C−F Bond Activation Mediated by Phosphorus Compounds

The activation and functionalization of C−F bonds has garnered significant attention in the scientific community as a strategy to mitigate toxicity and environmental concerns, as well as provide new pathways to agro- and pharmaceutical chemicals and materials. Although several transition-metal-based systems have been developed for this transformation, the use of main-group compounds remains less explored. In recent years, several strategies for C−F bond activation have focused on the use of phosphorus-based reagents. In this Minireview, an overview of strategies is provided that exploits P^V and P^III-based Lewis acids as well as P^III Lewis bases in frustrated Lewis pair (FLP) protocols for hydrodefluorination, C−C couplings and C−F derivatizations.     

Stereoselective Unsymmetrical 1,1-Diborylation of Alkynes with a Neutral sp2−sp3 Diboron Reagent

The incorporation of two distinct boryl groups at the same carbon center in organic molecules has attracted growing research interest due to its potential for facilitating controlled, precise synthesis through stepwise dual carbon-boron bond transformations. Here we report a method to access u nsymmetrical 1,1- d i b oryl a lkene (UDBA) stereoselectively via the reaction of readily available alkynes with a neutral sp²−sp³ diboron reagent (NHC) B H₂- B pin (NHC=N-heterocyclic carbene). Attributing to the chemically easily distinguishable nature of the sp² and sp³ boryl moieties, controllable stepwise derivatization of the resultant UDBAs is realized. This process leads to various multifunctionalized olefins and organoborons, such as acylboranes, which are difficult to prepare by other methods.     

Sustainable Synthesis of N−N Bond Bearing Organic Frameworks by Advanced Heterogeneous Metal Catalysis

The most significant task in synthetic organic chemistry is developing organic reactions from a green and sustainable perspective. In these ways, heterogeneous catalysts are essential in many organic reactions. Thus, this study focuses on the synthesis of N−N bond bearing organic frameworks, particularly aromatic azo compounds and indazoles, using a heterogeneous metal catalyst. Organic molecules containing nitrogen-nitrogen bonds are greatly sought after in the synthesis of dyes, photochemical switches, food additives, and bioactive chemicals due to industry and human requirements. Finally, this review provides an overview of synthetic routes for the formation of N−N bonds by using various heterogeneous metal catalytic systems.     

H2, B−H,and Si−H Bond Activation and Facile Protonolysis Driven by Pt-Base Metal Cooperation

We report a series of heterobimetallic Pt/Zn and Pt/Ca complexes to study the effect of proximity of a dicationic base metal on the organometallic Pt species. Varying degrees of Pt⋅⋅⋅Zn and Zn interaction with the bridging Me group are achieved, showcasing snapshots of a hypothetical process of retrotransmetalation from Pt to Zn. In contrast, only weak interactions were observed for Ca with a Pt-bound Me group. Activation of H₂, B−H and Si−H bonds leads to the formation of hydride-bridged Pt−H−Zn complexes, which is not observed in the absence of Zn, pointing out the importance of metal-metal cooperation. Reactivity of PtMe₂/M²⁺ with terminal acetylene, water and methanol is also studied, leading to facile protonation of one of the Me groups at the Pt center only when Zn is present. This study sheds light on various ways in which the presence of a 2+ metal cation significantly affects the reactivity of a common organoplatinum complex.     

The Na⋅⋅⋅B Bond in NaBH3−: A Different Type of Bond

A newly introduced Na−B bond in NaBH₃⁻ has been a challenge for the chemical bonding community. Here, a series of MBH₃⁻ (M=Li, Na, K) species and NaB(CN)₃⁻ are studied within the context of quantum chemical topology approaches. The analyses suggest that M–B interaction cannot be classified as an ordinary covalent, dative, or even simple ionic interaction. The interactions are controlled by coulombic forces between the metals and the substituents on boron, for example, H or CN, more than the direct M–B interaction. On the other hand, while the characteristics of the (3, −1) critical points of the bonds are comparable to weak hydrogen bonds, not covalent bonds, the metal and boron share a substantial sum of electrons. To the best of the author's knowledge, the characteristics of these bonds are unprecedented among known molecules. Considering all paradoxical properties of these bonds, they are herein described as ionic-enforced covalent bonds.     

