2-lithio-3-methoxy-1,3-dimethylcyclopentene: A synthetic equivalent of 2-lithio-1,3-dimethylcyclopentadiene and useful synthon for 6-alkyl-1,4-dimethylfulvenes and ansa-metallocene complexes

摘要

2-Bromo-3-methoxy-1,3-dimethylcyclopentene (3) is synthesized in 85% yield in 80 g scale by 1,2-addition of MeLi to 2-bromo-3-methyl-2-cyclopenten-1-one and subsequent in situ treatment of MeI. Addition of n-BuLi to the bromo-compound 3 in Et2O affords 2-lithio-3-methoxy-1,3-dimethylcyclopentene (1). Successive addition of RCHO [R = CH3. (CH2)(2)CH3, CH(CH3)(2), C(CH3)(3), H] and Met to the lithium compound and aqueous acidic work-up provide cyclopentadiene compounds, 2-[CHR(OMe)]-1,3Me(2)C(5)H(3) in 60-84% yields. Treatment of the cyclopentadienes with NaH or KH in pentane affords 1,4,6-trimethylfulvene, 6-n-propyl-1,4-dimethylfulvene, 6-isopropyl-1,4-dimethylfulvene, 6-tert-butyl-1,4-dimethylfulvene and 1,4-dimethylfulvene in 67-83% yields. Ligand for an ansa-metallocene complex, CH2C(1,3- Me2Cp)(2)ZrCl2, can be shortly afforded in 64% yield by reacting the lithium compound I with the newly synthesized 1,4-dimethylfulvene.