Density Functional (DFT) Study of the Anti−Syn Isomerization of the Butenyl Group in Cationic and Neutral (Butenyl)(butadiene)(monoligand)nickel(II) Complexes

Attractive Nonbonded Interactions Help Stabilize the Z Form of Alkenyl Anions

Alkenyl anions (^(-)H₂C-CH═CH-Y; Y = aliphatic, aromatic, amine, ether) are often thermodynamically stable in the sterically congested Z form. This preference for Z structures is used to control regiochemistry in organometallic and Grignard reactions, allyl amine rearrangements to enamines, and allyl ether rearrangements to enol ethers. Explanations for Z stability in alkenes (Y = CH₂R) typically invoke through-space attraction (Coulomb or charge transfer) between the formally anionic carbon C1 and the Y = CH₂R hydrogens. However, this explanation is difficult to generalize to amines and ethers. We suggest that the orbital-driven so-called "attractive nonbonded interactions" suggested to stabilize the Z forms of 1,2-difluoroethylene and 1-substituted propenes also help stabilize Z alkenyl anions. We present electronic structure calculations and surveys of the experimental literature to show these effects' relevance for alkenyl anions. Our results suggest new approaches for regiocontrol in reactions with alkenyl anion intermediates and motivate revisiting older orbital-based theories of "attractive nonbonded interactions".

cis-1,4-Selective Copolymerization of Ethylene and Butadiene: A Compromise between Two Mechanisms

Introducing ethylene units into polybutadiene backbones is an approach to synthesize advanced rubber materials, which has been a research challenge because of distinct polymerization mechanisms of the two monomers. To date, only trans-1,4- and 1,2-regulated copolymers have been obtained. Herein, we reported the unprecedented cis-1,4 selective copolymerization of ethylene and butadiene by using the thiophene-fused cyclopentadienyl-ligated scandium complexes. The effects of the sterics and electronics of the catalytic precursors as well as the monomer loading mode on the activity and selectivity as well as the sequence lengths were investigated, and the mechanism was elucidated. Thus a novel ethylene-based rubber material possessing a high molecular weight, 80 % cis-1,4 regularity and a T_g =-94 °C without an obvious melting point owing to short polyethylene sequences even at its content up to 45 mol %, was isolated. This new rubber material exhibited excellent anti-flowing performance and strong tensile strength.

DFT Studies on cis-1,4-Polymerization of Dienes Catalyzed by a Cationic Rare-Earth Metal Complex Bearing an Ancillary PNP Ligand

Dnsity functional theory (DFT) calculations have been carried out for the highly selective cis-1,4-polymerization of butadiene catalyzed by a cationic rare-earth metal complex bearing an ancillary PNP ligand. It has been found that the chain initiation and propagation of butadiene polymerization occurs via the favorable cis-1,4-insertion route. The trans-1,4 and 1,2-insertion are unfavorable both kinetically and thermodynamically. The chain growth follows the π-allyl-insertion mechanism. The analyses of energy decomposition of transition states indicate that the likelihood of rival insertion pathways is predominantly controlled by the interaction energy of butadiene with a metal center and the deformation energy of butadiene moiety. The electronic factor of the central metal has a decisive influence on the cis- vs. trans-insertion and the regioselectivity (cis-1,4- vs. cis-1,2-insertion) is mainly determined by steric hindrance. Tetrahydrofuran (THF) coordination made monomer insertion less favorable compared with THF-free case and had more noticeable impact on the trans-monomer insertion compared with the cis case. During the chain propagation, cis-insertion of monomer facilitates THF de-coordination and the THF molecule could therefore dissociate from the central metal.

Mechanistic insight into the selective trans-1,4-polymerization of butadiene by terpyridine–iron(II) complexes — A computational study

The density functional theory (DFT) method has been employed to unravel mechanistic intricacies of the 1,4-polymerization of 1,3-butadiene mediated by the [(η3-RC3H4)FeII(C15H11N3)(η2-C4H6)]+ terpyridine–iron(II) active catalyst species. The π-allyl-insertion mechanism is operative for chain growth, whilst the alternative σ-allyl-insertion mechanism has been explicitly demonstrated as being inoperable. This study elucidates the mechanism of cis–trans regulation and unveils the factors that govern the observed high trans-1,4 stereoselectivity, in particular, the discriminative role of allylic isomerization. An atactic trans-1,4-polydiene is expected from polymerization of a terminally monosubstituted butadiene, the experimental results of which have not been reported thus far.