Quantitative Assessment of B−B−B,B−Hb−B,and B−Ht Bonds: From BH3 to B12H122−

We report the thermodynamic stabilities and the intrinsic strengths of three-center-two-electron B−B−B and B−H_b−B bonds ( : bridging hydrogen), and two-center-two-electron B−H_t bonds ( : terminal hydrogen) which can be served as a new, effective tool to determine the decisive role of the intermediates of hydrogenation/dehydrogenation reactions of borohydride. The calculated heats of formation were obtained with the G4 composite method and the intrinsic strengths of B−B−B, B−H_b−B, and B−H_t bonds were derived from local stretching force constants obtained at the B3LYP-D2/cc-pVTZ level of theory for 21 boron-hydrogen compounds, including 19 intermediates. The Quantum Theory of Atoms in Molecules (QTAIM) was used to deepen the inside into the nature of B−B−B, B−H_b−B, and B−H_t bonds. We found that all of the experimentally identified intermediates hindering the reversibility of the decomposition reactions are thermodynamically stable and possess strong B−B−B, B−H_b−B, and B−H_t bonds. This proves that thermodynamic data and intrinsic B−B−B, B−H_b−B, and B−H_t bond strengths form a new, effective tool to characterize new (potential) intermediates and to predict their role for the reversibility of the hydrogenation/dehydrogenation reactions.     

Benzyl Borane NHC Adducts: Beyond B−C Bond Scission

Benzyl-substituted boronates and borates are widely employed as mild sources in radical or anionic transfer reactions of benzyl entities. In this process the B−C bond to the benzyl moiety is essentially ruptured. In contrast, reactions with retention of the B−C bond are poorly investigated although several other reactive sites in benzyl–boron systems are clearly inherent. In this respect, the novel reactivity of the representative borane adduct IiPr−BH₂Bn [IiPr=:C{N(iPr)CH}₂, Bn=CH₂C₆H₅] is demonstrated. Dihalogenation of the BH₂ entity is observed with BCl₃ and BBr₃, whereas BI₃ either affords IiPr−BHI₂ or proceeds with borylation of the aromatic phenyl ring to give a hydride-bridged bisborylated species. The photochemical mono- and dihalogenation of the benzylic CH₂ group was demonstrated with elemental bromine Br₂. The brominated product IiPr−BBr₂−CHBr−C₆H₅ was borylated at the benzylic carbon atom in an umpolung event with BI₃ to afford the zwitterion IiPr−BI−CH(BI₃)−C₆H₅.     

Carbon Monoxide Activation by a Molecular Aluminium Imide: C−O Bond Cleavage and C−C Bond Formation

Anionic molecular imide complexes of aluminium are accessible via a rational synthetic approach involving the reactions of organo azides with a potassium aluminyl reagent. In the case of K₂[( NON )Al(NDipp)]₂ ( NON =4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene; Dipp=2,6-diisopropylphenyl) structural characterization by X-ray crystallography reveals a short Al−N distance, which is thought primarily to be due to the low coordinate nature of the nitrogen centre. The Al−N unit is highly polar, and capable of the activation of relatively inert chemical bonds, such as those found in dihydrogen and carbon monoxide. In the case of CO, uptake of two molecules of the substrate leads to C−C coupling and C≡O bond cleavage. Thermodynamically, this is driven, at least in part, by Al−O bond formation. Mechanistically, a combination of quantum chemical and experimental observations suggests that the reaction proceeds via exchange of the NR and O substituents through intermediates featuring an aluminium-bound isocyanate fragment.     

