Oxidative addition to dimethylplatinum (II) compounds containing bulky nitrogen ligands: crystal structures of compounds [PtMe3I{(Me2NCH2CH2NCH)Ar}] (Ar = phenanthryl or anthryl)

Oxidative addition of methyl iodide to platinum (II) compounds [PtMe₂{(Me₂NCH₂CH₂NCH)Ar}] (Ar = phenanthryl or anthryl) produced the corresponding platinum (IV) compounds. Processes aimed at reducing the steric crowding at the coordination sphere of the platinum (IV) centre such as C-C restricted rotation of the pendant part of the ligand leading to rotamers and isomerisation of the CN moiety have been detected in solution. The obtained platinum (IV) compounds were characterised by elemental analyses, mass spectrometry and NMR spectroscopy. According to the crystallographic characterisation, the anthracene derivative gave an E conformer while a Z conformation was obtained for the phenanthrene derivative. In order to rationalize the experimental results, DFT calculations have been performed.     

Mechanistic pathways for oxidative addition of aryl iodides to the low-ligated diethanolamine palladium(0) complex in phosphine-free Heck reactions

A set of reactions of different activated olefins and aryl iodides with the trans-dichlorobis(diethanolamine-N)palladium(II) complex (trans-[PdCl₂(DEA)₂]) as a precatalyst was performed, in the presence of diethanolamine (DEA) as a weak base, and NaOEt as a strong base. It was established that the presence of NaOEt slightly lowered the yields, but significantly accelerated the reactions. This experimental finding is in agreement with our computational investigation that shows that significantly higher activation barrier is required for the preactivation reaction in the presence of a weak base than in the presence of a strong base. The reaction between the catalytically active DEA-Pd(0)-Cl complex, formed in the preactivation reaction, and iodobenzene was investigated using density functional theory. Two mechanisms for the oxidative addition of the activated complex were found. The first mechanism is based on a nucleophilic attack of Pd on I of iodobenzene, and yields an intermediate tetracoordinated Pd complex (aI2). The second mechanism begins with a nucleophilic attack of Pd on the benzene ring, and yields a tricoordinated intermediate complex (bI4). It was concluded, on the basis of structural and energetical properties of aI2 and bI4, that the second mechanism is significantly more favorable. It was shown that the oxidative addition requires noticeable lower activation energy than that required for the preactivation process. Thus, our investigations indicate that oxidative addition is not the rate determining step for the Heck reactions investigated in this work, but preactivation step.     

Cyclometallated platinum(II) compounds with imine ligands derived from amino acids: Synthesis and oxidative addition reactions

The reactions of [PtMe₂(μ-SMe₂)]₂ with imines 4-ClC₆H₄CHNCHRCO₂Me (R = H (1a), Me (1b), ⁱPr (1c), CH₂C₆H₄(4’-OH) (1d), C₆H₅ (1e), CH₂C₆H₅(1f)) derived from natural amino acids produced under mild conditions cyclometallated platinum(II) compounds [PtMe{κ²-(C,N)-4-ClC₆H₃CHNCHRCO₂Me}(SMe₂)] (2a-2f). These compounds gave the corresponding phosphine derivatives [PtMe{κ²-(C,N)-4-ClC₆H₃CHNCHRCO₂Me}(PPh₃)] (3a-3f). The corresponding cyclometallated platinum(IV) compounds [PtMe₂I{κ²-(C,N)-4-ClC₆H₃CHNCHRCO₂Me}(PPh₃)] (4) arising from intermolecular oxidative addition of methyl iodide were obtained with a high degree of stereo selectivity. Analogous results were obtained for imine 2,6-Cl₂C₆H₃CHNCH(CH₂C₆H₅)CO₂Me (1g) in a process involving intramolecular oxidative addition of a C-Cl bond. The obtained compounds were fully characterized including structure determinations for compounds 3f, 4d and 4f.     

