Automated Variable Electric-Field DFT Application for Evaluation of Optimally Oriented Electric Fields on Chemical Reactivity.

Recent theoretical work and experiments at molecular junctions have provided a strong conceptualization for the effects of oriented electric fields (OEFs) on organic reactions. Depending on the axis of application, OEFs can increase (or decrease) the reaction rate or distinguish between isomeric pathways. Despite the conceptual elegance of OEFs, which may be applied externally or induced locally, as tools for catalyzing organic reactions, implementation in synthetically relevant systems has been hampered by inefficiencies in evaluating reaction sensitivity to field effects. Herein, we describe the development of the Automated Variable Electric-Field DFT Application (A.V.E.D.A.) for streamlined evaluation of a reaction's susceptibility to OEFs. This open-source software was designed to be accessible for nonexpert users of computational and programming tools. Following initiation by a single command (and with no subsequent intervention) the Linux workflow manages a series of density functional theory calculations and mathematical manipulations to optimize local-minimum and transition-state structures in oriented electric fields of increasing magnitude. The resulting molecular and reaction dipole moments, field-perturbed geometries, and net effective activation energies are compiled for user interpretation. Ten representative pericyclic reactions that showcase the development and evaluation of A.V.E.D.A. are described.

Pyridine(diimine) Iron Diene Complexes Relevant to Catalytic [2+2]-Cycloaddition Reactions.

The synthesis, characterization, and catalytic activity of pyridine(diimine) iron piperylene and isoprene complexes are described. These diene complexes are competent precatalysts for (i) the selective cross-[2+2]-cycloaddition of butadiene or (E)-piperylene with ethylene and α-olefins and (ii) the 1,4-hydrovinylation of isoprene with ethylene. In the former case, kinetic analysis implicates the diamagnetic η4-piperylene complex as the resting state prior to rate-determining oxidative cyclization. Variable temperature 1H NMR and EXSY experiments established that diene exchange from the diamagnetic, 18e- complexes occurs rapidly in solution at ambient temperature through a dissociative mechanism. The solid-state structure of (Me(Et)PDI)Fe(η4-piperylene) (Me(Et)PDI = 2,6-(2,6-Me2-C6H3N═CEt)2C5H3N), was determined by single-crystal X-ray diffraction and confirmed the s-trans coordination of the monosubstituted 1,3-diene. Possible relationships between ligand-controlled diene coordination geometry, metallacycle denticity, and chemoselectivity of iron-mediated cycloaddition reactions are discussed.

Enantioselective Catalysis of an Anionic Oxy-Cope Rearrangement Enabled by Synergistic Ion Binding.

Charge-accelerated rearrangements present interesting challenges to enantioselective catalysis, due in large part to the competing requirements for maximizing reactivity (ion-pair separation) and stereochemical communication. Herein, we describe application of a synergistic ion-binding strategy to catalyze the anionic oxy-Cope rearrangement of a symmetric bis-styrenyl allyl alcohol in up to 75:25 e.r. Structure-reactivity-selectivity relationship studies, including linear free-energy-relationship analyses, with bifunctional urea catalysts indicate that H-bonding and cation-binding interactions act cooperatively to promote the chemo- and enantioselective [3,3]-rearrangement. Implications for catalyst designs applicable to other transformations involving oxyanionic intermediates are discussed.

Regio- and Diastereoselective Iron-Catalyzed [4+4]-Cycloaddition of 1,3-Dienes.

A family of single-component iron precatalysts for the [4+4]-cyclodimerization and intermolecular cross-[4+4]-cycloaddition of monosubstituted 1,3-dienes is described. Cyclooctadiene products were obtained with high regioselectivity, and catalyst-controlled access to either cis- or trans-diastereomers was achieved using 4-substituted diene substrates. Reactions conducted either with single-component precatalysts or with iron dihalide complexes activated in situ proved compatible with common organic functional groups and were applied on multigram scale (up to >100 g). Catalytically relevant, S = 1 iron complexes bearing 2-(imino)pyridine ligands, (RPI)FeL2 (RPI = [2-(2,6-R2-C6H3-N═CMe)-C5H4N] where R = iPr or Me, L2 = bis-olefin), were characterized by single-crystal X-ray diffraction, Mößbauer spectroscopy, magnetic measurements, and DFT calculations. The structural and spectroscopic parameters are consistent with an electronic structure description comprised of a high spin iron(I) center ( SFe = 3/2) engaged in antiferromagnetically coupling with a ligand radical anion ( SPI = -1/2). Mechanistic studies conducted with these single-component precatalysts, including kinetic analyses, 12C/13C isotope effect measurements, and in situ Mößbauer spectroscopy, support a mechanism involving oxidative cyclization of two dienes that determines regio- and diastereoselectivity. Topographic steric maps derived from crystallographic data provided insights into the basis for the catalyst control through stereoselective oxidative cyclization and subsequent, stereospecific allyl-isomerization and C-C bond-forming reductive elimination.

