MECHANISM AND SOLVENT CATALYSIS OF THE DEGENERATE 1,12-METALATIONS OF[1.1]FERROCENOPHANYLLITHIUM AND [1.1]FERROCENOPHANYLSODIUM STUDIED BYNMR-SPECTROSCOPY

Insight into the detailed mechanism of carbon lithiation by an organol ithium reagent and of carbon sodiation by an organosodium reagent has been obtained using [1.1]ferrocenophanyllithium (1) and [1.1]ferroceno phanylsodium (3), respectively. In tetrahydrofuran (THF) 1 and 3 under go rapid 1,12-proton transfer reactions which are coupled with 1,12-li thium ion and 1,12-sodium ion transfers, respectively. It is concluded that the degenerate rearrangement of 1 does not make use of a pseudor otation mechanism, but occurs by direct conversion of a syn-conformer to another syn-conformer. Activation parameters (Delta H-double dagger = 19 kJ mol(-1) and Delta S-double dagger = -93 J K-1 mol(-1)) for th e degenerate reaction of 1 in THF have been measured by dynamic NMR sp ectroscopy. The primary isotope effect (k(H)/k(D)) Of the 1,12-hydron transfer is 7.4 +/- 1.5 at 320 K. The degenerate rearrangement of 1 sh ows strong solvent dependence, e.g, the reaction is 4 x 10(3) times fa ster in THF than in dimethyltetrahydrofuran (DMTHF). Thus, the rearran gement 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 solven t molecule more than in the initial state. It is paired with a carbani on in which the proton is symmetrically located between the bridge car bons. 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 t hat contains one solvent molecule more than the initial complex.