Chain-walking Process Research Articles

Nickel-Catalyzed Removal of Alkene Protecting Group of Phenols, Alcohols via Chain Walking Process.

An efficient nickel-catalyzed removal of alkene protection group under mild condition with high functional group tolerance through chain walking process has been established. Not only phenolic ethers, but also alcoholic ethers can be tolerated with the retention of stereocenter adjacent to hydroxyl group. The new reaction brings the homoallyl group into a start of new type of protecting group.

Cationic para-benzhydryl substituted α-diimine nickel catalyzed ethylene and 1-decene polymerizations via controllable chain-walking

A series of α-diimine nickel complexes bearing para-benzhydryl groups, {[(2,6-R2-4-CHPh2C6H2N = C)2Nap]NiBr2, Nap: 1,8-naphthdiyl, R = Me (C1); R = Et (C2); R = iPr (C3)}, were synthesized and characterized. Upon activation with AlEt2Cl, these para-benzhydryl substituted catalysts conducted the ethylene and 1-decene polymerizations via controlled chain-walking process. These nickel complexes exhibited good thermal stability and high activities (up to 106 g PE (mol Ni h)−1) towards ethylene polymerization, producing high molecular weight polyethylenes with a relatively low degree of branching and narrow polydispersities (Mw/Mn ≤ 2.01). The catalytic activities [(0.81–2.42) × 106 g PE (mol Ni·h)−1], polyethylene molecular weights [Mn: (7.9–29.3) × 104 g mol−1] and total branching densities (57–102/1000C) can be tuned over a wide range. These controlled branching degrees of the obtained polyethylenes were decreased with increasing the bulkiness of the ligand and decreasing the polymerization temperature. Polymerization of 1-decene using these cationic nickel complexes generated branched poly(1-decene)s (69–87/1000C) with high molecular weights and narrow polydispersities (Mw/Mn = 1.18–1.55).

Synthesis of polyolefin elastomers from unsymmetrical α-diimine nickel catalyzed olefin polymerization

Due to a unique chain walking process, α-diimine nickel catalysts produce variously branched polyolefins using only ethylene as the feedstock.

Ethylene Polymerization by Dinuclear Xanthene-Bridged Imino- and Aminopyridyl Nickel Complexes

A series of xanthene-bridged dinucleating ligands bearing imino- and aminopyridyl moieties and their nickel complexes were synthesized and characterized. The properties of these dinuclear complexes in ethylene polymerization were studied in comparison with the corresponding mononuclear nickel complexes. The iminopyridyl dinuclear nickel complexes activated by methylaluminoxane (MAO) showed higher catalytic activities (up to 2.2 × 106 g of PE (mol Ni)−1 h–1), higher molecular weights, and produced polyethylene with much lower branching density (27/1000C) than their mononuclear analogues. Similar trends were observed for the aminopyridyl dinuclear complexes. A metal–metal cooperativity effect was proposed to be able to slow down the β-hydride elimination and the corresponding chain-walking process. These results clearly demonstrated the great potentials of dinuclear nickel catalysts with the xanthene-bridged coordination modes in controlling the ethylene polymerization process as well as the microstructures...

A unique Pd-catalysed Heck arylation as a remote trigger for cyclopropane selective ring-opening

Combining functionalization at a distant position from a reactive site with the creation of several consecutive stereogenic centres, including the formation of a quaternary carbon stereocentre, in acyclic system represents a pinnacle in organic synthesis. Here we report the regioselective Heck arylation of terminal olefins as a distant trigger for the ring-opening of cyclopropanes. This Pd-catalysed unfolding of the strained cycle, driving force of the chain-walking process, remarkably proved its efficiency and versatility, as the reaction proceeded regardless of the molecular distance between the initiation (double bond) and termination (alcohol) sites. Moreover, employing stereodefined polysubstituted cyclopropane vaults allowed to access sophisticated stereoenriched acyclic scaffolds in good yields. Conceptually, we demonstrated that merging catalytically a chain walking process with a selective C–C bond cleavage represents a powerful approach to construct linear skeleton possessing two stereogenic centres.

Palladium-Catalyzed Long-Range Deconjugative Isomerization of Highly Substituted α,β-Unsaturated Carbonyl Compounds.

The long-range deconjugative isomerization of a broad range of α,β-unsaturated amides, esters, and ketones by an in situ generated palladium hydride catalyst is described. This redox-economical process is triggered by a hydrometalation event and is thermodynamically driven by the refunctionalization of a primary or a secondary alcohol into an aldehyde or a ketone. Di-, tri-, and tetrasubstituted carbon-carbon double bonds react with similar efficiency; the system is tolerant toward a variety of functional groups, and olefin migration can be sustained over 30 carbon atoms. The refunctionalized products are usually isolated in good to excellent yield. Mechanistic investigations are in support of a chain-walking process consisting of repeated migratory insertions and β-H eliminations. The bidirectionality of the isomerization reaction was established by isotopic labeling experiments using a substrate with a double bond isolated from both terminal functions. The palladium hydride was also found to be directly involved in the product-forming tautomerization step. The ambiphilic character of the in situ generated [Pd-H] was demonstrated using isomeric trisubstituted α,β-unsaturated esters. Finally, the high levels of enantioselectivity obtained in the isomerization of a small set of α-substituted α,β-unsaturated ketones augur well for the successful development of an enantioselective version of this unconventional isomerization.

