Conventional and microwave synthesis of 2-fluoro-diazaborolidines and their reaction with lithium phosphanides

Condensation of N,N'-disubstituted ethylenediamines with BF3 center dot OEt2, in the presence or absence of an auxiliary base, gives mixtures of 2-fluoro-1,3,2-diazaborolidines and ammonium tetrafluoroborates, respectively. Using BF3 center dot NEt3 as the reactant allows the introduction of the boron source and the auxiliary in a single component, but suffers from the inhibition of the cyclisation by an excess of free amine formed as a by-product. In contrast, rapid and quantitative consumption of the starting materials is observed when the reaction is carried out with a 2:1 mixture of BF3 center dot NEt3 and BF3 center dot OEt2 per mol of ethylenediamine at elevated temperature. Extremely short reaction times are achieved by conducting the reaction in a superheated solution in a microwave reactor. The 2-fluoro-1,3,2-diazaborolidines formed under these conditions are readily isolated in high yields, and their synthetic usability is demonstrated by reactions with lithium phosphanides to give 2-phosphanyl-1,3,2-diazaborolidines. Both the F- and R2P-substituted N-heterocyclic boranes are fully characterised. In addition, the structural characterisation of an unprecedented BF3 complex of Hunig's base (iPr(2)EtN) and of a 1,3,2-diazaborolidine-BF3 complex is reported.