Olefin Exchange Research Articles

Synthesis of a Triphenylphosphinimide-Substituted Silirane as a "Masked" Acyclic Silylene.

Phosphinimides are long known as useful ligands in transition metal chemistry, but examples of these in low-valent silicon chemistry are rather rare. Hence, in this work, we report on the implementation of a triphenylphosphinimide moiety as a ligand of a novel silylene that is trapped as a silirane with cyclohexene. By performing activation reactions with B(p-Tol)3, HSiEt3, N2O, and NH3, we demonstrate that the silirane exhibits a silylene-like behavior, making it a "masked" silylene. Furthermore, we treated the silirane with ethylene, propylene, and trans-butene, which led to an olefin exchange. In the case of ethylene and propylene, an additional insertion of the olefin into the silicon-silicon bonds of the respective siliranes could be achieved. As the insertion of trans-butene was not feasible, we surmise that the scope of this reactivity is restricted by the steric demand of the olefin.

Measuring the Olefin‐to‐Pd(0) Bond Strength: A Kinetic Study Involving Olefin Exchange Reactions on Palladium(0) Complexes Bearing Isocyanide Ligands

AbstractIn this article, the synthesis and characterization of new palladium(η2‐olefin) complexes bearing isocyanides as spectator ligands are described. These derivatives have been tested on olefin exchange reactions, allowing to define a scale of strength of the palladium(0)‐olefin bond. In this way we have proved that dimethyl fumarate can be released by the metal center much more easily than the other olefins and that (Z)‐1,2‐ditosylethene is by far the most firmly anchored to palladium.When the sterically encumbered tetramethylethene‐1,1,2,2‐tetracarboxylate (tmetc) is involved in the olefin exchange, the reaction kinetics can be monitored by UV‐Vis spectrophotometry, showing that the process follows a second‐order rate law. This response indicates that the reaction mechanism is associative, as is also confirmed by a DFT study.

Synthesis and reactivity toward olefin exchange and oxidative addition of some platinum(0) olefin complexes with thioquinolines as spectator ligands

We have synthesized and fully characterized by NMR, IR and elemental analysis sixteen Pt(0) derivatives stabilized by four different thioquinoline spectator ligands and four different deactivated olefins. Moreover we have studied the reactivity and proposed a mechanism for olefin-olefin, olefin-alkyne exchange and oxidative addition of alkyl halides to this new class of Pt(0) derivatives based on detailed kinetic measurements. The kinetic study was carried out by NMR and UV–Vis techniques and in one case we have determined the activation parameters. Finally, we were able to resolve the solid state structure of a couple of diastereoisomers among the four possible isomers generated by the combination of two stereocenters.

Highly selective olefin-assisted palladium-catalyzed oxidative carbocyclization via remote olefin insertion.

A highly selective olefin-assisted palladium-catalyzed oxidative carbocyclization via remote olefin insertion to afford cyclohexenes has been developed. It was shown that the assisting olefin moiety was indispensable for the formation of the cyclohexene product. Furthermore, preliminary studies on chiral anion-induced asymmetrical carbocyclization-borylation of enallenes have been carried out.

β-Hydride Elimination at Low-Coordinate Gold(III) Centers.

This Article reports the first comprehensive study of β-hydride elimination at gold(III). The stability/fate of gold(III) alkyl species have been investigated experimentally and computationally. A series of well-defined cationic cyclometalated gold(III) alkyl complexes [(P,C)gold(III)(R)][NTf2] [(P,C) = 8-diisopropylphosphino-naphthyl; R = Me, nPr, nBu] have been synthesized and spectroscopically characterized. While the cationic gold(III) methyl derivative 3c is stable for days at room temperature, the gold(III) n-propyl and n-butyl complexes 3a,b readily undergo β-hydride elimination at low temperature to generate propylene and 2-butenes, respectively. The formation of internal olefins from the gold(III) n-butyl complex 3b shows that olefin isomerization takes place after β-hydride elimination. Computational studies indicate that this isomerization proceeds through a chain-walking mechanism involving a highly reactive gold(III) hydride intermediate and a sequence of β-hydride elimination/reinsertion into the Au-H bond. The reaction of the cationic gold(III) methyl complex 3c with ethylene was also explored. According to (1)H and (13)C NMR spectroscopy, a mixture of propylene, 1-butene, and 2-butenes is formed. DFT calculations provide detailed mechanistic insights and support the occurrence of migratory insertion of ethylene, β-hydride elimination, and olefin exchange at gold(III).

