Formation of BN Isosteres of Azo Dyes by Ring Expansion of Boroles with Azides

Abstract

Herein, we present the results of our investigations on the effect of ortho substitution of aryl azides on the ring-expansion reaction of boroles, five-membered unsaturated boron heterocycles. These studies led to the isolation of the first 1,2-azaborinine-substituted azo dyes, which are bright yellow solids. One of the derivatives, (E)-2-mesityl-1-(mesityldiazenyl)-3,4,5,6-tetraphenyl-1,2-azaborinine, was found to be unstable in solution and to transform through a Jacobsen-like reaction into an indazole and 1-hydro-1,2-azaborinine. DFT calculations were performed to shed light on possible mechanisms to rationalize the unexpected azo-azaborinine formation and to draw conclusions about the role played by the ortho substituents in the reaction.

1,2‐Azaborine: The Boron‐Nitrogen Derivative of ortho‐Benzyne

“…[3] The similarity of carbon and its BN derivatives is well documented in chemistry, including the graphite/hexagonal boron nitride, and benzene/1,2-dihydro-1,2-azaborine pairs, including their Dewar forms. [4] However, evidence for the existence of BN-arynes is scarce and indirect as it comes from a self-trapping experiment. [5] Here we show by direct spectroscopic techniques that 1,2-azaborine can exist as reactive intermediate.…”

O,N,B-Containing eight-membered heterocycles by ring expansion of boroles with nitrones

“…1, II). 37 During the reaction of 1,2,3,4,5-pentaphenylborole (1) with Me 3 SiN 3 , Martin et al trapped an eight-membered ring system in which all of the three nitrogen atoms of the azide are incorporated into the borole ring ( Fig. 1, III).…”

Ring expansion reactions of anti-aromatic boroles: a promising synthetic avenue to unsaturated boracycles

“…Experimental and computational mechanistic investigations on the reaction of TMSN 3 and pentaphenylborole 1 revealed that the reaction is initiated by adduct formation via coordination of the α--nitrogen to the boron center (Int11). 52 The differing reactivity can be rationalized by the α--nitrogen acting as the nucleophilic site in the production of 18, whereas bulky groups prevent coordination via this nitrogen, and as a consequence, the γ--nitrogen coordinates to give Int13 that subsequently undergoes a 1,1--insertion reaction to yield the azobenzene analogues 21 and 22. 50 A higher energy pathway originating from Int12 was also observed, producing 1,2--azaborine 18 as the thermodynamic product and N 2 as a byproduct.…”