Tridentate Lewis Acids: Silicon‐Functionalised 1,3,5‐Triethynylbenzene and 1,3,5‐Trivinylbenzene

Selectivity in Adduct Formation of a Bidentate Boron Lewis Acid

A bidentate boron Lewis acid based on 1,8-diethynylanthracene has been studied in detail with respect to its adduct formation with diamines and diphosphanes of different linker lengths between the donor functions. A clear correlation between the linker length of the bifunctional base and the formation of 1 : 1 adducts, 1 : 2 adducts or oligomers was found. The adducts were characterized in solution by NMR titration experiments and structurally by X-ray diffraction. In addition, adduct formation and competition experiments of the host system with ZR3 (Z=N, P; R=H, Me) demonstrated the generally higher stability of alkylphosphane adducts compared to alkylamine adducts with boron functions. The results provide a general insight into the adduct formation of bidentate Lewis acids with guests of different sizes as well as the differences in stability between borane-amine and borane-phosphane adducts.

Bidentate boron Lewis acids: synthesis by tin boron exchange reaction and host-guest complex formation

Four bidentate boron Lewis acids based on the 1,8-diethynylanthracene backbone have been synthesized by a tin-boron exchange reaction with various chloroboranes, yielding the products in good to excellent yields. Complexation experiments of the host compounds with pyridine, pyrimidine and TMEDA demonstrated striking differences in terms of formation and solubility of the supramolecular adducts. The host-guest complexes were investigated by multinuclear NMR spectroscopy and structurally characterized by X-ray diffraction experiments, illustrating the adaptation of the host system upon adduct formation with different neutral guest molecules.

Synthesis of bi- and tetradentate poly-Lewis acids by hydroalumination of poly-alkynyl-anthracene derivatives

Photo-dimers of 1,5-diethynylanthracene and 1,5-bis[(trimethylsilyl)ethynyl]anthracene, obtained by UV radiation of the monomers, were treated with the bulky aluminium hydride [(SiMe₃)₂HC]₂AlH in hydroalumination reactions. Hydroalumination of the tetraethynyl-substituted anthracene photo-dimers (syn and anti) led to fourfold aluminium-functionalized Janus-like products with two aluminium functions on each side oriented towards the same direction. The reactions of the monomeric anthracene derivatives with [(SiMe₃)₂HC]₂AlH afforded semi-flexible bidentate Lewis acids, which are interesting building blocks for molecular chains bearing multiple Lewis-acidic functions. The aluminium-functionalized anti-photo-dimer was treated with various donor molecules to gain insight into its host-guest chemistry. All poly-Lewis acids and their adducts were characterized by X-ray diffraction experiments, multinuclear NMR spectroscopy and by elemental analyses.

Synthesis of a bifunctional boron-Lewis acid and studies on host-guest chemistry using pyridine and TMPD

Based on the previously described bifunctional Lewis acid with a functional distance of the boron functions of 4.918(2) Å, the development of a further bifunctional, boron-containing Lewis acid with a wider functional distance is demonstrated. Again, a stannylated precursor was used and the Lewis-acidic boron functions were introduced by means of tin-boron exchange. The general suitability of this class of compounds for the formation of host-guest-complexes is demonstrated by NMR experiments and by solid-state structures using pyridine and TMPD (N 1,N 1,N 4,N 4-tetramethylbenzene-1,4-diamine) as Lewis basic guests. The influence of traces of moisture on the boron-containing, bifunctional Lewis acids was investigated by the structure elucidation of a decomposition product.

Co-Crystal Formation of Partially Fluorinated 1,3,5-Tris(phenylethynyl)benzenes

Several rigid 1,3,5-tris(phenylethynyl)benzenes with different fluorination patterns were synthesized through selective Sonogashira-Hagihara coupling reactions to analyze the packing behavior in solid-state structures. The aggregation is dominated by various intermolecular interactions between aryl substituents, triple bonds, C-H bonds and H⋅⋅⋅F contacts. Co-crystallization experiments for the analysis of preferred aryl-aryl-interactions led to 1 : 1 complexes. Intermolecular phenyl-perfluorophenyl interactions with short centroid-centroid distances are dominating these co-crystal structures. They lead to melting point increases of up to 49 °C for the co-crystals compared to the pure substances.

Bidentate Lewis Acids Derived from o-Diethynylbenzene with Group 13 and 14 Functions

Starting from 1,2-diethynylbenzene, a series of bidentate Lewis acids was prepared by means of hydrometalations, in particular hydrosilylation, hydroboration, hydroalumination and terminal metalation based on group 13 and 14 elements. In the case of terminal alkyne metalation, the Lewis-acidic gallium function was introduced using triethylgallium under alkane elimination. A total of six different Lewis acids based on a semiflexible organic scaffold were prepared, bearing -SiClMe2 , -SiCl2 Me, -SiCl3 , -B(C6 F5 )2 , -AlBis2 (Bis=bis(trimethylsilyl)methyl) and -GaEt2 as the corresponding functional units. In all cases, the Lewis acid functionalisation was carried out twice and the products were obtained in good to excellent yields. In the case of the twofold gallium Lewis acid, a different structural motif in the form of a polymer-like chain was observed in the solid state. All new bidentate Lewis acids were characterised by multinuclear NMR spectroscopy, CHN analysis and X-ray diffraction experiments.

Chalice-Type Tridentate Silicon Lewis Acids of C3 Symmetry in a Single Step Starting from Hexadehydrotribenzo[12]annulene

Tridentate Lewis acids with aligned functions were synthesized based on the rigid framework hexadehydrotribenzo[12]annulene. The backbone and its fluorinated analogue were synthesised in one-pot syntheses, with alkyne deprotection and Sonogashira cross coupling reaction being carried out in one step. Hydrosilylation of the annulene with chlorohydrosilanes proceeded highly selectively and afforded rigid poly-Lewis acids with three SiCl3 or SiCl2 Me substituents perfectly oriented to one side of the molecule in a single step. The progress of hydrosilylation was investigated by time-correlated NMR spectroscopic studies. The crystal structures show that the framework is symmetrically functionalised and the silyl substituents are aligned in one direction. To increase the acidity of the Lewis acids the chlorosilyl substituents were fluorinated with SbF3 . Further investigation of hydrometallation reactions (M=B, Al, Ga, Sn) did not lead to corresponding structures.

Bidentate Boron Lewis Acids: Selectivity in Host-Guest Complex Formation

Bidentate boron Lewis acids based on 1,8-diethynylanthracene were synthesised in two steps by initial stannylation of the terminal alkynes and subsequent tin-boron exchange with different chloroboranes. The reactions were very selective, and the target compounds were obtained in high purity and good to excellent yields. Complexation experiments of 1,8-bis[(diphenylboranyl)ethynyl]anthracene with nitrogen bases (pyridine, pyrimidine, TMEDA) afforded three stable adducts, which were structurally characterised by X-ray diffraction. Competition experiments demonstrated the selective exchange of guests, and quantum-chemical calculations provided information on their energetics. NMR experiments at low temperature gave insight into the dynamic behaviour of the TMEDA adduct.