Ligands For Rhodium Research Articles

Unusual Kinetics Induced by Ligands in Rhodium(III)-Catalyzed Dehydrogenative Olefination Reactions.

In this study, we investigate the effects of ligands on C-H activation during rhodium(III)-catalyzed C-H bond olefination reactions using well-defined [CpXRhIII] catalytic systems with three representative CpX (Cp (η5-C5H5), CpCF3 (η5-C5Me4CF3), and Cp* (η5-C5Me5)) ligands. Our results demonstrate that C-H activation as the rate-limiting step is significantly influenced by the steric properties of the CpX ligands. Moreover, we observe a dramatic acceleration of the simple [CpRhIII]-catalyzed C-H olefination reaction with acid coproducts such as HOAc, implying an autocatalytic C-H activation process.

Dioxaphosphabicyclooctanes: small caged phosphines from tris(hydroxymethyl)phosphine.

Dioxaphosphabicyclo[2.2.2]octanes (L1-L4) have been prepared in a one-pot reaction from tris(hydroxymethyl)phosphine and various α,β-unsaturated ketones. The non-volatile phosphines oxidise very slowly in air. They possess highly upfield 31P chemical shifts (-59 to -70 ppm), small cone angles (121-140°) and a similar electronic parameter to PPh3. Reaction of L1 with [Rh(acac)(CO)2] gave the complex [Rh(acac)(CO)(L1)] with a ν(CO) of 1981.5 cm-1, whereas reaction L1 with [Rh(CO)2Cl]2 gave [Rh(CO)(L1)2Cl] with a ν(CO) of 1979.9 cm-1, remarkably similar to the CO stretching frequencies reported for analogous PPh3 complexes. The cage phosphines were explored as ligands in rhodium catalysed hydroformylation of 1-octene. All of the ligands gave a linear selectivity to n-nonanal of 68%, regardless of the substituents. However the ligand substituents had a significant effect on the catalyst activity, with increased steric bulk around the coordination environment giving a three-fold increase in aldehyde yield. The phosphines undergo ligand subsitution with [Pd(MeCN)2Cl2] forming square planar trans-[Pd(L)2Cl2] complexes. Subsequent reduction with hydrazine furnishes homoleptic tetravalent [Pd(L1)4] which was applied as a catalyst in Suzuki-Miyaura couplings, furnishing the C-C coupled products in moderate yields.

Amide-functionalised phosphonium-based ionic liquids as ligands for rhodium(iii) extraction.

Seven new amide-functionalised phosphonium-based ionic liquids (APILs) with chloride anions are synthesised and applied to extraction of rhodium(iii) from HCl solution. The effects of structural modification of the APILs on the extraction performance are examined by liquid–liquid extraction using toluene as a diluent and the results compared with those for trihexyltetradecylphosphonium chloride ([P66614][Cl]), a typical commercial extractant. The performance of the APILs as rhodium(iii) extractants is influenced by three main factors: (1) the length of the alkyl chains attached to the P atom; (2) the length of the linker between the amide and phosphonium moiety; and (3) the type of amide group. A novel ligand, [3°C2P444][Cl], had outstanding performance in the effective recovery of rhodium(iii). Extraction of rhodium(iii) from a 1.0 mol dm−3 HCl solution with 0.5 mol dm−3 [3°C2P444][Cl] proceeded quantitatively (>98%) and the extraction efficiency was higher than that of the commercial extractant [P66614][Cl]. The mechanism of rhodium(iii) extraction by [3°C2P444][Cl] was investigated by slope analysis, UV-vis, and FT-IR spectroscopy. These results indicate that [RhCl4(H2O)2]− in aqueous solution is extracted by [3°C2P444][Cl] through an anion-exchange mechanism and slowly converted into a dimer, [Rh2Cl9]3−, in the organic phase.

