Reductive Elimination Reactions Research Articles

Synthesis, characterization and coordination properties of bis(alkyl)selenosalen ligands

New bis (alkyl) selenosalen podand ligands having Se2N2 donor sites have been synthesized by the condensation of unsymmetrical o-formylphenyl alkyl selenide (1-3) with ethylenediamine. The reaction of bis(alkyl)selenosalen podands with Pd(II) and Pt(II) afforded selenoether-selenolate coordination complexes 7-10via cleavage of one of the two Se-C(alkyl) bonds of bis(alkyl)selenosalen podands upon complexation. DFT calculations revealed that the cleavage of Se-C(alkyl) bonds occurred possibly via S(N)2 mechanism instead of a sequence of oxidative addition and reductive elimination reactions. The spectral data and elemental analyses confirmed the formation of selenoether-selenolate complexes. The structures of the podands N,N'-bis[(2-methylseleno)phenylmethylene]-1,2-ethanediamine (4), N,N'-bis[(2-decylseleno)phenylmethylene]-1,2-ethanediamine (5) and the selenoether-selenolate complex 8 have been determined by single crystal X-ray diffraction analysis. The crystal structure of 5 showed SeH interaction with a ladder like 3D supramolecular arrangement via interdigitation of long alkyl chains. Comparison of crystal packing of podands 4 and 5 indicates that the alkyl chain length has significant impact on the crystal packing. The platinum selenolate complex 8 shows a square planar arrangement around the Pt centre, where the Se atoms in the selenolate and the selenoether have nearly equal Pt-Se bond length.

Redox-Active Ligand-Mediated Oxidative Addition and Reductive Elimination at Square Planar Cobalt(III): Multielectron Reactions for Cross-Coupling

Square planar cobalt(III) complexes with redox-active amidophenolate ligands are strong nucleophiles that react with alkyl halides, including CH(2)Cl(2), under gentle conditions to generate stable square pyramidal alkylcobalt(III) complexes. The net electrophilic addition reactions formally require 2e(-) oxidation of the metal fragment, but there is no change in metal oxidation state because the reaction proceeds with 1e(-) oxidation of each amidophenolate ligand. Although the four-coordinate complexes are very strong nucleophiles, they are mild outer-sphere reductants. Accordingly, addition of alkyl- or phenylzinc halides to the five-coordinate organometallic complexes regenerates the square planar starting materials and extrudes C-C coupling products. The net 2e(-) reductive elimination reaction also occurs without a oxidation state change at the cobalt(III) center. Together these reactions comprise a complete, well-defined cycle for cobalt Negishi-like cross-coupling of alkyl halides with organozinc reagents.

Bimetallic Reductive Elimination from Dinuclear Pd(III) Complexes

In 2009, we reported C-halogen reductive elimination reactions from dinuclear Pd(III) complexes and implicated dinuclear intermediates in Pd(OAc)(2)-catalyzed C-H oxidation chemistry. Herein, we report results of a thorough experimental and theoretical investigation of the mechanism of reductive elimination from such dinuclear Pd(III) complexes, which establish the role of each metal during reductive elimination. Our results implicate reductive elimination from a complex in which the dinuclear core is intact and suggest that redox synergy between the two metals is responsible for the facile reductive elimination reactions observed.

C–C coupling reaction of pyridine derivatives at the dimethyl rare-earth metal cation [YMe 2(THF) 5] +: A DFT investigation

Applying DFT methods, the reductive elimination reaction of bis(pyridyl) rare-earth metal cation [Ln( η 2-C,N-pyridyl) 2(THF) 3] + (Ln = Y, La) was studied. The effect of both electron-donating and electron-withdrawing substituents in the para-position of the pyridine ring was considered. An alternative mechanism for the C–C coupling reaction between pyridine derivatives (pyridine, DMAP and 4-trifluoromethyl-pyridine) and [LnMe 2(THF) 5] + (Ln = Y or La) is suggested. The reaction involves a single electron reductive C–C coupling to form of Ln(II) complexes with a bipyridine radical anion.

