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

PMe(3) adds selectively to the central carbon of the η(3)- propargyl complex [C(5)Me(5)(CO)(2)Re(η(3)-CH(2)C≡CCMe(3))][BF(4)] (1-t-Bu) to form the metallacyclobutene [C(5)Me(5)(CO)(2)Re(CH(2)C(PMe(3))=CCMe(3))][BF(4)] (7). The rate of rearrangement of the metallacyclobutene 7 to η(2)-alkyne complex [C(5)Me(5)(CO)(2)Re(η(2)-Me(3)PCH(2)C≡CCMe(3))][BF(4)] (8) is is independent of phosphine concentration, consistent with a dissociative mechanism proceeding via η(3)-propargyl complex 1-t-Bu. The rate of this rearrangement is 480 times slower than the rate of exchange of PMe(3) with the labeled metallacyclobutene 7-d(9). This rate ratio provides an indirect measurement of the regioselectivity for addition of PMe(3) to the central carbon of η(3)-propargyl complex 1-t-Bu to give 7 compared to addition to a terminal carbon to give 8. The addition of PPh(3) to 1-t-Bu gives the metallacyclobutene [C(5)Me(5)(CO)(2)Re(CH(2)C(PPh(3))=CCMe(3))][BF(4)] (11). Low temperature (1)H NMR spectra provide evidence for an equilibrium between metallacyclobutene 11 and η(3)-propargyl complex 1-t-Bu (K(eq) ≈ 44 M(-1) at -46 °C and ΔG° (0 °C) = -1.2 ± 0.2 kcal mol(-1)).