Azaborabutadienes: Synthesis by Metal‐Free Carboboration of Nitriles and Utility as Building Blocks for B,N‐Heterocycles

Germaborene reactivity study - addition of carbon nucleophiles, cycloaddition reactions, coordination chemistry

MeNHC substituted germaborenium cation 2 was synthesized directly in reaction of bromo-substituted germaborene 1b with MeNHC. The adamantyl isonitrile substituted germaborenium cation 4 was obtained stepwise: substitution of the chloride atom against adamantyl isonitrile at the B-Cl unit in 1a, simultaneous migration of the chloride to the germanium atom followed by chloride abstraction using Na[BArF 4] gives the germaborenium cation 4. Substitution of the bromide atom in 1b against carbon monoxide followed by bromide abstraction using Ag[Al(OtBuF)4] leads to compound 6 exhibiting a B[double bond, length as m-dash]C double bond substituted at the boron atom by a germylium cation. Treating the germaborene [Ge[double bond, length as m-dash]B-Ph] (1c) with selenium, a cycloaddition product 7 was characterised featuring a GeBSe heterocycle. Carbon dioxide reacts with 1b to give a four membered ring molecule 8 as the product of a B-C and Ge-O bond formation. In reaction of 1b with dimethylbutadiene, a product 9 of a [2 + 4] cycloaddition was isolated. Transition metal fragments [Fe(CO)4 (10), CuBr (11), AuCl (12)] show coordination at the germaborene double bond. Molecular structures of the germaborene coordination compounds 10-12 are presented and the ligand properties are discussed. After treating the germaborene [Ge[double bond, length as m-dash]B-Br] (1b) with [Cp*Al]4, insertion of a Cp*Al moiety into the B-Br bond was found (13).

Derivatization of an Alkylideneborane with B═C Bond Cleavage

We present the synthesis, structural characterization, and reactivity of alkylideneborane 2, supported by π-donating N-heterocyclic imino and σ-donating N-heterocyclic carbene (NHC) ligands. The incorporation of these ligands effectively weakens the B═C bond strength, leading to enhanced reactivity. Consequently, selective cleavage of the B═C bond can be achieved using pyridine-N-oxide, sulfur, and selenium, resulting in the formation of 1,3-dioxa-2,4-diboretane 3, thioxoborane 4, and selenoborane 5, respectively. Furthermore, intriguing B═C bond insertions with CO2 and CS2 are observed, affording zwitterionic borenium/fluorenide 6 and dithiaboretane 7. The former species 6 is readily converted to transient oxoborane and imidazolium enolate, showcasing the bora-Wittig reaction of alkylideneborane. This investigation highlights the potential of alkylideneborane as a versatile building block for synthesizing novel organoboron compounds through unconventional transformations.

Electron-Precise Dicationic Tetraboranes: Syntheses, Structures and Rearrangement to an Alkylidene Borate-Borenium Zwitterion and a 1,3-Azaborinine

Carbene-stabilized symmetrical and unsymmetrical dicationic tetraboranes, featuring an electron-precise tetraborane chain, were synthesized and fully characterized. Reactions of these tetraboranes with reductants/bases give rise to different outcomes according to the conditions employed, including: 1) reduction and rearrangement of the tetraborane chain to give a zwitterionic alkylidene borate-borenium species; 2) cleavage of the tetraborane chain to afford a 1,3-azaborinine; and 3) reduction of the supporting ligands to provide a diamino dipotassium salt. The zwitterionic alkylidene borate-borenium species can be viewed as an analogue of the base-stabilized diborenes. NMR spectroscopy and DFT calculations reveal a highly polarized B-B bond in the zwitterionic alkylidene borate-borenium, in which the formal oxidation states of the boron atoms can be considered as -1 and +2. These results suggest the considerable potential of tetraboranes as synthons for low-valent boron species.

