Vanadium-Catalyzed Synthesis of Geometrically Defined Acyclic Tri- and Tetrasubstituted Olefins from Propargyl Alcohols

Metal-Free, Adjustable, and Recyclable Catalytic Systems for the Construction of C-C Bonds by Activating Propargylic Alcohols

Pyrano[3,2-c]coumarin derivatives and C3-substituted 4-hydroxycoumarins as important skeletal structures of active natural products and pharmaceutically relevant molecules have received increasing attention. However, developing an adjustable system for selectively synthesizing them is still a challenging task. Herein, sulfonic acid-functionalized ionic liquid was successfully used as the catalyst for the alkylation of 4-hydroxycoumarin derivatives with secondary aromatic propargylic alcohols using dimethyl carbonate as the green solvent, giving up to 98% yield. On the other hand, protonated imidazole-based ionic liquid-catalyzed cyclization was also selectively achieved with a nearly quantitative yield. Developed metal-free catalytic systems exhibited well adjustable and recyclable properties, avoiding the contamination of metal and halogen, reducing the neutralization after the reaction, and benefiting the separation between the catalyst and the product. New strategies were applied for performing the gram-scale reaction smoothly. The adjustable systems might occur through two different mechanisms involving propargylic or allenic carbocation and hydrogen bonding effects between the catalysts and the substrates.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Activation of o-Propargyl Alcohol Benzaldehydes under Acetalization Conditions for Intramolecular Electrophile Intercepted Meyer-Schuster Rearrangement

The reactivity of o-propargyl alcohol benzaldehydes has been increased tremendously toward Brønsted acid-catalyzed intramolecular electrophile intercepted Meyer-Schuster (M-S) rearrangement under acetalization conditions using trimethyl orthoformate (TMOF). The in situ formed acetal transfers the methoxy group intramolecularly to generate the M-S intermediate in even less reactive substrates, and the formed oxocarbenium ion makes the carbonyl more electrophilic for an effective intramolecular trapping of the M-S intermediate to furnish the indanone derivatives.

A Metal-Free Path to 2-Iodo-3-alkyl-1-arylbut-2-en-1-ones and Their Application to the Domino Synthesis of Functionalized 2H-Pyran-2-ones

We report a metal-free selective synthesis of 2-iodo-3-alkyl-1-arylbut-2-en-1-ones from propargylic alcohols that is enabled by N-iodosuccinimide. A variety of substituted propargylic alcohols are amenable to delivering the selective 2-iodoenone products in very good yields. The utility of the α-iodoenone derivatives is further extended by developing an efficient, novel, and new synthetic methodology for the synthesis of 3,5,6-trisubstituted 2H-pyran-2-ones. To the best of our knowledge, this protocol is the first of its kind to accomplish 3,5,6-trisubstituted 2H-pyran-2-ones through an unprecedented domino (formation of two C-C bonds and one C-O bond) one-pot process via intermolecular Heck coupling, base-driven Michael addition, and base-mediated double bond isomerization followed by cyclo-condensation. This protocol showed good compatibility with a wide range of iodoenones (18 examples) and 2H-pyran-2-ones (42 examples). Mechanistic studies indicate that palladium is only involved in the Heck coupling; the base solely drives the rest of the steps.

A metal-free tandem dehydrogenative α-arylation reaction of propargylic alcohols with 2-alkynylbenzaldoximes toward the synthesis of α-(4-bromo-isoquinolin-1-yl)-propenone skeletons

A tandem reaction of 2-alkynylbenzaldoximes with propargylic alcohols has been developed for the synthesis of α-(4-bromo-isoquinolin-1-yl)-propenones. Employing 2-alkynylbenzaldoximes as a precursor in the presence of Br₂ generates 4-bromo-isoquinoline-N-oxides. Subsequently, dehydroxylation of propargylic alcohols gives carbocation intermediates, which are trapped using the N-oxides, affording aryl-substituted α-enones.

