Rhodium(I) catalyzed decomposition of formic acid

Rh(C₆H₄PPh₂)(PPh₃)₂ catalyzes the decomposition of formic acid to CO₂ and H₂. The initial step is the oxidative addition of formic acid to produce the intermediate Rh(HCO₂)(PPh₃)₃, which probably is followed by β-hydride elimination, to produce CO₂ and RhH(PPh₃)₃. The latter reacts with formic acid to produce H₂ and to reform Rh(HCO₂)(PPh₃)₃.     

Rhodium catalyzed reductive esterification reactions

Reductive esterification occurs when unsaturated acids are treated with hydrogen in alcohol using either rhodium trichloride or the dimer of chloro(1,5-hexadiene)rhodium(I) as the catalyst. Saturated acids containing appropriate functional groups are also esterified under the same conditions.     

Pd-carbene catalyzed carbonylation reactions of aryl iodides

A series of carbene complexes [PdBr(2)((i)Pr(2)-bimy)L] (C2-C13) with different types of co-ligands (L) have been tested for their catalytic activities in the carbonylative annulation of 2-iodophenol with phenylacetylene in DMF to afford the respective flavone 2a. Complex C12 with an N-phenylimidazole co-ligand showed the best activity and also afforded high yields when the substrate scope was extended to other aryl or pyridyl acetylenes. In addition, catalyst C12 was also efficient in the carbonylative annulation of 2-iodoaniline with acid chlorides giving the desirable 2-substituted 4H-3,1-benzoxazin-4-ones (4) in good yields. Additionally, this Pd-NHC complex also proved to be a very efficient catalyst for the hydroxycarbonylation of iodobenzene derivatives at low catalyst loading and under low CO pressure. These results demonstrate the versatility and efficiency of this phosphine-free Pd(II)-NHC complex in different types of carbonylations of aryl iodides under mild conditions.     

Rhodium(I) carbonyl complexes of quinoline carboxaldehyde ligands and their catalytic carbonylation reaction

The dimeric rhodium precursor [Rh(CO)₂Cl]₂ reacts with quinoline (a) and its three isomeric carboxaldehyde ligands [quinoline-2-carboxaldehyde (b), quinoline-3-carboxaldehyde (c), and quinoline-4-carboxaldehyde (d)] in 1:2 mole ratio to afford complexes of the type cis-[Rh(CO)₂Cl(L)] (1a-1d), where L = a-d. The complexes 1a-1d have been characterised by elemental analyses, mass spectrometry, IR and NMR (¹H, ¹³C) spectroscopy together with a single crystal X-ray structure determination of 1c. The X-ray crystal structure of 1c reveals square planar geometry with a weak intermolecular pseudo dimeric structure (Rh?Rh = 3.573 Å). 1a-1d undergo oxidative addition (OA) with different electrophiles such as CH₃I, C₂H₅I and I₂ to give Rh(III) complexes of the type [Rh(CO)(COR)Cl(L)I] {R = -CH₃ (2a-2d), R = -C₂H₅ (3a-3d)} and [Rh(CO)Cl(L)I₂] (4a-4d) respectively. 1b exhibits facile reactivity with different electrophiles at room temperature (25 °C), while 1a, 1c and 1d show very slow reactivity under similar condition, however, significant reactivity was observed at a temperature ∼40 °C. The complexes 1a-1d show higher catalytic activity for carbonylation of methanol to acetic acid and methyl acetate [Turn Over Frequency (TOF) = 1551-1735 h⁻¹] compared to that of the well known Monsanto’s species [Rh(CO)₂I₂]⁻ (TOF = 1000 h⁻¹) under the reaction conditions: temperature 130 ± 2 °C, pressure 33 ± 2 bar, 450 rpm and time 1 h. The organometallic residue of 1a-1d was also isolated after the catalytic reaction and found to be active for further run without significant loss of activity.     

Ligand-free copper(I) catalyzed N- and O-arylation of aryl halides

A simple and industrially viable protocol for C-N and C-O coupling is reported here. Arylation of phenol, benzylamine and imidazole with aryl bromides is achieved using ligand-free Cu(I) halide salts in low catalytic amount (2.5 mol %).     

Synthesis of esters by rhodium(i) catalyzed borate ester-benzylic bromide carbonylation reactions

Benzyl halides react with trialkylborates and carbon monoxide, in the presence of 1,5-hexadienerhodium chloride dimer, to give esters In excellent yields. The reaction is applicable to the synthesis of primary, secondary and even tertiary esters.     