Four-Electron Reduction of O2 Using Distibines in the Presence of ortho-Quinones

This study, which aims to identify atypical platforms for the reduction of dioxygen, describes the reaction of O2 with two distibines, namely, 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethylxanthene and 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethyldihydroacridine, in the presence of an ortho-quinone such as phenanthraquinone. The reaction proceeds by oxidation of the two antimony atoms to the + V state in concert with reductive cleavage of the O2 molecule. As confirmed by 18O labeling experiments, the two resulting oxo units combine with the ortho-quinone to form an α,α,β,β-tetraolate ligand that bridges the two antimony(V) centers. This process, which has been studied both experimentally and computationally, involves the formation of asymmetric, mixed-valent derivatives featuring a stibine as well as a catecholatostiborane formed by oxidative addition of the quinone to only one of the antimony centers. Under aerobic conditions, the catecholatostiborane moiety reacts with O2 to form a semiquinone/peroxoantimony intermediate, as supported by NMR spectroscopy in the case of the dimethyldihydroacridine derivative. These intermediates swiftly evolve into the symmetrical bis(antimony(V)) α,α,β,β-tetraolate complexes via low barrier processes. Finally, the controlled protonolysis and reduction of the bis(antimony(V)) α,α,β,β-tetraolate complex based on the 9,9-dimethylxanthene platform have been investigated and shown to regenerate the starting distibine and the ortho-quinone. More importantly, these last reactions also produce two equivalents of water as the product of O2 reduction.

Cooperative Bond Activation and Catalytic CO2 Functionalization with a Geometrically Constrained Bis(silylene)-Stabilized Borylene

Metal-ligand cooperativity has emerged as an important strategy to tune the reactivity of transition-metal complexes for the catalysis and activation of small molecules. Studies of main-group compounds, however, are scarce. Here, we report the synthesis, structural characterization, and reactivity of a geometrically constrained bis(silylene)-stabilized borylene. The one-pot reaction of [(SiNSi)Li(OEt₂)] (SiNSi = 4,5-bis(silylene)-2,7,9,9-tetramethyl-9H-acridin-10-ide) with 1 equiv of [BBr₃(SMe₂)] in toluene at room temperature followed by reduction with 2 equiv of potassium graphite (KC₈) leads to borylene [(SiNSi)B] (1), isolated as blue crystals in 45% yield. X-ray crystallography shows that borylene (1) has a tricoordinate boron center with a distorted T-shaped geometry. Computational studies reveal that the HOMO of 1 represents the lone pair orbital on the boron center and is delocalized over the Si-B-Si unit, while the geometric perturbation significantly increases its energy. Borylene (1) shows single electron transfer reactivity toward tris(pentafluorophenyl)borane (B(C₆F₅)₃), forming a frustrated radical pair [(SiNSi)B]^•+[B(C₆F₅)₃]^•-, which can be trapped by its reaction with PhSSPh, affording an ion pair [(SiNSi)BSPh][PhSB(C₆F₅)₃] (3). Remarkably, the cooperation between borylene and silylene allows the facile cleavage of the N-H bond of aniline, the P-P bond in white phosphorus, and the C═O bond in ketones and carbon dioxide, thus representing a new type of main-group element-ligand cooperativity for the activation of small molecules. In addition, 1 is a strikingly effective catalyst for carbon dioxide reduction. Computational studies reveal that the cooperation between borylene and silylene plays a key role in the catalytic chemical bond activation process.

Boraalkenes Made by a Hydroboration Route: Cycloaddition and B=C Bond Cleavage Reactions

Hydroboration of styrene or vinylcyclohexane with the IMes(C₆ F₅ )BH⁺ cation followed by deprotonation provided a convenient synthetic entry to the [B]=CHCH₂ R boraalkenes 9 a and 9 b. The in situ generated IMes(SCN)BH⁺ system reacted similarly with 1,1-diphenylethene followed by deprotonation to give the isothiocyanato substituted boraalkene 9 c. The boraalkenes underwent [2+2] cycloaddition reactions with a small series of heterocumulenes to give the respective four-membered heterocycles. The [B]=CHCH₂ R+CO₂ cycloadducts 13 a and 13 b added the borane HB(C₆ F₅ )₂ with cleavage of the central B-C σ-bond. CS₂ underwent an unusual reaction with the boraalkenes, namely insertion into the B=C bond with formation of the borylated dithioketene acetal under complete rupture of the strong B=C double bond. The intermediate dithiobora-β-lactone type intermediate was isolated in the case of the isothiocyanato-boraalkene reaction and characterized by X-ray diffraction.

