This volume is number 51 of a very long series, first published in 1970 under the editorship of Professor Stuart Trippett and which subsequently has involved a wide a range of significant contributors to the literature of organophosphorus chemistry. This volume provides a selected review of papers in this area, published in the period from January 2020 to January 2021, and continues our efforts to provide an up-to-date survey of progress in this topic, which still generates a vast amount of research. We have again been fortunate in securing contributions from a team of international authors from the UK, Germany, Russia, Poland, Portugal, Italy, Hungary and India and it is noteworthy that the efforts of all our contributors have been achieved against a background of the Covid virus problems. We also note that Professor John Tebby has now decided to step down from his editorial role in this series and we thank him for his many years of service, both as an author and latterly as a co-editor.

Chapter 1 covers the literature published during 2020 relating to the synthesis and reactivity of tertiary phosphines, continuing a series in recent years from the group of Professor Andrey Karasik in Kazan. Although it has been necessary to continue to be selective in the choice of publications cited, it is hoped that the most significant developments have been noted. A significant number of review articles has been published in 2020 and some of these reviews are cited in the various sections of this report. The use of a wide range of tervalent phosphorus ligands in transition metal catalysis and in organocatalysis continues to be a major driver in the chemistry of traditional P–C-bonded phosphines which inspires the design of new phosphines of various structures and the tuning of their properties.

In Chapter 2 Mariette Pereira and colleagues have covered the most relevant literature published during the year 2020, related to tervalent phosphorus acid derivatives. Some of the most relevant concepts are reported, regarding synthetic methodologies to halogenophosphorus compounds, phosphorus esters (phosphinite, phosphonite and phosphites), phosphorus amides (aminophosphines, phosphoramidites and diamidophosphites) and mixed phosphorus compounds bearing two different tervalent phosphorus moieties. A critical selection of their applications is briefly assessed, with an emphasis on the catalytic applications of ligands and metal complexes.

In Chapter 3, György Keglevich gives an account of new developments in the synthesis and reactivity of phosphine chalcogenides, mainly oxides and sulfides, published during 2020. A series of new synthetic methods for phosphine oxides are discussed, followed by preparations utilizing simple P-reagents such as P(iii)- and P(v)-chlorides, and >P(O)H species. Their additions to unsaturated substrates and P–C coupling reactions, involving interesting variations, represent a hot topic. The modification of P-chalcogenides including deoxygenations are also shown. Optically active phosphine oxides and their complexes also form a usual part of this chapter.

In Chapter 4, Maurizio Selva and his group have reviewed the preparation, characterisation, reactivity and applications of phosphonium salts and ylides during 2020. The large number of cited references highlights the importance of P-based derivatives in synthetic and non-synthetic applications. Topics are organized to offer an introductory survey on the methods of preparation and characterisation of both groups of compounds, followed by an analysis of applicative and curiosity driven research. Again, the significant growth in the number of references highlighting the importance of phosphonium-based derivatives in synthetic- and non-synthetic applications, in particular their use as non-aqueous ionic liquid solvents (PILS), is noteworthy.

In Chapter 5, Balczewski, Owsianik and Turek cover advances in 2020 on phosphoric, phosphonic and phosphinic acid chemistry, showcasing their respective syntheses, reactions and applications (both medicinal and non-biological). Recent developments on natural products containing the phosphate group, such as the psychoactive psilocybin, or where phosphate chemistry has been used in the synthetic pathway but is not present in the final compound, for example islatravir, are also described, as is the trend to modify natural products by the inclusion of a phosphate group, exemplified here by examples of modified coumarins and warfarins. For phosphonates, investigations into the synthesis and cytotoxicity of α-aminophosphonates continues apace, whilst bisphosphonates and their acid derivatives have found applications as surface coatings for nanoparticles and as additional functionality in macrocyclic chelating agents. The use of phosphoric and phosphinic acids as flame retardants also receives due acknowledgement here. Each subsection concludes with developments in the corresponding nitrogen, sulfur and selenium analogues, together with a review of catalytic applications of these compounds, including their use in supported and asymmetric catalysis.

In Chapter 6, Gerd-V. Roeschenthaler and Romana Pajkert have again shown that the chemistry of penta- and hexa-coordinated phosphorus compounds continues to expand in its breadth and application.

Regarding this, the synthesis, properties and structural investigations of novel cage-phosphoranes, spiro-phosphoranes, fluorophosphoranes and spiro-oxaphosphetanes have been briefly discussed. Pentacoordinated phosphoranes have also been postulated as intermediates in the synthesis of dibenzophospholes, the carbofluorination of alkynes, Wittig-type reactions and also organocatalytic Mitsunobu reactions. Moreover, studies of novel hexacoordinated fluorinated phosphorus(v) compounds have been also evaluated.

Chapter 7 features a review by Vadapalli Chandrasekhar on developments in the synthesis and reactivity of phosphazenes, which once again is structured to cover three sub-topics published in the calendar year, 2020: the acyclic phosphazenes (iminophosphoranes), cyclophosphazenes and polyphosphazenes. Particular highlights of the chapter include the design of acyclic phosphazenes for the complexation of actinides, their use as super bases and in copolymerisation reactions, the material (notably flame retardancy) properties of cyclophosphazenes, a discussion of the related carbophosphazenes and their Group 11 and 12 coordination chemistry, and the final section which includes important discoveries in the applications of polyphosphazenes in drug delivery, catalysis (including the transformation of carbon dioxide into formate) and as cathode materials for batteries.

Finally, in Chapter 8, Goutam Brahmachari has provided an update on recent key developments during 2020 in the areas of green and sustainable chemistry and engineering, reviewing progress in green and energy-efficient synthetic approaches in organophosphorus chemistry. This area has now attained considerable growth, having become increasingly of interest among researchers working in all branches of chemical science, leading to developments in new green protocols for the synthesis of new and known organophosphorus compounds.

D. W. Allen

L. J. Higham

J. C. Tebby