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 σ-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 σ-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 σ-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 σ-SnSn bond of 1 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 σ-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
σ-bonds to CC multiple bonds via a 2e redox sequence and draw
thereby an unprecedented parallel with the group 10 metals.