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

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 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-d 9 . 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).