Structure, Reactivity, and Electronic Properties of [4]Ferrocenophanes and [4]Ruthenocenophanes Prepared via a Novel Heteroannular Cyclization Reaction

The reactions of 1,1′-bis((trimethylsilyl)ethynyl)ferrocene and 1,1′-bis((trimethylsilyl)-ethynyl)ruthenocene with catalytic quantities of alkali-metal methoxides in methanol directly afforded the highly unsaturated [4]metallocenophanes 1,1′-(1-methoxy-1,3-butadienylene)-ferrocene and 1,1′-(1-methoxy-1,3-butadienylene)ruthenocene, respectively, in high yields via a novel desilylation/heteroannular cyclization sequence. Analogously, 1,1′-bis((trimethylsilyl) ethynyl)octamethylferrocene reacted to give 1,1′-(1-methoxy-1,3-butadienylene)octamethylferrocene in high yield. The reactions of. ((trimethylsilyl)ethynyl)ferrocene and ((trimethylsilyl)ethynyl)ruthenocene under identical conditions afforded ethynylferrocene and ethynylruthenocene, respectively. Synthetic elaboration of the heteroannular bridge of the cyclization products provided a route to additional [4]metallocenophanes. Treatment of 1,1′-(1-methoxy-1,3-butadienylene)ferrocene with acidic silica gel afforded 1,1′-(4-oxo-1-butenylene) ferrocene. Reaction of 1, 1′-(4-oxo-1-butenylene)ferrocene with alane provided 1,1′-(1-butenylene)ferrocene, while reaction with sodium borohydride gave 1,1′-(4-hydroxyl-butenylene)ferrocene. Dehydration of 1,1′-(4-hydroxy-1-butenylene)ferrocene on activated alumina provided 1,1′-(1,3-butadienylene)ferrocene. Similar synthetic transformations were carried out to yield the analogous series of ruthenocenophanes and octamethylferrocenophanes. Voltammetric half-wave oxidation potentials were measured for all of the metallocenophanes in order to evaluate the electronic effect of the heteroannular bridges. X-ray crystal structure analyses were carried out on 1,1′-(1-methoxy-1,3-butadienylene)- ferrocene and 1,1′-(1-methoxy-1,3-butadienylene)ruthenocene. 1,1′-(1-Methoxy-1,3-butadienylene) ferrocene, C₁₅H₁₄FeO, crystallized in the orthorhombic space group Pcnb with a = 26.997(5) Å, b = 5.981(2) Å, c = 28.962(3) Å, Z = 16, and R = 0.072. 1,1′-(1-Methoxy-1,3-butadienylene)ruthenocene, C₁₅H₁₄RuO, crystallized in the monoclinic C2/c space group with a = 20.590(3) Å, b = 9.023(2) Å, c = 13.940(2) Å, β = 111.296(8)°, Z = 8, and R = 0.021.