Recent Developments on Diverse (Hetero)Arene Functionalization via Nucleophile‐triggered Umpolung of (Hetero)Aryl Phosphonium Salts

With the aim of developing a green synthesis of phosphonium ionic liquids (PILs), we prepared a series of methyltrialkylphosphonium methyl carbonates [Pn,n,n,1]+[O(CO)OCH3]- by the alkylation of trialkylphosphines with the non toxic dimethylcarbonate. These compounds proved to be active organocatalysts for a number of base-promoted organic reactions such as Michael, Henry, and Baylis-Hillman condensations, and transesterification reactions. In addition methyl carbonate salts were a convenient starting material to synthesise a large array of ionic liquids where the phosphonium cation was coupled to weakly basic anions such as bicarbonate, acetate, trifluoroacetate, phenate, chloride, bromide and many more. Mechanistic studies indicated that the anionic and the cationic partners of such ionic liquids acted cooperatively and independently as nucleophilic and electrophilic catalysts. The ambiphilic propensity of these salts was demonstrated by kinetically discriminating the contributions of the anion (nucleophilic catalyst) and of the cation (electrophilic catalyst) on the model solvent-free Baylis–Hillman dimerization of cyclohexenone.

Photocatalytic 1,3-dicarboxylation of unactivated alkenes with CO2

A Track-Based Molecular Synthesizer that Builds a Single-Sequence Oligomer through Iterative Carbon-Carbon Bond Formation

Organic sulfones are an important class of chemical compounds widely used in many research fields. The direct decarboxylative sulfonylation of carboxylic acids is attractive but challenging, particularly when iron is used as a metal catalyst. Herein, we describe a photoinduced iron-catalytic method for the synthesis of sulfones directly using carboxylic acids via a radical-based decarboxylation. This protocol is mild, highly efficient, and easy-to-operate. A broad scope of carboxylic acids and carbon electrophiles could be well tolerated. A mechanism involving the iron-catalyzed decarboxylation, radical transfer, single-electron reduction, and nucleophilic attack is proposed.

<p>Amide synthesis is a fundamental reaction in synthetic chemistry due to the abundance of amides in biologically active molecules, synthetic polymers, and commercial products. Coupling reagents that activate the carboxylic acid have become the most well-studied method to direct amide bond formation. Amongst these, carbodiimides, uronium and phosphonium salts, and benzotriazoles are the most common. These reactions, however, are often limited by poor atom economy, and toxic and expensive reagents. Therefore, synthetic chemists need a better method to form amides safely and efficiently. Organosilanes have been reported as greener, efficient alternative amide coupling reagents. Current work in this field often requires a stoichiometric (or more) amount of the silane. While stoichiometric amide coupling reactions are generally the methods of choice for their efficiency and practicality, catalytic amide coupling can present a new step toward greener methodologies to make amides. This thesis explores novel organosilanes that are investigated as catalysts for direct amidation. The silane catalysts are designed to have enhanced reactivity by incorporating a hydrogen bond donor (HBD) to facilitate the formation of a silyl ester intermediate and nucleophilic attack at the carbonyl by the amine coupling partner. The synthesis of various organosilanes have been investigated, including silanes that have an amide or urea, hydroxy, or protonated amine as a HBD. The silanes that have been successfully synthesized were tested as catalysts in direct amidation. Aryl and alkyl silanes, and di- and trisubstituted silanes were used in stoichiometric silane-mediated amide synthesis to study how the substitution on the silicon effects amidation.</p>

<p>Amide synthesis is a fundamental reaction in synthetic chemistry due to the abundance of amides in biologically active molecules, synthetic polymers, and commercial products. Coupling reagents that activate the carboxylic acid have become the most well-studied method to direct amide bond formation. Amongst these, carbodiimides, uronium and phosphonium salts, and benzotriazoles are the most common. These reactions, however, are often limited by poor atom economy, and toxic and expensive reagents. Therefore, synthetic chemists need a better method to form amides safely and efficiently. Organosilanes have been reported as greener, efficient alternative amide coupling reagents. Current work in this field often requires a stoichiometric (or more) amount of the silane. While stoichiometric amide coupling reactions are generally the methods of choice for their efficiency and practicality, catalytic amide coupling can present a new step toward greener methodologies to make amides. This thesis explores novel organosilanes that are investigated as catalysts for direct amidation. The silane catalysts are designed to have enhanced reactivity by incorporating a hydrogen bond donor (HBD) to facilitate the formation of a silyl ester intermediate and nucleophilic attack at the carbonyl by the amine coupling partner. The synthesis of various organosilanes have been investigated, including silanes that have an amide or urea, hydroxy, or protonated amine as a HBD. The silanes that have been successfully synthesized were tested as catalysts in direct amidation. Aryl and alkyl silanes, and di- and trisubstituted silanes were used in stoichiometric silane-mediated amide synthesis to study how the substitution on the silicon effects amidation.</p>

