Charge-tagged ligands: useful tools for immobilising complexes and detecting reaction species during catalysis

In recent years, charge-tagged ligands (CTLs) have become valuable tools in organometallic catalysis. Insertion of an ionic side chain into the molecular skeleton of a known ligand has become a useful protocol for anchoring ligands, and consequently catalysts, in polar and ionic liquid phases. In addition, the insertion of a cationic moiety into a ligand is a powerful tool that can be used to detect reaction intermediates in organometallic catalysis through electrospray ionisation mass spectrometry (ESI-MS) experiments. The insertion of an ionic tag ensures the charge in the intermediates independently of the ESI-MS. For this reason, these ligands have been used as ionic probes in mechanistic studies for several catalytic reactions. Here, we summarise selected examples on the use of CTLs as immobilising agents in organometallic catalysis and as probes for studying mechanisms through ESI-MS.     

Effect of the axial cysteine ligand on the electronic structure and reactivity of high-valent iron(IV) oxo-porphyrins (Compound I): a theoretical study

The effect of axial ligands on the reactivity of high-valent iron(IV) oxo-porphyrins (Compound I) was investigated using the B3LYP hybrid density functional method. We studied alkane hydroxylation using four models: Compound I with thiolate, imidazole, phenolate, and chloride anions as axial ligands. The first three ligands were employed as models for cysteinate, histidine, and tyrosinate, respectively. Our calculations show that anionic ligands and neutral ligands favor different electronic states for stationary points in the reaction coordinate, and the calculated energy barrier and energy of several reaction intermediates show similar values. A remarkable effect of axial ligands was found in the final product release step. Our calculations show that the thiolate ligand weakens a bond between heme and an alcohol. In contrast, the imidazole ligand significantly increases the interaction between heme and an alcohol, which causes the catalytic cycle to be less efficient.     

Synthesis of chiral sulfonamide/Schiff base ligands

We report a facile two step synthesis of chiral ligands for bonding to transition metals. The ligands are easily prepared from trans-1,2-diaminocyclohexane by reaction with sulfonyl chlorides to give amino-sulfonamide compounds. These intermediates are then condensed with salicylaldehyde derivatives to provide sulfonamide/Schiff base compounds which represent a new class of chiral ligands.     

Aryl transfer between Pd(II) centers or Pd(IV) intermediates in Pd-catalyzed domino reactions

A computational study has been performed to determine the mechanism of the key steps of Pd-catalyzed domino reactions in which C(sp2)-C(sp2) are formed from aryl and alkenyl halides. DFT calculations were done on model complexes of the proposed intermediates, with PH3 and H2O as ancillary ligands, to explore two possible mechanisms: the oxidative addition of aryl or alkenyl halides to palladacycles to give Pd(IV) intermediates, and the transmetalation-type reaction of aryl or alkenyl ligands between two Pd(II) centers, a palladacycle, and a Pd(II) complex formed by oxidative addition of aryl or alkenyl halides to Pd0. We have shown that oxidative addition of iodoethylene to Pd0 precursors is more favorable than oxidative addition to Pd(II) palladacycles, whereas transmetalation-type reactions between Pd(II) complexes are facile. Similar results were obtained with iodobenzene instead of iodoethylene and formamide as the ancillary ligand. These results suggest that Pd(IV) intermediates are not involved in these reactions.     

Phosphinoferrocenylaminophosphines as novel and practical ligands for asymmetric catalysis

[structure: see text] A new series of ligands with a novel phosphine-aminophosphine ligation design as depicted in structure 1 has been prepared on a ferrocenylethyl backbone. These BoPhoz ligands of structure 2 have afforded exceedingly high activity and enantioselectivity in the rhodium-catalyzed asymmetric hydrogenation of dehydro-alpha-amino acid derivatives, itaconic acids, and alpha-ketoesters. These air-stable ligands are readily prepared from cost-effective and non-pyrophoric intermediates.     