ChemInform Abstract: Development of Organopalladium(IV) Chemistry: Fundamental Aspects and Systems for Studies of Mechanism in Organometallic Chemistry and Catalysis

Palladium is one of the most widely studied elements in organometallic chemistry, partly owing to the important role of palladium complexes in organic synthesis and catalysis. Since the initial report of PdIMe3(bpy), a rich and diverse organopalladium(IV) chemistry has evolved, including reaction systems that are ideal for studying topics of contemporary interest, e.g., mechanisms of oxidative addition and reductive elimination reactions, selectivity in reductive elimination, and studies of alkyl group exchange between metal centers. An account of each of these topics is presented here, followed by an assessment of some of the possible roles of Pd(IV) in organic synthesis and catalysis. Palladium(IV) chemistry has developed rapidly since 1986 to include the isolation of alkyl-, benzyl-, (eta-l-allyl)-, and arylpalladium complexes. It is providing new comparisons of structure, solution dynamics, and reactivity among the nickel triad elements and is providing new systems for mechanistic studies.

ChemInform Abstract: Formation and Cleavage of Si-E Bonds (E: C, Si) by Oxidative Addition and Reductive Elimination Reactions

ChemInform Abstract: Formation and Cleavage of Si-E Bonds (E: C, Si) by Oxidative Addition and Reductive Elimination Reactions

ChemInform Abstract: Chemistry at Diplatinum Centers.

Publisher Summary This chapter describes dimeric complexes containing unsupported metal–metal bonds, followed by those bridged by simple ligands, such as halides or hydrides, then a number of compounds with bridging organic fragments. The number and variety of dimeric complexes have increased tremendously, however, with the availability of several bidentate ligands of a bite size suitable to coordinate in a bridging fashion. Most notable among these is bis(dipheny1phosphino) methane (dppm), and the remarkable versatility of this ligand has prompted the development of a number of analogs of dppm. Dppm and related ligands have been used to support dimeric complexes, containing platinum in the formal oxidation states 0, I, II, III, or IV. These ligands confer substantial stability on the diplatinum system, such that substitution, oxidative addition, and reductive elimination reactions within a dimeric framework can be investigated.

Pd-catalyzed enantioselective allyl-allyl cross-coupling.

The Pd-catalyzed cross-coupling of allylic carbonates and allylB(pin) is described. The regioselectivity of this reaction is sensitive to the bite angle of the ligand, with small-bite-angle ligands favoring the branched substitution product. This mode of regioselection is consistent with a reaction that operates by a 3,3' reductive elimination reaction. In the presence of appropriate chiral ligands, this reaction is rendered enantioselective and applies to both aromatic and aliphatic allylic carbonates.

Direct Synthesis of Titanium Complexes with Chelating cis‐9,10‐Dihydrophenanthrenediamide Ligands through Sequential CC Bond‐Forming Reactions from o‐Metalated Arylimines

A series of new titanium(IV) complexes with o-metalated arylimine and/or cis-9,10-dihydrophenanthrenediamide ligands, [o-C(6)H(4)(CH=NR)TiCl(3)] (R=2,6-iPr(2)C(6)H(3) (3 a), 2,6-Me(2)C(6)H(3) (3 b), tBu (3 c)), [cis-9,10-PhenH(2)(NR)(2)TiCl(2)] (PhenH(2)=9,10-dihydrophenanthrene; R=2,6-iPr(2)C(6)H(3) (4 a), 2,6-Me(2)C(6)H(3) (4 b), tBu (4 c)), [{cis-9,10-PhenH(2)(NR)(2)}{o-C(6)H(4)(HC=NR)}TiCl] (R=2,6-iPr(2)C(6)H(3) (5 a), 2,6-Me(2)C(6)H(3) (5 b), tBu (5 c)), have been synthesised from the reactions of TiCl(4) with o-C(6)H(4)(CH=NR)Li (R=2,6-iPr(2)C(6)H(3), 2,6-Me(2)C(6)H(3), tBu). Complexes 4 and 5 were formed unexpectedly from the reactions of TiCl(4) with two or three equivalents of the corresponding o-C(6)H(4)(CH=NR)Li followed by sequential intramolecular C-C bond-forming reductive elimination and oxidative coupling reactions. Attempts to isolate the intermediates, [{o-C(6)H(4)(CH=NR)}(2)TiCl(2)] (2), were unsuccessful. All complexes were characterised by (1)H and (13)C NMR spectroscopy, and the molecular structures of 3 a, 4 a-c, 5 a, and 5 c were determined by X-ray crystallography.

