Β-alkyl Elimination Research Articles

Demystifying group-4 polyolefin hydrogenolysis catalysis: Gaseous propane hydrogenolysis mechanism over the same catalysts

A kinetic/mechanistic investigation of gaseous propane hydrogenolysis over the single-site heterogeneous polyolefin depolymerization catalysts AlS/ZrNp2 and AlS/HfNp2 (AlS = sulfated alumina, Np = neopentyl), is use to probe intrinsic catalyst properties without the complexities introduced by time- and viscosity-dependent polymer medium effects. In a polymer-free automated plug-flow catalytic reactor, propane hydrogenolysis turnover frequencies approach 3,000 h-1 at 150 °C. Both catalysts exhibit approximately linear relationships between rate and [H2] at substoichiometric [H2] with rate law orders of 0.66 ± 0.09 and 0.48 ± 0.07 for Hf and Zr, respectively; at higher [H2], the rates approach zero-order in [H2]. Reaction orders in [C3H8] and [catalyst] are essentially zero-order under all conditions, with the former implying rapid, irreversible alkane binding/activation. This rate law, activation parameter, and DFT energy span analysis support a scenario in which [H2] is pivotal in one of two plausible and competing rate-determining transition states-bimolecular metal-alkyl bond hydrogenolysis vs. unimolecular β-alkyl elimination. The Zr and Hf catalyst activation parameters, ΔH‡ = 16.8 ± 0.2 kcal mol-1 and 18.2 ± 0.6 kcal mol-1, respectively, track the relative turnover frequencies, while ΔS‡ = -19.1 ± 0.8 and -16.7 ± 1.4 cal mol-1 K-1, respectively, imply highly organized transition states. These catalysts maintain activity up to 200 °C, while time-on-stream data indicate multiday activities with an extrapolated turnover number ~92,000 at 150 °C for the Zr catalyst. This methodology is attractive for depolymerization catalyst discovery and process optimization.

Ultrasmall amorphous zirconia nanoparticles catalyse polyolefin hydrogenolysis

Carbon–carbon bond cleavage reactions, adapted to deconstruct aliphatic hydrocarbon polymers and recover the intrinsic energy and carbon value in plastic waste, have typically been catalysed by metal nanoparticles or air-sensitive organometallics. Metal oxides that serve as supports for these catalysts are typically considered to be inert. Here we show that Earth-abundant, non-reducible zirconia catalyses the hydrogenolysis of polyolefins with activity rivalling that of precious metal nanoparticles. To harness this unusual reactivity, our catalytic architecture localizes ultrasmall amorphous zirconia nanoparticles between two fused platelets of mesoporous silica. Macromolecules translocate from bulk through radial mesopores to the highly active zirconia particles, where the chains undergo selective hydrogenolytic cleavage into a narrow, C18-centred distribution. Calculations indicated that C–H bond heterolysis across a Zr–O bond of a Zr(O)2 adatom model for unsaturated surface sites gives a zirconium hydrocarbyl, which cleaves a C–C bond via β-alkyl elimination.

Carbon-carbon bond activation by Mg, Al, and Zn complexes.

Examples of carbon-carbon bond activation reactions at Mg, Al, and Zn are described in this review. Several distinct mechanisms for C-C bond activation at these metals have been proposed, with the key C-C bond activation step occurring by (i) α-alkyl elimination, (ii) β-alkyl elimination, (iii) oxidative addition, or (iv) an electrocyclic reaction. Many of the known pathways involve an overall 2-electron redox process. Despite this, the direct oxidative addition of C-C bonds to these metals is relatively rare, instead most reactions occur through initial installation of the metal on a hydrocarbon scaffold (e.g. by a cycloaddition reaction or hydrometallation) followed by an α-alkyl or β-alkyl elimination step. Emerging applications of Mg, Al, and Zn complexes as catalysts for the functionalisation of C-C bonds are also discussed.