Electrophilic halogenations of propargyl alcohols: paths to α-haloenones, β-haloenones and mixed β,β-dihaloenones

The Meyer–Schuster rearrangement of propargyl alcohols or alkynols leading to α,β-unsaturated carbonyl compounds is well known. Yet, electrophilic halogenations of the same alkynols and their alkoxy, ester and halo derivatives are inconspicuous. This review on the halogenation reactions of propargyl alcohols and derivatives intends to give a perspective from its humble direct halogenation beginning to the present involving metal catalysis. The halogenation products of propargyl alcohols include α-fluoroenones, α-chloroenones, α-bromoenones and α-iodoenones, as well as β-haloenones and symmetrical and mixed β,β-dihaloenones. They are, in essence, tri and tetrasubstituted alkenes carrying halo-functionalization at the α- or β-carbon. This is a potential stepping stone for further construction towards challenging substituted alkenones via Pd-catalysed coupling reactions.

This review highlights the development of α-haloenone, β-haloenone and mixed β,β-dihaloenone formations from propargyl alcohols via direct electrophilic halogenations and metal catalysed-halonium interception rearrangements.

Origin of Ligand Effects on Stereoinversion in Pd-Catalyzed Synthesis of Tetrasubstituted Olefins

The mechanism and origin of ligand effects on stereoinversion of Pd-catalyzed synthesis of tetrasubstituted olefins were investigated using DFT calculations and the approach of energy decomposition analysis (EDA). The results reveal that the stereoselectivity-determining steps are different when employing different phosphine ligands. This is mainly due to the steric properties of ligands. With the bulkier Xantphos ligand, the syn/anti-to-Pd 1,2-migrations determine the stereoselectivity. While using the less hindered P(o-tol)₃ ligand, the 1,3-migration is the stereoselectivity-determining step. The EDA results demonstrate that Pauli repulsion and polarization are the dominant factors for controlling the stereochemistry in 1,2- and 1,3-migrations, respectively. The origins of differences of Pauli repulsion and polarization between the two stereoselective transition states are further identified.

Ligand-dependent stereoselective Suzuki-Miyaura cross-coupling reactions of β-enamido triflates

The stereoselective Suzuki–Miyaura cross-coupling of (Z)-β-enamido triflates is demonstrated. Depending on the nature of the ligand in the palladium catalyst, either retention or inversion of the configuration during the synthesis of β,β-diaryl-substituted enamides is observed. Thus, the method provides synthetic access to both isomers of the target enamides from (Z)-β-enamido triflates.

Stereoconvergent and -divergent Synthesis of Tetrasubstituted Alkenes by Nickel-Catalyzed Cross-Couplings

We report the development of a method to diastereoselectively access tetrasubstituted alkenes via nickel-catalyzed Suzuki-Miyaura cross-couplings of enol tosylates and boronic acid esters. Either diastereomeric product was selectively accessed from a mixture of enol tosylate starting material diastereomers in a convergent reaction by judicious choice of the ligand and reaction conditions. A similar protocol also enabled a divergent synthesis of each product isomer from diastereomerically pure enol tosylates. Notably, high-throughput optimization of the monophosphine ligands was guided by chemical space analysis of the kraken library to ensure a diverse selection of ligands was examined. Stereoelectronic analysis of the results provided insight into the requirements for reactive and selective ligands in this transformation. The synthetic utility of the optimized catalytic system was then probed in the stereoselective synthesis of various tetrasubstituted alkenes, with yields up to 94% and diastereomeric ratios up to 99:1 Z/E and 93:7 E/Z observed. Moreover, a detailed computational analysis and experimental mechanistic studies provided key insights into the nature of the underlying isomerization process impacting selectivity in the cross-coupling.

Lewis Acid-Driven Meyer-Schuster-Type Rearrangement of Yne-Dienone

Developed herein is a Cu(II)-catalyzed Meyer-Schuster-type rearrangement of alkyne-tethered cyclohexadienone for the construction of m-enone-substituted phenols. The reaction involves an uncommon 5-exo-trig 1,6-enyne cyclization of alkyne-tethered-cyclohexadienone, aromatization-triggered C-O bond cleavage, and an electrocyclic 4π-ring-opening of oxetene intermediate. This atom-efficient transformation provides access to a wide range of synthetically important α-(m-substituted phenol)-α,β-unsaturated ketones, featuring a broad scope with labile functional group tolerance. The gram-scale demonstration makes this transformation synthetically viable. The synthetic application of α,β-unsaturated ketones is also showcased.

