Catalytic Dehydrohalogenation Research Articles

Transition-Metal (Pd, Ni, Mn)-Catalyzed C-C Bond Constructions Involving Unactivated Alkyl Halides and Fundamental Synthetic Building Blocks.

The catalytic construction of C-C bonds between organohalide or pseudohalide electrophiles and fundamental building blocks such as alkenes, arenes, or CO are widely utilized metal-catalyzed processes. The use of simple, widely available unactivated alkyl halides in these catalytic transformations has significantly lagged behind the use of aryl or vinyl electrophiles. This difference is primarily due to the relative difficulty of activating alkyl halides with transition metals under mild conditions. This Account details our group's work toward developing a general catalytic manifold for the construction of C-C bonds using unactivated alkyl halides and a range of simple chemical feedstocks. Critical to the strategy was the implementation of new modes of hybrid organometallic-radical reactivity in catalysis. Generation of carbon-centered radicals from alkyl halides using transition metals offers a solution to challenges associated with the application of alkyl electrophiles in classical two-electron reaction modes. A major focus of this work was the development of general palladium-catalyzed carbocyclizations and intermolecular cross-couplings of unactivated alkyl halides (alkyl-Mizoroki-Heck-type reactions). Initial studies centered on the use of alkyl iodides in these processes, but subsequent studies determined that the use of an electron-rich ferrocenyl bisphosphine (dtbpf) enables the palladium-catalyzed carbocyclizations of unactivated alkyl bromides. Mechanistic studies of these reactions revealed interesting details regarding a difference in mechanism between reactions of alkyl iodides and alkyl bromides in carbocyclizations. These studies were consistent with alkyl bromides reacting via an autotandem catalytic process involving atom-transfer radical cyclization (ATRC) followed by catalytic dehydrohalogenation. Reactions of alkyl iodides, on the other hand, involved metal-initiated radical chain pathways. Recent studies have expanded the scope of alkyl-Mizoroki-Heck-type reactions to the use of a first-row transition metal. Inexpensive nickel precatalysts, in combination with the bisphosphine ligand Xantphos, efficiently activate alkyl bromides for both intra- and intermolecular C-C bond-forming reactions. The reaction scope is similar to the palladium-catalyzed system, but in addition, alkene regioisomeric ratios are dramatically improved over those in reactions with palladium, solving one of the drawbacks of our previous work. Initial mechanistic studies were consistent with a hybrid organometallic-radical mechanism for the nickel-catalyzed reactions. The novel reactivity of the palladium catalysts in the alkyl-Mizoroki-Heck-type reactions have also paved the way for the development of other C-C bond-forming processes of unactivated alkyl halides, including aromatic C-H alkylations as well as low-pressure alkoxycarbonylations. Related hybrid organometallic-radical reactivity of manganese has led to an alkene dicarbofunctionalization using alkyl iodides.

Palladium-Catalyzed Carbocyclizations of Unactivated Alkyl Bromides with Alkenes Involving Auto-tandem Catalysis.

The development of a general catalytic system for the palladium-catalyzed carbocyclization of unactivated alkyl bromides with alkenes is described. This approach uses a commercially available bisphosphine ligand and avoids the use of carbon monoxide atmosphere present in prior studies involving alkyl iodides. Detailed mechanistic studies of the transformation are performed, which are consistent with auto-tandem catalysis involving atom-transfer radical cyclization followed by catalytic dehydrohalogenation. These studies also suggest that reactions involving alkyl iodides may proceed through a metal-initiated, rather than metal-catalyzed, radical chain process.

Alkene Metathesis Catalysis: A Key for Transformations of Unsaturated Plant Oils and Renewable Derivatives

