Tetrafluoroborate anion B-F bond activation -: unusual formation of a nucleophilic heterocyclic carbene:BF3 adduct

Tetrafluoroborate anion boron-fluorine bond activation, yielding a boron trifluoride-nucleophilic heterocyclic carbene (NHC) adduct. is reported. Bis-[2-(2- (trifluoroborane)-3-methylimidazolin-2-yliden-1-yl)etliyl]ether (4) was obtained as a by-product from the reaction of di-silver-bis-[bis-(2-(3-methylimidazolin-2-ylideii-1-yl)ethyl)ether] di-tetrafluoroborate (3) and ZrCl4. The Ag-1(NHC)(BF4) complex was prepared from bis-(2-(3-methylimidazolium-1-yl)ethyl)ether diiodide (2) and silver(l) oxide; spectroscopic data suggests that it is a dinuclear species. Compounds 2-4 were spectroscopically characterised, the structures of the NHC:BF3 adduct and parent bis-imidazolium salt being confirmed by single crystal X-ray studies. The geometry of 4 is similar to that of previously reported NHC:BF3 adducts. Heating the NHC:BF3 adduct in dimethyl-d(6) sulfoxide (d(6)-DMSO) in the presence of CsF leads to complete hydrogen/deuterium substitution of the C-im(4.5) positions. It was found that the H/D substitution was readily effected at room temperature on the free carbene bis-(2-(3-methylimidazolin-2-yliden-1-yl)ethyl)ether. Prolonged heating was required to observe a similar H/D substitution in 3, preliminary to its decomposition, and (2-(3-methylimidazolium-1-yl)ethyl)ether tetrafluoroborate was found to be resistant to substitution. (C) 2003 Elsevier B.V. All rights reserved.