Producing Conventional and Transient Amino(mercapto)methyl Radicals by Addition of Muonium to a Crystalline Thioformamide (Mes*NHCH=S).

Muonium (Mu=µ+e-) is composed of a muon of light isotope of proton (µ+) and electron (e-) and can be used as a light surrogate for a hydrogen atom. In this paper, we investigated addition of muonium to a newly synthesized Mes*-substituted thioformamide (Mes*NHCH=S, Mes*=2,4,6-tBu3C6H2). Transverse-field muon spin rotation (TF-µSR) of a solution sample of the thioformamide confirmed addition of muonium to the sulfur atom leading to the corresponding C-centered radical [Mes*NHC(H)⋅-SMu]. Density functional theory (DFT) calculations assigned a conventional amino(mercapto)methyl radical, in which both nitrogen and carbon were slightly pyramidalized, and the calculated muon hyperfine coupling constant (hfcc) including the muon isotope effect was compatible with the experimentally determined parameter. However, the muon level-crossing resonance (µLCR) spectrum of an anisotropic crystalline sample indicated two paramagnetic species, and the major product showed the considerably larger muon hfcc compared with the conventional structure of the amino(mercapto)methyl radical. The unusual transient muoniated thioformamide with the larger muon hfcc that showed rapid relaxation could be only explained by a transient structure including planarization of the nitrogen and carbon atoms in Mes*NHC(H)⋅-SMu.

A path integral molecular dynamics study on the muoniated xanthene-thione molecule.

A positive Mu is a useful tool for investigating the spin density of radical species. The theoretical estimation of its behavior in a molecule requires the inclusion of a quantum effect due to the small mass of muonium. Herein, we performed ab initio a path integral molecular dynamics (PIMD) simulation, which accurately included a multi-dimensional quantum effect, for muoniated 9H-xanthene-9-thione (µXT). Our results showed that the quantum effect significantly increased the hyperfine coupling constant (HFCC) value of µXT, which qualitatively improved the calculated HFCC value, compared to the experimental one. In the PIMD simulation, the bond length between muonium and sulfur in µXT is longer than that between hydrogen and sulfur in a hydrogenated 9H-xanthene-9-thione (HXT), leading to a spin density transfer from XT (9H-xanthene-9-thione) to muonium due to neutral dissociations. Additionally, we found that the S-Mu bond in µXT prefers a structure perpendicular to the molecular plane, where the interaction between Mu and the singly occupied molecular orbital of µXT is the strongest. These structural changes resulted in a larger HFCC value in the PIMD simulation of µXT.

A π-Extension Process from 9-Phosphaanthracene Leading to Phosphatetraphenes and Phosphatetracenes.

Extension of π-system with phosphinine (phosphorine, phosphabenzene) has been of interest because of the expected higher HOMO and lower LUMO levels compared with the corresponding carbon congeners. In this paper, a π-extension process based on the 9-phosphaanthracene skeleton was demonstrated by synthesizing 12-phosphatetraphene and 9-phosphabenzo[f]tetraphene by utilizing the deaminative aromatization pathway. Starting from 3,5-bis(trifluoromethyl)aniline, we developed the dibromotriarylmethane precursors containing the 3,5-bis(trifluoromethyl)-2-bromophenyl unit, which would be slightly effective to increase the steric congestion around the fragile P=C bonds incorporated in the fused polyaromatic skeletons. The bis-trifluoromethyl 12-phosphatetraphenes have been synthesized together with the mono-trifluoromethyl derivative, and the planar 12-phosphatetraphene skeleton was confirmed. On the other hand, the CF3 -substituted 9-phosphabenzo[f]tetraphene displayed a remarkably twisted fused five ring system leading to the wavy structures incorporating phosphinine. Also, synthetic study of 5-phosphatetracene using the bis(trifluoromethyl)phenyl unit was attempted and the incomplete elimination of the amine indicated labile characters of the observed phosphorus congener of tetracene. The findings of this study would be informative for developing heavier congeners of polyaromatic hydrocarbons (PAHs) as well as the trifluromethyl effects.

Difluoromethylborates and Muonium for the Study of Isonitrile Insertion Affording Phenanthridines via Imidoyl Radicals.