Mechanism of Rhodium-Catalyzed Formyl Activation: A Computational Study.

Metal-catalyzed transfer hydroformylation is an important way of cleaving C-C bonds and constructing new double bonds. The newly reported density functional theory (DFT) method, M11-L, has been used to clarify the mechanism of the rhodium-catalyzed transfer hydroformylation reported by Dong et al. DFT calculations depict a deformylation and formylation reaction pathway. The deformylation step involves an oxidative addition to the formyl C-H bond, deprotonation with a counterion, decarbonylation, and β-hydride elimination. After olefin exchange, the formylation step takes place via olefin insertion into the Rh-H bond, carbonyl insertion, and a final protonation with the conjugate acid of the counterion. Theoretical calculations indicate that the alkalinity of the counterion is important for this reaction because both deprotonation and protonation occur during the catalytic cycle. A theoretical study into the formyl acceptor shows that the driving force of the reaction is correlated with the stability of the unsaturated bond in the acceptor. Our computational results suggest that alkynes or ring-strained olefins could be used as formyl acceptors in this reaction.

Complete H–D Exchange of Butene via D Absorbed in a Pd–Au Alloy

The H–D exchange reaction is one of the easiest ways to synthesize deuterium-labeled compounds. H–D exchange of olefins has been widely studied on metal surfaces such as Pd and Pt. However, H–D exchange of butene on these surfaces is hardly completed and always accompanied by hydrogenation or dehydrogenation, and its reaction mechanism is yet to be elucidated. In the present study, we investigated the reaction of cis-2- and 1-butenes with H and D atoms absorbed in the near-surface region of Pd70Au30(110) by using thermal desorption spectroscopy (TDS), reflection–absorption infrared spectroscopy (RAIRS), and TDS simulation. We show complete H–D exchange of both butene isomers without hydrogenation and dehydrogenation. Similar product yield distributions for both butene isomers indicate a fast di-σ bond migration on this surface, which is a key for the complete H–D exchange. Our results would lead to understanding and controlling the catalytic activity and selectivity of the Pd–Au alloy and Pd-related surfaces and would open up a new chemistry using absorbed hydrogen in combination with alloyed surfaces.

Reactivity of palladium olefin complexes with heteroditopic NHC–pyridine as spectator ligand toward olefin exchange

We have synthesized some derivatives bearing two strong σ-donor ligands simultaneously coordinated to the palladium centre by exploiting the lability of the pyridine wing of the (1-methyl-3-((6-methylpyridine-2-yl)methyl)-imidazole) (Me-NHC-CH2-Py) moiety as a spectator ligand in Pd(0) complexes stabilized by deactivated olefins. Unprecedented mixed complexes bearing the ligand L (L=triphenylphosphine (PPh3), 2,6-dimethylphenyl isocyanide (DIC), triethyl phosphite (P(OEt)3)) and the ligand Me–NHC–CH2–Py with the uncoordinated dangling pyridine wing were therefore obtained by reacting an equimolar amount of the ligands L with the complex [Pd(η2-olefin)(Me–NHC–CH2–Py)] (olefin=tetramethylethenetetracarboxylate (tmetc), fumaronitrile (fn), maleic anhydride (ma), naphthoquinone (nq)). Moreover, we have determined the equilibrium constants KE(ol) for the olefin exchange in the complex [Pd(η2-olefin)(Me–NHC–CH2–Py)] and the related olefin coordinative ability order.

Synthesis and Reactivity of Ester‐Functionalized 5‐Membered RhI‐κ2‐C,O‐Chelates and Their Relevance in Rh(cod)‐Mediated Carbene Polymerization

AbstractThe previous mechanistic studies of the Rh‐mediated polymerization of carbenes suggests the involvement of organometallic compounds that have been derived from Rh(diene) species that contain a five‐membered chelate ring of the type Rh{κ2‐C,O‐[–CH(COOR)–CH(Pol)–C(OR)=O–]} (Pol = polymer chain), which are characterized by the coordination of the β‐ester group of the growing polymer chain to the metal. Herein we present our efforts to characterize the possibly related RhI(cod){κ2‐C,O‐[–CH(COOR)–CHR′–C(OR)=O–]} (cod = 1,5‐cyclooctadiene) species. These structures can be generated by means of olefin exchange with an allyl complex and with the concomitant release of the corresponding 1,3‐diene, which is derived from the allyl ligand. The product is unstable and further reacts by means of a bimolecular C–H activation process to form a related dinuclear complex [(cod)Rh(μ‐{–CH(COOR)–CH–C(OR)=O–})2Rh(cod)], in which the carbon dianion of the dimethyl or diethyl succinate bridges two Rh(cod) moieties. The complex was synthesized in an independent way and was structurally characterized. We investigated the importance of these complexes in carbene polymerization.

