Ring Opening of a Cobalt-Stabilized Bornyl Cation: Mechanistic Study of the Alkyne−Dicobalt/Carbynyl−Tricobalt Cluster Transformation

Upon protonation with HBF4, [2-endo-((allyldimethylsilyl)ethynyl)borneol]Co2(CO)6 (2) suffers elimination of water or propene, to yield [2-((allyldimethylsilyl)ethynyl)born-2-ene]Co2(CO)6 (11) and [2-endo-((dimethylfluorosilyl)ethynyl)borneol]Co2(CO)6 (12), respectively, and, surprisingly, the tricobalt complex (2-norbornylidene)CHCCo3(CO)9 (13). In contrast, protonation of the terminal alkyne (2-endo-ethynylborneol)Co2(CO)6 (19), an anticipated precursor to 13, led instead to (2-ethynyl-2-bornene)Co2(CO)6 (21) and the ring-opened species (2-ethynyl-4-isopropyl-1-methylcyclohexa-1,3-diene)Co2(CO)6 (22). However, conversion of 19 to 13 was achievable upon prolonged heating at reflux in acetone, thereby also affording the corresponding alcohol [2-(2-hydroxybornyl)]CH2CCo3(CO)9 (20). A mechanistic rationale is offered for the formation of RCH2CCo3(CO)9 clusters upon protonation of alkyne complexes of the type (RC⋮CH)Co2(CO)6.