Density functional studies of influences of Ni triad metals and solvents on oxidative addition of MeI to [M(CH3)2(NH3)2] complexes and C-C reductive elimination from [M(CH3)3(NH3)2I] complexes

Three metal square planar complexes of the type [M(CH₃)₂(NH₃)₂] (M = Ni, Pd, Pt), with a systematic variation in the metals, are chosen to investigating their S_N2-type oxidative addition reactions with methyl iodide by using the B3LYP levels of theory. The oxidative addition was found to take place via a transition state with a nearly linear arrangement of the I-CH₃-M moiety. Solvation effects in these oxidative addition reactions were also investigated. Considering the nature of the metal centre and solvation effects, the following conclusions emerge: (i) addition of MeI is exothermic for all three metals, and Pt is predicted to react with a much lower barrier than either Pd or Ni. The results describe that the MeI addition would be expected to be more favourable with the complex bearing the third-row metal (platinum) as compared to the other triad metals, nickel or palladium, in which case a more strongly bound MeI adduct is formed with a lower activation barriers and the reaction being more exothermic; (ii) the reaction is very difficult to occur in low polar solvents, such as benzene, due to the high barrier which is induced by dissociation of iodide anion from methyl group, but the reaction easily occurs in polar solvents, such as acetonitrile; this is attributed to the ability of polar solvents to solvate and therefore stabilize the related polar intermediate ion pair. Ethane reductive elimination from the M(VI) complexes fac-[M(CH₃)₃(NH₃)₂I] were also studied, indicating that the Ni(IV) and Pd(IV) complexes are very prone to undergo the reductive elimination while the Pt(IV) analogous is less reactive towards the reductive elimination. The results indicate that in contrast to the Me-Me reductive elimination, the S_N2 oxidative addition reaction of MeI to M(II) is much less sensitive to the nature of the metal centre, suggesting that the nucleophilicity of M(II) in [M(CH₃)₂(NH₃)₂] does not change significantly as one moves from M = Ni to Pt.     

Formation of anionic palladium(0) complexes ligated by the trifluoroacetate ion and their reactivity in oxidative addition

As established previously for Pd(OAc)₂, Pd⁰ complexes are formed in situ from Pd(OCOCF₃)₂ and n equiv. triarylphosphines (4-Z-C₆H₄)₃P (Z = CF₃, F, Cl, H, CH₃; n ? 3). The phosphines are the intramolecular reducing agents and are oxidized to triarylphosphine oxides. The generated Pd⁰ complexes are anionic species ligated by the trifluoroacetate anion: Pd⁰(PAr₃)_n(OCOCF₃)⁻ (n = 2 or 3). Pd⁰(PAr₃)₂(OCOCF₃)⁻ is the reactive species involved in the oxidative addition to PhI. This leads to trans-PhPd(OCOCF₃)(PPh₃)₂, involved in equilibrium with the cationic complex trans-[PhPd(PPh₃)₂(DMF)]⁺, instead of the expected trans-PhPdI(PPh₃)₂ complex. The existence of anionic Pd⁰ complexes ligated by the acetate or trifluoroacetate ions delivered by the precursors Pd(OAc)₂ or Pd(OCOCF₃)₂, respectively, as well as their comparative reactivity in oxidative additions are consistent with theoretical DFT calculations.     

Synthesis of mono- and trinuclear palladium(II) complexes via oxidative addition of a bulky hexathioether containing a disulfide bond to palladium(0)

