Preface

This book, entitled Boron: Sensing, Synthesis and Supramolecular Self-Assembly, is part of the Monographs in Supramolecular Chemistry series edited by Philip Gale and Jonathan Steed and follows in the footsteps of Boronic Acids in Saccharide Recognition released in 2006 as part of the same series and edited by Sir J. Fraser Stoddart.¹  This present book extends the scope in that it covers all aspects of the supramolecular chemistry of boron and also includes many interesting developments in the use of boron-based synthetic building blocks.

The editorial team of James, Fossey and Li have assembled a diverse collection of chapters from world leading experts involved in the chemistry of boron from diverse areas of expertise, including synthetic, material and biological aspects of boron based systems. The chapters take us on a journey from molecules to polymers and from covalent to supramolecular bond forming reactions and interactions.

Whether you are visiting for the first time or as a regular visitor, we are delighted to welcome you to Fabulous Boron, we hope you enjoy your stay (Figure 1).

Boron is found in many everyday applications, from cleaning materials to glass, and its utility in the world of chemical synthesis and sensing has never been more important. Boron-containing materials are found in nature as minerals and have been well studied in this regard. Boron, in the eyes of the target reader of this book, may be more readily recognised in the chemical sciences for its utility and versatility in synthesis; boron is an important component in reducing agents, e.g. sodium borohydride and borane. Indeed it plays dual roles as both hydride source (BH₃) and a Lewis acid when one employs the CBS catalyst to asymmetrically reduce ketones, for example. It is the empty p-orbital of boron that bestows a Lewis acid character on the atom and one may also be familiar with boron’s use as a Lewis acid catalyst (e.g. boron trifluoride). It is as boronic acids or boronic esters that boron might be most revered by the synthetic chemistry community. These oxygen and carbon appended boron-containing species are key organic building blocks, as cross-coupling partners in palladium catalysed Suzuki–Miyaura reactions. They are vital to the world of small molecule synthesis for pharmaceuticals, agrochemicals and veterinary science. Readers are pointed to Chapters 3 and 13 of this book that pay particular attention to synthetic strategies exploiting boron.

In recent years the interplay between boronic acids and boronic esters has underpinned an explosion of self-assembly and supramolecular chemistry, which is also well covered by chapters within this book. Furthermore, the reversible binding of diols with boronic acids to form boronic esters has been exploited in the development of new chemical sensors or chemosensors for carbohydrates, including new sensing regimes for glucose. The fact boron is capable of functioning as a sensor for anions through conversion into a boronate emphasises the importance of this atom to the sensing community.

Sensing systems using boronic acids are of particular interest to the editors of this book,^2–6  who have established a track record in the area over a number of years. The editors, often in collaboration with authors of chapters in this book, have reported on boron-containing anion sensors,^7–9  carbohydrate sensors,^10–13  glycation recognition,^14,15  reactive oxygen species detection^16,17  and enantiodiscrimination sensors;^18–24  and mechanisms of sensing have included fluorescence, NMR spectroscopy, self-assembly, electrochemical responses and colorimetric assays (Figure 2).^25–28 

Shinkai and Kanekiyo discuss the development of boronic acid-based supramolecular systems. Supramolecular systems discussed include sugar-responsive gels, porphyrin–boronic acid, systems that exhibit guest-induced spectroscopic changes, two-dimensional self-assembly at the air–water interface, boronic acid-functionalized metal nanoparticles and boronic acid-appended polymers.

Anslyn and co-author’s provide a comprehensive description for the investigation of boron containing species by NMR spectroscopy. Indeed this chapter will be of broad interest in the field and provides both a handy first port of call and more in-depth analysis of issues pertaining to boron in NMR spectroscopy.

Blandin and Chavant discuss the use of boronic esters, other than pinacol, for use in cross-coupling reactions. They have identified an interesting alternative to the typical approaches that use hexyleneglycol, resulting in significant advantages that are discussed in detail in their chapter.

Yoon and Guo describe boronic acid systems that can bind to nucleophilic species, 1,2-diols, and arylboronates that are converted into the corresponding phenols by treatment with hydrogen peroxide. Resulting in fluorescent chemosensors for carbohydrates, dopamine, fluoride, metal ions and hydrogen peroxide.

Singaram and co-authors survey the development of boronic acids towards optical, continuous, sensing of saccharides. Since this group has made seminal contributions to the area it is pleasing to see the authors taking this opportunity to discuss their progress, the underpinning fundamental science and setting their contribution in context lets readers experience the full, impressive, story.

Zhao and co-workers describe boronic acid based systems for enantioselective fluorescent recognition. The scaffold used for the chiral sensors employs a fluorophore, arylboronic acid binding sites (two are best) and a linker (of appropriate size for the guest) between the two receptors.

The current understanding of synthesis and photophysical properties of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene derivatives (BODIPY dyes) and their important applications for molecular sensors are described by Zhu and Zhao. The often complicated synthetic routes required for the preparation of BODIPY derivatives has restricted their use as fluorescent reagents. However, Chapter 7 describes up-to-date methods for the preparation of many BODIPY derivatives. In addition, recent advances of using BODIPYs as signaling units for fluorescent probes selective towards different analytes are presented.

The latest developments in boronic acid chemistry for applications to electroanalysis are discussed by Marken. Boron and its derivatives used as Lewis acidic units have many applications for a range of nucleophilic targets in analytical electrochemistry. Chapter 8 reports recent progress in boronic acid-based electrochemical sensors both in solution processes and surface processes (on surfaces, in films, and in composites) for the detection of general biological analytes.

Boronic acids can be incorporated into polymers in order to improve polymer-polymer interactions and develop polymer based materials. The incorporation of specific boronic acid–diol interactions, dramatically improves the polymer properties. Such that, structural changes caused by the complexation are amplified and readily detected. Details of polymer–polymer interactions prompted by the boronic acid–diol interaction are reviewed in this chapter by Numata.

Guan and Zhang take a detailed look at boron-containing hydrogels with a view to their use as medical diagnostic tools. Their chapter comprehensively evaluates different assembly and application approaches across a diverse range of platforms.

Boronate affinity materials, such as macroporous monoliths, magnetic nanoparticles, mesoporous nanoparticles and molecularly imprinted polymers, for cis-diol-containing biomolecules’ recognition and separation has attracted significant attention over recent years. Liu and Li describe useful strategies for reducing the binding pH and enhancing the binding strength of the boronate affinity materials and also illustrate in detail their applications in recognition and separation of cis-diol-containing biomolecules including nucleosides, intact proteins and glycoproteins.

Kubo and Nishiyabu describe the use of reversible boronate esterification to build well-ordered microparticles through supramolecular polymerisation of benzene-1,4-diboronic acid with tetraols. When pentaerythritol is used as the tetraol component for self-assembly, thermodynamically stable flower-like microparticles are produced. Surface functionalisation enables formation of nanometal-deposited heterogeneous catalysts and white-light-emissive chemosensors.

Buckley’s chapter makes an impressive overview of the use of boron as a cross-coupling partner. The Suzuki–Miyaura reaction is one of the most versatile and important reactions in modern day organic synthesis and this chapter details some important recent cases in the area. Readers are taken through some of the principles of this chemistry and brought up-to-date with intriguing cases from the recent literature.