Dibenzothiophenium Salts: Practical Alternatives to Hypervalent I(III)-Based Reagents.

Abstract

ConspectusDuring the past few years, the interest among organic synthesis practitioners in the use of sulfonium salts has exponentially growth. This can arguably be attributed to a series of specific factors: (a) The recent development of more direct and efficient protocols for the synthesis of these species, which make sulfonium reagents of a wide structural variety easily available in multigram scale. (b) The recognition that the reactivity of these salts resembles that of hypervalent iodine compounds, and therefore, they can be used as effective replacement of such species in most of their applications. (c) Their intrinsic thermal stability and tolerance to air and moisture, which clearly surpass that of I(III)-reagents of analogue reactivity, and facilitate their purification, isolation as well-defined species, storage, and safely handling on larger scale. (d) Finally, the possibility to further functionalize sulfonium salts once the sulfur-containing platform has been incorporated. Specifically, this last synthetic approach is not trivial when working with hypervalent I(III)-species and facilitates the access to sulfonium salts with no counterpart in the I(III) realm.This renewed interest in sulfonium salts has led to the improvement of already existing transformations as well as to the discovery of unprecedented ones; in particular, by the development of protocols that incorporate sulfonium salts as partners in traditional cross-coupling and C-H activation steps or combine them with more modern technologies such as photocatalysis or electrosynthesis. In this Account, the reactivity of a series of sulfonium salts originally prepared in our laboratory will be outlined and compared to their I(III)-counterparts. Some of these reagents are now commercially available, and their use has started to spread widely across the synthetic chemistry community, helping to speed the process of identification of potentially bioactive products or new functionaliced materials. However, challenges still remain. The development of sulfonium reagents characterized by an optimal balance between reactivity and site-selectivity, or showing broader compatibility toward sensitive functional groups is still a need. In addition, the intrinsic stability of sulfonium salts often makes necessary the use of (sophisticated) catalysts that activate the latent reactivity hidden in their structures. Although a priori one can see this fact as a disadvantage, it might actually be decisive to harvest the full synthetic potential of sulfonium salts because their thermal stability will surely facilitate the preparation of operational reagents with no counterpart in the context of I(III)-chemistry. If this becomes true, sulfonium salts may contribute to the expediting of retrosynthetic disconnections that, to date, are impossible.