Cyclisation of α,ω-dienes promoted by bis(indenyl)zirconium sandwich and ansa-titanocene dinitrogen complexes

Metallation of a variety of alpha,omega-dienes has been explored with an eta(9),eta(5)-bis(indenyl)zirconium sandwich compound and an ansa-titanocene dinitrogen complex. The eta(9),eta(5)-bis(indenyl)zirconium sandwich compound, (eta(9)-C(9)H(5)-1,3-Pr(2))(eta(5)-C(9)H(5)-1,3-(i)Pr(2))Zr, served as an isolable source of Negishi's reagent and readily formed a kinetic mixture of cis and trans diastereomers of the corresponding zirconacyclopentanes upon diene metallation. For pure hydrocarbon substrates such as 1,6-heptadiene and 1,7-octadiene, an equimolar amount of cis and trans diastereomers were the kinetic products; isomerization to the thermodynamically favoured trans isomers was observed over time at ambient temperature or upon heating to 105 degrees C, respectively. By contrast, substitution of the methylene or ethylene spacer in the alpha,omega-diene with a fluorenyl group (e.g. 9,9-diallylfluorene) resulted in exclusive kinetic formation of the trans diastereomer. Amino-substituted dienes were also readily cyclised and one example was characterised by single-crystal X-ray diffraction. Similar studies were also conducted with the ansa-titanocene dinitrogen complex, [Me(2)Si(eta(5)-C(5)Me(4))(eta(5)-C(5)H(3)-3-(t)Bu)Ti](2)(mu(2),eta(1),eta(1)-N(2)), and both kinetic and thermodynamic selectivities evaluated. The use of a C(1) symmetric ansa-metallocene increases the number of isomeric possibilities. For diallyl tert-butyl amine, diene metallation was more selective than for the bis(indenyl)zirconium sandwich compound and isomerization was also more rapid. Preliminary functionalisation reactivity for both the zircona- and titanocycles was also explored.