The Mechanism of Phosphonium Ylide Alcoholysis and Hydrolysis: Concerted Addition of the O−H Bond Across the P=C Bond

Total synthesis, biological evaluation and biosynthetic re-evaluation of Illicium-derived neolignans

We report the first total syntheses of simonsol F (3), simonsinol (5), fargenin (4), and macranthol (6) in addition to syntheses of simonsol C (2), simonsol G (1), and honokiol (14). The syntheses are based upon a phosphonium ylide-mediated cascade reaction and upon natural product isomerization reactions which proceed through Cope rearrangements of putative biosynthetic dienone intermediates. As a corollary of the natural product isomerization reactions, we propose an alternative biosynthesis of honokiol (14), simonsinol (5), and macranthol (6) which unites the natural products in this family under a single common precursor, chavicol (7). Finally, we demonstrate that simonsol C (2) and simonsol F (3) promote axonal growth in primary mouse cortical neurons.

Selective Methylation of Nucleosides via an In Situ Generated Methyl Oxonium

A very mild and efficient procedure has been developed for the preparation of N-methylated uridine, pseudouridine, guanosine and inosine derivatives. This process was compatible with free hydroxyls within the ribose and did not require precautions on the protection or deprotection of other functionalities. The key to this extremely mild methylation without protection relied on the in situ generated methyl oxonium from the Wittig reagent and methanol. A putative mechanism for the selective methylation was also proposed.

Ring-opening polymerization of cyclic oligosiloxanes without producing cyclic oligomers

A long-standing problem associated with silicone synthesis is contamination of the polymer products with 10 to 15% cyclic oligosiloxanes that results from backbiting reactions at the polymer chain ends. This process, in competition with chain propagation through ring-opening polymerization (ROP) of cyclic monomers, was thought to be unavoidable and routinely leads to a thermodynamically controlled reaction mixture (polymer/cyclic oligosiloxanes = 85/15). Here, we report that simple alcohol coordination to the anionic chain ends prevents the backbiting process and that a well-designed phosphonium cation acts as a self-quenching system in response to loss of coordinating alcohols to stop the reaction before the backbiting process begins. The combination of both effects allows a thermodynamically controlled ROP of the eight-membered siloxane ring D4 without producing undesirable cyclic oligosiloxanes.

Synthesis and Functionalization of Thiophosphonium Salts: A Divergent Approach to Access Thioether, Thioester, and Dithioester Derivatives

Herein, we report a straightforward practical method for efficiently obtaining a diverse range of thiophosphonium salts. This method involves the direct coupling of commercially available thiols and aldehydes with Ph3P and TfOH. The setup is simple and carried out in a metal-free manner. The synthetic utility of these salts is demonstrated through various examples of C-P bond functionalizations, enabling the synthesis of thioether, deuterated thioether, thioester, and dithioester derivatives. These products, which serve as valuable building blocks, are obtained in high yields.

Diversity-Oriented Synthesis (DOS) of On-DNA Peptidomimetics from Acid-Derived Phosphonium Ylides

The DNA-encoded library (DEL) technology represents a revolutionary drug-discovery tool with unprecedented screening power originating from the association of combinatorial chemistry and DNA barcoding. The chemical diversity of DELs and its chemical space will be further expanded as new DNA-compatible reactions are introduced. This work introduces the use of DOS in the context of on-DNA peptidomimetics. Wittig olefination of aspartic acid-derived on-DNA Wittig ylide, combined with a broad substrate scope of aldehydes, led to formation of on-DNA α ${\alpha }$ , β ${\beta }$ -unsaturated ketones. The synthesis of on-DNA multi-peptidyl-ylides was performed by incorporating sequential amino acids onto a monomeric ylide. Di-, tri- and tetrameric peptidyl-ylides were validated for Wittig olefination and led to on-DNA α ${\alpha }$ , β ${\beta }$ -unsaturated-based peptidomimetics, an important class of intermediates. One on-DNA aryl Wittig ylide was also developed and applied to Wittig olefination for synthesis of on-DNA chalcone-based molecules. Furthermore, DOS was used successfully with electron-deficient peptidomimetics and led to the development of different heterocyclic cores containing on-DNA peptidomimetics.