Synthesis of 7H-Benzo[e]naphtho[1,8-bc]silines by Rhodium-catalyzed [2 + 2 + 2] Cycloaddition

An efficient synthesis of 7H-benzo[e]naphtho[1,8-bc]silines has been developed by a rhodium-catalyzed [2 + 2 + 2] cycloaddition of alkynyl(8-alkynyl-1-naphthyl)silanes with internal alkynes. High chemoselectivity can be realized by employing P(2-MeOC6H4)3 as the ligand for rhodium. Preliminary investigation of its asymmetric variant has also been described to create a silicon stereogenic center with relatively high enantioselectivity.

Control of Enantioselectivity in Rhodium(I) Catalysis by Planar Chiral Dibenzo[a,e]cyclooctatetraenes.

Planar chiral 5,11-disubstiuted dibenzo[a,e]cyclo-octatetraenes (dbCOTs) have been developed as the first useful chiral homologs to dbCOT-ligands for asymmetric applications. Methods enabling the preparation of such compounds on a gram-scale in enantiomerically pure form are described. Evaluated as ligands in rhodium(I)-catalyzed 1,4- and 1,2-arylation reactions, tertiary and quarternary stereogenic centers were formed with excellent yields and selectivities of up to >99 % ee. A catalytic asymmetric synthesis of a key cyclization precursor to (-)-penifulvin A highlights the system in an applied context.

Intramolecular C–C Coupling Reactions of Alkynyl, Vinylidene, and Alkenylphosphane Ligands in Rhodium(III) Complexes

Electrophilic attack with methyl trifluoromethanesulfonate or tetrafluoroboric acid, to new alkynyl rhodium complexes containing alkenylphosphanes, leads to butenynyl coupling products or to the unprecedented rhodaphosphacycle complexes [Rh(η5-C5Me5){κ4-(P,C,C,C)-iPr2PCH2C(═CH2)C(CH2R)C═C(R)}][BF4] (R = Ph (11a), p-tol (11b)). These complexes 11a,b can be explained as a result of the coupling of three organic fragments in the molecule, the alkynyl, the vinylidene, generated in situ by reaction with HBF4 (A), and the C–C double bond from the alkenylphosphane. DFT computational studies on the formation of complex 11a suggest the [2 + 2] intramolecular cycloaddition between the double bond of the allylphosphane and the Cα–Cβ of the vinylidene A as the most plausible pathway for this reaction.

Synthesis and evaluation of C2-symmetric bis-sulfinamides as effective ligands in rhodium catalyzed addition of arylboronic acids to cycloalkenones

Synthesis of novel C2-symmetric 1,2- 1,3- and 1,4- bis-sulfinamides and their use as effective ligands in rhodium (I) catalyzed asymmetric conjugate addition of arylboronic acids to cyclohexenone and cyclopentenones is described. C2-symmetry as well as chirality at the sulfur center in the ligand is crucial for the high enantioselectivity in the 1,4-addition reaction. It was also observed that the conjugate addition proceeded with good selectivity with Wilkinson's catalyst as the rhodium source.

Structural diversity of bimetallic rhodium and iridium half sandwich dithiolato complexes.

The synthesis of a range of rhodium(iii) and iridium(iii) half sandwich complexes with aryl dithiolato ligands of varying geometry and flexibility are reported. These include dinuclear [Cp*M(S-R-S)]2 complexes 3b and 4b, M = Rh, Ir; S-R-S = naphthalene-1,8-dithiolate (b) and four dinuclear complexes bearing bridging dithiolate ligands [(Cp*M)2(μ2-Cl)(μ2-S-R-S)]Cl 3c, 4c, 5b, 6b, M = Rh, Ir; S-R-S = naphthalene-1,8-dithiolate (b) or acenaphthene-5,6-dithiolate (c). The introduction of a less rigid biphenyl dithiolate backbone resulted in the tetranuclear dicationic complex [(Cp*Rh)4(S-R-S)3]Cl2 (3d), S-R-S = biphenyl-2,2'-dithiolate (d) with dithiolate ligands in two different bridging modes. All new complexes were fully characterised by multinuclear NMR, IR, Raman and MS spectroscopy and single crystal X-ray diffraction.

