Reductive Elimination Products Research Articles

Coumarin C-H Functionalization by Mn(I) Carbonyls: Mechanistic Insight by Ultra-Fast IR Spectroscopic Analysis.

Mn(I) C-H functionalization of coumarins provides a versatile and practical method for the rapid assembly of fused polycyclic pyridinium-containing coumarins in a regioselective manner. The synthetic strategy enables application of bench-stable organomanganese reagents in both photochemical- and thermal-promoted reactions. The cyclomanganated intermediates, and global reaction system, provide an ideal testing ground for structural characterization of the active Mn(I) carbonyl-containing species, including transient species observable by ultra-fast time-resolved spectroscopic methods. The thermodynamic reductive elimination product, solely encountered from reaction between alkynes and air-stable organometallic cyclomanganated coumarins, has enabled characterization of a critical seven-membered Mn(I) intermediate, detected by time-resolved infrared spectroscopy, enabling the elucidation of the temporal profile of key steps in the reductive elimination pathway. Quantitative data are provided. Manganated polycyclic products are readily decomplexed by AgBF4 , opening-up an efficient route to the formation of π-extended hybrid coumarin-pyridinium compounds.

Synthesis and reactivity of Pd(II) imidoyl complexes obtained by insertion of isocyanoferrocene into the Pd-C bonds of orthopalladated precursors.

While the multifaceted reactivity of organic isocyanides has been extensively demonstrated, that of their organometallic analogue, isocyanoferrocene (FcNC; Fc = ferrocenyl), has not yet been adequately explored. This contribution describes the syntheses of novel chelating Pd(II) imidoyl complexes, [(YCH2C6H4C(NFc)-κ2Y,C)PdCl(PR3)], by insertion of FcNC into the Pd-C bond of cyclopalladated precursors [(YCH2C6H4-κ2Y,C)PdCl(PR3)] (Y = Me2N, MeS, R = Ph, Me). The imidoyl complexes underwent facile alkylation with [Me3O][BF4] to produce the cationic aminocarbene complexes [{YCH2C6H4C(N(Me)Fc)-κ2Y,C}PdCl(PR3)][BF4]. All compounds were fully characterised using a combination of spectroscopic methods (NMR, FTIR and ESI MS), cyclic voltammetry and single-crystal X-ray crystallography. In addition, DFT calculations were used to describe the bonding in the two compound families. Analyses with intrinsic bond orbitals (IBOs) and the quantum theory of atoms in molecules (QTAIM) consistently pointed to the transformation of an X-type imidoyl C-ligand (σ-organyl) into an L-type carbene donor upon alkylation, which has only a minor structural consequence. Also reported is the unexpected conversion of the imidoyl complex [(Me2NCH2C6H4C(NFc)-κ2N,C)PdCl(PPh3)] into (Z)-2,2-dimethyl-1-(ferrocenylimino)isoindolin-2-ium tetrafluoroborate as a reductive elimination product, which was induced by Lewis and Brønsted acids.

Reversible C(sp3)-Si Oxidative Addition of Unsupported Organosilanes: Effects of Silicon Substituents on Kinetics and Thermodynamics.

The intermolecular oxidative addition of unactivated C(sp3)-Si bonds is reported for a family of organosilanes at a cationic pincer-supported iridium complex. To our knowledge, no examples of oxidative addition to give analogous unsupported (alkyl)metal silyl complexes have been previously reported. The generality of this transformation is excellent, with successful examples demonstrated for tetraorganosilanes, mono- and poly alkoxysilanes, and two siloxysilanes. Oxidative addition is found to be completely reversible, with the product of reductive elimination being subject to trapping by triethylsilane. The successful isolation of these metal silyl complexes has allowed for an in-depth kinetic analysis of C(sp3)-Si reductive elimination, a process with strong implications in both catalytic C-H silylation and olefin hydrosilylation. The apparent order of reactivity is SiMe3 > SiMe2(CF3) > SiMe2OSiMe3 > SiMe2OSiMe2OSiMe3 > SiMe2(OMe) > SiMe2(OEt) > SiMe(OMe)2. A DFT analysis of the oxidative addition products shows that the thermodynamic stability of the (alkyl)metal silyl complexes span a range of ca. 10 kcal·mol-1, which relate closely with the experimentally determined rates of C(sp3)-Si reductive elimination and trapping, though a clear kinetic distinction exists between methoxy- and siloxysilyl complexes.