Photochemical behavior of (bisphosphane)(η2-tolane)Pt0 complexes. Part B: An insight from DFT calculations

A series of various (bisphosphane)(η(2)-tolane)Pt(0) complexes, exhibiting a manifold of substitution patterns of the tolane ligand (5a-g) and different rigid bisphosphanes defining various P-Pt-P bite angles at the Pt center (9a-b) have been theoretically investigated using time-dependent density functional theory (TD-DFT). UV/Vis absorption spectra have been calculated in order to rationalize the photochemistry of the complexes. Metal-ligand charge transfer (MLCT) transitions from the Pt atom to the alkyne are assigned as the photochemical "active" states responsible for promoting the C(aryl)-C(ethynyl) bond activation. The steric, the electronic effects, as well as the P-Pt-P bite angle play an important role in determining the presence/absence of photochemical "active" states of d→π*(alk) character. Thus, electron-withdrawing substituted series and ortho-substituted complexes are best candidates to achieve C(aryl)-C(ethynyl) bond activation. C-Br bond cleavage is also theoretically rationalized. The observed photochemical C(aryl)-C(ethynyl) bond cleavage is, oppositely to C-Br bond activation, reversible under thermal conditions regaining the appropriate Pt(0) complexes by reductive elimination (see T. Weisheit, D. Escudero, H. Petzold, H. Görls, L. González and W. Weigand, Photochemical behavior of (bisphosphane)(η(2)-tolane)Pt(0) complexes in solution and in the solid state. Part A: Experimental considerations, Dalton Trans., 2010, 39, DOI: 10.1039/B925562a). In this part, we rationalize and clarify the thermal reductive elimination reactions via mechanistic DFT studies on the ground state.

Investigations of Iridium-Mediated Reversible C−H Bond Cleavage: Characterization of a 16-Electron Iridium(III) Methyl Hydride Complex

New iridium complexes of a tridentate pincer ligand, 2,6-bis(di-tert-butylphosphinito)pyridine (PONOP), have been prepared and used in the study of hydrocarbon C-H bond activation. Intermolecular oxidative addition of a benzene C-H bond was directly observed with [(PONOP)Ir(I)(cyclooctene)][PF(6)] at ambient temperature, resulting in a cationic five-coordinate iridium(III) phenyl hydride product. Protonation of the (PONOP)Ir(I) methyl complex yielded the corresponding iridium(III) methyl hydride cation, a rare five-coordinate, 16-valence electron transition metal alkyl hydride species which was characterized by X-ray diffraction. Kinetic studies of C-H bond coupling and reductive elimination reactions from the five-coordinate complexes have been carried out. Exchange NMR spectroscopy measurements established a barrier of 17.8(4) kcal/mol (22 degrees C) for H-C(aryl) bond coupling in the iridium(III) phenyl hydride cation and of 9.3(4) kcal/mol (-105 degrees C) for the analogous H-C(alkyl) coupling in the iridium(III) methyl hydride cation. The origin of the higher barrier of H-C(aryl) relative to H-C(alkyl) bond coupling is proposed to be influenced by a hindered rotation about the Ir-C(aryl) bond, a result of the sterically demanding PONOP ligand.

