Allyl Complexes Research Articles

Substrate-dependent stereospecificity in samarium-mediated allylic benzoate reductions

Abstract A series of experiments were performed to determine if SmI2(H2O)n reductions of allylic benzoates was stereospecific to alkene geometry. The results indicate that for alkyl-substituted alkenes, the reaction is stereospecific (i.e. the cis- and trans-isomers select for the opposite major diastereomer). However, the introduction of a phenyl substituent causes the reaction to become non-stereospecific, both cis- and trans-isomers converging to the same major diastereomer. A potential rationale is presented based on stabilization of a proposed organosamarium intermediate by the phenyl group that allows for isomerization of alkene geometry to a sterically preferred trans-configuration at the transition state. The data provide further insights into the mechanism of this reaction, the nature of allylic organosamarium complexes, and the unique reactivity of benzene rings in samarium-mediated transformations.

Synthesis, Structure, Dynamics, and Enantioface-Selective η3-Benzyl Coordination in the Chiral Rhodium Complexes Rh(diphos*)(η3-CH2Ph)

The rhodium benzyl complexes Rh(diphos*)(η3-CH2Ph) (1–14, diphos* = chiral bis(phosphine)), potential precursors for asymmetric catalysis, were prepared either by treatment of Rh(COD)(η3-CH2Ph) (15...

Rhodium(III)-Catalyzed Asymmetric [4+1] and [5+1] Annulation of Arenes and 1,3-Enynes: A Distinct Mechanism of Allyl Formation and Allyl Functionalization.

We report chiral RhIII cyclopentadienyl-catalyzed enantioselective synthesis of lactams and isochromenes through oxidative [4+1] and [5+1] annulation, respectively, between arenes and 1,3-enynes. The reaction proceeds through a C-H activation, alkenyl-to-allyl rearrangement, and a nucleophilic cyclization cascade. The mechanisms of the [4+1] annulations were elucidated by a combination of experimental and computational methods. DFT studies indicated that, following the C-H activation and alkyne insertion, a RhIII alkenyl intermediate undergoes δ-hydrogen elimination of the allylic C-H via a six-membered ring transition state to produce a RhIII enallene hydride intermediate. Subsequent hydride insertion and allyl rearrangement affords several rhodium(III) allyl intermediates, and a rare RhIII η4 ene-allyl species with π-agostic interaction undergoes SN2 '-type external attack by the nitrogen nucleophile, instead of C-N reductive elimination, as the stereodetermining step.

Tris(dicyclohexylamido) Group 4 Metal Allyl and Phenylacetylide Complexes – Synthesis and Characterization

The synthesis and characterization of organyl complexes derived from group 4 metal dicyclohexylamido complexes (Cy2N)3MCl (Cy: Cyclohexyl, M = Ti, Zr, Hf 1‐M) is presented. The allyl Grignard reagent is used to obtain the corresponding group 4 allyl complexes 2‐M, and lithium phenylacetylide is used to obtain the phenylacetylide complexes 3‐M. The thermal stability of complexes 2‐M and 3‐M is investigated. All complexes are characterized by NMR spectroscopy and single‐crystal X‐ray diffraction. Their molecular structures in solution and in the solid state are discussed.

Palladium(II)-η3 -Allyl Complexes Bearing N-Trifluoromethyl N-Heterocyclic Carbenes: A New Generation of Anticancer Agents that Restrain the Growth of High-Grade Serous Ovarian Cancer Tumoroids.

The first palladium organometallic compounds bearing N-trifluoromethyl N-heterocyclic carbenes have been synthesized. These η3 -allyl complexes are potent antiproliferative agents against different cancer lines (for the most part, IC50 values fall in the range 0.02-0.5 μm). By choosing 1,3,5-triaza-7-phosphaadamantane (PTA) as co-ligand, we can improve the selectivity toward tumor cells, whereas the introduction of 2-methyl substituents generally reduces the antitumor activity slightly. A series of biochemical assays, aimed at defining the cellular targets of these palladium complexes, has shown that mitochondria are damaged before DNA, thus revealing a behavior substantially different from that of cisplatin and its derivatives. We assume that the specific mechanism of action of these organometallic compounds involves nucleophilic attack on the η3 -allyl fragment. The effectiveness of a representative complex, 4 c, was verified on ovarian cancer tumoroids derived from patients. The results are promising: unlike carboplatin, our compound turned out to be very active and showed a low toxicity toward normal liver organoids.