Recent advances in the stereoselective synthesis of acyclic all-carbon tetrasubstituted alkenes

Alkenes bearing four carbon-based groups are ubiquitous motifs in chemical sciences due to their various applications from medicinal to materials chemistry, and as chemical platforms for the synthesis of complex, chiral molecules. As such, tremendous research efforts are currently ongoing in order to develop general procedures for the challenging stereoselective synthesis of all-carbon tetrasubstituted alkenes, especially for acyclic structures. Since classical approaches to carbon-carbon double bonds are not suitable for the high steric demand around tetrasubstituted alkenes, a variety of unique approaches to access these privileged functional groups have been developed in recent years. This review article highlights the most significant developments in the field from 2007 to 2020, with an emphasis on the mechanisms and remaining limitations of these contemporary methods. Specifically, recent advances in internal alkyne carbofunctionalizations, in multicomponent couplings or other cross-couplings from nucleophilic or electrophilic alkenyl partners, and in the development of miscellaneous methods, are discussed.

A benzene-bridged divanadium complex-early transition metal catalyst for alkene alkylarylation with PhI(O2CR)2via decarboxylation

The synthesis, structure and catalytic activity of a benzene-bridged divanadium complex were comprehensively studied. The reduction of (Nacnac)VCl2 (1) (Nacnac = (2,6-iPr2C6H3NCMe)2HC) supported by β-diketiminate with potassium graphite (KC8) by employing benzene as the solvent allows access to the benzene-bridged inverted-sandwich divanadium complex (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a), which can catalyze alkene alkylarylation with hypervalent iodine(iii) reagents (HIRs) via decarboxylation to generate regioselectively diverse indolinones. Furthermore, the mild nature of this reaction was amenable to a wide range of functionalities on alkenes and HIRs. Mechanistic studies revealed a relay sequence of decarboxylative radical alkylation/radical arylation/oxidative re-aromatization.

Iodo(III)-Meyer-Schuster Rearrangement of Propargylic Alcohols Promoted by Benziodoxole Triflate

Benziodoxole triflate (BXT), a cyclic iodine(III) electrophile, has been found to promote a rearrangement of propargylic alcohols into α,β-unsaturated ketones bearing an α-λ³-iodanyl group. This iodo(III)-Meyer-Schuster rearrangement proceeds under mild conditions and tolerates a variety of functionalized propargylic alcohols, thus complementing previously reported halogen-intercepted Meyer-Schuster rearrangement. The α-λ³-iodanylenones can be utilized for facile Pd-catalyzed cross-coupling for the synthesis of multisubstituted enones.

Regioselective synthesis of fused oxa-heterocycles via iodine-mediated annulation of cyclic 1,3-dicarbonyl compounds with propargylic alcohols

The synthesis of structurally diverse fused oxa-heterocycles is established through the iodine-mediated cascade annulation of cyclic 1,3-dicarbonyl compounds with propargylic alcohols.

Pd-Catalyzed Selective Chlorination of Acrylamides at Room Temperature

In this Letter, the transition-metal-catalyzed chlorination of alkenes is reported. In the presence of the commercially available and inexpensive N-chlorosuccinimide and without additive, the Pd-catalyzed chlorination of acrylamides by C-H bond activation was developed at room temperature under air. Under these mild reaction conditions, the versatility of the methodology was demonstrated as an array of acrylamides was functionalized to selectively provide the corresponding difficult-to-synthesize chlorinated olefins as a single Z stereoisomer. Mechanistic studies were conducted to get insights into the reaction mechanism, and post-functionalization reactions further demonstrated the synthetic utility of the approach toward the access to high value-added chlorinated compounds.

Catalytically Generated Vanadium Enolates Formed via Interruption of the Meyer-Schuster Rearrangement as Useful Reactive Intermediates

