FeCl3 -Catalyzed Ring-Closing Carbonyl-Olefin Metathesis

Probing the Catalytic Degradation of Unsaturated Polyolefin Materials via Fe-Based Lewis Acids-Initiated Carbonyl-Olefin Metathesis

Degradation and recyclability of polymeric materials, including extensively used polyolefins, are becoming increasingly necessary. Chemically stable saturated polyolefin backbones make their degradation frustratingly challenging. The current effective strategy is to create cleavable defects, e.g., C═C double bonds along the backbone, and subsequently depolymerize them via cross-metathesis reaction with olefins. High-value chemicals or reusable polymeric segments are obtained. This two-step protocol provides operable means for alleviating plastics problems. There are several approaches to introduce unsaturation into a polymer backbone, like dehydrogenation or copolymerization of olefins and conjugated dienes. However, for the second step, to conduct a cross-metathesis reaction, only noble metal catalysts can be used most of the time. Regardless of their limited availability, the fact that these organometallics are unfavorably sensitive to impurities would raise barriers in industrial practices. Herein we employed earth-abundant and inexpensive iron-based Lewis acids to initiate carbonyl-olefin metathesis reactions between ketone/aldehyde reagents and unsaturated polyolefins. After explorations in poly(diene)s and industrial thermoplastic elastomers, we extended this protocol to degrade low-density polyethylene (LDPE). Low-molecular weight PE wax-like products were obtained as useful chemicals. This catalytic degradation system is expected to enable the development of more efficient metathesis strategies to promote degradation of polyolefins and pave sustainable ways for reuse of polymeric materials.

Theoretical Investigations of Substrate Behavior in FeCl3-Catalyzed Carbonyl-Olefin Metathesis

FeCl3-catalyzed ring-closing carbonyl-olefin metathesis is a powerful method for the formation of cyclic olefins. Multiple substrate classes are known to display this reactivity; however, two substrates have been reported to form an oxetane, and do not undergo retro-[2 + 2] fragmentation into the cyclic olefin and a byproduct carbonyl. Specifically, phenanthrene producing polycyclic aromatic hydrocarbons yield an oxetane when electrophilic fluorine is introduced α to the substrate carbonyl. Herein, we report the application of quantum chemical modeling of enthalpies and NBO charges to investigate this divergent reactivity. In particular, the replacement of C-H bonds with C-F bonds eliminates hyperconjugative stabilization of the retro-[2 + 2] transition state. Taken together, this model suggests that charge stabilization at the reactive carbonyl carbon dictates the ability of the oxetane to fragment into the metathesis product. However, we also observe that electron-deficient carbonyls have a significantly lower barrier to Fe(III)-mediated oxetane formation. Balancing the factors implicated by our model, we predict the structures of possible metathesis-active molecules as well as oxetane-forming molecules.

Hyperconjugation-Driven Isodesmic Reaction of Indoles and Anilines: Reaction Discovery, Mechanism Study, and Antitumor Application

Isodesmic reactions, in which chemical bonds are redistributed between substrates and products, provide a general and powerful strategy for both biological and chemical synthesis. However, most isodesmic reactions involve either metathesis or functional-group transfer. Here, we serendipitously discovered a novel isodesmic reaction of indoles and anilines that proceeds intramolecularly under weakly acidic conditions. In this process, the five-membered ring of the indole motif is broken and a new indole motif is constructed on the aniline side, accompanied by the formation of a new aniline motif. Mechanistic studies revealed the pivotal role of σ→π* hyperconjugation on the nitrogen atom of the indole motif in driving this unusual isodesmic reaction. Furthermore, we successfully synthesized a diverse series of polycyclic indole derivatives; among quinolines, potential antitumor agents were identified using cellular and in vivo experiments, thereby demonstrating the synthetic utility of the developed methodology.