The 6-(difluoromethyl)phenanthridine unit is a highly attractive fluoroalkyl-substituted planar nitrogen heterocycle in pharmaceutical and agrochemical research. In this paper, we report that difluoromethylborates can be used as a source of difluoromethyl radicals for isonitrile insertion, leading to 6-(difluoromethyl)phenanthridines. Tuning the aryl substituents in the difluoromethylborates and oxidizing reagents enabled the synthesis of 6-(difluoromethyl)phenanthridines through the generation of difluoromethyl radical and spontaneous intramolecular cyclization of the CF2H-imidoyl radical intermediates. The presence of difluoromethyl radicals was experimentally confirmed, and the reaction mechanisms including imidoyl radical and prompt cyclization reactions could be supported theoretically. Furthermore, we obtained valuable information about the imidoyl radical intermediate by performing transverse-field muon spin rotation (TF-µSR) measurements of 2-isocyano-4'-methoxy-1,1'-biphenyl and using density functional theory (DFT) calculations to interpret the spectra. Muonium, a simple free radical, preferentially adds to the carbon atom of the isonitrile unit, yielding the corresponding imidoyl radical. The temperature dependence of the muon hyperfine coupling constant and the spin relaxation of the muoniated radical signal are compatible with the intramolecular cyclization of biaryl-substituted imidoyl radicals on the µs time scale.

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.

Copper-Catalyzed Enantioselective Hydrosilylation of gem-Difluorocyclopropenes Leading to a Stereochemical Study of the Silylated gem-Difluorocyclopropanes.

Optically active cyclopropanes have been widely investigated especially from the views of pharmaceutical and agrochemical industries, and substituting one of the methylenes with the difluoromethylene unit should be promising for developing novel biologically relevant compounds and functional materials. In this paper, the copper-catalyzed enantioselective hydrosilylation of gem-difluorocyclopropenes to provide the corresponding chiral gem-difluorocyclopropanes is presented. The use of copper(I) chloride, chiral ligands including bidentate BINAPs and monodentate phosphoramidites, and silylborane Me2 PhSi-Bpin accompanying sodium tert-butoxide in methanol was appropriate for the enantioselective hydrosilylation of the strained C=C double bond, and the resultant chiral difluorinated three-membered ring was unambiguously characterized. Subsequent activation of the silyl groups in enantio-enriched gem-difluorocyclopropanes showed substantial reduction of the enantiopurity, indicating cleavage of the distal C-C bond leading to the transient acyclic intermediates.

Muonium Addition to a peri-Trifluoromethylated 9-Phosphaanthracene Producing a High-Energy Paramagnetic π-Conjugated Fused Heterocycle.

In this communication, we report muon spin rotation/resonance (µSR) studies for understanding radical reactivity of 10-mesityl-1,8-bis(trifluoromethyl)-9-phosphaanthracene. Transverse-field muon spin rotation (TF-µSR) and muon avoided level-crossing resonance (µLCR) measurements successfully visualized a paramagnetic species produced by regioselective addition of muonium (Mu) to the skeletal phosphorus atom. Density functional theory (DFT) calculations for the P-muoniation product suggested two possible isomers. Whereas the most stable isomer including the envelope-type phosphorus heterocycle shows considerably different hyperfine coupling constants (hfcs) from those of the TF-µSR and µLCR, the metastable structure accompanying the almost planar tricyclic π-conjugated skeleton could simulate the experimentally determined hfcs. The metastable planar π-conjugated paramagnetic tricyclic-fused skeleton is promoted by the larger zero-point energy due to the light muon (µ+ ), one ninth of the proton mass.

Gold(I) Complexation of Phosphanoxy-Substituted Phosphaalkenes for Activation-Free LAuCl Catalysis.

The phosphanoxy-substituted phosphaalkene bearing the P=C-O-P skeleton can be prepared from diphosphene Mes*P=PMes* (Mes*=2,4,6-tBu3 C6 H2 ), and their use for catalysis is of interest. In this paper, complexation of the phosphanoxy-substituted phosphaalkenes with gold are investigated, and the catalytic activity of the mono- and bis(chlorogold) complexes are subsequently evaluated. Reaction of the P=C-O-P compound with (tht)AuCl (tht=tetrahydrothiophene) showed dominant coordination on the sp3 phosphorus, and complete coordination on the sp2 phosphorus required removal of tetrahydrothiophene. Atoms In Molecules (AIM) analysis based on the X-ray structure of the mono(chlorogold) complex indicated a pseudo coordinating interaction between the gold center and the P=C unit. The bis(chlorogold) complexes displayed conformational isomerism, and catalyzed the cycloisomerization/alkoxycyclization of 1,6-enyne and for hydration of terminal alkyne without activation treatment. Even the mono(chlorogold) complexes catalyzed the alkoxycyclization reactions without a silver co-catalyst, indicating that the alcohols were effective in activating the AuCl unit.