Efficient NH3-based process to remove chlorine from electron beam deposited ruthenium produced from (\u03b73-C3H5)Ru(CO)3Cl

The fabrication of Ru nanostructures by focused electron beam induced deposition (FEBID) requires suitable precursor molecules and processes to obtain the pure metal. So far this is problematic because established organometallic Ru precursors contain large organic ligands, such as cyclopentadienyl anions, that tend to become embedded in the deposit during the FEBID process. Recently, (η3-C3H5)Ru(CO)3X (X = Cl, Br) has been proposed as an alternative precursor because CO can easily desorb under electron exposure. However, allyl and Cl ligands remain behind after electron irradiation and the removal of the halide requires extensive electron exposures. Auger electron spectroscopy is applied to demonstrate a postdeposition purification process in which NH3 is used as a reactant that enhances the removal of Cl from deposits formed by electron irradiation of thin condensed layers of (η3-C3H5)Ru(CO)3Cl. The loss of CO from the precursor during electron-induced decomposition enables a reaction between NH3 and the Cl ligands that produces HCl. The combined use of electron-stimulated desorption experiments and thermal desorption spectrometry further reveals that thermal reactions contribute to the loss of CO in the FEBID process but remove only minor amounts of the allyl and Cl ligands.

Allyl palladium complexes bearing carbohydrate‐based N‐heterocyclic carbenes: Anticancer agents for selective and potent in vitro cytotoxicity

Novel allyl palladium compounds stabilized by carbohydrate‐based N‐heterocyclic carbenes (NHCs) were prepared and characterized by nuclear magnetic resonance, high‐resolution mass spectrometry and elemental analysis. The antiproliferative activity of the compounds was tested on a panel of different tumor lines, especially ovarian cancer and MRC‐5 human lung fibroblasts (nontumor cells). These experiments showed that both mixed NHC/PPh3 and NHC/PTA (PTA = 1,3,5‐triaza‐7‐phosphaadamantane) allyl complexes have IC50 (half maximal inhibitory concentration) values comparable and sometimes even significantly lower than cisplatin. Moreover, the mixed NHC/PTA allyl complexes exhibit good activity toward the seven tumor lines tested with a substantial inactivity against normal cells, a necessary condition to avoid the general cytotoxicity of a metallo‐drug. Furthermore, this subclass of compounds proved particularly active on the A549 lung cancer tumor line (up to 100‐fold more cytotoxic than cisplatin) and exhibited satisfactory cytotoxicity against KURAMOCHI and OVCAR3 cell lines, which are currently considered the best in vitro models for serous ovarian cancer, the most lethal tumor for women worldwide.

Synthesis and comparative study of the anticancer activity of η3-allyl palladium(II) complexes bearing N-heterocyclic carbenes as ancillary ligands

In this paper we report the synthesis and characterization (NMR, IR, elemental analysis and XRD) of fifteen new Pd(II) allyl complexes containing at least one N-heterocyclic carbene ligand.The synthesized compounds and others previously published by our research group, have been tested against the six tumor lines OVCAR5, A2780, A2780cis (ovarian cancer), DLD1 (colon cancer), A549 (lung cancer) and A375 (malignant melanoma).This comparative study, involving also complexes coordinating NS and NP heteroditopic ligands, shows that most of the tested compounds exhibit a much powerful antitumor activity than cisplatin on all tumor cell lines examined.In many cases, the antiproliferative activity on the A2780 (cisplatin-sensitive) and A2780cis (cisplatin-resistant) lines is comparable, suggesting a different mechanism of action of this class of molecules as compared to cisplatin and its derivatives.Moreover, a selected number of compounds among those reported are particularly promising since exhibit a markedly lower activity toward normal cells than to cancer cells.

Carbon-Carbon Bond Cleavage at Allylic Positions: Retro-allylation and Deallylation.