Cyclization Strategies in Carbonyl-Olefin Metathesis: An Up-to-Date Review

The metathesis reaction between carbonyl compounds and olefins has emerged as a potent strategy for facilitating swift functional group interconversion and the construction of intricate organic structures through the creation of novel carbon-carbon double bonds. To date, significant progress has been made in carbonyl-olefin metathesis reactions, where oxetane, pyrazolidine, 1,3-diol, and metal alkylidene have been proved to be key intermediates. Recently, several reviews have been disclosed, focusing on distinct catalytic approaches for achieving carbonyl-olefin metathesis. However, the summarization of cyclization strategies for constructing aromatic heterocyclic frameworks through carbonyl-olefin metathesis reactions has rarely been reported. Consequently, we present an up-to-date review of the cyclization strategies in carbonyl-olefin metathesis, categorizing them into three main groups: the formation of monocyclic compounds, bicyclic compounds, and polycyclic compounds. This review delves into the underlying mechanism, scope, and applications, offering a comprehensive perspective on the current strength and the limitation of this field.

Intermolecular C-C/C-N σ-bond metathesis enabled by visible light

Transition-metal-catalyzed double/triple bond metathesis reactions have been well-established due to the ability of transition-metal catalysts to readily interact with π bonds, facilitating the progression of the entire reaction. However, activating σ-bonds to induce σ-bond metathesis is more challenging due to the absence of π bonds and the high bond energy of σ bonds. In this study, we present a novel photo-induced approach that does not rely on transition metals or photosensitizers to drive C-C and C-N σ-bond metathesis reactions. This method enables the cross-coupling of tertiary amines with α-diketones via C-C and C-N single bonds cleavage and recombination. Notably, our protocol exhibits good compatibility with various functional groups in the absence of transition metals and external photosensitizers, resulting in the formation of aryl alkyl ketones and aromatic amides in good to high yields. To gain insights into the mechanism of this pathway, we conducted controlled experiments, intermediate trapping experiments, and DFT (Density Functional Theory) calculations. This comprehensive approach allowed us to elucidate the detailed mechanism underlying this transformative reaction.

Double strain-release enables formal C-O/C-F and C-N/C-F ring-opening metathesis

Metathesis reactions have been established as a powerful tool in organic synthesis. While great advances were achieved in double-bond metathesis, like olefin metathesis and carbonyl metathesis, single-bond metathesis has received less attention in the past decade. Herein, we describe the first C(sp3)-O/C(sp3)-F bond formal cross metathesis reaction between gem-difluorinated cyclopropanes (gem-DFCPs) and epoxides under rhodium catalysis. The reaction involves the formation of a highly electrophilic fluoroallyl rhodium intermediate, which is capable of reacting with the oxygen atom in epoxides as weak nucleophiles followed by C-F bond reconstruction. The use of two strained ring substrates is the key to the success of the formal cross metathesis, in which the double strain release accounts for the driving force of the transformation. Additionally, azetidine also proves to be a suitable substrate for this transformation. The reaction offers a novel approach for the metathesis of C(sp3)-O and C(sp3)-N bonds, presenting new opportunities for single-bond metathesis.

Enantioselective Ring-Closing Aminomethylamination of Allylic Aminodienes with Aminals Triggered by C-N Bond Metathesis

A conceptually novel strategy utilizing a cyclopalladated complex as an electrophile to activate the C-N bond for the C-N bond metathesis between allylamines and aminals is developed, which enables an efficient ring-closing aminomethylamination of allylic aminodienes and aminals. The reaction proceeds under mild reaction conditions and displays a remarkable scope. Utilizing a modified Trost-type diphosphine as the ligand, this method enables the efficient synthesis of 5-10-membered aminoallylated chiral N-heterocycles in good yields with high enantiomeric excess values.

