Alternative reaction pathways in domino reactions of hydrazinediidozirconium complexes with alkynes

Reaction of [Zr{(NAr)(2)N(py)}(NMe(2))(2)] (Ar=3,5-xylyl: 2 a, mesityl: 2 b) with one or two molar equivalents of 1,1-diphenylhydrazine gave the mixed amido/hydrazido(1-) complex [Zr{(NMes)(2)N(py)}(HNNPh(2))(NMe(2))] (3), the bis-hydrazido complex [Zr{(NMes)(2)N(py)}(HNNPh(2))(2)] (4), and, in the presence of excess 4-dimethylaminopyridine (DMAP), hexacoordinate hydrazinediidozirconium complexes [Zr{(NXyl)(2)N(py)}(=NN(Me)Ph)(dmap)(2)] (5) and [Zr{(NXyl)(2)N(py)}(=NNPh(2))(dmap)(2)] (6). The reaction of one equivalent of the zirconium-hydrazinediide [Zr{(NTBS)(2)N(py)}(NNPh(2))(py)] (1) with disubstituted alkynes at RT for 16 h led to the formation of seven-membered diazazirconacycles 7 a-7 e in high yields. Similar reactivity was observed by reacting bis-amido complex 2 b with one molar equivalent of the corresponding alkyne and diphenylhydrazine. The formation of the seven-membered zirconacycles implied a key coupling step that involved the alkyne and one of the aryl rings of the diphenylhydrazinediido ligand. In some cases, such as the reaction with 2-butyne, the corresponding metallacycle was only obtained in modest yields (45 % for the reaction with 2-butyne) and a second major product, vinylimido complex 9, was formed in almost equal amounts (42 %) by 1,2-amination (formal insertion of the alkyne). The formation of compounds 7 a and 9 followed in part the same sequence of reaction steps and a key intermediate, an azirinido complex, represented a "bifurcation point" in the reaction network. Reaction of 1.2 equivalents of several diarylhydrazines and various substituted alkynes (1 equiv) at ambient temperature (or at 80 °C) in the presence of 10 mol % [Zr{(NXyl)(2)N(py)}(NMe(2))(2)] (2 a) gave the corresponding indole derivatives. On the other hand, the replacement of 1,1-diarylhydrazines by 1-methyl-1-phenyl hydrazine led to head-to-head cis-1,3-enynes in good yields.