Reactions of diborenes with terminal alkynes: mechanisms of ligand-controlled -selective hydroalkynylation, cycloaddition and C[triple bond, length as m-dash]C triple bond scission.

The reactions of terminal acetylenes with doubly Lewis base-stabilised diborenes resulted in different outcomes depending on the nature of the ligands at boron and the conformation of the diborene (cyclic acyclic). N-heterocyclic carbene (NHC)-stabilised diborenes tended to undergo -selective hydroalkynylation at room temperature, whereas [2 + 2] cycloaddition was observed at higher temperatures, invariably followed by a C-N bond activation at one NHC ligand, leading to the ring-expansion of the initially formed BCBC ring and formation of novel boron-containing heterocycles. For phosphine-stabilised diborenes only [2 + 2] cycloaddition was observed, followed by a rearrangement of the resulting 1,2-dihydro-1,2-diborete to the corresponding 1,3-isomer, which amounts to complete scission of both the B[double bond, length as m-dash]B double and C[triple bond, length as m-dash]C triple bonds of the reactants.

Boron- versus Nitrogen-Centered Nucleophilic Reactivity of (Cyano)hydroboryl Anions: Synthesis of Cyano(hydro)organoboranes and 2-Aza-1,4-diborabutatrienes.

Cyclic alkyl(amino)carbene-stabilized (cyano)hydroboryl anions were synthesized by deprotonation of (cyano)dihydroborane precursors. While they display boron-centered nucleophilic reactivity towards organohalides, generating fully unsymmetrically substituted cyano(hydro)organoboranes, they show cyano-nitrogen-centered nucleophilic reactivity towards haloboranes, resulting in the formation of hitherto unknown linear 2-aza-1,4-diborabutatrienes.

Tuneable reduction of cymantrenylboranes to diborenes or borylene-derived boratafulvenes.

Whereas the reduction of N-heterocyclic carbene (NHC)-stabilised cymantrenyldibromoboranes, (NHC)BBr2Cym, in benzene results in the formation of the corresponding diborenes (NHC)2B2Cym2, a change of solvent to THF yields a borylene analogue of the form (NHC)2BCym, stabilised through a boratafulvene/borafulvenium conformation.

2,2'-Bipyridyl as a Redox-Active Borylene Abstraction Agent.

2,2'-Bipyridyl is shown to spontaneously abstract a borylene fragment (R-B:) from various hypovalent boron compounds. This process is a redox reaction in which the bipyridine is reduced and becomes a dianionic substituent bound to boron through its two nitrogen atoms. Various transition metal-borylene complexes and diboranes, as a well as a diborene, take part in this reaction.

Synthesis of polyheterocyclic 1,1-diboryltriazenes by γ-nitrogen insertion of azides into activated B-B single bonds.

The γ-nitrogen insertion of arylazides into the B-B bond of electron-rich cyclic μ-hydridodiboranes stabilised by one N-heterocyclic carbene (NHC) ligand leads to the expansion of the central C3B2 ring, yielding unsymmetrical polyheterocyclic 1,1-diboryltriazenes. The 2-benzyl-bridged analogues undergo further NHC ring expansion and thermally induced loss of N2.

Synthesis of Complex Boron-Nitrogen Heterocycles Comprising Borylated Triazenes and Tetrazenes Under Mild Conditions.

The reactions of organic azides with diaryl(dihalo)diboranes(4) were explored, resulting in the observation of a number of surprising reactivity patterns. The reaction of phenyl azide with 1,2-diaryl-1,2-dihalodiboranes(4) resulted in the formation of five-membered rings comprising diboryl-triazenes with retention of the boron-boron bond, while the reaction of the peculiar 1,1-di(9-anthryl)-2,2-difluorodiborane(4) with phenyl azide yielded a six-membered ring bearing a diboryl-triazene, whereby the B-B bond was ruptured by the insertion of an arylnitrene-like reactive intermediate. Both types of heterocycles feature unprecedented connectivity patterns and are very rare examples of boryl triazenes beyond the more common 1,2,3-triazolatoboranes.

