Scope and Limitation of the Microwave-Assisted Catalytic Wittig Reaction

Chiral Bisphosphine-Catalyzed Asymmetric Staudinger/aza-Wittig Reaction: An Enantioselective Desymmetrizing Approach to Crinine-Type Amaryllidaceae Alkaloids

An unprecedented chiral bisphosphine-catalyzed asymmetric Staudinger/aza-Wittig reaction of 2,2-disubstituted cyclohexane-1,3-diones is reported, enabling the facile access of a broad range of cis-3a-arylhydroindoles in high yields with excellent enantioselectivities. The key to the success of this work relies on the first application of chiral bisphosphine DuanPhos to the asymmetric Staudinger/aza-Wittig reaction. An effective reductive system has been established to address the challenging PV═O/PIII redox cycle associated with the chiral bisphosphine catalyst. In addition, comprehensive experimental and computational investigations were carried out to elucidate the mechanism of the asymmetric reaction. Leveraging the newly developed chemistry, the enantioselective total syntheses of several crinine-type Amaryllidaceae alkaloids, including (+)-powelline, (+)-buphanamine, (+)-vittatine, and (+)-crinane, have been accomplished with remarkable conciseness and efficiency.

A Facile Synthetic Method for Anhydride from Carboxylic Acid with the Promotion of Triphenylphosphine Oxide and Oxaloyl Chloride

A highly efficient synthesis reaction of carboxylic anhydrides catalyzed by triphenylphosphine oxide is described for the quick synthesis of a range of symmetric carboxylic anhydrides and cyclic anhydrides under mild and neutral conditions with a high yield. The system adopts the strong reactive intermediate Ph₃PCl₂ as the catalyst of carboxylic acid salt; driven by catalytic reaction, the synthesis takes a relatively short time to complete.

Wittig Olefination Using Phosphonium Ion-Pair Reagents Incorporating an Endogenous Base

Despite common perception, the use of strong bases in Wittig chemistry is utterly unnecessary: we report a series of novel ion-pair phosphonium carboxylate reagents which are essentially "storable ylides". These reagents are straightforwardly prepared in excellent yields, and their fluxional nature permits clean olefination of a broad range of aldehydes and even hemiacetals.

Main Group Redox Catalysis of Organopnictogens: Vertical Periodic Trends and Emerging Opportunities in Group 15

A growing number of organopnictogen redox catalytic methods have emerged-especially within the past 10 years-that leverage the plentiful reversible two-electron redox chemistry within Group 15. The goal of this Perspective is to provide readers the context to understand the dramatic developments in organopnictogen catalysis over the past decade with an eye toward future development. An exposition of the fundamental differences in the atomic structure and bonding of the pnictogens, and thus the molecular electronic structure of organopnictogen compounds, is presented to establish the backdrop against which organopnictogen redox reactivity-and ultimately catalysis-is framed. A deep appreciation of these underlying periodic principles informs an understanding of the differing modes of organopnictogen redox catalysis and evokes the key challenges to the field moving forward. We close by addressing forward-looking directions likely to animate this area in the years to come. What new catalytic manifolds can be developed through creative catalyst and reaction design that take advantage of the intrinsic redox reactivity of the pnictogens to drive new discoveries in catalysis?

Let's Make White Phosphorus Obsolete

Industrial and laboratory methods for incorporating phosphorus atoms into molecules within the framework of Green Chemistry are in their infancy. Current practice requires large inputs of energy, involves toxic intermediates, and generates substantial waste. Furthermore, a negligible fraction of phosphorus-containing waste is recycled which in turn contributes to negative environmental impacts, such as eutrophication. Methods that begin to address some of these drawbacks are reviewed, and some key opportunities to be realized by pursuing organophosphorus chemistry under the principles of Green Chemistry are highlighted. Methods used by nature, or in the chemistry of other elements such as silicon, are discussed as model processes for the future of phosphorus in chemical synthesis.

An Improved PIII/PV═O-Catalyzed Reductive C-N Coupling of Nitroaromatics and Boronic Acids by Mechanistic Differentiation of Rate- and Product-Determining Steps

Experimental, spectroscopic, and computational studies are reported that provide an evidence-based mechanistic description of an intermolecular reductive C–N coupling of nitroarenes and arylboronic acids catalyzed by a redox-active main-group catalyst (1,2,2,3,4,4-hexamethylphosphetane P-oxide, i.e., 1·[O]). The central observations include the following: (1) catalytic reduction of 1·[O] to P^III phosphetane 1 is kinetically fast under conditions of catalysis; (2) phosphetane 1 represents the catalytic resting state as observed by ³¹P NMR spectroscopy; (3) there are no long-lived nitroarene partial-reduction intermediates observable by ¹⁵N NMR spectroscopy; (4) the reaction is sensitive to solvent dielectric, performing best in moderately polar solvents (viz. cyclopentylmethyl ether); and (5) the reaction is largely insensitive with respect to common hydrosilane reductants. On the basis of the foregoing studies, new modified catalytic conditions are described that expand the reaction scope and provide for mild temperatures (T ≥ 60 °C), low catalyst loadings (≥2 mol%), and innocuous terminal reductants (polymethylhydrosiloxane). DFT calculations define a two-stage deoxygenation sequence for the reductive C–N coupling. The initial deoxygenation involves a rate-determining step that consists of a (3+1) cheletropic addition between the nitroarene substrate and phosphetane 1; energy decomposition techniques highlight the biphilic character of the phosphetane in this step. Although kinetically invisible, the second deoxygenation stage is implicated as the critical C–N product-forming event, in which a postulated oxazaphosphirane intermediate is diverted from arylnitrene dissociation toward heterolytic ring opening with the arylboronic acid; the resulting dipolar intermediate evolves by antiperiplanar 1,2-migration of the organoboron residue to nitrogen, resulting in displacement of 1·[O] and formation of the target C–N coupling product upon in situ hydrolysis. The method thus described constitutes a mechanistically well-defined and operationally robust main-group complement to the current workhorse transition-metal-based methods for catalytic intermolecular C–N coupling.

