An air-stable bisboron complex: a practical bidentate Lewis acid catalyst

We report an air-stable bisboron complex as an efficient catalyst for the inverse electron-demand Diels–Alder (IEDDA) reaction of 1,2-diazine as well as 1,2,4,5-tetrazine. Its stability towards air and moisture was demonstrated by NMR studies enabling its application in organic transformations without glovebox. A one-pot procedure for its synthesis was developed starting from 1,2-bis(trimethylsilyl)benzene greatly enhancing its practicality. Comparative reactions were carried out to evaluate its catalytic activity in IEDDA reactions of diazine including phthalazine as well as 1,2,4,5-tetrazine.


General information
All solvents were purchased from Fisher Scientific, Sigma-Aldrich or Acros and were used as received. Amines were distilled over calcium hydride before used. Aldehydes were distilled and stored over 4 Å molecular sieves. Technical grade solvents for extraction and column chromatography were bulb-to-bulb distilled prior to usage.

IEDDA reactions catalyzed by the air-stable bidentate Lewis acid catalyst B
General procedure A for IEDDA reactions of phthalazine: In a Schlenk tube charged with a stirring bar, the air-stable bidentate Lewis acid catalyst B (5.00 mol %) and the stated solvent were added under N 2 . Then, the phthalazine (1.00 equiv), dienophile (2.00 equiv; for enamines, generated in situ from aldehyde and amine) were added subsequently. The reaction mixture was stirred at the given temperature.
After the reaction was finished, the solvent was removed. The remaining residue was purified by flash column chromatography over SiO 2 to obtain the product.
Spectroscopic data for the title compound was consistent with the literature. Spectroscopic data for the title compound was consistent with the literature.

X-ray crystallography
All non-hydrogen atoms could be refined with anisotropic displacement parameters. EADP constraints were used for the refinement of some of the ethylacetate molecules. All hydrogen atoms could be refined using AFIX codes of SHELXL. The program Diamond was used for graphical representations [14]. The CCDC reference number is 1567302.