Regioselective Reactions of 1,3-Diynes with the Titanocene Vinylidene Fragment [Cp*2TiCCH2]. Crystal and Molecular Structure of Cp*2TiC(C⋮CSiMe3)C(SiMe3)CCH2

The titanocene vinylidene intermediate [Cp* 2 Ti C CH 2 ] ( 6 ) (Cp*: C 5 (CH 3 ) 5 ), formed by ethene or methane elimination from Cp* 2 TiCH 2 CH 2 C CH 2 ( 5 ) and Cp* 2 Ti(CH CH 2 )(CH 3 ) ( 7 ), respectively, reacts with 1,3-diynes RC⋮C-C⋮CR (R = Me ( 13a ), Ph ( 13b ), SiMe 3 ( 13c ), CMe 3 ( 13d )) by a [2 + 2]-cycloaddition, giving metallacyclobutenes Cp* 2 TiC(C⋮CR) CRC CH 2 ( 9a − d ). Using the unsymmetrical 1,3-diyne RC⋮CC⋮CR 1 (R = CMe 3, R 1 = SiMe 3 ( 13e )), a 9:1 mixture of the metallacyclobutenes Cp* 2 TiC(C⋮CR) CR 1 C CH 2 ( 9e ) and Cp* 2 TiC(C⋮CR 1 ) CRC CH 2 ( 9e ‘) is formed. In all cases, exclusive formation of one regioisomer exhibiting the C⋮CR substituent in the α-position of the metallacycle is observed. The regioselectivity of the formation of 9 is attributed to the polarity of the C⋮C bond as well as the stereochemical conditions, proven by ab initio calculations at the Hartree−Fock (HF) level of theory with the 3-21G(*) standard basis set and molecular modeling. The X-ray structure of 9c is presented. Subsequent reactions of 9 with an excess of the titanocene intermediate 6, to form binuclear metallacycles, are not observed.