Selective [1,4]-Hydrovinylation of 1,3-Dienes with Unactivated Olefins Enabled by Iron Diimine Catalysts.

The selective, intermolecular [1,4]-hydrovinylation of conjugated dienes with unactivated α-olefins catalyzed by α-diimine iron complexes is described. Value-added "skipped" diene products were obtained with exclusive [1,4]-selectivity, and the formation of branched, ( Z)-olefin products was observed with no evidence for alkene isomerization. Mechanistic studies conducted with the well-defined, single-component iron precatalyst (MesDI)Fe(COD) (MesDI = [2,4,6-Me3-C6H2-N═CMe]2); COD = 1,5-cyclooctadiene) provided insights into the origin of the high selectivity. An iron diene complex was identified as the catalyst resting state, and one such isoprene complex, (iPrDI)Fe(η4-C5H8), was isolated and characterized. A combination of single crystal X-ray diffraction, Mößbauer spectroscopy, magnetic measurements, and DFT calculations established that the complex is best described as a high-spin Fe(I) center ( SFe = 3/2) engaged in antiferromagnetic coupling to an α-diimine radical anion ( SDI = -1/2), giving rise to the observed S = 1 ground state. Deuterium-labeling experiments and kinetic analyses of the catalytic reaction provided support for a pathway involving oxidative cyclization of an alkene with the diene complex to generate an iron metallacycle. The observed selectivity can be understood in terms of competing steric interactions in the transition states for oxidative cyclization and subsequent ß-hydrogen elimination.

Chiral Thioureas Promote Enantioselective Pictet-Spengler Cyclization by Stabilizing Every Intermediate and Transition State in the Carboxylic Acid-Catalyzed Reaction.

An investigation of the mechanism of benzoic acid/thiourea co-catalysis in the asymmetric Pictet-Spengler reaction is reported. Kinetic, computational, and structure-activity relationship studies provide evidence that rearomatization via deprotonation of the pentahydro-ß-carbolinium ion intermediate by a chiral thiourea·carboxylate complex is both rate- and enantioselectivity-determining. The thiourea catalyst induces rate acceleration over the background reaction mediated by benzoic acid alone by stabilizing every intermediate and transition state leading up to and including the final selectivity-determining step. Distortion-interaction analyses of the transition structures for deprotonation predicted using density functional theory indicate that differential π-π and C-H···π interactions within a scaffold organized by multiple hydrogen bonds dictate stereoselectivity. The principles underlying rate acceleration and enantiocontrol described herein are expected to have general implications for the design of selective transformations involving deprotonation of high-energy intermediates.

On- and Off-Cycle Catalyst Cooperativity in Anion-Binding Catalysis.

Chiral, neutral H-bond donors have found widespread use as catalysts in enantioselective reactions involving ion-pair intermediates. Herein, a systematic mechanistic study of a prototypical anion-binding reaction, the thiourea-catalyzed enantioselective alkylation of α-chloroethers, is detailed. This study reveals that the catalyst resting state is an inactive dimeric aggregate that must dissociate and then reassemble to form a 2:1 catalyst-substrate complex in the rate-determining transition structure. Insight into this mode of catalyst cooperativity sheds light on the practical limitations that have plagued many of the H-bond donor-catalyzed reactions developed to date and suggests design strategies for new, highly efficient catalyst structures.

Mechanism-Guided Development of a Highly Active Bis-thiourea Catalyst for Anion-Abstraction Catalysis.

We describe the rational design of a linked, bis-thiourea catalyst with enhanced activity relative to monomeric analogues in a representative enantioselective anion-abstraction reaction. Mechanistic insights guide development of this linking strategy to favor substrate activation though the intramolecular cooperation of two thiourea subunits while avoiding nonproductive aggregation. The resulting catalyst platform overcomes many of the practical limitations that have plagued hydrogen-bond-donor catalysis and enables use of catalyst loadings as low as 0.05 mol %. Computational analyses of possible anion-binding modes provide detailed insight into the precise mechanism of anion-abstraction catalysis with this pseudo-dimeric thiourea.