A deprotonation pathway to reactive [B]CH2 boraalkenes

The BH compounds IMes(Ar^F)BH(NTf₂) (Ar^F: C₆F₅ or FpXyl) were converted to the IMes(Ar^F)BCH₂ boraalkenes in a three step reaction sequence of B-methylation with methyllithium, hydride abstraction and deprotonation. The BCH₂ boraalkenes reacted with elemental sulfur to give a thiaborirane product. They underwent [2+2] cycloaddition reactions with carbon dioxide or sulfur dioxide to give four-membered boron containing heterocycles. The boraalkenes added strongly Lewis acidic boranes at their CH₂ carbon atoms. The corresponding HB(C₆F₅)₂/boraalkene adduct reduced carbon monoxide to a -OCH(C₆F₅)- moiety inside a five-membered heterocycle at the B-CH₂-B template. The boraalkenes reacted with the [(Me₂S)AuCl] reagent to form the corresponding (boraalkene)AuCl complexes.

An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry

A one-pot synthesis of a CAAC-stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two-electron reduction steps from an adduct of CAAC and B2 Br4 (SMe2 )2 . Theoretical studies revealed that this diborene has a considerably smaller HOMO-LUMO gap than those of reported NHC- and phosphine-supported diborenes. Complexation of the diborene with [AuCl(PCy3 )] afforded two diborene-AuI π complexes, while reaction with DurBH2 , P4 and a terminal acetylene led to the cleavage of B-H, P-P, and C-C π bonds, respectively. Thermal rearrangement of the diborene gave an electron-rich cyclic alkylideneborane, which readily coordinated to AgI via its B=C double bond.

N-Heterocyclic Carbene Stabilized 1-Bora-1,3-butadienes

Deprotonation of [(NHC)(Fmes)B-allyl]⁺ borenium cations (NHC, IMes (a) or IMe₂ (b); Fmes, 2,4,6-(CF₃)₃C₆H₂) provides an easy entry to the NHC-stabilized 1-bora-1,3-butadienes. They feature a planar s-trans-conformation just like 1,3-butadiene. The 1-borabutadiene 7a undergoes hydroboration reactions; the HB(C₆F₅)₂ hydroboration product is trapped with CO or an isonitrile to give the respective cyclic zwitterionic borenium-borate enolate or enamide products. 1-Borabutadiene 7b undergoes 1,4-chalcogenation with elemental sulfur or selenium, and it gives the six-membered heterocyclic 1,4-addition product with the S═O bond of sulfur dioxide. Compound 7b served as a precursor for the formation of a borylated η³-allyl ligand at Ru. 7b formed a Rh complex by reaction with [Rh(ethylene)₂Cl]₂. It subsequently underwent an intramolecular C-H activation reaction to a mixture of η³-methyl-boraallyl Rh complex isomers.

Facile access to halogenated cationic BC-centered organoborons isoelectronic with alkenyl halides

A facile strategy to prepare halogenated cationic BC-centered organoboron analogues of alkenyl halides via the reaction between carbodiphosphoranes (CDPs) and aryl dihaloboranes is reported. Structural comparison of the halogen and alkoxy derivatives, as well as detailed computational studies, corroborates the polarized double dative bonding nature of the central BC double bonds.

