Rhodium Carbene Complexes Research Articles

Synthesis and catalytic properties of N -functionalised carbene complexes of rhodium(I)

Reaction of [RhCl(COD)]2, with 1,3-dialkylimidazolinylidene (1) or 1,3-dialkylbenzimidazolinylidene (2) resulted in the formation of rhodium(I) 1,3-dialkylimidazolin-2-ylidene (3a-c) and 1,3-dialkylbenzimidazolin-2-ylidene (4a,b) complexes. Triethylsilane reacts with acetophenone derivatives in the presence of catalytic amounts of RhCl(COD)(1,3-dialkylimidazolin-2-ylidene) or RhCl(COD)(1,3-dialkylbenzimidazolin-2-ylidene) to give the corresponding silylethers in good yield (57–98%).

Hydroaminomethylation with Novel Rhodium—Carbene Complexes: An Efficient Catalytic Approach to Pharmaceuticals.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Hydroaminomethylation with Novel Rhodium–Carbene complexes: An Efficient Catalytic Approach to Pharmaceuticals

Starting from [{Rh(cod)Cl}(2)] and 1,3-dimesitylimidazole-2-ylidenes the novel [RhCl(cod)(carbene)] complexes 1-5 have been synthesized, characterized, and tested in the hydroaminomethylation of aromatic olefins. The influence of different ligands and reaction parameters on the catalytic activity was investigated in detail applying 1,1-diphenylethylene and piperidine as a model system. The scope and limitations of the novel catalysts is shown in the preparation of 16 biologically active 1-amino-3,3-diarylpropenes. In general, high chemo- and regioselectivity as well as good yields of the desired products were achieved.

Stereodynamics and Asymmetric Hydrosilylation with Chiral Rhodium Complexes Containing a Monodentate N-Heterocyclic Carbene

Imidazolinium salts derived from (1S,2S)-diphenylethylenediamine were used to prepare chiral N-heterocyclic carbene (NHC) complexes of rhodium of the form (cod)Rh(NHC)Cl via silver transmetalation. The neutral complexes were readily converted to cationic amine analogues [(cod)Rh(NHC)(L)]SbF6, where L = isoquinoline and 3,5-lutidine. The NHC ligands contain unsymmetrically substituted wingtips that give rise to chiral axes; the conformations of the substituents, as well as other ligands, are influenced by the fixed chirality in the backbone of the ligand. The stereodynamics of both the neutral and cationic complexes were studied, and it was observed that strong conformational preferences exist both in solution and in the solid state. The interplay of the various chiral elements provides the environment that eventually controls the enantioselectivity in the catalytically active species. These complexes are active catalysts for the hydrosilylation of acetophenone with diphenylsilane and furnished (S)-1-phenylethanol in up to 58% enantiomeric excess.

Polymer‐Supported N‐Heterocyclic Carbene—Rhodium Complex Catalyst for the Addition of Arylboronic Acids to Aldehydes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Chromium(0)–rhodium(I) metal exchange: Synthesis and X-ray structure of new Fischer (NHC)carbene complexes of rhodium(I)

An easy approach to Fischer (NHC)carbene complexes of rhodium(I) 3 from methoxy- and aminocarbene complexes of chromium 1 and (NHC)(cod)RhCl ( 2) is described. The process involves the transfer of the carbene unit and a CO ligand from chromium to rhodium. The X-ray analysis is provided for 3d and the preliminary results on their thermal stability and reactivity toward alkynes and allenes are also reported.

Neutral and Cationic Fluorinated N-Heterocyclic Carbene Complexes of Rhodium and Iridium

Transmetalation of the structurally characterized silver N-heterocyclic carbene (NHC) salt [Ag(IpFMe)2]+ (1, IpFMe = 1-pentafluorobenzyl-3-methylimidazol-2-ylidene) to [Rh(COD)Cl]2 affords Rh(IpFMe)(COD)Cl (2). The analogous neutral iridium complex, Ir(IpFMe)(COD)Br (5), may be formed upon elimination of MeOH from [Ir(COD)(OMe)]2 in the reaction with 1-pentafluorobenzyl-3-methylimidazolium bromide. Thermolysis of [Ag(IpFMe)2]+ with either 5 or [Ir(COD)Cl]2 yields the cationic bis-NHC complex [Ir(IpFMe)2(COD)](AgX2) (6, X = Cl, Br), which exists as a mixture of conformers in solution. Addition of CO or AgOTf to 2 gives Rh(IpFMe)(CO)2Cl (3) and Rh(IpFMe)(COD)(OTf) (4), respectively. The molecular structures of compounds 1−6 have been determined by X-ray crystallography.

