Science of Synthesis

DOI: 10.1055/sos-SD-201-00001

de Vries, J. G., Science of Synthesis: Stereoselective Synthesis, (2011) 1, 1.

It must have been around 1980. I was living in the Boston area doing a postdoc at Brandeis University after obtaining a Ph.D. in The Netherlands on asymmetric ketone reductions using a chiral dihydropyridine. Back then we called it bio-mimetic chemistry; the word organocatalysis had yet to be invented. The occasion was a visit of Ryoji Noyori to MIT where Barry Sharpless was his host. The latter had just invented the titanium-catalyzed asymmetric epoxidation of allylic alcohols and Noyori gave a brilliant lecture on the asymmetric reduction of aromatic ketones using a combination of lithium aluminum hydride and BINOL at −100 °C. Ruthenium–BINAP was still in the making. The packed lecture room was boiling with excitement; we all knew that this was a historic moment. Thirty years later, it may be hard to imagine what the excitement was about. Not many people realize that as little as thirty years ago there simply was no catalytic method to obtain a chiral alcohol with high enantioselectivity, and there was no catalytic method to obtain an epoxide with high enantioselectivity. These were inconceivable breakthroughs at the time.

Today, we have a wealth of catalytic methods to choose from to perform a range of reactions types with impressive stereo- and enantioselectivities. Among these transformations, those on alkenes and other unsaturated carbon—carbon bonds are highly attractive as they are additions, and thus by nature highly atom-economic. There is an astonishing range of metal precursors and chiral ligands available, and some ligands can even be synthesized in large numbers by a robot. Organocatalysis is catching up, although reaction rates are often still too low for industrial applications. But this is a rapidly moving field where many new developments can be expected in the coming years. Enzymatic catalysis has made great strides in the past 10 years with many highly enantioselective processes utilized in the plant. In this volume, its discussion is limited to epoxidation and aza- and oxa-Michael reactions. Although nature produces a multitude of alkenes, particularly in the oleochemical and terpenoid areas, they are rarely modified further.

In Section ‌1.1‌, Kilian Muñiz has expertly reviewed what is largely a Sharpless legacy: the asymmetric dihydroxylation and aminohydroxylation reactions. In addition he describes the much more recent diamination and dibromination reactions.

In Section ‌1.2‌, Kazuhiro Matsumoto, Tsutomu Katsuki, and Isabel Arends discuss the stunning progress that has been made in the area of alkene epoxidation; here a broad palette of homogeneous catalysis, organocatalysis, and biocatalysis can be applied.

In Section ‌1.3‌, Stefano Colonna and Dario Perdicchia review the epoxidation of enones using hydrogen peroxide. Of course a large part is devoted to the Julià–Colonna reaction, an organocatalytic reaction avant la lettre. But in all fairness, other organocatalytic and metal-catalyzed approaches are described equally well.

In Section ‌1.4‌, Jeff Johnston, together with Hubert Muchalski, describes the much more recent stereoselective aziridination reactions that are intimately connected with the use of hypervalent iodine compounds, another area that has taken off quite recently. In spite of the relatively small size of the chapter, the diversity of technologies used is quite remarkable.

Dave Ager has a knack of being in the right place when the action takes place. He has seen it all from Knowlesʼ l-DOPA process during his Monsanto years up to the use of MonoPhos while working for DSM and everything in between. It is clear that he was an obvious author for Section ‌1.5‌, which describes the (asymmetric) hydrogenation of C=C bonds. Not surprisingly, the focus is very much on the ligands. However, the substrate scope has also made tremendous advances in the past 10 years.

Yong-Gui Zhou is archetypical for a steadily growing group of Chinese scientists who are rapidly rising to world fame in the area of stereoselective synthesis at this very moment. In Section ‌1.6‌ he and Sheng-Mei Lu describe the area to which he has made many contributions: the stereoselective hydrogenation of arenes and heteroarenes. Benzoannulated hetarenes seem to be a done thing, but who is finally going to crack the pyridine problem?

In Section ‌1.7‌, John Brown and Bao Nguyen review stereoselective catalytic hydroboration and diboration reactions. The final products are chiral alcohols and amines. John Brown was one of the pioneers in the rhodium-catalyzed enantioselective hydroboration reaction. The diboration reaction is of recent date and is still developing in interesting new directions.

