Regioselective reactions of 1,3-diynes with the titanocene vinylidene fragment [Cp*Ti-2=C=CH2]. Crystal and molecular structure of Cp*2TiC(C CSiMe(3))=C(SiMe(3))C=CH2

The titanocene vinylidene intermediate [Cp*Ti-2=C=CH2] (6)(Cp*: C-5(CH3)5), formed by ethene or methane elimination from Cp*2TiCH2CH2C=CH2 (5) and Cp*Ti-2(CH=CH2)(CH3) (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*2TiC(C=CR)=CRC=CH2 (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*2TiC(C=CR)=CR(1)C=CH2 (9e) and Cp*2TiC-(C=CR(1))=CRC =CH2 (9e') is formed. In all cases, exclusive formation of one regioisomer exhibiting the C=CR substituent in the alpha-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.