This account presents the importance of ruthenium-catalysed alkene cross-metathesis for the catalytic transformations of biomass derivatives into useful intermediates, especially those developed by the authors in the Rennes (France) catalysis team in cooperation with chemical industry. The cross-metathesis of a variety of functional alkenes arising from plant oils, with acrylonitrile and fumaronitrile and followed by catalytic tandem hydrogenation, will be shown to afford linear amino acid derivatives, the precursors of polyamides. The exploration of cross-metathesis of bio-sourced unsaturated nitriles with acrylate with further catalytic hydrogenation has led to offer an excellent route to α,ω-amino acid derivatives. That of fatty aldehydes has led to bifunctional long chain aldehydes and saturated diols. Two ways of access to functional dienes by ruthenium-catalyzed ene-yne cross-metathesis of plant oil alkene derivatives with alkynes and by cross-metathesis of bio-sourced alkenes with allylic chloride followed by catalytic dehydrohalogenation, are reported. Ricinoleate derivatives offer a direct access to chiral dihydropyrans and tetrahydropyrans via ring closing metathesis. Cross-metathesis giving value to terpenes and eugenol for the straightforward synthesis of artificial terpenes and functional eugenol derivatives without C=C bond isomerization are described.

Reductive Degradation of Polychlorinated Phenols by Pd/C‐Formate: An Ecoefficient Remediation Method for Aqueous Chlorinated Phenols

AbstractA new catalytic dehydrohalogenation method for chlorinated phenols is described, which can be used to break down chlorinated pollutants in wastewater. It uses a system of Pd‐C as catalyst with sodium formate as reducing agent. This economic method is easy to perform with a complete degradation of the pollutant within 12 to 30 h at room temperature. The ecoefficieny of the procedure is compared with eleven alternative methods showing the special advantages of the method.

Catalytic dehydrohalogenation of alkyl halides by Nb, Mo, Ta, and W halide clusters with an octahedral metal framework and by a Re chloride cluster with a triangular metal framework

Abstract The catalytic dehydrohalogenation of fluoro-, chloro-, bromo-, and iodopentanes to yield pentene is studied in a gas flow reactor under 1 atm of helium or hydrogen. The catalysts used are crushed crystals of molecular Nb, Mo, Ta, and W halide clusters possessing an octahedral metal framework and of a solid state Re chloride cluster possessing a triangular metal framework. All of the former clusters develop catalytic activity for dehydrohalogenation of 1-halogenated pentenes yielding 1-pentene and cis- and trans-2-pentenes in a helium or hydrogen stream when the clusters are preliminarily treated at 300 °C. The Re cluster develops catalytic activity for the same reaction in a stream of helium above 175 °C, while in a stream of hydrogen at 300 °C, it is reduced to metallic Re to catalyze both dehydrohalogenation and hydrodehalogenation (hydrogenolysis) concurrently, yielding pentenes and pentane. Even the C–F bond of 1-fluoropentane is hydrogenolyzed by the Re metal.

A catalyst that plays multiple roles: asymmetric synthesis of beta-substituted aspartic acid derivatives through a four-stage, one-pot procedure.

We report a new method for the catalytic, asymmetric synthesis of beta-substituted aspartic acid derivatives in which the nucleophilic catalyst serves up to four discrete roles in a one-pot procedure: catalytic dehydrohalogenation of acid chlorides to form ketenes; catalytic dehydrohalogenation of alpha-chloroamines to form the corresponding imines; catalyzed [2 + 2]-cycloaddition to produce intermediate acyl beta-lactams; and finally, nucleophilic ring opening to afford optically enriched beta-substituted aspartic acids in high enantioselectivity and diastereoselectivity.

Non-linear chemical dynamics in the activation of haloalkanes by iron(III)

Catalytic dehydrohalogenation of dihaloalkanes with Fe(III) and its mechanism has been investigated. Damped oscillations of the reaction mixture were observed when the reactions were carried out at 50°C in evacuated systems. We have described the main features of this behavior taking into account the organometallic intermediates, the reversibility of the reaction and the catalyst transformation.

Direct formation of cyclohexene via the gas phase catalytic dehydrohalogenation of cyclohexyl halides

