Metallation of a variety of α,ω-dienes has been explored with an η 9,η 5-bis(indenyl)zirconium sandwich compound and an ansa-titanocene dinitrogen complex. The η 9, η 5-bis(indenyl)zirconium sandwich compound, (η 9- C 9H 5-1,3-Pr 2)(η 5-C 9H 5-1,3- iPr 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 °C, respectively. By contrast, substitution of the methylene or ethylene spacer in the α, ω-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 2Si(η 5-C 5Me 4)(η 5-C 5H 3-3- tBu)Ti] 2(μ 2, η 1,η 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.
All Science Journal Classification (ASJC) codes
- Inorganic Chemistry