Organolanthanide-Mediated Intermolecular Hydroamination of 1,3-Dienes: Mechanistic Insights from a Computational Exploration of Diverse Mechanistic Pathways for the Stereoselective Hydroamination of 1,3-Butadiene with a Primary Amine Supported by anansa-Neodymocene-Based Catalyst

The complete catalytic reaction course for the organolanthanide-mediated intermolecular hydroamination of 1,3-butadiene and n-propylamine by an archetypical [Me2Si(eta5-Me4C5)2NdCH(SiMe3)2] precatalyst was critically scrutinized by employing a reliable gradient-corrected DFT method. A free-energy profile of the overall reaction is presented that is based on the thorough characterization of all crucial elementary steps for a tentative catalytic cycle. A computationally verified, revised mechanistic scenario is proposed which is consistent with the experimentally derived empirical rate law and accounts for crucial experimental observations. It involves kinetically mobile reactant association/dissociation equilibria and facile, reversible intermolecular diene insertion into the Nd-amido bond, linked to turnover-limiting protonolysis of the eta3-butenyl-Nd functionality. The computationally predicted effective kinetics (Delta(tot) = 11.3 kcal mol(-1), Delta(tot) = -35.7 e.u.) are in reasonably good agreement with experimental data for the thoroughly studied hydroamination of alkynes. The thermodynamic and kinetic factors that determine the almost complete regio- and stereoselectivity of the mechanistically diverse intermolecular 1,3-diene hydroamination have been unraveled. The present computational study complements experiments because it allows, first, a more detailed understanding and a consistent rationalization of the experimental results for the hydroamination of 1,3-dienes and primary amines and, second, enhances the insights into general mechanistic aspects of organolanthanide-mediated intermolecular hydroamination.

Organolanthanide-Mediated Intramolecular Hydroamination/Cyclization of Conjugated Aminodienes: A Computational Exploration of Diverse Mechanistic Pathways for the Regioselective Generation of Functionalized Azacycles Supported by a Lanthanocene-Based Catalyst Complex

The complete catalytic reaction course for the organolanthanide-mediated intramolecular hydroamination/cyclization (IHC) of (4E,6)-heptadien-1-amine by a prototypical achiral Cp*(2)LaCH(TMS)(2) precatalyst is critically scrutinized by employing a gradient-corrected DFT method. The condensed free-energy profile for the overall reaction, comprised of thermodynamic and kinetic aspects of individual elementary steps, is presented. A computationally verified, revised mechanistic scenario has been proposed, which is consistent with the empirical rate law, accounts for crucial experimental observations, and provides a first understanding of the origin of the measured negative DeltaS(++). It involves rapid substrate association/dissociation equilibria and facile intramolecular diene insertion, linked to turnover-limiting protonolysis of the eta(3)-butenyl-Ln functionality, with the amine-amidodiene-Ln adduct complex representing the catalyst's resting state. The thermodynamic and kinetic factors that determine the high regio- and stereoselectivity of the mechanistically diverse IHC of aminodienes have been elucidated. These achievements allow a deeper understanding and a consistent rationalization of the experimental results for aminodiene IHC and furthermore enhance the insights into general mechanistic aspects of the organolanthanide-mediated cycloamination.

[Ni0]-Catalyzed Co-oligomerization of 1,3-Butadiene and Ethylene: A Theoretical Mechanistic Investigation of Competing Routes for Generation of Linear and Cyclic C10-Olefins

A detailed theoretical investigation of the mechanism for the [Ni(0)]-catalyzed co-oligomerization of 1,3-butadiene and ethylene to afford linear and cyclic C(10)-olefins is presented. Crucial elementary processes have been carefully explored for a tentative catalytic cycle, employing a gradient-corrected density functional theory (DFT) method. The favorable route for oxidative coupling starts from the prevalent [Ni(0)(eta(2)-butadiene)(2)(ethylene)] form of the active catalyst through oxidative coupling between the two eta(2)-butadienes. The initial eta(3),eta(1)(C(1))-octadienediyl-Ni(II) product is the active precursor for ethylene insertion, which preferably takes place into the syn-eta(3)-allyl-Ni(II) bond of the prevalent eta(3)-syn,eta(1)(C(1)),Delta-cis isomer. The insertion is driven by a strong thermodynamic force, giving rise entirely to eta(3),eta(1),Delta-trans-decatrienyl-Ni(II) forms, with the eta(3)-anti,eta(1),Delta-trans isomer almost exclusively generated. Occurrence of allyl,eta(1),Delta-cis isomers, however, is precluded on both kinetic and thermodynamic grounds, thereby rationalizing the observation that cis-DT and cis,cis-CDD are never formed. Linear and cyclic C(10)-olefins are generated in a highly stereoselective fashion, with trans-DT and cis,trans-CDD as the only isomers, along competing routes of stepwise transition-metal-assisted H-transfer (DT) and reductive CC elimination under ring closure (CDD), respectively, that start from the prevalent eta(3)-anti,eta(1),Delta-trans-decatrienyl-Ni(II) species. The role of allylic conversion in the octadienediyl-Ni(II) and decatrienyl-Ni(II) complexes has been analyzed. As a result of the detailed exploration of all important elementary steps, a theoretically verified, refined catalytic cycle is proposed and the regulation of the selectivity for formation of linear and cyclic C(10)-olefins is elucidated.

