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

Abstract

1-Alkynyl(diethy1amino)dimethylsilanes [C=CR: R = Me (la), nBu (lb), SiMe, ( l c ) ] react with triethylborane stereoselectively by 1,l-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 'H-, '*B-, 13C-, 15N-, and 2gSi-NMR data.Heterocycles containing reactive silicon-and boron-element bonds are of considerable interest for further synthetic use. In organotin chemistry, 1,1-organoboration of 1 -alkynyltin compounds proved to be extremely useful in heterocyclic syntheses"], whereas similar strategies in organosilicon chemistry have so far been demonstrated only for four types of heterocyclic systems, namely 1,2-dihydro-1,2,5-disilaborepine~[~], ~iloles[~-~], 1,l '-spirobisiloles[61, and recently for the synthesis of heterobicyclic boron com-pound~[~].The synthesis of 2,5-dihydro-l,2,5-azoniasilaboratoles of the type 2 was described previously by the route shown in eq. (1)[31. In this paper we report on a more direct way [eq.(3)], using 1,1 -organoboration, by starting from l-alkynyl-(diethy1amino)dimethylsilanes 1 and triethylborane, Et3B. The compounds 1 were prepared in high yield according to eq. (2), whereas in the literature differing routes such as the aminolysis of 1 -alkynylchlorodimethylsilanes[sl or 1 -alkynyl(trifluorosulfonato)dimethylsilane[9] are described.The synthetic potential of the heterocycles 2 was tested in reactions with some protic reagents such as various azoles, ethanol, and water. Results and Discussion Preparative ResultsThe 1-alkynyl(diethy1amino)dimethylsilanes 1 are colorless liquids, easy to purify by distillation, but sensitive to moisture. In contrast to the corresponding tin comthey are not in equilibrium with the symmetrically substituted silanes, even at a temperature of + HCI J(c)-NaCl I i a i b i : R Me "Bu SiMe, ,NEt2 ENf2 + Et,B I_) Me2SI(;BEt, (3) \ Me,Si RnEt \ \ \R 2 a, b, c I a, b, c 100-126°C. The reaction of 1 with Et,B proceeds slowly (several days) at 100°C, either in pure Et3B or in toluene, and gives the compounds 2 in high yield [eq. (3)].The 2,5-dihydro-l,2,5-azoniasilaboratoles 2 are colorless liquids and can be distilled at reduced pressure. They are sensitive to traces of moisture, but can be stored for several

1,1‐Ethylboration of alkyn‐1‐yl‐chloro(methyl)silanes: alkenes with chloro(methyl)silyl and diethylboryl groups in cis positions

“…8 Alkenes bearing organometallic substituents at the C C bond are attractive reagents for further transformations. Various methods have recently been reported for the preparation of alkenes with boryl and silyl substituents.…”

Intramolecular Reaction of Silanol and Triarylborane: Boron−Aryl Bond Cleavage and Formation of a Si−O−B Heterocyle

“…The intramolecular interaction between boryl groups and heteroatom-functionalized silyl groups has been precisely investigated by Wrackmeyer et al with multinuclear NMR spectroscopy . They prepared a number of compounds in which a heteroatom-functionalized silyl group and a boryl group were bonded to an alkene skeleton in a cis fashion and revealed that such compounds formed unique silicon−heteroatom−boron bridges.…”

Synthesis of 3,4,5‐Trisubstituted Isoxazoles via Sequential [3 + 2] Cycloaddition/Silicon‐Based Cross‐Coupling Reactions.

“…Data for this compound matched that from the literature. 6 In an oven dried a 35 mL 1-neck round-bottom flask was combined N,N-diethyl-1,1dimethyl-1-(1-propynyl)silanamine (6.54 g, 38.6 mmol, 1.0 equiv), methanol (2.35 mL, 57.9 mmol, 1.5 equiv) and Et 2 O (20 mL). This reaction was stirred for 1 h at room temperature and then purified by fractional distillation to afford 3.26 g (66%) of a clear, colorless liquid.…”