Enolate chemistry is one of the most fundamental strategies for the formation of carbon-carbon and carbon-heteroatom bonds. Classically, this has been accomplished through the use of stoichiometric quantities of strong base and cryogenic reaction temperatures. However, these techniques present issues related to enolate regioselectivity and functional group tolerance. While more modern methods utilizing stoichiometric activating agents have overcome some of these limitations, these processes add additional steps and suffer from poor atom economy. While certain classes of highly acidic nucleophiles have enabled the development of elegant and general catalytic solutions to address all of these limitations, functionalizing less acidic nucleophiles remains difficult.To overcome these challenges, we developed an alternative general approach for the formation and subsequent functionalization of metal enolates that leverages catalytic amounts of Lewis acid and entirely avoids the need for exogenous base or stoichiometric additives. To do so, we re-engineered the classical Meyer-Schuster rearrangement, which normally converts propargylic alcohols into α,β-unsaturated carbonyl compounds. By careful control of reaction conditions and by selection of an appropriate vanadium-oxo catalyst, the transient metal enolates formed via the 1,3-transposition of propargylic or allenylic alcohols can be guided away from simple protonation reaction pathways and toward more synthetically productive carbon-carbon, carbon-halogen, and carbon-nitrogen bond-forming processes.By utilizing readily available propargylic and allenylic alcohols as our starting materials and relying on a catalytic 1,3-transposition to generate metal enolates in situ, all issues related to the regioselectivity of enolate formation are resolved. Likewise, utilization of a simple isomerization for enolate formation results in a highly efficient process that can be 100% atom economical. The mild reaction conditions employed also allow for remarkable chemoselectivity. Functional groups not typically conducive to enolate chemistry, such as alkynyl ketones, methyl ketones, free alcohols, and primary alkyl halides, are all well tolerated. Finally, by varying the substitution patterns of the alcohol starting materials, enolates of ketones, esters, and even amides are all accessible.Utilizing this strategy starting from propargylic alcohols, we have developed functionalization reactions that produce highly substituted and geometrically defined α-functionalized α,β-unsaturated carbonyl compounds. Such processes include aldol, Mannich, and electrophilic halogenation reactions, as well as dual catalytic reactions wherein catalytically generated vanadium enolates are trapped with catalytically generated palladium π-allyl electrophiles. In the case of allenylic alcohols, we have developed complementary aldol, Mannich, halogenation, and dual catalytic processes to generate α'-functionalized α,β-unsaturated carbonyl products.The results described in this work showcase the power and generality of our alternative approach to enolate chemistry. Additionally, we point out unaddressed challenges in the field and invite other groups to help innovate in these areas.

Brønsted Acid-Catalyzed General Petasis Allylation and Isoprenylation of Unactivated Ketones

Brønsted acid-catalyzed general Petasis allylation and isoprenylation of unactivated ketones were developed by using o-hydroxyaniline and the corresponding pinacolyl boronic esters. This robust methodology provided access to a broad variety of quaternary homoallylic amines and dienyl amines in high yields, proved to be applicable to a gram-scale synthesis, and allowed the synthesis of a potentially bioactive quaternary homoallylic aminodiol.

Selective synthesis of α-olefins by dehydration of fatty alcohols over alumina–thoria mixed catalysts

The selective and high-yield production of α-olefins by alcohol dehydration is challenging because the isomerization and polymerization olefin products are more thermodynamically stable.

Reactivity patterns of vanadium(iv/v)-oxo complexes with olefins in the presence of peroxides: a computational study

Vanadium porphyrin complexes are naturally occurring substances found in crude oil and have been shown to have medicinal properties as well. Little is known on their activities with substrates; therefore, we decided to perform a detailed density functional theory study on the properties and reactivities of vanadium(iv)- and vanadium(v)-oxo complexes with a TPPCl8 or 2,3,7,8,12,13,17,18-octachloro-meso-tetraphenylporphyrinato ligand system. In particular, we investigated the reactivity of [VV(O)(TPPCl8)]+ and [VIV(O)(TPPCl8)] with cyclohexene in the presence of H2O2 or HCO4-. The work shows that vanadium(iv)-oxo and vanadium(v)-oxo are sluggish oxidants by themselves and react with olefins slowly. However, in the presence of hydrogen peroxide, these metal-oxo species can be transformed into a side-on vanadium-peroxo complex, which reacts with substrates more efficiently. Particularly with anionic axial ligands, the side-on vanadium-peroxo and vanadium-oxo complexes produced epoxides from cyclohexene via small barrier heights. In addition to olefin epoxidation, we investigated aliphatic hydroxylation mechanisms by the same oxidants and some oxidants show efficient and viable cyclohexene hydroxylation mechanisms. The work implies that vanadium-oxo and vanadium-peroxo complexes can react with double bonds through epoxidation, and under certain conditions also undergo hydroxylation, but the overall reactivity is highly dependent on the equatorial ligand, the local environment and the presence or absence of anionic axial ligands.