Characterization and dynamics of substituted ruthenacyclobutanes relevant to the olefin cross-metathesis reaction.

The reaction of the phosphonium alkylidene [(H(2)IMes)RuCl(2)=CHP(Cy)(3))](+) BF(4)(-) with propene, 1-butene, and 1-hexene at -45 °C affords various substituted, metathesis-active ruthenacycles. These metallacycles were found to equilibrate over extended reaction times in response to decreases in ethylene concentrations, which favored increased populations of α-monosubstituted and α,α'-disubstituted (both cis and trans) ruthenacycles. On an NMR time scale, rapid chemical exchange was found to preferentially occur between the β-hydrogens of the cis and trans stereoisomers prior to olefin exchange. Exchange on an NMR time scale was also observed between the α- and β-methylene groups of the monosubstituted ruthenacycle (H(2)IMes)Cl(2)Ru(CHRCH(2)CH(2)) (R = CH(3), CH(2)CH(3), (CH(2))(3)CH(3)). EXSY NMR experiments at -87 °C were used to determine the activation energies for both of these exchange processes. In addition, new methods have been developed for the direct preparation of metathesis-active ruthenacyclobutanes via the protonolysis of dichloro(1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)(benzylidene) bis(pyridine)ruthenium(II) and its 3-bromopyridine analogue. Using either trifluoroacetic acid or silica-bound toluenesulfonic acid as the proton source, the ethylene-derived ruthenacyclobutane (H(2)IMes)Cl(2)Ru(CH(2)CH(2)CH(2)) was observed in up to 98% yield via NMR at -40 °C. On the basis of these studies, mechanisms accounting for the positional and stereochemical exchange within ruthenacyclobutanes are proposed, as well as the implications of these dynamics toward olefin metathesis catalyst and reaction design are described.

A Unique Functional Group Transformation of Planar Chiral Diolefinic Organonitrogen Cycles Utilizing PtCl2(2,4,6-trimethylpyridine) Complexes

The planar chiral diolefinic organonitrogen cycles 1 underwent stereospecific and group-selective olefin exchange with 3 to give 4, whose molecular structure was characterized on the basis of NMR and X-ray diffraction. The bonded (E)-olefin moieties in 4 were robust enough to allow untouched (Z)-olefin to undergo epoxidation and hydrogenation to give the corresponding PtCl2(η2-olefin) complexes, whose treatment with PPh3 smoothly liberated their olefinic parts, which would be otherwise difficult to synthesize.

Why Are (NN2)Ni Pincer Complexes Active for Alkyl−Alkyl Coupling: β-H Elimination Is Kinetically Accessible but Thermodynamically Uphill

Isomerization and olefin exchange experiments show that β-H elimination is kinetically viable but thermodynamically unfavorable in [(MeNN2)Ni-alkyl] complexes. The intermediacy of Ni-hydride species was corroborated by a trapping experiment. The alkyl complex [(MeNN2)Ni-propyl] catalyzes olefin isomerization.

Concerning the potential reversibility of carbometalation in alkoxide-directed Ti(Oi-Pr)4-mediated reductive cross-coupling of homoallylic alcohols with aromatic imines

In an attempt to understand the nature of selectivity in Ti-mediated reductive cross-coupling between homoallylic alcohols and imines, we investigated whether thermodynamic equilibration of the presumed organometallic intermediate plays a role in selectivity. No evidence could be found for olefin exchange in preformed azatitanacyclopentanes--an observation that is consistent with a model based on kinetically controlled selective carbometalation.

