Selective Electrocatalytic Degradation of Ether-Containing Polymers

Leveraging electrochemistry to degrade robust polymeric materials has the potential to impact society's growing issue of plastic waste. Herein, we develop an electrocatalytic oxidative degradation of polyethers and poly(vinyl ethers) via electrochemically mediated hydrogen atom transfer (HAT) followed by oxidative polymer degradation promoted by molecular oxygen. We investigated the selectivity and efficiency of this method, finding our conditions to be highly selective for polymers with hydridic, electron-rich C-H bonds. We leveraged this reactivity to degrade polyethers and poly(vinyl ethers) in the presence of polymethacrylates and polyacrylates with complete selectivity. Furthermore, this method made polyacrylates degradable by incorporation of ether units into the polymer backbone. We quantified degradation products, identifying up to 36 mol % of defined oxidation products, including acetic acid, formic acid, and acetaldehyde, and we extended this method to degrade a polyether-based polyurethane in a green solvent. This work demonstrates a facile, electrochemically-driven route to degrade polymers containing ether functionalities.

2-Nitro-cyclopropyl-1-carbonyl Compounds from Unsaturated Carbonyl Compounds and Nitromethane via Enolonium Species

2-Nitrocyclopropanes bearing ketones, amides, esters, and carboxylic acids in the 1 position may be accessed as single diastereoisomers in one operation from the corresponding unsaturated carbonyl compounds. The source of the nitro-methylene component is nitromethane. The reaction proceeds at room temperature under mild conditions. The products may be converted into, e.g., cyclopropyl-amino acids in a single step. Both nitrocyclopropanes and amino-cyclopropanes are unique moieties found in biologically active compounds and natural products.

2-Fluoroenones via an Umpolung Morita-Baylis-Hillman Reaction of Enones

Several methods have been reported for the formation of 2-fluoroenones. However, all these methods involve laborious multiple-step sequences with resulting low overall yields. In this paper, we report the first formal enone-α-H to F substitution, leading to 2-fluoroenones in a single step from ubiquitous enones in 63-90% yield. The reaction is applicable to a wide range of aromatic and alkenyl enones and is carried out at room temperature using HF-pyridine complex as the fluoride source. Mechanistic investigations support that the reaction takes place through a rare umpolung Morita-Baylis-Hillman-type mechanism.

Total stereocontrolled synthesis of a novel pyrrolizidine iminosugar

Herein we describe a versatile approach to the pyrrolizidine alkaloids skeleton by tailoring our original strategy already used for the pyrrolidine iminosugars synthesis. The key steps are the regio- and stereoselective azidolysis of the suitable chiral vinyl epoxide and then asymmetric dihydroxylation of the corresponding azido alcohol by using (DHQ)₂AQN as the ligand. Further optimized elaborations addressed to the closure of the two rings allowed us to achieve the target iminosugar with complete stereocontrol. The wide range of pyrrolizidine iminosugars' biological properties make them a key focus of new drug research and therefore the development of synthetic strategies for obtaining them is of decisive importance.

Direct Umpolung Morita-Baylis-Hillman like α-Functionalization of Enones via Enolonium Species

Herein we report on the umpolung of Morita-Baylis-Hillman type intermediates and application to the α-functionalization of enone C-H bonds. This reaction gives direct access to α-chloro-enones, 1,2-diketones and α-tosyloxy-enones. The latter are important intermediates for cross-coupling reaction and, to the best of our knowledge, cannot be made in a single step from enones in any other way. The proposed mechanism is supported by spectroscopic studies. The key initial step involves conjugate attack of an amine (DABCO or pyridine), likely assisted by hypervalent iodine acting as a Lewis acid leading to formation of an electrophilic β-ammonium-enolonium species. Nucleophilic attack by acetate, tosylate, or chloride anion is followed by base induced elimination of the ammonium species to give the noted products. Hydrolysis of α-acetoxy-enones lead to formation of 1,2-diketones. The α-tosyl-enones participate in Negishi coupling reactions under standard conditions.

Azido-Enolonium Species in C-C and C-N Bond-Forming Coupling Reactions

Vinyl azides react with boron trifluoride activated Koser's hypervalent iodine reagent to afford azido-enolonium species. These previously unknown azido-enolonium species react efficiently with aromatic compounds, allyltrimethylsilane, and azoles under mild conditions, with no need for a transition-metal catalyst, forming C-C and C-N bonds to give a variety of α-functionalized ketones. The intermediacy of the proposed azido-enolonium species is supported by spectroscopic studies.

Hypervalent iodine reactions utilized in carbon–carbon bond formations

This review covers recent developments of hypervalent iodine chemistry in dearomatizations, radicals, hypervalent iodine-guided electrophilic substitution, arylations, photoredox, and more.