Three-Component Synthesis of Dioxaphosphorane-Fused Diphosphacycles Exhibiting Unique Dynamic Fluorescence "On/Off" Properties

Rigidly planar polycyclic phosphacycles featuring an internal dioxaphosphorane are promising photofunctional materials. However, the lack of efficient synthetic methods resulted in limited structural diversities which significantly hampered extensive study. Herein, we report a straightforward three-component synthesis of novel dioxaphosphorane-fused diphosphacycles with distinctive photophysical properties. Control experiments and theory calculations were performed to account for a plausible reaction mechanism. We also systematically investigated the structure-property relationships of these unprecedented platforms by combining experiments (X-ray analysis, optical and redox properties) and theoretical computations. Based on their unique structure and properties, a novel fluorescent switch for pH sensing was revealed by a dynamic ring-opening/ring-closing process.

Synthesis of trans-stilbenes via phosphine-catalyzed coupling reactions of benzylic halides

An efficient and practical phosphine-catalyzed homo-coupling reaction of benzyl chlorides is described. The reactions proceed smoothly in the presence of CsF/B(OMe)₃ and NaH as the base, respectively, to provide trans-stilbenes in good yields with a broad scope. Unsymmetrical stilbenes are also generated from the reactions of benzyl chlorides with phosphonium salts. Several P-based key intermediates have been detected by NMR and HRMS analyses, which shed light on the postulated catalytic cycle. In the presence of different bases, the transformations involve two different pathways, in which phenylcarbene and phosphonium alkoxide are considered as key intermediates, respectively. The two pathways are complementary in synthesis but different in mechanisms. The synthetic utility, including gram-scale reactions and straightforward access to π-conjugated molecules, has been demonstrated as well.

P(v) intermediate-mediated E1cB elimination for the synthesis of glycals

Glycals are highly versatile and useful building blocks in the chemistry of carbohydrate and natural products. However, the practical synthesis of glycals remains a long-standing and mostly unsolved problem in synthetic chemistry. Herein, we present an unprecedented approach to make a variety of glycals using phosphonium hydrolysis-induced, P(v) intermediate-mediated E1cB elimination. The method provides a highly efficient, practical and scalable strategy for the synthesis of glycals with good generality and excellent yields. Furthermore, the strategy was successfully applied to late-stage modification of complex drug-like molecules. Additionally, the corresponding 1-deuterium-glycals were produced easily by simple t BuONa/D2O-hydrolysis-elimination. Mechanistic investigations indicated that the oxaphosphorane intermediate-mediated E1cB mechanism is responsible for the elimination reaction.

Synthesis of Pentacoordinated Spiro[4.4]phosphoranes by Reaction of Cyclic Phosphazenyl Anions with Epoxides: Study of their P-Remote Functionalization and Hydrolysis

The synthesis of a new family of pentacoordinated spiro[4.4]phosphoranes stabilized by the ortho-benzamide bidentate ligand (oBA, -C6H4-2-C(O)NH-) through reaction of Cα,C ortho -dilithiated phosphazenes with oxiranes is reported. Mixtures of epimers that differ in configuration at the phosphorus atom were obtained with moderate to high yields and diastereoselectivities. C3-Disubstituted derivatives could be separated, which provided access to enantiopure products arising from the reaction with (R)-2-phenyloxirane. Stereomutation was observed by heating the spirophosphoranes at 100 °C. Directed ortho-lithiation of spirophoshoranes followed by electrophilic quench reactions including carbon-based and heteroatom-based electrophiles afforded derivatives functionalized in a remote position with respect to the phosphorus atom. Benzoic acid catalyzed hydrolysis of spirophosphoranes furnished ortho-(γ-hydroxyalkylphosphoryl)benzamides by cleavage of the P–O and P–N bonds with retention of the phosphorus configuration. In contrast, alkaline hydrolysis led to the formation of γ-hydroxyphosphinic acids and benzamide.