π Back-Bonding in Dibenzyl-β-diketiminato Copper Olefin Complexes

N,N'-Dibenzyl-4-amino-2-iminopent-3-ene, nacnacBnH, was obtained in a one-step synthesis starting from benzylamine and acetylacetone. Reaction of nacnacBnH with CuOtBu in the presence of various Lewis bases gave the corresponding copper complexes (nacnacBn)CuL (L: 2, styrene; 3, η2-acrylonitrile; 4, allyl phenyl ether; 5, stilbene; 7, xylylisonitrile; 8, triphenylphosphine). With diphenylacetylene (DPA) the dimeric complex {(nacnacBn)Cu}2(μ-DPA), 6, was obtained. In the presence of excess DPA, 6 coordinates additional acetylene to form the monomeric complex (nacnacBn)Cu(DPA), 6b. All complexes, with the exception of 4 and 6b, were characterized by X-ray diffraction studies. Structural and spectroscopic data indicate that π back-bonding in 2−8 is still weak when compared to other transition metals, but stronger than in most Cu(I) complexes. Olefin exchange experiments indicate preferred binding of electron-deficient olefins. Reaction of 3 with morpholine did not yield any hydroamination products, in agreem...

A Mechanistic Investigation on Copolymerization of Ethylene with Polar Monomers Using a Cyclophane-Based Pd(II) α-Diimine Catalyst

A detailed mechanistic investigation of the copolymerization of ethylene and methyl acrylate (MA) by a Pd(II) cyclophane-based alpha-diimine catalyst is reported. Our previous observations of unusually high incorporations of acrylates in copolymerization using this catalyst (J. Am. Chem. Soc. 2007, 129, 10062) prompted us to conduct a full mechanistic study on ethylene/MA copolymerization, which indicates a dramatic departure from normal Curtin-Hammett kinetic behavior as observed in copolymerization using the normal Brookhart type of Pd(II) alpha-diimine catalysts. Further investigation reveals that this contrasting behavior originates from the axial blocking effect of the cyclophane ligand hindering olefin substitution and equilibration. In equilibrium studies of ethylene with nitriles, the cyclophane catalyst was found to more strongly favor the linearly binding nitrile ligands as compared to the standard acyclic Pd(II) alpha-diimine catalysts. Ethylene exchange rates in the complexes [(N--N)PdMe(C(2)H(4))](+) (N--N = diimine) were measured by 2D EXSY NMR spectroscopy and found to be over 100 times slower in the cyclophane case. Measurement of the slow equilibration of ethylene, methyl acrylate, and 4-methoxystyrene in cyclophane-based Pd(II) olefin complexes by (1)H NMR and fitting of the obtained kinetic plots allowed for the estimation of exchange rates and equilibrium constants of the olefins. After extrapolation to typical polymerization temperature, DeltaG(double dagger) = 20.6 and 16.4 kcal/mol for ethylene-methyl acrylate exchange in the forward (ethylene displacement by methyl acrylate) and reverse directions, respectively. These values are of similar magnitude to the previously determined migratory insertion barriers of ethylene (DeltaG(double dagger) = 18.9 kcal/mol) and methyl acrylate (DeltaG(double dagger) = 16.3 kcal/mol) under equivalent conditions, but contrast strongly to the rapid olefin exchange seen in the Brookhart acyclic catalyst. The large barrier to olefin exchange hinders olefin pre-equilibrium, decreasing the cyclophane catalyst's ability to preferentially incorporate one monomer (in this case ethylene) over the other, thus giving rise to high comonomer incorporations.

ChemInform Abstract: Olefin Exchange Mediated Cyclopropanation of Nitriles with Homoallylic Alcohols.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

M‐Terphenylaluminum and ‐gallium Compounds: Synthesis and Conversion into Low‐Coordinate Organogallium Cations

