R. Beckhaus , J. Sang, U. Englert, U. Böhme
Organometallics 1996, 15, 4731-4736
The titanocene vinylidene intermediate [Cp*2Ti=C=CH2] (6) (Cp*: C5(CH3)5), formed by ethene or methane elimination from Cp*2Ti-CH2CH2-C=CH2 (5) and Cp*2Ti(CH=CH2)(CH3) (7), respectively, reacts with 1,3-diynes R-CC-CC-R (R = Me 13a, Ph 13b, SiMe3 13c, CMe3 13d) by [2+2]-cycloaddition, giving metallacyclobutenes Cp*2Ti-C(CC-R)=CR-C=CH2 (9a-9d). Using the unsymmetrical 1,3-diyne R-CC-CC-R1 (R = CMe3, R1 = SiMe3 13e), a 9:1 mixture of the metallacyclobutenes Cp*2Ti-C(CC-R)=CR1-C=CH2 (9e) and Cp*2Ti-C(CC-R1)=CR-C=CH2 (9e') is formed. In all cases, exclusive formation of one regioisomer exhibiting the CC-R-substituent in the a-position of the metallacycle is observed. The regioselectivity of the formation of 9 is attributed to the polarity of the CC-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.