A Base-Stabilized Silyliumylidene Cation as a Ligand for Rhodium and Tungsten Complexes

The reaction of the base-stabilized silyliumylidene triflate [LSi(DMAP)]OTf (1·OTf; L = PhC(NtBu)2, DMAP = 4-dimethylaminopyridine) with [Rh(cod)Cl]2 and W(CO)5THF afforded [{L(DMAP)Si}2{μ-Rh(μ-Cl)2Rh(cod)}](OTf)2 (2) and [L(DMAP)Si→W(CO)5]OTf (3), respectively. Their crystal structures determined by X-ray crystallography show that the silyliumylidene cation acts as a two-electron donor coordinating to the rhodium and tungsten atoms.

Rhodium‐Catalysed Tandem Hydroformylation/Arylation Reaction with Boronic Acids

AbstractA new efficient multicatalytic process involving a single catalyst to promote tandem hydroformylation/arylation reactions is disclosed. The effect of the rhodium ligand was evaluated and the rhodium/triphenylphosphine catalytic system was selected to apply the methodology to different olefins and boronic acids. High yields (up to 89%) and good to excellent isomer ratios (up to 98:2) were achieved using aryl olefins as starting materials. This new methodology allows the preparation of secondary alcohols, from simple olefins, and paves the way for the synthesis of high‐value products, namely vinylindole and anethole derivatives.magnified image

ChemInform Abstract: α,β‐Divinyl Tetrahydropyrroles as Chiral Chain Diene Ligands in Rhodium(I)‐Catalyzed Enantioselective Conjugated Additions.

AbstractThe divinyl tetrahydropyrrole is a highly efficient ligand in the asymmetric conjugated addition of various arylboronic acids to cyclic α,β‐unsaturated carbonyl compounds.

ChemInform Abstract: Vicinal‐Diamine‐Based Chiral Chain Dienes as Ligands for Rhodium(I)‐Catalyzed Highly Enantioselective Conjugated Additions.

AbstractThe novel diene DAH effectively catalyzes the reaction of a variety of cyclic ketones and boronic acids in combination with KOH.

α,β-Divinyl Tetrahydropyrroles as Chiral Chain Diene Ligands in Rhodium(I)-Catalyzed Enantioselective Conjugated Additions

A series of α,β-divinyl tetrahydropyrroles, synthesized by asymmetric allylic C-H bond activation/conjugated diene addition reaction of ene-2-dienes, were found to be very efficient chiral chain diene ligands in the rhodium-catalyzed conjugated addition of organoboronic acids to various α,β-unsaturated compounds, achieving the desired chiral adducts with good to excellent yields and ee values.

Vicinal-Diamine-Based Chiral Chain Dienes as Ligands for Rhodium(I)-Catalyzed Highly Enantioselective Conjugated Additions

A variety of readily accessible vicinal-diamine-based chiral chain dienes were successfully synthesized and utilized as steering ligands for rhodium-catalyzed conjugated additions of organoboronic acids to α,β-unsaturated carbonyl compounds to afford the desired adducts in good to excellent yields and ee's.

Phthalazin-2-ylidenes As Cyclic Amino Aryl Carbene Ligands in Rhodium(I) and Iridium(I) Complexes

The direct metalation of phthalazinium halides with [Rh(tBuO)(COD)]2 or [Ir(tBuO)(COD)]2 led to the isolation of stable [MCl(phthalazin-2-ylidene)X(COD)] (M = Rh, Ir) as a new family of cyclic amino aryl carbene transition metal complexes. The X-ray diffraction analysis of several representatives systematically revealed a relatively short C(aryl)−C(carbene) bond distance of 1.44−1.46 A, shorter than the typical C(sp2)−C(sp2) single bond, indicating some electron density delocalization from the π system of the aryl ring over the vacant carbene orbital.