Olefination of Aromatic Carbonyls via Site-Specific Activation of Cycloalkanone Ketals

Skeletal editing is an important strategy in organic synthesis as it modifies the carbon backbone to tailor molecular structures with precision, enabling access to compounds with specific desired properties. Skeletal editing empowers chemists to transform synthetic approaches of target compounds across diverse applications from drug discovery to materials science. Herein, we introduce a new skeletal editing method to convert readily available aromatic carbonyl compounds into valuable unsaturated carboxylic acids with extended carbon chains. Our reaction setup enables a cascade reaction of enolization-[2+2]cycloaddition-[2+2]cycloreversion between aromatic carbonyl compounds and ketals of cyclic ketones to generate unsaturated carboxylic acids as ring-opening products. Through a simple design, our substrates are specifically activated to react at predetermined positions to enhance selectivity and efficiency. This practical method offers convenient access to versatile organic building blocks as well as provides fresh insights into manipulating traditional reaction pathways for new synthetic applications.

InCl3-catalyzed intramolecular carbonyl-olefin metathesis

An efficient and novel synthetic strategy for the generation of different carbocyclic moieties by ring closing carbonyl-olefin metathesis is reported. Herein, we describe a sustainably attractive protocol for one of the most powerful carbon-carbon bond-forming reactions, based on solvent-reduction, use of InCl3 catalyst, and microwave irradiation, affording target compounds with yields up to 96%.

Controlling Catalyst Behavior in Lewis Acid-Catalyzed Carbonyl-Olefin Metathesis

Lewis acid-catalyzed carbonyl-olefin metathesis has introduced a new means for revealing the behavior of Lewis acids. In particular, this reaction has led to the observation of new solution behaviors for FeCl3 that may qualitatively change how we think of Lewis acid activation. For example, catalytic metathesis reactions operate in the presence of superstoichiometric amounts of carbonyl, resulting in the formation of highly ligated (octahedral) iron geometries. These structures display reduced activity, decreasing catalyst turnover. As a result, it is necessary to steer the Fe-center away from inhibiting pathways to improve the reaction efficiency and augment yields for recalcitrant substrates. Herein, we examine the impact of the addition of TMSCl to FeCl3-catalyzed carbonyl-olefin metathesis, specifically for substrates that are prone to byproduct inhibition. Through kinetic, spectroscopic, and colligative experiments, significant deviations from the baseline metathesis reactivity are observed, including mitigation of byproduct inhibition as well as an increase in the reaction rate. Quantum chemical simulations are used to explain how TMSCl induces a change in catalyst structure that leads to these kinetic differences. Collectively, these data are consistent with the formation of a silylium catalyst, which induces the reaction through carbonyl binding. The FeCl3 activation of Si-Cl bonds to give the silylium active species is expected to have significant utility in enacting carbonyl-based transformations.

Catalytic Olefin-Imine Metathesis: Cobalt-Enabled Amidine Olefination with Enaminones

Organic metathesis reactions allow for expedient assembly of diverse molecular skeletons and appendages through the exchange of molecular fragments. The olefin-imine variant of this process, in particular, can expand the synthetic toolbox for manipulating carbon-carbon and carbon-nitrogen bonds but has thus far been achieved only on a stoichiometric metal-mediated basis. Herein, we report the development of a catalytic olefin-imine metathesis reaction, featuring cobalt-catalyzed amidine olefination with enaminones and a versatile product synthon enabling further structural diversification.

Ring-closing C-O/C-O metathesis of ethers with primary aliphatic alcohols

In canonical organic chemistry textbooks, the widely adopted mechanism for the classic transetherifications between ethers and alcohols starts with the activation of the ether in order to weaken the C-O bond, followed by the nucleophilic attack by the alcohol hydroxy group, resulting in a net C-O/O-H σ-bond metathesis. In this manuscript, our experimental and computational investigation of a Re₂O₇ mediated ring-closing transetherification challenges the fundamental tenets of the traditional transetherification mechanism. Instead of ether activation, the alternative activation of the hydroxy group followed by nucleophilic attack of ether is realized by commercially available Re₂O₇ through the formation of perrhenate ester intermediate in hexafluoroisopropanol (HFIP), which results in an unusual C-O/C-O σ-bond metathesis. Due to the preference for the activation of alcohol rather than ether, this intramolecular transetherification reaction is therefore suitable for substrates bearing multiple ether moieties, unparalleled by any previous methods.