The Cation-π Interaction in Small-Molecule Catalysis.

Catalysis by small molecules (≤1000 Da, 10(-9)  m) that are capable of binding and activating substrates through attractive, noncovalent interactions has emerged as an important approach in organic and organometallic chemistry. While the canonical noncovalent interactions, including hydrogen bonding, ion pairing, and π stacking, have become mainstays of catalyst design, the cation-π interaction has been comparatively underutilized in this context since its discovery in the 1980s. However, like a hydrogen bond, the cation-π interaction exhibits a typical binding affinity of several kcal mol(-1) with substantial directionality. These properties render it attractive as a design element for the development of small-molecule catalysts, and in recent years, the catalysis community has begun to take advantage of these features, drawing inspiration from pioneering research in molecular recognition and structural biology. This Review surveys the burgeoning application of the cation-π interaction in catalysis.

Secondary-sphere preorganization enables nickel-catalyzed nitrile hydroboration.

Herein, we describe nickel-catalyzed nitrile hydroboration with pinacolborane, wherein a tethered NHC-pyridonate ligand enables efficient catalysis (5 mol% [Ni], ≤6 h reaction time) at room temperature. Mechanistic studies, including isolation of the catalytically relevant intermediates, shed light on the cooperative role of the ligand in activating both reagents simultaneously.

Distinguishing Competing Mechanistic Manifolds for C(acyl)-N Functionalization by a Ni/N-Heterocyclic Carbene Catalyst System.

Carboxylic acid derivatives are appealing alternatives to organohalides as cross-coupling electrophiles for fine chemical synthesis due to their prevalence in biomass and bioactive small molecules as well as their ease of preparation and handling. Within this family, carboxamides comprise a versatile electrophile class for nickel-catalyzed coupling with carbon and heteroatom nucleophiles. However, even state-of-the-art C(acyl)-N functionalization and cross-coupling reactions typically require high catalyst loadings and specific substitution patterns. These challenges have proven difficult to overcome, in large part due to limited experimental mechanistic insight. In this work, we describe a detailed mechanistic case study of acylative coupling reactions catalyzed by the commonly employed Ni/SIPr catalyst system (SIPr = 1,3-bis(2,6-di-isopropylphenyl)-4,5-dihydroimidazol-2-ylidine). Stoichiometric organometallic studies, in situ spectroscopic measurements, and crossover experiments demonstrate the accessibility of Ni(0), Ni(I), and Ni(II) resting states. Although in situ precatalyst activation limits reaction efficiency, the low concentrations of active, SIPr-supported Ni(0) select for electrophile-first (closed-shell) over competing nucleophile-first (open-shell) mechanistic manifolds. We anticipate that the experimental insights into the nature and controlling features of these distinct pathways will accelerate rational improvements to cross-coupling methodologies involving pervasive carboxamide substrate motifs.

Iron-catalysed synthesis and chemical recycling of telechelic 1,3-enchained oligocyclobutanes.

Closed-loop recycling offers the opportunity to mitigate plastic waste through reversible polymer construction and deconstruction. Although examples of chemical recycling of polymers are known, few have been applied to materials derived from abundant commodity olefinic monomers, which are the building blocks of ubiquitous plastic resins. Here we describe a [2+2] cycloaddition/oligomerization of 1,3-butadiene to yield a previously unrealized telechelic microstructure of (1,n'-divinyl)oligocyclobutane. This material is thermally stable, has stereoregular segments arising from chain-end control, and exhibits high crystallinity even at low molecular weight. Exposure of the oligocyclobutane to vacuum in the presence of the pyridine(diimine) iron precatalyst used to synthesize it resulted in deoligomerization to generate pristine butadiene, demonstrating a rare example of closed-loop chemical recycling of an oligomeric material derived from a commodity hydrocarbon feedstock.

Iron-Catalyzed Vinylsilane Dimerization and Cross-Cycloadditions with 1,3-Dienes: Probing the Origins of Chemo- and Regioselectivity.

