Dimorpholinoacetylene and Its Use for the Synthesis of Tetraaminocyclobutadiene Species

The new diaminoacetylene (DAA) dimorpholinoacetylene (3) was prepared from 1,1-dimorpholinoethene (1) by bromination to form the dibromoketene aminal 2, which upon lithiation afforded 3 through a Fritsch-Buttenberg-Wiechell rearrangement. Heating 3 at elevated temperatures resulted in a complete conversion into the dimer 1,1,2,4-tetramorpholino-1-buten-3-yne (4), which was used for the synthesis of four-membered cyclic bent allene (CBA) transition-metal complexes of the type [(CBA)MLn ] (5-7; MLn =AuCl, RhCl(COD), RhCl(CO)2 ; CBA=1,3,4,4-tetramorpholino-1,2-cyclobutadiene; COD=1,5-cyclooctadiene). The reaction of 3 with tetraethylammonium bromide gave 1,2,3,4-tetramorpholinocyclobutenylium bromide (8), which reacted with bromine to form 1,2,3,4-tetra(morpholino)cyclobutenediylium bis(tribromide) (9). Compound 9 represents the first fully characterized compound containing a tetraaminocyclobutadiene dication and displays a nearly planar C4 N4 core as shown by X-ray diffraction analysis. Detailed quantum chemical calculations were performed to assess the aromaticity of tetraaminocyclubutadiene dications by employing the Nucleus Independent Chemical Shift (NICS) method and current density analysis.

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.

Incorporation of Boron into Uranium Metallacycles: Synthesis, Structure, and Reactivity of Boron-Containing Uranacycles Derived from Bis(alkynyl)boranes

The reaction of uranacyclopropene complex (C₅ Me₅ )₂ U[η² -1,2-C₂ (SiMe₃ )₂ ] with B-aryl bis(alkynyl)borane PhB(C≡CPh)₂ led to the first six-membered uranium metallaboracycle, while the reaction with B-amino bis(alkynyl)borane (Me₃ Si)₂ NB(C≡CPh)₂ afforded an unexpected uranaborabicyclo[2.2.0] complex via [2+2] cycloaddition. The reaction with CuCl revealed the non-innocent property of the rearranged bis(alkynyl)boron species towards oxidant. The reactions with isocyanide DippNC: (Dipp=2,6-iPr₂ -C₆ H₃ ) and isocyanate tBuNCO afforded the novel uranaborabicyclo[3.2.0] complexes. All new complexes have been structurally characterized. DFT calculations were performed to provide more insights into the electronic structures and the reaction mechanism.

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. They are also the product of a unique type of aryl migration from a boron center to the phenyl azide γ-nitrogen center. Lastly, the substitution of 1,2-diaryl-1,2-dihalodiboranes(4) with azide groups, using trimethylsilyl azide as the transfer reagent, yielded boryl-tetrazaboroles and diboryldiazadiboretidines (as side-products), invoking the intermediacy of the first N-boryl-substituted iminoboranes, which are BN isosteres of monoborylated alkynes. The synthetic results are complemented with mechanistic proposals derived from quantum-chemical calculations.

Stereo- and regio-selective synthesis of silicon-containing diborylalkenes via platinum-catalyzed mono-lateral diboration of dialkynylsilanes

A highly chemoselective platinum-catalyzed mono-lateral diboration of dialkynylsilanes for the construction of silicon-tethered alkynyl diborylalkenes is described, in which tris(4-methoxyphenyl)phosphine was found to be an effective ligand for the cis-addition of diboron agents to the silicon-tethered alkynes, and the chiral ligand (AFSi-Phos)-mediated diboration of dialkynylsilanes resulted in the desymmetric construction of silicon-stereogenic centers with promising enantioselectivity.

Dimerisation of Dipiperidinoacetylene: Convenient Access to Tetraamino-1,3-Cyclobutadiene and Tetraamino-1,2-Cyclobutadiene Metal Complexes

The reaction of 1,2-dipiperidinoacetylene (1) with 0.5 equivalents of SnCl2 or GeCl2 ⋅dioxane afforded the 1,2,3,4-tetrapiperidino-1,3-cyclobutadiene tin and germanium dichloride complexes 2 a and 2 b, respectively. A competing redox reaction was observed with excess amounts of SnCl2 , which produced a tetrapiperidinocyclobutadiene dication with two trichlorostannate(II) counterions. Heating neat 1 to 110 °C for 16 h cleanly produced the dimer 1,3,4,4-tetrapiperidino-3-buten-1-yne (3); its reaction with stoichiometric amounts of SnCl2 or GeCl2 ⋅dioxane furnished the 1,3,4,4-tetrapiperidino-1,2-cyclobutadiene tin and germanium dichloride complexes 4 a and 4 b, respectively. Transition-metal complexes containing this novel four-membered cyclic bent allene (CBA) ligand were prepared by reaction of 3 with [(tht)AuCl], [RhCl(CO)2 ]2 , and [(Me3 N)W(CO)5 ] to form [(CBA)AuCl] (5), [(CBA)RhCl(CO)2 ] (6), and [(CBA)W(CO)5 ] (7). The molecular structures of all compounds 2-7 were determined by X-ray diffraction analyses, and density functional theory (DFT) calculations were carried out to rationalise the formation of 3 and 4 a.

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) B₂F₄, B₂Cl₄, and B₂Br₄ with a Lewis basic platinum(0) complex led to the isolation of the cis-bis(difluoroboryl) complex cis-[(Cy₃P)₂Pt(BF₂)₂] (1) and the novel borylborato complexes trans-[(Cy₃P)₂Pt{B(X)-BX₃}] (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) F₂B-BMes₂ and the new derivative F₂B-BAn₂ (An = 9-anthryl) toward a range of platinum(0) complexes was investigated. Reactions with relatively nonbulky platinum(0) complexes led to the formation of unsymmetrical cis-bis(boryl) complexes cis-[(R₃P)₂Pt(BF₂)(BMes₂)] (6, R = Me; 7, R = Et) as well as the first example of a fourfold-unsymmetrical bis(boryl) complex, [(Me₃P)(Cy₃P)Pt(BF₂)(BMes₂)] (12). The use of a more bulky Pt complex provided access to the unprecedented dinuclear bis(boryl) complexes [{( iPr₃P)Pt}₂(μ-BF₂)(μ-BAr₂)] (8, Ar = Mes; 9, Ar = An), which feature two different μ₂-bridging boryl ligands.

Dimerization of boryl- and amino-substituted acetylenes by B2C2 four-membered ring formation

Boryl- and amino-substituted acetylenes bearing diphenylboryl or 9-borabicyclononyl groups were synthesized. X-ray diffraction analyses revealed the dimerized structures of these acetylenes via the formation of B2C2 four-membered rings. Spectroscopic studies and DFT calculations indicated that these dimers can dissociate to afford monomeric acetylenes, and that the equilibrium constant for the dissociation depends on the structure of the boryl substituents.

Cleavage of BN triple bonds by main group reagents

A silylene and a 1,2-diaminoalkyne respectively cleave the strong BN triple bond in iminoboranes.

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

A new cyclopentadienyl-containing dialane is prepared and found to undergo dialumination reactions with unsaturated species.