Reactions of saturated NHC•boranes with electron deficient alkyne and alkene. Borirane formation, their reactivities, and coupling of tetracyanoethylene

Abstract

In this study, we describe the reactions of five- and six-membered saturated N-heterocyclic carbene boranes (NHC•BH3) with electron deficient alkene and alkyne. Notably, both five- and six-membered saturated NHCs form boriranes upon reaction of NHC•BH3 with dimethyl acetylene dicarboxylate (DMAD). However, their reactivity with HCl differs: the five-membered saturated NHC-stabilized borirane undergoes ring opening in toluene/THF, whereas the six-membered saturated NHC-stabilized borirane undergoes ring expansion, forming exclusively a boralactone irrespective of solvent. Interestingly, when acetonitrile is used as a solvent, the five-membered saturated NHC-stabilized borirane yields both ring-opening and ring-expansion products. To remove the hydrogen attached to the boron atom in boriranes, we reacted the five-membered saturated NHC-stabilized borirane with AlCl3 and B(C6F5)3, resulting in azaborolium cation or adduct formation, respectively. Transitioning from an electron-deficient alkyne to an alkene and reacting the six-membered saturated NHC•BH3 with tetracyanoethylene (TCNE) led to the unusual coupling of two TCNE molecules.