Cobalt complexes Lewis acid catalysis

Lewis-acid catalysis is effective in intermolecular as well as intramolecular /zomo-Diels-Alder reactions. Thus, complex polycyclic compounds 93 have been obtained in good yield by the cycloaddition of norbornadiene-derived dienynes 92 by using cobalt catalyst, whereas no reaction occurred under thermal conditions [91] (Scheme 3.18). 

Studies of Lewis acid catalysis have been carried out with kinetically labile metal ions (Cu(II), Ni(II), Zn(II) etc.) and with kinetically inert metal centres such as cobalt(III). The use of inert cobalt(III) complexes in kinetic studies presents many advantages as the metal complex is well defined and it is often possible to obtain more detailed mechanistic information from such systems. 

The Nicholas reaction [103] has been successfully exploited for a novel glyco-sylation reaction based on the internal delivery concept (Scheme 31a). The suitably O-protected glycop3rranoside derivative 111 bearing the cobalt-complexed 4-alkoxy-6-phenyl-5-hexynoate residue at the anomeric position affords the corresponding propynyl cation 112 under Lewis acid catalysis. The intermediate 112 collapses spontaneously to form the oxoniiun ion delivering the cobalt-complexed y-lactone 113. The oxocarbenium cation is then intramolecularly captured by its counter alkoxy anion leading to glycoside 114. 

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