence of a rhodium complex proceeded stereo‐ and chemoselectively to afford exocyclic α,β‐enoates.     

A New Method for Constructing Quaternary Carbon Centres: Tandem Rhodium‐Catalysed 1,4‐Addition/Intramolecular Cyclisation

The efficient tandem rhodium‐catalysed 1,4‐addition/cyclisation of 1,1′‐alkenes using arylzinc chlorides is described. The simple one‐step synthesis of substituted cyclopentanone and cyclohexanone derivatives is performed from acyclic precursors using relatively low catalyst loadings under mild conditions. A new quaternary carbon centre is created during the cyclisation step.     

Tandem Catalysis: Access to Ketones from Aldehydes and Arylboronic Acids via Rhodium‐Catalyzed Addition/Oxidation

Direct cross‐coupling reactions of aromatic aldehydes with arylboronic acids afforded ketones in high yields and under mild conditions in the presence of a rhodium catalyst, acetone and a base. This new reaction, involving a formal aldehyde C H bond activation, is believed to proceed via a tandem process involving addition of the organometallic species to the aldehyde followed by oxidation by β‐hydride transfer.     

Synthesis,Rhodium Complexes and Catalytic Applications of a New Water‐Soluble Triphenylphosphane‐Modified β‐Cyclodextrin

A new triphenylphosphane based on a β‐cyclodextrin skeleton (PM‐β‐CD‐OTPP) was synthesized. This ligand can be dispersed in water by using the nanoprecipitation method. Transmission electron microscopy and NMR spectroscopy showed that PM‐β‐CD‐OTPP is aggregated in water and forms a stable dispersion. Its aqueous solubility can be dramatically increased in the presence of selected water‐soluble guests by formation of inclusion complexes. Associated to a rhodium precursor, PM‐β‐CD‐OTPP is able to generate soluble rhodium species in water. In addition, NMR experiments showed that the cyclodextrin cavity remains accessible for a guest even when PM‐β‐CD‐OTPP is coordinated to rhodium. Finally, this ligand was efficient for rhodium‐catalyzed hydrogenation and hydroformylation performed in aqueous medium.     

Enantioselective Hydrogenation of α‐Dehydroamino Acid Esters Catalyzed by Rhodium Complexes with Chiral Bisaminophosphine Ligands

A highly efficient strategy for the synthesis of a series of chiral bisaminophosphine ligands was well established with several remarkable features. The synthetic utility of these ligands was explored for rhodium‐catalyzed asymmetric hydrogenations of α‐dehydroamino acid esters. Up to 98% ee values were achieved for the enantioselective synthesis of aminocarboxylic acids and their derivatives, which are very important chiral building blocks for the synthesis of a variety of natural products and biologically active molecules.     

Optically Pure 1,2‐Bis[(o‐alkylphenyl)phenylphosphino]ethanes and Their Use in Rhodium‐Catalyzed Asymmetric Hydrogenations of α‐(Acylamino)acrylic Derivatives

Optically pure (S,S)‐1,2‐bis[(o‐alkylphenyl)phenylphosphino]ethanes 1a–d were prepared in four steps from phenyldichlorophosphine via phosphine‐boranes as the intermediates. The rhodium complexes 5a–d of these diphosphines were used for the asymmetric hydrogenations of α‐(acylamino)acrylic derivatives including β‐disubstituted derivatives. Markedly high enantioselectivity (78–>99%) was observed for the reduction of β‐monosubstituted derivatives. β‐Disubstituted derivatives were also reduced in considerably high enantioselectivity (up to 90%). The single crystal X‐ray analysis of the rhodium complex 5c of (S,S)‐1,2‐bis[phenyl(5′,6′,7′,8′‐tetrahydronaphthyl)phosphino]ethane ( 1c) revealed its δ‐type structure with face orientation of the two tetrahydronaphthyl groups and edge orientation of the two phenyl groups. This conformation corresponds to that of the rhodium complex of 1,2‐bis[(o‐methoxyphenyl)phenylphosphino]ethane (DIPAMP); the rhodium complex of (R,R)‐DIPAMP, whose chirality at phosphorus is opposite that of 5c , exhibits a λ‐type structure with the face orientation of the two o‐methoxyphenyl groups and the edge orientation of the two phenyl groups. The conformational similarity of these rhodium complexes as well as the stereochemical outcome in the asymmetric hydrogenations means that the coordinative interaction of the methoxy group of DIPAMP with rhodium metal is not the main factor that affects asymmetric induction.