Mechanism of an Organocatalytic Cope Rearrangement Involving Iminium Intermediates: Elucidating the Role of Catalyst Ring Size

Theoretical Study on the Mechanism of the Electrocatalytic CO2 Reduction to Formate by an Iron Schiff Base Complex

The iron(III) chloride compound 6,6'-di(3,5-ditert-butyl-2-hydroxybenzene)-2,2'-bipyridine (Fe(tbudhbpy)Cl) can effectively catalyze the electrochemical CO2 reduction in N,N-dimethylformamide. Density functional calculations were conducted to investigate the mechanism and unravel the governing factors of product selectivity. The results suggest that the initial catalyst, Fe(tbudhbpy)Cl (formally FeIII-Cl), undergoes two reduction steps, accompanied by the dissociation of Cl-, leading to the formation of the active ferrous radical intermediate 2 (formally FeI). Without phenol, 2 attacks CO2 to generate the FeIII-carboxylate intermediate FeIII-CO2, followed by a one-electron reduction to generate FeII-CO2, which reacts with another CO2 to produce CO. This aligns with the experimental result that CO is the main product when the phenol is absent. In contrast, when phenol is presented, the triple reduced species 3 is protonated at its ligand N site to yield 3pt(N) (formally Fe0-NH), which subsequently performs a nucleophilic attack on CO2 to afford formate. This process occurs via an orthogonal electron/proton transfer mechanism, where two electrons and one proton are transferred from the ligand to the CO2 moiety. The redox noninnocent nature of the ligand is thus crucial for formate formation, as it facilitates electron and proton shuttling, enabling 3pt(N) to attack CO2 through this unusual mechanism effectively.

Dual Activation Modes Enable Bifunctional Catalysis of Aldol Reactions by Flexible Dihydrazides

Hydrazides are known to catalyze reactions of α,β-unsaturated aldehydes via transient iminium formation. The iminium intermediate displays enhanced electrophilicity, which facilitates conjugate additions and cycloadditions. We observed that a hydrazide embedded in a seven-membered ring catalyzes homoaldol condensation of a simple aldehyde in a process that displays an approximate second-order dependence on the hydrazide. This finding suggests a dual activation mechanism involving both iminium and enamine intermediates. The unexpected nucleophilic activation mode led us to examine a series of simple dihydrazides for bifunctional catalysis of the aldol condensation. The two cyclic hydrazide units were connected via linear polymethylene linkers, which are inherently flexible. Substantial increases in initial reaction rate, relative to a monohydrazide, were observed for polymethylene linkers of sufficient length, with a maximum at 10 methylenes. Reactions promoted by this dihydrazide displayed an approximate first-order dependence on catalyst concentration, which suggested a bifunctional catalytic mechanism. Even for a dihydrazide with an 18-methylene linker, a substantial increase in relative initial rate was observed. These observations show that significant coordination can be achieved between two catalytically active moieties even when these moieties are connected via many flexible bonds.

Reactivity in Friedel-Crafts aromatic benzylation: the role of the electrophilic reactant

Density functional theory is employed in understanding the reactivity in the TiCl4 catalyzed Friedel-Crafts benzylation of benzene with substituted benzyl chlorides in nitromethane solvent. A series of ten substituted (in the aromatic ring) benzyl chlorides are characterized by theoretical reactivity indices. The theoretical parameters are juxtaposed to experimental relative rates of benzylation. It is established that the carbon-chlorine ionic bond dissociation energy and the Hirshfeld charge at the chlorine atom for the benzyl chlorides reactants - describe quite satisfactorily the reactivity trends. These results provide further insights into the factors governing reactivity in EAS reactions, which so far have been mostly focused on rate variations induced by changes in the structure of the aromatic substrate. The EAS benzylation investigated is quite unusual since, in contrast to most EAS reactions, the latest experimental kinetic results suggest that the aromatic substrate does not participate in the kinetic equation of the process. To shed more light on this unexpected result, we also conducted a theoretical study on the mechanistic pathway by applying M06-2X density functional computations combined with several basis sets: 6-311+G(d,p), 6-311+G(2df,2p), and def2-TZVPP. Because of the well-known difficulties in evaluating realistic free energy barriers for organic reactions, we tested two solvent models in determining the barrier for the TiCl4-catalyzed Friedel-Crafts benzylation of benzene by benzyl chloride. Since all methods employed did not provide satisfactory results for the free energy barriers, we used a combination of theoretically estimated enthalpy barriers and the available (from kinetic experiments) entropy contribution. This approach enabled us to verify that indeed the rate of this EAS reaction does not depend on the nature of the aromatic substrate. The computations revealed the structure and relative energies of the critical structures along the mechanistic pathway. Four intermediates were established along the reaction route.

