Isolation and Reactivity of 1,4,2-Diazaborole

Abstract

An isolable 1,4,2-diazaborole derivative was synthesized and structurally characterized. X-ray diffraction analysis and computational studies revealed a delocalization of 6π-electrons over the BC2N2 five-membered ring, which thus indicates the aromatic property. The reactivity toward electrophiles such as MeOTf and selectfluor was also investigated.

Ring Expansion, Photoisomerization, and Retrocyclization of 1,4,2‐Diazaboroles

“…Although a plethora of RE, PI, and RC reactions have been documented, such reactions of aromatic B,N heterocycles have scarcely been described thus far. Recently, we reported the first isolation of a 1,4,2‐diazaborole . During the course of our continuous study of 1,4,2‐diazaborole chemistry, we discovered unprecedented reactivity of 1,4,2‐diazaboroles along with their skeletal deformation.…”

Ring Expansion, Photoisomerization, and Retrocyclization of 1,4,2‐Diazaboroles

“…Thep roposed mechanism for the formation of 7 is as follows (Scheme 3): Initially, 4 undergoes photoinduced retro-[2+ +3]-cycloaddition to generate 1,4,2-diazaborol-3-imine intermediate 8 along with ethylene.Subsequent 1,3-dipolar cycloaddition between 8 and ethylene occurs to form 7.W epostulated that the 1,3-dipolar cycloaddition step could possibly be prevented by installing ab ulkier group on the boron atom, which would allow detection of intermediate 8.T otest this hypothesis,wecarried out the photoreaction of 9,which bears abulky mesityl group on the Batom. [17] Upon irradiation of as olution of 9 in benzene,t he color of the solution turned to greenish yellow from pale yellow (Scheme 4). In the 1 HNMR spectrum, as inglet corresponding to ethylene was observed at 5.25 ppm (see Figure S22), whereas the 11 BNMR spectrum showed anew broad peak for product 10 around 48.6 ppm (see Figure S24), at significantly lower field than that of 9 (18.4 ppm).…”

“…[17] Treatment of the imino-Nheterocyclic carbene 1 with boron tribromide (2 equiv) in hexane at room temperature readily afforded compound 2 in 92 %yield (Scheme 1a). [17] Treatment of the imino-Nheterocyclic carbene 1 with boron tribromide (2 equiv) in hexane at room temperature readily afforded compound 2 in 92 %yield (Scheme 1a).…”

“…Recently,w er eported the first isolation of a1 ,4,2-diazaborole. [17] During the course of our continuous study of 1,4,2-diazaborole chemistry,w e discovered unprecedented reactivity of 1,4,2-diazaboroles along with their skeletal deformation. Herein, we show av ariety of transformations of the B,N-heterocyclic framework of 1,4,2-diazaborole derivatives through ring expansion, photoisomerization, and retrocyclization reactions.…”

“…To begin ar eactivity study of 1,4,2-diazaboroles,w e attempted to synthesize an ew 1,4,2-diazaborole by the reported synthetic procedure. [17] Treatment of the imino-Nheterocyclic carbene 1 with boron tribromide (2 equiv) in hexane at room temperature readily afforded compound 2 in 92 %yield (Scheme 1a). Next, 2 was reduced with potassium graphite (KC 8 ;5equiv) in toluene.A fter workup,aB,Ndihydroindole derivative 3 containing two boron atoms was obtained as aw hite solid in 72 %y ield, and was fully characterized by the standard spectroscopic means and Xray diffraction analysis.I nt he 11 BNMR spectrum of 3, ab road signal due to the overlap of two peaks appeared at 29 ppm, at significantly lower field than that (À7.28 ppm) of the cationic part of 2.The solid-state molecular structure of 3 revealed the formation of as ix-membered ring by the incorporation of an additional boron atom into the carbene ring of 2 ( Figure 1a).…”