Alessandro Bismuto

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) are among the most nucleophilic (σ donating) and also electrophilic (π-accepting) stable carbenes known to date. These properties allow them to activate a variety of small molecules and enthalpically strong bonds, to stabilize highly reactive main-group and transition-metal diamagnetic and paramagnetic species, and to bind strongly to metal centers, which gives rise to very robust catalysts. The most important results published up to the end of 2013 are briefly summarized, while the majority of this Review focuses on findings reported within the last three years.

CAACs—versatile, readily available, and storable: Combined with most of the elements of the periodic table cyclic (alkyl)(amino)carbenes (CAACs), have influenced various fields of chemistry, and a lot remains to be discovered.

A detailed mechanistic investigation identified the stepwise nature of the 1,3-aryl shift, which enables our recently disclosed Al³⁺-catalyzed insertion of unactivated alkynes into the sp²–sp³ C−C bond of benzyl alcohols. The selectivity for the rearranged product was found to be induced by the continued coordination of the aluminum catalyst to the rearranging species, which is encouraged by a reversible background reaction. This participation of the catalyst beyond the ionization step is unique in the realm of carbocation driven reactions and opens up the possibility of a catalyst-induced chiral induction in the future. Furthermore, the study represents a rare example of detailed mechanistic analysis of a reaction with a product selectivity that changes with increasing conversion.

Never let go aluminum! Continued participation of the catalyst in a carbocation-driven reaction proved responsible for the selective induction of a highly unusual 1,3-carbon shift. Detailed mechanistic analysis explains why a vinyl cation undergoes an unexpected rearrangement, resulting in the net insertion of unactivated alkynes into a C−C bond in benzylic alcohols.

A borylidene–phosphorane, the lightest phosphoranylidene, which is stabilized by an N-heterocyclic carbene ligand, was synthesized and fully characterized. Experimental electron density analysis and DFT calculations indicate an enhanced ylene character (rather than an ylide character) with an exceptionally strong BP π-back bonding related to the less electronegative boron compared to phosphorus.

Opposites attract: A phosphorus bora-ylide stabilized by an N-heterocyclic carbene ligand was synthesized. Experimental electron density analysis and DFT calculations indicate enhanced phosphorane character (rather than ylide character) with an exceptionally strong BP π-back bonding compared to related compounds; this is due to the electronegativity difference between boron and phosphorus atoms.

Borinium ions, that is, two-coordinate boron cations, are the most electron-deficient isolable boron compounds. As borinium ions have only four formal valence electrons on boron, they should show a strong tendency to accept electron pairs on the boron atom to fill its valence shell. Thus chemical reactions of borinium ions are expected to give products in which the coordination number of boron is increased from two to three or four. However, contrary to this expectation, we found that the dimesitylborinium ion (Mes₂B⁺) undergoes twofold 1,2-carboboration reactions with two equivalents of diphenylacetylene to yield an unprecedented borinium ion (1⁺) with two substituted vinyl groups on the boron center. NMR spectroscopy and X-ray diffraction analysis of 1⁺, together with electronic-structure calculations, revealed that the positive charge is delocalized over the entire π-conjugated system. The fact that the chemical transformation of a borinium ion gives rise to a different borinium ion without a change in the coordination number is remarkable and should provide new insight into the chemistry of the Group 13 elements.

Still hungry: A two-coordinate diarylborinium ion undergoes twofold 1,2-carboboration with two equivalents of diphenylacetylene to form an unprecedented divinylborinium ion. NMR spectroscopy and X-ray diffraction analysis, together with theoretical calculations, revealed that the positive charge of the divinylborinium ion is delocalized over the entire π-conjugated system.

para-Selective C−H borylation of benzamides and pyridines has been achieved by cooperative iridium/aluminum catalysis. A combination of iridium catalysts commonly employed for arene C−H borylation and bulky aluminum-based Lewis acid catalysts provides an unprecedented strategy for controlling the regioselectivity of C−H borylation to give variously substituted (hetero)arylboronates, which are versatile synthetic intermediates for complex multi-substituted aromatic compounds.

Collect. Select. Reflect: para-Selective C−H borylation of benzamide and pyridine adducts is controlled by a combination of iridium and bulky aluminum-based Lewis acid catalysts. Variously substituted (hetero)arylboronates were prepared, which are versatile synthetic intermediates for complex multi-substituted aromatic compounds.