The oxidative addition reactions of a bulky hexathioether containing a disulfide bond, TbtS(o-phen)S(o-phen)SS(o-phen)S(o-phen)STbt (1) (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, o-phen = o-phenylene), to a palladium(0) complex were studied. In the reaction of 1 with 3 molar amounts of [Pd(PPh₃)₄], a trinuclear palladium(II) complex, [Pd₃{S(o-phen)S}₂{(o-phen)STbt}₂(PPh₃)₂] (2), was formed via three-step palladium insertion reaction including unusual C(aryl)-S bond cleavages. On the other hand, the reaction of 1 with an equimolar amount of [Pd(PPh₃)₄] afforded mononuclear palladium(II) complex having a pseudo-octahedral structure, [Pd{S(o-phen)S(o-phen)STbt}₂] (3). The hexa-coordinated geometry for the palladium center in 3 was confirmed by the atoms in molecule (AIM) analysis, which revealed the presence of the bond critical points between the central Pd atom and the S atoms at the axial positions. In contrast to the bulky system, the reaction of Ph-substituted hexathioether, PhS(o-phen)S(o-phen)SS(o-phen)S(o-phen)SPh (4), with an equimolar amount of [Pd(PPh₃)₄] gave a palladium(II) complex having square-planar structure, [Pd{S(o-phen)S(o-phen)SPh}₂] (5). Theoretical calculations revealed that there is no remarkable difference among the energies of isomers of [Pd{S(o-phen)SPh}₂], 6a-syn, 6a-anti, 6b-syn, and 6b-anti. This result suggests that a reason for the preference of the trans-anti-conformation in 3 is the steric repulsion between the bulky Tbt groups, and that of the cis-syn-conformations in 5 and 6 is the intermolecular interactions.     

Highly regioselective hydroselenation and double-bond isomerization of terminal alkynes with benzeneselenol catalyzed by bis(triphenylphosphine)palladium(II) dichloride

Bis(triphenylphosphine)palladium(II) dichloride (PdCl₂(PPh₃)₂) catalyzes regioselective addition of benzeneselenol to terminal alkynes and the subsequent double-bond isomerization to afford the corresponding internal alkenyl selenides in good yields.     

Thermal characterization and thermal degradation of poly(norbornene-2,3-dicarboxylic acid dialkyl esters) synthesized by vinyl addition polymerization

The thermal properties and degradation behaviors of poly(norbornene-2,3-dicarboxylic acid dialkyl esters) (PNB-dialkyl esters) (alkyl = Me (PNB-Me), Et (PNB-Et), Pr (PNB-Pr), and Bu (PNB-Bu)) were investigated by thermogravimetric analysis (TGA) in dynamic conditions and by infrared (IR) spectroscopy. The PNB-dialkyl esters show good thermal stability up to 350 °C, and the thermal stability decreases in the order Me > Et > Pr > Bu with the increase in size of side chain. The effect of side-chain size on the thermal degradation behaviors of PNB-dialkyl esters is evidenced by one-step thermal degradation profile for PNB-Me while two-step thermal degradation profile for PNB-Et, PNB-Pr, and PNB-Bu. Transformation is deduced to undergo β-hydrogen elimination and formation of anhydride group in the first stage of thermal degradation reaction according to TGA and IR results for PNB-Et, PNB-Pr, and PNB-Bu. The apparent activation energy and thermal degradation model of PNB-dialkyl esters are estimated by means of Ozawa-Flynn-wall method and Phadnis-Deshpande method, respectively.     

Platinum(II)-catalyzed addition of alcohols to alkynes

A simple, high-yielding synthesis of acetals from the platinum(II)-catalyzed addition of alcohols to alkynes is described. The regioselectivity of the method and its mechanism are also discussed.     

Kinetic study of the oxidative addition of methyl iodide to Vaska’s complex in ionic liquids

Oxidative addition of methyl iodide to Vaska’s complex in the ionic liquids 1-butyl-3-methylimidazolium triflate [C₄mim][OTf], [C₄mim] bis(trifluormethylsulfonyl)imide [Tf₂N], and N-hexylpyridinium [C₆pyr][Tf₂N] occurred cleanly to give the expected Ir(III) oxidative addition product. Pseudo-first order rate constants were determined for the oxidative addition reaction in each solvent ([Vaska’s] = 0.25 mM, [CH₃I] = 37.5 mM). The observed rate constants under these conditions were 5-10 times slower than the rate seen in DMF. At high methyl iodide concentrations (>23 mM), the expected first order dependence on methyl iodide was not observed. In each ionic liquid, there was no change in the reaction rates within experimental error over the methyl iodide concentration range of 23-75 mM. At lower methyl iodide concentration, a decrease in rate was observed in [C₄mim][Tf₂N] with decreasing concentration of methyl iodide.     