Highly Chemo-, Regio-, and Stereoselective [3+2]-Cyclization of Activated and Deactivated Allenes with Alkenyl Fischer Carbene Complexes: A Straightforward Access to Alkylidenecyclopentanone Derivatives

A broad range of functionalized 5-alkylidenecyclopentene derivatives are synthesized by the rhodium(I)-catalyzed [3+2]-cyclization reaction of chromium alkenyl(methoxy)carbene complexes 1 and activated allenes. Thus, amidocyclopentenes 4a-n are readily available from N-allenylamides 2a-c, while phenoxyallene 2e gives access to phenoxycyclopentenes 6. In turn, the cyclization reaction with (alkoxycarbonyl)allenes 3 leads to (alkoxycarbonyl)methylidenecyclopentenes 7-10. In terms of selectivity, most cyclization reactions take place with complete chemo-, regio-, and diastereoselectivity. Representative cycloadducts are efficiently hydrolyzed to the corresponding 2-alkylidenecyclopentanones 11a-e without tautomerization or isomerization. Finally, a tentative reaction pathway is proposed that involves the rhodium(I) carbene complexes as the species responsible for the [3+2]-cyclization.

New Rhodium(I) Carbene Complexes from Carbene Transfer Reactions

The Rh(I) complexes RhCl(diene)(NHC) have been synthesized from Ag(I) carbene precursors, where NHC is one of the N-heterocyclic carbenes 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), 3-methyl-1-phenacylimidazol-2-ylidene (MAI), and 3-methyl-1-picolylimidazol-2-ylidene (MPI), and diene is either NBD or COD. The complexes are characterized by elemental analysis, 1H NMR and IR spectroscopies, mass spectrometry, and, in the case of RhCl(NBD)(IMes) and RhCl(COD)(MAI), X-ray crystallography. The new Ag(I) carbene complexes [Ag(IPr)2]PF6 and [Ag(IMes)2]PF6 are also reported.

Polymer-supported N-heterocyclic carbene–rhodium complex catalyst for the addition of arylboronic acids to aldehydes

A novel polymer-supported N-heterocylic carbene (NHC)–rhodium complex was prepared from chloromethyl polystyrene (CM PS) resin using a simple procedure. This polymer-supported NHC–rhodium complex was used as a catalyst for the addition of arylboronic acids to aldehydes affording arylmethanols in excellent yields.

Synthesis, Structure Determination, and Hydroformylation Activity of N‐Heterocyclic Carbene Complexes of Rhodium.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Unsymmetrically N,N′-substituted saturated carbenes: synthesis, reactivity and preparation of a rhodium(i) carbene complex

A versatile synthesis of unsymmetrically N,N'-substituted saturated carbenes is described. The novel racemic imidazolidin-2-ylidenes rac-5 have been synthesized by reductive desulfurization of the corresponding imidazolidin-2-thiones rac-4. The thiones were prepared in two reaction steps from aldimines and secondary amines. Three different substituents at N1, N3 and C4 of the five-membered N-heterocyclic ring can be introduced by choice of suitable aldimines and secondary amines. The dimerization behaviour (diaminocarbene/enetetramine equilibrium) for the unsymmetrically N,N'-substituted imidazolidin-2-ylidenes has been investigated by NMR spectroscopy. Unsymmetrically N-iPr and N-iBu substituted N-heterocyclic carbenes undergo a slow dimerization, whereas N-tBu substituted derivatives are stable as monomeric carbenes indefinitely. The carbene ligand rac-5d has been coordinated to rhodium(I) to give the square-planar rhodium carbene complex [Cl(cod)Rh(rac-5d)]rac-6d which has been characterized by an X-ray diffraction analysis.

