On the Dual Reactivity of a Nucleophilic Dihydrido-Diborane: Reaction at the B−B Bond and/or the B−H Bond

Cationic Group 13 and 14 Element Clusters

Anionic and neutral clusters dominate the cluster chemistry of group 13 and 14 elements, many of which have become classic textbook examples of main group element clusters. However, facilitated by the development of unreactive, weakly coordinating anions, the number of known group 13 and 14 cationic cluster compounds has risen rapidly in recent years. Hence, this review aims to give an overview over this research field, which arouses increasing interest owing to the often unusual structures of the cationic clusters, as well as their application in bond activation chemistry. Challenges of the cluster formation are discussed and suitable starting materials are presented, as well as syntheses, structures and the rich follow-up chemistry of (also mixed) group 13 and 14 cluster cations.

Small Ring Molecules Comprising 3-6 Boron Atoms: An Account on Synthesis, Structure, and Orbital Engineering

ConspectusUnlike carbon, boron does not usually form ring compounds due to its electron-deficiency-driven affinity toward polyhedral geometries. The polyhedral boranes having closo-, nido-, arachno-, or hypho-shapes can be structurally and electronically correlated using various electron counting rules developed by Wade, Mingos, and one of us. However, in the last few decades, boron chemistry progressed significantly toward ring systems. In this regard, three of our research groups have made significant contributions to the development of boron ring molecules through different synthetic approaches. While the Ghosh group generally starts from transition metal (TM) stabilized boron species, the Himmel group typically starts from electron-deficient TM-free boron ring compounds. On the other hand, the Jemmis group studies boron rings and their analogous structures computationally and develops electron counting rules to describe them. Over the past few years, through different synthetic approaches, several boron ring molecules have been prepared by our research groups and others. Recently, the Ghosh group has reported the synthesis of an almost planar B6-ring that is stabilized by a TM template. Similarly, the B3-, B4-, and B5-rings have also been stabilized in the coordination spheres of early and late TMs. The recent work of Himmel has uncovered some remarkable diversity in the structures and bonding of B3 and B4 rings, along with their redox reactions. The well-known hydrocarbon analogues of these borane rings, i.e., two-dimensional aromatic compounds [C3H3]+, [C5H5]-, [C6H6], etc., are governed by Hückel's (4n + 2) π-electron rule. However, planar or nearly planar borane rings are not seriously thought of as achievable targets. One of the reasons for this is the influence of the Rudolph diagram in the thought process of chemists that the nido- and arachno-structures generated from closo-polyhedral boranes must also be three-dimensional (3D) fragments. However, this is not the only possibility. Flat arachno- and nido-boranes reminiscent of their organic counterparts follow from an equivalent of the Rudolph diagram. Therefore, this Account is very much necessary for the boron community, in particular, to design and synthesize 3-6 membered boron rings or beyond. This Account aims to highlight significant ongoing experimental and theoretical results in this area from our groups, in addition to relevant works from other groups wherever appropriate. This will also bring into focus various ways in which the flat Bn-systems can be stabilized, such as the utilization of TM or main group caps, utilization of various Lewis bases, edge-condensation of small rings, control over the electron count, and orbital engineering.

1,2-Dialkynyldiboranes(4): B-B versus CC bond reactivity

The reactivity of three 1,2-dialkynyl-1,2-diaminodiborane(4) derivatives, B₂(NMe₂)₂(CCR)₂ (R = H, Me, SiMe₃), towards small molecules known to react with both B-B and CC bonds was studied. With arylazides nitrene insertion into the B-B bond with concomitant loss of N₂ was kinetically favoured in all cases. While reactions with sterically unhindered hydroboranes proceeded unselectively, sterically encumbered dimesitylborane cleanly added to both alkynyl moieties, resulting in the first examples of 1,2-divinyldiboranes(4). In the presence of catalytic amounts of Pd/C room-temperature hydrogenation at 1 bar led to oxidative B-B bond cleavage and yielded the fully hydrogenated alkyl(amino)hydroborane products. These could be prevented from dimerising and isolated by complexation with an NHC ligand. Finally, stepwise halogenation of the B-B bond and the alkynyl groups afforded first the corresponding alkynyl(amino)haloboranes and then the amino(halo)(1,2-dihalovinyl)boranes.

Derivatization of an especially electron-rich diborane

Starting with electron-rich ditriflato-diborane B₂(hpp)₂(OTf)₂ (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate), novel symmetric and unsymmetric diboranes B₂(hpp)₂X₂ and B₂(hpp)₂XY with X,Y = Br, NCS, N₃ or OTf are synthesized by substitution reactions with S_N1 mechanisms. The stability of the unsymmetric diboranes with respect to dismutation equilibria is evaluated.