Theoretical studies of the oxidative addition of PhBr to Pd(PX3)2 and Pd(X2PCH2CH2PX2) (X = Me, H, Cl)

The density functional theory calculations were used to study the influence of the substituent at P on the oxidative addition of PhBr to Pd(PX₃)₂ and Pd(X₂PCH₂CH₂PX₂) where X = Me, H, Cl. It was shown that the C_ipso-Br activation energy by Pd(PX₃)₂ correlates well with the rigidity of the X₃P-Pd-PX₃ angle and increases via the trend X = Cl < H < Me. The more rigid the X₃P-Pd-PX₃ angle is, the higher the oxidative addition barrier is. The exothermicity of this reaction also increases via the same sequence X = Cl < H < Me. The trend in the exothermicity is a result of the Pd(II)-PX₃ bond strength increasing faster than the Pd(0)-PX₃ bond strength upon going from X = Cl to Me. Contrary to the trend in the barrier to the oxidative addition of PhBr to Pd(PX₃)₂, the C_ipso-Br activation energy by Pd(X₂PCH₂CH₂PX₂) decreases in the following order X = Cl > H > Me. This trend correlates well with the filled d_π orbital energy of the metal center. For a given X, the oxidative addition reaction energy was found to be more exothermic for the case of X₂PCH₂CH₂PX₂ than for the case of PX₃. This effect is especially more important for the strong electron donating phosphine ligands (X = Me) than for the weak electron donating phosphine ligands (X = Cl).     

Dimethylplatinum (II) complexes with isocyanocoumarin ligands: the crystal structure of cis-dimethylbis-(7-diethylamino-3-isocyanocoumarin)platinum(II)

Platinum complexes that contain isocyanocourmarin ligands have been prepared. [Pt₂Me₄(μ-SMe₂)₂] and [PtPh₂(SMe₂)₂] react with ligands L, (L = 7-diethylamino-3-isocyanocoumarin, Idc; 7-isocyano-4-methylcoumarin, Mic; 7-isocyano-4-trifluoromethylcoumarin, Tic; 3-chloro-4-methyl-7-isocyanocoumarin, Cmic), to give PtR₂L₂, monomers in high yield. The NMR and IR spectra of these complexes are consistent with cis stereochemistry. The UV-Vis absorption spectra of the complexes show bands assigned to ligand-centered transitions. Excitation into the absorption bands of the Idc complexes gives emission at room temperature in methylene chloride solution. The oxidative addition reaction of two of these complexes with methyl iodide has been studied. Platinum (IV) species with fac geometry have been isolated and characterized. Cis-dimethylbis-(7-diethylamino-3-isocyanocoumarin)platinum(II) was characterized by X-ray diffraction.     

Vinyl sulfone- and vinyl sulfoxide-modified tetrahydrofurans: a preliminary account of the enantiomeric synthesis of and diastereoselectivity of addition to new classes of Michael acceptors

Enantiomerically pure 2-hydroxymethylene substituted-2,5-dihydro-3-(arylsulfonyl)- and 2-hydroxymethylene substituted-2,5-dihydro-3-(arylsulfinyl)-furans have been prepared from easily accessible carbohydrate derivatives for the first time. The strategy for accessing both these sulfones and sulfoxides is more efficient than the methods reported so far for the synthesis of this type of compounds. Hydroxymethylene group is sufficient to impose diastereoselectivity on the addition of a wide range of nucleophiles to vinyl sulfone-modified tetrahydrofurans. The benzyl protected hydroxymethylene group also suppresses the influence of chirally pure sulfoxides as two diastereomeric vinyl sulfoxide-modified tetrahydrofurans afforded the Michael adducts with same configurations at C-3 and C-4; this has been established by oxidizing the adducts, which were found to be identical to the products obtained by adding the same nucleophiles to the corresponding vinyl sulfones. These highly reactive Michael acceptors may be considered as a new addition to the arsenals of synthetic chemists interested in the functionalization of tetrahydrofurans.     

Enantioselective conjugate addition of diethylzinc to acyclic enones using a copper phosphino-phenol complex as catalyst

Newly designed phosphino-phenol 1c was found to be an efficient chiral auxiliary for copper-mediated asymmetric conjugate addition of diethylzinc to acyclic enones. High enantioselectivity up to >99% ee was achieved in the reaction of acyclic α,β-enones.     

