Hydride Intermediates Research Articles

Regioselectivity in aqueous palladium catalysed hydroxycarbonylation of styrene: a catalytic and mechanistic study

Regioselectivity control was studied in palladium catalysed hydroxycarbonylation of styrene in neat water with water-soluble phosphines, mostly trisulfonated triphenylphosphine, TPPTS, but also N-bis(N',N'-diethyl-2-aminoethyl)-4-aminomethylphenyl-diphenylphosphine, N3P. The factor giving the highest changes in regioselectivity in the TPPTS system, under similar reaction conditions, is the temperature. In the N3P case, only a minor variation in the n/i ratio as a function of temperature is observed. Insitu normal- and high-pressure NMR experiments were performed to obtain further information about the catalytic cycle and the reaction intermediates. Two palladium hydride intermediates, a palladium eta3-benzylic complex and both the branched and linear palladium acyl complexes were identified in the HP NMR experiments. The hydroxycarbonylation in water using styrene as a substrate operates using a hydride mechanism for pathways leading to both linear and branched product. Insertion of styrene in the palladium-hydride bond gives an eta3-benzyl compound. A high CO pressure gives a kinetic preference for the iso-acyl in the next step. In the TPPTS system, at moderate temperatures, the hydrolysis of the iso-acyl is faster than its conversion to the thermodynamically more stable n-acyl. A low n/i therefore requires high pressures and reasonably low temperatures. The N3P ligand always favours the linear product since isomerisation of the iso-acyl to the n-acyl in this system is fast under all conditions investigated.

Unusual Effect of Diethyl Zinc and Triisobutylaluminium in Ethylene/1‐Hexene Copolymerisation using an MgCl2‐Supported Ziegler‐Natta Catalyst

AbstractSummary: The use of triisobutylaluminium and diethyl zinc as cocatalyst and chain transfer agent respectively in ethylene/1‐hexene copolymerisation, carried out with a Ziegler‐Natta catalyst of type MgCl2/TiCl4/diisobutyl phthalate, has been found to give significantly higher comonomer incorporation than was obtained when triethylaluminium was used in combination with diethyl zinc. Triisobutylaluminium reacts readily with diethyl zinc, yielding metallic zinc, most likely via the formation of intermediate hydride species. In contrast, the use of 2,6‐dimethylpyridine as external donor in this system has been found to have relatively little effect on comonomer incorporation and distribution, indicating that a change from tendentially isospecific to syndiospecific propagation in propene polymerisation is not accompanied by a major improvement in comonomer distribution in ethylene/1‐hexene copolymerisation.1H NMR spectrum of the liquid product of the reaction between triisobutylaluminium and diethyl zinc.image1H NMR spectrum of the liquid product of the reaction between triisobutylaluminium and diethyl zinc.

Molybdenum−Sulfur Dimers as Electrocatalysts for the Production of Hydrogen at Low Overpotentials

(CpMomu-S)2S2CH2, 2, and related derivatives serve as electrocatalysts for the reduction of protons with current efficiencies near 100%. The kinetics of the electrochemical reduction process has been studied, and the effects of varying the proton source, the solvent, the cyclopentadienyl substituents, and the sulfur substituents on the catalyst have been examined. The reduction of excess p-cyanoanilinium tetrafluoroborate under a hydrogen atmosphere in 0.3 M Et4NBF4/acetonitrile buffered at pH 7.6 is catalyzed by 2 at -0.64 V versus ferrocene, with an overpotential of 120 mV. Protonation of the sulfido ligand in 2 is an initial step in the catalytic process, and the rate-determining step at high acid concentrations appears to be the elimination of dihydrogen. The elimination may occur either from adjacent hydrosulfido sites or from a hydrosulfido-molybdenum hydride intermediate.