Stereoselective mechanochemical synthesis of thiomalonate Michael adducts via iminium catalysis by chiral primary amines

The study presents a novel approach utilizing iminium salt activation and mild enolization of thioesters, offering an efficient and rapid synthesis of Michael adducts with promising stereoselectivity and marking a significant advancement in mechanocatalysis. The stereoselective addition of bisthiomalonates 1-4 to cyclic enones and 4-chlorobenzylideneacetone proceeds stereoselectively under iminium activation conditions secured by chiral primary amines, in contrast to oxo-esters as observed in dibenzyl malonate addition. Mild enolization of thioesters allows for the generation of Michael adducts with good yields and stereoselectivities. Reactions in a ball mill afford product formation with similar efficacy to solution-phase reactions but with slightly reduced enantioselectivity, yet they yield products in just one hour compared to 24 or even 168 hours in solution-based reactions. It is noteworthy that this represents one of the early reports on the application of iminium catalysis using first-generation chiral amines under mechanochemical conditions, along with the utilization of easily enolizable thioesters as nucleophiles in this transformation.

Mimicking enzymatic cation-π interactions in hydrazide catalyst design: access to trans-decalin frameworks

Chiral bicyclic hydrazide organocatalysts have previously been shown to catalyze the cyclization of (Z)-polyene substrates with high enantioselectivity, but with poor selectivity for the corresponding (E)-polyenes. Here we demonstrate that diazapane carboxylates bearing terphenyl groups efficiently catalyze (E)-polyene bicyclization with enantioselectivities up to 94 : 6 er and with high diastereoselectivity for trans-decalin formation. The catalysts function by simultaneously initiating the cyclization via iminium ion formation and stabilizing intermediates/transition states by cation-π interactions.

An Organocatalytic Oxy-Cope/Michael Cascade Reaction

Ethyl diazepane carboxylate catalyzes the oxy-Cope rearrangement of 4-hydroxy- and 4-alkoxy-1,5-hexadiene-2-carboxaldehydes via iminium ion activation. The resulting intermediate undergoes an intramolecular Michael reaction to furnish cyclopentane-containing products. The reaction proceeds with a range of substrates, including both cyclic and acyclic substrates, and tolerates substitution on the vinyl substituent. Substrates fused on a cycloalkane framework undergo net ring expansion/cyclopentannulation with a high degree of stereocontrol via chairlike transition states. The reaction extends iminium organocatalysis to the oxy-Cope rearrangement, embedded within a complexity-generating cascade transformation.

Theoretical Perspectives in Organocatalysis

It is clear that the field of organocatalysis is continuously expanding during the last decades. With increasing computational capacity and new techniques, computational methods have provided a more economic approach to explore different chemical systems. This review offers a broad yet concise overview of current state-of-the-art studies that have employed novel strategies for catalyst design. The evolution of the all different theoretical approaches most commonly used within organocatalysis is discussed, from the traditional approach, manual-driven, to the most recent one, machine-driven.

Mechanism and selectivity of photocatalyzed CO2 reduction by a function-integrated Ru catalyst

The phosphine-substituted Ru(II) polypyridyl complex, [Ru^II-(tpy)(pqn)(MeCN)]²⁺ (RuP), was disclosed to be an efficient photocatalyst for the reduction of CO₂ to CO with excellent selectivity. In this work, density functional calculations were performed to elucidate the reaction mechanism and understand the origin of selectivity. The calculations showed that RuP was first excited to the singlet excited state, followed by intersystem crossing to produce a triplet species (3RuIII(L˙-)-S), which was then reduced by the sacrificial electron donor BIH to generate a Ru^II(L˙^-) intermediate. The ligand of Ru^II(L˙^-) was further reduced to produce a Ru^II(L^2-) intermediate. The redox non-innocent nature of the tpy and pqn ligands endows the Ru center with an oxidation state of +2 after two one-electron reductions. Ru^II(L^2-) nucleophilically attacks CO₂, in which two electrons are delivered from the ligands to CO₂, affording a Ru^II-COOH species after protonation. This is followed by the protonation of the hydroxyl moiety of Ru^II-COOH, coupled with the C-O bond cleavage, resulting in the formation of Ru^II-CO. Ultimately, CO is dissociated after two one-electron reductions. Protonation of Ru^II(L^2-) to generate a Ru^II-hydride, a critical intermediate for the production of formate and H₂, turns out to be kinetically less favorable, even though it is thermodynamically more favorable. This fact is due to the presence of a Ru²⁺ ion in the reduced catalyst, which disfavors its protonation.

