Introduction to p-block Lewis acids in organic synthesis

There is no doubt that the range of chemical transformations for use in organic chemistry broadened dramatically through the 20^th century. This has largely been based on the evolution of transition metal mediated reactions. During that same time, main group element chemistry adopted a largely secondary role. For example, the development of reactions such as hydroboration or hydrosilylation demonstrated the value of the incorporation of p-block elements into organic compounds as such derivatives served as versatile functional or protecting groups. Similarly, while the use of transition metal catalysts was widespread, much of this chemistry was tuned by the controlled design of nitrogen and/or phosphorus-based donor ligands. Indeed, this relegation of main group chemistry to an ancillary role was engrained in the mindset of chemists based on a century of terrific success in the development of transition metal-based catalysts. However, in the past two decades, several advancements have inspired a renaissance in main group chemistry.

From my perspective, the work of Piers¹ on borane mediated hydrosilylation in the late 1990s was seminal in this renaissance of the p-block chemistry. This was followed by our discovery of frustrated Lewis pairs² and their applications in metal-free hydrogenations, a decade later. Harder's discovery³ of alkali-metal based hydrogenation catalysts emerged shortly thereafter. All these findings illustrated the ability of main group species to participate in the activation of small molecules and thus mimic in some fashion the reactivity of transition metals, a concept articulated by Power.⁴ These early studies stimulated a great deal of research from which a diverse array of applications of main group compounds in synthetic organic chemistry have emerged.

In addition to the novelty of the concept of main group catalysts, several other factors have stimulated research around main group reagents in synthesis. Firstly, main group species are typically less costly and toxic than conventional precious metal catalysts. The lower cost is associated with the higher abundance of most main group elements, but this also reduces the carbon footprint associated with their use, an issue of global concern currently. In addition, the lower toxicity also reduces purification costs for products targeted for human consumption. Perhaps of greater fundamental importance however is the fact that the reactivity of main group reagents provides us with a new set of tools to construct molecules, gives us both alternatives and new avenues of reactivity that are complementary to existing protocols.

In this themed collection, we have focused on one facet of this renaissance of main group chemistry, specifically, p-block Lewis acids in organic synthesis. Herein, we have assembled a collection of contributions from researchers from around the world, who are developing new applications of Lewis acidic main group reagents in synthetic organic chemistry. These manuscripts cover a broad range of p-block elements in diverse chemical transformations including such reactions as the reduction of multiple bonds and C–C bond formations. In addition, several reviews highlight recent developments pointing the way to new synthetic strategies. Collectively these papers illustrate a range of strategies that p-block reagents and catalysts can provide for synthetic chemistry.

The varied contributions in this themed collection provide continuing evidence of the renaissance and impact of main group chemistry. Indeed, this on-going evolution of p-block reactivity is expanding the classical toolbox for organic chemistry with new reagents, catalysts, and protocols.

References

1. D. J. Parks and W. E. Piers, J. Am. Chem. Soc., 1996, 118, 9440–9441.
2. G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan, Science, 2006, 314, 1124–1126.
3. J. Spielmann, F. Buch and S. Harder, Angew. Chem., Int. Ed., 2008, 47, 9434–9438.
4. P. P. Power, Nature, 2010, 463, 171–177.

---