Synthesis of a 1,3-diphosphacyclobutane-2,4-diyl from Mes*C[triple bond]P

Synthesis and Characterization of an All-Germanium Analogue of Cyclobutane-1,3-diyl

A tetragermacyclobutane-1,3-diyl was prepared and structurally characterized via the reduction of chlorogermylene-coordinated germylgermylene with potassium graphite, which represents the first all-germanium analogue of cyclobutane-1,3-diyl. Single-crystal X-ray analysis of the tetragermacyclobutane-1,3-diyl disclosed that it adopts a planar-cis structure, which is different from those reported all-silicon cyclobutane-1,3-diyls. DFT calculations revealed that both the bulky substituents on the germanium atoms and the tethering of the amido groups are important for the planar cis-configuration of 5.

Photoswitchable and long-lived seven-membered cyclic singlet diradicals for the bioorthogonal photoclick reaction

Annularly 1,3-localized singlet diradicals are energetic and homolytic intermediates, but commonly too short-lived for widespread utilization. Herein, we describe a direct observation of a long-lived and seven-membered singlet diradical, oxepine-3,6-dione-2,7-diyl (OXPID), via spectroscopic experiments and also theoretical evidence from computational studies, which is generated via photo-induced ring-expansion of 2,3-diaryl-1,4-naphthoquinone epoxide (DNQO). The photo-generated OXPID reverts to the thermally stable σ-bonded DNQO with t1/2 in the μs level, thus constituting a novel class of T-type molecular photoswitches with high light-energy conversion efficiency (η = 7.8-33%). Meanwhile, the OXPID is equilibrated to a seven-membered cyclic 1,3-dipole as an electronic tautomer that can be captured by ring-strained dipolarophiles with an ultrafast cycloaddition rate (k2CA up to 109 M-1 s-1). The T-type photoswitchable DNQO is then exploited to be a highly selective and recyclable photoclick reagent, enabling spatiotemporal-resolved bioorthogonal ligation on living cell membranes via a tailored DNQO-Cy3 probe.

Muon Spin Rotation/Resonance (μSR) for Studying Radical Reactivity of Unsaturated Organophosphorus Compounds

The positive muon (μ+ ) can be regarded as a light isotope of proton and has been an important tool to study radical reactions of organic compounds. Recently, muons have been applied to produce short-lived paramagnetic species from the heavier unsaturated organic molecules including the p-block elements. This article overviews recent muon spin rotation/resonance (μSR) studies on the phosphorus analogs of alkenes, anthracenes, and cyclobutane-1,3-diyls together with the fundamentals of μSR. The acyclic phosphaalkene of P=C and phosphasilenes of P=Si can accept muonium (Mu=[μ+ e- ]) at the heavier double bonds, and the corresponding radicals have been characterized. The phosphorus atom in 9-phosphaanthracene, whose P=C double bond is stabilized by the peri-substituted CF3 groups, predominantly captures muonium to provide the corresponding paramagnetic fused heterocyclic system. The peri-trifluoromethyl groups are functional to promote the unprecedented light isotope effect of muon providing the planar three-cyclic molecular structure to consume the increased zero-point energy. The formally open-shell singlet 1,3-diphosphacyclobutane-2,4-diyl unit can accept muonium at the (ylidic) phosphorus or the skeletal radicalic carbon, and the corresponding paramagnetic phosphorus heterocycles can be characterized by μSR. The findings on these muoniation processes to the unsaturated phosphorus-containing compounds will contribute not only to development of novel paramagnetic functional species but also to progress on muon science.

Concerted addition of aldehydes to the singlet biradical [P(μ-NTer)]2

The reaction of the singlet biradical [P(μ-NTer)]₂ with various aldehydes selectively yielded the corresponding [2.1.1]-bicyclic addition products in a very fast reaction. All products were fully characterized, including by NMR and vibrational spectroscopy as well as single-crystal X-ray diffraction. The mechanism of the addition was investigated theoretically using high-level ab initio methods (CCSD(T) with triple- and quadruple-zeta basis sets) and corresponds to a concerted cycloaddition reaction with a very low activation barrier. For comparison, the mechanisms of the literature-known cycloadditions of H₂, alkenes, and alkynes were also studied, indicating a similar reaction profile for all unsaturated reactants.