Theoretical Insight into Palladium(II)-Counterion-Ligand Cooperative Regiodivergent Syntheses of Indolo[3,2-c]coumarins and Benzofuro[3,2-c]quinolinones from Diphenylethyne Derivatives.

With two distinct active sites, 2-hydroxy-2'-amino-diphenylethyne derivatives can offer benzofuro[3,2-c]quinolinones via the O-attack/N-carbonylation cyclization or indolo[3,2-c]coumarins via the N-attack/O-carbonylation cyclization. This work presents a density functional theory-based computational study to understand the mechanism and origin of the palladium(II)-catalyzed regiodivergent reactivity of diphenylethyne derivatives. It is indicated that the reaction features a palladium(II)-counterion-ligand cooperative catalysis. The O-attack/N-carbonylation cyclization mainly benefits from the inductive effect of the rigid electron-withdrawing bidentate nitrogen ligand and the stabilization of the 3c-4e bond between the trifluoroacetate (TFA) anion and the hydroxyl group in the substrate for the precursor and transition state, while the viability of the N-attack/O-carbonylation cyclization stems intrinsically from the stronger nucleophilicity of the N atom as well as the important π-π interaction between the flexible electron-rich bidentate phosphine ligand and the substrate. Moreover, these calculations propose an unconventional reductive elimination mechanism for the transformation from Pd(II) to Pd(0), where the intramolecular nucleophilic attack of the N/O atom on the carbonyl C atom results in the formal reductive elimination product. The calculated overall barriers of 14.8 kcal/mol for Pd(TFA)2 with the bidentate nitrogen ligand and 23.9 kcal/mol for Pd(OTf)2 with the bidentate phosphine ligand are qualitatively consistent with the mild experimental conditions.

NHC Nickel Catalyzed Hiyama‐ and Negishi‐Type Cross‐Coupling of Aryl Fluorides and Investigations on the Stability of Nickel(II) Fluoroaryl Alkyl Complexes

The reactivity of [Ni(iPr2Im)4(µ‐COD)] 1 (iPr2Im = 1,3‐diisopropyl‐imidazolin‐2‐ylidene, COD = 1,4‐cyclooctadiene) in Hiyama‐ and Negishi‐type cross‐coupling reactions as well as the synthesis of several novel nickel fluoroaryl alkyl complexes is reported. Hiyama coupling of 1.1 equiv. perfluoroaromatics and 1 equiv. PhSi(OR)3 (R = Me, Et) with 5 mol‐% of 1 as catalyst leads to the C–C coupling product ArF–Ph in good to fair yields. In presence of the additive NMe4F alkoxy transfer from PhSi(OR)3 to the perfluoroarene occurs to yield ArF–OR and PhSiF(OR)2. Negishi cross‐coupling between C6F6 or C7F8 (1 equiv.), diorganozinc reagents [ZnR2] (R = Me, Et) (2.1 equiv.) and 5 mol‐% 1 as the catalyst in toluene at 115 °C leads to ArF–R only in traces. However, NMR experiments revealed that nickel alkyl complexes are readily formed from the reaction of trans‐[Ni(iPr2Im)2(F)(ArF)] with [ZnR2] (R = Me, Et). In course of these investigations, a series of novel nickel alkyl complexes trans‐[Ni(iPr2Im)2(R)(ArF)] (R = Me, ArF = C6F5 2, C7F7 3, C12F9 4; R = Et, ArF = C6F5 5, C7F7 6, C12F9 7) have been synthesized in stoichiometric reactions starting from trans‐[Ni(iPr2Im)2(F)(ArF)] (ArF = C6F5, C7F7, C12F9) and [ZnR2] (R = Me, Et) in thf at –78 °C. As these nickel alkyl complexes 2–7 are stable at room temperature in solution for several days with respect to reductive elimination, their thermal stability was investigated. Heating trans‐[Ni(iPr2Im)2(Me)(C6F5)] 2 for 24 hours at 100 °C leads to 91 % unreacted complex 2 and only traces of reductive elimination product, i.e. C6F5Me, are formed. Furthermore, the nickel ethyl complex trans‐[Ni(iPr2Im)2(Et)(C6F5)] 5 is also very stable, even with respect to β‐hydride elimination. After heating this complex to 100 °C for 24 hours there is still 26 % unreacted 5 left.