Catalytic carbon-carbon bond cleavage and carbon-element bond formation give new life for polyolefins as biodegradable surfactants

Summary Catalytic methods that introduce functional groups via carbon–carbon bond cleavage steps have typically been limited to moieties activated by strain or by directing groups, with few transformations engaging the bonds of only sp 3 -hybridized carbon atoms in saturated hydrocarbons. Here, we report the conversion of catenated carbon chains in polyolefins, which currently represent >50% of discarded plastics, into shorter aliphatic alkylaluminum species via a sequence of zirconation via C–H bond activation, β-alkyl elimination for carbon–carbon bond cleavage, and heterobimetallic alkyl group exchange for carbon–aluminum bond formation. The versatility of aliphatic alkylaluminum species is exemplified by their subsequent conversion into high-value fatty acids or alcohols, which have applications as biodegradable surfactants and detergents. A technoeconomic analysis indicates that fatty alcohols produced from discarded polyolefins are cost competitive with conventional syntheses. Thus, this process could ameliorate economic and environmental challenges of the plastic-waste crisis.

Probing β-alkyl elimination and selectivity in polyolefin hydrogenolysis through DFT

A long chain substrate with [(SiO)3ZrH] has been investigated to elucidate selectivity rules in β-alkyl elimination. DFT studies indicate that polypropylene preferentially undergoes β-Me elimination.

The catalytic cleavage of carbon-carbon double bond in polychloroprene induced by Schwartz's reagent via chlorine self-assisted β-alkyl elimination mechanism

The carbon-carbon double bonds (CC) in polychloroprene (PCP) was broken down by Schwartz's reagent ([Cp2ZrClH]n) under mild conditions. The reaction mechanism for cleaving CC bonds in PCP was studied in detail. It was found that the cleavage pathway was chlorine self-assisted β-alkyl elimination reaction, namely, β-alkyl elimination was promoted while chlorine in PCP was eliminated by releasing Cp2ZrCl2. The molecular weights of chain-scission products were controlled ranging from starting molecular weights of PCP to 0.2 kg mol−1; at the same time, microstructures of chain-scission products were similar to chain structures of original PCP. In addition, chain-scission products could be chain-end functionalized by electrophiles quenching chain scission reaction. More importantly, efficient catalytic chain cleavage was achieved under the synergistic effect of [Cp2ZrClH]n with both LiH and H2.

Cp2ZrHCl induced catalytic chain scission of diene-based polymers under mild conditions: Influence of chemical environment around C=C bonds

Controlled chain scissions of diene-based polymers, including cis-1,4-polybutadiene (cPB), cis-1,4-polyisoprene (cPIP), trans-1,4-polyisoprene (tPIP), 1,4-polychloroprene (PCP) and polycyclooctene (PCOE), were carried out under mild conditions via the hydrozirconation between C=C bonds and bis(cyclopentadienyl) zirconium hydrochloride (Cp2ZrHCl). The molecular weights (Mn) of the chain-scission products mainly depended on the loadings of Cp2ZrHCl and reaction time regardless of reaction media and temperature; however, cis/trans configuration ratio of C=C bonds in chain-scission products was influenced by reaction media, temperature and time. By means of NMR spectroscopy, chain-scission mechanism was confirmed to be β-alkyl elimination reaction. Deuterium (2H) tracing experiments showed that the hydrogenation of C=C bonds was a detrimental competing reaction to β-alkyl elimination. Furthermore, the hydrogenation was strongly influenced by steric and electronic effects around C=C bonds. However, chemical environments around C=C bonds did not affect chain-end functionalization of products, as chain-end alkylzirconium complexes generated from β-alkyl elimination were facilely quenched by electrophiles to obtain chain-end functionalized products. More importantly, under the assistance of hydrogen (H2), highly-efficient catalytic chain scission was achieved. Even more, cross-linked rubbers (such as cross-linked cPIP) could be also cleaved catalytically into soluble oligomers.

β-Alkyl Elimination: Fundamental Principles and Some Applications.