Methane oxidative carbonylation catalyzed by rhodium chalcogen halides over carbon supports

Gas phase carbonylation of methane is studied in the presence of molecular oxygen over pure carbon carriers and carbon supported rhodium chalcogen halides. Activated carbons and fullerene blacks have been used as carbon supports. XPS and IR-spectroscopy data show the formation of rhodium chalcogen halides in solids prepared by different methods. We have found that the productivity of acetic acid by carbon supported rhodium chalcogen halides depends strongly on the carbon carrier and the method of the catalyst preparation. Namely, the catalyst with highest productivity for the acetic acid is prepared by synthesizing the rhodium chalcogen halide over the carbon support followed by thermal destruction. We have also found that rhodium chalcogen halides over activated carbons are more active compared with fullerene supported catalysts.     

Palladium‐catalyzed carbonylation of quaternary ammonium halides to tertiary amides

Catalytic carbonylation of quaternary ammonium salts under anhydrous conditions was investigated using palladium catalyst. The carbonylation of tetramethylammonium iodide was chosen as a model reaction and studied systematically. Ligand‐free PdCl₂ showed efficient catalytic performance for this transformation. A palladium catalyst loading as low as 0.05 mol% was sufficient for high yield (96.9%) of N,N‐dimethylacetamide, corresponding to a turnover frequency of 242 h^?1. Under optimum conditions, several other quaternary ammonium halides were also carbonylated to corresponding tertiary amides in moderate to excellent yields. The catalytic activity of commercial palladium on activated carbon (Pd/C) catalyst was also evaluated. The Pd/C catalyst exhibited high activity for this carbonylation reaction and could be recycled six times with a slight decrease in activity. Furthermore, mechanistic considerations concerning Pd‐catalyzed carbonylation of quaternary ammonium halides were also discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Rhodium(I) catalyzed four-component reaction of Fischer alkenyl carbene complexes and 1,1-diphenylallene

Alkyl substituted chromium Fischer carbene complexes react with 1,1-diphenylallene in the presence of rhodium(I) catalysts (10 mol%) to yield highly substituted dienyl indenone derivatives. In this process a catalytic chromium(0)-rhodium(I) exchange occurs, four new C-C bonds are created, and four-components (two allenes, the carbene ligand and one CO ligand) are joined in a chemo- and regioselective manner.     

Phase-transfer reactions catalyzed by polymer-supported crown ethers

Polymer-supported crown ethers were prepared from chloromethylated or ω-bromoalkylated polystyrene resins and hydroxymethylbenzo-18-crown-6 and 15- or 18-membered monoazacrown ethers. Effects of the cavity size of crown ethers, the degree of crosslinking, the degree of ring substitution, particle size, spacer chains, and solvents on the activity of the polymer-supported crown ethers in the reaction of 1-chloro- or 1-bromo-octane with aqueous NaI or KI were investigated and mechanisms of the reaction were discussed in terms of mass transfer, intraparticle diffusion, and intrinsic reactivity.     

Palladium catalyzed reductive homocoupling reactions of aromatic halides in dimethyl sulfoxide (DMSO) solution

Biaryls were obtained in good to excellent yields from the palladium catalyzed reductive homocoupling reactions of various aryl iodides and bromides in dimethyl sulfoxide (DMSO) solution without the need for any additional reducing reagents. Pd(dppf)Cl₂ is the most effective among the screened palladium catalysts for the homocoupling reactions. Fluorides, carbonates, acetates and hydroxides can be used as bases at promoting the palladium catalyzed reductive homocoupling of aryl halides in DMSO solution. X-ray photoelectron spectroscopic (XPS) analysis shows that the oxidative Pd²⁺(dppf) species can be reduced into the Pd⁰(dppf) active species by solvent DMSO molecules to furnish the catalytic cycle, indicating that DMSO plays a dual role as both solvent and reducing reagent. A plausible reaction mechanism has been discussed. Elimination of additional reducing reagents will not only reduce the reaction operation cost, but will also simplify the product separation and purification.     

Rhodium(III)-catalyzed oxidative carbonylation of benzamides with carbon monoxide

An efficient strategy for the oxidative carbonylation of aromatic amides via C-H/N-H activation to form phthalimides using an Rh(III) catalyst has been developed. The reaction shows a preference for C-H bonds of electron-rich aromatic amides and tolerates a variety of functional groups.     

The mechanism of transition metal catalyzed carbonylation of allyl halides: A theoretical investigation

The mechanism of the carbonylation reaction of allyl halides catalyzed by nickel (Ni(CO)₄) and palladium (Cl₂Pd(PPh₃)₂) complexes is theoretically investigated at the DFT level using the hybrid B3LYP functional. The favored reaction path to carbonylation corresponds, for both catalysts, to a direct attack of the halogen on the metal. This affords η¹ intermediates that can undergo the final carbonylation step. It is also possible to obtain the acyl product (β,γ-unsaturated acyl halides) from η² and/or η³ intermediates. However, in this case, the barrier of the rate-determining step to carbonylation is much higher. Since a channel on the potential surface connects rather easily the η² or η³ intermediates to the η¹ intermediates, an alternative and competitive path leading to the acyl products can originate from the η² or η³ intermediates, follow the η²/η³ → η¹ transformation, then undergo the final carbonylation step.     