An Isolable Gallium-Substituted Nitrilimine and its Reactivity with B−H,Si−H and B−B Bonds

Reaction of the Ga(I) compound NacNacGa ( 9 ) with the diazo compound N₂CHSiMe₃ affords the nitrilimine compound NacNacGa(N-NCSiMe₃)(CH₂SiMe₃) ( 10 ). Carrying out this reaction in the presence of pyridine does not lead to C−H activation on the transient alkylidene NacNacGa=CHSiMe₃ but generates a metallated diazo species NacNacGa(NHN=CHSiMe₃)(CN₂SiMe₃) ( 13 ) that further rearranges into the isonitrile compound NacNacGa(NHN=CHSiMe₃)(N(NC)SiMe₃) ( 15 ). Reactions of 10 with the silane H₃SiPh and the borane HBcat furnished products of 1,3 addition to the nitrilimine moiety NacNacGa{N(ER_n)NCSiMe₃}(CH₂SiMe₃), whereas reaction with the diborane B₂cat₂ gave the product of formal nitrene insertion into the B−B bond. DFT calculations suggest that the interaction of 9 with N₂CHSiMe₃ proceeds through intermediate formation of an alkylidene compound that undergoes CH activation with a second molecule of N₂CHSiMe₃. Insertion into the B−B bond likely proceeds through an initial 1,3-addition of the diborane, followed by boryl migration to the former nitrene center.     

B−B Bond Nucleophilicity in a Tetraaryl μ-Hydridodiborane(4) Anion

The tetraaryl μ-hydridodiborane(4) anion [ 2 H]⁻ possesses nucleophilic B−B and B−H bonds. Treatment of K[ 2 H] with the electrophilic 9-H-9-borafluorene (HBFlu) furnishes the B₃ cluster K[ 3 ], with a triangular boron core linked through two BHB two-electron, three-center bonds and one electron-precise B−B bond, reminiscent of the prominent [B₃H₈]⁻ anion. Upon heating or prolonged stirring at room temperature, K[ 3 ] rearranges to a slightly more stable isomer K[ 3 a ]. The reaction of M[ 2 H] (M⁺=Li⁺, K⁺) with MeI or Me₃SiCl leads to equimolar amounts of 9-R-9-borafluorene and HBFlu (R=Me or Me₃Si). Thus, [ 2 H]⁻ behaves as a masked [:BFlu]⁻ nucleophile. The HBFlu by-product was used in situ to establish a tandem substitution-hydroboration reaction: a 1:1 mixture of M[ 2 H] and allyl bromide gave the 1,3-propylene-linked ditopic 9-borafluorene 5 as sole product. M[ 2 H] also participates in unprecedented [4+1] cycloadditions with dienes to furnish dialkyl diaryl spiroborates, M[R₂BFlu].     

Isolable Anionic,Neutral and Cationic Radicals from Reactions of N,N′-Dimesityldiamidocarbene and Lewis Acids

B(C₆F₅)₃ undergoes nucleophilic attack by N,N′-dimesityldiamidocarbene (DAC) with fluoride transfer to the boron center, resulting in a new zwitterion ( 1 ). This B−F fluoride can be replaced or abstracted to give the corresponding hydride ( 2 ) or triflate ( 3 ) derivatives or the corresponding cation ( 4 ). These species are reduced with KC₈ or Cp₂Co to give isolable anionic and neutral radicals ( 5 – 8 ). Similarly, the [Ph₃C] cation undergoes nucleophilic attack by DAC resulting in the spontaneous formation of the radical cation ( 9 ).     

