Synthesis of Complex Boron–Nitrogen Heterocycles Comprising Borylated Triazenes and Tetrazenes Under Mild Conditions

Abstract

The reactions of organic azides with diaryl­(dihalo)­diboranes(4) were explored, resulting in the observation of a number of surprising reactivity patterns. The reaction of phenyl azide with 1,2-diaryl-1,2-dihalodiboranes(4) resulted in the formation of five-membered rings comprising diboryl-triazenes with retention of the boron–boron bond, while the reaction of the peculiar 1,1-di­(9-anthryl)-2,2-difluorodiborane(4) with phenyl azide yielded a six-membered ring bearing a diboryl-triazene, whereby the B–B bond was ruptured by the insertion of an arylnitrene-like reactive intermediate. Both types of heterocycles feature unprecedented connectivity patterns and are very rare examples of boryl triazenes beyond the more common 1,2,3-triazolatoboranes. They are also the product of a unique type of aryl migration from a boron center to the phenyl azide γ-nitrogen center. Lastly, the substitution of 1,2-diaryl-1,2-dihalodiboranes(4) with azide groups, using trimethylsilyl azide as the transfer reagent, yielded boryl-tetrazaboroles and diboryldiazadiboretidines (as side-products), invoking the intermediacy of the first N-boryl-substituted iminoboranes, which are BN isosteres of monoborylated alkynes. The synthetic results are complemented with mechanistic proposals derived from quantum-chemical calculations.

Diborane(4) Azides: Surprisingly Stable Sources of Transient Iminoboranes

“…Our recent work on the attempted synthesis of 1,2(diaryl)‐diborane(4) azide derivatives indicates that such compounds tend to be short‐lived and spontaneously eliminate N 2 , giving rise to a chemistry that is best described as oligomerization of previously unknown boryl‐substituted iminoboranes (Figure ). Attempts at forming diborane(4) azides from tetrahalodiboranes(4) and their derivatives showed even less controllable decomposition reactions that have so far only yielded intractable mixtures, all of which are indicative of the breakage of the B−B bond to a large extent in the products (see Supporting Information).…”

“…Attempts at forming diborane(4) azides from tetrahalodiboranes(4) and their derivatives showed even less controllable decomposition reactions that have so far only yielded intractable mixtures, all of which are indicative of the breakage of the B−B bond to a large extent in the products (see Supporting Information). The predisposition of diborane(4) azides to form transient nitrene‐like species was hypothesized to be due to the intramolecular activation of the azide, by analogy with borane‐triggered decompositions of some azides . A reasonable strategy to hinder such intramolecular activation would be the saturation of the empty p orbital at boron through π‐donation, for example with amine functional groups.…”

“…These new species are the apparent dimerization products of transient, endocyclic iminoboranes. These iminoboranes are rare examples of borylated iminoboranes and the first examples of borylimino(amino)boranes. Indeed, while borepines are not uncommon, seven‐membered rings bearing the NBNBN sequence have, to our knowledge, never been reported, although related seven‐membered boron‐nitrogen‐containing heterocycles have been reported in the context of ring‐expansion reactions of boroles and in carbene‐diazadiborinine adducts .…”

“…In conclusion, we report the first isolable diborane(4) azides. Alongside our previous work, these results suggest that the formation of a nitrene‐like moiety and its insertion into a B−B bond tend to be the most accessible decomposition pathway of diborane(4) azides. The fact that temperatures well above 100 °C are required to decompose compounds 1 and 2 strongly supports our hypothesis that the electronic saturation of the boron centers in diborane(4) azides, provided in this study by amine substituents at boron, should lead to significant stabilization.…”

Diboran(4)azide als stabile Quelle für kurzlebige Iminoborane

“…Bereits in früheren Arbeiten haben wir versucht, Azid‐substituierte 1,2‐Diaryldiborane(4) zu generieren und zu isolieren. Es hat sich jedoch gezeigt, dass dieser Verbindungstyp sehr kurzlebig ist und zur spontanen Freisetzung von Stickstoff neigt.…”