Radical Reactivity of the Biradical [⋅P(μ-NTer)2 P⋅] and Isolation of a Persistent Phosphorus-Cantered Monoradical [⋅P(μ-NTer)2 P-Et]

The activation of C-Br bonds in various bromoalkanes by the biradical [⋅P(μ-NTer)2 P⋅] (1) (Ter=2,6-bis-(2,4,6-trimethylphenyl)-phenyl) is reported, yielding trans-addition products of the type [Br-P(μ-NTer)2 P-R] (2), so-called 1,3-substituted cyclo-1,3-diphospha-2,4-diazanes. This addition reaction, which represents a new easy approach to asymmetrically substituted cyclo-1,3-diphospha-2,4-diazanes, was investigated mechanistically by different spectroscopic methods (NMR, EPR, IR, Raman); the results suggested a stepwise radical reaction mechanism, as evidenced by the in-situ detection of the phosphorus-centered monoradical [⋅P(μ-NTer)2 P-R].< To provide further evidence for the radical mechanism, [⋅P(μ-NTer)2 P-Et] (3Et⋅) was synthesized directly by reduction of the bromoethane addition product [Br-P(μ-NTer)2 P-Et] (2 a) with magnesium, resulting in the formation of the persistent phosphorus-centered monoradical [⋅P(μ-NTer)2 P-Et], which could be isolated and fully characterized, including single-crystal X-ray diffraction. Comparison of the EPR spectrum of the radical intermediate in the addition reaction with that of the synthesized new [⋅P(μ-NTer)2 P-Et] radical clearly proves the existence of radicals over the course of the reaction of biradical [⋅P(μ-NTer)2 P⋅] (1) with bromoethane. Extensive DFT and coupled cluster calculations corroborate the experimental data for a radical mechanism in the reaction of biradical [⋅P(μ-NTer)2 P⋅] with EtBr. In the field of hetero-cyclobutane-1,3-diyls, the demonstration of a stepwise radical reaction represents a new aspect and closes the gap between P-centered biradicals and P-centered monoradicals in terms of radical reactivity.

Chemistry of Several Sterically Bulky Molecules with P=P, P=C, and C≡P Bond

Several sterically protected, low-coordinate organophosphorus compounds with P=P, P=C, and C≡P bond are described in this study. Molecules such as diphosphenes, phosphaalkenes, 1-phosphaallenes, 1,3-diphosphaallenes, 3,4-diphosphinidenecyclobutenes, and phosphaalkynes are stabilized with an extremely bulky 2,4,6-tri-t-butylphenyl (Mes*) group. The synthesis, structures, physical, and chemical properties of these molecules are discussed, together with some successful applications in catalytic organic reactions.

An Open-Shell Singlet SnI Diradical and H2 Splitting

The first SnI diradical [(ADCPh )Sn]2 (4) based on an anionic dicarbene (ADCPh ={CN(Dipp)}2 CPh; Dipp=2,6-iPr2 C6 H3 ) scaffold has been isolated as a green crystalline solid by KC8 reduction of the corresponding bis-chlorostannylene [(ADCPh )SnCl]2 (3). The six-membered C4 Sn2 -ring of 4 containing six π-electrons shows a diatropic ring current, thus 4 may also be regarded as the first 1,4-distannabenzene derivative. DFT calculations suggest an open-shell singlet (OS) ground state of 4 with a remarkably small singlet-triplet energy gap (ΔEOS-T =4.4 kcal mol-1 ), which is consistent with CASSCF (ΔES-T =6.6 kcal mol-1 and diradical character y=37 %) calculations. The diradical 4 splits H2 at room temperature to yield the bis-hydridostannylene [(ADCPh )SnH]2 (5). Further reactivity of 4 has been studied with PhSeSePh and MeOTf.