Convenient Preparation and Structure Determination of Air- and Moisture-Tolerant Difluoromethylborates.

Convenient and reliable synthetic methods for difluoromethylborates have been established. The intermediary generated difluoromethylsilicate species from TMSCF2 H (TMS=trimethylsilyl) and potassium tert-butoxide were allowed to react with pinBPh (Me4 C2 O2 BPh) in the presence of 18-crown-6 to give the corresponding borate compound [pinB(Ph)CF2 H]- K+ (18-crown-6) as an air- and moisture-tolerant solid. The unambiguously determined crystal structure of [pinB(Ph)CF2 H]- K+ (18-crown-6) revealed that the difluoromethylborate unit partially coordinated on the potassium ion. Reaction of [pinB(Ph)CF2 H]- K+ (18-crown-6) with potassium difluoride (KHF2 ) in acetic acid enabled substitution of the pinacol unit and phenyl group with fluorides, and gave (difluoromethyl)trifluoroborate [F3 BCF2 H]- K+ (18-crown-6) in a good yield. The crystal structure of air- and moisture-tolerant [F3 BCF2 H]- K+ (18-crown-6), which would be a promising reagent for synthesis of various difluoromethylboron species, showed a polyrotaxane-like polymeric structure based on the K⋅⋅⋅F interactions between the K+ (18-crown-6), CF2 H, and BF3 units.

Scalable Orbital Tuning of the 1,3-Diphosphacyclobutane-2,4-diyl Unit of Singlet Biradicaloid.

The phosphino groups in 1,3-diphosphacyclobutane-2,4-diyl in the singlet biradicaloid play important roles in determining the physicochemical characteristics of the particular open-shell phosphorus heterocycles. Herein, we demonstrate that the introduction of para-substituted phenyl groups to the phosphorus atom induces useful photoabsorption and redox characteristics, as confirmed by the Hammett equation. These findings provided reliable methods for realizing desirable physicochemical properties of the open-shell singlet phosphorus heterocyclic chromophores based on programmed tuning of the molecular orbital energy levels of the particular open-shell singlet (non-Kekulé) heterocyclic unit. In addition, use of the thiophene substituent allowed confirmation of the orbital tuning by catenation of the P-heterocyclic units, with the >P-C4H2S-P< bridging component inducing splitting of the degenerate LUMO energy levels and destabilizing of the cationic species upon oxidation.

Diels-Alder reactions of 1-phosphabutadienes: a highly selective route to P[double bond, length as m-dash]C-substituted phosphacyclohexenes.

Kinetically stabilized 1-phosphahaloprenes (2-halo-1-phosphabutadienes) as well as 1-phosphaisoprene undergo a hitherto unknown phospha-Diels-Alder dimerization of the P[double bond, length as m-dash]C-C[double bond, length as m-dash]C units upon heating. The [4+2] cyclodimerization is highly stereo- and regio-selective. The phosphaalkene-substituted phosphacyclohexene product is an unprecedented P(sp2),P(sp3) ligand that is of interest in polymer/materials science and catalysis.

Synthetic Methodologies for Perfluoroaryl-Substituted (Diaryl)methylphosphonates, -Phosphinates via SNAr Reaction.

The new synthetic methodologies for perfluoroaryl-substituted (diaryl)methylphosphonates, -phosphinates via nucleophilic aromatic substitution (SNAr) were developed. Benzylphosphonate and α-fluorobenzylphosphonate reacted with a wide variety of perfluoroarenes via SNAr reaction. The reaction took place quickly and gave perfluoroarylated phosphonates in high yields. Highly diastereoselective SNAr reaction with binaphthyl-based chiral phosphinates was further carried out.

Design of Phosphinic Acid Catalysts with the Closest Stereogenicity at the α-Position: Synthesis and Application of α-Stereogenic Perfluoroalkyl Phosphinic Acid Catalysts.

