Chemistry of propargyldicobalt cations: Recent developments in the Nicholas and related reactions

The chemistry of propargyldicobalt cations, also known as the Nicholas reac tion, is reviewed, with a focus on the developments since 1995. Advances in the understanding of the fundamental properties, such as structure, stabil ity, and reactivity, of both the hexacarbonyl complexes and those bearing o ther ligands are discussed. All reactions involving propargyl cation dicoba lt complexes are covered, including those stemming from ionization of propa rgylic leaving groups and those created by electrophilic addition to enyne complexes. Migration reactions involving either initiation or termination b y propargyl cation complexes are included, as are the generation and reacti ons of propargyldicobalt radicals. Cyclization reactions employing these ca tions have received much attention, in cases with the alkynedicobalt unit l ocated in both exocyclic and endocyclic positions, and these reports are de scribed. Particular attention is paid to preparation of medium ring cycloal kyne complexes and their heterocyclic analogues. In addition, there is disc ussion of the progress in the in selectivity of these reactions, especially in terms of introduction of asymmetry at the propargylic site. Finally, re cent applications of Nicholas reaction chemistry in the synthesis of natura l products and related compounds are reported.