Allyl Complexes Research Articles

Synthesis, Characterization, Solution Behavior and Theoretical Studies of Pd(II) Allyl Complexes with 2-Phenyl-3H-indoles as Ligands

The study of the reactivity of three 2-phenyl-3H-indole ligands of general formulae C8H3N-2-(C6H4-4-R1)-3-NOMe-5-R2 (1) [with R1 = H, R2 = OMe (a); R1 = R2 = H (b) or R1 = Cl, R2 = H (c)] with [Pd(η3-1-R3C3H4)(μ-Cl)]2 (R3 = H or Ph) has allowed us to isolate two sets of new Pd(II)-allyl complexes of general formulae [Pd(η3-1-R3C3H4)(1)Cl] {R3 = H (2) or Ph (3)}. Compounds 2a–2c and 3a–3c were characterized by elemental analyses, mass spectrometry and IR spectroscopy. The crystal structures of 2a, 3a and 3b were also determined by X-ray diffraction. 1H-NMR studies reveal the coexistence of two (for 2a–2c) or three (for 3a–3c) isomeric forms in CD2Cl2 solutions at 182 K. Additional studies on the catalytic activity of mixtures containing [Pd(η3-C3H5)(μ-Cl)]2 and the parent ligand (1a–1c) in the allylic alkylation of (E)-3-phenyl-2-propenyl (cinnamyl) acetate with sodium diethyl 2-methylmalonate as well as the stoichiometric reaction between compounds 3a and 3c with the nucleophile reveal that in both cases the formation of the linear trans- derivative is strongly preferred over the branched product. Computational studies at a DFT level on compound 3a allowed us to compare the relative stability of their isomeric forms present in solution and to explain the regioselectivity of the catalytic and stoichiometric processes.

Molybdenum carbonyl complexes as HER electrocatalysts

Four cyclopentadienyl and allylic molybdenum carbonyl complexes have been prepared and tested as proton reduction electrocatalysts using trifluoroacetic acid as the proton source in acetonitrile. Cyclic voltammetry studies showed that these complexes are able to function as moderate electrocatalysts with overpotentials of about 0.9 V and catalytic efficiency of up to 60%. The average turnover number and turnover frequency for each of the molybdenum catalyst measured over a five-hour period have been calcualted to be in the range of 16–24 and 3.2 h−1 to 5.4 h−1 respectively for 1 mM catalyst and 20 acid equivalent. A mechanism to account for the proton reduction process is also proposed based on the electrochemical and spectroscopic measurements.

Synthesis of new triazolyl-oxazoline chiral ligands and study of their coordination to Pd(II) metal centers

We report an improved protocol for the synthesis of TryOx, a family of N,N chiral ligands in which a 1,2,3-triazol-4-yl moiety bears a chiral 2-oxazoline as the substituent in 4 position. TryOxs were successfully employed for the preparation of cationic Pd(II)-allyl complexes of formulation [Pd(allyl)(TryOx)]+ in which they behave as bidentate N,N chelating ligands. On the other hand, reaction of TryOxs with an equimolecular amount of [PdCl2(CH3CN)2] leads to the formation of neutral complexes which according to NMR, ESI-MS and molar mass determination (VPO) appear to be dimeric species of formulation [PdCl(µCl)(TryOx)]2 in which monodentate TryOxs link to palladium with the N atom of the oxazoline ring. X-rays diffraction studies show that upon crystallization these dimers transform into monomeric compounds of formulation [PdCl2(TryOx)], where the ligands behave as N,N chelating.Treatment of the [PdCl(µCl)(TryOx)]2 dimers with an equivalent of TryOx affords mononuclear neutral dichloro species of formulation [PdCl2(TryOx)2]. X-ray diffraction studies show that the two TryOx units are equivalent behaving as monodentate ligands coordinated to the metal through the oxazoline N atom.

A Bridging bis-Allyl Titanium Complex: Mechanistic Insights into the Electronic Structure and Reactivity.

