As for the catalyst precursor of this reaction, the low-valent Nb(III) complex, NbCl 3 (DME), is highly efficient. When the Ta(III) analog, TaCl 3 (DME), was used as a catalyst, preferential formations of the alkyne cyclotrimerization products (5a) (33% yield) as well as the dimerization product, 1,4-di-tert-butyl-1-buten-3-yne (9% yield), were observed, but the desired 1,3-cyclohexadiene adduct was not produced at all (entry 8). Other Nb(IV), Nb(V) and V(III) complexes such as ...
