Cyclizations of 1,2-Bis(pinacolylboryl)acetylene at Zirconocene

In the reaction of Cp2Zr(py)((2)-Me3SiC2SiMe3) (1Zr) (Cp = (5)-cyclopentadienyl) with the bis-BPin-substituted alkyne (2) (BPin = pinacolylboryl = 4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl) via the zirconacyclopropene Cp2Zr(py)((2)-PinBC(2)BPin) (3), or alternatively by the reduction of Cp2ZrCl2 with magnesium in the presence of two equivalents of 2, the zirconacyclopentadiene Cp2Zr[C-4(BPin)(4)] (4) was obtained. It was of interest to see how substituents influence the special side reactions of metallacyclopentadienes and if those would limit their synthetic application. Complex 4 reacts with (Ph3P)(2)NiCl2 and 3-hexyne to give the hexasubstituted benzene 1,2-ethyl-3,4,5,6-pinacolylboryl-benzene (6). During this procedure the heterometallic ferrol-type complex CpZr[CpNiC4(BPin)(4)]Cl-2 (7) is observed as a side product. In this complex, one Cp ligand moves from Zr to Ni, from which the Ph3P ligands are eliminated. The nickelacyclopentadiene is (4)-bonded to the vacant site of the former zirconocendichloride. The formation of 7 is connected with the inhibition for the formation of 6. Compounds 7 and 6 were characterized by NMR spectroscopy and X-ray crystal structure analysis.