Coordination insertion reactions of acrylonitrile into Pd–H and Pd–methyl bonds in a diimine-palladium(II) system

Acrylonitrile (AN) displaces the ethyl ether ligand of the cationic complex [Pd(N-N)Me(Et 2O)] + (N-N = (2,6-( i-Pr) 2C 6H 3)-N CMeCMe N-(2,6-( i-Pr) 2C 6H 3)) to form the N-bonded AN complex [Pd(N-N)Me(AN)] +, which exists as two interconverting rotamers. On standing or heating, [Pd(N-N)Me(AN)] + undergoes 2,1-insertion to give [Pd(N-N)(CH(CN)CH 2Me)(AN)] +, which undergoes β-hydrogen elimination to give the intermediate hydride, [Pd(N-N)H(AN)] +, which in turn inserts AN to give the cyanoethyl complex [Pd(N-N)(CH(CN)Me)] +. Dimerization of the [Pd(N-N)(CH(CN)CH 2CH 3)] + moiety via bridging nitrile groups also occurs, giving the dicationic species [ Pd ( N - N ) ( CH ( CN ) CH 2 Me ) ] 2 2 + . Although [Pd(N-N)Me(AN)] + does behave as a typical Brookhart ethylene polymerization catalyst, it does not catalyze AN polymerization and in fact added AN suppresses ethylene polymerization.

Hydrogenation properties and structural change of Hf xZr 7− xNi 10 ( [formula omitted

Hf 7Ni 10 has the same crystal structure as Zr 7Ni 10, which shows several unusual hydrogenation properties such as reversible hydrogenation by irreversible phase transformation and structural change caused by hydrogenation followed by dehydrogenation, and its hydrogenation properties have been unknown. The phase relation and hydrogenation properties of Hf x Zr 7− x Ni 10 alloys were investigated in the compositional range from Zr 7Ni 10 ( x = 0 ) to Hf 7Ni 10 ( x = 7 ). All the samples were obtained by arc melting followed by heat treatment in vacuum. The constituent phase was identified by the combination of X-ray diffraction and microscopic analysis and hydrogenation properties were examined by differential scanning calorimetry and Sieverts’ method. No structural change from orthorhombic Cmca type to another one such as tetragonal I 4 / m m m type and orthorhombic Pbca type was observed for the addition of Hf and Hf was substituted for Zr in Zr 7Ni 10 in all the compositional range we examined. The lattice constant of the aimed phase decreased with an increase in Hf content. The measurements using differential scanning calorimetry indicated that the Hf-substituted alloys showed the same phase transformation as Zr 7Ni 10 in hydrogenation, that is the unusual phase transformation that an intermediate hydride phase appears only in dehydrogenation but not in hydrogenation. The transformation temperature decreased with an increase in Hf content.

Ti-Catalyzed Selective Isomerization of Terminal Mono-substituted Olefins

Transition metal complexes have been utilized in the homogeneous catalysis because of selectivity and catalytic activity under mild conditions. The isomerization of olefins catalyzed by transition metal complexes constitutes one of important type of reaction in organometallic chemistry. The isomerization of olefins occurs either by a metal hydride addition-elimination or by a π-allyl metal hydride intermediate. HCo(CO)4, [(C2H4)2RhCl]2, Ni[P(OEt)3]4, and PtCl2(PPh3)2-SnCl2 are effective catalysts for isomerization of olefins via a metal hydride addition-elimination mechanism, and Fe3(CO)12 catalyzed isomerization of 3-ethyl-1pentene and isomerization of 1-heptene catalyzed by (PhCN)2PdCl2 occur via a π-allyl metal hydride mechanism. The cis/trans ratio of 2-butene obtained from isomerization of 1-butene by RhH(CO)(PPh3)3 has also been investigated. The skeletal isomerization of olefins catalyzed by (R3P)2NiCl2 is developed such as conversion of cis-1,4hexadiene to trans-2-methyl-1,3-pentadiene. Titanium complexes serve as an effective catalysts for a variety of reactions such as hydroalumination, hydroboration, and hydrogenation of unsaturated hydrocarbons. We have been interested in the selective reactions of unsaturated hydrocarbons by using titanium and zirconium compounds. The reagent system composed of LiAlH4/Cp2TiCl2 ≤ 2 in the molar ratio promotes the isomerization of 1-octene, but the detailed reaction for isomerization of olefins has not been reported. We report here a selective isomerization of olefins with low valent titanium complex generated from Cp2TiCl2 (Cp=cyclopentadienyl) and LiAlH4.

