Activation of an alkyne: the preparation, structure, and reactivity of an acetylenic amidinium ion derived from an N-heterocyclic carbene

Abstract

(Bromoethynyl)benzene dissolved in hexane reacts with 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) to produce the acetylenic amidinium salt [Ph-C≡C-IMes][Br]. Treatment of the bromide saltwithKPF6 results in the isolation of solid [Ph-C≡C-IMes][PF6].Both salts have been characterized spectroscopically and their structures determined using X-ray crystallography. Crystals of a hydrolysis product were also isolated from this reaction on one occasion and its structure is reported. This led us to explore the reactivity of the bromide salt further. It was found to react rapidly with sodium azide (NaN3), azidotrimethylsilane ((CH3)3Si-N3), cyanotrimethylsilane ((CH3)3Si-CN), and also, surprisingly, with IMes. All of the addition products have been characterized using X-ray crystallography. The results provide insight into the reactivity of acetylenic amidinium cations.