Pseudodiborenes: hydride-bridged diboranes(5) as two-electron reductants of chalcogens.

The reactivity of two nucleophilic neutral sp2-sp3 diboranes towards chalcogens is presented herein. Both diboranes(5) serve as two-electron reductants, incorporating oxygen, sulfur and selenium atoms. Treatment with chalcogen sources results in the oxidative insertion of one chalcogen atom into the B-B single bond, while depending on the negative inductive effect of the chalcogen and the boron bound aryl substituent further N-heterocyclic carbene (NHC) ring expansion and hydride migration can occur.

Spontaneous trans-Selective Transfer Hydrogenation of Apolar Boron-Boron Double Bonds.

The transfer hydrogenation of N-heterocyclic carbene (NHC)-supported diborenes with dimethylamine borane proceeds with high selectivity for the trans-1,2-dihydrodiboranes. DFT calculations, supported by kinetic studies and deuteration experiments, suggest a stepwise proton-first-hydride-second reaction mechanism via an intermediate μ-hydrodiboronium dimethylaminoborate ion pair.

Reactivity of Tetrahalo- and Difluorodiboranes(4) toward Lewis Basic Platinum(0): Bis(boryl), Borylborato, and Doubly Boryl-Bridged Platinum Complexes.

The reaction of the tetrahalodiboranes(4) BF, BCl, and BBr with a Lewis basic platinum(0) complex led to the isolation of the cis-bis(difluoroboryl) complex cis-[(CyP)Pt(BF)] (1) and the novel borylborato complexes trans-[(CyP)Pt{B(X)-BX}] (2, X = Cl; 3, X = Br), respectively. The trans influence of the borylborato group was found to be one of the strongest ever observed experimentally. Furthermore, the reactivity of little-explored diaryldifluorodiboranes(4) FB-BMes and the new derivative FB-BAn (An = 9-anthryl) toward a range of platinum(0) complexes was investigated.

Bottleable Neutral Analogues of [B H ] as Versatile and Strongly Binding η Donor Ligands.

Herein we report the discovery that two bottleable, neutral, base-stabilized diborane(5) compounds are able to bind strongly to a number of copper(I) complexes exclusively through their B-B bond. The resulting complexes represent the first known complexes containing unsupported, neutral σ diborane ligands. Single-crystal X-ray analyses of these complexes show that the X-Cu moiety (X=Cl, OTf, C F ) lies opposite the bridging hydrogen atom of the diborane and is near perpendicular to the B-B bond, interacting almost equally with both boron atoms and causing a B-B bond elongation.

Alkylideneborate zwitterions and C-C coupling by atypical diboration of electron-rich alkynes.

The combination of electron-rich diaminoalkynes and ditopic Lewis acids diboranes(4) leads to unusual uncatalysed diboration reactions involving internal Lewis adduct, zwitterion, and C-C bond formation. The products are novel multicyclic, charge-separated compounds with intramolecular dative bonds.

Reaction of Dihalodiboranes(4) with a N-Heterocyclic Silylene: Facile Construction of 1-Aryl-2-Silyl-1,2-Diboraindanes.

Dihalodiboranes(4) react with a N-heterocyclic silylene (NHSi) to generate NHSi adducts of 1-aryl-2-silyl-1,2-diboraindanes, as was confirmed by X-ray crystallography, featuring the functionalization of both B-X (X=halogen) bonds and a sp - or sp -C-H bond under mild conditions. Coordination of a third NHSi to the proposed 1,1-diaryl-2,2-disilyldiborane(4) intermediates, generated by a twofold B-X insertion, may be crucial for the C-H borylation that leads to the final products. Notably, our results demonstrate the first C-H borylation with a strong B-F bond activated by silylene insertion.