Cobalt-Catalysed Cyclisations

Intramolecular Pauson-Khand reactions are often regioselective because it is physically impossible for the molecule to cyclise any other way. Pauson-Khand disconnection of bicyclic 111 reveals an allyl ether 112 of the alcohol 113, easy to make from acetylene and cyclohexanone. In the cobalt-catalysed cyclisation, only one regioisomer is possible and this, the TM111, is formed in an excellent 80% yield.28... 

Ring aromatic steroids (72) have been constructed by sequential cobalt-catalysed cyclisation of enetriynes (69), and intramolecular Diels-Alder reaction of the resulting o-quinodimethanes (71) derived... 

Let us now discuss some applications of microemulsions in catalytic processes. It has been shown in [298] that the use of microemulsions instead of organic solvents for electrochemical reactions is advantageous from both economical and ecological reasons. The electrode/fluid interface in microemulsions probably consists of a dynamic layer of surfactant molecules packed more loosely on the electrode than in aqueous solutions. Microemulsions provide good yields of carbon-carbon addition products in reactions catalysed by cobalt complexes when preparing vitamin B 2. Excellent stereo-selective control in microemulsions made with the cationic surfactant cetyl trimethyl ammonium bromide was demonstrated for the catalytic cyclisation of 2-(4-bromobutyl)-2-cycIohexene-l-one to 1-decalone. Electrochemical synthesis may be a viable future approach to environmentally friendly chemical methods. 

Cobalt complexes of a-alkynylpyranosides undergo acid catalysed epimerisation by a mechanism involving an intermediate acyclic cation. The intermediate in this process may be trapped by nucleophiles to give derivatives which after suitable modification, may be induced to undergo thermodynamically controlled cyclisation to furnish dehydrooxepanes (Scheme 8) <94T12883>. Decomplexation may be readily carried out with iodine. 

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