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

In the reaction of Cp 2 Zr(py)(η 2 ‐Me 3 SiC 2 SiMe 3 ) ( 1Zr ) (Cp = η 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 Cp 2 Zr(py)(η 2 ‐PinBC 2 BPin) ( 3 ), or alternatively by the reduction of Cp 2 ZrCl 2 with magnesium in the presence of two equivalents of 2 , the zirconacyclopentadiene Cp 2 Zr[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 (Ph 3 P) 2 NiCl 2 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[CpNiC 4 (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 Ph 3 P ligands are eliminated. The nickelacyclopentadiene is η 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.