Treatment of the dinuclear compound [{Ti(η5-C5Me5)Cl2}2(μ-O)] with allylmagnesium chloride provides the formation of the allyltitanium(III) derivative [{Ti(η5-C5Me5)(μ-C3H5)}2(μ-O)] (1), structurally identified by single-crystal X-ray analysis. Density functional theory (DFT) calculations confirm that the electronic structure of 1 is a singlet state, and the molecular orbital analysis, along with the short Ti-Ti distance, reveal the presence of a metal-metal single bond between the two Ti(III) centers. Complex 1 reacts rapidly with organic azides, RN3 (R = Ph, SiMe3), to yield the allyl μ-imido derivatives [{Ti(η5-C5Me5)(CH2CH═CH2)2}2(μ-NR)(μ-O)] [R = Ph(2), SiMe3(3)] along with molecular nitrogen release. Reaction of 2 and 3 with H2 leads to the μ-imido propyl species [{Ti(η5-C5Me5)(CH2CH2CH3)2}2(μ-NR)(μ-O)] [R = Ph(4), SiMe3(5)]. Theoretical calculations were used to gain insight into the hydrogenation mechanism of complex 3 and rationalize the lower reactivity of 2. Initially, the μ-imido bridging group in these complexes activates the H2 molecule via addition to the Ti-N bonds. Subsequently, the titanium hydride intermediates induce a change in hapticity of the allyl ligands, and the nucleophilic attack of the hydride to the allyl groups leads to metallacyclopropane intermediates. Finally, the proton transfer from the amido group to the metallacyclopropane moieties affords the propyl complexes 4 and 5.

Design of a Highly Active Pd Catalyst with P,N Hemilabile Ligands for Alkoxycarbonylation of Alkynes and Allenes: A Density Functional Theory Study.

In the palladium-catalysed methoxycarbonylation of technical propyne, the presence of propadiene poisons the hemilabile Pd(P,N) catalyst. According to density functional theory calculations (B3PW91-D3/PCM level), a highly stable π-allyl intermediate is the reason for this catalyst poisoning. Predicted regioselectivities suggest that at least 11 % of propadiene should yield this allyl intermediate, in which the reaction gets stalled under the turnover conditions due to an insurmountable methanolysis barrier of 25.8 kcal mol-1 . The results obtained for different ligands and substrates are consistent with the available experimental data. A new ligand, (6-Cl-3-Me-Py)PPh2 , is proposed, which is predicted to efficiently control the branched/linear selectivity, avoiding rapid poisoning (with only 0.2 % of propadiene being trapped as the Pd allyl complex), and to tremendously increase the catalytic activity by decreasing the overall barrier to 9.1 kcal mol-1 .

Halide metathesis in overdrive: mechanochemical synthesis of a heterometallic group 1 allyl complex.

As a synthesis technique, halide metathesis (n RM + M'Xn → RnM' + n MX) normally relies for its effectiveness on the favorable formation of a metal halide byproduct (MX), often aided by solubility equilibria in solution. Owing to the lack of significant thermodynamic driving forces, intra-alkali metal exchange is one of the most challenging metathetical exchanges to attempt, especially when conducted without solvent. Nevertheless, grinding together the bulky potassium allyl [KA']∞ (A' = [1,3-(SiMe3)2C3H3]–) and CsI produces the heterometallic complex [CsKA'2]∞ in low yield, which was crystallographically characterized as a coordination polymer that displays site disorder of the K+ and Cs+ ions. The entropic benefits of mixed Cs/K metal centers, but more importantly, the generation of multiple intermolecular K…CH3 and Cs…CH3 interactions in [CsKA'2]∞, enable an otherwise unfavorable halide metathesis to proceed with mechanochemical assistance. From this result, we demonstrate that ball milling and unexpected solid-state effects can permit seemingly unfavored reactions to occur.

Synthesis, Characterization, and Spectroscopic Properties of Allylic Ruthenium(II) Complexes of a Highly Conjugated Perinone

Highly conjugated perinones were evaluated as proligands with [Ru(η6‐arene)Cl2]2 precursors (arene = p‐cymene or benzene). None of them, except itaco‐perinone (IP) bearing one exocyclic methylene group, were able to form coordination compounds. Expected η1‐coordination through the lone pair of the nitrogen or oxygen atoms of the perinone did not occur. Instead a deprotonation reaction involving the exocyclic methylene was observed and the corresponding [Ru(η6‐arene)(η3‐IP)Cl] complex was isolated in moderate yields. Mechanistic studies revealed that the base‐promoted isomerization of itaco‐perinone to citraco‐perinone prevented higher yields in the synthesis of the allylic complexes. Additionally, it was observed that IP can dimerize through the exocyclic methylene group, indicating high reactivity of this carbon–carbon double bond. Electronic absorption and emission properties of the perinones and organometallic compounds were studied.