This review describes organometallic compounds and materials that are capable of mediating a rarely encountered but fundamentally important reaction: β-alkyl elimination at the metal-Cα-Cβ-R moiety, in which an alkyl group attached to the Cβ atom is transferred to the metal or to a coordinated substrate. The objectives of this review are to provide a cohesive fundamental understanding of β-alkyl-elimination reactions and to highlight its applications in olefin polymerization, alkane hydrogenolysis, depolymerization of branched polymers, ring-opening polymerization of cycloalkanes, and other useful organic reactions. To provide a coherent understanding of the β-alkyl elimination reaction, special attention is given to conditions and strategies used to facilitate β-alkyl-elimination/transfer events in metal-catalyzed olefin polymerization, which provide the well-studied examples.

Controlled Chain‐Scission of Polybutadiene by the Schwartz Hydrozirconation

Controlled chain-scission of polybutadiene (PB), polyisoprene, and poly(styrene-co-butadiene), induced by bis(cyclopentadienyl) zirconium hydrochloride (Cp(2)ZrHCl), was revealed at room temperature. The chain-scission reaction of linear PB was studied by means of GPC, NMR spectroscopy, and MALDI-TOF-MS. It was confirmed that the molecular weights of degraded products were quasi-quantitatively controlled by Cp(2)ZrHCl loading, irrespective of the starting PB, whereas the microstructure of PB chains was crucial to the scission reaction. The hydrozirconation of model molecules indicated that the existence of an internal double bond in compounds with multiple double bonds was essential for chain cleavage. The chain-cleavage mechanism was proposed to involve hydrozirconation of internal double bonds in PB chains and β-alkyl elimination. Furthermore, metallocene-catalyzed chain-scission by a chain-transfer reaction was developed. It is believed that the reported chain scission offers a promising pathway for end-group functionalization by chain cleavage and presents a new application of Schwartz's reagent.

Low temperature depolymerization from a copper-based aqueous vinyl polymerization system

A low temperature strategy for the aqueous polymerization of N-isopropylacrylamide (NIPAM) has been developed. The radical polymerization system is featured with the exclusive use of CuCl and tris(2-dimethylaminoethyl)amine (Me6TREN), devoid of any externally added organic atom transfer radical polymerization (ATRP) initiator. The polymerization reaction is tolerant of variation on the N-substitution pattern, as demonstrated by the synthesis of a homopolymer of acrylamide (AM) and a copolymer of NIPAM and N,N-dimethylacrylamide (DMAA). Importantly, an intriguing phenomenon of depolymerization has been observed at 0 °C with the addition of either 2,2,6,6-tetramethylpiperidinooxy (TEMPO) or 1,4-benzoquinone (BQ) to the polymerization system. A control experiment carried out with conventional 2,2′-azobisisobutyronitrile (AIBN) radical system indicates that copper is required in the depolymerization process. The depolymerization reaction is therefore proposed to proceed through β-alkyl elimination from CuII coordination center. These observations provide a likely starting point for the establishment of new synthetic paradigms for the precision recycling of vinyl monomers under mild and benign conditions.

Formation of a Bridging Planar Trimethylenemethane Dianion from a Neopentyl Precursor via Sequential β-Alkyl Elimination and C−H Activation

The reaction of neopentyllithium, Me3CCH2Li, with [(C5Me5)2Sm][(mu-Ph)2BPh2], 1, was investigated as a route to the unsolvated alkyl, [(C5Me5)2Sm(CH2CMe3)]x, and found to generate the first f element trimethylenemethane dianion complex, [(C5Me5)2Sm]2[mu-eta3:eta3-C(CH2)3], 2. Formation of the [C4H6]2- trimethylenemethane ligand from the [C5H11]1- neopentyl precursor can be explained by a combination of a beta-methyl elimination reaction to form isobutene and [(C5Me5)2SmMe]3, 3, with subsequent C-H activation reactions. This sequence has been modeled in several ways, including the synthesis of 2 from reactions of 3 with CH2=CMe2 and 3 with the 2-methylallyl complex, (C5Me5)2Sm[CH2C(Me)CH2], 4.