Exploring C(sp3)–C(sp3) reductive elimination from an isolable iron metallacycle

A six-coordinate iron metallacyclopentane, (phen)2Fe(CH2)4, supported by two 1,10-phenanthroline (phen) ligands, has been synthesized and structurally and spectroscopically characterized. The complex is diamagnetic and an idealized octahedral geometry was observed in the solid state. The electronic structure of (phen)2Fe(CH2)4 was determined by a combination of X-ray diffraction, Mössbauer spectroscopy, and DFT analyses and is best described as a low-spin Fe(III) center antiferromagnetically coupled to a radical anion delocalized equally over both phen ligands. The reactivity of (phen)2Fe(CH2)4 under different conditions was explored. Thermolysis or photolysis promoted elimination reactions and mixtures of isomeric butenes and butane were observed. Reactions of (phen)2Fe(CH2)4 with ethylene and isoprene yielded 3% and 11% of reductive elimination product cyclobutane, respectively, along with butane and butene isomers. Addition of π-accepting ligands such as carbon monoxide, maleic anhydride, or 1,4-benzoquinone to (phen)2Fe(CH2)4 promoted C(sp3)-C(sp3) reductive elimination as judged by high selectivity for cyclobutane formation. Two electron oxidation of (phen)2Fe(CH2)4 with two equivalents of ferrocenium tetraphenylborate also exclusively yielded cyclobutane in 95% yield. The electronic structure and reactivity of related bis(bipyridine) iron dialkyl compounds previously isolated by Kochi and co-workers were also revisited and their electronic structures revised based on structural, spectroscopic and computational data.

Photolytic Reactivity of Organometallic Chromium Bipyridine Complexes.

Known stable [Cr(bpy)2(Ph)2](BPh4) complexes undergo reductive elimination of biphenyl with visible-light photolysis using household incandescent or compact fluorescent light bulbs. A series of [Cr(R-bpy)2(Ar)2](X) complexes (R = H or CMe3; Ar = Ph, C6H4-CMe3, or C6H4-OMe; X = I, BPh4, or PF6) were prepared, and the effect of varying the bipyridine and aryl ligands on the UV-visible spectra and electrochemistry of the chromium(III) complexes was investigated. Photolysis of a mixture of two different bis(aryl) complexes gave only the homocoupled biaryl products by 1H NMR and gas chromatography/mass spectrometry analysis. The initial product of photoinduced reductive elimination of [Cr(bpy)2(Ar)2](PF6) was trapped with bipyridine to generate [Cr(bpy)3](PF6) and with benzoyl peroxide to form [Cr(bpy)2(O2CPh)2](PF6). The latter chromium(III) bis(benzoate) complex was also synthesized by the addition of bipyridine and PhCO2H to Cp2Cr, followed by air oxidation. The neutral Cr(bpy)(S2CNMe2)Ph2 complex also generated biphenyl upon visible-light photolysis. While the treatment of Cr(tBu-bpy)(dpm)Cl2 [dpm = (OCtBu)2CH] with AgO2CPh gave trans-Cr(tBu-bpy)(dpm)(O2CPh)2, reaction of the dichloro precursor with PhMgCl produced anionic [Cr(tBu-bpy)Ph3]- with [Mg(dpm)(THF)4]+ as the countercation, with both complexes characterized by single-crystal X-ray diffraction. Protonolysis of Cr(bpy)Ph3(THF) with 8-hydroxyquinoline produced Cr(bpy)(quin)Ph2, which generated biphenyl under visible-light photolysis, and the initial product of reductive elimination was trapped by bipyridine or benzoyl peroxide. A related Cr(bpy)(quin)2 complex was synthesized by protonolysis of Cr(bpy)[N(SiMe3)2]2 and characterized by single-crystal X-ray diffraction.

Alkoxide Migration at a Nickel(II) Center Induced by a π-Acidic Ligand: Migratory Insertion versus Metal-Ligand Cooperation.

