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

Abstract

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.

Regioselectivity, Stereoselectivity and Catalysis in Intermolecular Pauson-Khand Reactions: Teaching an Old Dog New Tricks

“…117 While tandem catalysis employing intramolecular Pauson-Khand reactions has been recently explored in several cases, 118 only little information was known about intermolecular tandem processes. Towards this goal we investigated the sequential Pauson-Khand reaction/transfer hydrogenation outlined in Scheme 69.…”

Solvent‐Mediated Generation of Cobalt‐Cluster‐Stabilised Propargyl Cations and Radicals: Allyl Migration versus Peroxide Formation

“…[23] In the present case, one might have expected formation of the tertiary alcohol 15, but elimination of water led directly to the observed product 11, which has been unequivocally characterised by X-ray crystallography.…”

“…[31] Since dichloromethane is not normally considered to be an electron-donating solvent, it is possible that radical formation was initiated by single-electron transfer from a cobalt radical fragment generated during the final step in the formation of the trinuclear complex 24. [23] In the crystalline state, the propargylallene dimer exhibits a 50:50 disorder, such that the modelled overlapped molecular orientations ( Figure 7) generate pseudo C i symmetry. This same centrosymmetric disorder phenomenon is also found in the corresponding bis(trimethylsilyl)propargylallene.…”

The alkyne-anion promoted ring-contraction of hexachlorotropone: synthesis and structure of [trimethylsilyl(pentachlorobenzoyl)ethyne]-hexacarbonyldicobalt

“…These have encompassed cobalt and molybdenum complexes of formally nonclassical systems, such as the 2-bornyl or 2-fenchyl cations [1][2][3][4][5], cobalt and iron derivatives of the cyclopentadienyl, indenyl and fluorenyl cations which possess 4p, 8p and 12p electrons, respectively, and are formally antiaromatic [6], and also chromium-stabilized benzyl cations [7]. In such cases, the cationic carbon leans toward the relatively electron-rich metal so as to alleviate the electron deficiency by direct overlap with a filled metal orbital [8].…”