Neutral cyclic sp2-sp3 and sp3-sp3 diboranes from N,N'-dicyclohexylcarbodiimide insertion into 1,2-dichlorodiboranes(4)

We report the synthesis and structural characterization of new neutral cyclic sp2-sp3 and sp3-sp3 diboranes from the reaction of N,N'-dicyclohexylcarbodiimide with 1,2-dichlorodiboranes(4) at room temperature. The present study is the first example of an insertion reaction of carbodiimide into diborane(4).

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].

Trishomoaromatic (B3 N3 Ph6 ) Dianion: Characterization and Two-Electron Reduction

Benzene, a common aromatic compound, can be converted into an unstable antiaromatic 8π-electron intermediate through two-electron reduction. However, as an isoelectronic equivalent of benzene, borazine (B₃ N₃ Ph₆ ), having weak aromaticity, undergoes a totally different two-electron reduction to afford (B₃ N₃ R₆ )^2- homoaromatic compounds. Reported here is the synthesis of homoaromatic (B₃ N₃ Ph₆ )^2- by the reduction of B₃ N₃ Ph₆ with either potassium or rubidium in the presence of 18-crown-6 ether. Theoretical investigations illustrate that two electrons delocalize over the three boron atoms in (B₃ N₃ Ph₆ )^2- , which is formed by the geometric and orbital reorganization and exhibits (π,σ)-mixed homoaromaticity. Moreover, (B₃ N₃ Ph₆ )^2- can act as a robust 2e reductant for unsaturated compounds, such as anthracene, chalcone, and tanshinones. This 2e reduction is of high efficiency and selectivity, proceeds under mild reaction conditions, and can regenerate neutral borazine.

trans-Selective Insertional Dihydroboration of a cis-Diborene: Synthesis of Linear sp3 -sp2 -sp3 -Triboranes and Subsequent Cationization

The reaction of aryl- and amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3-trans-dihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis-1,2-μ-H-3-hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base-stabilized B₃ H₄ ⁺ analogues.

Boron-Boron Dehydrocoupling of Boranes Initiated by Reaction with Iodine

A new boron-boron dehydrocoupling strategy was established, providing convenient access to some diborane(4) compounds starting from simple borane adducts under mild conditions. In contrast to the traditional pathway using a reducing reagent, the reduction from B^III to B^II was paradoxically initiated by the addition of the oxidation-reagent iodine. A reaction pathway for this unusual reaction was proposed based on quantum-chemical calculations.

Four Boron Atoms, Four Positive Charges, and Four Skeletal Electrons: A Fluorescent σ-Aromatic Tetraborane(4)

Reaction of a ditriflatodiborane compound with the Lewis acids AlCl₃ or GaCl₃ leads to abstraction of the two triflate substituents and dimerization of the resulting dicationic diborane to give a σ-aromatic tetracationic tetraborane with a planar, rhomboid B₄ core. The compound exhibits four skeletal σ-electrons involved in two (3c,2e) bonds and represents the first stable fourfold base-stabilized [B₄ H₄ ]⁴⁺ analogue. The product is isolated from the reaction mixture in the form of bright orange crystals that display fluorescence. Further analysis shows that the new tetraborane(4) is stabilized in the solid state by the lattice energy. It exhibits an extremely high electron affinity and is only stable in solution after one-electron reduction to the radical cation.

A redox-active diborane platform performs C(sp3)-H activation and nucleophilic substitution reactions

Targeted C(sp³)–H activation or nucleophilic substitution reactions have been achieved through the interaction of a diborane dianion with haloalkanes.

Organoboranes are among the most versatile and widely used reagents in synthetic chemistry. A significant further expansion of their application spectrum would be achievable if boron-containing reactive intermediates capable of inserting into C–H bonds or performing nucleophilic substitution reactions were readily available. However, current progress in the field is still hampered by a lack of universal design concepts and mechanistic understanding. Herein we report that the doubly arylene-bridged diborane(6) 1H₂ and its B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B-bonded formal deprotonation product Li₂[1] can activate the particularly inert C(sp³)–H bonds of added H₃CLi and H₃CCl, respectively. The first case involves the attack of [H₃C]^– on a Lewis-acidic boron center, whereas the second case follows a polarity-inverted pathway with nucleophilic attack of the B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B double bond on H₃CCl. Mechanistic details were elucidated by means of deuterium-labeled reagents, a radical clock, α,ω-dihaloalkane substrates, the experimental identification of key intermediates, and quantum-chemical calculations. It turned out that both systems, H₃CLi/1H₂ and H₃CCl/Li₂[1], ultimately funnel into the same reaction pathway, which likely proceeds past a borylene-type intermediate and requires the cooperative interaction of both boron atoms.

On the dual reactivity of a Janus-type mesoionic dipole: experiments and theoretical validation

A mesoionic bicycle, easily synthesized from a proteinogenic amino acid, l-leucine, behaves as both thiazolium-olate and diazolium-olate dipoles, as unveiled by its dipolar cycloadditions.