Formation and Cycloaddition Reactions of a Reactive Boraalkene Stabilized Internally by N-Heterocyclic Carbene

The synthesis of element-carbon double bonds is of great importance for the development and understanding of reactive π-bonded systems in chemistry. The seven-membered heterocyclic system 4 b is readily made by internal C-H activation at a pendent isopropyl methyl group of the respective [(IPr)C₆ F₅ BH]⁺ borenium ion. Subsequent deprotonation with the IMes carbene gives the neutral cyclic boraalkene system 5 b. The B=C double bond in compound 5 b adds carbon dioxide, CS₂ , sulfur dioxide, phenyl isocyanate, an acetylenic ester or two NO molecules to give the corresponding four-membered ring annulated heterocycles. With sulfur or selenium 5 b gives the respective three-membered ring systems. N₂ O reacts with 5 b to give a mixture of the related oxaborirane 18 and a unique [B]OH containing diazoalkane product 19.

Towards deeper understanding of multifaceted chemistry of magnesium alkylperoxides

Despite considerable progress in the multifaceted chemistry of non-redox-metal alkylperoxides, the knowledge about magnesium alkylperoxides is in its infancy and only started to gain momentum. Harnessing the well-defined dimeric magnesium tert-butylperoxide [(^f5BDI)Mg(μ-η²:η¹-OOtBu)]₂ incorporating a fluorinated β-diketiminate ligand, herein, we demonstrate its transformation at ambient temperature to a spiro-type, tetranuclear magnesium alkylperoxide [(^f5BDI)₂Mg₄(μ-OOtBu)₆]. The latter compound was characterized by single-crystal X-ray diffraction and its molecular structure can formally be considered as a homoleptic magnesium tert-butylperoxide [Mg(µ-OOtBu)₂]₂ terminated by two monomeric magnesium tert-butylperoxides. The formation of the tetranuclear magnesium alkylperoxide not only contradicts the notion of the high instability of magnesium alkylperoxides, but also highlights that there is much to be clarified with respect to the solution behaviour of these species. Finally, we probed the reactivity of the dimeric alkylperoxide in model oxygen transfer reactions like the commonly invoked metathesis reaction with the parent alkylmagnesium and the catalytic epoxidation of trans-chalcone with tert-butylhydroperoxide as an oxidant. The results showed that the investigated system is among the most active known catalysts for the epoxidation of enones.

Effectiveness of the Oxygenation over Classical Protonolysis Reactions: A Case of Alkylzinc Complexes Incorporating an Aminoalcoholate Ligand

Alkylzinc aminoalcoholates have emerged as powerful catalysts in organic synthesis and polymerization processes. Despite extensive research, difficulties in the rational design of these catalytic systems and in-depth understanding of their modes of action have hitherto been encountered. Most of the major obstacles stem largely from the relatively limited knowledge of the structure-activity relationship of zinc catalysts. In fact, the key active species are often generated in situ via the protonolysis of the alkylzinc precursors, which precludes their isolation and detailed characterization. Herein, the effectiveness of the oxygenation over the classical protonolysis in the synthesis of zinc alkylperoxides stabilized by an aminoalcoholate ligand is demonstrated. The controlled oxygenation of a tert-butylzinc complex incorporating a pridinolum (prinol) ligand leads to well-defined a dinuclear adduct of a (prinol)ZnOOtBu moiety with the parent tBuZn(prinol) complex and a novel dimer [tBuOOZn(prinol)]₂ with terminal alkylperoxide groups. The observed reaction outcomes strongly depend on the reaction conditions. Although sparse examples of heteroleptic adducts of the [RZn(L)]_x [ROOZn(L)]_y -type are known, the herein reported homoleptic [ROOZn(L)]_x aggregate is unprecedented. Strikingly, comparative studies involving reactions between tBuZn(prinol) and tert-butylhydroperoxide or ethanol revealed that the respective seemingly simple zinc alkylperoxides, or zinc alkoxides, respectively, are not accessible via the classical alcoholysis. We believe that these game-changing results concerning multifaceted chemistry of organozinc aminoalcoholates should pave the way for more rational development of various Zn-based catalytic systems.