Copolymerization of Carbon Monoxide with Cyclic Compounds Catalyzed by Transition Metal Complexes

This article reviews recent discovery and progress of copolymerization of CO with cyclic monomers such as methylenecyclopropanes and propylene oxide promoted by transition metal complexes. 2-Aryl-1-methylenecyclopropanes react with CO in the presence of the Pd catalyst under mild conditions (1 atm, room temperature) to afford the polyketones. The reaction involves alternating insertion of the two monomers and opening of the three-membered ring of the cyclic monomers. Mechanistic study of the polymer growth revealed that 2-aryl-1-methylenecyclopropane undergoes 1,2-insertion into the Pd–acyl bond and subsequent β-alkyl elimination of the cyclopropylmethylpalladium group. A bicyclic methylenecyclopropane, 7-methylenebicyclo[4.1.0]heptane, also reacts with CO in the presence of the Pd complexes with diimine ligands to afford the polyketone with six-membered rings in the repeating unit. The NMR spectra of the polyketone indicate the regulated tacticity of the monomer units. Co complexes and a mixture of Co and Ru complexes with carbonyl ligands promote the copolymerization of CO with propylene oxide in the presence of amine additives under CO pressure at 50 atm. Structures of the produced polyesters vary depending on the reaction conditions.

遷移金属錯体触媒を用いる炭素‐炭素結合の選択的切断反応 β‐炭素脱離反応を中心に

The development of efficient methods for the selective cleavage of carbon-carbon bonds catalyzed by transition metal complexes is a central and challenging subject of modern organic synthesis. Alkene insertion into metal-alkyl bonds has been recognized as a fundamental step in many catalytic reactions. The reverse reaction, i.e., carbon-carbon bond cleavage via β-carbon elimination, has recently received growing attention. Since Watson and Roe reported the first example of β-methyl elimination of (C5Me5) 2LuCH2CH (CH3) 2, several examples of reversible β-alkyl insertion-elimination at both early and late transition metal centers have been reported. Recently, the reactions which involve catalytic carbon-carbon bond cleavage via β-alkyl elimination are in progress. Here, both stoichiometric and catalytic carbon-carbon bond cleavages via β-alkyl elimination as a key step of the reaction are summarized from the synthetic, industrial and environmental point of view.

Catalytic Hydrogenolysis at Low Temperature and Pressure of Polyethylene and Polypropylene to Diesels or Lower Alkanes by a Zirconium Hydride Supported on Silica-Alumina: A Step Toward Polyolefin Degradation by the Microscopic Reverse of Ziegler-Natta Polymerization.

Cleavage and polymerization with the same catalyst: The catalyst system named in the title, which cleaves polyethylene and polypropylene under a hydrogen atmosphere, is also capable of polymerizing ethylene or propylene. This shows the close relationship between olefin insertion and β-alkyl elimination [Eq. (a), (P)=polymer chain(P)].

Catalytic partial oxidation of alkanes on silver in fluidized bed and monolith reactors