A concise modular synthesis of 2,5-diethynyl-3,4-dibutyl-thiophene-bridged back-to-back terpyridine ligands

The efficient synthesis of soluble and rigid terpyridine-based ditopic ligands bearing an increasing number of 2,5-diethynyl-3,4-dibutylthiophene (DEDBT) modules has demonstrated the advantages of a single convergent strategy based on a double coupling in a final step of monoterpyridine building blocks carrying the adequate number of thiophene modules with a diiodo-substituted thiophene subunit. This protocol enjoys the advantages of both efficiency and versatility and requires pivotal intermediates, which were produced by a step-by-step implementation of monoterpyridine fragments with a key thiophene intermediate carrying an iodo function, a propargylic-protecting group, and two butyl-solubilizing fragments. One set of experimental conditions is required to produce all the intermediates and the final ligands. Oxidative dimerization of monosubstituted terpyridine skeletons bearing one or two thiophene substituents and a terminal alkyne function, in the presence of cupric salts and oxygen, afforded the homotopic ligands with a central dithienylbutadiyne spacer. Optical properties for the new oligomers have been investigated and are discussed in terms of effective conjugation length and pi-electron conjugation. Upon increasing the number of interspersed DEDBT units, a significant lowering in energy of absorption and fluorescence transitions as well as of the quantum yields is observed.     

Metallomacrocycles That Incorporate Cofacially Aligned Diimide Units

Pyromellitic diimide and naphthalene diimide moieties were incorporated into hemilabile phosphanylalkyl thioether ligands. These ligands reacted with [Cu(CH₃CN)₄]PF₆ and [Rh(NBD)Cl]₂ (NBD=norbornadiene) by the weak‐link approach to form condensed intermediates. Upon reaction of each diimide ligand with these transition‐metal precursors, the two diimide units became cofacially aligned within a supramolecular macrocyclic architecture. The introduction of ancillary ligands to each of these condensed intermediates caused the weak thioether–metal bonds to break, thus generating a large macrocycle in which the distance between diimide units is significantly larger than for the condensed intermediates. The two Rh^I cationic condensed intermediates were characterized by single‐crystal X‐ray diffraction studies, and the electrochemical activity of these macrocycles was demonstrated with the naphthalene diimide–Cu^I macrocycles.     

Recent advances in iridium-catalysed asymmetric hydrogenation of unfunctionalised olefins

In this account, the recent advances which have been made in asymmetric iridium-catalysed hydrogenation are reviewed. The first part focuses on our own studies of bicyclic pyridine–phosphinite ligands. These ligands have greatly enhanced the application range of asymmetric hydrogenation and, for the first time, have allowed highly enantioselective hydrogenation of simple, alkyl-substituted olefins and substituted furans. In the second part of this account, experimental and computational mechanistic studies are discussed. Whether the catalytic cycle proceeds via Ir(I)–Ir(III) intermediates or via Ir(III)–Ir(V) intermediates is still the subject of debate.     

Detection and mechanistic relevance of transient ligand radicals formed during [Ru(bpy)2(OH2)]2O4+-catalyzed water oxidation

Mechanistic proposals to account for the reactivity of water-oxidizing ruthenium diimine complexes have often invoked participation of covalently hydrated or pseudobase intermediates formed by reaction of solvent with the polypyridyl ligands. Probing for these intermediates has proven difficult because the concentrations of detectable reactive species are very low under commonly used experimental conditions. However, we have recently found that these transients accumulate in photocatalytic oxidation systems at neutral pH. In this work, we show that the reaction rates of these transient species correlate with catalytic activity and, therefore, that they meet minimal kinetic criteria to be true reaction intermediates.     

Copper Complexes with Diazoolefin Ligands and their Photochemical Conversion into Alkenylidene Complexes

Homometallic copper complexes with alkenylidene ligands are discussed as intermediates in catalysis but the isolation of such complexes has remained elusive. Herein, we report the structural characterization of copper complexes with bridging and terminal alkenylidene ligands. The compounds were obtained by irradiation of Cu^I complexes with N-heterocyclic diazoolefin ligands. The complex with a terminal alkenylidene ligand required isolation in a crystalline matrix, and its structural characterization was enabled by in crystallo photolysis at low temperature.     