New routes to low-coordinate iron hydride complexes: The binuclear oxidative addition of H 2

The oxidative addition and reductive elimination reactions of H 2 on unsaturated transition-metal complexes are crucial in utilizing this important molecule. Both biological and man-made iron catalysts use iron to perform H 2 transformations, and highly unsaturated iron complexes in unusual geometries (tetrahedral and trigonal planar) are anticipated to give unusual or novel reactions. In this paper, two new synthetic routes to the low-coordinate iron hydride complex [L tBuFe(μ-H)] 2 are reported. Et 3SiH was used as the hydride source in one route by taking advantage of the silaphilicity of the fluoride ligand in three-coordinate L tBuFeF. The other synthetic method proceeded through the binuclear oxidative addition of H 2 or D 2 to a putative Fe(I) intermediate. Deuteration was verified through reduction of an alkyne and release of the deuterated alkene product. Mössbauer spectra of [L tBuFe(μ-H)] 2 indicate that the samples are pure, and that the iron(II) centers are high-spin.

Reactivity of diacetylplatinum(II) complexes: Oxidative addition of halogens and hydrogen halides

Diacetylplatinum(II) complexes [Pt(COMe) 2( ▪)] ( ▪ = bpy, 3a; 4,4′- t-Bu 2-bpy, 3b), obtained by the reaction of [Pt(COMe) 2X(H)( ▪)] with NaOH in CH 2Cl 2/H 2O, were found to undergo oxidative addition reactions with halogens (Br 2, I 2) yielding the platinum(IV) complexes ( trans, OC-6-13)/( cis, OC-6-32) [Pt(COMe) 2X 2( ▪)] ( ▪ = bpy, X = Br, 4a/4b; I, 4c/ 4d; ▪ = 4,4′- t-Bu 2-bpy, X = Br, 4e/4f; I, 4g/ 4h). The diastereoselectivity of the reactions proved to be strongly dependent on the solvent. The oxidative addition of (SCN) 2 resulted in the formation of ( OC-6-13)-[Pt(COMe) 2(SCN) 2( ▪)] ( ▪ = bpy, 4i; 4,4′- t-Bu 2-bpy, 4j). In a reaction the reverse of their formation, the diacetylplatinum(II) complexes 3 underwent oxidative addition with anhydrous HX (X = Cl, Br, I), prepared in situ from Me 3SiX/H 2O, yielding diacetyl(hydrido)platinum(IV) complexes [Pt(COMe) 2X(H)( ▪)] ( ▪ = bpy, X = Cl, 5a; Br, 5b; I, 5c; ▪ = 4,4′- t-Bu 2-bpy, X = Cl, 5d; Br, 5e; I, 5f). Furthermore, diacetyldihaloplatinum complexes 4 were found to undergo reductive elimination reactions in boiling methanol yielding acetylplatinum(II) complexes [Pt(COMe)X( ▪)] ( ▪ = bpy, X = Br, 6b; I, 6c; ▪ = 4,4′- t-Bu 2-bpy, X = Br, 6e; I, 6f). All complexes were characterized by microanalysis, IR and 1H and 13C NMR spectroscopy. Additionally, the bis(thiocyanato) complex 4j was characterized by single-crystal X-ray diffraction analysis.

Palladium-Catalyzed Oxidative Coupling of 2-Alkylfurans with Olefins through C−H Activation: Synthesis of Difurylalkanes

The course of the palladium-catalyzed coupling of 2-alkylfurans with olefins through C−H activation is influenced by the nature of the solvent. At room temperature, with acetic acid as solvent and benzoquinone as oxidant, the usual Heck product, 2-cinnamyl-5-methylfuran, is obtained in low yield. The use of a AcOH/CH3CN mixture as solvent induced the formation of difurylalkanes in fair to high yields. Allylarenes and styrenes led to the formation of β,β-difurylalkanes as major compounds, while acrylates afforded selectively β,β-difuryl esters. Mechanism studies have shown that these transformations do not occur through the processes usually involved in the formation of diarylalkanes. According to ESI-MS studies and labeling experiments, two consecutive C−H activations of 2-alkylfurans by Pd(II) are followed by an insertion of the alkene, a migration of a furan ring involving β-elimination/insertion steps, and finally a reductive elimination reaction.

