INSERTION OF ALKYNES WITH ELECTRON-WITHDRAWING SUBSTITUENTS INTO A RHENIUM-RHENIUM BOND - EVIDENCE FOR AN INTRAMOLECULAR INSERTION WITH A TRANSSTEREOCHEMISTRY

The reaction of the dirhenium complex Re2-(CO)9(NCMe) (1) with electrophilic alkynes, R'C=CCO2R, proceeds by displacement of the NCMe ligand and addition of the alkyne with insertion Into the rhenium-rhenium bond to yield the dimetalated olefin complexes Re(CO)4-[trans-mu-R'C=C(CO2R)]Re(CO)5 (R = Me, R' = H (2a), Me (2b); R = Et, R' = C02Et (2c)) having a trans stereochemistry at the C-C double bond. The carboxylate group is coordinated to one of the rhenium atoms to form a metallacycle. Insensitivity to radical scavengers and the absence of crossover in a reaction of a mixture of labeled and unlabeled reagent indicate that the insertion of the alkyne into the metal-metal bond occurs by an intramolecular process. Crystal data for 2a: space group P2(1)/n, a = 6.644 (1) angstrom, b = 30.005 (8) angstrom, c = 18.036 (2) angstrom, beta = 96.09 (1)-degrees, Z = 8, 2468 reflections, R = 0.033.