Allylic boranes are chemist’s best friends: Reactivity, applications, new opportunities

Halogenoborane mediated allene cyclooligomerization

“…The parent allene, propadiene, has been cyclotrimerized at a variety of transition metal catalysts, but this usually gave predominately 1,2,4-trimethylenecyclohexane and only minor amounts of the 1,3,5-trimethylenecyclohexane isomer 1 2,3. Compound 1 had been synthesized in stoichiometric reaction sequences 4. We recently developed the metal-free, HB(C 6 F 5 ) 2 catalysed formation of isomerically pure 1 by allene cyclotrimerization under mild conditions 5.…”

“…We have started to use the allene cyclotrimerization products 1 and 14 as the starting materials for the conversion to the respective arene isomers. It is known that 1,3,5-trimethylene-cyclohexane is resistant to thermal isomerization, but it was reported that it could be isomerized by treatment with acid 2a,13. We repeated this reaction: treatment of 1 with 5 mol% of p -toluene sulfonic acid in toluene solution at r.t. for 1.5 h cleanly converted 1 to mesitylene (Scheme 5).…”