Cleavage of Unstrained C(sp2)—C(sp2) Single Bonds with Ni0 Complexes Using Chelating Assistance

Abstract

The reaction of Ni(COD)2 with a ligand containing a biphenyl subunit (P(iPr)2-O-Ph-Ph-O-P(iPr)2, L1) resulted in a mixture of compounds, the main product being [L1]Ni(COD), which could be quantitatively transferred to product 2 using bis(p-toluyl)acetylene. The possible activation products, cis- and trans-3 (3c and 3t, respectively), which would result from the cleavage of the bridging C—C single bond were synthesized independently. The mechanism of the cis/trans-isomerization was examined, activation parameters determined and trapping tests performed. Reaction of (PPh3)2Ni(CO)2 with L1 gave a novel complex 4 by a ligand exchange reaction. This compound, when heated to 95 °C under a 5 bar atmosphere of CO for several days, slowly incorporated CO into the bridging C—C single bond. This behavior as well as isotopic labeling experiments with 13CO indicated that, following a dissociation of CO in complex 4, an interaction between the metal center and the biphenyl moiety is induced, leading to activation. The reaction of Ni(COD)2 with P(iPr)2-O-Ph-CO-Ph-O-(P(iPr)2 (L3), which contains a benzophenone moiety, resulted in the quantitative activation of the PhC—CO bond at 20 °C, leading to complexes 3c/t (trans/cis 80/20 at 20 °C). The mechanism was examined and intermediates of it were identified as well as activation parameters for their further reaction determined. Reaction of (PPh3)2Ni(CO)2 with L3 gave complex 5, which is stable at 20 °C. Complexes 3c/t were shown to react quantitatively to give compound 5 under an atmosphere of CO. Selective isotopic labeling with 13CO proved that an activation similar to that observed with Ni(COD)2 occurs in complex 5 at 95 °C but much more slowly and is accompanied by the formation of complex 4. Molecular structures of the novel complexes 2, 3c, 4, and 5 have been determined by single crystal X-ray diffraction studies.