Synthesis and Reactivity of o-Phosphane Oxide Substituted Aryl(hydro)borates and Aryl(hydro)boranes

Bis-Ortho-Donor-Modification of Boracyclic π-Electron Systems beyond Steric Protection to Produce Thermally Activated Delayed Fluorescence Materials

Polycyclic π-conjugated compounds that contain tricoordinate boron atoms at their periphery represent an attractive class of materials with electron-accepting character. Their molecular design generally requires the introduction of a bulky aryl group onto the boron atom, where it provides predominantly kinetic stabilization. The addition of extra functionality to the aryl group on the boron atom can be expected to further expand the potential utility of this class of materials. Herein, we report the synthesis of a series of boracyclic π-conjugated molecules with firm ortho B⋅⋅⋅N nonbonding interactions by introducing N-containing electron-donors at the ortho-positions of the aryl group on the boron atom. X-ray crystallographic analysis revealed that the combination of a planar boracyclic π-skeleton with only sp2 carbons and a strong electron-donating phenothiazine moiety results in a particularly short B⋅⋅⋅N distance. Theoretical study provided insights into the inherent nature of the B⋅⋅⋅N interaction. Owing to their donor-acceptor (D-A) structures, these molecules exhibit substantially red-shifted fluorescence in solution, albeit that the fluorescence quantum yields (ΦF) are low. In contrast, when incorporated into films, these compounds exhibit thermally activated delayed fluorescence (TADF) with improved ΦF values. Organic light-emitting diodes (OLEDs) fabricated using the ortho-donor-substituted derivatives exhibit orange-red electroluminescence.

Accessing unusual heterocycles: ring expansion of benzoborirenes by formal cycloaddition reactions

Benzoborirenes are a very rare class of strained boron heterobicyclic systems. In this study a kinetically stabilized benzoborirene 1 is shown to react with multiple bonds of trimethylphosphine oxide, acetaldehyde, and tert-butyl isonitrile. The (2 + 2) cycloaddition product with trimethylphosphine oxide, benzo[c][1,2,5]oxaphosphaborole, has a long apical PO bond (194.0 pm) that must be considered on the border line between ionic and covalent according to the natural bond orbital, quantum theory of atoms in molecules, and compliance matrix approaches to the description of chemical bonding. The coordination compound between the benzoborirene and phosphine oxide was observed by NMR spectroscopy at 213 K. The Lewis acidity of 1 is similar to that of B(OCH2CF3)3 and B(C6F5)3 based on the 31P{1H} NMR chemical shift of the Lewis acid base complexes with trimethylphosphine oxide at 213 K. Benzoboriene 1 does not react with acetone, but forms a (2 + 2) cycloaddition product, an oxaborole, with acetaldehyde. In contrast, it undergoes a double (2 + 1) reaction with tert-butyl isonitrile to yield a boro-indane derivative under mild conditions. The observed reactivity of 1 is in agreement with computational analyses of the respective potential energy surfaces.

Photodissociative Modules that Control Dual-Emission Properties in Donor-π-Acceptor Organoborane Fluorophores

Donor-π-acceptor fluorophores that consist of an electron-donating amino group and an electron-accepting triarylborane moiety generally exhibit substantial solvatochromism in their fluorescence while retaining high fluorescence quantum yields even in polar media. Herein, we report a new family of this compound class, which bears ortho-P(=X)R2 -substituted phenyl groups (X=O or S) as a photodissociative module. The P=X moiety that intramolecularly coordinates to the boron atom undergoes dissociation in the excited state, giving rise to dual emission from the corresponding tetra- and tricoordinate boron species. The susceptibility of the systems to photodissociation depends on the coordination ability of the P=O and P=S moieties, whereby the latter facilitates dissociation. The intensity ratios of the dual emission bands are sensitive to environmental parameters, including temperature, solution polarity, and the viscosity of the medium. Moreover, precise tuning of the P(=X)R2 group and the electron-donating amino moiety led to single-molecule white emission in solution.

Metal-Free Phosphorus-Directed Borylation of C(sp2 )-H Bonds

Spectacular progress has recently been achieved in transition metal-catalyzed C-H borylation of phosphines as well as directed electrophilic C-H borylation. As shown here, P-directed electrophilic borylation provides a new, straightforward, and efficient access to phosphine-boranes. It operates under metal-free conditions and leverages simple, readily available substrates. It is applicable to a broad range of backbones (naphthyl, biphenyl, N-phenylpyrrole, binaphthyl, benzyl, naphthylmethyl) and gives facile access to various substitution patterns at boron (by varying the boron electrophile or post-derivatizing the borane moiety). NMR monitoring supports the involvement of P-stabilized borenium cations as key intermediates. DFT calculations reveal the existence and stabilizing effect of π-arene/boron interactions in the (biphenyl)(i-Pr)₂ P→BBr₂ ⁺ species.