Heterocyclopentanediyls vs Heterocyclopentadienes: A Question of Silyl Group Migration

The reaction of the singlet biradical [P(μ-NHyp)]₂ (Hyp = hypersilyl, (Me₃Si)₃Si) with different isonitriles afforded a series of five-membered N₂P₂C heterocycles. Depending on the steric bulk of the substituent at the isonitrile, migration of a Hyp group was observed, resulting in two structurally similar but electronically very different isomers. As evidenced by comprehensive spectroscopic and theoretical studies, the heterocyclopentadiene isomer may be regarded as a rather unreactive closed-shell singlet species with one localized N═P and one C═P double bond, whereas the heterocyclopentanediyl isomer represents an open-shell singlet biradical with interesting photochemical properties, such as photoisomerization under irradiation with red light to a [2.1.0]-housane-type species.

Impact of the macrocyclic structure and dynamic solvent effect on the reactivity of a localised singlet diradicaloid with π-single bonding character

Localised singlet diradicals are key intermediates in bond homolysis processes. Generally, these highly reactive species undergo radical–radical coupling reaction immediately after their generation. Therefore, their short-lived character hampers experimental investigations of their nature. In this study, we implemented the new concept of “stretch effect” to access a kinetically stabilised singlet diradicaloid. To this end, a macrocyclic structure was computationally designed to enable the experimental examination of a singlet diradicaloid with π-single bonding character. The kinetically stabilised diradicaloid exhibited a low carbon–carbon coupling reaction rate of 6.4 × 10³ s⁻¹ (155.9 μs), approximately 11 and 1000 times slower than those of the first generation of macrocyclic system (7.0 × 10⁴ s⁻¹, 14.2 μs) and the parent system lacking the macrocycle (5 × 10⁶ s⁻¹, 200 ns) at 293 K in benzene, respectively. In addition, a significant dynamic solvent effect was observed for the first time in intramolecular radical–radical coupling reactions in viscous solvents such as glycerin triacetate. This theoretical and experimental study demonstrates that the stretch effect and solvent viscosity play important roles in retarding the σ-bond formation process, thus enabling a thorough examination of the nature of the singlet diradicaloid and paving the way toward a deeper understanding of reactive intermediates.

An extremely long-lived localised singlet diradical with π-single bonding character is found in a macrocyclic structure that retards the radical–radical coupling reaction by the “stretch and solvent-dynamic effects”.

Incomplete Electrocyclization of a Sterically Hindered 1,4-Diphosphabutadiene

Butadiene is the simplest neutral acyclic closed-shell π-conjugated system and is typically sufficiently stable enough to avoid electrocyclization to cyclobutene. In contrast, most congeners of butadiene containing heavier elements are easily converted into the corresponding 4-membered cyclobutene system. Herein, we demonstrate that the gauche 1,4-diphosphabutadiene (P=C-C=P) skeleton in a sterically encumbered 2,3-bis(phosphanylidene)-1,4-disilinane can be remarkably perturbed due to "incomplete electrocyclization" where P=C-C=P partially form the corresponding 1,2-dihydrodiphosphete (3,4-diphosphacyclobutene) by [2+2] electrocyclization. ³¹ P NMR data obtained in solution indicated that the coexistence of a closed ring substantially reduces the open-ring characteristics of the P=C-C=P moiety. However, the ³¹ P CP-MAS spectrum of 2,3-bis(phosphanylidene)-1,4-disilinane showed that the P=C-C=P structure is predominant in the solid-state. Single-crystal X-ray analysis revealed that decreasing the temperature promoted the generation of small amounts of incomplete 1,2-dihydrodiphosphete system in the crystalline state. Furthermore, the 1,2-dihydrodiphosphete units disappeared upon warming the single crystal, and this unique solid-state electrocyclization reaction was reversible.