Synthesis, Spectroscopy, and Hydroformylation Activity of Sterically Demanding, Phosphite‐Modified Cobalt Catalysts

AbstractThe dinuclear complex [Co2(CO)6{P(O‐2,4‐t‐Bu2C6H3)3}2] (6) has been synthesised and fully characterised. X‐Ray crystal‐structure analysis revealed a trans‐diaxial geometry, no bridging carbonyls, and CoCo and CoP bond lengths of 2.706(5) and 2.134(4) Å, respectively. The hydroformylation of pent‐1‐ene in the presence of 6 was studied at 120–180° at pressures between 20 and 80 bar Syngas. High‐pressure (HP) spectroscopy (IR, NMR) was used to detect potential hydride intermediates. HP‐IR Studies revealed the formation of [CoH(CO)3{P(O‐2,4‐t‐Bu2C6H3)3}] (2) at ca. 105°, with no significant amount of [CoH(CO)4] detectable. The intermediate 2 was synthesised and characterised. The formation of the undesired complex [CoH(CO)2{P(O‐2,4‐t‐Bu2C6H3)3}2] was completely suppressed due to the large cone angle of the sterically demanding phosphite.

Olefin hydrogenation using diimine pyridine complexes of Co and Rh

Square-planar cobalt diimine pyridine complexes LCoR (L = 2,6-[RN CMe] 2C 5H 3N; R = n-C 6H 13 for L hex, 2,6-( i-Pr) 2C 6H 3 for L dip) are active in the hydrogenation of monosubstituted and disubstituted olefins; sterically more hindered trisubstituted olefins do not react. For the L dipCo system, a diamagnetic hydride intermediate was observed, although a small amount of paramagnetic product is also formed upon reaction of L dipCoR with H 2. DFT studies suggest a traditional hydrogenation cycle starting with LCoH, except that intermediate LCo(R)(H 2) transfers a hydrogen atom directly from H 2 to the alkyl group in a σ-bond metathesis step, without going through a discrete Co III intermediate. Autoclave experiments show that conversion is not linear in catalyst intake. Diffusion limitation was ruled out as an explanation, and we propose a concentration-dependent catalyst decay. At low catalyst intake conversion rates up to 2 × 10 4 (mol octene/mol Co/bar/h) can be reached. Reducing the steric bulk at the imine positions (L dip → L hex), or changing the metal from cobalt to rhodium, do not alter the activity or specificity of the hydrogenation much. For the L hexCo and L dipRh systems, no diamagnetic products corresponding to L dipCoH were observed.

