Selective propargylic C(sp)-H activation of methyl-substituted alkynes [2 + 2] cycloaddition at a titanium imido template.

The reaction of the titanium imido complex with 2-butyne leads to the formation of the titanium azadiene complex at ambient temperature instead of yielding the archetypical [2 + 2] cycloaddition product (titanaazacyclobutene) which is usually obtained by combining titanium imido complexes and internal alkynes. The formation of is presumably caused by an initial propargylic C(sp)-H activation step and quantum chemical calculations suggest that the outcome of this unexpected reactivity is thermodynamically favored. The previously reported titanaazacyclobutene (which is obtained by reacting with 1-phenyl-1-propyne) undergoes a rearrangement reaction at elevated temperature to give the corresponding five-membered titanium azadiene complex .