Cross-coupling of dissimilar ketone enolates via enolonium species to afford non-symmetrical 1,4-diketones

Due to their closely matched reactivity, the coupling of two dissimilar ketone enolates to form a 1,4-diketone remains a challenge in organic synthesis. We herein report that umpolung of a ketone trimethylsilyl enol ether (1 equiv) to form a discrete enolonium species, followed by addition of as little as 1.2–1.4 equivalents of a second trimethylsilyl enol ether, provides an attractive solution to this problem. A wide array of enolates may be used to form the 1,4-diketone products in 38 to 74% yield. Due to the use of two TMS enol ethers as precursors, an optimization of the cross-coupling should include investigating the order of addition.

α-N-Heteroarylation and α-Azidation of Ketones via Enolonium Species

Enolonium species, resulting from the umpolung of ketone enolates by Koser's hypervalent iodine reagents activated by boron trifluoride, react with a variety of nitrogen heterocycles to form α-aminated ketones. The reactions are mild and complete in 4-5 h. Additionally, α-azidation of the enolonium species takes place using trimethylsilyl azide as a convenient source of azide nucleophile.

Chiral isoxazolidine-mediated stereoselective umpolung α-phenylation of methyl ketones

Asymmetric nucleophilic phenylation of N-alkoxyenamine with triphenylaluminium is achieved for the formation of enantioenriched α-phenyl ketones through a one-pot process.

Chemoselective Intermolecular Cross-Enolate-Type Coupling of Amides

A new approach for the synthesis of 1,4-dicarbonyl compounds is reported. Chemoselective activation of amide carbonyl functionality and subsequent umpolung viaN-oxide addition generates an electrophilic enolonium species that can be coupled with a wide range of nucleophilic enolates. The method conveys broad functional group tolerance on both components, does not suffer from formation of homocoupling byproducts and avoids the use of transition metal catalysts.

Metal-Free Formal Oxidative C-C Coupling by In Situ Generation of an Enolonium Species

Much contemporary organic synthesis relies on transformations that are driven by the intrinsic, so-called "natural", polarity of chemical bonds and reactive centers. The design of unconventionally polarized synthons is a highly desirable strategy, as it generally enables unprecedented retrosynthetic disconnections for the synthesis of complex substances. Whereas the umpolung of carbonyl centers is a well-known strategy, polarity reversal at the α-position of a carbonyl group is much rarer. Herein, we report the design of a novel electrophilic enolonium species and its application in efficient and chemoselective, metal-free oxidative C-C coupling.

Applications of oxazolidinones as chiral auxiliaries in the asymmetric alkylation reaction applied to total synthesis

In this review, a number of applications of chiral oxazolidinones in the asymmetric alkylation reaction applied to total synthesis are described.

Facile Installation of 2-Reverse Prenyl Functionality into Indoles by a Tandem N-Alkylation-Aza-Cope Rearrangement Reaction and Its Application in Synthesis

An unprecedented tandem N-alkylation-ionic aza-Cope (or Claisen) rearrangement-hydrolysis reaction of readily available indolyl bromides with enamines is described. Due to the complicated nature of the two processes, an operationally simple N-alkylation and subsequent microwave-irradiated ionic aza-Cope rearrangement-hydrolysis process has been uncovered. The tandem reaction serves as a powerful approach to the preparation of synthetically and biologically important, but challenging, 2-reverse quaternary-centered prenylated indoles with high efficiency. Notably, unusual nonaromatic 3-methylene-2,3-dihydro-1H-indole architectures, instead of aromatic indoles, are produced. Furthermore, the aza-Cope rearrangement reaction proceeds highly regioselectively to give the quaternary-centered reverse prenyl functionality, which often produces a mixture of two regioisomers by reported methods. The synthetic value of the resulting nonaromatic 3-methylene-2,3-dihydro-1H-indole architectures has been demonstrated as versatile building blocks in the efficient synthesis of structurally diverse 2-reverse prenylated indoles, such as indolines, indole-fused sultams and lactams, and the natural product bruceolline D.

Rh(II)-Catalyzed Reactions of Diazoesters with Organozinc Reagents

Rh(II)-catalyzed reactions of diazoesters with organozinc reagents are described. Diorganozinc reagents participate in reactions with diazo compounds by two distinct, catalyst-dependent mechanisms. With bulky diisopropylethyl acetate ligands, the reaction mechanism is proposed to involve initial formation of a Rh-carbene and subsequent carbozincation to give a zinc enolate. With Rh2(OAc)4, it is proposed that initial formation of an azine precedes 1,2-addition by an organozinc reagent. This straightforward route to the hydrazone products provides a useful method for preparing chiral quaternary α-aminoesters or pyrazoles via the Paul-Knorr condensation with 1,3-diketones. Crossover and deuterium labeling experiments provide evidence for the mechanisms proposed.