Diphenylphosphide-Bridged Diiron Derivatives of [Fe2(η5-C5H5)2(μ-H)(μ-PPh2)(CO)2

The complex trans-[Fe2Cp2(μ-H)(μ-PPh2)(CO)2] is obtained in 91% yield by refluxing toluene solutions of [Fe2Cp2(CO)4] (Cp = η5-C5H5) and the secondary phosphine PPh2H. This compound isomerizes upon irradiation with visible−UV light under a CO atmosphere to yield cis-[Fe2Cp2(μ-H)(μ-PPh2)(CO)2]. The above hydride complexes react under photochemical conditions with 1 equiv of secondary phosphines PR2H (R = Et, Ph) to give the corresponding monocarbonyl compounds [Fe2Cp2(μ-PPh2)(μ-PR2)(μ-CO)] via the hydride intermediates [Fe2Cp2(μ-H)(μ-PPh2)(CO)(PR2H)] (detected and isolated for R = Et). Deprotonation of trans-[Fe2Cp2(μ-H)(μ-PPh2)(CO)2] with LiBu gives the binuclear anion [Fe2Cp2(μ-PPh2)(CO)2]-. This highly nucleophilic carbonylate reacts rapidly with [AuCl(PiPr3)] or MeI to give the corresponding gold diiron cluster [AuFe2Cp2(μ-PPh2)(CO)2(PiPr3)] or methyl derivative [Fe2Cp2(Me)(μ-PPh2)(μ-CO)(CO)2], respectively. Both hydrides cis- and trans-[Fe2Cp2(μ-H)(μ-PPh2)(CO)2] can be reversibly oxidized at low temperature to the corresponding cation radicals cis- and trans-[Fe2Cp2(μ-H)(μ-PPh2)(CO)2]+. At room temperature, however, the trans dicarbonyl cation isomerizes to its cis isomer, which in turn experiences a degradation process involving the reductive elimination of the bridging groups. The structures of the new complexes are analyzed on the basis of the corresponding IR and NMR (1H, 31P and 13C) spectroscopic data. The nature of the new radical cations is analyzed also on the basis of cyclic voltammetry and ESR measurements.

Dehalogenation of Hexachlorocyclohexanes and Simultaneous Chlorination of Triethylsilane Catalyzed by Rhodium and Ruthenium Complexes

Complexes RhCl(PPh3)3 and RhH2Cl(PiPr3)2 catalyze the dehalogenation of γ-hexachlorocyclohexane to benzene and the simultaneous chlorination of HSiEt3 to ClSiEt3. In the presence of RhCl(PPh3)3, the dehalogenation is inhibited by addition of PPh3. 1H and 31P{1H} NMR spectroscopy studies at variable temperature, on the behavior of the complex RhCl(PPh3)3 in the presence of γ-hexachlorocyclohexane and/or HSiEt3, indicate that the catalytic reaction involves the initial interaction of the catalyst with the silane and that chloro−hydride intermediates related to RhH2Cl(PPh3)3 are the main species during the catalysis. The complexes RuHCl(PPh3)3, RuH2Cl2(PiPr3)2, and RuHCl(η2-H2)(PiPr3)2 also catalyze the dehalogenation of γ-hexachlorocyclohexane and the simultaneous chlorination of HSiEt3 to ClSiEt3. In these cases, the dehalogenation products are 6/4 cyclohexene/cyclohexane (RuHCl(PPh3)3) and 4/6 cyclohexene/benzene (RuH2Cl2(PiPr3)2 and RuHCl(η2-H2)(PiPr3)2) mixtures. The dehalogenations of α- and δ-hexachlo...

Synthesis, crystal structure and hydroformylation activity of triphenylphosphite modified cobalt catalysts.

The dinuclear complex [Co(2)(CO)(6)[P(OPh)3]2] (2) has been synthesised and was fully characterised. The solid state structure revealed a trans diaxial geometry, no bridging carbonyls, and Co-Co and Co-P bond lengths of 2.6722(4) and 2.1224(4) Angstrom, respectively. Catalysed hydroformylation of 1-pentene with 2 was attempted at temperatures in the range 120 to 210 degrees C and pressures between 34 and 80 bar. High pressure spectroscopy (HP-IR and HP-NMR) was used to detect hydride intermediates. High pressure infrared (HP-IR) studies revealed the formation of [HCo(CO)(3)P(OPh)3] (4) at ca. 110 degrees C, but at higher temperatures absorption bands corresponding to [HCo(CO)(4)]() were observed. The hydride intermediate 4 has also been synthesised and characterised. Upon increased ligand concentration, HP-IR studies showed the formation of new carbonyl absorption bands due to a higher substituted cobalt carbonyl complex-[HCo(CO)(2)[P(OPh)3]2] (5), which is believed to be catalytically less active. Complex 5 has been synthesised independently and was fully characterised. A low temperature crystal structural study of 5 revealed a trigonal bipyramidal structure with a trans H-Co-CO arrangement and two equatorial phosphite ligands, the Co-P bond lengths being 2.1093(8) and 2.1076(8)[Angstrom], respectively.