Two pathways of alkoxide migration occurring at a nickel(II) center supported by a PPP ligand (PPP- = P[2-PiPr2-C6H4]2-) are presented in this Article. In the first route, the addition of a π-acidic ligand to a (PPP)Ni alkoxide species reveals the formation of a P-O bond. This reaction occurs via metal-ligand cooperation (MLC) involving a 2-electron reduction at nickel. To demonstrate a P-O bond formation, a nickel(II) isopropoxide species (PPP)Ni(OiPr) (4) was prepared. Upon addition of a π-acidic isocyanide ligand CNtBu, a nickel(0) isocyanide species (PPOiPrP)Ni(CNtBu) (6b) was generated; P-O bond formation occurred via reductive elimination (RE). When CO is present, migratory insertion (MI) occurs instead. The reaction of 4 with CO(g) results in the formation of (PPP)Ni(COOiPr) (5), representing an alternative pathway. The corresponding RE product (PPOiPrP)Ni(CO) (6a) can be independently produced from the substitution reaction of {(PPOiPrP)Ni}2(μ-N2) (3) with CO(g). While two different carbonylation pathways in 4 seem feasible, C-O bond forming migratory insertion singly occurs. Regeneration of a (PPP)Ni moiety via a P-O bond cleavage was demonstrated by treating 3 with CO2(g). The formation of (PPP)Ni(OCOOiPr) (7) clearly shows that an isopropoxide group migrates onto the bound CO2 ligand, and a P-Ni moiety is regenerated.

Fundamental Differences between Group 8 Metals: Unexpected Oxidation State Preferences and Mechanisms in Ruthenium Borylene Complex Formation.

The reaction of the salts K[Ru(CO)3 (PMe3 )(SiR3 )] (R=Me, Et) with Br2 BDur or Cl2 BDur (Dur=2,3,5,6-Me4 C6 H) leads to both boryl and borylene complexes of divalent ruthenium, the former through simple salt elimination and the latter through subsequent CO loss and 1,2-halide shift. The balance of products can be altered by varying the reaction conditions; boryl complexes can be favored by the addition of CO, and borylene complexes by removal of CO under vacuum. All of these products are in competition with the corresponding (aryl)(halo)(trialkylsilyl)borane, a reductive elimination product. The Ru(II) borylene products and the mechanisms that form them are distinctly different from the analogous reactions with iron, which lead to low-valent borylene complexes, highlighting fundamental differences in oxidation state preferences between iron and ruthenium.

Dinuclear gold(i) dithio- and diselenophosph(in)ate complexes forming mononuclear gold(iii) oxidative addition complexes and reversible chemical reductive elimination products

Dinuclear gold(i) dithio- and diselenophosph(in)ate complexes were prepared to serve as precursors for subsequent oxidative addition (OA) chemistry following reaction with mild oxidant iodine, I2. The new OA products circumvented the formation of the expected dinuclear Au(ii) complexes, but instead formed novel chelating mononuclear square-planar gold(iii) products of the type [AuI2{E2PR2}] (R = (CH2)2Ph; E = S, 2; E = Se, 3) and [AuI2{Se2P(OR)2}] (R = Et, 4; (i)Pr, 5) directly. We further demonstrate that this process is chemically reversible as all the Au(iii) complexes undergo chemical reductive elimination to the starting dinuclear Au(i) complexes in the presence of SnI2 as determined by (119)Sn and (31)P NMR. The complexes (2-5) were all prepared from the reaction between I2 and the corresponding dinuclear gold(i) precursor material. All new complexes were characterized by (1)H, (31)P, (77)Se (for E = Se) NMR, infrared spectroscopy, elemental analysis and single crystal X-ray diffraction.

Oxidatively Induced Reductive Elimination from a Chromium(III) Bis(aryl) Complex

The previously reported high-spin Cr(II) compounds CpCr(C6H4CH2NMe2) (1) and CpCr[C(Ph)C(Ph)C6H4CH2NMe2] (2) were investigated as precursors to CpCr(III) complexes. Single-electron oxidation of 1 was used to prepare CpCr(C6H4CH2NMe2)(X) for X = I (3), OTs (4), O2CPh (5), OCMe2Ph (6), SPh (7). Similarly, CpCr[C(Ph)C(Ph)C6H4CH2NMe2](X) for X = I (8), SPh (9) were obtained from 2. Reactions of 4 with PhCH2MgCl or Mg(C6H4Me)2 reagents gave the Cr(III) benzyl (10) and p-tolyl (11) complexes, respectively. The corresponding reaction of 4 with Mg(CH2CMe3)2 led to isolation of 12, a bimetallic complex with a bridging neopentylidene group. While the reaction of 7 or 10 with AgOTs gave Cr(III) tosylate complex 4, the reaction of 11 and AgOTs led to the product of reductive elimination. The structures of 1, 2, 6–8, 11, and 12 were elucidated using single-crystal X-ray diffraction.