Coordination-Induced N−H Bond Splitting of Ammonia and Primary Amine of CuI−MOFs

We report a porous three-dimensional anionic tetrazolium based Cu^I−MOF 1 , which is capable of cleaving the N−H bond of ammonia and primary amine, as well as the O−H bond of H₂O along with spontaneous H₂ evolution. In the gas-solid phase reaction of 1 with ammonia and water vapor, Cu^I−MOF 1 was gradually oxidized to NH₂−Cu^II−MOF and OH−Cu^II−MOF, through single-crystal-to-single-crystal (SCSC) structural transformations, which was confirmed by XPS, PXRD and X-ray single-crystal diffraction. Density functional theory (DFT) demonstrated that Cu^I−MOF could lower N−H bond dissociation free energy of ammonia through coordination-induced bond weakening and promote H₂ evolution by the reduction potential of 1 . To our knowledge, this is the first example of MOFs that activate ammonia and amine in gas-solid manner.     

RASS-Enabled S/P−C and S−N Bond Formation for DEL Synthesis

DNA encoded libraries (DEL) have shown promise as a valuable technology for democratizing the hit discovery process. Although DEL provides relatively inexpensive access to libraries of unprecedented size, their production has been hampered by the idiosyncratic needs of the encoding DNA tag relegating DEL compatible chemistry to dilute aqueous environments. Recently reversible adsorption to solid support (RASS) has been demonstrated as a promising method to expand DEL reactivity using standard organic synthesis protocols. Here we demonstrate a suite of on-DNA chemistries to incorporate medicinally relevant and C−S, C−P and N−S linkages into DELs, which are underrepresented in the canonical methods.     

Electrosynthetic C−O Bond Activation in Alcohols and Alcohol Derivatives

Alcohols and their derivatives are ubiquitous and versatile motifs in organic synthesis. Deoxygenative transformations of these compounds are often challenging due to the thermodynamic penalty associated with the cleavage of the C−O bond. However, electrochemically driven redox events have been shown to facilitate the C−O bond cleavage in alcohols and their derivatives either through direct electron transfer or through the use of electron transfer mediators and electroactive catalysts. Herein, a comprehensive overview of preparative electrochemically mediated protocols for C−O bond activation and functionalization is detailed, including direct and indirect electrosynthetic methods, as well as photoelectrochemical strategies.     

P−P Condensation and P−N/P−P Bond Metathesis: Facile Synthesis of Cationic Tri- and Tetraphosphanes

[L_C^RP((PhP)₂C₂H₄)][OTf] ( 4 a,b [OTf]) and [L_C^iPrP(PPh₂)₂][OTf] ( 5 b [OTf]) were prepared from the reaction of imidazoliumyl-substituted dipyrazolylphosphane triflate salts [L_C^RP(pyr)₂][OTf] ( 3 a,b [OTf]; a : R=Me, b =iPr; L_C^R=1,3-dialkyl-4,5-dimethylimidazol-2-yl; pyr=3,5-dimethylpyrazol-1-yl) with the secondary phosphanes PhP(H)C₂H₄P(H)Ph) and Ph₂PH. A stepwise double P−N/P−P bond metathesis to catena-tetraphosphane-2,3-diium triflate salt [(Ph₂P)₂(L_C^MeP)₂][OTf]₂ ( 7 a [OTf]₂) is observed when reacting 3 a [OTf] with diphosphane P₂Ph₄. The coordination ability of 5 b [OTf] was probed with selected coinage metal salts [Cu(CH₃CN)₄]OTf, AgOTf and AuCl(tht) (tht=tetrahydrothiophene). For AuCl(tht), the helical complex [{(Ph₂PPL_C^iPr)Au}₄][OTf]₄ ( 9 [OTf]₄) was unexpectedly formed as a result of a chloride-induced P−P bond cleavage. The weakly coordinating triflate anion enables the formation of the expected copper(I) and silver(I) complexes [( 5 b )M(CH₃CN)₃][OTf]₂ (M=Cu, Ag) ( 10 [OTf]₂, 11 [OTf]₂).     

New Organotin Compounds Including a Tin–Zinc Bond

Abstract

The reaction of dialkyltinhydrides with dialkylzinc results in a tin metal bond formation. Crystal structures of 1,1,1,2,3,3,3-hepta-phenyl-2-(ethylzincio)tristannane *TMEDA 1, 1,1,2,2,3,3,4,4-octaphenyl-1,4-(ethylzincio)tetrastannane*2TMEDA 2, and triphenyl(phenylzincio)stannane*TMEDA 3 could be obtained.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.