Formation and reactions of metal–metal bonded heterobimetallic (C5H4PR2)-bridged (Zr–Ir) and (Zr–Rh) complexes: evidence for the participation of metal hydride intermediates

Treatment of the complexes [(C(5)H(4)PR(2))(2)Zr(CH(3))(2)](b: R = isopropyl; c: R = cyclohexyl) with the reagent HIr(CO)(PPh(3))(3) (2b) yield the heterobimetallic complexes [mu-C(5)H(4)PR(2))(2)(H(3)C-Zr-Ir(CO)(PPh(3)))] (4b, 4c) with evolution of methane. The reaction of the -PPh(2) substituted analogue with initially yields an intermediate [(H(3)C)(2)Zr(mu-C(5)H(4)PPh(2))(2)Ir(H)(CO)(PPh(3))] 5a, that still contains both methyl groups at zirconium and does not contain a metal-metal bond. At room temperature, the intermediate reacts further with methane formation to eventually yield the (Zr-Ir) complex 4a. The corresponding [mu-C(5)H(4)PR(2))(2)(H(3)C-Zr-Rh(CO)(PPh(3)))] complexes 3a (R = Ph) and 3b (R = isopropyl) react cleanly with isopropyl alcohol to liberate methane and yield the corresponding [mu-C(5)H(4)PR(2))(2)(Me(2)CHO-Zr-Rh(CO)(PPh(3)))] products (7a, 7b). Carefully monitoring the reaction of with Me(2)CHOH by NMR revealed that the Zr-Rh functionality is attacked first to give the intermediate [Me(Me(2)CHO)Zr([micro sign]-C(5)H(4)PR(2))(2)Rh(H)(CO)(PPh(3))] (6b). This intermediate then reacts further to cleave off methane and re-form the (Zr-Rh) metal-metal bond to yield the product 7b. The tetrametallic mu-oxo-(Zr-Rh) metallocene derivate 11a was obtained starting from the (Zr-Rh) complex 3a and it was characterized by X-ray diffraction. It may be that this reaction is also initiated by H-OH addition to the [Zr-Rh] metal-metal bond.

Palladium catalyzed transformations of monoterpenes: stereoselective deuteriation and oxidative dimerization of camphene

Camphene undergoes a highly regio and stereoselective palladium catalyzed deuteriation in deuteriated acetic acid solutions of Pd(OAc) 2. NMR reveals that an outward oriented vinylic hydrogen is selectively exchanged for 2H, resulting in 90% camphene- d 1 (ca. 100% stereoselectivity) and 10% camphene- d 2 at 75% conversion of camphene (6 h, 25 °C). Neither π-allyl nor π-olefin palladium complexes are formed in detectable concentrations during the reaction, whereas palladium hydride (singlet at −6.86 ppm) and palladium deuteride (singlet at −6.78 ppm) intermediates have been detected by 1H and 2H NMR, respectively. At higher temperature, oxidative coupling of camphene readily occurs giving the ( E, E)-diene, i.e., bis(3,3-dimethyl-2-norbornylidene)ethane, which formally originates by abstracting the outward oriented vinylic hydrogens and coupling the resulting fragments of two camphene molecules. The reaction is catalytic at palladium in the Pd(OAc) 2–LiNO 3(cat)–O 2 and Pd(OAc) 2–benzoquinone systems. Similar mechanisms for the deuteriation and oxidative coupling of camphene are proposed, which involve the formation of σ-vinyl palladium hydride intermediates. No deuteriation neither oxidative coupling of limonene, myrcene and β-pinene were observed under the same conditions.

