The Reaction between Diazadienes and Element Tribromides EBr3 (E = P, B) Revisited: Metal‐Free Synthesis of Halogenated N‐Heterocyclic Phosphanes and Boranes

Reactions of selected diazadienes with PBr3 and BBr3 in the presence of a tertiary amine yielding N‐heterocyclic phosphanes (NHPs) and N‐heterocyclic boranes (NHBs) were studied. It is demonstrated that heterocycle formation occurs even without an amine or another auxiliary reagent, and the amine acts mainly as scavenger of Br2 formed as by‐product, but also that the additive speeds up reactions and has an influence on the product selectivity. In extension of the results of earlier studies it is shown that the reactions can follow two different pathways to give either 2‐bromo‐ or 2,4‐dibromo‐substituted heterocycles. Computational studies enabled to propose a reaction mechanism, which relates the observed behaviour to the availability of two competing reaction channels and provides also a rational explanation for the different behaviour of ECl3, EBr3, and EI3 (E = B, P) in cycloaddition reactions with diazadienes, which is empirically well established. In the borane series, a tribromo‐NHB is formed in a follow‐up reaction between the initial products and Br2, which seems to be the first example of an electrophilic backbone functionalization in an NHB. The 2‐bromo derivative is formed exclusively when PPh3 is used as auxiliary reagent. Selected products are isolated and fully characterised, proving the synthetic utility of the reactions studied.