The efficient production of fine chemicals from simple and readily available bulk materials such as simple alkenes is an important research area in synthetic organic chemistry. Lower alkenes are produced by one of the most common chemical processes (i.e. hydrocarbon cracking) on over 500 million metric tons scale. Development of catalytic methods which enable the direct use of these abundant organic molecules as carbon sources is highly demanded for practical and selective chemical reactions. However, despite several previous examples, the use of feedstock alkenes in synthetically useful transformations remains a challenging task.Catalytic aldehyde allylation plays an important role in synthetic organic chemistry. Traditionally, nucleophilic allylmetal species as stoichiometric starting materials have contributed to the development of this catalytic process. Despite usefulness of this approach, however, there is room for improvement in the overall efficiency, redox/atom/step economy, and environmental sustainability. The catalytic generation of nucleophilic allylmetal species in situ from simple and feedstock alkenes via allylic sp3 C-H bond activation would be a straightforward approach. We envisioned that radical sp3 C-H bond activation followed by oxidative interception of the resulting carbon-centered radical by a metal complex is a promising strategy for the catalytic generation of organometallic species from inert substrates. We supposed the ternary hybrid catalysis comprising a photoredox catalyst, a hydrogen-atom-transfer (HAT) catalyst, and a metal catalyst would realize catalytic allylation of aldehydes with simple alkenes.We planned to use a combination of an acridinium photoredox catalyst (Mes-Acr+) and a thiophosphoric imide (TPI) HAT catalyst to activate an allylic C-H bond of the alkenes through the HAT process. Under visible light irradiation, single-electron oxidation of TPI by a photo-excited acridinium catalyst (*Mes-Acr+) generates a sulfur-centered radical (RS•). The RS• radical abstracts the allylic C-H bond of alkenes, affording allyl radicals. Based on the mechanism of Nozaki-Hiyama allylation, we expected that a chromium(II) complex catalyst oxidatively intercepted allyl radicals, giving allylchromium(III) species. This species reacts with aldehydes via a Zimmerman-Traxler transition state to produce chromium alkoxides in an anti-selective fashion. Protonolysis by the proton generated in the HAT catalysis should afford the target homoallylic alcohols and chromium(III) species. Single-electron reduction by the photoredox catalyst (Mes-Acr•) would close the catalytic cycle with regenerating a chromium(II) complex catalyst and the oxidized form of the photoredox catalyst (Mes-Acr+).Based on this mechanism, we studied the substrate scope with a combination of 5 mol% CrCl2, 10 mol% TPI catalyst (HAT) and 5 mol% acridinium photoredox catalyst (PC). First, the scope of alkenes was investigated. In all cases, the branched constitutional isomer was produced exclusively (>20/1). The reactions of C4 feedstocks, 1-butene and 2-butenes which are annually produced on a 105 metric ton scale, with benzaldehyde afforded the product in 94% and 100% yield, respectively, with high diastereoselectivity (14-16/1). This crotylation reaction using butenes as a starting material will significantly streamline polyketide synthesis. 3-Methyl-1-butene selectively afforded the product containing a quaternary carbon. Branched C6 alkenes were also competent substrates. Other alkenes, including cyclic, halide-containing, and ether-containing, all afforded the product in high yield. The aldehyde scope was then investigated by fixing the alkene substrate to 2-butene. A series of aromatic aldehydes bearing halogens, electron-withdrawing groups, and a boronate ester afforded the corresponding products with high diastereoselectivity. A methyl substituent at the ortho-, meta-, and para positions of the aromatic ring did not affect the results. The reactions of aliphatic aldehydes, including those containing potentially sensitive functional groups (e.g. amine, alkyl halide, sulfide and benzyl oxy moiety), also proceeded efficiently. Next, we investigated the application to highly functionalized substrates. Propranolol derivative bearing free hydroxy group were competent substrate. A dipeptide and a nucleic acid derivative containing coordinating polar functional groups such as amides and an adenine heterocycle did not affect the reaction progress. Therefore, the allylation of aldehydes mediated by the ternary hybrid catalysis realized high functional group compatibility and chemoselectivity using easily available feedstock alkenes.In conclusion, we developed a ternary hybrid catalysis, enabling catalytic allylation of aldehydes using simple alkenes, under visible light irradiation at room temperature. This hybrid catalysis is applicable to various types of unactivated alkenes including feedstock lower alkenes. Due to the high chemoselectivity and mild conditions, the reaction was applicable to highly functionalized bioactive molecules. The asymmetric variant enabled facile access to enantiomerically and diastereomerically enriched homoallylic alcohols by introducing a chiral ligand to the chromium catalyst. Further studies to expand the scope are ongoing. Figure 1

Flavin has long been known to function as a single electron reductant in biological settings, but this reactivity has rarely been observed with flavoproteins used in organic synthesis. Here we describe the discovery of an enantioselective radical dehalogenation pathway for α-bromoesters using flavin-dependent 'ene'-reductases. Mechanistic experiments support the role of flavin hydroquinone as a single electron reductant, flavin semiquinone as the hydrogen atom source, and the enzyme as the source of chirality.

