2,5‐Dihydro‐1,2,5‐azoniasilaboratole Derivatives – Useful Starting Materials in Heterocyclic Synthesis

1‐Alkynyl(diethylamino)dimethylsilanes [CC̊: R = Me ( 1a ), n Bu ( 1b ), SiMe 3 ( 1c )] react with triethylborane stereoselectively by 1,1‐ethyloboration to give the alkenes with the boryl and silyl group in cis ‐positions at the C=C bond. Owing to the strongly intramolecular coordinative N‐B bond, these products are 2,5‐dihydro‐1,2,5‐azoniasilaboratoles ( 2a‐c ). Protic reagents such as azoles (indole, pyrazole, imidazole, triazole, indazole, benzotriazole) react with 2 to give diethylamine and the respective N ‐azolyl derivatives 3–8 which contain tetracoordinate boron in the case of 4–8 . Compound 7 , derived from indazole, was characterized by an X‐ray analysis. With ethanol, the corresponding 2,5‐dihydro‐1,2,5‐oxoniasilaboratoles 9 are obtained. Treatment of 2 with water affords 1,2,5‐oxasilaborolanes 11 , presumably via an intermediate 10 with the structure of a 2,5‐dihydro‐1,2,5‐oxoniasilaboratole. All products were characterized by their 1 H‐, 11 B‐, 13 C‐, 15 N‐, and 29 Si‐NMR data.