Chiral N-phosphino sulfinamide ligands in rhodium(I)-catalyzed [2+2+2] cycloaddition reactions

The combination of cationic rhodium(I) complexes with N-phosphino tert-butylsulfinamides (PNSO) ligands is efficient for catalytic intra- and intermolecular [2+2+2] cycloaddition reactions. PNSO ligands are a new class of chiral bidentate ligands, which have the characteristic of combining the easily accessible sulfur chirality with the coordinating capacity of phosphorous. Cycloaddition of open-chained and macrocyclic E-enediynes with these chiral complexes have proved to be highly efficient in terms of yields, giving moderate enantiomeric excesses of the corresponding cyclohexadiene derivatives. In addition Rh(I)/PNSO complexes catalyzed the intermolecular cycloaddition of diynes with monoalkynes in mild reaction conditions and short reaction times.

ChemInform Abstract: One Step Synthesis of Chiral Olefins via Asymmetric Diamination and Their Applications as Ligands for Rhodium(I)‐Catalyzed 1,4‐Additions.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Ferrocenyl-Aryl Based trans-Chelating Diphosphine Ligands: Synthesis, Molecular Structure and Application in Enantioselective Hydrogenation

Potentially trans-chelating diphosphine ligands based on a ferrocenyl-aryl backbone were synthesised in a four-step sequence and the molecular structures in the solid state of two representatives were determined by X-ray diffraction. High throughput screening of these ligands in rhodium-, ruthenium- and iridium-mediated hydrogenations of a variety of alkenes and ketones revealed that these ligands can deliver high enantioselectivity for alkenes (up to 98% ee) but are less selective when ketones are used as the substrates. The coordination behaviour of one ligand in its square planar palladium and platinum dichloride complexes was studied by 31 P NMR and only trans-chelated complexes, together with oligomeric by-products, were observed. Reaction with the (p-cymene)ruthenium dichloride dimer, [RuCl 2 (p-cymene)] 2 , resulted in a mixture of diastereomeric complexes.

Room-Temperature Rhodium-Catalyzed Asymmetric 1,4-Addition of Potassium Trifluoro(organo)borates

For the first time the room-temperature rhodium-catalyzed asymmetric 1,4-addition of potassium aryltrifluoroborates to alpha,beta-unsaturated substrates is described. Thanks to the use of a chiral diene as ligand for rhodium and triethylamine as base, to facilitate transmetalation of the boron species, high yields and enantioselectivities were generally achieved. Moreover, the use of such tetravalent boron species offers some improvements compared to the use of boronic acids in term of stability and ease of purification.

Impact of Incorporating Substituents onto the P‐o‐Anisyl Groups of DiPAMP Ligand on the Rhodium(I)‐Catalyzed Asymmetric Hydrogenation of Olefins

AbstractThe introduction of 1,2‐bis[(o‐anisyl)(phenyl)phosphino]ethane (DiPAMP) as a P‐stereogenic ligand for rhodium(I)‐catalyzed hydrogenation by Knowles et al. came after their evaluation of several diphosphines. However, no in‐depth study was carried out on incorporating various substituents on its P‐o‐anisyl groups. In this work, we have prepared a large series of enantiopure and closely related DiPAMP analogues possessing various substituents (MeO, TMS, t‐Bu, Ph, fused benzene ring) on the o‐anisyl rings. The new ligands were evaluated in rhodium‐catalyzed hydrogenation of several model substrates: methyl α‐acetamidoacrylate, methyl (Z)‐α‐acetamidocinnamate, methyl (Z)‐β‐acetamidocrotonate, dimethyl itaconate, and atropic acid. They displayed enhanced activities and increased enantioselectivities, particularly the P‐(2,3,4,5‐tetra‐MeO‐C6H)‐substituted ligand (4MeBigFUS). Interestingly enough, 88% ee was obtained in the hydrogenation of atropic acid using the Rh‐(4MeBigFUS) catalyst under mild conditions (10 bar H2, room temperature) versus 7% ee using Rh‐DiPAMP. Conversely, the ligand possessing P‐(2,6‐di‐MeO‐C6H3) groups proved to slow down considerably the hydrogenation. X‐Ray structures of their corresponding Rh complexes are presented and discussed.