Toward Homogeneous Brønsted-Acid-Catalyzed Intramolecular Carbonyl-Olefin Metathesis Reactions

The carbonyl-olefin metathesis (COM) reaction is an attractive approach for the formation of a new carbon-carbon double bond from a carbonyl precursor. In principle, this reaction can be promoted by the activation of the carbonyl group with a Brønsted acid catalyst; however, it is often complicated as a result of unwanted side reactions under acidic conditions. Thus, there have been only a very few examples of Brønsted-acid-catalyzed COM reactions, all of which required specially designed setups. Herein, we report a new practical homogeneous Brønsted-acid-catalyzed protocol using nitromethane, a readily available solvent, to promote intramolecular ring-closing COM reactions.

FeCl3-Catalyzed Synthesis of Tanshinlactone Analogues from 1H-Indene-1,2,3-triones and Alkynes

Tanshinlactone has been found in several natural products and biologically active compounds. Herein, a new FeCl₃-catalyzed strategy using 1H-indene-1,2,3-triones and alkynes as starting materials is reported to obtain various tanshinlactone derivatives. This protocol has the advantages of involving low cost, environmentally benign catalysts, simple reaction conditions, and a broad range of substrates.

Metal-Free Ring-Opening Metathesis Polymerization with Hydrazonium Initiators

The ring-opening metathesis polymerization (ROMP) of cyclopropenes using hydrazonium initiators is described. The initiators, which are formed by the condensation of 2,3-diazabicyclo[2.2.2]octane and an aldehyde, polymerize cyclopropene monomers by a sequence of [3+2] cycloaddition and cycloreversion reactions. This process generates short chain polyolefins (M_n ≤9.4 kg mol^-1 ) with relatively low dispersities (Đ≤1.4). The optimized conditions showed efficiency comparable to that achieved with Grubbs' 2^nd generation catalyst for the polymerization of 3-methyl-3-phenylcyclopropene. A positive correlation between monomer to initiator ratio and degree of polymerization was revealed through NMR spectroscopy.

A Removable Acyl Group Promoted the Intramolecular Dehydro-Diels-Alder Reaction of Styrene-Ynes: Highly Chemoselective Synthesis of Aryldihydronaphthalene Derivatives

A removable acyl group promoted the intramolecular didehydro-Diels-Alder reaction of styrene-ynes under mild reaction conditions is proposed. The reaction is free of metals and catalysts, is easy to perform, and exhibits good functional group tolerance, providing a highly chemoselective approach for obtaining the valuable aryldihydronaphthalene derivatives.