Z-Selective phosphine promoted 1,4-reduction of ynoates and propynoic amides in the presence of water

Phosphine-mediated reductions of substituted propynoic esters and amides in the presence of water yield the partially reduced α,β-unsaturated esters and amides with high Z-selectivity. The competitive in situ Z to E-isomerization of the product in some cases lowers the Z to E ratios of the isolated α,β-unsaturated carbonyl products. Reaction time and the amounts of phosphine and water in the reaction mixture are the key experimental factors which control the selectivity by preventing or reducing the rates of Z- to E-product isomerization. Close reaction monitoring enables isolation of the Z-alkenes with high selectivities. The computational results suggest that the reactions could be highly Z-selective owing to the stereoselective formation of the E-P-hydroxyphosphorane intermediate.

Organophosphorus zwitterions engaged in a conjugated macrocycle on fullerene

Organophosphorus zwitterions are one of the most important but elusive intermediates for carbon-carbon bond formation in synthetic chemistry and biology. However, a lack of isolated examples due to their lability has hampered in-depth understanding of structures and their reaction mechanisms. In this study, we crystallographically reveal the solid-state structure of a phosha-Michael adduct engaged in a cage-opened C₆₀ skeleton, which is formed as a kinetic product. This compound exhibits dark brown colour in solution with an intense absorption band that extends to 1000 nm, reflecting intramolecular charge transfer transitions. From the 1,2-dicarbonyl moiety on the conjugated orifice, β-oxo-phosphorus ylide is formed as a thermodynamic product. The reaction mechanism that has long been disputed is examined by experimental and theoretical studies, showing a pathway which includes an S_N2 reaction as a key step instead of the hitherto considered carbene pathway.

Synthesis of Aryldiphenylphosphine Oxides by Quaternization of Tertiary Diphenylphosphines with Aryl Bromides Followed by the Wittig Reaction

A two-step method for the synthesis of aryldiphenylphosphine oxides from tertiary diphenylphosphines and aryl bromides is developed. The first step is the quaternization of methyldiphenylphosphine or benzyldiphenylphosphine with aryl bromides. This quaternization can be nickel-catalyzed (metal-free in some cases), and tolerate of a variety of functional groups, furnishing quaternary phosphonium salts in 48-90% yields. The second step is Wittig reactions of these quaternary phosphonium salts with furan-2-carbaldehyde or p-chlorobenzaldehyde to provide aryldiphenylphosphine oxides in 27-90% yields. The use of the Wittig reaction for the synthesis of tertiary phosphine oxides is in contrast to its traditional use for the synthesis of olefins, leaving tertiary phosphine oxide as a byproduct. This quaternization-Wittig method can be applied to synthesize aryldiphenylphosphine oxides that are difficult to access by the alkaline hydrolysis of aryltriphenylphosphonium salts, especially those bearing an electron-deficient aryl group. The ligand-coupling mechanism for the alkaline hydrolysis of (p-acylphenyl)triphenylphosphonim salts is also discussed.

Reduction of Activated Alkenes by PIII /PV Redox Cycling Catalysis

The carbon-carbon double bond of unsaturated carbonyl compounds was readily reduced by using a phosphetane oxide catalyst in the presence of a simple organosilane as the terminal reductant and water as the hydrogen source. Quantitative hydrogenation was observed when 1.0 mol % of a methyl-substituted phosphetane oxide was employed as the catalyst. The procedure is highly selective towards activated double bonds, tolerating a variety of functional groups that are usually prone to reduction. In total, 25 alkenes and two alkynes were hydrogenated to the corresponding alkanes in excellent yields of up to 99 %. Notably, less active poly(methylhydrosiloxane) could also be utilized as the terminal reductant. Mechanistic investigations revealed the phosphane as the catalyst resting state and a protonation/deprotonation sequence as the crucial step in the catalytic cycle.

