Activation of a σ-SnSn Bond at Copper, Followed by Double Addition to an Alkyne

Many synthetically useful copper-catalyzed transformations involve the activation of apolar or weakly polar sigma-bonds (E-H and E-E' bonds, with E = C, B, Si, Sn, etc.). Yet, little is known so far about the associated elementary steps, and it is highly desirable to gain better knowledge regarding the way sigma-bonds can be activated by copper to help further development in this area. To this end, we became interested in investigating the coordination and activation of apolar or weakly polar sigma-bonds at copper using chelating assistance. Here we report investigations of gold and copper complexes deriving from the diphosphine-stannane [Ph2P(o-C6H4)Me2Sn SnMe2(o-C6H4)PPh2] 1. The sigma-SnSn bond of I readily undergoes oxidative addition at both gold and copper, giving bis(stannyl) Au+ and Cu+ complexes 2 and 3. Coordination of 1 to CuBr leads to the neutral complex 4 which features more sigma-SnSn complex character. The ability of complex 3 to undergo insertion reactions with alkynes was then examined. With methyl propiolate, a clean reaction occurred, and the bis-stannylated alkene copper complex 5 was isolated. The structures of ligand 1 and complexes 2-5 have been unambiguously determined by multinuclear NMR spectroscopy and crystallography. These results substantiate the ability of copper to promote the addition of apolar sigma-bonds to CC multiple bonds via a 2e redox sequence and draw thereby an unprecedented parallel with the group 10 metals.