Unexpected differences in the reactivity between MPh 2Cl 2 (M = Pb or Sn) and benzil bis(thiosemicarbazone). X-ray crystal structure of benzil bis(thiosemicarbazonate)lead(II)

The differences in the reactivity of diphenyllead and diphenyltin dichloride with benzil bis(thiosemicarbazone) LH 6 are explored. The reaction of PbPh 2Cl 2 afforded the complex [Pb(LH 4)] 1, containing the ligand doubly deprotonated and without the phenyl groups bonded to the metal. The reaction with SnPh 2Cl 2 in the absence of solvent yielded the complex [Sn(LH 4)Cl 2] 2. The lead(II) complex, also obtained by the reaction with lead(II) nitrate, was formed by a redistribution of the organolead derivative followed by a reductive elimination reaction, while the tin one was formed by a dephenylation process. Reactions in solution of the organotin derivative are complicated and depending on the working conditions different compounds could be isolated: the organostannoxane [Sn 2Ph 4ClO(OH)] 2, which was confirmed by X-ray crystallography, or a complex containing a triazine-3-thione ligand, formed by the loss of one of the thiosemicarbazone arms with subsequent cyclisation. The compounds were characterized by IR, mass spectrometry, 1H, 13C and 119Sn NMR spectroscopy in solution and in the solid state. The structure of [Pb(LH 4)] has been studied by X-ray diffraction and the ligand acts as a N 2S 2 planar bideprotonated ligand, with the lead(II) ion localized 1.3858 Å over this plane.

Metal‐Mediated Formation of Carbon–Halogen Bonds

Organic halides represent basic starting materials for numerous metal-catalyzed organic transformations. Generally, the carbon-halogen is broken in the first step, that is, an oxidative addition reaction, of the catalytic cycle. On the other hand, very little is known about the reverse reaction, carbon-halogen reductive elimination from a transition-metal center. In this Concept article, we describe the examples of C(sp(3))-halide and C(sp(2))-halide reductive-elimination reactions which demonstrate that this type of reactivity can be quite common in organometallic chemistry. Although the thermodynamic driving force for the formation of carbon-halogen bonds is relatively small, the kinetic barrier for these reactions can also be low. Thus, C-halide reductive elimination can compete favorably with the more established organic transformations, such as C-C reductive elimination.

Synthesis, characterization and catalytic activity of organolanthanide(II) complexes with heterocyclic-functionalized fluorenyl ligands

Two series of new organolanthanide(II) complexes with tetrahydro-2H-pyranyl- or N-piperidineethyl-functionalized fluorenyl ligands were synthesized via one-electron reductive elimination reaction. Treatments of [(Me 3Si) 2N] 3Ln III(μ-Cl)Li(THF) 3 with 2 equiv. of C 5H 9OCH 2C 13H 9 ( 1) or C 5H 10NCH 2CH 2C 13H 9 ( 2), respectively, in toluene at about 80 °C produced, after workup, the corresponding organolanthanide(II) complexes with formula [η 5:η 1-C 5H 9OCH 2C 13H 8] 2Ln II (Ln = Yb ( 3), Ln = Eu ( 4)) and [η 5:η 1-C 5H 10NCH 2CH 2C 13H 8] 2Ln II (Ln = Yb ( 5), Ln = Eu ( 6)) in good yields. All the compounds were fully characterized by spectroscopic methods and elemental analyses. The structures of complexes 3, 4, and 6 were additionally determined by single-crystal X-ray analyses. It represents the first example of solvent-free organolanthanide(II) complexes with fluorenyl ligands. The catalytic properties of the organolanthanide(II) complexes on the polymerization of ε-caprolactone and methyl methacrylate have been studied. The temperatures, solvents and coordination effects on the catalytic activities of the complexes were examined.