Dioxygen Activation and Pyrrole α-Cleavage with Calix[4]pyrrolato Aluminates: Enzyme Model by Structural Constraint

The present work describes the reaction of triplet dioxygen with the porphyrinogenic calix[4]pyrrolato aluminates to alkylperoxido aluminates in high selectivity. Multiconfigurational quantum chemical computations disclose the mechanism for this spin‐forbidden process. Despite a negligible spin–orbit coupling constant, the intersystem crossing (ISC) is facilitated by singlet and triplet state degeneracy and spin–vibronic coupling. The formed peroxides are stable toward external substrates but undergo an unprecedented oxidative pyrrole α‐cleavage by ligand aromatization/dearomatization‐initiated O−O σ‐bond scission. A detailed comparison of the calix[4]pyrrolato aluminates with dioxygen‐related enzymology provides insights into the ISC of metal‐ or cofactor‐free enzymes. It substantiates the importance of structural constraint and element–ligand cooperativity for the functions of aerobic life.

Boron-based stepwise dioxygen activation with 1,4,2,5-diazadiborinine

Activation of dioxygen (O2) by 1,4,2,5-diazadiborinine 1 is reported. Two boron centers in 1 undergo a formal [4 + 2] cycloaddition with O2 at room temperature affording a bicyclo[2.2.2] molecule 2 featuring a B-O-O-B unit. Treatment of 2 with an additional equivalent of 1 leads to the cleavage of the O-O bond in 2 concomitant with the formation of two B-O bonds to yield 4 involving the extremely rare B4C2N2O2 ten-membered rings. A series of these reactions demonstrate the stepwise scission of the O[double bond, length as m-dash]O π-bond and the O-O σ-bond of O2.

Reaction of B2 (o-tol)4 with CO and Isocyanides: Cleavage of the C≡O Triple Bond and Direct C-H Borylations

The reaction of highly Lewis acidic tetra(o-tolyl)diborane(4) with CO afforded a mixture of boraindane and boroxine by the cleavage of the C≡O triple bond. ¹³ C labeling experiments confirmed that the carbon atom in the boraindane stems from CO. Simultaneously, formation of boroxine 3 could be considered as borylene transfer to capture the oxygen atom from CO. The reaction of diborane(4) with ^t Bu-NC afforded an azaallene, while the reaction with Xyl-NC furnished cyclic compounds by direct C-H borylations.

Transition-Metal-Like Behavior of Monovalent Boron Compounds: Reduction, Migration, and Complete Cleavage of CO at a Boron Center

The borylene-carbonyl moiety in [bis(silylene)B(CO)][WBr(CO)₅ ] shows diverse reactivity. Reduction, migration, and complete cleavage of CO have been observed at the boron center, leading to the formation of new types of borylenes. These reactions not only serve as new methods for the synthesis of various stable borylenes, but also demonstrate that main-group-element compounds can mimic the behavior of transition-metal complexes.

Probing a General Rule towards Thermodynamic Stabilities of Mono BN-doped Lower Polyenes

The BN-doped organic analogues are interesting as aliphatic amineboranes for hydrogen storage, precursors for aromatic borazines and adsorbent cage azaboranes. However, BN-doped aliphatic polyenes remained undeveloped. Herein, we perform theoretical calculations on two mono BN-doped aliphatic lower polyenes, 1,3-butadiene and 1,3,5-hexatriene. A general rule is proposed, i.e., isomers with terminal nitrogen and directly BN-connected, N-B(R), in particular, are of significant thermodynamic stability as compared with their inverse analogues (where boron is at the terminal position). The N-B(R) type isomers are found to be the most stable ones in both polyenes. Isomers with terminal B and N are of intermediate stability. Highly destabilized isomers are those with one terminal methylene group and one terminal heteroatom in the butadiene series, and two terminal methylene groups in the hexatriene series. Rules established here may lead researchers to synthesize isomers with particular thermodynamic stability.