Ring opening of a cobalt-stabilized bornyl cation: Mechanistic study of the alkyne-dicobalt/carbynyl-tricobalt cluster transformation

摘要

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-enel-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-l-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 (RCdropCH)Co-2(CO)(6). [References: 40]