Merging Carbonyl Addition with Photocatalysis

The carbonyl group stands as a fundamental scaffold and plays a ubiquitous role in synthetically important chemical reactions in both academic and industrial contexts. Venerable transformations, including the aldol reaction, Grignard reaction, Wittig reaction, and Nozaki-Hiyama-Kishi reaction, constitute a vast and empowering synthetic arsenal. Notwithstanding, two-electron mechanisms inherently confine the breadth of accessible reactivity and topological patterns.Fostered by the rapid development of photoredox catalysis, combing well-entrenched carbonyl addition and radicals can harness several unique and increasingly sustainable transformations. In particular, unusual carbon-carbon and carbon-heteroatom disconnections, which are out of reach of two-electron carbonyl chemistry, can be conceived. To meet this end, a novel strategy toward the utilization of simple carbonyl compounds as intermolecular radical acceptors was developed. The reaction is enabled by visible-light photoredox-initiated hole catalysis. In situ Brønsted acid activation of the carbonyl moiety prevents β-scission from occurring. Furthermore, this regioselective alkyl radical addition reaction obviates the use of metals, ligands, or additives, thus offering a high degree of atom economy under mild conditions. On the basis of the same concept and the work of Schindler and co-workers, carbonyl-olefin cross-metathesis, induced by visible light, has also been achieved, leveraging a radical Prins-elimination sequence.Recently, dual chromium and photoredox catalysis has been developed by us and Kanai, offering a complementary approach to the revered Nozaki-Hiyama-Kishi reaction. Leveraging the intertwined synergy between light and metal, several radical-to-polar crossover transformations toward eminent molecular motifs have been developed. Reactions such as the redox-neutral allylation of aldehydes and radical carbonyl alkylation can harvest the power of light and enable the use of catalytic chromium metal. Overall, exquisite levels of diastereoselectivity can be enforced via highly compact transition states. Other examples, such as the dialkylation of 1,3-dienes and radical carbonyl propargylation portray the versatile combination of radicals and carbonyl addition in multicomponent coupling endeavors. Highly valuable motifs, which commonly occur in complex drug and natural product architectures, can now be accessed in a single operational step. Going beyond carbonyl addition, seminal contributions from Fagnoni and MacMillan preconized photocatalytic HAT-based acyl radical formation as a key aldehyde valorization strategy. Our group articulated this concept, leveraging carboxy radicals as hydrogen atom abstractors in high regio- and chemoselective carbonyl alkynylation and aldehyde trifluoromethylthiolation.This Account, in addition to the narrative of our group and others' contributions at the interface between carbonyl addition and radical-based photochemistry, aims to provide core guiding foundations toward novel disruptive synthetic developments. We envisage that extending radical-to-polar crossovers beyond Nozaki-Hiyama-Kishi manifolds, taming less-activated carbonyls, leveraging multicomponent processes, and merging single electron steps with energy-transfer events will propel eminent breakthroughs in the near future.

Hydrogen Bonding Networks Enable Brønsted Acid-Catalyzed Carbonyl-Olefin Metathesis

Synthetic chemists have learned to mimic nature in using hydrogen bonds and other weak interactions to dictate the spatial arrangement of reaction substrates and to stabilize transition states to enable highly efficient and selective reactions. The activation of a catalyst molecule itself by hydrogen-bonding networks, in order to enhance its catalytic activity to achieve a desired reaction outcome, is less explored in organic synthesis, despite being a commonly found phenomenon in nature. Herein, we show our investigation into this underexplored area by studying the promotion of carbonyl-olefin metathesis reactions by hydrogen-bonding-assisted Brønsted acid catalysis, using hexafluoroisopropanol (HFIP) solvent in combination with para-toluenesulfonic acid (pTSA). Our experimental and computational mechanistic studies reveal not only an interesting role of HFIP solvent in assisting pTSA Brønsted acid catalyst, but also insightful knowledge about the current limitations of the carbonyl-olefin metathesis reaction.

Expanding the Protecting Group Scope for the Carbonyl Olefin Metathesis Approach to 2,5-Dihydropyrroles

Chiral pyrrolidine derivatives are important building blocks for natural product synthesis. Carbonyl olefin metathesis has recently emerged as a powerful tool for the construction of such building blocks from chiral amino acid derivatives. Here, we demonstrate that the supramolecular resorcinarene catalyst enables access to chiral 2,5-dihydropyrroles under Brønsted acid catalysis. Moreover, this catalytic system even tolerated Lewis-basic-protecting groups like mesylates that are not compatible with alternative catalysts. As expected for conversion inside a closed cavity, the product yield and selectivity depended on the size of the substrates.

Brønsted Acid-Catalyzed Carbonyl-Olefin Metathesis: Synthesis of Phenanthrenes via Phosphomolybdic Acid as a Catalyst

Compared with the impressive achievements of catalytic carbonyl-olefin metathesis (CCOM) mediated by Lewis acid catalysts, exploration of the CCOM through Brønsted acid-catalyzed approaches remains quite challenging. Herein, we disclose a synthetic protocol for the construction of a valuable polycycle scaffold through the CCOM with the inexpensive, nontoxic phosphomolybdic acid as a catalyst. The current annulations could realize carbonyl-olefin, carbonyl-alcohol, and acetal-alcohol in situ CCOM reactions and feature mild reaction conditions, simple manipulation, and scalability, making this strategy a promising alternative to the Lewis acid-catalyzed COM reaction.

