Metal-stabilized carbocations derived from monoterpenes: dicobalt hexacarbonyl complexes of alkynyl-verbenols

Nucleophilic attack by alkynyl anions on the verbenone skeleton occurs on the face opposite to that occupied by the gem-dimethyl group. The resulting alkynols react with dicobalt octacarbonyl to form tetrahedral (μ2-alkyne)Co2(CO)6 clusters in which the tricarbonylcobalt moieties are diastereotopic, and treatment with dppm tags each of these vertices with a 31P NMR label. Protonation yields a single cobalt-stabilized carbocation, 13a, in which the cobalt is situated on the side bearing the methylene bridge, thus avoiding the gem-dimethyl group, and pulls the capping phenyl substituent proximate to the hydrogen atom at C(3). All new molecules were characterized by NMR spectroscopy and by X-ray crystallography for syn-trimethylsilylethynylverbenol, 6, syn-ethynyl-verbenol, 7, and for the (2-phenylethynyl-4-methylene-6,6-dimethyl-(1S,5S)-bicyclo[3.1.1]hept-2-ene)Co2(CO)4(dppm) cluster, 12. The 2-ethynyl-4-methylene-6,6-dimethyl-bicyclo[3.1.1]hept-2-ene ligand, 15, was also prepared and characterized for the first time.