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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 chemistry of the heavier Ae metals turned out to be substantially more difficult than simply reproducing the Grignard method with Ca instead of Mg.

1.2.2 Synthesis of Group 1 Organometallics

A short overview of synthetic methods to pure organoalkali metal compounds is shown in Scheme 1.1 (for comprehensive reviews, see [9, 10]). The synthesis of nBuLi directly from its metal and nBuCl stands central to the further development of lithium chemistry. The highly Brønsted basic nBu- anion (approximate pKa of CH3 in butane?~?50 [11, 12]) is able to deprotonate a large variety of organic substrates with pKa's up to 45 (e.g. benzene) provided the nBuLi reagent is sufficiently activated by polar cosolvents (see Section 1.2.3). For problematic deprotonation reactions, the even stronger base tBuLi is routinely used (approximate pKa of CH in 2-methylpropane?~?52 [11, 12]). Direct substrate deprotonation with alkyllithium reagents has the great advantage that the alkane side product is volatile and easily removed. Its disadvantage is that the selectivity of product formation is controlled by the substrate's acidity. In some cases, however, complex formation between substrate and alkyllithium reagent controls the selectivity of deprotonation by the complex-induced-proximity-effect (CIPE) [13] influencing the kinetics and thermodynamics of product formation. The well-known Li-halogen exchange route allows exact regiocontrol of the deprotonation reaction. In polar solvents, this conversion is even extremely fast at -80?°C. The mechanism proceeds through a hypervalent C-X-nBu intermediate because only heavier halogens (X = Br and I) can be used. Also, for these reactions, it holds that the product carbanion should be more stable than the nBu- anion. For the identity exchange reaction, C6F5I?+?C6F5Li, the hypervalent intermediate [Li+·(TMEDA)2][C6F5-I-C6F5-], has been structurally characterized [14]. The Li-halogen exchange is limited to lithiations of sp2 C atoms because sp3 C-X groups are prone to SN2 substitution. The product isolation is sometimes complicated by the subsequent SN2 reaction of the organolithium product with the side product nBuX. Another method for selective lithiation is Li-Sn exchange, which proceeds through a hypervalent intermediate with retention at Sn and C [15].

Scheme 1.1 Overview of common synthetic routes to organolithium compounds.

Reagents based on the heavier alkali metals Na and K cannot be prepared by the direct reaction of an organohalide with the metal because of Wurtz coupling. A simple route for the preparation of the highly reactive superbases nBuNa and nBuK consists of mixing nBuLi with the higher metal alkoxide tBuOM (M = Na and K) [16]. This metal exchange reaction is based on the hard-soft-acid-base (HSAB) principle: the smaller (harder) Li+ prefers to interact with the smaller (harder) O- of the alkoxide, whereas the larger (softer) Na+ and K+ prefer interaction with the larger (softer) C- in the carbanion. As the early pioneers in this area, Lochmann and Schlosser [17], disputed this invention, these reagents are known as Lochmann or Schlosser bases. However, their roots go back much further when Morton and Chester reported the highly basic properties of RNa/LiOR mixtures [18]. The more correct naming of these mixtures as "superbases" reveals that these reagents are indeed very powerful deprotonation reagents. Their very high reactivity originates from their longer (and weaker) bonds, which facilitate the kinetics of the bond-breaking and bond-making processes. Use of heterobimetallic reagents consisting of an alkalimetal and Mg, Zn, or Al brought the synthetic utility of alkali metal reagents even to a higher level [19]. Template-controlled interaction with the substrate results in deprotonation at unusually mild conditions or with unusual regioselectivities and often polydeprotonations can be achieved.

1.2.3 Synthesis of Group 2 Organometallics

With few exceptions, preparative methods for the organometallic complexes of group 2...