The gas phase dehalogenation of cyclohexyl chloride and cyclohexyl bromide (where 423 K≤ T≤523 K) promoted using silica and zeolite supported nickel catalysts in the presence of hydrogen is presented as a viable one step route for the production of cyclohexene. Cyclohexene is generated via the internal elimination of the corresponding hydrogen halide where the process is 100% selective at T≤473 K. At higher temperatures, cyclohexane and benzene were isolated in the product mixture as a result of the combination of catalytic hydrogenolysis, hydrogenation and dehydrogenation steps. Cyclohexene yield was subject to a short term reversible decline with time-on-stream due to a surface poisoning by the hydrogen halide that was produced but the presence of hydrogen served to displace the inorganic halide and extend the productive lifetime of the catalyst. Bromine removal from cyclohexyl bromide was found to be more facile while the use of a higher loaded (15.2% (w/w) as opposed to 1.5% (w/w)) nickel silica is shown to result in appreciably higher dehydrohalogenation rates. Both Na/Y and Ni–Na/Y zeolites promoted cyclohexene formation but exhibited an irreversible deactivation which is attributed to pore blockage by occluded coke. With a view to optimising catalyst efficiency, the effect on cyclohexene yield of varying such process parameters as reaction time and temperature and reactant(s) partial pressures were studied and the catalytic data are compared with thermodynamic predictions.

Removal of chlorinated organic compounds from water by catalytic dehydrohalogenation

A study on the dehydrochlorination of trichloroethylene (TCE) with H 2, by using palladium-on-carbon catalysts, is reported. The dehydrohalogenation reaction was performed under mild conditions either on TCE in water solution or on TCE preadsorbed on the carbon-supported catalyst. The results of this latter method suggest the use of palladium-on-carbon as the bifunctional material: for the adsorption of chlorinated compounds from water and their subsequent catalytic dehydrohalogenation. These environmentally friendly processes for TCE disposal have been evaluated in both, batch and continuous experiments.

Catalytic dehydrohalogenation of polychlorinated biphenyls Part II: Studies on a continuous process

A convenient method for the environmentally friendly disposal of polychlorinated biphenyls (PCB), based on catalytic dehalogenation under mild conditions, is reported. By using 0.5% Pd on carbon in a fixed bed reactor at H 2 atmospheric pressure and at 453–503 K it was possible to obtain high conversion of PCB to HCl and hydrocarbons, mainly biphenyl.

Production of vinyl chloride by catalytic dehydrohalogenation

Modified zeolites useful in the catalytic cracking of 1,2-dichloroethane to form vinyl chloride are disclosed. The modified zeolites have a Si:Al ratio of 10 or more and are treated with Zn or alkaline earth metal ions. A method for use of such catalysts in a cracking process is described.

Synthesis of Poly(m-aniline) by Dehydrohalogenation of m-Chloroaniline

Abstract A reddish-orange colored polymer was synthesized by catalytic dehydrohalogenation of m-chloroaniline. The polymer was confirmed to have a poly(m-aniline) structure which will be of our interest as a basic skeleton for ferromagnetic polymer.

Preparation de l'acide méso diamino‐2, 6 heptane‐dioique‐1, 7‐[3H‐3, 4, 5] (méso‐diamino pimelique‐[3H‐3, 4, 5

AbstractMeso 2, 6‐diamino [3H‐3, 4, 5] 1, 7‐heptanedioc acid‐ (meso [3H‐3, 4, 5] DAP) specific activity 30 Ci/mM was obtained by catalytic dehydrohalogenation of meso 3‐chloro and 4‐chloro DAP. LL, DD and meso DAP were analytically separated by cellulose thin layer chromatography (methanol‐water‐7 : 3).

Catalytic dehydrohalogenation on alumina modified with alkali-metal chlorides

The stopped-flow technique was used to elucidate the mechanistic scheme of dehydrobromination of 1-bromo-2-methylpropane on alumina modified with alkali-metal chlorides. The main product was 2-methylpropene, and it was found to be formed both directly from 1-bromo-2-methylpropane, and through isomerization of the latter to 2-bromo-2-methylpropane. Small amounts of water adsorbed on the surfaces increase the catalytic activity and decrease the retention volumes of the various substances. Hydrogen bromide produced in the reactions does not seem to affect the catalysts. Rate constants at various temperatures and Arrhenius parameters have been determined in order to compare the catalytic activity of the surfaces used.