Facile addition of E–H bonds to a dicarbondiphosphide

Charge transfer at P atoms in an N-heterocyclic carbene stabilized 6π-electron aromatic dicarbondiphosphide 1 has been observed upon interaction with a variety of small molecule substrates that feature a polar E–H bond (E = C, N, and O).

Dynamic solvent effects in radical-radical coupling reactions: an almost bottleable localised singlet diradical

Localised singlet diradicals are key intermediates in bond homolysis, which plays a crucial role in chemical reactions. However, thorough experimental analyses of the reaction dynamics and chemical properties are generally difficult because bond formation is rapid, even under low-temperature matrix conditions. In this study, the effects of solvent and pressure on the lifetimes of long-lived singlet diradicals with bulky substituents were investigated. The solvent dynamic effect was revealed to provide control over the rate constant of radical-radical coupling reactions, and an almost bottleable singlet diradical with a lifetime of ∼2 s at 293 K was obtained.

Bolaamphiphilic properties and pH-dependent micellization of quercetin polyglycoside

Quercetin polyglycoside as a new bolaamphiphile is prepared via a one-pot grafting polymerization technique using sugar-based cyclic sulfite. Micelles comprising quercetin polyglycoside exhibit special pH-effects, in which the polyglycoside moieties on the surface of the micelle serve as a steric protecting group to endow chemical stabilization.

Metastable phosphorus neutral monoradical: a key intermediate in the bicyclic cage formation

A key intermediate in the formation of a bicyclic cage formed between a biradical (LCP)₂ (L = carbene) and an unsaturated substrate via a [2 + 2] cycloaddition reaction has been isolated and fully characterized including by X-ray diffraction analysis.

Theoretical Study on Open-Shell Singlet Character and Second Hyperpolarizabilities in Cofacial π-Stacked Dimers Composed of Weak Open-Shell Antiaromatic Porphyrins

From the analysis based on the broken-symmetry density functional theory (DFT) calculations, we in this study propose a strategy to enhance the open-shell characters and third-order nonlinear optical (NLO) properties of π-stacked dimers composed of antiaromatic molecules with weak open-shell characters. For this purpose, we here constructed cofacial π-stacked dimer models composed of aromatic and antiaromatic Ni^II porphyrins in order to examine the π-π stacking distance (R) dependence of the diradical characters (y) and static second hyperpolarizabilities (γ). The antiaromatic porphyrin dimers are found to have intermediate y around R∼3.3 Å, the result of which originates in the unique intermolecular interactions between the antiaromatic monomers. Static γ along the stacking direction of such antiaromatic porphyrin dimers with intermediate diradical characters are shown to be enhanced significantly as compared to those of the isolated monomers and the aromatic porphyrin dimers.

Observation of a Metastable P-Heterocyclic Radical by Muonium Addition to a 1,3-Diphosphacyclobutane-2,4-diyl

A 1,3-diphosphacyclobutane-2,4-diyl contains a unique unsaturated cyclic unit, and the presence of radical-type centers have been expected as a source of functionality. This study demonstrates that the P-heterocyclic singlet biradical captures muonium (Mu=[μ⁺ e^- ]), the light isotope of a hydrogen radical, to generate an observable P-heterocyclic paramagnetic species. Investigation of a powder sample of 2,4-bis(2,4,6-tri-t-butylphenyl)-1-t-butyl-3-benzyl-1,3-diphosphacyclobutane-2,4-diyl using muon (avoided) level-crossing resonance (μLCR) spectroscopy revealed that muonium adds to the cyclic P₂ C₂ unit. The muon hyperfine coupling constant (A_μ ) indicated that the phosphorus atom bearing the t-butyl group trapped muonium to provide a metastable P-heterocyclic radical involving the ylidic MuP(<)=C moiety. The observed regioselective muonium addition correlates the canonical formula of 1,3-diphosphacyclobutane-2,4-diyl.