Synthesis, Isolation, and Characterization of an Organometallic Triiodopalladium(IV) Complex. Quantitative and Regioselective Synthesis of Two C–I Reductive Elimination Products

Iodine and the pincer complex [Pd(O,N,C-L)I], where L is the monoanionic ligand resulting from deprotonation of the acetyl group of the dimethylmonoketal of 2,6-diacetylpyridine, are in equilibrium at low temperatures with the palladium(IV) complex [Pd(O,N,C-L)I(3)], which can be isolated at -40 °C and characterized by (1)H NMR spectroscopy and X-ray diffraction studies, in spite of its great instability. When the same reaction is carried out at room temperature, a quantitative reductive elimination process occurs, giving L-I, which in the presence of water affords L'-I, resulting from hydrolysis of L-I.

Allylic C–H bond activation and functionalization mediated by tris(oxazolinyl)borato rhodium(i) and iridium(i) compounds

Allylic C-H bond oxidative addition reactions, mediated by tris(oxazolinyl)borato rhodium(I) and iridium(I) species, provide the first step in a hydrocarbon functionalization sequence. The bond activation products To(M)MH(η(3)-C(8)H(13)) (M = Rh (1), Ir (2)), To(M)MH(η(3)-C(3)H(5)) (M = Rh (3), Ir (4)), and To(M)RhH(η(3)-C(3)H(4)Ph) (5) (To(M) = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) are synthesized by reaction of Tl[To(M)] and the corresponding metal olefin chloride dimers. Characterization of these group 9 allyl hydride complexes includes (1)H-(15)N heteronuclear correlation NMR experiments that reveal through-metal magnetization transfer between metal hydride and the trans-coordinated oxazoline nitrogen. Furthermore, the oxazoline (15)N NMR chemical shifts are affected by the trans ligand, with the resonances for the group trans to hydride typically downfield of those trans to η(3)-allyl and tosylamide. These group 9 oxazolinylborate compounds have been studied to develop approaches for allylic functionalization. However, this possibility is generally limited by the tendency of the allyl hydride compounds to undergo olefin reductive elimination. Reductive elimination products are formed upon addition of ligands such as CO and CN(t)Bu. Also, To(M)RhH(η(3)-C(8)H(13)) and acetic acid react to give To(M)RhH(κ(2)-O(2)CMe) (8) and cyclooctene. In contrast, treatment of To(M)RhH(η(3)-C(3)H(5)) with TsN(3) (Ts = SO(2)C(6)H(4)Me) gives the complex To(M)Rh(η(3)-C(3)H(5))NHTs (10). Interestingly, the reaction of To(M)RhH(η(3)-C(8)H(13)) and TsN(3) yields To(M)Rh(NHTs)(H)OH(2) (11) and 1,3-cyclooctadiene viaβ-hydride elimination and Rh-H bond amination. Ligand-induced reductive elimination of To(M)Rh(η(3)-C(3)H(5))NHTs provides HN(CH(2)CH=CH(2))Ts; these steps combine to give a propene C-H activation/functionalization sequence.

Carbon−Hydrogen Bond Oxidative Addition of Partially Fluorinated Aromatics to a Ni(PiPr3)2Synthon: The Influence of Steric Bulk on the Thermodynamics and Kinetics of C−H Bond Activation