Catalytic dehydrohalogenation of bromobutanes studied by stopped-flow gas chromatography

Dehydrohalogenation reactions of bromobutanes on alumina modified with potassium bromide were studied by the stopped-flow technique. Isomerization and polymerization reactions of the products were kept to a minimum. The reactions were first-order with respect to the reactant. From 1-bromobutane all four possible alkenes were produced, 1-butene being the main product. From 1-bromo-2-methyl-propane almost exclusively 2-methylpropene was obtained, whereas 2-bromobutane gave cis- and trans-2-butene in about equal amounts. The heat of adsorption of the first two bromides was determined and this was correlated with the Arrhenius parameters to interpret the compensation effect observed. From this correlation certain conclusions about the mechanism of the reactions are drawn. The results are consistent with a two-step mechanism, the second step being rate determining.

Chemistry of the Transition Metals. II. Reaction of Branched Alkyl Chlorides with Titanium

Except for t-butyl chloride, the 1- and 2-methyl substituted ethyl chlorides reacted at ambient temperatures with clean titanium surfaces to produce mixtures of the corresponding alkanes and alkenes; t-butyl chloride gave only isobutene. At elevated temperatures (80–150 °C depending on reactant) the branched alkyl chlorides underwent clean, catalytic dehydrochlorination on hydrochlorinated titanium films to give gaseous olefin and hydrogen chloride, in contrast to the previously observed dehydrohalogenation of n-alkyl halides, where metal was consumed continuously in a non-catalytic fashion.The kinetics of the dehydrochlorination were exceptionally simple and reproducible for a heterogeneous reaction and all reactions were first order in reactant. The reactivity order was t-butyl > i-propyl > neopentyl > i-butyl (ca. 106:6:3:1). Although a slight dependence of activation energy on reactant pressure was observed, all reactants gave a limiting activation energy of 15.5 ± 0.5 kcal/mol and the large variation in activity is attributed to differences in the Arrhenius pre-exponential factor.It is proposed that the non-catalytic and catalytic dehydrohalogenation reactions proceed by different mechanisms, the former via a dissociative chemisorption of the carbon–halogen bond and the latter via a concerted, surface-assisted HCl elimination.

The Catalytic Dehydrohalogenation of Haloethanes on Solid Acids and Bases

Abstract The elimination reactions of haloethanes and halopropanes on solid acids and bases, including a series of metal sulfates supported by silica gel, were studied at 300°C by means of the pulse technique in order to clarify the mechanism of the catalytic dehydrohalogenation. The trans-1,2-dichloroethylene/cis-1,2-dichloroethylene (trans-1,2/cis-1,2) selectivities from 1,1,2-trichloroethanes as well as the activities of the metal sulfates supported by silica gel were well correlated with the electronegativities of the metal ions. Generally speaking, the rates of the dehydrochlorination from chloroalkanes were nearly equal to those of the dehydrobromination from bromoalkanes on the solid acids, but the latter were larger than the former on solid bases. The reactivities of the alkylhalides on solid acids and bases were examined from the electronic view-point, and we tried to correlate them with some reactivity indexes. A kinetic isotope effect with regard to hydrogen was observed on solid bases, but not on solid acids, in the dehydrobromination of 1,2-dibromoethane. From these facts, it was concluded that the dehydrohalogenation proceeds through a carbonium-ion mechanism on solid acids and via an E2 step-by-step mechanism on solid bases.

Preparation of tritium‐labelled D,L‐2,6‐diaminopimelic acid

AbstractOn hydrogenation of the 2,6‐bis(phenylhydrazone) of 2,6‐diketopimelic acid with carrier‐free tritium a very low specific activity was achieved, due to the labile C‐T‐bonds in the α‐positions. 2,6‐Diaminopimelic‐4−3H acid of spec. activity 790 mCi/mmole was obtained by catalytic dehydrohalogenation of 4‐chloro‐2,6‐diaminopimelic acid in 1N ammonia. A new, simple apparatus for catalytic tritiation is also described.

The catalytic dehydrohalogenation of alkyl halides by fused zinc chloride and other molten chlorides

The thermal and catalyzed eliminations of hydrogen chloride from ethyl, isopropyl, and n-propyl chloride have been studied in a stirred flow reactor at atmospheric pressure. The catalysts employed were molten zinc chloride alone, and binary mixtures containing zinc chloride and other metallic chlorides. Both the thermal and catalyzed reactions follow first-order kinetics between 260 and 500 °C. It is suggested that the catalyzed reaction occurs largely on the surface of the molten salt and that the active catalytic species is molecular zinc chloride.