Synthesis of phosphine oxide based amphiphilic molecules via ring-opening Wittig olefination of a macrocyclic phosphoranylidene and their property study as non-ionic surfactants

A novel ring-opening Wittig olefination approach was developed for the synthesis of amphiphilic phosphine oxides (PO) as non-ionic surfactants. The approach concurrently introduces the crucial functional groups (lipophilic chain and phosphine oxide moiety) present in the known PO surfactants and additional hydrophilic group (i.e., ethylene glycol units) in one step via Wittig olefination of a macrocyclic phosphoranylidene. A series of novel PO compounds were obtained from a variety of aldehydes and selected compounds were examined for their physiochemical properties (surface tension, critical micelle concentration and interfacial tension) and also for their abilities to form emulsions as non-ionic surfactants.

A novel ring-opening Wittig olefination approach was developed for the synthesis of amphiphilic phosphine oxides (PO) as non-ionic surfactants.

P-B Desulfurization: An Enabling Method for Protein Chemical Synthesis and Site-Specific Deuteration

Cysteine-mediated native chemical ligation is a powerful method for protein chemical synthesis. Herein, we report an unprecedentedly mild system (TCEP/NaBH₄ or TCEP/LiBEt₃ H; TCEP=tris(2-carboxyethyl)phosphine) for chemoselective peptide desulfurization to achieve effective protein synthesis via the native chemical ligation-desulfurization approach. This method, termed P-B desulfurization, features usage of common reagents, simplicity of operation, robustness, high yields, clean conversion, and versatile functionality compatibility with complex peptides/proteins. In addition, this method can be used for incorporating deuterium into the peptides after cysteine desulfurization by running the reaction in D₂ O buffer. Moreover, this method enables the clean desulfurization of peptides carrying post-translational modifications, such as phosphorylation and crotonylation. The effectiveness of this method has been demonstrated by the synthesis of the cyclic peptides dichotomin C and E and synthetic proteins, including ubiquitin, γ-synuclein, and histone H2A.

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.

A convenient access to N-phosphonio-substituted NHC metal complexes [M = Ag(i), Rh(i), Pd(ii)]

NHC Pd(ii) complexes featuring a propyl bridge were ionized by the introduction of a phosphonium moiety, which may coordinate to the metal center or remain pendant.

Synthesis, Properties and Stereochemistry of 2-Halo-1,2λ⁵-oxaphosphetanes

Results of research into four-membered 2-halo-1,2λ⁵-oxaphosphetane phosphorus(V)-heterocycles are presented. The preparation of 2-halo-1,2λ⁵-oxaphosphetanes by reaction of P-haloylides with carbonyl compounds is described. The mechanism of asynchronous [2+2]-сycloaddition of ylides to aldehydes was proposed on the base of low-temperature NMR investigations. 2-Halo-1,2λ⁵-oxaphosphetanes were isolated as individual compounds and their structures were confirmed by ¹Н-, ¹³C-, ¹⁹F- and ³¹Р-NMR spectra. These compounds are convenient reagents for preparing of various organic and organophosphorus compounds hardly available by other methods. Chemical and physical properties of the 2-halo-1,2λ⁵-oxaphosphetanes are reviewed. The 2-chloro-1,2λ⁵-oxaphosphetanes, rearrange with formation of 2-chloroalkyl-phosphonates or convert into trans-phosphorylated alkenes depending on the substituents at the α-carbon atom. Prospective synthetic applications of 2-halo-1,2λ⁵-oxaphosphetanes are analyzed. The 2-halo-1,2λ⁵-oxaphosphetanes may be easily converted to various alkenylphosphonates: allyl- or vinylphosphonates, phosphorus ketenes, thioketenes, ketenimines.