The reaction of (PiPr3)2NiCl2 with the anthracene adduct (THF)3Mg(η2-C14H10) in THF provides the anthracene adduct (PiPr3)2Ni(η2-C14H10). In aromatic solvents (benzene, toluene, mesitylene) a thermal equilibrium exists between the bis(phosphine)nickel(0) anthracene adduct, (PiPr3)2Ni(η2-C14H10), and the monophosphine solvent adduct, (PiPr3)Ni(η6-solvent). The reaction of (PiPr3)2Ni(η2-C14H10) with 1,2,4,5-C6F4H2 affords the C−H activation product trans-(PiPr3)2NiH(2,3,5,6-C6F4H). The thermodynamic C−F activation product is not obtained even after hours of heating at 100 °C. Similar reactions with 1,2,3,5-C6F4H2 and pentafluorobenzene produce the desired C−H activation products, trans-(PiPr3)2NiH(2,3,4,6-C6F4H) and trans-(PiPr3)2NiH(C6F5), respectively, in >95% yield. The reaction with 1,2,3,4-tetrafluorobenzene did not produce an observable C−H activation product. Unlike previously reported analogous C−H activation products with Ni(PEt3)2 synthons, the bulkier Ni(PiPr3)2 moiety did not provide observable mononuclear or dinuclear η2-fluoroarene adducts. Solutions of Ni(COD)2 with 2 equiv of triisopropylphosphine and 1,2,4,5-C6F4H2 reacted to give the 1,5-cyclooctadiene insertion and rearrangement product, (η3-C8H13)Ni(PiPr3)(2,3,5,6-C6F4H). The same reaction with 1,2,3,5- and 1,2,3,4-C6F4H2 afforded analogous compounds, which demonstrates that C−H bond activation is kinetically accessible at room temperature with 1,2,3,4-tetrafluorobenzene despite the presence of a single ortho-fluorine substituent adjacent to the site of activation. The room-temperature reactions of the C−H activation products (PiPr3)2NiH(ArF) (ArF = 2,3,5,6-C6F4H; C6F5) with 3-hexyne provided a mixture of the alkyne adduct (PiPr3)2Ni(η2-EtC≡CEt), with the liberation of ArFH, and the insertion product (PiPr3)2Ni(CEt═CHEt)(ArF), even in the presence of excess fluorinated aromatic ArFH. The reaction of (PiPr3)2Ni(η2-EtC≡CEt) with 1,2,4,5-C6F4H2 resulted in no reaction at room temperature, but heating at 50 °C provided the insertion product (PiPr3)2Ni(CEt═CHEt)(ArF) as the initial product, followed by the product of reductive elimination, 1,2,4,5-C6F4H-CEt═CHEt. In contrast, the reaction of (PEt3)2Ni(η2-EtC≡CEt) with 1,2,4,5-C6F4H2 at 80 °C slowly produced (PEt3)2Ni(CEt═CHEt)(2,3,5,6-C6F4H), but very little 1,2,4,5-C6F4H-CEt═CHEt.

UV-Vis, HPLC, and 1H-NMR studies of the substitution reactions of some Pt(IV) complexes with 5′-GMP and L-histidine

Substitution reactions of [PtCl4(en)] and [PtCl4(dach)] with guanosine-5′-monophosphate (5′-GMP) and L-histidine were studied by different experimental methods. By UV-Vis spectrophotometry, these reactions were investigated under pseudo-first-order conditions at 310 K in 25 mmol Hepes buffer (pH = 7.2) and 10 mmol NaCl to prevent the hydrolysis of the complexes. [PtCl4(en)] reacts slightly faster than the [PtCl4(dach)]. Also, L-histidine is a better nucleophile than 5′-GMP. Final 1H-HMR spectra of the substitution of Pt(IV) were in a good agreement with the spectra of Pt(II) complexes with the same nucleophiles, confirming the assumption of the reduction of Pt(IV) complexes during substitution. The same reactions were studied by high-performance liquid chromatography comparing the chromatograms during the reaction. The changes in the intensity of signals and their retention time show that at the end of substitution, there is only one dominant product in the system. We conclude that substitution of these Pt(IV) complexes is followed by rapid reductive elimination and final product is substituted Pt(II) complex.

Quantitative synthesis and full characterization of the first isolated and stable pincer palladium(iv) complexes. Quantitative and regioselective synthesis of the C–X (X = Cl, Br) reductive elimination products

The pincer complexes [Pd(O(1),N(1),C(1)-L)X], where X = Cl, Br and L is the monoanionic ligand resulting from deprotonation of the acetyl methyl group of the monoketal of 2,6-diacetylpyridine (dap), react with excess of Cl(2) or Br(2) affording, quantitatively, the Pd(IV) complexes [Pd(O(1),N(1),C(1)-L)X(3)], which have been characterized by X-ray diffraction, and their decomposition that quantitatively affords the reductive elimination products L-X has been studied.

