Sitting Out the Halogen Dance. Room-Temperature Formation of 2,2′-Dilithio-1,1′-dibromoferrocene. TMEDA and Related Lithium Complexes: A Synthetic Route to Multiply Substituted Ferrocenes

The clean room-temperature synthesis of 2,2′-dilithio-1,1′-dibromoferrocene from 1,1′-dibromoferrocene is reported. When this dilithium compound is quenched with electrophiles, the synthesis of 2,2′-disubstituted-1,1′-dibromoferrocene is facilitated. For example, quenching with 1,2-dibromohexafluoropropane as an electrophile precursor gives 1,1′,2,2′-tetrabromoferrocene in high yield. The similar dilithiation reaction of 1,1′,2,2′-tetrabromoferrocene produces 3,3′-dilithio-1,1′,2,2′-tetrabromoferrocene, which in turn furnishes 1,1′,2,2′,3,3′-hexabromoferrocene again in high yield. Essentially the bromines are added in pairs beginning with the readily available 1,1′-dibromoferrocene. All 2,2′-dihalogeno-1,1′-dibromoferrocenes have been obtained and characterized. The reaction sequence when it is continued in an iterative fashion should ultimately afford decabromoferrocene; however, highly brominated products such as octabromoferrocene, nonabromoferrocene, and decabromoferrocene are not isolated cleanly because of their poorer solubility, as the synthetic method has been optimized in nonpolar solvents. Just as 1,1′-dibromoferrocene has played an important role in the broader synthesis of other ferrocenes, it is fully expected that 1,1′,2,2′-tetrabromoferrocene and 1,1′,2,2′,3,3′-hexabromoferrrocene will play similar roles.