The development of C-C bond-cleaving transformations is an issue in modern organic chemistry that is as challenging as it is important. Among these transformations, the retro-allylation and deallylation of allylic compounds are uniquely intriguing methods for the cleavage of C-C σ bonds at the allylic position. Retro-allylation is regarded as a prospective method for the generation of highly valuable regio- and stereodefined allylic metal compounds. Because the C-C cleavage proceeds via a favorable six-membered chairlike transition state, the regio- and stereochemical information on the starting homoallylic alcohols can be transferred onto the products. Moreover, retro-allylation can also be achieved using enantioselective C-C cleavage powered by chiral catalysts for the synthesis of enantiomerically enriched compounds. As a result of these attractive features, retro-allylation has wide utility in regio-, stereo-, and enantioselective synthesis. Deallylation is C-C σ-bond cleavage involving the departure of an allylic fragment and the formation of a relatively stable carbanion or radical, and it proceeds via either oxidative addition to a low-valent metal or an addition/β-elimination cascade. The removal of the versatile allylic group might seem to be unproductive; however, this unique transformation offers the opportunity of using the allylic group as a protective group for acidic C-H bonds. This Review aims to exhibit the synthetic utility as well as the uniqueness of these two C-C σ-bond cleavage methods by presenting a wide range of transformations of allylic compounds with the aid of main group metals, transition-metal catalysts, and radical species.

Sesquicarbene Complexes: Bonding at the Interface Between M–C Single Bonds and M═C Double Bonds

Allylic dimetalated complexes [M2C(vinyl)]+ (M = Au(IPr) and Cr(CO)5–) incorporate a new coordination mode of carbon. Digold complexes of this type have recently been detected experimentally. The intrinsic bond orbitals, partial charges, and structural parameters of the gold complexes and of chromium analogs were studied computationally and compared to those of the respective monometalated species and hydrocarbons. This showed that such digold complexes have a carbene character at both Au–C bonds comparable to typical carbene complexes of gold. Dichromium complexes with their stronger π-backdonation compete for interaction with carbon’s π-orbital; each of the chromium atoms partakes in double bonding that is significant but weaker than that in the carbene analogs. Containing two M–C bonds on the interface between single and double bonds, these bridged complexes can be conceived as “sesquicarbene complexes”. The π-system acted in a very adaptive manner and employed additional stabilization of the vinyl system only where needed. Significant carbene character is found simultaneously in both M–C bonds at the same carbon center. The discovery of these complexes with relatively strong double bond character between one carbon and two metal atoms could bring unusual single-carbon-centered organometallic cascade reactions to the horizon.

Nickel(II) allyl complexes with anionic N-heterocyclic carbene-borate ligands

Deprotonation of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IDipp) with n-butyllithium at the 4-position and addition of B(C6F5)3 afforded the lithium-carbene borate complex [(WCA-NHC)Li(toluene)] with a weakly coordinating anionic (WCA) moiety. The reaction of the lithiocarbene with the dimeric allyl complex [(η3-C3H5)Ni(μ-Cl)]2 in tetrahydrofuran (THF) afforded the chloronickelate complex [Li(THF)n][(WCA-IDipp)NiCl(η3-C3H5)] (1 ≤ n ≤ 3), which was used to generate the complex [(WCA-IDipp)Ni(η3-C3H5)], which is expected to achieve stabilization through an intramolecular arene-metal interaction (“π-face donation”) involving the N-aryl groups of the carbene ligand, by elimination of LiCl in toluene solution. The labile complex [(WCA-IDipp)Ni(η3-C3H5)], which undergoes decomposition by C–H activation and/or reductive elimination, can be stabilized by addition of carbon monoxide or phosphines to form novel neutral complexes of the type [(WCA-IDipp)NiL(η3-C3H5)] (L = CO, PMe3, PMe2Ph, PMePh2).