Structure and catalytic activity of rhodium(I) carbene complexes in polymerization of phenylacetylene

Rh(I) carbene complexes of [RhX(bmim)(η 4-1,5-cod)] type (bmim = 1-butyl-3-methyl imidazolium cation, X = Cl 2, Br 3, I 4), obtained in the reaction of [Rh(OMe)(η 4-1,5-cod)] 2 ( 1) with [bmim]X ionic liquids, catalyzed polymerization of phenylacetylene (PA) to cis-polyphenylacetylene (PPA) in CH 2Cl 2 and in ionic liquids. The yield of PPA increased and molecular weight ( M w) decreased after addition of phosphorus ligands PPh 3 or P(OPh) 3. Complex 4 reacted with P(OPh) 3 giving cis-[RhI(bmim)(P(OPh) 3) 2] ( 5) complex which catalyzed oligomerization but not polymerization of PA.

Formation of chiral ionic liquids and imidazol-2-ylidene metal complexes from the proteinogenic aminoacid l-histidine

Treatment of the amino acid derivative Bz-His-OMe with excess n-propyl bromide gave the corresponding histidinium salt [Bz-His( n-propyl) 2-OMe +Br −]. It features a melting point of 39 °C and may serve as a useful readily available optically active ionic liquid. Its subsequent treatment with silver oxide gave the corresponding l-histidine derived chiral N-heterocyclic carbene complex [“(carbene) 2Ag · AgBr 2”]. Transmetallation by treatment with Pd(CH 3CN) 2Cl 2 or [Rh(cod)Cl] 2 led to the formation of the respective chiral late metal imidazol-2-ylidene complexes [“(carbene) 2PdCl 2”] and [“(carbene)RhCl(cod)”], respectively. Four diastereomers of the square planar palladium system were observed. Due to the additional chirality center in the l-histidine-derived “Arduengo-carbene ligand” two diastereomers of the rhodium carbene complex were formed.

1Pyridine and phosphine functionalised N-heterocyclic carbene complexes of rhodium and iridium

The pyridine functionalised N-heterocyclic carbene complexes [M(κ 2-NC DIPP)(COD)] +A −, NC DIPP = 3 - ( 2 , 6 - Pr 2 i C 6 H 3 ) - 1 - [ 2 - ( 3 - picolyl ) ] - imidazol - 2 - ylidene , A - = [ Ar 4 F B ] - , [{3,5-(CF 3) 2C 6H 2} 4B] −, M = Rh, 1b, Ir, 4, have been prepared in two steps by reaction of the [M(COD)Cl] 2 with the isolated NC DIPP to [Rh(κ 1-NC DIPP)(COD)Cl], 2, and [Ir(κ 2-NC DIPP)(COD)Cl], 3, followed by anion exchange with Na +A −. The phosphine functionalised N-heterocyclic carbene complex [Rh(PCH 2C mes) 2]Br, 6, PCH 2C mes = 1-(diphenylphosphino-methyl)-3-(2,4,6-Me 3C 6H 2)-imidazol-2-ylidene, was prepared by the reaction of the [Rh(COD)Cl] 2 with the corresponding N-heterocyclic carbene generated in situ. Monomeric [Rh(PCH 2CH 2C DIPP)(acac)], 7 was prepared by an analogous reactions from [Rh(COE) 2(acac)]. In contrast, phosphine functionalised N-heterocyclic carbene complexes of iridium (I) were not easily accessible. However, the reaction of [Ir(COD)(μ-Cl)2(μ-H)]2 with PCH 2CH 2C mes gave complex [Ir(COD)(PCH 2CH 2C mes )Br], 8, in which the carbene is coordinated to the metal in an ‘abnormal’ mode.

Synthesis and Structure of N-Heterocyclic Carbene Complexes with Tethered Olefinic Groups: Application of the Ruthenium Catalyst in Olefin Metathesis

