Regiocontrolled functionalization of seven-membered ring via nucleophilic cycloheptenyl-η3-allyltitanium complexes

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Abstract
A new "electron-reversed" method for the functionalization of a seven-membered ring is developed. Cycloheptenyl-η3-allyltitanocene complexes derived from cycloheptatriene react with aldehydes or carbon dioxide to afford, respectively, the mixture of isomeric 1,4- and 1,3-cycloheptadienyl carbinols 3a–e and 2a–e or methyl esters 7 and 6. The 3/2 and 7/6 ratios increase with increasing steric hindrance in the electrophile. Moreover, these may be significantly increased by introducing tert-butyl substituents on the Cp rings of the starting titanocene dichloride. The regiochemistry of the reaction was rationalized in terms of the equilibrium between the two envisageable allylic complexes. The reaction constitutes the first direct entry to functionalized 1,4-cycloheptadienes, some of which are related to biologically active compounds. Keywords: η3-allyltitanium complexes, cycloheptadiene derivatives, regiocontrol, electron reversal.