Aromatic 1,4,2,3-Diazadiborole Featuring an Unsymmetrical B=B Entity: A Versatile Synthon for Unusual Boron Heterocycles.

The replacement of a CC unit with an isoelectronic BN unit in aromatic systems can give rise to molecules and materials with fascinating properties. We report here the synthesis, characterization, and reactivity of a 1,4,2,3-diazadiborole species, 2, featuring an unprecedented 6π-aromatic BN-heterocyclic moiety that is isoelectronic to cyclopentadienide (Cp-). Bearing an unsymmetrical B=B entity, 2 exhibits reactivity toward oxidants, protic reagents, electrophiles, and unsaturated substrates. This reactivity facilitates the synthesis of a variety of novel mono- and bicyclic organoboron derivatives through mechanisms including ring retention, cleavage/recombination, annulation, and expansion. These findings reveal innovative synthetic routes to BN-embedded aromatic compounds via desymmetrization, affording unique building blocks for synthetic chemistry.

Stable Ketenyl Anions via Ligand Exchange at an Anionic Carbon as Powerful Synthons.

Under an atmosphere of carbon monoxide (CO), a (phosphino)diazomethyl anion salt [[P]-CN2 ][K(18-C-6)(THF)] (1) ([P]=[(CH2 )(NDipp)]2 P; 18-C-6=18-crown-6; Dipp=2,6-diisopropylphenyl) undergoes a facile N2 /CO exchange reaction giving the (phosphino)ketenyl anion salt [[P]-CCO][K(18-C-6)] (2). Oxidation of 2 with elemental Se affords the (selenophosphoryl)ketenyl anion salt [P](Se)-CCO][K(18-C-6)] (3). These ketenyl anions feature a strongly bent geometry at the P-bound carbon and this carbon atom is highly nucleophilic. The electronic structure of the ketenyl anion [[P]-CCO]- of 2 is examined by theoretical studies. Reactivity investigations demonstrate 2 as a versatile synthon for derivatives of ketene, enolate, acrylate and acrylimidate moieties.

Geometrically Constrained Organoboron Species as Lewis Superacids and Organic Superbases.

This report unveils an advancement in the formation of a Lewis superacid (LSA) and an organic superbase by the geometrical deformation of an organoboron species towards a T-shaped geometry. The boron dication [2]2+ supported by an amido diphosphine pincer ligand features both a large fluoride ion affinity (FIA>SbF5 ) and hydride ion affinity (HIA>B(C6 F5 )3 ), which qualifies it as both a hard and soft LSA. The unusual Lewis acidic properties of [2]2+ are further showcased by its ability to abstract hydride and fluoride from Et3 SiH and AgSbF6 respectively, and effectively catalyze the hydrodefluorination, defluorination/arylation, as well as reduction of carbonyl compounds. One and two-electron reduction of [2]2+ affords stable boron radical cation [2]⋅+ and borylene 2, respectively. The former species has an extremely high spin density of 0.798e at the boron atom, whereas the latter compound has been demonstrated to be a strong organic base (calcd. pKBH + (MeCN)=47.4) by both theoretical and experimental assessment. Overall, these results demonstrate the strong ability of geometric constraining to empower the central boron atom.

Crystalline Neutral Diboron Analogues of Cyclopropanes.

Although Schleyer's computations in 1979 predicted that the ground state of the parent diborirane features a planar-tetracoordinate carbon atom (anti van't Hoff-Le Bel geometry), this work demonstrates that substitution of C coupled with N-heterocyclic carbene (NHC) coordination provides access to isolable diborirane derivatives 3 and 4 with van't Hoff-Le Bel geometry. Species 3 and 4 are isoelectronic with cyclopropane and the highly strained B2 C rings feature 2c-2e bent σ bonds. Consequently, the B-B and B-C bonds in 3 are cleaved in the presence of hydride, proton, and chalcogens. The former two reactions gave NHC-coordinated fluorenyldihydridoborane 5 and dichlorofluorenylborane 6, respectively, whereas the latter transformations afforded novel thiaborirane 8 and selenaborirane 9. In addition, transfer hydrogenation of 3 with ammonia borane (H3 N⋅BH3 ) led to the formation of (µ-hydrido)diborane 7 via selective cleavage of the B-B bond. These reactivities show potential for their future application in organic synthesis.

Unveiling Hetero-Enyne Reactivity of Aryliminoboranes: Dearomative Hetero-Diels-Alder-Like Reactions.

Being isoelectronic with alkynes, iminoboranes with a polar B≡N triple bond have been exclusively investigated as a potent 1,2-dipole in synthetic chemistry. Herein, we disclose the unprecedented reactivity of aryliminoboranes via the BNCC π conjugation, namely hetero-enyne behavior. This allows for facile dearomative Diels-Alder-like reactions of aryliminoboranes with aldehydes. This cycloaddition features mild conditions, is catalyst-free, and has a broad substrate scope and good functional group tolerance. Kinetic and computational studies reveal its second-order reaction and concerted cyclization mechanism. This report unveils new synthetic application of iminoboranes beyond their classical reaction patterns.

N-Heterocyclic Carbenes as Promotors for the Rearrangement of Phosphaketenes to Phosphaheteroallenes: A Case Study for OCP to OPC Constitutional Isomerism.