An η3‐Bound Allyl Ligand on Magnesium in a Mechanochemically Generated Mg/K Allyl Complex

AbstractMilling two equivalents of K[1,3‐(SiMe3)2C3H3] (=K[A′]) with MgX2 (X=Cl, Br) produces the allyl complex [K2MgA′4] (1). Crystals grown from toluene are of the solvated species [((η6‐tol)K)2MgA′4] ([1⋅2(tol)]), a trimetallic monomer with both bridging and terminal (η1) allyl ligands. When recrystallized from hexanes, the unsolvated 1 forms a 2D coordination polymer, in which the Mg is surrounded by three allyl ligands. The C−C bond lengths differ by only 0.028 Å, indicating virtually complete electron delocalization. This is an unprecedented coordination mode for an allyl ligand bound to Mg. DFT calculations indicate that in isolation, an η3‐allyl configuration on Mg is energetically preferred over the η1‐ (σ‐bonded) arrangement, but the Mg must be in a low coordination environment for it to be experimentally realized. Methyl methacrylate is effectively polymerized by 1, with activities that are comparable to K[A′] and greater than the homometallic magnesium complex [{MgA′2}2].

An η3 -Bound Allyl Ligand on Magnesium in a Mechanochemically Generated Mg/K Allyl Complex.

Milling two equivalents of K[1,3-(SiMe3 )2 C3 H3 ] (=K[A']) with MgX2 (X=Cl, Br) produces the allyl complex [K2 MgA'4 ] (1). Crystals grown from toluene are of the solvated species [((η6 -tol)K)2 MgA'4 ] ([1⋅2(tol)]), a trimetallic monomer with both bridging and terminal (η1 ) allyl ligands. When recrystallized from hexanes, the unsolvated 1 forms a 2D coordination polymer, in which the Mg is surrounded by three allyl ligands. The C-C bond lengths differ by only 0.028 Å, indicating virtually complete electron delocalization. This is an unprecedented coordination mode for an allyl ligand bound to Mg. DFT calculations indicate that in isolation, an η3 -allyl configuration on Mg is energetically preferred over the η1 - (σ-bonded) arrangement, but the Mg must be in a low coordination environment for it to be experimentally realized. Methyl methacrylate is effectively polymerized by 1, with activities that are comparable to K[A'] and greater than the homometallic magnesium complex [{MgA'2 }2 ].

Element-Element Bond Formation upon Oxidation and Reduction.

The redox chemistry of [(Cp′′′Co)2(μ,η2:η2‐E2)2] (E=P (1), As (2); Cp′′′=1,2,4‐tri(tert‐butyl)cyclopentadienyl) was investigated. Both compounds can be oxidized and reduced twice. That way, the monocations [(Cp′′′Co)2(μ,η4:η4‐E4)][X] (E=P, X=BF4 (3 a), [FAl] (3 b); E=As, X=BF4 (4 a), [FAl] (4 b)), the dications [(Cp′′′Co)2(μ,η4:η4‐E4)][TEF]2 (E=P (5), As (6)), and the monoanions [K(18‐c‐6)(dme)2][(Cp′′′Co)2(μ,η4:η4‐E4)] (E=P (7), As (8)) were isolated. Further reduction of 7 leads to the dianionic complex [K(18‐c‐6)(dme)2][K(18‐c‐6)][(Cp′′′Co)2(μ,η3:η3‐P4)] (9), in which the cyclo‐P4 ligand has rearranged to a chain‐like P4 ligand. Further reduction of 8 can be achieved with an excess of potassium under the formation of [K(dme)4][(Cp′′′Co)2(μ,η3:η3‐As3)] (10) and the elimination of an As1 unit. Compound 10 represents the first example of an allylic As3 ligand incorporated into a triple‐decker complex.

Nickel Tetracarbonyl as Starting Material for the Synthesis of NHC‐stabilized Nickel(II) Allyl Complexes