Hydrosilylation of alkenes and ketones catalyzed by nickel(II) indenyl complexes

Abstraction of Cl– from the complexes (indenyl)Ni(PPh3)Cl generates cationic species that are effective precatalysts for the hydrosilylation of some olefins and ketones. For instance, the mixture of (1-Me-indenyl)Ni(PPh3)Cl and NaBPh4 (or methylaluminoxane) reacts at room temperature with ca. 100 equiv. each of PhSiH3 and styrene to produce [1-phenyl-1-ethyl](phenyl)silane, PhCH(CH3)(SiPhH2), in 50%–80% yield. The same system can also catalyze the hydrosilylation of 1-hexene and norbornene, but the products arising from these substrates consist of mixtures of regio- and stereoisomers. On the other hand, ketone hydrosilylation is regiospecific, giving the corresponding silyl ethers in high yields. A number of experimental observations have indicated that the initially generated Ni-based cation is not the catalytically active species. Indeed, the cationic initiators may be replaced by LiAlH4 or AlMe3, which generate the corresponding Ni-H or Ni-Me derivatives, respectively. Moreover, the observed regioselectivity for the addition of PhSiH3 to styrene (i.e., predominant addition of the silyl fragment to the α-C) is opposite of what would be expected if the reaction mechanism involved carbocationic intermediates. A new mechanism is proposed in which the active species is a Ni-H species originating from the transfer of H– from PhSiH3 to the initially generated Ni cation. Key words: hydrosilylation, nickel indenyl complexes, cationic complexes, hydride intermediates.

Another unusual phenomenon for Zr 7Ni 10: structural change in hydrogen solid solution and its conditions

Zr 7Ni 10 has three hydrogen occlusion phases, α, β and γ, and the following unusual features are known for the phase transitions in the Zr 7Ni 10–H 2 system: (1) The intermediate hydride phase ( β) appears only during dehydrogenation but not during hydrogenation, and (2) The continuous hydrogen solid solution phase ( α) exhibits a much higher hydrogen solubility during hydrogenation than during dehydrogenation. In order to clarify the mechanism about the difference in the hydrogen solubility of the α phase, the relation between the pressure-composition isotherms and corresponding structural change has been examined by a conventional volumetric method and X-ray diffraction. Through the examination, we discovered that the crystal structure of the α phase, which undergoes hydrogenation followed by dehydrogenation, is different from that of its pure metal phase, where the crystal structure of the dehydrogenated α phase changes from an orthorhombic structure to a tetragonal structure. The conditions causing the structural change were then examined, and it has been found that the α phase maintains its original orthorhombic structure as long as it is hydrogenated so as not to absorb enough hydrogen to change it to the hydride with a higher hydrogen content ( γ). The phenomenon can be understood as one of the hydrogen-assisted phase transitions such as hydrogen-induced amorphization (HIA) in the sense that the phase transition requires hydrogenation under special conditions.

Catalytic Reduction of Carbonyl Functional Groups in 2-Propanol by Molybdocene Hydrides

The catalytic reduction of acetophenone in 2-propanol by the organometallic complex [Cp2Mo(μ-OH)2 MoCp2](OTs)2 (I) proceeds through a molybdocene hydride intermediate. Activation parameters for the reduction of acetophenone to 1-phenylethyl alcohol in 2-propanol-d8 are 19 kcal mol-1 and −26 cal mol-1 K-1 for ΔH⧧ and ΔS⧧, respectively. These parameters suggest 2-propanol and I are involved in a C−H activation step to form a molybdocene monohydride that may be the active species in the hydrogenation catalysis. When the catalysis reaction was performed with (CH3)2CHOD as the solvent, 1-deuterio-1-phenylethyl alcohol was almost quantitatively formed. This is consistent with a mechanism where a Cp2Mo hydride, formed in the C−H activation of (CH3)2CHOD, is converted to the deuteride by the solvent's alcoholic deuteron. The labeling experiment further supports the a molybdocene hydride as the catalytic species.