On the origin of regio- and stereoselectivity in the rhodium-catalyzed vinylarenes hydroboration reaction

We studied the hydroboration of vinylarenes using rhodium complexes bearing atropoisomeric ligands. For the first time, an NMR spectroscopy study of the styrene and catecholborane addition to the precursor of catalyst [Rh(COD)(L-L)]BF(4), where L-L = (R)-BINAP and (R)-QUINAP, showed evidence of the structure of intermediates involved in the catalytic cycle. On the basis of this evidence, and using DFT calculations and QM/MM strategies, we investigated the origin of regio- and stereoselectivity. We determined the structure and stability of the key intermediates for several ligands and substrates and found excellent agreement between the relative stability of the intermediates and the experimentally observed trends. Using model systems, we analyzed the role of the steric and electronic features of the ligands and the substrates in detail.     

Acrylate formation via metal-assisted C-C coupling between CO2 and C2H4: reaction mechanism as revealed from density functional calculations

The reaction path for the formation of a binuclear hydrido-acrylate complex in a CO(2)-C(2)H(4) coupling process is explored in detail by locating the key intermediates and transition states on model potential energy surfaces derived from density functional calculations on realistic models. The formation of the new C-C bond is shown to take place via oxidative coupling of coordinated CO(2) and C(2)H(4) ligands resulting in a metalla-lactone intermediate, which can rearrange to an agostic species allowing for a beta-hydrogen shift process. The overall reaction is predicted to be clearly exothermic with all intermediates lying below the reactants in energy, and the highest barrier steps correspond to C-C coupling and beta-hydrogen transfer. The phosphine ligands are found to play an important role in various phases of the reaction as their dissociation controls the coordination of CO(2), the formation of the agostic intermediate, and the dimerization process; furthermore, their presence facilitates the oxidative coupling by supplying electrons to the metal center. Our results provide a theoretical support for the reaction mechanism proposed from experimental observations. The effect of the solvent medium on the relative energy of reaction intermediates and transition states is examined and found important in order to predict reliable energetics.     

Theoretical study of the biologically important dioxo diiron diamond core structures

Density functional theory calculations were used to investigate synthetic complexes with diiron dioxo diamond cores and models for intermediates in the catalytic cycle of methane monooxygenase (MMO). The synthetic complexes share an antiferromagnetically coupled diiron dioxo/hydroxo diamond core structure with the oxidized and reduced intermediates (H_ox and H_red, respectively) of MMO. The DFT (B3P86) calculations on model complexes of the synthetic models, with ferromagnetic coupling, reproduce the crystal structure data to within 0.05 Å and 5° for the diamond core parameters. The crystal structures of H_ox extracted from two different bacteria (Bath and OB3b) indicate that H_ox has either two bridging hydroxy ligands or one hydroxy and one water bridge. The B3P86 calculations strongly suggest that both bridging ligands in H_ox are hydroxy groups. The carboxylate shift established in the crystal structures of H_red was calculated to be a minimum at the BP86 level of theory.     

过渡金属/磷配体催化不对称氢化反应研究进展

正手性过渡金属配合物催化的不对称氢化是合成手性药物、农药和精细化工中间体的重要方法.到目前为止,已经有一些过渡金属/配体配合物催化的不对称氢化反应得到工业化应用,典型的实例如孟山都公司采用手性双齿膦配体DIPAMP生产L     

pi-Acidic alkene ligand effects in Pd-catalysed cross-coupling processes: exploiting the interaction of dibenzylidene acetone (dba) and related ligands with Pd(0) and Pd(II)

pi-Acidic alkene (olefin) ligands positively influence Pd-catalysed cross-coupling processes, interacting with both palladium(0) and palladium(ii) species, in some cases stabilising key catalytic intermediates. Rates of oxidative addition and reductive elimination are both affected. In certain cases, beta-hydrogen elimination can be slowed down by pi-acidic alkenes, which opens up new reaction pathways (e.g. interception of sigma-alkylpalladium(ii) species by appropriate nucleophiles). pi-Acidic alkene ligands can act independently or in a synergistic fashion with another two-electron donor ligand (e.g. amine, phosphine or N-heterocyclic carbene). The purpose of this perspective article is to highlight the impressive results that can be obtained using pi-acidic alkene ligands, with a particular focus on dibenzylidene acetone (dba) derivatives. Other types of alkene ligands, e.g. macrocyclic alkenes, are also discussed.     

