Dicobaltoctacarbonyl species in organic synthesis : a novel route into tricyclic fused ring systems

This thesis describes the modern uses of cobalt carbonyl complexes in organic synthesis, by focusing primarily on the Pauson-Khand Reaction (PKR). This is where an alkyne, alkene and a dicobalt complex react in a (2+2+1) cycloaddition producing an cyclopentenone adduct. The cyclopentadienone products of the PKR were used in the synthesis of hetero fused tricyclic ring systems using a diastereoselective and regioselective chemical synthesis via the addition of soft organometallic nucleophiles: The nucleophilic addition of such species allowed a spontaneous cyclocondensation that gave fused ring systems with known biological activity prevalent in a wide range of natural products. This unique synthesis was then applied to non-hetero fused tricyclic ring systems affording a triquinane skeleton analogue which possess significant levels of biological activity. These syntheses were achieved by way of a three-step sequence after an initial PKR. A 1,4-Michael addition was followed by a 1,5-cyclisation and catalytic oxidation of an olefin resulting in the target tricyclic compound. Secondly, an investigation was performed into the regioselectivity of the PKR, using di-substituted alkynes with a high degree of conjugation, which were prepared by transition-metal catalysed cross couplings reactions. The isolation of the corresponding dicobalt alkyne complexes, and their gjto-tricyclodecadienone products were used to assist in elucidating the mechanism by which the PKR proceeds. The final Chapter describes the uses and mechanism, by which dinorbornadienedicobalttetracarbonyl (DDTC) was formed as a side product of the PKR. This side product was then used to promote the PKR. Surprisingly, only alkynes with electronegative oxygen atoms worked and diastereoselective control for the synthesis of cyclopentadienones was maintained at similar levels observed within the PKR with dicobaltoctacarbonyl