Carbonyl-Olefin Metathesis

This Review describes the development of strategies for carbonyl-olefin metathesis reactions relying on stepwise, stoichiometric, or catalytic approaches. A comprehensive overview of currently available methods is provided starting with Paternò-Büchi cycloadditions between carbonyls and alkenes, followed by fragmentation of the resulting oxetanes, metal alkylidene-mediated strategies, [3 + 2]-cycloaddition approaches with strained hydrazines as organocatalysts, Lewis acid-mediated and Lewis acid-catalyzed strategies relying on the formation of intermediate oxetanes, and protocols based on initial carbon-carbon bond formation between carbonyls and alkenes and subsequent Grob-fragmentations. The Review concludes with an overview of applications of these currently available methods for carbonyl-olefin metathesis in complex molecule synthesis. Over the past eight years, the field of carbonyl-olefin metathesis has grown significantly and expanded from stoichiometric reaction protocols to efficient catalytic strategies for ring-closing, ring-opening, and cross carbonyl-olefin metathesis. The aim of this Review is to capture the status quo of the field and is expected to contribute to further advancements in carbonyl-olefin metathesis in the coming years.

Combined Theoretical and Experimental Investigation of Lewis Acid-Carbonyl Interactions for Metathesis

The coordination of a carbonyl to a Lewis acid represents the first step in a wide range of catalytic transformations. In many reactions it is necessary for the Lewis acid to discriminate between starting material and product, and as a result, how these structures behave in solution must be characterized. Herein, we report the application of computational modeling to calculate properties of the solution interactions of acetone and benzaldehyde with FeCl₃. Using these chemical models, we can predict spectral features in the carbonyl region of infrared (IR) spectroscopy. These simulated spectra are then directly compared to experimental spectra generated via titration-IR. We observe good agreement between theory and experiment, in that, between 0 and 1 equiv carbonyl with respect to FeCl₃, a pairwise interaction dominates the spectra. When >1 equiv carbonyl is present, our theoretical model predicts two possible structures composed of 4:1 carbonyl to FeCl₃, for acetone as well as benzaldehyde. When these predicted spectra are compared with titration-IR data, both structures contribute to the observed solution interactions. These findings suggest that the resting state of FeCl₃-catalyzed carbonyl-based reactions employing simple substrates starts as a Lewis pair, but this structure is gradually consumed and becomes a highly ligated, catalytically less active Fe-centered complex as the reaction proceeds. An analytical model is proposed to quantify catalyst inhibition due to equilibrium between 1:1 and 4:1 carbonyl:Fe complexes.

Brønsted-Acid-Catalyzed Intramolecular Carbonyl-Olefin Reactions: Interrupted Metathesis vs Carbonyl-Ene Reaction

Lewis acid catalysts have been shown to promote carbonyl-olefin metathesis through a critical four-membered-ring oxetane intermediate. Recently, Brønsted-acid catalysis of related substrates was similarly proposed to result in a transient oxetane, which fragments within a single elementary step via a postulated oxygen-atom transfer mechanism. Herein, careful quantum chemical investigations show that Brønsted acid (triflic acid, TfOH) instead invokes a mechanistic switch to a carbonyl-ene reaction, and oxygen-atom transfer is uncompetitive. TfOH's conjugate base is also found to rearrange H atoms and allow isomerization of the carbocations that appear after the carbonyl-ene reaction. The mechanism explains available experimental information, including the skipped diene species that appear transiently before product formation. The present study clarifies the mechanism for activation of intramolecular carbonyl-olefin substrates by Brønsted acids and provides important insights that will help develop this exciting class of catalysts.