Tandem Long Distance Chain-Walking/Cyclization via RuH2(CO)(PPh3)3/Brønsted Acid Catalysis: Entry to Aromatic Oxazaheterocycles.

A novel route to 1,3-oxazaheterocycles based on cooperative Ru-H/Brønsted acid catalysis is reported. The use of the commercially available RuH2(CO)(PPh3)3 complex allows for an efficient long distance chain-walking process while the Brønsted acid is responsible for generation of an electrophilic iminium ion which is trapped intramolecularly by an alcohol moiety. The alcohol, besides its nucleophilic function, also plays an important role in the stabilization of the Ru catalyst.

Scope and mechanism in palladium-catalyzed isomerizations of highly substituted allylic, homoallylic, and alkenyl alcohols.

Herein we report the palladium-catalyzed isomerization of highly substituted allylic alcohols and alkenyl alcohols by means of a single catalytic system. The operationally simple reaction protocol is applicable to a broad range of substrates and displays a wide functional group tolerance, and the products are usually isolated in high chemical yield. Experimental and computational mechanistic investigations provide complementary and converging evidence for a chain-walking process consisting of repeated migratory insertion/β-H elimination sequences. Interestingly, the catalyst does not dissociate from the substrate in the isomerization of allylic alcohols, whereas it disengages during the isomerization of alkenyl alcohols when additional substituents are present on the alkyl chain.

Reactions of Cationic Palladium α-Diimine Complexes with Nitrogen-Containing Olefins: Branched Polyethylene with Carbazole Functionalities

The reactivity of three nitrogen-containing olefins with a cationic (α-diimine)Pd−Me species was studied to evaluate possibilities for their catalytic copolymerization with ethylene. Allyl dimethyl amine gives 1,2-insertion into the Pd−Me bond, but the resulting five-membered chelate is inert toward ethylene. Olefins bearing carbazole substituents insert to form three-membered chelate complexes after a chain-walking process. These chelates are reactive toward ethylene, and N-pentenylcarbazole was successfully copolymerized with ethylene to yield branched polyethylene copolymers bearing carbazole functionalities. The fluorescence of these copolymers is dependent on the amount of incorporated comonomer.

Ethylene polymerization and oligomerization catalyzed by bulky β-diketiminato Ni(II) and β-diimine Ni(II) complexes/methylaluminoxane systems

β-Diketiminato complexes Ni{(N(Ar)C(Me)) 2CH}Br ( 9), Ni{(N(Ar)C(Me)) 2CH} Pph 3Br ( 10) and β-diimine complexes Ni{(N(Ar)C(Me)) 2CH 2}Br 2 ( 5) (Ar = 2,6-iPr 2C 6H 3 ( a), 2,6-Me 2C 6H 3 ( b)) were used as catalyst precursors for ethylene polymerization in the presence of methylaluminoxane (MAO). High molecular weight ethylene polymers as well as short chain oligomers (C4–C8) were simultaneously produced from the catalysis reactions. Ethylene polymers obtained by using these β-N–N Ni(II)/MAO catalyst systems are mainly methyl branched. Small amounts of even number branches were also observed in the 13C NMR spectra of the obtained ethylene polymers, which are believed generating from the incorporation of simultaneously produced α-olefins. Except methyl branch and the branches derived from the incorporation of α-olefin oligomers, the formation of other branch types via the chain walking process is not favored in β-diketiminato Ni(II) systems.

Control of branch formation in ethylene polymerization by a [Ni(eta3-2-MeC3H4)(diimine)] PF6 / DEAC catalyst system

The polymerization of ethylene mediated by [Ni(eta³-2-MeC3H4){ArN=C(H)C(H)=NAr}] PF6, Ar = 2,6-C6H3iPr2 /DEAC catalyst precursor under mild reaction conditions (reaction temperature between -10 ºC and 25 ºC and ethylene pressure between 109 and 1520 kPa) yields high molecular weight branched polyethylene. The degree of branching was modulated by a careful choice of reaction conditions. Thus, at 0 ºC and 109 kPa, the branching degree was 17 branches/1000 backbone carbon atoms and at 25 ºC, it went up to 90 branches/1000 backbone carbon atoms. The nature of the observed branches (methyl, ethyl and longer), their quantity and distribution along the polymer backbone chain can be rationalized in terms of a chain walking process and control of the extend of isomerization by the steric hindrance of the growing chain.