Organometallics 2011, 30, 5–6 DOI: 10.1021/om100982z
5
Can We Predict the Future of Organometallic Chemistry?† Irina P. Beletskaya*,‡ and Valentine P. Ananikov*,§ ‡
Chemistry Department, Lomonosov Moscow State University, Vorob’evy gory, Moscow 119899, Russia, and §Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow 119991, Russia Received October 13, 2010
An essay is presented about the future development of organometallic chemistry and the role of transition-metal catalysis.
† Part of the special issue Future of Organometallic Chemistry. *To whom correspondence should be addressed. I.P.B.: fax, þ7 495 9393618; e-mail,
[email protected]. V.P.A.: fax, þ7 499 1355328; e-mail,
[email protected].
oversight can perhaps be attributed to the complicated mechanistic picture of the catalytic reactions and some bias within the organometallic community at the time toward experimental and theoretical studies of the mechanisms of multistage catalytic transformations. The key turning point was reached with development and understanding of the mechanism of palladium-catalyzed Csp2-Csp2 and Csp2-Csp bond formation, followed by Csp2-heteroatom bond formation reactions. Palladium-catalyzed transformations opened a new era in organic synthesis, which in a short period of time was extended to other transition-metal complexes. Transition-metal catalysis rapidly grew from the point of a convenient synthetic approach to a leading force in organic chemistry. The importance of catalytic cross-coupling chemistry and the Heck reaction was established with a recent Nobel Prize (2010). Chiral ligands gave rise to asymmetric hydrogenation, an area also highlighted by a Nobel Prize (2001), and facilitated preparation of other classes of chiral organic molecules with high enantiomeric purities. Quite naturally, this area currently has captured the attention of numerous pharmaceutical companies and has brought up new challenges. Carbene complexes were of great utility in solving the problem of olefin metathesis, again highlighted with a Nobel Prize (2005). In this regard, one should take into account that carbene species themselves represent a relatively “young” phenomenon in chemistry. The Huisgen chemistry of dipolar cycloaddition became incredibly popular for joining two small molecular units together in a predictable manner, with an outstanding success of Cu-catalyzed reactions between alkyne and azide groups. A renaissance of Ullmann chemistry has shown that coppercatalyzed reactions are rather often more convenient than those of their corresponding palladium analogues and, obviously, are more cost efficient (as, for example, C-heteroatom bond formation). Without getting back to our past, it is not possible to predict the future; this is true not only for our history but also for attempts to predict the future of science. If we want to understand the future, the preceding development of organometallic chemistry should be recalled. It is organometallic chemistry that allowed approaching valence theory at the beginning stages of chemical science. The preparation of metal compounds by Frankland with different numbers of
r 2011 American Chemical Society
Published on Web 01/04/2011
Prediction is very difficult, especially about the future of science. Our experience and knowledge obviously show that no one could possibly know what is going to happen. Not long ago, even at the middle of the last century, it was hardly possible to imagine the changes that catalysis by transitionmetal complexes might introduce into the modern face of organic chemistry and organic synthesis. Indeed, from the point of view of our recent history some decades ago, would it be possible to foresee how many new reactions will be discovered? What outstanding tools would be developed for organic synthesis? How impetuous the earliest days of cross-coupling chemistry would be and what a surprising exponential growth would follow afterward? Could we imagine thousands of publications on SuzukiMiyaura, Sonogashira, Heck, and carbon-heteroatom bond formation reactions snowballing into avalanche? Moreover, it would have been hard to imagine the carbonylation of organic halides at 1 atm pressure with industrial efficiency. It is equally hard to imagine the development of activation of multiple carbon-carbon bonds and elementelement/element-hydrogen bonds (E-E and E-H) to a certain level that allows atom-economic addition reactions to be carried out with complete stereo- and regioselectivity. It is still not easy to realize that enantioselective synthesis is a part of everyday laboratory practice, rendering complicated natural product molecules more and more accessible. Meanwhile, transition-metal-catalyzed transformations were well-known at earlier times in the last century, with several excellent examples available among the processes of industrial importance and those feasible in research laboratories. To name a few, there are hydrogenation and hydroformylation, the Wacker process, hydrocyanation of 1,3-dienes, and, of course, polymerization, which attained their practical value owing to catalysis by transition-metal complexes (mostly as homogeneous catalytic systems in solution with soluble metal complexes). For a long time these excellent catalytic reactions were far away from the needs of organic synthesis and were overlooked in the development of the toolbox of organic chemists. This
pubs.acs.org/Organometallics
6
Organometallics, Vol. 30, No. 1, 2011
