Conventional and Microwave Synthesis of 2-Fluoro-diazaborolidines and Their Reaction with Lithium Phosphanides

4 years ago

Conventional and Microwave Synthesis of 2-Fluoro-diazaborolidines and Their Reaction with Lithium Phosphanides

Dietrich Gudat, Christian Bäucker, Max Deimling, Manuel Kaaz, Martin Nieger, Johannes Bender, Simon König, Simon H. Schlindwein

Condensation of N,N′-disubstituted ethylenediamines with BF3·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·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·NEt3 and BF3·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 Hünig's base (iPr2EtN) and of a 1,3,2-diazaborolidine–BF3 complex is reported. 2-Fluoro-1,3,2-diazaborolidines are prepared in one step from amines and BF3·OEt2/BF3·NEt3 by conventional and microwave-assisted syntheses, and their synthetic use is illustrated by their conversion into 2-phosphanyl-diazaborolidines.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/ejic.201700559