Synthetic and Spectroscopic Investigations Enabled by Modular Synthesis of Molecular Phosphaalkyne Precursors

Carbodiimide and Isocyanate Hydroboration by a Cyclic Carbodiphosphorane Catalyst

We report hydroboration of carbodiimide and isocyanate substrates catalyzed by a cyclic carbodiphosphorane catalyst. The cyclic carbodiphosphorane outperformed the other Lewis basic carbon species tested, including other zerovalent carbon compounds, phosphorus ylides, an N-heterocyclic carbene, and an N-heterocyclic olefin. Hydroborations of seven carbodiimides and nine isocyanates were performed at room temperature to form N-boryl formamidine and N-boryl formamide products. Intermolecular competition experiments demonstrated the selective hydroboration of alkyl isocyanates over carbodiimide and ketone substrates. DFT calculations support a proposed mechanism involving activation of pinacolborane by the carbodiphosphorane catalyst, followed by hydride transfer and B-N bond formation.

Phosphinidenes: Fundamental Properties and Reactivity

Phosphinidenes are heavy congeners of nitrenes that have been broadly used as in situ reagents in synthetic phosphorus chemistry and also serve as versatile ligands in coordination with transition metals. However, the detection of free phosphinidenes is largely challenged by their high reactivity and also the lack of suitable synthetic methods, rendering the knowledge about the fundamental properties of this class of low-valent phosphorus compounds limited. Recently, an increasing number of free phosphinidenes bearing prototype structural and bonding properties have been prepared for the first time, thus enabling the exploration of their distinct reactivity from the nitrene analogues. This Concept article will discuss the experimental approaches for the generation of the highly unstable phosphinidenes and highlight their distinct reactivity from the nitrogen analogues so as to stimuate future studies about their potential applications in phosphorus chemistry.

Synthesis of Phosphet-2-one Derivatives via Phosphinidene Transfer to Cyclopropenones

We report the first synthesis and structural characterization of free, uncomplexed phosphet-2-ones. These unsaturated four-membered phosphacycles were prepared by phosphinidene transfer from dibenzo-7-phosphanorbornadiene compounds, RPA (A = C14H10, anthracene), to cyclopropenones in yields of up to 89%. Theoretical studies suggest that the reaction proceeds through ketene-ylide and ketene-phosphaalkene intermediates. Further transformations of the phosphet-2-ones led to the isolation of more phosphet-2-ones and 1,2-dihydrophosphetes, including two furanone derivatives which are postulated to be produced by intramolecular phosphine-catalyzed [3 + 2] annulations.

A Triphenylphosphine-Based Microporous Polymer for a Wittig Reaction Cycle in the Solid State

The Wittig reaction is a key step in industrial processes to synthesise large quantities of vitamin A and various other important chemicals that are used in daily life. This article presents a pathway to achieve the Wittig reaction in a solid network. A highly porous triphenylphosphine-based polymer was applied as a solid Wittig reagent that undergoes, in a multi-step cycle, in total six post-synthetic modifications. This allowed for regeneration of the solid Wittig reagent and reuse for the same reaction cycle. Of particular industrial relevance is that the newly developed material also enables a simple way of separating the product by filtration. Therefore, additional costly and difficult separation and purification steps are no longer needed.

Hydrogen-Bonded π Complexes between Phosphaethyne and Hydrogen Chloride

The highly labile complexes between phosphaethyne (HCP) and hydrogen chloride (HCl) with 1:1 and 1:2 stoichiometries have been generated in Ar and N₂ matrices at 10 K through laser photolysis of the molecular precursors 1-chlorophosphaethene (CH₂PCl) and dichloromethylphosphine (CH₃PCl₂), respectively. The IR spectrum of the 1:1 complex suggests the preference of a single "T-shaped" structure in which HCl acts as the hydrogen donor that interacts with the electron-rich C≡P triple bond. In contrast, three isomeric structures for the 1:2 complex bearing a core structure of the "T-shaped" 1:1 complex are present in the matrix. The spectroscopic identification of these rare HCP π-electron complexes is supported by D-isotope labeling and the quantum chemical calculations at the CCSD(T)-F12a/cc-pVTZ-F12 level of theory.

Frustrated Lewis Pair Adduct of Atomic P(-1) as a Source of Phosphinidenes (PR), Diphosphorus (P2 ), and Indium Phosphide

We report phosphinidenes (PR) stabilized by an intramolecular frustrated Lewis pair (FLP) chelate. These adducts include the parent phosphinidene, PH, which is accessed via thermolysis of coordinated HPCO. The reported FLP-PH species acts as a springboard to other phosphorus-containing compounds, such as FLP-adducts of diphosphorus (P₂ ) and InP₃ . Our new adducts participate in thermal- or light-induced phosphinidene elimination (of both PH and PR, R=organic group), transfer P₂ units to an organic substrate, and yield the useful semiconductor InP at only 110 °C from solution.

