A Primer in Frustrated Lewis Pair Hydrogenation

Concepts to Applications

Douglas Stephan

• Publisher: Royal Society of Chemistry
• Year: 2021
• eBook ISBN: 9781839165061
• Edition: 1
• Topic: Naturwissenschaften
• Subtopic: Industrielle & technische Chemie

Written for senior undergraduate and graduate students, as well as those chemists unfamiliar with "frustrated Lewis pairs (FLPs)", this text serves as an introduction to the both the concept and application of FLPs in hydrogenation catalysis.

The book begins by setting the stage as it was around the turn of the century. While the dramatic impacts of catalysis were undeniable, there was dependence of such technologies on transition metal chemistry. Contrastingly, the chemistry of main group elements was dominated by Lewis acid-base behavior. However, these perceptions were altered with the discovery of the ability of "frustrated Lewis pairs (FLPs)" to activate dihydrogen. The basic features, and mechanisms of action of these unique main group systems are chronicled, while the evolution of the initial findings to applications in catalytic hydrogenation is discussed. A subsequent chapter focuses on the breadth of organic substrates for which borane based FLP hydrogenations are effective. This is furthered with a chapter on the metal-free enantioselective reductions. FLP chemistry is not limited to systems involving boranes and this book also catalogues a range of FLP hydrogenation catalysts involving elements from across the periodic table. The final chapter in the book describes other directions of interest where the application of the concept of FLPs hydrogenation are beginning to emerge.

Ideal for course use and self study, this book provides synthetic chemists with a fresh, expert introduction to the field. The reader will be left recognizing that hydrogenation catalysts is a domain no longer limited to transition metals.

1 Context, Background, and Discovery

This chapter provides a discussion of the state of catalysis as it was at the turn of the 20th to the 21st century, providing a brief history of this important chemical concept. Why transition metals are so well suited to catalysis is considered. This is contrasted with the then prevalent perception of main group element chemistry at the time. We discuss the role of main group compounds as Lewis acids and bases, as well as several examples of chemical reactivity that contravene the expected behavior. These examples prompted us to consider the possibility that combinations of Lewis acids and bases could mimic the reactivity of transition metals, unveiling of the notion of ‘frustrated Lewis pairs’ (FLPs). Such select combinations of main group donors and acceptors are shown to activate dihydrogen in addition to other small molecules.

1.1 Chapter Overview

To begin a discussion of ‘frustrated Lewis pair (FLP) chemistry’, we set the stage as it was at the turn of the 20th to 21st century. At that time, it was clear that the implementations of catalytic processes had been a major driver in the evolution of chemistry and consequently in many technological advances in society. Herein, we briefly describe the history of this important chemical concept beginning with its inception over 100 years ago. We discuss why transition metals are so well suited to catalysis, focusing on their unique ability to act concurrently as both electron donors and acceptors. This is contrasted with the perceptions of main group elements, where generally molecules containing these elements act as either Lewis acids or Lewis bases. We also discuss examples of chemical reactivity that contravene reaction pathways expected based on the simple concepts of Lewis acidity and basicity. These examples prompted us to consider the possibility that combinations of Lewis acids and bases could mimic the reactivity of transition metals. This led to the initial unveiling of the notion of ‘frustrated Lewis pairs (FLPs)’ in which select combinations of main group donors and acceptors could activate dihydrogen in addition to other small molecules.

1.2 Our Chemical World

While it is shocking to compare our lives today with that of our parents, grandparents, or great grandparents, there is no doubt that over the last century, technological advances have dramatically transformed the way we live. Modern life, in both work and play, is replete with the tools, products, and materials derived from science and engineering that have either eased or enriched our lives. What is perhaps less widely appreciated, is the role that fundamental chemistry plays in cultivating the development and commercialization of such technologies. Indeed, it is not an overstatement to say that the nature of modern society has been shaped in large part by activities initiated in research laboratories around the world. Life-saving drugs, synthetic materials, plastics, solar energy, the internet, computers, and cell phones are just a few important examples of the endless array of modern technologies where chemistry is at the root of these advances, in whole or in part. Drugs, agrochemicals, and polymers have obvious roots in fundamental organic chemistry. For other technological advances, such as the ability to miniaturize circuitry, the precise control of the properties of new materials, the generation of vibrant colors on a display or the ability to harvest solar energy, the link to chemistry is perhaps less obvious to the uninitiated. Nonetheless, such advances are derived from an understanding of chemical properties and how to efficiently make and alter compounds or materials for a specific purpose. Ultimately, the recipe for cutting-edge technologies has involved a mix of fundamental chemistry, innovative engineering, and creative entrepreneurship.

1.3 A Brief History of Catalysis

Catalysis is an advance in chemistry that has contributed disproportionately to our abilities to synthesize new products over the past century. The origin of this transformative concept traces back to Ostwald (Figure 1.1), who was awarded the 1909 Nobel Prize for conceiving the notion of ‘catalytic phenomena as a means to accelerate chemical processes’. Subsequent work by Sabatier ^1,2 (1912 Nobel Prize, shared with Grignard) was similarly rewarded three years later. In his seminal findings, Sabatier experimentally showed that amorphous transition metals, principally nickel, can be used to catalyze the hydrogenation of unsaturated organic molecules. This finding provided a powerful method for efficient access to a range of organic compounds and initiated the field of heterogeneous catalysis. In 1918, Haber ³ was recognized for the catalytic production of ammonia, the precursor to fertilizer. This discovery has been described as the most important of the 20th century, as the Haber process provides the capability to feed the world's ever-growing population.

Figure 1.1 Nobel Laureates in chemical catalysis.

Fifty years later, Ziegler ⁴ and Natta ^5,8 (1963 Nobel Prize) uncovered heterogeneous catalysts for the polymerization of ethylene and propylene to the respective polymers. While these findings ushered in the industrial production of plastics, this field has since broadened, providing a range of synthetic polymers that have found applications in a wide array of consumer products.

In the 1960s, the emergence of organometallic chemistry brought molecular chemistry to catalysis. In this regard, a classical example is the work of Wilkinson (1973 Nobel Prize, shared with Fischer) and coworkers. These researchers showed that the rhodium complex, (Ph₃P)₃RhCl, acts as a pre-catalyst for the hydrogenation of unsaturated organic compounds in sol...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Dedication
• Preface
• Contents
• 1 Context, Background, and Discovery
• 2 The Nature of Frustrated Lewis Pairs
• 3 Borane-based FLP Hydrogenations
• 4 Borane-based Asymmetric FLP Hydrogenations
• 5 Structural Variations of FLPs
• 6 Other Directions for FLP Hydrogenations
• Subject Index