Preface

Just as it has done for many years, organometallic and metalloorganic chemistry continues to play a vital role for synthetic chemists in the twenty-first century. It offers some of the most effective ways of regiospecifically elaborating organic systems or of harnessing the potential of small molecules. Nevertheless, chemists constantly encounter challenges and target new systems which are not amenable to existing reagents. It is issues such as aggressive nucleophilicity or the implications of temperature instability or solvent sensitivity of many traditional organometallic bases, which have driven much of the research discussed in this volume. A major emergent theme covered is the potential of new, heterobimetallic systems. In particular, the problems of understanding just how reagents predicated on the action of two different metals operate. Synergy vs. cooperativity will be looked at in depth.

The new and often unique reactivities of these more complex reagents (or reagent mixtures) and their ability to achieve more effective chemical transformations under increasingly mild conditions are discussed. To do this, a broad view of organometallic chemistry is adopted, taking in organooxides, amides and the like as appropriate. Mostly, combinations of main group metals will be looked at but, in particular when considering environmental applications of bimetallic systems, coverage will extend to the d-block. Divided into nine chapters, the volume broadly covers three main fields; structural chemistry in the solid state, understanding often catalytic processes by monitoring reaction pathways, and synthetic applications.

The first three chapters are dominated by the role of crystallography in understanding organometallic reagents and, in particular, heterobimetallics. Throughout the book, a range of crystal structures are reproduced. For ease, these omit minor disorder, lattice solvent and all but the most chemically relevant hydrogens. Chapter 1 focuses on the work in the laboratory where a new generation of so-called ate or synergic bases were originally developed. The advent of Kondo's non-nucleophilic zincates is considered and the implications for selective deprotonation are described. The major emphasis then switches to some of the most recent work in this area, where lithium cuprates have been studied and the idea of harnessing the oxidative flexibility of copper in coupling chemistry has been developed. If the first chapter shows anything it is that truly understanding the nature of many heterobimetallic reagents is vital to predicting or rationalizing their behaviour, and this frequently necessitates crystallographic determination. Chapter 2 focuses on this through the prism of synergistic main group combinations. In particular, a more expansive view of the less strongly polarizing metal component of these systems is considered, enabling the demonstration of some mechanistic variation in formally synergistic systems. This introduces competition between concerted transformations by individually heterobimetallic reagents and stepwise conversions, where new reactivity is enabled in spite of individual steps proceeding through the action of much more traditional reagents that cooperate. Though Chapter 2 tends towards the applications of these new systems, the theme is drawn out further in Chapter 3, where magnesiate chemistry begins to explore the seminal advances made by Mulvey and latterly O'Hara in what was originally termed 'inverse crown ether' chemistry but is now more broadly thought of in terms of a methodology for templating selective metalation. This has empowered chemists to achieve selectivities that contrast with those associated with established directing effects. Thereafter, looking elsewhere in group 2 allows an exploration of recent advances in calciate, strontiate, and bariate complexes.

Chapter 4 is the first of two chapters dominated by a catalytic emphasis on our consideration of applied chemistry. It does this, though, through the lens of understanding chemical processes. It directs the reader towards the role of computation in understanding how heterobimetallic complexes that now take in d-block as well as main group elements can work, and also the applications of these systems in environmentally relevant polymerization processes dependent on the activation of small molecules. As is pointed out, proving positive cooperative effects is easily done, though understanding the nature and origin of a cooperative mechanism remains difficult, with concerted or consecutive actions of different metals potentially competing, and then with variance between formal metal-metal bonding or metals tethered by bridging ligands. In all this it is important to bear in mind that limited direct observation of structures will typically be possible on catalytic species in operando. Catalysis remains the dominant theme in Chapter 5, but more through the prism of p-block chemistry, and of group 13 in particular. Noting that the domain of catalysis has been dominated by d-block metals, this chapter records that the beginning of the twenty-first century has seen a shift towards main group element alternatives. This is the context then in which the remarkable reactivity of group 13 compounds have entered the catalytic arena. While tuning of their Lewis acidity and electrophilicity is possible by varying metal coordination number, it is the conversion of covalent complexes into cationic derivatives that will dominate. The chapter provides an overview of recent developments in the synthetic, characterization, computation and reactivity studies and applications in catalysis of cationic complexes. It will cover boron for completeness, but focus on aluminium and the higher group 13 elements.

Following the introduction of convenient spectroscopic handles in Chapter 5, multinuclear NMR studies allow the interrogation of solution processes and intermediates in Chapter 6. This picks up ideas hinted at but not fully explored in Chapters 1-3. The chapter revisits early studies in the NMR spectroscopic analysis of recognizably traditional organolithium and lithium amide systems, but then brings that work up-to-date, looking at the recent elegant advances in our understanding of the solution dynamics of enolates. The main focus of this chapter, though, relates to recent developments in diffusion-ordered spectroscopy. Again, this is contextualized though reference to the early work of Williard, but then shifts to modern lithium amide chemistry capable of being done under aerobic conditions. Most particularly, though, modern diffusion methods have allowed us to better understand the real solution chemistries of heterobimetallic formulations. This has been greatly enabled by the work of Stalke, raising the bar where the accuracy of molecular weight determination in solution is concerned. In consequence, diffusion analysis and multinuclear spectroscopy has prompted a much truer understanding of the aggregation and solvation of highly applied systems-for example the turbo reagents of Knochel.

The final set of chapters deal with up-to-date aspects of (mostly main group) synthetic applications of organometallic and metalloorganic chemistry. Chapter 7 is focused on the chemistry of boryl anions. In these, the nucleophilicity at the group 13 centre contrasts with the more common Lewis acid chemistry of boron, enabling a range of boryl-substituted organic, main group, and organometallic compounds to be prepared. Chapter 8 deals very much with synthetic organic chemistry as accessed by ate complexes. A range of ate complexes are discussed, but the chapter opens, appropriately enough, with work done in Sendai in the early 1990s. Initial discussion of sterically demanding tetramethylzincate in a range of transformations eventually shifts to the advent of organoamidozincates that are structurally covered in earlier chapters. From there, the evolution of aluminate and cyanocuprate congeners follows naturally. Again, structural aspects of these species feature elsewhere, but the discussion here focuses on their potential in aromatic derivatization. The chapter also continues a theme from the previous one. Efforts to exert control over the reactivity of boryl anions through ate complexes are presented in the context of novel nucleophilic boration. Lastly, Chapter 9 finishes the book as it started, looking at organocopper compounds. Copper-mediated processes are well established, prompting much organocopper structural chemistry. Whereas efforts have often focused on alkynyl- and arylorganocopper complexes, the emphasis here is instead on traditionally unstable mono-alkenyl and butadienylcopper compounds. The potential of steric, coordinative and synergistic strategies for preventing decomposition are examined.

Overall, this book seeks to bring the reader up-to-date with developments in the field of modern polar organometallic chemistry, particularly in the context of the emergent areas of synergic and cooperative species. The recent advances in our understanding of the operation of these systems are presented, but the evolution of the area is described, offering a context and explaining the need and rationale for the advent of new reagents. The hope is that this is done in a way that makes the text accessible to students as well as academics and industrial scientists, arming them with a broad understanding of how these systems present new opportunities to the synthetic chemist.

A. E. H. Wheatley
Cambridge, UK

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