Isolation and structural characterization of 1-zirconacyclopent-3-yne, five-membered cyclic alkynes.

Stable five-membered cyclic alkynes were synthesized, isolated, and fully characterized. Divalent zirconium species, bis(cyclopentadienyl)zirconium(II) equivalent, reacted with (Z)-1,4-disubstituted 1,2,3-butatrienes [(Z)-R-CH=C=C=CH-R, 1a: R = (CH3)3Si-, 1b: R = tert-C4H9-] to give 1-zirconacyclopent-3-yne compounds (2a-b) in good yields. X-ray diffraction analysis revealed their five-membered cyclic structure with a sufficiently short triple bond to regard this compound as a metallacyclopentyne.