A novel, useful in situ synthesis for NHC nickel allyl halide complexes [Ni(NHC)(η3‐allyl)(X)] starting from [Ni(CO)4], NHC and allyl halides is presented. The reaction of [Ni(CO)4] with (i) one equivalent of the corresponding NHC and (ii) with an excess of the corresponding allyl chloride at room temperature leads with elimination of carbon monoxide to complexes of the type [Ni(NHC)(η3‐allyl)(X)]. This approach was used to synthesize the complexes [Ni(tBu2Im)(η3‐H2C‐C(Me)‐CH2)(Cl)] (2), [Ni(iPr2ImMe)(η3‐H2C‐C(Me)‐CH2)(Cl)] (3), [Ni(iPr2Im)(η3‐H2C‐C(Me)‐CH2)(Cl)] (4), [Ni(iPr2Im)(η3‐H2C‐C(H)‐C(Me)2)(Br)] (5), [Ni(Me2ImMe)(η3‐H2C‐C(Me)‐CH2)(Cl)] (6), and [Ni(EtiPrImMe)(η3‐H2C‐C(Me)‐CH2)(Cl)] (7). The complexes 1 to 7 were characterized using NMR and IR spectroscopy and elemental analysis, and the molecular structures are provided for 2 and 7. The allyl nickel complexes 1–7 are stereochemically non‐rigid in solution due to (i) NHC rotation about the nickel‐carbon bond, (ii) allyl rotation about the Ni–η3‐allyl axis and (iii) π–σ–π allyl isomerization processes. The allyl halide complexes can be methylated as was demonstrated by the methylation of a number of the complexes [Ni(NHC)(η3‐allyl)(X)] with methylmagnesium chloride or methyllithium, which led to isolation of the complexes [Ni(Me2Im)(η3‐H2C‐C(Me)‐CH2)(Me)] (8), [Ni(tBu2Im)(η3‐H2C‐C(Me)‐CH2)(Me)] (9), [Ni(iPr2ImMe)(η3‐H2C‐C(Me)‐CH2)(Me)] (10), [Ni(iPr2Im)(η3‐H2C‐C(Me)‐CH2)(Me)] (11), [Ni(iPr2Im)(η3‐H2C‐C(H)‐C(Me)2)(Me)] (12), and [Ni(EtiPrImMe)(η3‐H2C‐C(Me)‐CH2)(Me)] (13). These complexes were fully characterized including X‐ray molecular structures for 10 and 11.

Palladium Complexes Based on Ylide-Functionalized Phosphines (YPhos): Broadly Applicable High-Performance Precatalysts for the Amination of Aryl Halides at Room Temperature.

Palladium allyl, cinnamyl, and indenyl complexes with the ylide‐substituted phosphines Cy3P+−C−(R)PCy2 (with R=Me (L1) or Ph (L2)) and Cy3P+−C−(Me)PtBu2 (L3) were prepared and applied as defined precatalysts in C−N coupling reactions. The complexes are highly active in the amination of 4‐chlorotoluene with a series of different amines. Higher yields were observed with the precatalysts in comparison to the in situ generated catalysts. Changes in the ligand structures allowed for improved selectivities by shutting down β‐hydride elimination or diarylation reactions. Particularly, the complexes based on L2 (joYPhos) revealed to be universal precatalysts for various amines and aryl halides. Full conversions to the desired products are reached mostly within 1 h reaction time at room temperature, thus making L2 to one of the most efficient ligands in C−N coupling reactions. The applicability of the catalysts was demonstrated for aryl chlorides, bromides and iodides together with primary and secondary aryl and alkyl amines, including gram‐scale applications also with low catalyst loadings of down to 0.05 mol %. Kinetic studies further demonstrated the outstanding activity of the precatalysts with TOF over 10.000 h−1.

Crystal structures of the first diselenophosphinate molybdenum(II) allyl complexes: [Mo(CH3CN)(κ3-C3H5)](CO)2(κ2-Se2PPh2)] and [Mo(κ3-C3H5)(CO)2(κ2-Phen)(κ1-Se2PPh2)