organic groups, R2Zn, R3Al, R4Sn, as well as tetravalent carbon by Kekule, gave the fundamental basis for further development. Studies of organometallic chemistry, especially reactions of organotin and organomercury derivatives, gave rise to formulation of the SE1-SE2 mechanism; electrophilic substitution is a counterpoint to the usage of SN1-SN2 nucleophilic substitution. The chemistry of carbanions and the theory of carbanionic ionic pairs also are directly related to organometallic chemistry. Altogether, it was a valuable contribution to the organometallic chemistry of nontransition metals. In addition, we must take into account reactions involving Grignard reagents, and later reactions involving lithium, zinc, and aluminum organic derivatives: neither past nor present chemistry can be imagined without this chapter. However, catalysis is now the main subject of organometallic chemistry and organic synthesis. Just a few decades ago most would say homogeneous catalysis by soluble complexes of transition metals only, but this mentality is already in our past. A fascinating revolution is taking place with the emergence of heterogenized catalysts and especially nanosized catalysts, including immobilized nanoparticles on a solid support, on soluble polymers, dendrimers, micelles, etc. Not long ago the homogeneous and heterogeneous catalysis worlds, each having their well-known advantages and disadvantages, were considered different research areas with marginal interplay between them. Research groups were concentrated in either area and rather often even did not hear about problems and achievements in the other catalysis world. Currently these areas have become as close as ever possible. Heterogeneous catalysis has now gained the best properties of homogeneous catalysis: high selectivity and efficiency and the ability to carry out asymmetric transformations. On the other hand, homogeneous catalysis has attained such characteristics as stability, easy separation, and even recycling. Of course, the mechanistic picture has become more complicated, with several issues remaining unresolved about the real nature of the catalytic species and possible in situ interconversions between the homogeneous and heterogeneous catalysts. A good question is “what next?” for organometallic chemistry in the near future. Definitely, different answers will be given from the points of view of inorganic and organic chemists. Most likely, inorganic chemists will dream about new materials with various novel physical properties and practical applications. Organic chemists will first of all think about organometallic chemistry and transition-metal catalysis toward the development of organic synthesis. In our Russian tradition, organometallic chemistry was always considered as a part of organic chemistry, which was different from coordination chemistry. Next, the contribution of organic and organometallic chemistry (including main-group chemistry of Si, P, B, Ge, Se, etc.) in the construction of smart materials cannot be neglected. Rapid progress can be expected in the design of conductors, superconductors, semiconductors, molecular
Beletskaya and Ananikov
magnetics, electronic devices, nonlinear optic devices, organic light-emitting devices, solar cells, photonics, spintronics, fuel cells, and sensors (to name a few). An organic and organometallic chemistry approach is especially powerful for creating multifunctional and hybrid materials, whose integrated properties will be superior to the individual properties of the components. It is really hard to believe how deep changes in our abilities to synthesize, manipulate, and produce new matter may become the reality of tomorrow. For the students entering our laboratories now anything seems possible. Concerning the immediate future of catalysis, we believe more and more industrial applications will benefit from implementation of new techniques in catalysis and from the preparation of new transition-metal complexes serving as catalysts. To our knowledge this will also include syntheses of enantiomerically pure compounds. It is not uncommon to expect that transition metal complexes themselves (both reagents and catalysts) will be generated or assembled in situ directly under the reaction conditions. Expensive and rare metals should be replaced by simple and readily available analogues. Understanding the mechanism of catalytic reactions is a rational way to find suitable replacements (Pd, Pt, Rh f Cu, Ni, Co, Fe) and encourage the further development of catalysts (Ru, Re, Au, Ag, Ln). For several important organic reactions (such as reduction and oxidation, formation of new bonds, cyclization, etc.), where a considerable mechanistic knowledge has been gained, rapid elaboration of new, highly efficient catalysts operating under mild conditions may be anticipated without time-consuming trial and error screening. Thanks to the development of a diverse range of fascinating ligands, the behavior of catalytic metal centers can be controlled to promote the desired chemical transformation and suppress the side reactions. Selective activation of Csp2-H bonds, as well as Csp3-H bonds, will introduce low-cost natural starting materials into organic synthesis. Cracking of C-C bonds under relatively mild conditions will solve economic and ecological problems in various aspects of modern life. In this area, catalysis by nanoparticles promises to be particularly efficient, since it relies not only on metal-dependent catalytic activity but also on size and shape dependence in the variation of catalytic properties. Morphology-oriented catalyst design is undoubtedly a very intriguing puzzle related to metal particles. Organometallic chemistry, as a bridge between inorganic and organic chemistry, brings up the subjects and models for theoretical studies, including the nature of chemical bonds, reaction mechanisms, bioinspired processes, and control of enzymatic catalysis. Organic derivatives of metals and organometallic catalysts will be the unfailing sources of new materials and technologies in the near future. Whether our attempts to predict the future of organometallic chemistry will be successful or not, we can clearly say that this science will remain an area of active ongoing research, where knowledge and intuition are equally important. Skill and luck will guide the research, with plenty of space for inspiration and art.