Chiral C2-symmetric phosphinic acids were designed based on sterically demanding and helical chiral perfluoroalkyl groups at the closest α-position advancing asymmetric reaction environment and catalytic activity. The perfluoroalkyl catalysts, [(CF3)2F2] and [(C2F5)2F2] phosphinic acids, were synthesized via a stereoselective addition/cyclization sequence of methyl phosphinate and deoxofluorination. These new classes of Brønsted acid catalysts were applied to an asymmetric Friedel-Crafts reaction to give up to 89% yield and 82% R-enantioselectivity, which is higher than those obtained with the parent phosphoric acid (42% and 55.5% S).

Regiocontrolled Heptafluoroisopropylation of Aromatic Halides by Copper(I) Carboxylates with Heptafluoroisopropyl-Zinc Reagents.

Copper(I)-mediated heptafluoroisopropylation of aryl halides (ArX: X = I, Br) is demonstrated using copper(I) carboxylates and a bis(heptafluoroisopropyl)zinc reagent Zn( i-C3F7)2(dmf)2, prepared from heptafluoroisopropyl iodide and diethylzinc. The air-tolerant solid heptafluoroisopropylzinc reagent is advantageous to conduct simple synthetic operations and successful to give the corresponding heptafluoroisopropyl arene derivatives via transmetalation to copper(I) center. The newly developed copper(I)-mediated heptafluoroisopropylation process can be advanced to the copper(I)-catalysis by silver carboxylate salts and complementary to the precedent radical-based processes.

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.

gem-Digold Acetylide Complexes for Catalytic Intermolecular [4 + 2] Cycloaddition: Having Two Gold Centers Is Better for Asymmetric Catalysis.

Gold(I)-catalyzed highly enantioselective intermolecular [4 + 2] cycloaddition is shown with ynones and cyclohexadiene. Various bicyclo[2.2.2]octadiene derivatives are produced in high yields (up to 99%) with good enantioselectivity (up to 96% ee). Key to the success is generation of the gem-digold terminal alkyne as a catalytic on-cycle species. As proof of the gem-digold catalysis, a positive nonlinear effect is clarified between the ee's of the ligand and the cycloadduct.

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 (Aµ ) 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.

CF3 -Inspired Synthesis of Air-Tolerant 9-Phosphaanthracenes that Feature Fluorescence and Crystalline Polymorphs.

9-Phosphaanthracene (dibenzo[b,e]phosphorin, acridophosphine) has attracted interest as one of the heavier acenes. Herein, we demonstrate an efficient synthetic process that provides air-tolerant 1,8-bis(trifluoromethyl)-9-phosphaanthracenes. The sterically encumbered and electron-withdrawing trifluoromethyl (CF3 ) groups are quite advantageous not only to stabilize the intrinsically unstable heavier unsaturated phosphorus atom but also to facilitate construction of the phosphinine skeleton based on a putative increase in aromaticity. The isolated 9-phosphaanthracenes allowed characterization of their fluorescence functionality and planar heteroanthracene frameworks. The crystal structures of 9-phosphaanthracenes are remarkably dependent on the aryl substituents at the 10 position; anthryl-substituted 9-phosphaanthracene showed unique polymorphs that induced different-colored crystals.

Study of the Air-Tolerant 1,3-Diphosphacyclobutane-2,4-diyl through the Direct Arylation.

Installing π-functional substituents on the skeletal phosphorus atoms of the air-tolerant 1,3-diphosphacyclobutane-2,4-diyl unit are promising for tuning the open-shell singlet P-heterocyclic chromophore. The sterically encumbered 1,3-diphosphaCycloButen-4-yl Anion (CBA), generated from the phosphorus-carbon triple bond, was available for the regioselective arylation via nucleophilic aromatic substitution (SN Ar) reaction, addition to arynes, and single-electron transfer (SET) process affording the corresponding P-arylated 1,3-diphosphacyclobutane-2,4-diyls. The photo-absorption and redox properties correlated with the effects of the aryl substituents on the 1,3-diphosphacyclobutane-2,4-diyl unit. The X-ray analyses enabled not only to discuss the metric parameters but also to visualize the radicalic electrons via the electron-density distribution analysis. The electron-donating character of the P-heterocyclic chromophores induced the p-type semiconductor behavior. Detection of hydrogen fluoride via formation of the 1λ5 ,3λ5 -diphosphete derivative was also developed.

Stable (sila)difluoromethylboranes via C-F activation of fluoroform derivatives.

Lithium 1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaborol-2-uide activates the C-F linkage of fluoroform (CF3H) and the Ruppert-Prakash reagent (CF3SiMe3) to provide difluoromethyl-substituted boranes as air-stable compounds.