Alkyl,Silyl, and Phosphane Ligands—Classical Ligands in Nonclassical Bonding Modes

Alkyl, silyl, and phosphane ligands are amongst the most familiar and ubiquitous ligands in organometallic and coordination chemistry. The C, Si, and P donor atoms of these ligands are sp³‐hybridized and the ligands are related to each other by the isolobal analogy: (CR₃)−(SiR₃)−PR₃. Herein, we demonstrate that although a number of unusual observations concerning the reactivity and bonding of these ligands appears unrelated at first sight, they in fact provide offer an exiting and consistent picture that may form the basis for new paradigms. The characterization of stable complexes in which alkyl, silyl, and phosphane ligands behave as symmetrical bridges confirms that there is no inherent thermodynamic instability associated with these bonding situations, and, in fact, reactivity studies suggest that these ligands should be able to bridge between metal centers in reaction intermediates or transition states.     

The synthesis of N,O-ferrocenyl pyrrolidine-containing ligands and their application in the diethyl- and diphenylzinc addition to aromatic aldehydes

A facile route to a series of planar chiral N,O-ferrocenyl pyrrolidine-containing ligands with varying substituents at the nitrogen and oxygen donor atoms is described. The oxygen donor atom was introduced via a diastereoselective ortho-metalation of N-methylpyrrolidinyl and N-allylpyrrolidinyl ferrocene intermediates and was quenched with various ketones. The nitrogen substituent was varied through deallylation and subsequent derivatization of a secondary pyrrolidine. The efficacy of these novel ligands was investigated in the enantioselective addition of diethylzinc and diphenylzinc to aromatic aldehydes. The ligands proved highly effective in the diethylzinc addition to benzaldehyde that resulted in high yields of up to 99% and enantioselectivities (ee's) of up to 95%. The role of planar chirality was explored and the results indicated that the planar chirality, and not the central chirality, of the ferrocenyl ligands was the dominant stereo-controlling element. Employment of a mixed ethyl-phenylzinc reagent in the phenylation of aromatic aldehydes led to a mixture of the two additional products, and the phenylated product was obtained in up to 37% ee.     

P-H activation using alkynylgold substrates: steric and electronic effects

The susceptibility of a prototypical hydrogen phosphonate to undergo P-H activation upon treatment with alkynylgold complexes has been studied. Dynamic solution behavior was observed during reactions involving triphenylphosphine ligated substrates and was attributed to rapid phosphine exchange between the alkynylgold starting material and the gold phosphonate product. The use of bulky biaryldialkylphosphine ligands eliminated the fluxional behavior, but did not significantly slow the rate of P-H activation. Similarly, changing the supporting ligand to an N-heterocyclic carbene did not significantly slow the rate of the reaction. Despite a number of reports outlining the functionalization of propargyl alcohols using gold catalysts, incorporating these groups into the architecture of the alkynylgold substrates did not alter the product distributions. Although the chemistry tolerated a range of supporting ligands, incorporating electron donating groups into the alkyne increased the rate of the reaction while electron-withdrawing groups slowed the reaction. A possible mechanism for the process includes a transition state containing significant pi-contribution from the alkyne. Due to the high yields of gold phosphonates obtained in this chemistry as well as the mild conditions of the reactions, the interception of intermediates/catalysts by substrates or ligands containing labile P-H donors is an issue that must be circumvented when designing or developing a gold catalyzed reaction that proceeds through alkynylgold intermediates.     

Halide-induced supramolecular ligand rearrangement

A novel reaction involving the halide-induced rearrangement of ligands within supramolecular Rh(I) complexes containing hemilabile ligands is presented. Three analogous bis- and trishemilabile ligands have been synthesized to construct bi- and trimetallic Rh(I) macrocyclic complexes. An intentionally added halide source results in the formal rotation of only one hemilabile ligand along the axis that is perpendicular to the plane defined by the aryl backbone of the hemilabile ligands. X-ray structures, as determined by X-ray crystallography, of key intermediates and products are presented.