AbstractThe reaction of two or 3 equiv. of m‐terphenyllithium with MCl3 or AlH3·NMe3 (M = Al, Ga) affords the bis(terphenyl)aluminum and ‐gallium chloride compounds [{2,6‐(4‐tBuC6H4)2C6H3}2GaCl] (1), [{2,6‐(3,5‐Me2C6H3)2C6H3}2GaCl] (2), [{2,6‐(4‐tBuC6H4)2C6H3}3Ga] (3), [(2,6‐Mes2C6H3)2AlCl] (5), and [(2,6‐Mes2C6H3)2AlH] (7; Mes = 2,4,6‐Me3C6H2). While the gallium compounds can be obtained at room temperature, heating is required for the aluminum derivatives. The dibutyl compounds [(2,6‐Mes2C6H3)GaBu2] (8) and[(2,6‐Dipp2C6H3)GaBu2] (9; Dipp = 2,6‐iPr2C6H3) have also been synthesized by the reaction of m‐terphenyllithium with ClGaBu2. A metathesis reaction of 2 with Li[Al{OCH(CF3)2}4] gives the ionic species [(2,6‐Mes2C6H3)2Ga]+[Li{Al[OCH(CF3)2]4}2]– (11). The cationic butyl(terphenyl)gallium compounds [(2,6‐Mes2C6H3)GaBu]+ ([13]+) and [(2,6‐Dipp2C6H3)GaBu]+ ([14]+) have been prepared by butanide abstraction with the trityl salts of the weakly coordinating anions [B(C6F5)4]–, [CHB11Br6Me5]–, and [CHB11Cl11]–. These ionic species are stable at room temperature for days or weeks, although their combinations with the borate anion [B(C6F5)4]– suffer from C6F5 migration, which is slow at room temperature and faster at elevated temperatures. The compound [14]+[CHB11Cl11]– is stable at 70 °C for at least 23 h. Addition of 1‐octene to a solution of [14]+[CHB11Cl11]– in C6D6 results in olefin exchange and formation of the octyl species [(2,6‐Dipp2C6H3)Ga(octyl)]+ ([17]+), and a slow alkylation of the solvent to afford various octylbenzenes. Hydrolysis of 8 and [13]+[CHB11Br6Me5]– gives the compounds [{(2,6‐Mes2C6H3)GaBu(μ‐OH)}2] (10) and [(BuGa)4(μ‐OH)6]2+[CHB11Br6Me5]–2 (16). All compounds have been characterized by 1H and 13C{1H} NMR spectroscopy and mass spectrometry, and compounds 3, 5, 7, 10, and 16·4.5C6H6 have also been characterized by single‐crystal X‐ray crystallography. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Non-equilibrium effects in the hydrogenation-mediated isomerization mechanism of olefins during cavitating ultrasound processing

The process whereby cis-olefins isomerize to their trans form is generally understood as occurring through C–H activation of surface bound alkyl radical species. Here we present aqueous phase deuteration results of cis-2-buten-1-ol on Raney Nickel. In the context of the accepted olefin isomerization mechanism, our results illustrate that transition-state theory can accurately model the competition between C–H and C–D activation for olefin exchange (isomerization) for the case of conventional catalytic processing. This is the case also for a catalytic process that includes cavitating ultrasound, although the model then requires a much higher vibrational temperature (at least ∼800 K) in order to simulate the selectivity of the deuterium exchange process. Thus, cavitating ultrasound likely incorporates a high level of molecular vibrational excitation, suggesting that the vibrational temperature is not in equilibrium with the thermal (e.g., translational) temperature as the chemistry proceeds along a traditional reaction path.

Molecular‐Level Design of Efficient Microporous Materials Containing Metal Carboxylates: Inclusion Complex Formation with Organic Polymer, Gas‐Occlusion Properties, and Catalytic Activities for Hydrogenation of Olefins

AbstractFor Abstract see ChemInform Abstract in Full Text.

“Inverse-Electron-Demand” Ligand Substitution: Experimental and Computational Insights into Olefin Exchange at Palladium(0)

The mechanism of olefin substitution at palladium(0) has been studied, and the results provide unique insights into the fundamental reactivity of electron-rich late transition metals. A systematic series of bathocuproine-palladium(0) complexes bearing trans-beta-nitrostyrene ligands (ns(X) = X-C(6)H(4)CH=CHNO(2); X = OCH(3), CH(3), H, Br, CF(3)), (bc)Pd(0)ns(X) (3(X)), was prepared and characterized, and olefin-substitution reactions of these complexes were found to proceed by an associative mechanism. In cross-reactions between (bc)Pd(ns(CH)()3) and ns(X) (X = OCH(3), H, Br, CF(3)), more-electron-deficient olefins react more rapidly (relative rate: ns(CF)()3 > ns(Br) > ns(H) > ns(OCH)()3). Density functional theory calculations of model alkene-substitution reactions at a diimine-palladium(0) center reveal that the palladium center reacts as a nucleophile via attack of a metal-based lone pair on the empty pi orbital of the incoming olefin. This orbital picture contrasts that of traditional ligand-substitution reactions, in which the incoming ligand donates electron density into an acceptor orbital on the metal. On the basis of these results, olefin substitution at palladium(0) is classified as an "inverse-electron-demand" ligand-substitution reaction.