In Section ‌1.8‌, Ilan Marek, together with Ahmad Basheer, describe the wondrous world of catalyzed carbometalation reactions, a research area where he is one of the leading investigators. Being a novice in this field, I could first hand test the usefulness of this chapter to the uninitiated. I am happy to say that the authors passed with flying colors.

The carbonylation, hydroformylation, and hydrocyanation reactions described in Section ‌1.9‌ are typically used in industry much more than in academia. Thus, we were very happy to have ex-Shell co-worker Piet van Leeuwen, who, after being a professor in homogeneous catalysis at the University of Amsterdam, has now started his post-retirement career at the ICIQ, Tarragona. Thus, this is a chapter written by someone who knows both the theoretical as well as the applied side and in addition is a master of didactics. The result reads like a mini handbook.

Hydrovinylation and hydroarylation, covered in Section ‌1.10‌, are nice examples of reactions where C—C bonds are made without any waste. If only we had more of those. This section is a successful Euregional collaboration (the Euregion is where Germany, The Netherlands, and Belgium meet): authors Walter Leitner and Giancarlo Franció are from the RWTH Aachen and Paul Alsters is one of my colleagues at DSM in Geleen. Although the scope of these reactions is still limited, there is quite a bit of development in the field. This gives a taste for more.

In Section ‌1.11‌, we are treated to more waste-free C—C bond formation. Kristin Schleicher and Timothy Jamison describe this fast growing class of reductive coupling reactions where two π-systems are joined via the action of a reducing agent. Initially, stoichiometric reagents such as triethylborane or diethylzinc were employed, but increasingly hydrogen is also used for these reactions, which makes them highly atom efficient.

Arguably the most versatile of all the reactions described in this volume are the conjugate addition reactions between Michael acceptors and a variety of C-, N-, O-, and S-nucleophiles. Section ‌1.12‌ has been a major effort to put together and indeed is the longest chapter in this volume. An excellent job has been done by Mimi Hii and Bao Nguyen (metal catalysis and organocatalysis), and Dick Janssen and Wiktor Szymański (biocatalysis). It should also be mentioned that Bao Nguyen is co-author in two chapters of this volume.

Stereoselective hydroamination reactions described in Section ‌1.13‌ by Kai Hultzsch and Alexander Reznichenko are again of relatively recent origin. And although we had set a minimum of 90% ee for reactions to be described in Stereoselective Synthesis, Kai smuggled in quite a few examples where the ee is lower because there simply are not that many >90% examples of this reaction. This remains an interesting challenge as the preparation of chiral amines directly from alkenes is a highly desirable reaction also from the applied perspective.

Not many people know that the first industrially applied asymmetric catalysis reaction was a copper-catalyzed asymmetric cyclopropanation that originated from Noyoriʼs work as a graduate student in Nozakiʼs group and later developed by Aratani into an industrial process for Sumitomo. These days a variety of copper, rhodium, and cobalt catalysts are known for this reaction, in addition to the many stereoselective variants based on stoichiometric zinc. Marie-Noelle Roy, Vincent Lindsay, and André Charette did an excellent job in reviewing this chemistry in Section ‌1.14‌.

Given the fact that alkenes are not inherently chiral, there is a surprising range of asymmetric and diastereoselective metathesis reactions available. Shawn Collins did a great job of describing all the different variants in Section ‌1.15‌. Obviously, they include kinetic resolutions and desymmetrizations of trienes and also of mono-enes in a tandem asymmetric ring-opening/cross metathesis reaction, and many other variants. The products are interesting cycloalkenes or unsaturated heterocycles.

Although there is quite a bit of literature on stereoselective radical reactions, the number of true experts in this area is rather low. Thus, we were very happy when Mukund Sibi agreed to write Section ‌1.16‌ together with Seiji Hata. A large part of the chapter is devoted to diastereoselective reactions, although enantioselective reactions as well as organocatalytic approaches are becoming more important. In addition, there is a thoughtful inclusion of a subchapter on the replacement of the popular but toxic tin reagents.

Tamio Hayashi and Jin Wook Han have written the final chapter, Section ‌1.17‌ on asymmetric hydrosilylation reactions. Many classes of alkenes, including aliphatic ones, can now be hydrosilylated with excellent enantiomeric excesses. Both authors have made major contributions to this field, guaranteeing a high quality throughout.

So looking back over a period of 30 years, we see that an astonishing revolution in stereoselective synthesis has taken place, resulting in the fact that most of the reactions described in these volumes can be effortlessly carried out by beginning students as the majority of important ligands are now commercially available.