Iron-Catalyzed Carbonyl–Alkyne and Carbonyl–Olefin Metathesis Reactions

Construction of carbon–carbon bonds is one of the most important tools for the synthesis of complex organic molecules. Among multiple possibilities are the carbonyl–alkyne and carbonyl–olefin metathesis reactions, which are used to form new carbon–carbon bonds between carbonyl derivatives and unsaturated organic compounds. As many different approaches have already been established and offer reliable access to C=C bond formation via carbonyl–alkyne and carbonyl–olefin metathesis, focus is now shifting towards cost efficiency, sustainability and environmentally friendly metal catalysts. Iron, which is earth-abundant and considered as an eco-friendly and inexpensive option in comparison to traditional metal catalysts, fulfils these requirements. Hence, the focus of this review is on recent advances in the iron-catalyzed carbonyl–alkyne, carbonyl–olefin and related C–O/C–O metathesis reactions. The still large research potential for ecologically and economically attractive and sustainable iron-based catalysts is demonstrated.

Synthesis of 1,2-Dihydroquinolines via Hydrazine-Catalyzed Ring-Closing Carbonyl-Olefin Metathesis

The synthesis of 1,2-dihydroquinolines by the hydrazine-catalyzed ring-closing carbonyl-olefin metathesis (RCCOM) of N-prenylated 2-aminobenzaldehydes is reported. Substrates with a variety of substitution patterns are shown. With an acid-labile protecting group on the nitrogen atom, in situ deprotection and autoxidation furnish quinoline. In comparison with related oxygen-containing substrates, the cycloaddition step of the catalytic cycle is shown to be slower, but the cycloreversion is found to be more facile.

Ring-opening carbonyl-olefin metathesis of norbornenes

A computational and experimental study of the hydrazine-catalyzed ring-opening carbonyl-olefin metathesis of norbornenes is described. Detailed theoretical investigation of the energetic landscape for the full reaction pathway with six different hydrazines revealed several crucial aspects for the design of next-generation hydrazine catalysts. This study indicated that a [2.2.2]-bicyclic hydrazine should offer substantially increased reactivity versus the previously reported [2.2.1]-hydrazine due to a lowered activation barrier for the rate-determining cycloreversion step, a prediction which was verified experimentally. Optimized conditions for both cycloaddition and cycloreversion steps were identified, and a brief substrate scope study for each was conducted. A complication for catalysis was found to be the slow hydrolysis of the ring-opened hydrazonium intermediates, which were shown to suffer from a competitive and irreversible cycloaddition with a second equivalent of norbornene. This problem was overcome by the strategic incorporation of a bridgehead methyl group on the norbornene ring, leading to the first demonstrated catalytic carbonyl-olefin metathesis of norbornene rings.

Intermolecular Carbonyl-olefin Metathesis with Vinyl Ethers Catalyzed by Homogeneous and Solid Acids in Flow

The carbonyl-olefin metathesis reaction has experienced significant advances in the last seven years with new catalysts and reaction protocols. However, most of these procedures involve soluble catalysts for intramolecular reactions in batch. Herein, we show that recoverable, inexpensive, easy to handle, non-toxic, and widely available simple solid acids, such as the aluminosilicate montmorillonite, can catalyze the intermolecular carbonyl-olefin metathesis of aromatic ketones and aldehydes with vinyl ethers in-flow, to give alkenes with complete trans stereoselectivity on multi-gram scale and high yields. Experimental and computational data support a mechanism based on a carbocation-induced Grob fragmentation. These results open the way for the industrial implementation of carbonyl-olefin metathesis over solid catalysts in continuous mode, which is still the origin and main application of the parent alkene-alkene cross-metathesis.

Characterizing Lewis Pairs Using Titration Coupled with In Situ Infrared Spectroscopy

Lewis acid-activation of carbonyl-containing substrates is a fundamental basis for facilitating transformations in organic chemistry. Historically, characterization of these interactions has been limited to models equivalent to stoichiometric reactions. Here, we report a method utilizing in situ infrared spectroscopy to probe the solution interactions between Lewis acids and carbonyls under synthetically relevant conditions. Using this method, we were able to identify 1:1 complexation between GaCl3 and acetone and a highly ligated complex for FeCl3 and acetone. The impact of this technique on mechanistic understanding is illustrated by application to the mechanism of Lewis acid-mediated carbonyl-olefin metathesis in which we were able to observe competitive binding interactions between substrate carbonyl and product carbonyl with the catalyst.

