Quantitative Determination of the Regioselectivity of Nucleophilic Addition to eta(3)-Propargyl Rhenium Complexes and Direct Observation of an Equilibrium between eta(3)-Propargyl Rhenium Complexes and Rhenacyclobutenes

摘要

PMe3 adds selectively to the central carbon of the eta(3)-propargyl complex [C5MC5(CO)(2)Re(eta(3)- CH,C CCMe3)1[BF4] (1-t-Bu) to form the metallacyclobutene [C5Me5(CO)(2)Re(CH2C(PMe)= CCMe3)][BF4] (7). The rate of rearrangement of the metallacyclobutene 7 to eta(2)-alkyne complex [C5Me5(CO)(2)Re(eta(2)-Me3PCH2C CCMe3)][BF4] (8) is independent of phosphine concentration, consistent with a dissociative mechanism proceeding via eta(3)-propargy] complex 1-t-Bu. The rate of this rearrangement is 480 times slower than the rate of exchange of PMe3 with the labeled metal lacyclobutene 749. This rate ratio provides an indirect measurement of the regioselectivity for addition of PMe3 to the central carbon of eta(3)-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 metal lacyclobutene [C5Me5(CO)(2)Re(CH2C(PPh3)=CCMe3)][BF4] (11). Low-ternperature H-1 NMR spectra provide evidence for an equilibrium between metal lacyclobutene 11 and eta(3)-propargyl complex 1-t-Bu (K-eq approximate to 44 M-1 at -46 degrees C and Delta G degrees(0 degrees C) = -1.2 +/- 0.2 kcal mol(-1)).