Mechanism and Solvent Catalysis of the Degenerate 1,12-Metalations of [1.1]Ferrocenophanyllithium and [1.1]Ferrocenophanylsodium Studied by NMR Spectroscopy

Abstract

Insight into the detailed mechanism of carbon lithiation by an organolithium reagent and of carbon sodiation by an organosodium reagent has been obtained using [1.1]ferrocenophanyllithium (1) and [1.1]ferrocenophanylsodium (3), respectively. In tetrahydrofuran (THF) 1 and 3 undergo rapid 1,12-proton transfer reactions which are coupled with 1,12-lithium ion and 1,12-sodium ion transfers, respectively. It is concluded that the degenerate rearrangement of 1 does not make use of a pseudorotation mechanism, but occurs by direct conversion of a syn-conformer to another syn-conformer. Activation parameters (ΔH⧧ = 19 kJ mol-1 and ΔS⧧ = −93 J K-1 mol-1) for the degenerate reaction of 1 in THF have been measured by dynamic NMR spectroscopy. The primary isotope effect (kH/kD) of the 1,12-hydron transfer is 7.4 ± 1.5 at 320 K. The degenerate rearrangement of 1 shows strong solvent dependence, e.g. the reaction is 4 × 103 times faster in THF than in dimethyltetrahydrofuran (DMTHF). Thus, the rearrangement may be catalyzed by THF in DMTHF. The catalysis is first order in THF at low concentrations of THF. The results show that in the rate-limiting activated complex the lithium ion is coordinating one solvent molecule more than in the initial state. It is paired with a carbanion in which the proton is symmetrically located between the bridge carbons. Compound 3 shows a behavior similar to that of 1, but it is more fluxtional. It also shows a solvent catalysis that is weaker than for 1. It is concluded that 3 also exchanges using an activated complex that contains one solvent molecule more than the initial complex.