Dimethylaminoalkyl and Methoxyalkyl Substituted Tetramethylcyclopentadienes: Synthesis of Novel Polydentate Ligands

The synthesis of new tetramethylcyclopentadienes with a further functionalized carbon side chain, X(CH₂)_nC₅Me₄H (X = Cl, NMe₂, OMe; n = 1, 2), is described. Treatment of 2-chloro-1-dimethylaminoethane with potassium tetramethylcyclopentadienide leads to the formation of a mixture of [2-(N,N-dimethylamino)-ethyl]tetramethylcyclopentadienes 3. Similarly, the reaction of 2-chloro-1-(p-toluenesulfonyl)ethane with lithium tetramethylcyclopentadienide yields a mixture of (2-chloroethyl) tetramethylcyclopentadienes 5. Due to geminal disubstitution, no isomers with an allylic hydrogen atom are present. Dicondensation of heteroatom-substituted esters, Me₂N(CH₂)_nCO₂R and MeO(CH₂)_nCO₂R (n = 1, 2), with 2-butenyllithium gives rise to the corresponding dienoles 8a-d. Subsequent acid catalysed dehydration and ring closure leads to various new functionalized tetramethylcyclopentadienes 9a-d in good yields. Contrary to 3 and 5, only isomers with allylic hydrogen atoms are obtained. Thus, these compounds are suitable candidates for the synthesis of peralkylated cyclopentadienyl systems possessing an N- or O-substituted side chain.