Azide-alkyne cycloadditions with an electronically activated alkyne: indole formation via 1-aryl-1,2,3-triazole-derived imino carbenes.

We report that the use of a diaminoalkyne in the azide-alkyne cycloaddition with aryl azides leads to 3H-indoles under mild, uncatalysed conditions. Computations reveal that N2 extrusion from, in one case, isolable triazoles is facile, generating imino carbenes, which undergo intramolecular aryl C-H bond activation and give 3H-indoles as products.

Ring expansion of alumoles with organic azides: selective formation of six-membered aluminum-nitrogen heterocycles.

Aside from simple Lewis acid-base chemistry, the reaction chemistry of aluminacyclopentadienes, which are commonly referred to as aluminoles or simply alumoles, remains relatively underdeveloped. To date, few attempts to extend their inherent insertion and cycloaddition reactivity to the construction of stable aluminum-containing heterocycles have been reported. Herein, we demonstrate the selective ring expansion of a cyclopentadienyl-substituted alumole with a series of organic azides to form unsaturated six-membered AlN heterocycles. Depending on the substituent on the azide, the reaction proceeds either with or without loss of dinitrogen, leading to incorporation of only the "NR" unit of the azide or the entire azo substituent into the periphery of the heterocycle. A deeper understanding of these ring expansion reactions is reached through computational studies, illustrating deviations in the mechanism as a function of the organic azide employed.

Borabicyclo[3.2.0]heptadiene: A Fused Bicyclic Isomer of Borepin.

A new isomer of borepin, identified as 2-borabicyclo[3.2.0]hepta-3,6-diene by single-crystal X-ray diffraction and heteronuclear NMR spectroscopy, was obtained by reaction of 1-mesityl-2,3,4,5-tetraphenylborole with the diaminoalkyne bis(piperidyl)acetylene. Analogous reactions of the alkyne with other borole derivatives generated seven-membered borepins, in which a second equivalent of the alkyne was found to insert into the exocyclic B-C bond. Results of mechanistic DFT studies as well as differences in the reactivity of the boroles toward the diaminoalkyne are discussed.

Dialumination of unsaturated species with a reactive bis(cyclopentadienyl) dialane.

A new bis(cyclopentadienyl) dialane is prepared, which shows controlled, selective dialumination reactions with a conventional alkyne, an electron-rich alkyne, and an azide. The reactions provide structurally diverse products, featuring a range of aluminium coordination numbers, cyclopentadienyl binding modes, and cyclic motifs. The variable nature of the bonding in the Cp*Al units allows a range of binding modes depending on the electronic requirements of the Al atom and provides new possibilities to the chemistry of dialanes, as demonstrated by the isolation of a double internal Lewis adduct with "ring-slipped" Cp* rings in this work.

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.

Abnormal Tin-Boron Exchange in the Attempted Synthesis of a Borylated Borole.

Boroles are important motifs within functional materials. With the aim to prepare a pinacolboryl-substituted derivative, the metallacycle transfer from corresponding zirconium and tin precursors has been explored. We show that the reaction of 1,1-dimethyl-2,3,4,5-tetrapinacolborylstannole with dichloro(phenyl)borane does not provide the desired borole, but instead a stannyl-substituted 1-chloroboracyclopent-3-ene. Spectroscopic and structural details of this highly functionalized boracycle indicate that intramolecular interactions between the tin and oxygen atoms of the boryl substituents may account for the unexpected outcome of the tin-boron exchange reaction.

Elucidating the Reactivity of Vicinal Dicarbenoids: From Lewis Adduct Formation to B-C Bond Activation.

The reactivity of the diaminoacetylene Pip-C≡C-Pip (Pip=piperidyl=NC5 H10 ) towards phenyldichloro- and triphenylborane is presented. In the case of the less Lewis acidic PhBCl2 , the first example of a double Lewis adduct of a vicinal dicarbenoid is reported. For the more Lewis acidic triphenylborane, coordination to the bifunctional carbene leads to a mild B-C bond activation, resulting in a syn-1,2-carboboration. Ensuing cis/trans isomerization yields a novel ethylene-bridged frustrated Lewis pair (FLP). The compounds were characterized using multinuclear NMR spectroscopy, structural analysis, and mass spectrometry. Reactivity studies of both isomers with the N-heterocyclic carbene 1,3-dimethylimidazol-2-ylidene (IMe) aided in elucidating the proposed isomerization pathway. DFT calculations were carried out to elucidate the reaction mechanism. The rather low free energy of activation is consistent with the observation that the reaction proceeds smoothly at room temperature.

Mild and Complete Carbonyl Ligand Scission on a Mononuclear Transition Metal Complex.

The complete scission of the carbon-oxygen bond of carbon monoxide, while frequently observed on bulk metals and with bimetallic and cluster transition metal complexes, is unknown with monometallic systems. Reaction of a zerovalent iron bis(borylene) complex with a cyclic (alkyl)(amino)carbene revealed a highly selective intramolecular cleavage of the C-O bond of a carbonyl ligand at room temperature, leading to the formation of a highly unusual iron complex containing a base-stabilized (bora)alkylideneborane ligand. DFT investigation of the reaction mechanism suggested that the two Lewis acidic borylene boron atoms cooperate to cleave the C-O multiple bond.