The selective, intermolecular, homodimerization and cross-cycloaddition of vinylsilanes with unbiased 1,3-dienes, catalyzed by a pyridine-2,6-diimine (PDI) iron complex is described. In the absence of a diene coupling partner, vinylsilane hydroalkenylation products were obtained chemoselectively with unusual head-to-head regioselectivity (up to >98% purity, 98:2 E/Z). In the presence of a 4- or 2-substituted diene coupling partner, under otherwise identical reaction conditions, formation of value-added [2+2]- and [4+2]-cycloadducts, respectively, was observed. The chemoselectivity profile was distinct from that observed for analogous α-olefin dimerization and cross-reactions with 1,3-dienes. Mechanistic studies conducted with well-defined, single-component precatalysts (MePDI)Fe(L2) (where MePDI = 2,6-(2,6-Me2-C6H3N═CMe)2C5H3N; L2 = butadiene or 2(N2)) provided insights into the kinetic and thermodynamic factors contributing to the substrate-controlled regioselectivity for both the homodimerization and cross cycloadditions. Diamagnetic iron diene and paramagnetic iron olefin complexes were identified as catalyst resting states, were characterized by in situ NMR and Mössbauer spectroscopic studies, and were corroborated with DFT calculations. Stoichiometric reactions and computational models provided evidence for a common mechanistic regime where competing steric and orbital-symmetry requirements dictate the regioselectivity of oxidative cyclization. Although distinct chemoselectivity profiles were observed in cross-cycloadditions with the vinylsilane congeners of α-olefins, these products arose from metallacycles with the same connectivity. The silyl substituents ultimately governed the relative rates of ß-H elimination and C-C reductive elimination to dictate final product formation.

[4+4]-Cycloaddition of Isoprene for the Production of High-Performance Bio-Based Jet Fuel.

Isoprene was efficiently converted to 1,6-dimethyl-1,5-cyclooctadiene (DMCOD) by selective [4+4]-cycloaddition with a catalyst formed by in situ reduction of [(MePI)FeCl(µ-Cl)]2 (MePI = [2-(2,6-(CH3)2-C6H3-N=C(CH3))-C4H5N]). DMCOD was isolated in 92% yield, at the preparative scale, with a catalyst loading of 0.025 mol%, and a TON of 3680. Catalytic hydrogenation of DMCOD yielded 1,4-dimethylcyclooctane (DMCO). The cyclic structure and ring strain of DMCO afforded gravimetric and volumetric net heats of combustion 2.4 and 9.2% higher, respectively, than conventional jet fuel. In addition, the presence of methyl branches at two sites resulted in a -20 °C kinematic viscosity of 4.17 mm2 s-1, 48 % lower than the maximum allowed value for conventional jet fuel. The ability to derive isoprene and related alcohols readily from abundant biomass sources, coupled with the highly efficient [Fe]-catalyzed [4+4]-cycloaddition described herein, suggests that this process holds great promise for the economical production of high-performance, bio-based jet fuel blendstocks.

Synergistic Ion-Binding Catalysis Demonstrated via an Enantioselective, Catalytic [2,3]-Wittig Rearrangement.

Sigmatropic rearrangements number among the most powerful complexity-building transformations in organic synthesis but have remained largely insensitive to enantioselective catalysis due to the diffuse nature of their transition structures. Here, we describe a synergistic ion-binding strategy for asymmetric catalysis of anionic sigmatropic rearrangements. This approach is demonstrated with the enantioselective [2,3]-Wittig rearrangement of α-allyloxy carbonyl compounds to afford highly enantioenriched homoallylic alcohol products. Chiral thiourea catalysts are shown to engage reactive anions and their countercations through a cooperative set of attractive, noncovalent interactions. Catalyst structure-reactivity-selectivity relationship studies and computational analyses provide insight into catalyst-substrate interactions responsible for enantioinduction and allude to the potential generality of this catalytic strategy.

Anion-Abstraction Catalysis: The Cooperative Mechanism of α-Chloroether Activation by Dual Hydrogen-Bond Donors.

We provide here a detailed mechanistic characterization of the electrophile-activation step in a representative thiourea-catalyzed enantioselective reaction proposed to involve generation of ion-pair intermediates. Comparison of catalyst-promoted substrate epimerization with catalytic alkylation points to the participation of a common intermediate in both pathways and provides conclusive evidence for anion abstraction via an SN1-like pathway involving the cooperative action of two catalyst molecules.

Conformational Control of Chiral Amido-Thiourea Catalysts Enables Improved Activity and Enantioselectivity.

While aryl pyrrolidinoamido-thioureas derived from α-amino acids are effective catalysts in a number of asymmetric transformations, they exist as mixtures of slowly interconverting amide rotamers. Herein, the compromising role of amide bond isomerism is analyzed experimentally and computationally. A modified catalyst structure that exists almost exclusively as a single amide rotamer is introduced. This modification is shown to result in improved reactivity and enantioselectivity by minimizing competing reaction pathways.