Arylation methods based on the generation and use of aryl radicals have been a rapidly growing field of research in recent years and currently represent a powerful strategy for carbon – carbon and carbon – heteroatom bond formation. The progress in this field is related to advances in the methods for generation of aryl radicals. The currently used aryl radical precursors include aryl halides, aryldiazonium and diaryliodonium salts, arylcarboxylic acids and their derivatives, arylboronic acids, arylhydrazines, organosulfur(II, VI) compounds and some other compounds. Aryl radicals are generated under mild conditions by single electron reduction or oxidation of precursors induced by conventional reagents, visible light or electric current. A crucial role in the development of the radical arylation methodology belongs to photoredox processes either catalyzed by transition metal complexes or organic dyes or proceeding without catalysts. Unlike the conventional transition metal-catalyzed arylation methods, radical arylation reactions proceed very often at room temperature and have high functional group tolerance. Without claiming to be exhaustive, this review covers the most important advances of the current decade in the generation and synthetic applications of (het)aryl radicals. Examples of reactions are given and mechanistic insights are highlighted.The bibliography includes 341 references.

Catalytic [2+2+2] Tandem Cyclization with Alkyl Substituted Methylene Malonate Enabling Concise Total Synthesis of Four Malagasy Alkaloids

The reversible conversion between a phosphine and a phosphonium salt has been achieved by external stimuli of light and heat. Two 2-phosphinoazobenzenes were successfully synthesized by desulfurization of the corresponding phosphine sulfides. One of the phosphines bearing an azo group was in equilibrium with an inner phosphonium salt and showed thermochromism, which is derived from the change of the equilibrium constant depending on the temperature. While the 2-phosphinoazobenzene reacted as a usual triarylphosphine, its reaction with water gave phosphine oxide bearing a hydrazine moiety via a mechanism similar to the Mitsunobu reaction. The 2-phosphinoazobenzene bearing a methyl group at the 4'-position of azobenzene was isomerized to the Z-isomer by irradiation. The Z-isomer was neither in equilibrium with an inner phosphonium salt nor hydrolyzed, in contrast to the E-isomer, because its geometry is difficult for an intramolecular nucleophilic attack. Photoisomerization caused the switching of the unique reactivity toward water. Such phosphines in equilibrium with the inner phosphonium salts are expected to be useful to control organic reactions by taking advantage of the photoisomerization of the azobenzene moiety.

[55795-18-1] C22H20BrP (MW 395.27) InChI = 1S/C22H20P.BrH/c1-2-3-19-23(20-13-7-4-8-14-20,21-15-9-5-10-16-21)22-17-11-6-12-18-22;/h2-19H,1H2;1H/q+1;/p-1/b19-3+; InChIKey = SSZVQMPGJGQLLP-SHGZZJJLSA-M (reagent used for preparation of conjugated dienes by 1,4-addition of a nucleophile followed by Wittig reaction) Physical Data: colorless solid; mp 186–188.5 °C.1 Solubility: soluble in THF.1-3, 5, 6 Preparative Methods: the title reagent can be prepared by the reaction of triphenylphosphine and (E)-1,4-dibromo-2-butene in benzene, the precipitated phosphonium salt is collected and is treated with anhydrous sodium carbonate in chloroform;1 lithium diisopropylamide in THF at −78 °C can also be used for the dehydrobromination step.2 Handling, Storage, and Precautions: the reagent is moisture and light sensitive, and is best used immediately after its preparation and isolation.

For Abstract see ChemInform Abstract in Full Text.

Reactions of nucleophilic active phosphacumulenes with iso(thio)cyanate compounds were performed. The reaction products depend on the nature of the reagent, substrate and the condition of the reaction used. New heterocyclic 4-membered or 6-membered sulfur and nitrogen compounds such as phosphoranylidene thietane, azetidine and thiazinane were obtained. On the other hand, the stable phosphonium ylides with the iso(thio)cyanate afforded phosphoranylidene thiocarbamoyl derivatives. Possible reaction mechanisms are considered and the structural assignments are based on compatible analytical and spectroscopic data.

Dominocyclisierung zweier Phosphinidenmoleküle: Die Methylierung des nucleophilen Phosphinidens 1 mit Methyltriflat liefert den ungewöhnlichen Phosphoniumkäfig 2, was für das Phospheniumion 3 als reaktive Zwischenstufe spricht.