DFT computational study of the mechanism of allyl halides carbonylation catalyzed by nickel tetracarbonyl

A theoretical investigation at the DFT(B3LYP) level on the carbonylation reaction of allyl bromide catalyzed by nickel tetra-carbonyl Ni(CO)(4) is discussed. The computational results show the following: (i) Three main steps characterize the catalytic cycle: (a) an oxidative addition step, (b) a carbonylation step, and (c) a reductive elimination step where the acyl product is obtained and the catalyst is regenerated. (ii) Both Ni(CO)(3) and Ni(CO)(4) complexes can behave as "active" catalytic species. (iii) The oxidative addition leads to the formation of either eta(3) or eta(1)-allyl nickel complexes, which are involved in a fast equilibrium. (iv) The carbonylation occurs much more easily on the eta(1) than on the eta(3) intermediates.     

Rhodium(I) derivatives of diethylbispyrazolylborate

Bis-substituted rhodium(I) polypyrazolylborates of the type L₂RhBPz₂Et₂ (L = CO or RNC where R = p-CH₃C₆H₄, t-C₄H₉ or p-CH₃C₆H₄SO₂CH₂) have been prepared and characterized and hence the analogous rhodium(III) derivatives by oxidative addition reactions with iodine, iodomethane, or mercury(II) chloride.     

Palladium complex-catalyzed carbonylation of organic halides in the presence of cyclic ethers: Halohydrin ester synthesis

Cyclic ethers were cleaved by palladium complex-catalyzed carbonylation of organic halides to give halohydrin esters.     

Rhodium(I) indazole and indazolate complexes

The preparation of cationic indazole (HIdz) rhodium(I) complexes of the types [(diolefin)Rh(HIdz)₂]ClO₄ and [(CO)₂Rh(HIdz)₂]ClO₄ is described. Neutral binuclear rhodium(I) complexes of the type [Y₂Rh(μ-Idz)]₂ (Y₂  COD, TFB, NBD, (CO)₂ or (CO)(PPh₃)) are obtained by treating the corresponding complexes [Y₂RhCl]₂ with indazole and organic or inorganic bases. The cationic mononuclear derivatives react with the solvated species [Y₂Rh(acetone)_x]ClO₄ in the presence of triethylamine to give neutral binuclear complexes of the types [(CO)₂Rh(μ-Idz)₂Rh(diolefin)], [(Ph₃P)(CO)Rh(μ-Idz)₂Rh(diolefin)] and [(diolefin)Rh(μ-Idz)Rh(diolefin′)] (diolefin  COD, TFB or NBD; diolefin′  COD or TFB). Alternative methods for the synthesis of the binuclear complexes are also described.     

Rhodium(I) Tristyrylphosphine Cyclooctadiene Complexes

Rhodium complexes with unsaturated phosphines as ligands were studied by NMR spectroscopy. Tris[(E,Z,Z)-styryl)phosphine is a stronger complex-forming agent compared with tris[(Z,Z,Z)-styryl)phosphine in view of the easier accessibility of its lone electron pair. The composition of the complexes and their NMR parameters suggest a square-planar structure with cis phosphine ligands.     

Rhodium(I) carbonyl complexes of mono selenium functionalized bis(diphenylphosphino)methane and bis(diphenylphosphino)amine chelating ligands and their catalytic carbonylation activity

The chelate complexes of the types (1) and (2) have been synthesized and characterized by IR and NMR spectroscopy. The lower shift of the ν(P-Se) bands and downfield shift of the ³¹P-{¹H}NMR signals for both P(III) and P(V) atoms in 1 and 2 compared to the corresponding free ligands indicate chelate formation through selenium donor. 1 and 2 show terminal ν(CO) bands at 1977 and 1981 cm⁻¹, respectively, suggesting high electron density at the metal center. The molecular structure of 2 has been determined by single-crystal X-ray diffraction. The rhodium atom is at the center of a square planar geometry having the phosphorus and selenium atoms of the chelating ligand at cis-position, one carbonyl group trans- to selenium and one chlorine atom trans- to phosphorus atom. 1 and 2 undergo oxidative addition (OA) reaction with CH₃I to produce acyl complexes (3) and (4), respectively. The kinetics of the OA reactions reveal that 1 undergoes faster reaction by about 4.5 times than 2. The catalytic activity of 1 and 2 in carbonylation of methanol was higher than that of the well known species [Rh(CO)₂I₂]⁻ and 2 shows higher catalytic activity compared to 1.