REACTIONS OF ALKYNES HAVING ELECTRON-WITHDRAWING SUBSTITUENTS WITH RE2(CO)9(NCME) - FORMATION OF TRANS DIMETALATED OLEFINS BY ALKYNE INSERTION INTO AN RE-RE BOND

The reaction of the dirhenium complex Re2(CO)9(NCMe), 1, with electrophilic alkynes, RC=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-RC=C(CO2R')]Re(CO)5, R = H, R'= Me, 2a, R = Me, R'= Me, 2b, and R = CO2Et, R' = Et, 2c, that have a trans stereochemistry at the C-C double bond. The carboxylate group is coordinated to one of the rhenium atoms through the ketonic oxygen atom to form a metallacycle. The reaction is insensitive to radical scavengers, solvent polarity, and light. The absence of crossover in a reaction of a mixture of labeled and unlabeled 1 is consistent with the insertion of the alkyne into the metal-metal bond by an intramolecular mechanism. Compound 2a was found to add CO (reversibly) to open the metallacycle and form the simple dimetalated olefin complex Re(CO)5[trans-mu-HC=C(CO2Me)]Re(CO)5, 3. When irradiated, 2 a underwent a shift of the alkenyl hydrogen atom to the carboxylate-substituted carbon atom to form the carboxylate-substituted vinylidene complex Re2(CO)8[mu-C=H(CO2Me)], 4, in which the ketonic oxygen atom of the carboxylate group is coordinated to one of the metal atoms. The metal-metal bond in 4 was cleaved (reversibly) by addition of CO at 1 atm/25-degrees-C to yield the complex Re(CO)9[mu-2C-CH(CO2Me)], 5, and under a high pressure of CO (1000 psi/100-degrees-C), compound 5 underwent a CO insertion (reversible) into one of the Re-C bonds to form the complex Re(CO)4[mu-O=CC=CH(CO2Me)]Re(CO)5, 6. Compounds 1, 2a, and 3-6 were characterized by single-crystal X-ray diffraction analysis. Crystal data are as follows. For 1: space group = P2(1)/c, a = 7.126(1) angstrom, b = 30.083(4) angstrom, c = 15.008(2) angstrom, beta = 93.71 (1)-degrees, Z = 8, 2731 reflections, R = 0.035. 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(l)-degrees, Z = 8, 2468 reflections, R = 0.033. For 3: space group = P1BAR, a = 10.730(2) angstrom, b = 11.533(2) angstrom, c = 9.544(2) angstrom, alpha = 114.14(l)-degrees, beta = 114.93(2)-degrees, gamma = 90.68(2)-degrees, Z = 2, 2012 reflections, R = 0.039. For 4: space group = P1BAR, a = 8.563(1) angstrom, b = 11.632(2) angstrom, c = 8.237(2) angstrom, alpha = 102.81(l); beta = 96.47(1)-degrees, gamma = 89.10(l); Z = 2,1636 reflections, R = 0.022. For 5: space group = P1BAR, a = 10.553(2) angstrom, b = 11.588(1) angstrom, c = 7.3536(9) angstrom, alpha = 104.228(9); beta = 90.31(1)-degrees, gamma = 84.13(1); Z = 2, 1760 reflections, R = 0.025. For 6: space group = P1BAR, a = 9.894(1) angstrom, b = 12.434(2) angstrom, c = 8.748(2) angstrom, alpha = 108.98(1)-degrees, beta = 113.593(9)-degrees, gamma = 77.69(l)-degrees, Z = 2, 2067 reflections, R = 0.032.