Hexacarbonyl dicobalt, Nicholas reaction

Later on, Schreiber used consecutively these two reactions in the key step for the synthesis of diterpene (-i-)-epoxydictimene (73), starting from natural (R)-pulegone [116,117]. This approach was built on their preliminary studies on Lewis acid mediated intermolecular Nicholas reactions [118]. They prepared functionalized enyne 70 bearing a mixed acetal. This compound was transformed into its dicobalt-hexacarbonyl complex and, in the presence of a carefully selected Lewis acid, it formed a stabilized carbocation by release of the more accessible ethyl moiety. This cation reacted intramolecularly with the allylsilane giving the central eight membered ring of the natural product... 

The total syntheses of (+)-secosyrins 1 and 2 was achieved and their relative and absolute stereochemistry was unambiguously established by C. Mukai and co-workers. To construct the spiro skeleton of these natural products, the intramolecular Nicholas reaction was utilized. The alkyne substrate was first converted to the dicobalt hexacarbonyl complex by treatment with Co2(CO)8 in ether. Exposure of the resulting complex to boron trifluoride etherate at room temperature brought about the ring closure with inversion of configuration at C5 to afford the expected tetrahydrofuran derivative. The minor product was the C5 epimer which was formed only in 15% yield. 

Nicholas reaction Trapping of dicobalt hexacarbonyl-stabilized propargylic cations with various nucleophiles. 314... 

Nicholas, K. M. Chemistry and synthetic ntility of cobalt-complexed propargyl cations. (1987). Acc. Chem. Res., 20, 207-214. Teobald, B. J. (2002). The Nicholas reaction the use of dicobalt hexacarbonyl-stabilised propargylic cations in synthesis. Tetrahedron, 58, 4133 170. 

Tyrrell demonstrated a three step tandem sequence involving an intermolecular Nicholas reaction, intramolecular Nicholas reaction, and a cationic cyclization. Treatment of silyl enol ether 55 with hexacarbonyl(propiolaldehyde diethyl acetal) dicobalt and boron trifluoride provides cobalt-alkyne complex 56. Exposure of this material to tetrafluoroboric acid promotes an intramolecular Nicholas reaction to form the second six-membered ring. Alkyne decomplexation with ceric ammonium nitrate enables the final cyclization step to yield the target tricycle 57." ... 

Nicholas published an Organic Syntheses paper highlighting the preparation of 2-(l-methyl-2-propynyl)cyclohexanone (65) from the reaction of 1-trimethylsiloxycyclohexene (62) with hexacarbonyl (l-methyI-2-propy-nylium)dicobalt tetrafluoroborate (63) and subsequent decomplexation of cobalt complexed alkyne 64. This report also includes procedures for the synthesis of the two starting materials (62 and 63). ... 

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