Quantitative Determination of the Regioselectivity of Nucleophilic Addition to η3-Propargyl Rhenium Complexes and Direct Observation of an Equilibrium Between η3-Propargyl Rhenium Complexes and Rhenacyclobutenes

摘要

PMe3 adds selectively to the central carbon of the η³– propargyl complex [C5Me5(CO)2Re(η³-CH2C≡CCMe3)][BF4] >(1-t-Bu) to form the metallacyclobutene [C5Me5(CO)2Re(CH2C(PMe3)=CCMe3)][BF4] >(7). The rate of rearrangement of the metallacyclobutene >7 to η²-alkyne complex [C5Me5(CO)₂Re(η²-Me₃PCH₂C≡CCMe₃)][BF₄] >(8) is is independent of phosphine concentration, consistent with a dissociative mechanism proceeding via η³-propargyl complex >1-t-Bu. The rate of this rearrangement is 480 times slower than the rate of exchange of PMe₃ with the labeled metallacyclobutene >7-d₉. This rate ratio provides an indirect measurement of the regioselectivity for addition of PMe₃ to the central carbon of η³–propargyl complex >1-t-Bu to give >7 compared to addition to a terminal carbon to give >8. The addition of PPh₃ to >1-t-Bu gives the metallacyclobutene [C₅Me₅(CO)₂Re(CH₂C(PPh₃)=CCMe₃)][BF₄] >(11). Low temperature ¹H NMR spectra provide evidence for an equilibrium between metallacyclobutene >11 and η³-propargyl complex >1-t-Bu (K_eq ≈ 44 M⁻¹ at −46 °C and ΔG° (0 °C) = −1.2 ± 0.2 kcal mol⁻¹).