Reversible Oxidative Addition and Reductive Elimination of Fluorinated Disulfides at Gold(I) Thiolate Complexes: A New Ligand Exchange Mechanism

Reaction of HAuCl4 x 3 H2O with excess HSAr (Ar = C6F5 or C6F4H) in ethanol, followed by addition of [Et4N]Cl, produced [Et4N][Au(SAr)4] (Ar = C6F5 (1a) or C6F4H (1b)) as red crystalline solids in high yield. These complexes are rare examples of homoleptic gold(III) thiolate complexes. The crystal structures 1 show square planar geometry at the gold center with elongated Au-S bonds. Both complexes undergo reversible reductive elimination/oxidative addition processes in solution via thermal and photochemical pathways. Equilibrium constant and photostationary state measurements indicate that the relative importance of the two pathways depends on the nature of the aromatic groups. The metal-containing reductive elimination products, [Et4N][Au(SAr)2] (Ar = C6F5 (2a) or C6F4H (2b)), were confirmed by both independent synthesis and crystallographic characterization. Cross-reactions between either 1 or 2 and various disulfides led to ligand exchange via an addition-elimination process, a previously unknown reaction pathway for ligand exchange at gold(I) centers.

Alkyl Carbon−Nitrogen Reductive Elimination from Platinum(IV)−Sulfonamide Complexes

Platinum(IV) complexes containing monodentate sulfonamide ligands, fac-(dppbz)PtMe(3)(NHSO(2)R) (dppbz = o-bis(diphenylphosphino)benzene; R = p-C(6)H(4)(CH2)(3)CH(3) (1a), p-C(6)H(4)CH(3) (1b), CH(3) (1c)), have been synthesized and characterized, and their thermal reactivity has been explored. Compounds 1a-c undergo competitive C-N and C-C reductive elimination upon thermolysis to form N-methylsulfonamides and ethane, respectively. Selectivity for either C-N or C-C bond formation can be achieved by altering the reaction conditions. Good yields of the C-N-coupled products were observed when the thermolyses of 1a-c were conducted in benzene-d(6). In contrast, exclusive C-C reductive elimination occurred upon themolysis of 1a,b in nitrobenzene-d(5). When the thermolyses of 1a were performed in the presence of sulfonamide anion NHSO2R- in benzene-d(6), ethane elimination was completely inhibited and C-N reductive elimination products were formed in high yield. Mechanistic studies support a two-step reaction pathway involving initial dissociation of NHSO(2)R(-) from the platinum center, followed by nucleophilic attack of this anion on a methyl group of the resulting five-coordinate platinum(IV) cation to form MeNHSO(2)R and (dppbz)PtMe(2). C-C reductive elimination to form ethane occurs directly from the five-coordinate Pt(IV) cation.

Investigation of the Thermal Decomposition of [biphen(QPiPr)Pt(alk)2]: An Entry to C–C Single Bond Activation?