Insight into fragmentation of a phosphirane to form phosphinidene complexes: an illustration with the 1-phenylselenylphosphirane W(CO)5 complex

Density functional theory (DFT) calculations with 1-phenylselenylphosphirane complex 1 provide an insight into phosphirane fragmentation to phosphinidene complexes. FMO and ELF analyses show that the cleavage of two P-C σ bonds of phosphirane proceeds via an asynchronous concerted pathway. Transient [PhSeP-W(CO)₅] was generated by dissociation of 1 at 90 °C and trapped with different reagents. The 1-phenoxylphosphirane complex undergoes [1 + 2] retroaddition at a comparatively higher temperature which implies that the lone pair of the adjacent atom center of phosphorus plays a major role in phosphirane fragmentation.

Staudinger Reactivity and Click Chemistry of Anthracene ()-Based Azidophosphine N3P

11-Azido-9,10-dihydro-9,10-phosphanoanthracene (N₃PA) has been demonstrated recently as a transfer reagent for molecular phosphorus mononitride (PN) because it easily dissociates at room temperature into dinitrogen (N₂), PN, and anthracene (A). Here we report further reactivity studies of the N₃PA molecule including strain-promoted 1,3-dipolar cycloaddition with cyclooctyne and Staudinger-type reactivity. Calculations at the DLPNO-CCSD(T)/cc-pVTZ//PBE0-D3(BJ)/cc-pVTZ level of theory indicate that the click reaction is faster than the dissociation of N₃PA. The Staudinger-type reactivity enabled transfer of the NPA fragment to a base-stabilized silylene. The previously reported intermediate of vanadium trisanilide with an NPA ligand could be isolated in 61% yield and structurally characterized in a single-crystal X-ray diffraction experiment. In line with the previously reported phosphinidene reactivity of the transient vanadium phosphorus mononitride complex, thermolysis or irradiation of the complex leads to A elimination and formation of the corresponding vanadium PN dimer or trimer, respectively.

Phosphenic isocyanate (O2PNCO): Gas-phase generation, characterization, and photodecomposition reactions

Phosphenic isocyanate (O2PNCO), a novel phosphorus-containing small molecule has been generated by thermolysis of a dioxaphospholane-based precursor. The characterization of O2PNCO with IR and UV-vis spectroscopy in solid N2 and...

Phosphorus-Atom Transfer from Phosphaethynolate to an Alkylidyne

A low-spin and mononuclear vanadium complex, (Me nacnac)V(CO)(η2 -P≡Ct Bu) (2) (Me nacnac- =[ArNC(CH3 )]2 CH, Ar=2,6-i Pr2 C6 H3 ), was prepared upon treatment of the vanadium neopentylidyne complex (Me nacnac)V≡Ct Bu(OTf) (1) with Na(OCP)(diox)2.5 (diox=1,4-dioxane), while the isoelectronic ate-complex [Na(15-crown-5)]{([ArNC(CH2 )]CH[C(CH3 )NAr])V(CO)(η2 -P≡Ct Bu)} (4), was obtained via the reaction of Na(OCP)(diox)2.5 and ([ArNC(CH2 )]CH[C(CH3 )NAr])V≡Ct Bu(OEt2 ) (3) in the presence of crown-ether. Computational studies suggest that the P-atom transfer proceeds by [2+2]-cycloaddition of the P≡C bond across the V≡Ct Bu moiety, followed by a reductive decarbonylation to form the V-C≡O linkage. The nature of the electronic ground state in diamagnetic complexes, 2 and 4, was further investigated both theoretically and experimentally, using a combination of density functional theory (DFT) calculations, UV/Vis and NMR spectroscopies, cyclic voltammetry, X-ray absorption spectroscopy (XAS) measurements, and comparison of salient bond metrics derived from X-ray single-crystal structural characterization. In combination, these data are consistent with a low-valent vanadium ion in complexes 2 and 4. This study represents the first example of a metathesis reaction between the P-atom of [PCO]- and an alkylidyne ligand.