Gold Sulfonium Benzylide Complexes Undergo Efficient Benzylidene Transfer to Alkenes.

The gold sulfonium benzylide complexes [(P1)AuCHPh(SR1 R2 )]+ {B[3,5-CF3 C6 H3 ]4 }- [P1=P(tBu)2 o-biphenyl; R1 , R2 =-(CH2 )4 - (1 a); R1 =Et, R2 =Ph (1 b); R1 =R2 =Ph (1 c)] were synthesized by reaction of the gold α-chloro benzyl complex (P1)AuCHClPh with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and excess sulfide. Complexes 1 undergo efficient benzylidene transfer to alkenes and DMSO under mild conditions without external activation. Kinetic analysis of the reaction of 1 c with styrene was consistent with the intermediacy of the cationic gold benzylidene complex [(P1)AuCHPh]+ (I).

Key Factors for High Diastereo- and Enantioselectivity of Umpolung Cyclizations of Aldehyde-Containing Allylpalladium Intermediates

Abstract Two palladium/chiral diphosphine-catalyzed umpolung cyclizations of aldehyde-containing allylic acetates and allenes with arylboronic acid are fully investigated to establish key factors in their high stereoselectivities. Both cyclization reactions afford cis-disubstituted pyrrolidine and tetrahydrofuran. These occur in high diastereo- and enantioselectivities through a common cationic (Z)-η1-allylpalladium, toward which a ring strain generated in the cyclization step leading to trans-isomers biases the equilibrium through η3-η1-η3-complex in the former cyclization. Varied diastereoselectivities were observed in the formation of five-membered carbocycles and six-membered heterocycles. These reflect release of a ring strain generated in the cyclization step leading to trans-isomers and a different distribution of the (Z)- and the (E)-η1-allylpalladium intermediates generated by the oxidative addition of allylic acetates to Pd(0) or carbopalladation of allenes, respectively. A sterically demanding substituent at the center of the allyl moiety is necessary for high diastereo- and enantioselectivity. The enantioselectivity of the former cyclization was lowered by the presence of organometallic reductants or reagents, possibly causing the formation of neutral η1-allylpalladium species. We used a chiral allylic acetate containing (E)-deuterium-labeled alkene to demonstrate that the electrophilic attack of the aldehyde to the allyl ligand occurred on the side where the palladium existed, consistent with the Zimmerman-Traxler transition state.

Focused Electron Beam-Induced Deposition and Post-Growth Purification Using the Heteroleptic Ru Complex (η3-C3H5)Ru(CO)3Br.

Focused electron beam-induced deposition using the heteroleptic complex (η3-C3H5)Ru(CO)3Br as a precursor resulted in deposition of material with Ru content of 23 at. %. Transmission electron microscopy images indicated a nanogranular structure of pure Ru nanocrystals, embedded into a matrix containing carbon, oxygen, and bromine. The deposits were purified by annealing in a reactive 98% N2/2% H2 atmosphere at 300 °C, resulting in a reduction of contaminants and an increase of the Ru content to 83 at. %. Although a significant volume loss of 79% was found, the shrinkage was observed mostly for vertical thickness (around 75%). The lateral dimensions decreased much less significantly (around 9%). Deposition results, in conjunction with previous gas-phase and condensed-phase surface studies on the electron-induced reactions of (η3-C3H5)Ru(CO)3Br, provide insights into the behavior of allyl, carbonyl, and bromide ligands under identical electron beam irradiation.

Carboxylation Reactions Using Carbon Dioxide as the C1 Source via Catalytically Generated Allyl Metal Intermediates.