We have examined catalytic oxidation of alkanes on supported Ag catalysts at high temperatures (>800°C) in both fluidized bed and monolithic reactors at contact times of 50–200 and 5–10 ms, respectively. The results obtained on Ag are compared with Pt for both fluidized beds and monoliths. Oxidation of CH 4, C 2H 6, and i-C 4H 10 are examined on 0.25–0.5 wt.% Ag α-Al 2O 3 in fluidized beds while CH 4 and C 2H 6 oxidations and ammoxidations are examined on 4–35 wt.% Ag monoliths supported on α-Al 2O 3. We find that alkanes do not dissociate as easily on Ag as on Pt, and in both types of reactors it is not possible to keep CH 4 ignited on Ag at any feed composition without the presence of NH 3 which is used for light-off. C 2H 6 and i-C 4H 10 are much more reactive than CH 4, and in fluidized beds, we observe high selectivities to olefins (60–75%) at 80 to 90% conversion with no problems of extinction. Also, there is no carbon buildup or evidence of any Ag loss from the catalyst at the high temperatures (ca. 800°C) over at least 16 h of operation. On the Ag monoliths under ammoxidation conditions (alkane + NH 3 + O 2) used for HCN synthesis on Pt monoliths and gauzes, <0.005% HCN is produced from both CH 4 and C 2H 6. For C 2H 6 oxidation, external heat input is necessary to keep the monoliths ignited with C 2H 4 being the dominant product (64% selectivity). Although the performance of Ag is very close to that of Pt, it is difficult to keep C 2H 6 ignited which extinguishes quickly below 800°C. At very high temperatures (>1100°C), metal losses are observed for Ag monoliths. We also examined Au and PtAu monoliths for C 2H 6 oxidation. Although the monoliths can be ignited using NH 3 and O 2, they extinguish as soon as C 2H 6 is introduced. Reactions on Ag appear to be initiated by oxidative H-abstraction from the alkane by a strongly bound oxygen species on the surface to form a surface alkyl. β-H or β-alkyl elimination reactions of the adsorbed alkyl then dominate just as on Pt leading to olefins rather than cracking to C s and H s.

Olefins by Catalytic Oxidation of Alkanes in Fluidized-Bed Reactors

The production of ethylene or syngas from ethane and olefins from propane, n-butane, and isobutane in the presence of air or O2 at atmospheric pressure has been examined over 100 μm α-Al2O3 beads coated with noble metals in a static fluidized bed reactor at contact times from 0.05 to 0.2 s. Variations in feed composition, preheating temperature, and flow rate were examined. For ethane on Pt, we observe selectivities to ethylene in excess of 72%, with conversions above 85% at the oxidative dehydrogenation stoichiometry (alkane/O2 = 2.0) for a 62% single-pass yield which is 5% higher than previously observed on Pt monoliths. On Rh, CO, and H2 (syngas) production dominates with CO and H2 selectivities up to 90% at nearly 100% ethane conversion. With propane and n-butane on Pt, we observe 55-60% and 65-70% selectivity to olefins, respectively, with >90% conversion for both alkanes. Ethylene and propylene are the major olefin products for both alkanes. Ethylene production dominates at higher temperatures and longer contact times while propylene production is favored at lower temperatures and shorter contact times. With isobutane on Pt, we observe 60-70% olefins with 80% conversion with isobutylene and propylene as the dominant olefins. For all alkanes, C2H2 formation was ≤ 0.8% and C4H6 and aromatics were less than 0.05%. Although all experiments were carried out in a regime predicted to be severely coke forming, no carbon buildup was observed on any of the catalysts and the catalysts exhibit no evidence of deactivation over several days. A simple reaction mechanism appears to explain the observed product distributions. Reactions are initiated by oxidative dehydrogenation of the alkane by adsorbed oxygen to form a surface alkyl. On Pt, β-hydrogen and β-alkyl elimination reactions of adsorbed alkyl dominate, which lead to olefins rather than cracking to Cs and Hs. On Rh, Ni, and Pd β-hydrogen elimination is not preferred and cracking to Cs and Hs ultimately leads to syngas production (Rh, Ni) or carbon deposition (Pd).

Olefin insertion and CH activation reactions of organolanthanides

Three general reaction pathways of organolanthanides will be discussed, illustrated by chemistry of the complex M(η 5-C 5Me 5) 2R (M = Lu, Yb, Y; R = H, CH 3). There are: 1. olefin insertion into the MC or MH bonds, 2. β-alkyl elimination - the reverse of olefin insertion into the MC bonds - and 3. CH activation reactions. In particular, the results of a mechanistic study of the exchange reaction of Lu(η 5-C 5Me 5) 2CH 3 with methane, 13C, will be presented.