A reaction of [Mo(CH3CN)2(κ3-C3H5)(CO)2(Br)] (complex 1) and (Et4N)[Se2PPh2] (complex 2) produces a κ2-diselenophosphinate molybdenum [Mo(CH3CN)(κ3-C3H5)(CO)2(κ2-Se2PPh2)] complex (complex 3). Treatment of 3 with 1,10-phenanthroline, C12H8N2, yields a κ1-diseleno­phosphinate [Mo(κ3-C3H5)(CO)2(κ2-Phen)(κ1-Se2PPh2)] complex (complex 4). The coordination geometry around the molybdenum atom of complexes 3 and 4 is octahedral with aceto­nitrile, allyl, two carbonyls, and two selenium atoms and allyl, two carbonyls, two nitrogen atoms of the 1,10-phenanthroline ligand, and one selenium atom, occupying six-coordination sites, respectively. To the best of our knowledge, complexes 3 and 4 are the first diselenophosphinate molybdenum allyl complexes. Complexes 3 and 4 are characterized through the elemental analysis, fast atom bombardment mass spectrometry, 31P NMR spectroscopy, and single crystal X-ray diffraction.

Bonding and electronic structures in dinuclear (X)[(Ind)M2L2] complexes (M = Ni, Pd, L = CO, PEt3, X = Cl, Allyl, Ind = indenyl, Cp = cyclopentadienyl): analogy between four-electron donor ligands

The calculations of bimetallic complexes of the type (X)[(Ind)M2(L)2] (M = Ni, Pd, L = CO, (PEt3) and X = Allyl, Cp and indenyl) have been done using two DFT functionals, namely BP86 and B3LYP*. The allyl, Cp and indenyl ligands adopt the same η3-coordination mode with a π bond and can be considered to be isolobal, while the chloride acts as σ- and π-donor. The computed structural and energetic parameters and energy decomposition yield chemically useful information. We report that the metal–metal bond distances are slightly sensitive to the electron donation and electron π-backdonation as the isolobal prediction suggests. Changing the metal from Ni to Pd has the result of increasing the metal–metal bond distance, decreasing the natural population of Pd and the weakness interactions between the X− ligand and the [(Ind)M2(L)2]+ fragment. The results showed that the four ligands behave quite similarly in terms of bonding, coordination mode and donation and π-backdonation properties highlighted by the orbitals’ populations and the energy decomposition. However, the strength of interactions can be classified as follows: Cl < Cp ≈ Ind < Allyl. In all the complexes studied, the M22+ moiety adopts a single metal–metal bonding attributing the 16-MVE configuration to each M(I) cation.

A Highly Asymmetric Gold(III) η3 -Allyl Complex.

A highly asymmetric AuIII η3‐allyl complex has been generated by treating Au(η1‐allyl)Br(tpy) (tpy=2‐(p‐tolyl)pyridine) with AgNTf2. The resulting η3‐allyl complex has been characterized by NMR spectroscopy and X‐ray crystallography. DFT calculations and variable temperature 1H NMR suggest that the allyl ligand is highly fluxional.

Isomerization‐Asymmetric Hydrogenation Sequence Converting Racemic β‐Ylidenecycloalkanols into Stereocontrolled β‐Substituted Cycloalkanols Using a Ru Catalytic System with Dual Roles

AbstractRacemic β‐ylidenecycloalkanols were transformed into thecis‐β‐substituted cycloalkanols with high enantio‐ and diastereoselectivities through an isomerization‐asymmetric hydrogenation sequence with the (4,4′‐bi‐1,3‐benzodioxole)‐5,5′‐diylbis[di(3,5‐xylyl)phosphine (DM‐Segphos)/2‐dimethylamino‐1‐phenylethylamine (DMAPEN)‐ruthenium(II) catalyst; such transformation hardly proceeded by single‐step asymmetric hydrogenation. The reaction was usually carried out with a substrate‐to‐catalyst molar ratio of 500 under 4 to 10 atm of H2to afford the products incis/transratio up to 99:1 and 98% ee. Mechanistic experiments suggested that this catalytic system reversibly formed two reactive species, types (I) and (II), through a ruthenacyclic amide intermediate. The amide complex and allylic alcohol reacted to afford the allylic alkoxide complex with partial or full removal of diamine (type (I)), and this type (I) complex catalyzed isomerization of the allylic alcohols into the racemic α‐substituted ketones. The RuH2complex with chelation of diamine (type (II)) formed by reaction of the amide complex and hydrogen promoted asymmetric hydrogenation of racemic α‐substituted ketone into the stereocontrolled β‐substituted cycloalkanols through dynamic kinetic resolution.magnified image