Proton to hydride umpolung at a phosphonium center via electron relay: a new strategy for main-group based water reduction

Generation of dihydrogen from water splitting, also known as water reduction, is a key process to access a sustainable hydrogen economy for energy production and usage. The key step is the selective reduction of a protic hydrogen to an accessible and reactive hydride, which has proven difficult at a p-block element. Although frustrated Lewis pair (FLP) chemistry is well known for water activation by heterolytic H-OH bond cleavage, to the best of our knowledge, there has been only one case showing water reduction by metal-free FLP systems to date, in which silylene (SiII) was used as the Lewis base. This work reports the molecular design and synthesis of an ortho-phenylene linked bisborane-functionalized phosphine, which reacts with water stoichiometrically to generate H2 and phosphine oxide quantitatively under ambient conditions. Computational investigations revealed an unprecedented multi-centered electron relay mechanism offered by the molecular framework, shuttling a pair of electrons from hydroxide (OH-) in water to the separated proton through a borane-phosphonium-borane path. This simple molecular design and its water reduction mechanism opens new avenues for this main-group chemistry in their growing roles in chemical transformations.

Heteroelement Analogues of Benzoxaborole and Related Ring Expanded Systems

The review covers the chemistry of organoboron heterocycles structurally related to benzoxaboroles where one of the carbon atoms in a boracycle or a fused benzene ring is replaced by a heteroelement such as boron, silicon, tin, nitrogen, phosphorus, or iodine. Related ring expanded systems including those based on naphthalene and biphenyl cores are also described. The information on synthetic methodology as well as the basic structural and physicochemical characteristics of these emerging heterocycles is complemented by a presentation of their potential applications in organic synthesis and medicinal chemistry, the latter aspect being mostly focused on the promising antimicrobial activity of selected compounds.

Twenty-five years of bis-pentafluorophenyl borane: a versatile reagent for catalyst and materials synthesis

Highlights of the extensive chemistry and applications of bis-pentafluorophenyl borane (“Piers’ borane”) from the 25 years since its first appearance are featured.

One-Step Conversion of Potassium Organotrifluoroborates to Metal Organoborohydrides

This letter describes the one-step conversion of heteroatom-substituted potassium organotrifluoroborates (KRBF₃) to metal monoorganoborohydrides (MRBH₃) using alkali metal aluminum hydrides. The method tolerates a variety of functional groups, expanding MRBH₃ diversity. Hydride removal with Me₃SiCl in the presence of dimethylaminopyridine (DMAP) affords the organoborane·DMAP (RBH₂·DMAP) adducts.

A Highly Reactive Geminal P/B Frustrated Lewis Pair: Expanding the Scope to C-X (X=Cl, Br) Bond Activation

The geminal frustrated Lewis pair tBu2 PCH2 B(Fxyl)2 (1; Fxyl=3,5-(CF3 )2 C6 H3 ) is accessible in 65 % yield from tBu2 PCH2 Li and (Fxyl)2 BF. According to NMR spectroscopy and X-ray crystallography, 1 is monomeric both in solution and in the solid state. The intramolecular P⋅⋅⋅B distance of 2.900(5) Å and the full planarity of the borane site exclude any significant P/B interaction. Compound 1 readily activates a broad variety of substrates including H2 , EtMe2 SiH, CO2 /CS2 , Ph2 CO, and H3 CCN. Terminal alkynes react with heterolysis of the C-H bond. Haloboranes give cyclic adducts with strong P-BX3 and weak R3 B-X bonds. Unprecedented transformations leading to zwitterionic XP/BCX3 adducts occur on treatment of 1 with CCl4 or CBr4 in Et2 O. In less polar solvents (C6 H6 , n-pentane), XP/BCX3 adduct formation is accompanied by the generation of significant amounts of XP/BX adducts. FLP 1 catalyzes the hydrogenation of PhCH=NtBu and the hydrosilylation of Ph2 CO with EtMe2 SiH.

Capturing HBCy2: Using N,O-Chelated Complexes of Rhodium(I) and Iridium(I) for Chemoselective Hydroboration

1,3-N,O-chelated complexes of Rh(I) and Ir(I) cooperatively and reversibly stabilized the B-H bond of HBCy2 to afford six-membered metallaheterocycles (M=Rh (7) or Ir (8)) having a δ-[M]⋅⋅⋅H-B agostic interaction. Treatment of these Shimoi-type borane adducts 7 or 8 with both an aldehyde and an alkene resulted in chemoselective aldehyde hydroboration and reformation of the 1,3-N,O-chelated starting material. The observed chemoselectivity is inverted from that of free HBCy2 , which is selective for alkene hydroboration.

Synthesis of P-stereogenic diarylphosphinic amides by directed lithiation: transformation into tertiary phosphine oxides via methanolysis, aryne chemistry and complexation behaviour toward zinc(ii)

A general synthesis of P-stereogenic compoundsviaDoLi–electrophilic quenching of phosphinic amides and subsequent derivatizations is reported.

The phosphinoboration reaction

The synthesis of phosphinoboronate esters containing a single P-B bond is reported, together with preliminary reactivity studies towards a range of organic substrates. These compounds add readily to aldehydes, ketones, aldimines, and α,β-unsaturated enones to give primarily the corresponding 1,2-addition products containing a new P-C bond. The first examples of transition-metal-catalyzed phosphinoborations of C-C multiple bonds in which P-C and B-C bonds are formed in a single step are also disclosed; allenes react by a highly regioselective 1,2-addition whereas terminal alkynes undergo a formal 1,1-addition.