Catalytic amounts of B(C₆F₅)₃ promote the ring opening and subsequent isomerization of a series of unactivated cyclopropanes to afford terminal olefins in good yields when a hydrosilane and 2,6-dibromopyridine are employed as additives.

Opening inert cyclopropanes: B(C₆F₅)₃-catalyzed ring-opening and isomerization reactions of aryl-, alkyl-, and vinyl-substituted cyclopropanes are described. These reactions generate terminal olefins after a sequential process of C−C bond cleavage, 1,2-hydride migration, and B(C₆F₅)₃ dissociation. The addition of 2,6-dibromopyridine and Ph₃SiH is crucial for the reactivity.

The first single-component N-heterocyclic silylene borane 1 (LSi-R-BMes₂; L=PhC(N^tBu)₂; R=1,12-xanthendiyl spacer; Mes=2,4,6-Me₃C₆H₂), acting as a frustrated Lewis pair (FLP) in small-molecule activation, can be synthesized in 65 % yields. Its HOMO is largely localized at the silicon(II) atom and the LUMO has mainly boron 2p character. In small-molecule activation 1 allows access to the intramolecular silanone–borane 3 featuring a Si=OB interaction through reaction with O₂, N₂O, or CO₂, and formation of silanethione borane 4 from reaction with S₈. The Si^II center in 1 undergoes immediate hydrogenation if exposed to H₂ at 1 atm pressure in benzene, affording the silane borane 5-H₂, L(H₂)Si-R-BMes₂. Remarkably, no H₂ activation occurs if the single silylene LSiPh and Mes₃B intermolecularly separated are exposed to dihydrogen. Unexpectedly, the pre-organized Si–B separation in 1 enables a metal-free dehydrogenation of H₂O to give the silanone–borane 3 as reactive intermediate.

A single for two: The first single-component N-heterocyclic silylene borane activates a variety of small molecules, such as O₂, N₂O, CO₂, H₂, and H₂O. Notably, the activation of H₂O by 1 makes it a metal-free system for dehydrogenation of H₂O.

The first deprotonation of a borohydride anion was achieved by treatment of [BH(CN)₃]⁻ with strong non-nucleophilic bases, which resulted in the formation of alkali-metal salts of the tricyanoborate dianion B(CN)₃²⁻ in up to 97 % yield and 99.5 % purity. [BH(CN)₃]⁻ is less acidic than (Me₃Si)₂NH but a stronger acid than iPr₂NH. Less sterically hindered, more nucleophilic bases such as PhLi and MeLi mostly attack a CN group under formation of imine dianions [RC(N)B(CN)₃]²⁻, which can be hydrolyzed to ketones of the [RC(O)B(CN)₃]⁻ type. The boron-centered nucleophile B(CN)₃²⁻ reacts with CO₂ and CN⁺ reagents to give salts of the [B(CN)₃CO₂]²⁻ dianion and the tetracyanoborate anion [B(CN)₄]⁻, respectively, in excellent yields.

Attack at boron: In the first deprotonation of a hydridoborate anion, the readily accessible [BH(CN)₃]⁻ anion reacts with strong non-nucleophilic bases to yield the boron-centered nucleophile B(CN)₃²⁻ in high yield and purity. The B(CN)₃²⁻ dianion reacts with CN⁺ sources to give the tetracyanoborate anion and with CO₂ to yield the carboxylic acid derivative [B(CN)₃CO₂]²⁻, which is a promising starting material for cyanoborate chemistry and materials science.

Radical anions of a diphosphene with two boryl substituents were isolated and characterized by single-crystal X-ray diffraction, electron spin resonance (ESR), and UV/Vis absorption spectroscopy as well as DFT calculations. Structural analysis of the radical anions revealed an elongation of the P=P bond and a contraction of the B−P bonds relative to the neutral diphosphene. The UV/Vis spectra of these radical anions showed a strong absorption in the visible region, which was assigned to SOMO-related transitions on the basis of DFT calculations. The ESR spectra revealed that the hyperfine coupling constant with the phosphorus nuclei is the smallest that has been reported thus far. The results of the DFT calculations furthermore suggest that this should be attributed to a soaking of electron spin to the vacant p orbitals of the boryl substituents.

Radical anions of a diphosphene with two boryl substituents were isolated and characterized by X-ray analysis, ESR, and UV/Vis spectroscopy as well as DFT calculations. All structural parameters suggest that the electron spin is soaked to the vacant p orbitals of the boryl substituents.