Advances in the cobalt carbonyl-mediated and -catalyzed cyclizations of enynes

Cycloaddition reactions of enynes as catalyzed or mediated by cobalt carbonyl complexes generating various carbocyclic frameworks form the bases of this work (Scheme 1). A major portion of these studies have been devoted to investigations on development of catalytic protocols for the Pauson-Khand reaction under one atmosphere of CO using Co2(CO) 8, Co4(CO)12 and enyne-Co2(CO) 6 complexes as cobalt carbonyl sources to provide bicyclopentenones. We have developed convenient and efficient procedures with Co2(CO) 8, and enyne-Co2(CO)6 complexes whose efficiencies were enhanced by cyclohexylamine. These studies have led to an observation that efficient Pauson-Khand cyclizations could be achieved using substoichiometric amounts of Co2(CO)8 under a N2 atmosphere, interestingly, only in the presence of the mine. Most notably, we have discounted the common belief that Co4(CO)12 is inactive toward the Pauson-Khand reaction and have made a significant improvement on protocols employing this cobalt cluster.; Studies on the application of the stoichiometric version have also demonstrated the utility and versatility of this protocol. Formation of enantiopure spiro{lcub}cyclopropane-1,4 '-bicyclo[3.3.0]oct-1-en-3-dione{rcub} was achieved by using a sterically-encumbered tartrate-derived C2-symmetric chiral ketal, which further illustrates the power of chiral auxiliaries in this cyclization. In a number of cases, generation of medium-sized fused or bridged tricyclic enones was likewise accomplished via the Pauson-Khand reaction, which represents the first report on the general utility of this reaction in the formation of medium-sized macrocycles.; Finally, this dissertation also includes studies on reductive Pauson-Khand reaction and reductive cyclization of enynes . Our studies have constituted the first account of a direct formation of bicyclopentanones from a sequential cyclocarbonylation of enynes and 1,4-reduction of the resulting bicyclopentenones using Co4(CO)12 in iso-propanol. In the presence of hydrosilanes an unprecedented interruption of the normal Pauson-Khand reaction pathway of enyne-Co2(CO) 6 complexes provide monocyclic alkenes.