Early Main Group Metal Catalysis

Concepts and Reactions

Sjoerd Harder

• Publisher: Wiley-VCH
• Year: 2019
• Print ISBN: 9783527344482
• eBook ISBN: 9783527818037
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Early Main Group Metal Catalysis gives a comprehensive overview of catalytic reactions in the presence of group 1 and group 2 metals. Chapters are ordered to reaction type, contain educational elements and deal with concepts illustrated by examples that cover the main developments. After a short introduction on polar organometallic chemistry and synthesis of early main group metal complexes, a variety of catalytic reactions are described, e.g. polymerization of alkenes, hydroamination and phosphination reactions, hydrosilylation, hydroboration and hydrogenation catalysis, as well as enantioselective and Lewis-acid catalysis. The book addresses organic chemists and researchers in industry interested in the state-of-the-art and new possibilities of early main group metal catalysis as well as newcomers to the field. Written by a team of leaders in the field, it is a very welcome addition to the area of main group metal chemistry, and to the field of catalysis.

1
Introduction to Early Main Group Organometallic Chemistry and Catalysis

Sjoerd Harder

University Erlangen-Nürnberg, Inorganic and Organometallic Chemistry, Egerlandstrasse 1, 91058 Erlangen, Germany

1.1 Introduction

Although organometallic complexes of the early main groups are well known for their very high reactivity and challenging isolation, they are among the first studied during the pioneering beginnings of the field. Their high nucleophilicity and Brønsted basicity have made them to what they are today: strong polar reagents that are indispensible in modern organic synthesis. It is exactly this high reactivity that has made them potent catalysts for organic transformations that are generally catalyzed by transition metal complexes. Despite their lack of partially filled d-orbitals and their inability to switch reversibly between oxidation states, the scope of their application in catalysis is astounding. As the early main group metal catalysis only started to become popular since the beginning of this century, it is still a young field with ample opportunities for further development. This introductory chapter is specifically written for new graduate students in the field. It gives a very compact overview of the history of early main group organometallic chemistry, synthetic methods, bonding and structures, analytical methods, solution dynamics, and some preliminary low-valent chemistry. This forms the basis for understanding their use in catalysis for which the basic steps are described in the second part of this chapter. For further in-depth information, the reader is referred to the individual chapters in this book.

1.2 s-Block Organometallics

1.2.1 Short History

The organometallic chemistry of the highly electropositive early main group metals could not have started without the isolation of these elements in the metallic state. Being only available in nature in the form of their salts, the invention of electricity and electrolysis in the beginning of 1800s has been the key to their isolation. The legendary Humphry Davy (1778–1829) can claim the discovery of at least seven s-block metals: the alkali metals Li, Na, and K and the alkaline earth (Ae) metals Mg, Ca, Sr, and Ba.

Most organometallic classes start with Frankland's well-known synthesis of Et₂Zn in 1847 [1] by reaction of Zn and EtI, which was originally an attempt to isolate the Et radical. It is, however, less known that before this experiment, Frankland tried to isolate the Et radical by mixing K and EtI, a reaction that was found to be very violent producing a variety of gaseous products [2]. Wanklyn reacted Frankland's Et₂Zn with Na and isolated the zincate Na⁺ZnEt₃⁻, which, based on its very high reactivity, was described as a solution of EtNa in Et₂Zn. Although not metal pure, this is likely the first preparation of an alkylsodium compound. Numerous early organometallic pioneers attempted to prepare organosodium complexes directly by reaction of the metal with R–X (R = alkyl or aryl; X = Cl, Br, and I) but never isolated the organosodium products that were found to be fleeting intermediates to Wurtz (or Wurtz–Fittig) R–R coupling products.

Switching to the less reactive metal Li, Wilhelm Schlenk was the first to isolate group 1 alkylmetal complexes in a pure form. Reduction of Me₂Hg with Li metal gives metallic mercury and MeLi as a white powder. Schlenk describes the pyrophoric nature of this powder in beautiful words, clearly demonstrating his fascination for these compounds [3]. Although Schlenk is not just the “man behind the flask” [4] but certainly also a major pioneer in organometallic chemistry of the alkali metals, it was Karl Ziegler who developed the simple mercury-free route to organolithium reagents [5], which is the method of choice even today for the synthesis of nBuLi: nBuCl + 2Li⁰ → nBuLi + LiCl. Because alkyllithium reagents are less reactive than Na or K reagents, Wurtz coupling is limited. Also, their much higher solubility in apolar organic solvents added to their successful isolation. The latter strong alkyllithium base is not only the starting point for modern organolithium chemistry but also for the development of superbases based on the heavier alkali metals.

Grignard developed organometallic chemistry of group 2 metals and reported in 1900 a similar protocol to prepare organomagnesium reagents: R–X + Mg → RMgX [6]. Foreseeing a great future for these potent reagents, Grignard was awarded the Nobel Prize for this milestone discovery already in 1912. Similar to group 1 metals, the development of the organometallic chemistry of the heavier group 2 metals was found to be more challenging. Beckmann mentioned already in 1905 the first synthetic routes to arylcalcium halides [7] but Gilman had problems reproducing these results [8]. Development of the organometallic chemis...

Table of contents

• Cover
• Table of Contents
• Preface
• 1 Introduction to Early Main Group Organometallic Chemistry and Catalysis
• 2 Polymerization of Alkenes and Polar Monomers by Early Main Group Metal Complexes
• 3 Intramolecular Hydroamination of Alkenes
• 4 Molecular s-Block Catalysts for Alkene Hydrophosphination and Related Reactions
• 5 H—N and H—P Bond Addition to Alkynes and Heterocumulenes
• 6 Early Main Group Metal-Catalyzed Hydrosilylation of Unsaturated Bonds
• 7 Early Main Group Metal Catalyzed Hydrogenation
• 8 Alkali and Alkaline Earth Element-Catalyzed Hydroboration Reactions
• 9 Dehydrocoupling and Other Cross-couplings
• 10 Enantioselective Catalysis with s-Block Organometallics
• 11 Early Main Group Metal Lewis Acid Catalysis
• 12 Enantioselective Group 2 Metal Lewis Acid Catalysis
• 13 Miscellaneous Reactions
• Index
• End User License Agreement