GRAPHICAL ABSTRACT     

High-Performance Near-Infrared Chlorinated Rylenecarboximide Fluorophores via Consecutive C−N and C−C Bond Formation

A new class of near-infrared (NIR) fluorophores, PAI , is obtained by consecutive C−N/C−C bond formation between diphenylamines and 9,10-dibromoperylenecarboximide. Owing to the rigid structure, extended π-conjugation and pronounced push-pull substitution, these fluorophores show emission maxima up to 804 nm and large Stokes shifts. The extraordinarily high fluorescence quantum yields from 47 % to 70 % are attributed to chloro substitution in the bay positions of the perylene core. These characteristics, together with high photostability, qualify them as useful NIR emitters for applications as biomarkers and security inks.     

Enhanced Stability and Optical Absorption in the Perovskite-Based Compounds MA1−xCsxPbI3−yBry

Organometal halide perovskites have been outstanding from enormous amount of functional materials thanks to their highly cost-effective processability and prominent light harvesting capacity. Unfortunately, poor long-term stability seriously hinders their further development. The recent experimental observations suggest that Cesium is a promising candidate to enhance the stability of MAPbI₃. To explore the inherent mechanism, a first-principles investigation based on density functional theory, including hybrid functional, has been performed to analyze the electronic and optical properties of perovskite series MA_0.75Cs_0.25PbI_3−yBr_y. The results indicate that perovskite compound MA_0.75Cs_0.25PbI₂Br is significantly superior to the other doped series in terms of optical absorption within the visible-light range. In the meanwhile, both Bader charge analysis and charge density distribution show that the compound of MA_0.75Cs_0.25PbI₂Br is the most stable among all the doped perovskite series. Moreover, it is clearly manifested that the impact of cesium is mainly embodied in the enhancement of the stability rather than in the improvement of optical absorption. Our study sheds a new light on screening new-type light harvesting materials, and provides theoretical insight into the rationale design of highly efficient and stable photovoltaic devices based on these functional materials.     

Deprotonation of a Seemingly Hydridic Diborane(6) To Build a B−B Bond

Deprotonation of the doubly arylene-bridged diborane(6) derivative 1 H₂ with (Me₃Si)₃CLi or (Me₃Si)₂NK gives the B−B σ-bonded species M[ 1 H] in essentially quantitative yields (THF, room temperature; M=Li, K, arylene=4,4′-di-tert-butyl-2,2′-biphenylylene). With nBuLi as the base, the yield of Li[ 1 H] drops to 20 % and the 1,1-bis(9-borafluorenyl)butane Li[ 2 H] is formed as a side product (30 %). In addition to the 1,1-butanediyl fragment, the two boron atoms of Li[ 2 H] are linked by a μ-H bridge. In the closely related molecule Li[ 3 H], the corresponding μ-H atom can be abstracted with (Me₃Si)₃CLi to afford the B−B-bonded conjugated base Li₂[ 3 ] (THF, 150 °C; 15 %). Li[ 1 H] and Li[ 2 H] were characterized by NMR spectroscopy and X-ray crystallography.     

Multicomponent Aromatic and Benzylic Mannich Reactions through C−H Bond Activation

Multicomponent Mannich reactions through C−H bond activation are described. These transformations allowed for the straightforward generation of densely substituted benzylic and homo-benzylic amines in good yields. The reaction involves a reaction between two transient species: an organometallic species, generated by transition-metal-catalyzed sp² or sp³ C−H bond activation and an in situ generated imine. The use of an acetal as an aldehyde surrogate was found essential for the reaction to proceed. The process could be successfully applied to Rh^III-catalyzed sp² C−H bond functionalization and extended to Cu^II-catalyzed sp³ C−H bond functionalization.