Five-membered zircona- and titanacycloallenes; their diverse reactivity depending upon the position of substituents

Five-membered metallacycloallene compounds that are 2,5-disubstituted 1-zirconacyclopenta-2,3-dienes were prepared from (E)-1,4-bis(trialkylsilyl)but-1-en-3-ynes in good yields. The structures of these compounds were determined by X-ray diffraction study. Contrary to our previous study on 2,4-disubstituted 1-zirconacyclopenta-2,3-dienes, which gave alkynyl alcohols by the reaction with ketones after hydrolysis, the 2,5-bis(trialkylsilyl) complexes reacted with ketones to afford allenyl alcohols. Reactions with a nitrile gave a pyrolle compound probably via the formation of seven-membered 1-aza-2-zirconacyclohepta-3,4,7-triene ring structures. When the (E)-1,4-bis(trialkylsilyl)but-1-en-3-ynes were treated with titanium isopropoxide/n-butyllithium followed by reaction with nitriles, the corresponding alkynyl ketones were obtained.