Gold (I)-catalyzed Claisen rearrangement of allenyl vinyl ethers; A synthetic and mechanistic approach

During the last decade, homogeneous gold catalysis emerged as one of the most efficient tools for C-C and C-X (O, N, S ...) formations in the organic chemist's tool box. Gold catalysis enriched the world of synthetic organic chemistry with a large number of synthetically useful methods and opened the way for new horizons in the world of homogenous catalysis and organic chemistry. Gold catalysis has been successfully employed in a wide variety of organic transformations and further increased their synthetic utility.;Claisen rearrangement was one of the reactions that has been successfully employed in gold catalysis. Although this reaction was discovered almost 100 years ago, this reaction is still considered as one of the most valuable and an important method for C-C bond formations, and is still an area of intensive study until now.;As a program directed toward the development of new metal-catalyzed reactions, we were interested in the development of a metal-catalyzed Claisen rearrangement of allenyl vinyl ethers as a method for the synthesis of highly substituted 1,3-dienes. Gold catalysts were superior to other traditional metal catalysts and successfully catalyzed the rearrangement of different allenyl vinyl ethers into the corresponding 1,3-dienes. The N-heterocyclic carbene gold(I) chloride catalyst, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidenegold(I)chloride (IPrAuCl) successfully solved the regioselectivity problem which was encountered upon using the phosphine-based ligands.;The remarkable difference between NHC- and phosphine-based gold catalysts on the regioselectivity of the reaction, prompted us to further investigate the effect of ligands on the reactivity of the gold cation and on the interaction between the metal center and the rearranging substrate. The mechanism of the reaction has been also intensively studied, since the originally proposed mechanism which was similar to those proposed for similar type of reactions failed to explain a lot of the experimental results and an alternative mechanism based on kinetic studies was proposed.