Effect of coordinating ligands on the Pauson-Khand cycloaddition: Trapping of an intermediate

Article abstract of DOI:10.1021/ja00069a017

The Pauson-Khand cobalt-mediated cycloaddition has become a synthetically important reaction. Several modifications have increased the utility of the reaction, including reports that tertiary amine N-oxides greatly accelerate the rate of cycloaddition in both intra- and some intermolecular reactions. To date, there has been no direct evidence to support the mechanistic hypotheses for either the thermal or amine oxide promoted reactions beyond complete identification of the hexacarbonyl alkyne complex. While the amine oxide promoted reaction is normally greatly accelerated over the analogous thermal one, in the presence of amine oxides, the rate of cycloaddition of a 1,6-enyne may be enhanced even further by the presence of sulfur, nitrogen, or oxygen in the homopropargylic or bishomopropargylic position. A series of substrates were prepared in an attempt to identify the accelerating effect. Upon heating, these same heteroatom-substituted substrates reacted faster than those without coordinating heteroatoms. The rate-accelerating effects have been rationalized within the context of the proposed mechanism. During the course of studies with sulfur-substituted substrates, analysis by thin layer chromatography revealed the formation of a new complex which then appeared to be converted to the bicyclic cyclopentenone. Normally, under Pauson-Khand reaction conditions, no intermediates are observed. In the case of cobalt complex 1, which required the longest reaction time for complete transformation to cyclopentenone, the intermediate complex was isolable, and its structure was verified by NMR spectroscopy.