Isolierbare, neutrale Analoga des [B 2 H 5 ] − ‐Ions als vielseitige und stark bindende η 2 ‐Donorliganden

Metathesis by Partner Interchange in σ-Bond Ligands: Expanding Applications of the σ-CAM Mechanism

In 2007 two of us defined the σ-Complex Assisted Metathesis mechanism (Perutz and Sabo-Etienne, Angew. Chem. Int. Ed. 2007, 46, 2578-2592), that is, the σ-CAM concept. This new approach to reaction mechanisms brought together metathesis reactions involving the formation of a variety of metal-element bonds through partner-interchange of σ-bond complexes. The key concept that defines a σ-CAM process is a single transition state for metathesis that is connected by two intermediates that are σ-bond complexes while the oxidation state of the metal remains constant in precursor, intermediates and product. This mechanism is appropriate in situations where σ-bond complexes have been isolated or computed as well-defined minima. Unlike several other mechanisms, it does not define the nature of the transition state. In this review, we highlight advances in the characterization and dynamic rearrangements of σ-bond complexes, most notably alkane and zincane complexes, but also different geometries of silane and borane complexes. We set out a selection of catalytic and stoichiometric examples of the σ-CAM mechanism that are supported by strong experimental and/or computational evidence. We then draw on these examples to demonstrate that the scope of the σ-CAM mechanism has expanded to classes of reaction not envisaged in 2007 (additional σ-bond ligands, agostic complexes, sp2 -carbon, surfaces). Finally, we provide a critical comparison to alternative mechanisms for metathesis of metal-element bonds.

B-B vs. B-H Bond Activation in a (μ-Hydrido)diborane(4) Anion upon Cycloaddition with CO2 , Isocyanates, or Carbodiimides

The intriguing (μ‐hydrido)diboranes(4) with their prominent pristine representative [B₂H₅]⁻ have mainly been studied theoretically. We now describe the behavior of the planarized tetraaryl (μ‐hydrido)diborane(4) anion [1H]⁻ in cycloaddition reactions with the homologous series of heterocumulenes CO₂, iPrNCO, and iPrNCNiPr. We show that a C=O bond of CO₂ selectively activates the B−B bond of [1H]⁻, while the μ‐H ligand is left untouched ([2H]⁻). The carbodiimide iPrNCNiPr, in contrast, neglects the B−B bond and rather adds the B‐bonded H⁻ ion to its central C atom to generate a formamidinate bridge across the B₂ pair ([3]⁻). As a hybrid, the isocyanate iPrNCO combines the reactivity patterns of both its congeners and gives two products: one of them ([4H]⁻) is related to [2H]⁻, the other ([5]⁻) is an analog of [3]⁻. We finally propose a mechanistic scenario that rationalizes the individual reaction outcomes and combines them to a coherent picture of B–B vs. B–H bond activation.

Synthesis of Boron Analogues of Enamines via Hydroamination of a Boron-Boron Triple Bond

An N-heterocyclic-carbene-stabilized diboryne undergoes rapid, high-yielding and catalyst-free hydroamination reactions with primary amines, yielding 1-amino-2-hydrodiborenes, which can be considered boron analogues of enamines. The electronics of the organic substituent at nitrogen influence the structure and further reactivity of the diborene product. With electron-rich anilines, a second hydroamination can occur at the diborene to generate 1,1-diamino-2,2-dihydrodiboranes. With isopropylamine, the electronic influence of the alkyl substituent upon the diborene leads to an unprecedented boron-mediated intramolecular N-dearylation reaction of an N-heterocyclic carbene unit.

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

The tetraaryl μ‐hydridodiborane(4) anion [2H]⁻ possesses nucleophilic B−B and B−H bonds. Treatment of K[2H] 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[2H] (M⁺=Li⁺, K⁺) with MeI or Me₃SiCl leads to equimolar amounts of 9‐R‐9‐borafluorene and HBFlu (R=Me or Me₃Si). Thus, [2H]⁻ 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[2H] and allyl bromide gave the 1,3‐propylene‐linked ditopic 9‐borafluorene 5 as sole product. M[2H] also participates in unprecedented [4+1] cycloadditions with dienes to furnish dialkyl diaryl spiroborates, M[R₂BFlu].

Facile Synthesis of a Stable Dihydroboryl {BH2 }- Anion

While the one-electron reduction of (CAAC^Me )BH₂ Br (CAAC^Me =1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) yields a hydride-shift isomer of the corresponding tetrahydrodiborane, a further reversible reduction leads to the first stable parent boryl anion, [(CAAC^Me )BH₂ ]^- , which acts as a powerful boron nucleophile.