Frontier Orbital Consistent Quantum Capping Potential (FOC-QCP) for Bulky Ligand of Transition Metal Complexes

Chemically reasonable models of PR3 (R = Me, Et, iPr, and tBu) were constructed to apply the post Hartree-Fock method to large transition metal complexes. In this model, R is replaced by the H atom including the frontier orbital consistent quantum capping potential (FOC-QCP) which reproduces the frontier orbital energy of PR3. The steric effect is incorporated by the new procedure named steric repulsion correction (SRC). To examine the performance of this FOC-QCP method with the SRC, the activation barriers and reaction energies of the reductive elimination reactions of C2H6 and H2 from M(R1)2(PR2(3))2 (M = Ni, Pd, or Pt; R1 = Me for R2 = Me, Et, or iPr, or R1 = H for R2 = tBu) were evaluated with the DFT[B3PW91], MP4(SDQ), and CCSD(T) methods. The FOC-QCP method reproduced well the DFT[B3PW91]- and MP4(SDQ)-calculated energy changes of the real complexes with PMe3. For more bulky phosphine, the SRC is important to present correct energy change, in which the MP2 method presents reliable steric repulsion correction like the CCSD(T) method because the systems calculated in the SRC do not include a transition metal element. The monomerization energy of [RhCl(PiPr3)2]2 and the coordination energies of CO, H2, N2, and C2H4 with [RhCl(PiPr3)2]2 were theoretically calculated by the CCSD(T) method combined with the FOC-QCP and the SRC. The CCSD(T)-calculated energies agree well with the experimental ones, indicating the excellent performance of the combination of the FOC-QCP with the SRC. On the other hand, the DFT[B3PW91]-calculated energies of the real complexes considerably deviate from the experimental ones.

Competitive Aryl−Iodide vs Aryl−Aryl Reductive Elimination Reactions in Pt(IV) Complexes: Experimental and Theoretical Studies

The platinum(IV) complex trans-(dmpe)Pt(IV)(Ar)2I2 (2, dmpe = 1,2-dimethylphosphinoethane, Ar = 4-FC6H4) rapidly reacts, upon moderate heating in solution under ambient light, via two distinct pathways: isomerization to the corresponding cis-isomer (3) and Ar-I reductive elimination to give (dmpe)Pt(II)(Ar)I (4). Complex 3 undergoes, upon prolonged heating at high temperatures, an exclusive Ar-Ar reductive elimination reaction to give (dmpe)Pt(II)I2. Experimental and DFT studies showed that the 2-to-3 isomerization proceeds via three pathways: photochemical or thermal phosphine chelate opening and a mechanism involving cleavage of the Pt-I bond. The isomerization reaction is significantly slowed down but not stopped in the absence of light or in the presence of an excess of tetra-n-butylammonium iodide. On the other hand, the Ar-I reductive elimination from 2 proceeds via the Pt(delta+)-I(delta-) ion pairlike transition state. Use of the rigid dmpe analogue 1,2-dimethylphosphinobenzene (dmpbz) as the ligand shuts down the chelate ring-opening isomerization pathway and enables faster Ar-I reductive elimination thus making the latter reaction the major reaction route for the dmpbz supported trans-diiodo Pt(IV) complex 8.

Reactivity of Pd(II) Complexes with Electrophilic Chlorinating Reagents: Isolation of Pd(IV) Products and Observation of C−Cl Bond-Forming Reductive Elimination

This communication describes the reactions of PhICl2 and N-chlorosuccinimide with PdII(phpy)2 (phpy = 2-phenylpyridine) to afford the novel PdIV oxidative addition products PdIV(phpy)2(Cl)2 and PdIV(phpy)2(Cl)(succinimide), respectively. These PdIV species are remarkably stable at room temperature. However, upon thermolysis at 80 °C, they undergo competing C−Cl, C−C, and C−N bond-forming reductive elimination reactions. Both the accessibility and the observed reactivity of these complexes provide support for a PdII/IV mechanism in Pd-catalyzed halogenation reactions.