Recent advances in catalytic Wittig-type reactions based on P(III)/P(V) redox cycling

Numerous organic transformations are based on the use of stoichiometric amounts of phosphorus reagents. The formation of phosphane oxides from phosphanes is usually the thermodynamic driving force for these reactions. The stoichiometric amounts of phosphane oxide which are formed as by-products often significantly hamper the product purification. Organophosphorus catalysis based on P(III)/P(V) redox cycling aims to address these problems. Herein we present our recent advances in developing catalytic Wittig-type reactions. More specifically, we reported our results on catalytic Wittig reactions based on readily available Bu3P=O as pre-catalyst as well as the first microwave-assisted version of this reaction and the first enantioselective catalytic Wittig reaction utilizing chiral phosphane catalysts. Further developments led to the implementation of catalytic base-free Wittig reactions yielding highly functionalized alkylidene and arylidene succinates.

Phosphonium ylide catalysis: a divergent diastereoselective approach to synthesize cyclic ketene acetals [thia(zolidines/zinanes)] from β-ketothioamides and dihaloalkanes

Phosphonium ylides are being reported here as a catalyst for the formation of thiazolidines and 1,3-thiazinanes from β-ketothioamides with dihaloalkanesvia[3 + 2] and [3 + 3] annulations under metal-free conditions.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Bridged [2.2.1] bicyclic phosphine oxide facilitates catalytic γ-umpolung addition-Wittig olefination

A novel bridged [2.2.1] bicyclic phosphine oxide, devised to circumvent the waste generation and burdens of purification that are typical of reactions driven by the generation of phosphine oxides, has been prepared in three steps from commercially available cyclopent-3-ene-1-carboxylic acid. The performance of this novel phosphine oxide was superior to those of current best-in-class counterparts, as verified experimentally through kinetic analysis of its silane-mediated reduction. It has been applied successfully in halide-/base-free catalytic γ-umpolung addition-Wittig olefinations of allenoates and 2-amidobenzaldehydes to produce 1,2-dihydroquinolines with good efficiency. One of the 1,2-dihydroquinoline products was converted to known antitubercular furanoquinolines.

A novel and highly efficient esterification process using triphenylphosphine oxide with oxalyl chloride

Triphenylphosphine oxide (TPPO) and oxalyl chloride ((COCl)2) are used as novel and high-efficiency coupling reagents for the esterification of alcohols with carboxylic acids via the TPPO/(COCl)2 system at room temperature for 1 h. The reaction represents the first TPPO-promoted esterification under mild and neutral conditions with excellent yields. Furthermore, we proposed a plausible mechanism with the help of 31P NMR spectroscopy.

Efficient phosphine-mediated formal C(sp3)–C(sp3) coupling reactions of alkyl halides in batch and flow

The Wittig-type chemical procedure is adapted to efficiently facilitate alkyl–alkyl coupling reactions in batch and flow.

Catalytic Wittig and aza-Wittig reactions

This review surveys the literature regarding the development of catalytic versions of the Wittig and aza-Wittig reactions. The first section summarizes how arsenic and tellurium-based catalytic Wittig-type reaction systems were developed first due to the relatively easy reduction of the oxides involved. This is followed by a presentation of the current state of the art regarding phosphine-catalyzed Wittig reactions. The second section covers the field of related catalytic aza-Wittig reactions that are catalyzed by both phosphine oxides and phosphines.

Novel Base-Free Catalytic Wittig Reaction for the Synthesis of Highly Functionalized Alkenes

A highly efficient catalyst system for base-free catalytic Wittig reactions has been developed and optimized. Initially, several potential (pre)catalysts as well as different silanes as reducing agents were screened. A system based on a readily available phosphine oxide as precatalyst and trimethoxy silane as reducing agent proved to be optimal. The effect of various Brønsted acidic additives was studied. Subsequently, the reaction conditions were optimized and standard reaction conditions were determined. Under these conditions the scope of this new protocol was evaluated. Nine activated olefins and 33 aldehydes were converted into 42 highly functionalized alkenes. Notably, aromatic, aliphatic as well as heteroaromatic aldehydes could be converted, giving the desired products in isolated yields up to 99 % and with good to excellent E/Z selectivities. These results underline the remarkable efficiency of this protocol considering the complexity of the reaction mixture and the four reaction steps that proceed in parallel in one pot.