The concept of isomerism is essential to chemistry and allows defining molecules with an identical composition but different connectivity (bonds) between their atoms (constitutional isomers) and/or a different arrangement in space (stereoisomers). The reaction of phosphanyl ketenes, (NHP)-P=C=O (NHP=N-heterocyclic phosphenium) with N-heterocyclic carbenes (NHCs) leads to phosphaheteroallenes (NHP)-O-P=C=NHC in which the PCO unit has been isomerized to OPC. Based on the isolation of several intermediates and DFT calculations, a mechanism for this fundamental isomerisation process is proposed.

One-, two-, and three-electron reduction of a cyclic alkyl(amino)carbene-SbCl3 adduct.

A cyclic alkyl(amino)carbene readily reacts with SbCl3 to form the corresponding Sb(III) adduct. One-electron reduction gives rise to the first example of a neutral antimony-centered radical characterized in solution. Two-electron reduction affords a Lewis base stabilized chloro-stibinidene, whereas three-electron reduction gives an antimony diatomic species capped by two carbenes. The radical has been characterized by EPR spectroscopy, while the structure of the other three species has been ascertained by single-crystal X-ray diffraction. In these four species, the formal oxidation state of the metalloid diminishes from III, to II, to I, and finally 0.

Deprotonation of a borohydride: synthesis of a carbene-stabilized boryl anion.

An acidic hydride! Thanks to the presence of a π-acceptor cyclic alkyl amino carbene and of two electron-withdrawing nitrile groups, a borohydride reacts with a base to give a carbene-stabilized boryl anion, which reacts with carbon and metal electrophiles at the boron center. Dipp = 2,6-diisopropylphenyl, KHMDS = potassium bis(trimethylsilyl)amide.

Unraveling the reactivity of a cationic iminoborane: avenues to unusual boron cations.

A cationic terminal iminoborane [Mes*N[triple bond, length as m-dash]B ← IPr2Me2][AlBr4] (3+[AlBr4]-) (Mes* = 2,4,6-tri-tert-butylphenyl and IPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) has been synthesized and characterized. The employment of an aryl group and N-heterocyclic carbene (NHC) ligand enables 3+[AlBr4]- to exhibit both B-centered Lewis acidity and BN multiple bond reactivities, thus allowing for the construction of tri-coordinate boron cations 5+-12+. More importantly, initial reactions involving coordination, addition, and [2 + 3] cycloadditions have been observed for the cationic iminoborane, demonstrating the potential to build numerous organoboron species via several synthetic routes.

(Phosphanyl)phosphaketenes as building blocks for novel phosphorus heterocycles.

Although BH3 simply coordinates the endocyclic P of (phospholidino)phosphaketene 1Dipp , the bulkier B(C6F5)3 gives rise to a zwitterionic diphosphirenium, which is a novel type of 2π-electron aromatic system as shown by the calculated NICS values. While the reaction of 1Dipp with Na[PCO(dioxane) x ] is unselective, the same reaction with the sterically bulky (phospholidino)phosphaketene 1Ar** [Ar** = 2,6-bis[di(4-tert-butylphenyl)methyl]-4-methylphenyl selectively affords a sodium bridged dimer containing a hitherto unknown λ3,λ5,λ3-triphosphete core. The latter formally results from "P-" addition to a 1,3-P/C-dipole. Similarly, adamantyl isonitrile adds to 1Dipp giving a 4-membered phosphacycle. In contrast to 1, the phosphaketene derived from the electrophilic diazaphospholidine-4,5-dione is unstable and reacts with a second molecule of Na[PCO(dioxane) x ] to afford a 1,3,4-oxadiphospholonide derivative.

Isolation of Au-, Co-η1PCO and Cu-η2PCO complexes, conversion of an Ir-η1PCO complex into a dimetalladiphosphene, and an interaction-free PCO anion.

Sodium phosphaethynolate reacts with [MCl(PDI)] (M = Co, Ir; PDI = pyridinediimine) to give metallaphosphaketenes, which in the case of iridium rearranges into a dimetalladiphosphene, via CO migration from phosphorus to the metal. Two different bonding modes of the PCO anion to CAAC-coinage metal complexes [CAAC: cyclic (alkyl)(amino)(carbene)] are reported, one featuring a strong Au-P bond and the other an η2 coordination to copper. The gold complex appears to be mostly unreactive whereas the copper complex readily reacts with various organic substrates. A completely free PCO anion was structurally characterized as the [Cu(La)2]+ (OCP)- salt. It results from the simple displacement of the PCO unit of the cationic (CAAC)Cu(PCO) complex by a second equivalent of CAAC.

Isolation of a Lewis base stabilized parent phosphenium (PH2(+)) and related species.

The parent phosphenium ion (PH2(+)), and even any phosphenium salts bearing a hydrogen (HRP(+)), have never been observed. The addition of trifluoromethanesulfonic acid and carbon electrophiles to an NHC-PH adduct, featuring a very bulky NHC, allows for the preparation and isolation of such compounds. Computational investigations show that most of the positive charge is localized at phosphorus, demonstrating the phosphenium nature of this compound. Further, the PH moiety is able to utilize one or both of its lone pairs to bind to one or two metals.

An efficient synthetic route to stable bis(carbene)borylenes [(L1)(L2)BH].

Two-electron reduction of bis(carbene) boronium salts allows for the preparation of unsymmetrically substituted nucleophilic boron derivatives of type (L1)(L2)BH, which are characterized by X-ray crystallography. A single electron reduction of the same starting materials leads to the corresponding boron-centered radical cations (L1)(L2)BH˙(+), X(-).