The stabilized iminium catalyzed (E)-polyene cyclization: trapping of non-activated terminating groups enabled by cation–π interactions

A cyclic hydrazide catalyst bearing a pendant anthracene catalyzes the polyene cyclization of 1,5-hexadiene-2-carboxaldehydes. Bicyclic closure proceeds in substrates with non-activated terminating groups that fail to react with simple hydrazide catalysts. Computational analysis shows that stabilization through cation–π interactions throughout the reaction sequence leads to the enhanced reactivity of the catalyst.

Recent Advances in Catalytic [3,3]-Sigmatropic Rearrangements

Carbon–carbon bond formation by [3,3]-sigmatropic rearrangement is a fundamental and powerful method that has been used to build organic molecules for a long time. Initially, Claisen and Cope rearrangements proceeded at high temperatures with limited scopes. By introducing catalytic systems, highly functionalized substrates have become accessible for forming complex structures under mild conditions, and asymmetric synthesis can be achieved by using chiral catalytic systems. This review describes recent breakthroughs in catalytic [3,3]-sigmatropic rearrangements since 2016. Detailed reaction mechanisms are discussed to enable an understanding of the reactivity and selectivity of the reactions. Finally, this review is inspires the development of new cascade reaction pathways employing catalytic [3,3]-sigmatropic rearrangement as related methodologies for the synthesis of complex functional molecules.

Heavy-Atom Kinetic Isotope Effects: Primary Interest or Zero Point?

Chemists have many options for elucidating reaction mechanisms. Global kinetic analysis and classic transition-state probes (e.g., LFERs, Eyring) inevitably form the cornerstone of any strategy, yet their application to increasingly sophisticated synthetic methodologies often leads to a wide range of indistinguishable mechanistic proposals. Computational chemistry provides powerful tools for narrowing the field in such cases, yet wholly simulated mechanisms must be interpreted with great caution. Heavy-atom kinetic isotope effects (KIEs) offer an exquisite but underutilized method for reconciling the two approaches, anchoring the theoretician in the world of calculable observables and providing the experimentalist with atomistic insights. This Perspective provides a personal outlook on this synergy. It surveys the computation of heavy-atom KIEs and their measurement by NMR spectroscopy, discusses recent case studies, highlights the intellectual reward that lies in alignment of experiment and theory, and reflects on the changes required in chemical education in the area.

[3,3] Ring Rearrangement of Oxo- or Aza-Bridged Bicyclo[3.2.1]octene-Based 1,5-Dienes

We report that oxo- or aza-bridged alkylidenemalononitrile-cycloheptenes undergo a [3,3] ring rearrangement to yield cyclopenta-fused dihydro-furans or pyrroles. Described herein are the origins of the serendipitous discovery, scope studies, and representative functional group interconversion chemistry.

A comparison of hydrogen release kinetics from 5- and 6-membered 1,2-BN-cycloalkanes

The reaction order and Arrhenius activation parameters for spontaneous hydrogen release from cyclic amine boranes, i.e., BN-cycloalkanes, were determined for 1,2-BN-cyclohexane (1) and 3-methyl-1,2-BN-cyclopentane (2) in tetraglyme. Computational analysis identified a mechanism involving catalytic substrate activation by a ring-opened form of 1 or 2 as being consistent with experimental observations.

The reaction order and Arrhenius activation parameters for spontaneous hydrogen release from cyclic amine boranes, i.e., BN-cycloalkanes, were determined for 1,2-BN-cyclohexane (1) and 3-methyl-1,2-BN-cyclopentane (2) in tetraglyme.

Rapid Evaluation of the Mechanism of Buchwald-Hartwig Amination and Aldol Reactions Using Intramolecular 13C Kinetic Isotope Effects

A practical approach is introduced for the rapid determination of ¹³C kinetic isotope effects that utilizes a "designed" reactant with two identical reaction sites. The mechanism of the Buchwald-Hartwig amination of tert-butylbromobenzene with primary and secondary amines is investigated under synthetically relevant catalytic conditions using traditional intermolecular ¹³C NMR methodology at natural abundance. Switching to 1,4-dibromobenzene, a symmetric bromoarene as the designed reactant, the same experimental ¹³C KIEs are determined using an intramolecular KIE approach. This rapid methodology for KIE determination requires substantially less material and time compared to traditional approaches. Details of the Buchwald-Hartwig amination mechanism are investigated under varying synthetic conditions, namely a variety of halides and bases. The enantioselectivity-determining step of the l-proline catalyzed aldol reaction is also evaluated using this approach. We expect this mechanistic methodology to gain traction among synthetic chemists as a practical technique to rapidly obtain high-resolution information regarding the transition structure of synthetically relevant reactions under catalytic conditions.