Reaction of phenylacetylene with (butadiene)tantalocene cation

Treatment of the (s-trans-η4-butadiene)tantalocene cation complex 3 (with [CH3B(C6F5)3−] anion) with phenylacetylene results in the formation of the regioisomeric alkyne–butadiene coupling products 4a (featuring the phenyl substituent in the β-position) and 4b (Ph at the α-carbon) in a 70:30 ratio. The minor insertion product 4b was characterized by an X-ray crystal structure analysis.