The use of carbon dioxide (CO2) is an important issue with regard to current climate research and the Earth's environment. Transition metal-catalyzed carboxylation reactions using CO2 are highly attractive. This review summarizes the transition metal-catalyzed carboxylation reactions of organic substrates with CO2 via allyl metal intermediates. First, carboxylation reactions via transmetalation are reviewed. Second, catalytic carboxylation reactions using allyl electrophiles and suitable reducing agents are summarized. The last section discusses the catalytic carboxylation reactions via addition reactions, affording allyl metal intermediates.

(CDC)–Rhodium-Catalyzed Hydroallylation of Vinylarenes and 1,3-Dienes with AllylTrifluoroborates

Catalytic site-selective hydroallylation of vinyl arenes and 1,3-dienes is reported. Transformations are promoted by a readily accessible bidentate carbodicarbene–rhodium complex and involve commercially available allyltrifluoroborates and an alcohol. The reaction is applicable to vinyl arenes and aryl- or alkyl-substituted 1,3-dienes (30 examples). Allyl addition products are generated in 40–78% yield and in up to >98:2 site selectivity. Reaction outcomes are consistent with the intermediacy of a Rh(III)–hydride generated by protonation of Rh(I) by an acid. A number of key mechanistic details of the reaction are presented: (1) Deuterium scrambling into the product and starting alkene indicates reversible Rh(III)–H migratory insertion. (2) A large primary kinetic isotope effect is observed. (3) With substituted allyltrifluoroborates (e.g., crotyl-BF3K) mixtures of site isomers are generated as a result of transmetalation followed by Rh–(allyl) complex equilibration, consequently disproving outer-sphere addition of the allyl nucleophile to Rh(III)–(η3-allyl). (4) Stereochemical analysis of a cyclohexadiene allyl addition product supports a syn Rh(III)–hydride addition. (5) A Hammett plot shows a negative slope. Finally, utility is highlighted by a iodocyclization and cross metathesis.

Ruthenium(II)-Catalyzed Migratory C–H Allylation/Hydroamination Cascade for the Synthesis of Rutaecarpine Analogues

An unprecedented allyl migration from a remote position of a quinazoline moiety through a ruthenium(II) shuttle is reported. This present cascade reaction is initiated through the formation of an η3-ruthenium–allyl complex followed by C–H allylation at the ortho position of the 2-aryl moiety. Finally, hydroamination with the quinazolinone -NH group, which is formed through tautomerization of the quinazoline, furnishes the annulation product. This exceedingly fast cascade reaction is complete within 10 minutes to provide rutaecarpine analogues in a single operation.

Kinetics and Mechanism of Thermal Decomposition of Bis(η3-Allyl)Nickel Complexes

The kinetics of thermal decomposition of bis(η3-allyl) nickel complexes in various media is studied. The specific features of the mechanism are determined, including the combination of the stages of trans-cis isomerization of Niall2 and the bimolecular decomposition of the cis-isomer with diallyl formation. The effect of autocatalytic decomposition of complexes with metallic nickel is been detected. The qualitative dependences of the process rate on the nature of the solvent and the structure of the allyl ligand are determined. The activation parameters of individual steps, consistent with quantum chemical calculations, are found.

CuH-Catalyzed Enantioselective Ketone Allylation with 1,3-Dienes: Scope, Mechanism, and Applications.

Chiral tertiary alcohols are important building blocks for the synthesis of pharmaceutical agents and biologically active natural products. The addition of carbon nucleophiles to ketones is the most common approach to tertiary alcohol synthesis but traditionally relies on stoichiometric organometallic reagents that are difficult to prepare, sensitive, and uneconomical. We describe a mild and efficient method for the copper-catalyzed allylation of ketones using widely available 1,3-dienes as allylmetal surrogates. Homoallylic alcohols bearing a wide range of functional groups are obtained in high yield and with good regio-, diastereo-, and enantioselectivity. Mechanistic investigations using density functional theory (DFT) implicate the in situ formation of a rapidly equilibrating mixture of isomeric copper(I) allyl complexes, from which Curtin-Hammett kinetics determine the major isomer of the product. A stereochemical model is provided to explain the high diastereo- and enantioselectivity of this process. Finally, this method was applied to the preparation of an important drug, ( R)-procyclidine, and a key intermediate in the synthesis of several pharmaceuticals.

Mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl complexes as precatalysts for direct arylation of azoles with aryl bromides

A series of mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl complexes with general formula [(NHC)Pd(η3-allyl)]2(μ2-N-heterocycles)(BF4)2 were prepared in one pot based on anion metathesis of (NHC)Pd(η3-allyl)Cl complexes and then ligand replacement with N-heterocycles [N-heterocycles = pyrazine (pyz), 4,4′-bipyridine (bpy) and trans-4,4′-bipyridylethylene (bpe)]. The solid-state structures shown dinuclear structures with two palladium(II) centers holding together by bridged N-heterocycles. Initially investigation of the obtained complexes as precatalysts for direct CH bond arylation of azoles with aryl bromides was carried out.

Preparation and catalytic properties of the triphenylarsine and triphenylstibine-stabilized tri-heteroleptic NHC[sbnd]Pd‒allyl complexes

A series of triphenylarsine and triphenylstibine-stabilized tri-heteroleptic NHCPd‒allyl complexes were synthesized and characterized. The solid-state structures of the complexes shown mononuclear carbene palladium complexes, in which, each palladium centre was coordinated by an N-heterocyclic carbene, the η3-coordinated allyl moiety and an As or Sb donor. Further investigation of the Pd complexes as catalysts in Sonogashira coupling reaction was carried out and the obtained complexes exhibited good reactivities for aryl bromides.

From Chelating to Bridging Ligands: N-Sulfonyliminoimidazolium Ylides as Precursors to Anionic N-Heterocyclic Carbene Ligands

The synthesis and investigation of a new family of anionic N-heterocyclic carbene (NHC) ligands, based on N-sulfonyliminoimidazolium ylides, is reported. Their corresponding silver complexes have been synthesized and present distinctive geometries compared to the previously reported ligands derived from the N-acyliminoimidazolium family. A comparison of the two ligand families was performed with palladium(II) allyl complexes to investigate their binding mode preference. The N-sulfonyl derived ligands were found to strongly favor bridging binding instead of chelation. This strong preference was exploited to synthesize a novel copper(I)–palladium(II) heterotrimetallic complex.

η3-Allyl Coordination at Pb(II)

Allylmagnesium chloride and methyl-propenyl-magnesium bromide were reacted with bulky substituted organolead and organotin halides (Ar*PbBr)2, (Ar′PbBr)2, (Ar*SnCl)2 (Ar* = 2,6-trip2C6H3-, trip = 2,4,6-triisopropylphenyl, Ar′ = 2,6-mes2C6H3-, mes = 2,4,6-trimethylphenyl). The allyl ligand coordinates in an η3-coordination mode at organoplumbylene fragments. In the solid state as well as in solution, η3-coordination was characterized by crystal structure analysis and Saunders’ isotopic perturbation technique. For the plumbylene Ar*Pb(C3H5), a solid state 207Pb magic angle spinning (MAS) NMR spectrum could be obtained. The isotropic chemical shift is −435 ppm, and the magnitude of the 207Pb chemical shift tensor of 7000(500) ppm is among the greatest observed experimentally. The methylallyl ligand coordinated at a plumbylene fragment exhibits two short and one long Pb–C interaction. In reaction with aniline, the allyl ligand reacts as a leaving group to give amidoplumbylenes.

Sterics versus electronics: Imine/phosphine-oxide-based nickel catalysts for ethylene polymerization and copolymerization

A series of imine/phosphine-oxide ligands was synthesized and characterized. These ligands are easily prepared in a modular fashion, with positions that are independently tunable. The corresponding nickel allyl complexes were synthesized and serve as single-component catalysts for the homopolymerization of ethylene. It is generally believed that sterically bulky substituents lead to high polymer molecular weights during olefin polymerization. In this system, however, catalysts bearing the sterically bulky dibenzhydryl substituent afforded polyethylenes with much lower molecular weights than those bearing the sterically unhindered isopropyl substituent. Based on control experiments, theoretical calculations, and literature evidence, we propose that electronic effects override steric effects and play dominant roles in this nickel system. Efficient copolymerizations of ethylene with methyl 10-undecenoate and 6-chloro-1-hexene were also facilitated by this system without the need for any cocatalyst or protecting agent.