Overcrowded aminophospanitrenes: a case study

In a search for stable phosphinonitrenes generated by thermally induced decomposition of the azidophosphines 8a, b, different cyclodiphosp(V)azene products were generated. While in the case of 8a the expected phosphinonitrene dimer 10 could be obtained, from 8b, which has the sterically more demanding TMP substituents, product 11, was obtained and characterized. DFT calculations have revealed that the primarily formed phosphinonitrene 9b is unstable against loss of TMP˙, and the resulting radical dimerizes to the biradicaloid 1,3-diaza-2,4-diphosphetane-2,4-diyl 13. Compound 13 then dimerizes after ring opening, yielding in the ring system, which provides the final product upon reaction with the starting azidophosphine.

Sterically protected organophosphorus compounds of unusual structures

The character of bis(2,4,6-tri-tert-butylphenyl)diphosphene is described experimentally and theoretically. The diphosphene is stabilized by steric protection and the structure can be characterized by spectroscopic as well as crystallographic analyses. Theoretical calculation on the diphosphene strongly suggests that the P=P bond is an isolated double bond and that the P–C bonds are single covalent bond. The reactivity has been investigated including photolysis, oxidation, sulfurization, selenation, transition-metal complex formation, and carbene addition. Plausible mechanistic scheme for the reaction of the diphosphene with 3,4,5,6-tetrachlorocyclohexa-3,5-diene-1,2-dione (or tetrachloro-o-benzoquinone) to a pentavalent spiro product is discussed based on the product analysis.

Access to Air-Stable 1,3-Diphosphacyclobutane-2,4-diyls by an Arylation Reaction with Arynes

Tuning of the physicochemical properties of the 1,3-diphosphacyclobutane-2,4-diyl unit is attractive in view of materials applications. The use of arynes is shown to be effective for installing relatively electron rich aryl substituents into the open-shell singlet P-heterocyclic system. Treatment of the sterically encumbered 1,3-diphosphacyclobuten-4-yl anion with ortho-silylated aryl triflates in the presence of fluoride under appropriate conditions afforded the corresponding 1-aryl 1,3-diphosphacyclobutane-2,4-diyls. The air-stable open-shell singlet P-heterocycles exhibit considerable electron-donating character, and the aromatic substituent influences the open-shell character, which is thought to be related to the property of p-type semiconductivity. The P-arylated 1,3-diphosphacyclobutane-2,4-diyl systems can be further utilized as detectors of hydrogen fluoride (HF), which causes a remarkable change in their photoabsorption properties.

Orbital phase design of diradicals

Over the last three decades the rational design of diradicals has been a challenging issue because of their special features and activities in organic reactions and biological processes. The orbital phase theory has been developed for understanding the properties of diradicals and designing new candidates for synthesis. The orbital phase is an important factor in promoting the cyclic orbital interaction. When all of the conditions: (1) the electron-donating orbitals are out of phase; (2) the accepting orbitals are in phase; and (3) the donating and accepting orbitals are in phase, are simultaneously satisfied, the system is stabilized by the effective delocalization and polarization. Otherwise, the system is less stable. According to the orbital phase continuity requirement, we can predict the spin preference of π-conjugated diradicals and relative stabilities of constitutional isomers. Effects of the intramolecular interaction of bonds and unpaired electrons on the spin preference, thermodynamic and kinetic stabilities of the singlet and triplet states of localized 1,3-diradicals were also investigated by orbital phase theory. Taking advantage of the ring strains, several monocyclic and bicyclic systems were designed with appreciable singlet preference and kinetic stabilities. Substitution effects on the ground state spin and relative stabilities of diradicals were rationalized by orbital interactions without loss of generality. Orbital phase predictions were supported by available experimental observations and sophisticated calculation results. In comparison with other topological models, the orbital phase theory has some advantages. Orbital phase theory can provide a general model for both π-conjugated and localized diradicals. The relative stabilities and spin preference of all kinds of diradicals can be uniformly rationalized by the orbital phase property. The orbital phase theory is applied to the conformations of diradicals and the geometry-dependent behaviors. The insights gained from the orbital phase theory are useful in a rational design of stable 1,3-diradicals.