AbstractThe reaction of [(COD)PtCl2] with biphen(QPiPr)(Q′PiPr) (Q, Q′ = O, CH2O) yields cis‐[{biphen(QPiPr)(Q′PiPr)}PtCl2]. Treatment of trans‐[PtCl2(ethene)(py)] with biphen(QPiPr)(Q′PiPr) (Q, Q′ = O, CH2O) gives trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] for Q or Q′ ≠ O. No trans/cis isomerization can be observed for these compounds after 24 h at 105 °C in [D8]toluene. In the case of Q = Q′ = O a trans/cis isomerization takes place at room temperature. X‐ray structures of a molecule in the solid state were determined for the compounds cis/trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] with Q or Q′ ≠ O. In the case of Q = O and Q′ = CH2O the closer of the two bridging C atoms in the biphenyl fragment of the ligand is only 3.42 Å away from the Pt center but does not show a coupling to it in the 13C NMR spectrum. Treatment of both cis‐ and trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] with two equivalents of ethyl‐ or n‐propylmagnesium chloride gives cis‐[{biphen(QPiPr)(Q′PiPr)}PtAlk2]; the X‐ray structures of cis‐[{biphen(CH2OPiPr)2}PtEt2] and cis‐[{biphen(OPiPr)2}Pt(nPr)2] are discussed. These complexes give the related alkene complexes as a result of β‐H elimination/reductive elimination if heated to temperatures above 90 °C in [D8]toluene, but none of the complexes with Q or Q′ ≠ O is stable at 95 °C. The thus‐formed Pt(alkene) complex loses the alkene (ethene or propene) in a consecutive reaction. For [{biphen(CH2OPiPr)2}PtEt2], selective deuteration of the CH2O group and the Pt‐ethyl group proves that scrambling of deuterium into the iPr methyl groups occurs (by a CH activation/insertion/β‐H elimination/reductive elimination sequence) after the β‐H elimination and before the loss of ethene, which at 95 °C is almost as fast as the loss of ethene. In the final alkene‐free 14 e complex a fast dynamic C–H activation of the iPr CH3 protons by the Pt center (one H at a time) can be detected [an Isotopic Perturbation of Resonance (IPR) is found for the partially deuterated compound], which most probably prevents a further reaction of the complex (e.g. activation of the bridging C–C single bond in the biphenyl backbone or permanent coordination of PPh3). H2, however, is activated by the final complex, and the use of D2 proves that deuterium is incorporated into the iPr groups of the ligand. The reaction of [(COD)PtCl2] with o‐Br‐phen(QPiPr) gives cis‐[PtCl2{o‐Br‐phen(QPiPr)}2] (X‐ray structure reported), which forms cis‐[Pt{phen(QPiPr)}2] upon treatment with sBuLi. The reaction of trans‐[PtCl2(ethene)(py)] with o‐I‐phen(QPiPr) gives trans‐[PtCl2{o‐I‐phen(QPiPr)}2] (X‐ray structure reported), which also forms cis‐[Pt{phen(QPiPr)}2] upon treatment with sBuLi. cis‐[Pt{phen(CH2OPiPr)}2] (X‐ray structure reported) slowly isomerizes quantitatively at 95 °C in toluene to the trans complex. No product of reductive elimination can be detected up to 150 °C in mesitylene. cis‐[Pt{phen(OPiPr)}2] (X‐ray structure reported) forms two products ― the trans isomer (X‐ray structure reported) and most likely a dimeric trans‐configured complex ― upon heating to 95 °C. Again, no reductive elimination is observed regardless of whether the reaction is performed under argon or ethene atmosphere up to 150 °C in mesitylene. All materials were characterized by means of 1H, 2H, 13C, 31P, and 195Pt NMR spectroscopy, FAB mass spectrometry, FT‐IR spectroscopy, elemental analysis, and X‐ray crystallography. DFT calculations (B3PW91/LANL2DZ) are also included for the most important compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Aryl-Fluoride Reductive Elimination from Pd(II): Feasibility Assessment from Theory and Experiment

DFT methods were used to elucidate features of coordination environment of Pd(II) that could enable Ar-F reductive elimination as an elementary C-F bond-forming reaction potentially amenable to integration into catalytic cycles for synthesis of organofluorine compounds with benign stoichiometric sources of F(-). Three-coordinate T-shaped geometry of Pd(II)Ar(F)L (L = NHC, PR(3)) was shown to offer kinetics and thermodynamics of Ar-F elimination largely compatible with synthetic applications, whereas coordination of strong fourth ligands to Pd or association of hydrogen bond donors with F each caused pronounced stabilization of Pd(II) reactant and increased activation barrier beyond the practical range. Decreasing donor ability of L promotes elimination kinetics via increasing driving force and para-substituents on Ar exert a sizable SNAr-type TS effect. Synthesis and characterization of the novel [Pd(C(6)H(4)-4-NO(2))ArL(mu-F)](2) (L = P(o-Tolyl)(3), 17; P(t-Bu)(3), 18) revealed stability of the fluoride-bridged dimer forms of the requisite Pd(II)Ar(F)L as the key remaining obstacle to Ar-F reductive elimination in practice. Interligand steric repulsion with P(t-Bu)(3) served to destabilize dimer 18 by 20 kcal/mol, estimated with DFT relative to PMe(3) analog, yet was insufficient to enable formation of greater than trace quantities of Ar-F; C-H activation of P(t-Bu)(3) followed by isobutylene elimination was the major degradation pathway of 18 while Ar/F- scrambling and Ar-Ar reductive elimination dominated thermal decomposition of 17. However, use of Buchwald's L = P(C(6)H(4)-2-Trip)(t-Bu)(2) provided the additional steric pressure on the [PdArL(mu-F)](2) core needed to enable formation of aryl-fluoride net reductive elimination product in quantifiable yields (10%) in reactions with both 17 and 18 at 60 degrees over 22 h.