Theoretical study on the mechanism of iron carbonyls mediated isomerization of allylic alcohols to saturated carbonyls.

The conversion of allylic alcohols to enols mediated by Fe(CO)(3) has been studied through density functional theoretical calculations. From the results obtained a complete catalytic cycle has been proposed in which the first intermediate is the [(allyl alcohol)Fe(CO)(3)] complex. This intermediate evolves to the [(enol)Fe(CO)(3)] complex through two consecutive 1,3-hydrogen shifts involving a pi-allyl hydride intermediate. The highest Gibbs energy transition state corresponds to the partial decoordination ot the enol ligand prior to the coordination of a new allyl alcohol molecule that regenerates the first intermediate. Alternative processes for the [(enol)Fe(CO)(3)] complex such as [Fe(CO)(3)]-mediated enol-aldehyde transformation and enol isomerization have also been considered. The results obtained show that the former process is unfavourable, whereas the enol isomerization may compete with the enol decoordination step of the catalytic cycle.

Unusual 1‐Alkyne Dimerization/Hydrogenation Sequences Catalyzed by [Ir(H)2(NCCH3)3(P‐i‐Pr3)]BF4: Evidence for Homogeneous‐Like Mechanism in Imidazolium Salts

AbstractThe reaction of complex [Ir(H)2(NCCH3)3(P‐i‐Pr3)]BF4 (1) with excess of 1‐alkynes such as t‐BuC≡CH and PhC≡CH gave the butadiene compounds Ir[η4‐(R)2C4H4](NCCH3)2(P‐i‐Pr3)}BF4 (R=t‐Bu, 5; R=Ph, 6). Compound 5 was obtained as a single isomer containing a 1,3‐disubstituted butadiene ligand, whereas 6 was formed as a 7:1 mixture of isomers containing 1,3‐ and 1,4‐disubstituted butadienes, respectively. Spectroscopic observations showed alkenyl hydride reaction intermediates, consistent with a double insertion/C‐C coupling sequence. Complexes 5 and 6 were found to react with dihydrogen to give 1 and alkenes resulting from the partial hydrogenation of the butadiene moieties. This dimerization/hydrogenation sequence has been found to be the major reaction of t‐BuC≡CH under conditions of homogeneous hydrogenation whereas that of PhC≡CH produced styrene and ethylbenzene as major products. Similar selectivity was observed for these hydrogenations using organic/ionic liquid biphasic conditions with toluene/BMIM⋅BF4, suggesting reaction mechanisms similar to those operating under homogeneous conditions. This conclusion is also supported by the spectroscopic observation of alkenyl hydride intermediates during the formation of 6 in BMIM⋅BF4 as solvent.

Alkyl isomerisation in three-coordinate iron(II) complexes.

The tertiary to iso-butyl isomerisation of three-coordinate iron(II) diketiminate complexes is reported and a hydride intermediate is proposed on the basis of exchange experiments.

Transition metal hydrides as active intermediates in hydrogen transfer reactions

Catalytic hydrogen transfer reactions involving transfer hydrogenation of ketones, imines as well as Oppenauer-type oxidation of alcohols occur via active transition metal hydride intermediates. In the RuCl 2(PPh 3) 3-catalyzed hydrogen transfer reaction a dramatic rate enhancement by base was observed. It was found that the role of base is to generate a highly active dihydride catalyst RuH 2(PPh 3) 3. The mechanism of Ru-, Rh-, and Ir-catalyzed hydrogen transfer was probed by using α-deuterated alcohol as hydrogen donor and measuring the amount of deuterium transferred to the keto carbon of the hydrogen acceptor. Two different mechanisms are proposed for the transition metal-catalyzed hydrogen transfer, one via a monohydride (giving a high D-content) and another via a dihydride (giving about half of D-content).