The internal rotation of acyl groups in acylferrocenes. A 1H and 13C DNMR study

Abstract

The barriers to acyl group rotation in a series of acylferrocenes, including 1,1′-diacetylferrocene, have been determined by 1H and 13C DNMR. Complete lineshape analysis over wide temperature ranges gave ΔG ≠ 173 8.79 ± 0.03 kcal mol −1, ΔH ≠ 7.5 ± 0.3 kcal mol −1 and ΔS ≠ −8 ± 2 cal mol −1 K −1 for [ 2H]formlyferrocene and ΔG ≠ 173 8.51 ± 0.02 kcal mol −1, ΔH ≠ 7.1 ± 0.2 kcal mol −1 and ΔS ≠ −8 ± 2 cal mol −1 for acetylferrocene. The unexpected high barrier in diferrocenylketone (ΔG ≠ 143 6.8 ± 0.2 kcal mol −1 is interpreted in terms of strain in the transition state, and the relatively low barrier in 1,1′-diacetylferrocene (ΔG ≠ 163 7.8 ± 0.3 kcal mol −1) is ascribed to an interannular effect.