[Ni(0)L]-catalyzed cyclodimerization of 1,3-butadiene: a comprehensive density functional investigation based on the generic [(C(4)H(6))(2)Ni(0)PH(3)] catalyst

We present a comprehensive theoretical investigation of the mechanism for cyclodimerization of butadiene by the generic [bis(butadiene)Ni(0)PH(3)] catalyst employing a gradient-corrected DFT method. We have explored all critical elementary steps of the whole catalytic cycle, namely, oxidative coupling of two butadienes, reductive elimination under ring closure, and allylic isomerization. Oxidative coupling of two butadienes in the [bis(butadiene)Ni(0)L] complex and reductive elimination in the [(bis(eta(3))-octadienediyl)Ni(II)L] species take place under different stereocontrol, which makes isomerization indispensable. Commencing from a preestablished equilibrium between several configurations of the [(octadienediyl)Ni(II)L] complex, the major cyclodimer products, namely, VCH, cis-1,2-DVCB, and cis,cis-COD, are formed along competing reaction paths via reductive elimination, which is found to be the overall rate-determining step. Careful exploration of different possible conceivable routes revealed that bis(eta(1)) species are not involved as critical intermediates either in reductive elimination or in isomerization along the most feasible pathway. The regulation of the selectivity of the cyclodimer formation based on both thermodynamic and kinetic considerations is outlined.

Reaction mechanism and structure-reactivity relationships in the stereospecific 1,4-polymerization of butadiene catalyzed by neutral dimeric allylnickel(II) halides [Ni(C3H5)X]2 (X- = Cl-, Br-, I-): a comprehensive density functional theory study

For the first time, a comprehensive and consistent picture of the catalytic cycle of 1,4-polymerization of butadiene with neutral dimeric allylnickel(II) halides [Ni(C3H5)X]2 (X = Cl- (I), Br- (II), and I- (III)) as single-site catalysts has been derived by means of quantum chemical calculations that employ a gradient-corrected density-functional method. All crucial reaction steps of the entire catalytic course have been scrutinized, taking into account butadiene pi complex formation, symmetrical and asymmetrical splitting of dimeric pi complexes, cis-butadiene insertion, and anti-syn isomerization. The present investigation examines, in terms of located structures, energies and activation barriers, the participation of postulated intermediates, in particular it aimed to clarify whether monomeric or dimeric species are the catalytically active species. Prior qualitative mechanistic assumptions are substituted by the presented theoretically well-founded and detailed analysis of both the thermodynamic and the kinetic aspects, that substantially improve the insight into the reaction course and enlarge them with novel mechanistic proposals. From a mechanistic point of view, all three catalysts exhibit common characteristics. First, chain propagation occurs by cis-butadiene insertion into the pi-butenylnickel(II) bond with nearly identical intrinsic free-energy activation barriers. Second, the reactivity of syn-butenyl forms is distinctly higher than that of anti forms. Third, the chain-propagation step is rate-determining in the entire polymerization process, and the pre-established anti-syn equilibrium can always be regarded as attained. Accordingly, neutral dimeric allylnickel(II) halides catalyze the formation of a stereo-regular trans-1,4-polymer under kinetic control following the k1t channel with butenyl(halide)(butadiene)NiII complexes being the catalytically active species. Production of a stereoregular cis-1,4-polymer with allylnickel chloride can only be explained by making the k2c channel accessible by the formation of polybutadienyl(butadiene) complexes, which is accompanied by the coordination of the next double bond in the growing chain to the NiII center.