A density functional theory study of the oxidative addition of methyl iodide to square planar [Rh(acac)(P(OPh)3)2] complex and simplified model systems

The transition state for the oxidative addition reaction [Rh(acac)(P(OPh)₃)₂] + CH₃I, as well as two simplified models viz. [Rh(acac)(P(OCH₃)₃)₂] and [Rh(acac)(P(OH)₃)₂], are calculated with the density functional theory (DFT) at the PW91/TZP level of theory. The full experimental model, as well as the simplified model systems, gives a good account of the experimental Rh-ligand bond lengths of both the rhodium(I) and rhodium(III) β-diketonatobis(triphenylphosphite) complexes. The relative stability of the four possible rhodium(III) reaction products is the same for all the models, with trans-[Rh(acac)(P(OPh)₃)₂(CH₃)(I)] (in agreement with experimental data) as the most stable reaction product. The best agreement between the theoretical and experimental activation parameters was obtained for the full experimental system.     

New routes to low-coordinate iron hydride complexes: The binuclear oxidative addition of H2

The oxidative addition and reductive elimination reactions of H₂ on unsaturated transition-metal complexes are crucial in utilizing this important molecule. Both biological and man-made iron catalysts use iron to perform H₂ 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. Et₃SiH 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 H₂ or D₂ 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.     

The homologous conjugate addition of thiols to electron-deficient cyclopropanes catalyzed by a calcium(II) complex

The synthesis of γ-sulfanyl malonates was achieved through the addition of thiols to electron deficient cyclopropanes. These reactions are catalyzed by calcium acetylacetonate, Ca(acac)₂. A variety of electron rich and electron deficient thiols were added without the need for prior activation or exogenous base. The thiol additions to donor–acceptor cyclopropanes bearing electron-rich and electron-deficient aromatic and heteroaromatic groups proceeded in good to excellent yields.     

Oxidative addition of organic halides to zerovalent palladium complexes containing rigid bidentate nitrogen ligands

Zerovalent complexes of the type Pd(Ar-BIAN)(alkene), i.e. complexes containing the rigid bidentate nitrogen ligands bis(arylimino) acenaphthene (Ar = p-Tol, p-MeOC₆H₄, o-Tol,o,o′-Me₂C₆H₃, o,o′-ⁱPr₂C₆H₃) and an electron-poor alkene have been shown to react with a variety of (organic) halides RX, including methyl, benzyl, aryl, acyl and allylic halides, to give the corresponding square planar divalent Pd(R)X(Ar-BIAN) or [Pd(η³-allyl)(Ar-BIAN)]X complexes. The new complexes obtained have been fully characterized and their fluxional behaviour in solution studied by ¹H NMR spectroscopy. The rate of oxidative addition of iodomethane to Pd(p-Tol-BIAN)(alkene) complexes was found to decrease with increasing Pd-alkene bond strength, i.e. dimethyl fumarate fumaronitrile, but oxidative addition to the fumaronitrile complex was accelerated by irradiation with a mercury lamp. Oxidative addition of allylic ha     

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

Diacetylplatinum(II) complexes [Pt(COMe)₂()] ( = bpy, 3a; 4,4′-t-Bu₂-bpy, 3b), obtained by the reaction of [Pt(COMe)₂X(H)()] with NaOH in CH₂Cl₂/H₂O, were found to undergo oxidative addition reactions with halogens (Br₂, I₂) yielding the platinum(IV) complexes (trans, OC-6-13)/(cis, OC-6-32) [Pt(COMe)₂X₂()] ( = bpy, X = Br, 4a/4b; I, 4c/4d;  = 4,4′-t-Bu₂-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-Bu₂-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 Me₃SiX/H₂O, yielding diacetyl(hydrido)platinum(IV) complexes [Pt(COMe)₂X(H)()] ( = bpy, X = Cl, 5a; Br, 5b; I, 5c;  = 4,4′-t-Bu₂-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-Bu₂-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.