Evidence for a Rapid Degenerate Hetero-Cope-Type Rearrangement in [CpW(S)2S-CH2-CHCH2]

Treatment of the salt [PPh4]+[Cp*W(S)3]− (6) with allyl bromide gave the neutral complex [Cp*W(S)2S‐CH2‐CHCH2] (7). The product 7 was characterized by an X‐ray crystal structure analysis. Complex 7 features dynamic NMR spectra that indicate a rapid allyl automerization process. From the analysis of the temperature‐dependent NMR spectra a Gibbs activation energy of ΔG≠ (278 K)≈13.7±0.1 kcal mol−1 was obtained [ΔH≠≈10.4±0.1 kcal mol−1; ΔS≠≈−11.4 cal mol−1 K−1]. The DFT calculation identified an energetically unfavorable four‐membered transition state of the “forbidden” reaction and a favorable six‐membered transition state of the “Cope‐type” allyl rearrangement process at this transition‐metal complex core. Allyl automerization: The title complex undergoes rapid allyl automerization, proceeding via a six‐membered transition state, as identified by DFT calculations. A detailed temperature‐dependent NMR study provides a Gibbs activation energy of about 14 kcal mol−1.