Transition Metal Catalysis in Aerobic Alcohol Oxidation
I welcome this new contribution in the RSC Green Chemistry book series on the use of transition metal catalysis in the aerobic oxidation of alcohols. It is timely and resonates well with all aspects of the Green Chemistry agenda. Safe, selective oxidation of alcohols is a fundamentally and strategically important process for all molecule makers who are involved in serving Society's needs for new functional materials. More than any other reaction, this process provides an opportunity and sets the benchmark for the discovery of new hetero- and homogeneous catalysts under aerobic conditions.
These reactions, in combination with sustainable solvent selection, consideration of costs and recyclability of the systems, provide a step change in how compounds will be prepared on scale in the future. As evidenced by the chapters in this book, the field is evolving rapidly. Clearly, nanoparticulate materials, multi-catalytic systems and novel immobilization methods are becoming increasingly important. Pleasingly, all these developments pay respect to our desire for greatly improved safe and sustainable chemical practices. Many of the chapters emphasize the robustness of the processes to facilitate scaling-up of the reactions while also addressing the issues that arise during downstream processing.
Potentially hazardous components during these oxidation reactions relate to the safe use of oxygen gas. This is addressed by using air or, even more wisely, oxygen gas further diluted with nitrogen. Although some practitioners in the area are also naturally disposed to criticize previous methods or prior art in order to justify incremental advances, the real importance of any new procedure usually derives from the selectivity of the transformations when compared directly with other methods. This is an area that is not always appreciated, yet is the key to the success of any new oxidative strategy. The trends in the use of these methods are to harness synergistic effects that arise by combination with supporting materials or using the special characteristics that arise from tuneable nanoparticle aggregation. Indeed, in general it is the physical aspects of the new reagents that play a greater and increasing role than previously suspected. Likewise, quantitative recovery of expensive transition metal catalysts is becoming a crucial component of any new procedure. It is apparent from the various chapters that the methods by which oxygen gas is introduced into the reaction environment will have a noticeable impact both on future reaction design and on the precise experimental conditions. In this regard, flow chemistry methods are increasingly being touted as providing improved reaction kinetics through better mixing and gas introduction.
These are clearly interesting times, where the pace of change in these new procedures is leading to real innovation in solving these fundamentally and intrinsically important reactions.
While many challenges still need to be faced in terms of selectivity, compatibility, cost and robustness, this new textbook provides an excellent starting point for the discovery of novel aerobic oxidation processes using transition metal catalysis that can even go beyond those of simple alcohol oxidations.
Steven V. Ley
University of Cambridge, UK