Adventures in gold-catalyzed cascade reactions and rearrangements.

摘要

Gold-catalyzed organic transformations have been a hot topic of research in synthetic organic community over the last few years. Amazingly, most of the reactions can be performed under mild conditions using a catalytic amount of gold complexes. One of the widely reported reactions using gold catalysts is heterocyclization that involves an activation of a p system followed by a nucleophilic attack by a hetero atom. The Nazarov reaction is another important class of reactions which has gained considerable attention in the past few years. Mechanistic studies performed by the Frontier group helped to shed light on the Nazarov reaction of a, a'-activated substrates under catalytic conditions.;We were able to combine the concept of gold-catalyzed furan formation with the Nazarov reaction of a-alkoxy substrates using gold(III) catalysts. Under mild reaction conditions, in the presence of gold(III) catalysts, alpha-alkoxy,alpha'-alkynyl divinyl ketones underwent a heterocyclization triggered Nazarov cascade reaction to give a synthetically important fused bicyclo compounds. A solvent dependent mechanistic dichotomy was observed while performing these reactions. Moreover, a computational study was undertaken to decipher the mechanism underlying the cascade process.;The above study of the cascade heterocyclization-Nazarov reaction led to the discovery of coordinating preferences of the gold(I) cation in the presence of electronically different p systems. This result was in contrast to the commonly believed concept of gold(I) cation complexing to the alkyne moiety in the presence of other p- systems. An exhaustive computational and experimental study suggested that our hypothesis was based on sound scientific principles.;The coordinating preference of gold(I) to vinyl ethers, found during the above investigation, prompted us to examine the mechanism of the gold-catalyzed propargyl Claisen rearrangement reported by the Toste group. We were surprised to find that all the reactions worked extremely fast at room temperature in presence of various gold(I) catalysts. However, when reactions were performed at low catalyst concentration, we found that there was a significant substituent effect from the groups attached to the carbinol carbon. A further inquiry into the mechanism of this reaction led us to a breakthrough that changed the perception of the previously conceived mechanism. The reaction was catalyzed by two gold(I) cations as opposed to generally believed one gold(I) cation.;An investigation into the electronic nature of gold(I) catalysts was undertaken using propargyl Claisen rearrangement as a probe. The electronic nature of gold(I) was explored as a function of reaction rate observed for different propargyl Claisen substrates. Different gold catalysts were studied through this reaction based on the nature of the ligand and anion attached on the gold. To our best knowledge, we are the first to take steps towards tuning gold(I) catalysts to perform functional group specific reactions.