31P NMR Chemical Shift Tensors: Windows into Ruthenium Phosphinidene Complex Electronic Structures

A series of octamethylcalix[4]pyrrole/ruthenium phosphinidene complexes (Na₂[1=PR]) can be accessed by phosphinidene transfer from the corresponding RPA (A = C₁₄H₁₀, anthracene) compounds (R = ^tBu, ⁱPr, OEt, NH₂, NMe₂, NEt₂, NⁱPr₂, NA, dimethylpiperidino). Isolation of the tert-butyl and dimethylamino derivatives allowed comparative studies of their ³¹P nuclear shielding tensors by magic-angle-spinning solid-state nuclear magnetic resonance spectroscopy. Density functional theory and natural chemical shielding analyses reveal the relationship between the ³¹P chemical shift tensor and the local ruthenium/phosphorus electronic structure. The general trend observed in the ³¹P isotropic chemical shifts for the ruthenium phosphinidene complexes was controlled by the degree of deshielding in the δ₁₁ principal tensor component, which can be linked to the σ_RuP/π_RuP* energy gap. A "δ₂₂-δ₃₃ crossover" effect for R = ^tBu was also observed, which was caused by different degrees of deshielding associated with polarizations of the σ_PR and σ_PR* natural bond orbitals.

Synthesis of α,δ-Disubstituted Tetraphosphates and Terminally-Functionalized Nucleoside Pentaphosphates

The anion [P₄O₁₁]^2-, employed as its bis(triphenylphosphine)iminium (PPN) salt, is shown herein to be a versatile reagent for nucleophile tetraphosphorylation. Treatment under anhydrous conditions with an alkylamine base and a nucleophile (HNuc¹), such as an alcohol (neopentanol, cyclohexanol, 4-methylumbelliferone, and Boc-Tyr-OMe), an amine (propargylamine, diethylamine, morpholine, 3,5-dimethylaniline, and isopropylamine), dihydrogen phosphate, phenylphosphonate, azide ion, or methylidene triphenylphosphorane, results in nucleophile substituted tetrametaphosphates ([P₄O₁₁Nuc¹]^3-) as mixed PPN and alkylammonium salts in 59% to 99% yield. Treatment of the resulting functionalized tetrametaphosphates with a second nucleophile (HNuc²), such as hydroxide, a phenol (4-methylumbelliferone), an amine (propargylamine and ethanolamine), fluoride, or a nucleoside monophosphate (uridine monophosphate, deoxyadenosine monophosphate, and adenosine monophosphate), results in ring opening to linear tetraphosphates bearing one nucleophile on each end ([Nuc¹(PO₃)₃PO₂Nuc²]^4-). When necessary, these linear tetraphosphates are purified by reverse phase or anion exchange HPLC, yielding triethylammonium or ammonium salts in 32% to 92% yield from [PPN]₂[P₄O₁₁]. Phosphorylation of methylidene triphenylphosphorane as Nuc¹ yields a new tetrametaphosphate-based ylide ([Ph₃PCHP₄O₁₁]^3-, 94% yield). Wittig olefination of 2',3'-O-isopropylidene-5'-deoxy-5'-uridylaldehyde using this ylide results in a 3'-deoxy-3',4'-didehydronucleotide derivative, isolated as the triethylammonium salt in 54% yield.

A Condensed, Scalable Synthesis of Racemic Koningic Acid

The natural product koningic acid (KA) is a selective covalent inhibitor of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a critical node in the glycolysis pathway. While KA is available commercially, sources are limited and its cost becomes rapidly prohibitive beyond the milligram scale. Additionally, a practical and flexible synthetic route to KA and analogs remains to be developed. Here we detail a new route that is operationally safer, scalable and offers a five-step reduction in the previously reported longest linear sequence.

Organoiron- and Fluoride-Catalyzed Phosphinidene Transfer to Styrenic Olefins in a Stereoselective Synthesis of Unprotected Phosphiranes

Catalytic phosphiranation has been achieved, allowing preparation of trans-1-R-2-phenylphosphiranes (R = t-Bu: 1-t-Bu; i-Pr: 1-i-Pr) from the corresponding dibenzo-7-(R)-7-phospha-norbornadiene (RPA, A = C₁₄H₁₀, anthracene) and styrene in 73% and 57% isolated yields, respectively. The cocatalyst system requires tetramethylammonium fluoride (TMAF) and [Fp(THF)][BF₄] (Fp = Fe(η⁵-C₅H₅)(CO)₂). In the case of the t-Bu derivative, the reaction mechanism was probed using stoichiometric reaction studies, a Hammett analysis, and a deuterium labeling experiment. Together, these suggest the intermediacy of iron-phosphido FpP(F)(t-Bu) (2), generated independently from the stoichiometric reaction of [Fp(t-BuPA)][BF₄] with TMAF. Two other plausible reaction intermediates, [Fp(t-BuPA)][BF₄] and [Fp(1-t-Bu)][BF₄], were prepared independently and structurally characterized.