Microwave assisted intramolecular heck cyclization of aryl chlorides

Abstract

Since its discovery in the 1970's, the Heck reaction has become an incredibly powerful synthetic tool in organic chemistry. Under the influence of a palladium catalyst, the Heck reaction forms carbon-carbon bonds between an alkene or an alkyne, and an unsaturated halide. Because the Heck reaction proceeds under mild conditions, functional group tolerance is quite high. As such, the intramolecular Heck reaction in particular has been used extensively to construct complex molecular frameworks and natural products, such as those pioneered by L. E. Overman et al. Challenging Heck reactions of this nature have traditionally been accomplished with iodides and bromides. Aryl chlorides have not typically been used in the Heck reaction because oxidative addition occurs more easily with bromides and iodides. However, the use of aryl chlorides is highly desirable, as they are more cost effective and readily available over alternative reagents. Within the last decade, G. C. Fu and others have made significant advances in activating aryl chlorides for use in the Heck reaction. We have investigated the utility of the intramolecular Heck reaction with aryl chlorides as substrates under microwave irradiation. Herein we report that the product mixture of this reaction is influenced by three key reaction conditions, solvent, catalyst loading, and identity of the phosphine ligands. Prepared from simple, commercially available reagents, tricyclic 1,3-dienes, allenes, and single olefins were isolated in good yields. Additionally, the success of this methodology extends to a more general bicyclic scaffold, where allene products were also obtained. At the present time, and to the best of our knowledge, this work represents the first example of allenic compounds prepared under these conditions.