Silver(I) and rhodium(I) complexes bearing the bisallyl-substituted N-heterocyclic carbene ligand (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (allyl2SIMes) have been prepared in a straightforward synthesis. The reaction of (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H-imidazol-1-ium tetrafluoroborate (1a) with Ag2O affords the ionic biscarbene complex [(allyl2SIMes)2Ag]+BF4- (2), while the reaction of (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H-imidazol-1-ium chloride (1b) with Ag2O leads to the monocarbene complex (allyl2SIMes)AgCl (3). Sequential treatment of 1a with KOtBu and dimeric [RhCl(cod)]2 (cod = cyclooctadiene) yields the rhodium carbene complex (allyl2SIMes)RhCl(cod) (4). However, the reaction of 1a with the first-generation Grubbs catalyst (PCy3)2Cl2RuC(H)Ph (Cy = cyclohexyl) leads to ring-closing metathesis of the two allylic groups, yielding 1,3-bis(2,4,6-trimethylphenyl)-3a,4,7,7a-tetrahydro-3H-benzimidazol-1-ium tetrafluoroborate (5). Subsequent reaction of this new imidazolium salt with KOtBu and 1 equiv of (PCy3)Cl2RuC(H)(C6H4OiPr-2) forms [1,3-bis(2,4,6-trimethylphenyl)-3a,4,7,7a-tetrahydro-3H-benzimidazolin-2-ylidene]dichloro(2-isopropanolatobenzylidene)ruthenium(II) (8). All new complexes have been thoroughly characterized, including X-ray crystallographic analyses of 2, 3, and 8. The most intriguing feature of 8 is the presence of an innocent CC bond that is part of a highly active olefin metathesis catalyst, which offers many options for further functionalization of the ligand backbone. The catalytic activity of complex 8 has been evaluated for the ring-closing metathesis of N,N-diallyl-4-toluenesulfonamide.

Synthesis, structure determination, and hydroformylation activity ofN-heterocyclic carbene complexes of rhodium

The effect of ancillary phosphine ligands on the structure and hydroformylation activity of Rh-N-heterocyclic carbene complexes of type [Rh(IMes)(PR3)(CO)Cl] and [Rh(SIMes)(PR3)(CO)Cl] is described. Very high selectivities for the branched isomer (>95:5) were obtained in the hydroformylation of vinylarenes in all cases except for R = OPh. The new complexes were characterized spectroscopically and by X-ray crystallography.Key words: hydroformylation, rhodium, N-heterocyclic carbene, IMes.

Rhodium-benzimidazolidin-2-ylidene catalyzed addition of arylboronic acids to aldehydes

Six rhodium–carbene complexes (2a–f) have been prepared and the new compounds characterized by C, H, N analyses, 1H-n.m.r. and 13C-n.m.r. Phenylboronic acid reacts with aldehydes in the presence of a catalytic amount of rhodium(I)–carbene complex, RhCl(COD)(1,3-dialkylbenzimidazolidin-2-ylidene), (2a–f), to give the corresponding aryl secondary alcohols in good yields.

Synthesis and structure of N-heterocyclic carbene complexes of rhodium and iridium derived from an imidazolium-linked cyclophane

This paper describes the synthesis, spectroscopic and structural characterisation, and electrochemical behaviour of some rhodium and iridium complexes of the form LM(X 1)(X 2) +, where L is a chelating bis(carbene) derived from an imidazolium-linked ortho-cyclophane. The complexes where X 1/X 2 = 1,5-cycooctadiene or norbornadiene were prepared from the imidazolium-linked cyclophane and the appropriate metal source. In these complexes, the M–L bonding was quite robust, but the diene could be displaced by CO to give the dicarbonyl complexes LM ( CO ) 2 + , from which one or both carbonyl ligands could be displaced by monodentate or bidentate phosphines, respectively. Structural studies revealed only minor variations in the cyclophane unit upon exchange of the ancillary ligands, in each case the rhodium complex being isomorphous with its iridium analogue. In cyclovoltammetric studies of LRh(dppe) +, reversible Rh(I/II) and Rh(II/III) redox couples were observed. The other rhodium complexes displayed more complex electrochemical behaviours and did not undergo simple reversible redox reactions.

Chelate and Pincer Carbene Complexes of Rhodium and Platinum Derived from Hexaphenylcarbodiphosphorane, Ph3PCPPh3

The reaction of [(cod)RhCl]2 with Ph3P=C=PPh3 (1) gave the bidentate Rh(I) carbene complex, (cod)Rh[eta2-C{P(C6H4)Ph2}{PPh3}] (2), in which one of the Ph groups in 1 underwent orthometalation to form the chelate. Displacement of cod by 2 equiv of PMe3 transformed 2, via a second orthometalation event, into the Rh(III) C,C,C pincer carbene complex, HRh(PMe3)2[eta3-C{P(C6H4)Ph2}2] (3). The reaction of [Me2Pt(SMe2)]2 with 1 led directly to the analogous C,C,C pincer carbene complex of Pt(II), (Me2S)Pt[eta3-C{P(C6H4)Ph2}2] (4). DFT calculations on a model form of 3 suggest a net single sigma-bonding interaction between Rh and an sp2-hybridized carbene center, with a HOMO that is predominantly carbene pz in character.