AuCl3 -Catalyzed Ring-Closing Carbonyl-Olefin Metathesis

Compared with the ripeness of olefin metathesis, exploration of the construction of carbon-carbon double bonds through the catalytic carbonyl-olefin metathesis reaction remains stagnant and has received scant attention. Herein, a highly efficient AuCl₃ -catalyzed intramolecular ring-closing carbonyl-olefin metathesis reaction is described. This method features easily accessible starting materials, simple operation, good functional-group tolerance and short reaction times, and provides the target cyclopentenes, polycycles, benzocarbocycles, and N-heterocycle derivatives in good to excellent yields.

Investigation of Lewis Acid-Carbonyl Solution Interactions via Infrared-Monitored Titration

Lewis acid-activation of carbonyl-containing substrates is broadly utilized in organic synthesis. In order to facilitate the development of novel reaction pathways and understand existing methods, it is necessary to determine the solution interactions between Lewis acids and Lewis bases. Herein, we report the application of in situ infrared spectroscopy and solution conductivity toward the identification of the solution structures formed when a range of carbonyl compounds are combined with catalytically active metal halide Lewis acids under synthetically relevant conditions. These data are consistent with formation of Lewis acid-dependent complexes, where metals of low relative Lewis acidity display no ground state interaction with carbonyls. Conversely, we observed the formation of polyligated complexes when stronger Lewis acids (SnCl₄, TiCl₄, ZrCl₄, FeCl₃, and AlCl₃) were treated with ketones, aldehydes, and esters. This collection of observations is intended to assist the synthetic chemist in the design of new catalysts and the development of novel methods.

Zinc-Catalyzed Alkyne-Carbonyl Metathesis of Ynamides with Isatins: Stereoselective Access to Fully Substituted Alkenes

A zinc-catalyzed intermolecular alkyne-carbonyl metathesis reaction of ynamides with isatins followed by an amide to ester conversion has been developed, which produces the indolone derivatives with a fully substituted alkene species in good to high yields. The salient features of this reaction include the following: mild reaction conditions, an inexpensive zinc catalyst, a broad substrate scope, the excellent regiocontrol and stereoselectivity, and amenable to the gram scale.

Synthesis of 2H-Chromenes via Hydrazine-Catalyzed Ring-Closing Carbonyl-Olefin Metathesis

The catalytic ring-closing carbonyl-olefin metathesis (RCCOM) of O-allyl salicylaldehydes to form 2H-chromenes is described. The method utilizes a [2.2.1]-bicyclic hydrazine catalyst and operates via a [3+2]/retro-[3+2] metathesis manifold. The nature of the allyl substitution pattern was found to be crucial, with sterically demanding groups such as adamantylidene or diethylidene offering optimal outcomes. A survey of substrate scope is shown along with a discussion of mechanism supported by DFT calculations. Steric pressure arising from syn-pentane minimization of the diethylidene moiety is proposed to facilitate cycloreversion.

Catalytic, transannular carbonyl-olefin metathesis reactions

Transannular carbonyl-olefin metathesis reactions complement existing procedures for related ring-closing, ring-opening, and intermolecular carbonyl-olefin metathesis. We herein report the development and mechanistic investigation of FeCl₃-catalyzed transannular carbonyl-olefin metathesis reactions that proceed via a distinct reaction path compared to previously reported ring-closing and ring-opening protocols. Specifically, carbonyl-ene and carbonyl-olefin metathesis reaction pathways are competing under FeCl₃-catalysis to ultimately favor metathesis as the thermodynamic product. Importantly, we show that distinct Lewis acid catalysts are able to distinguish between these pathways to enable the selective formation of either transannular carbonyl-ene or carbonyl-olefin metathesis products. These insights are expected to enable further advances in catalyst design to efficiently differentiate between these two competing reaction paths of carbonyl and olefin functionalities to further expand the synthetic generality of carbonyl-olefin metathesis.