Tutorial pubs.acs.org/Organometallics Tutorial on Oxidative Addition Jay A. Labinger* Beckman Institute and Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States ABSTRACT: This tutorial introduces oxidative addition as a reactivity pattern and organizing principle for organometallic chemistry. The history, characteristics, and scope of oxidative addition are briefly surveyed, followed by a detailed examination of the variety of mechanisms found for the oxidative addition of alkyl halides and their relevance to practical applications. ■ INTRODUCTION comprehensive review but rather as an introduction to the The recognition of oxidative addition as a common pattern of topic, such as might be presented in a lecture, as part of a reactivity has played a central role in the development of course on organometallic chemistry. Accordingly, the tone is organometallic chemistry over the second half of the 20th rather informal, and citations have been limited to a moderate number of historically significant papers. Those interested in century. The starting point for modern organotransition-metal following up on any aspects can find more details and thorough chemistry is usually taken as the discovery and structural referencing elsewhere; Hartwig’s recent textbook5 is a good characterization of ferrocene in the early 1950s (of course, starting point. there were many important earlier contributions). That inspired a large amount of new chemistry during the next decade or ’ soso much, in fact, that it was not easy to codify it in any ■ VASKA S COMPOUND AND OXIDATIVE ADDITION rational manner. There was as yet no well-delineated set of The square-planar, d8, 16-electron Ir(I) complex trans-IrCl- ff fi reactivity patterns that had served so e ectively as organizing (CO)(PPh3)2 was rst synthesized rather serendipitously, principles over the preceding century of organic chemistry.1 apparently6by Vaska and DiLuzio in 1961.7 That report Notably, the two most popular organometallic chemistry included the reaction with HCl. A year later the same authors 8 textbooks of the 1960s were arranged according to periodic described analogous additions of Cl2 and, especially, H2. It group2 or ligand type.3 Either can be useful for categorizing soon became clear that Vaska’s compound, as it subsequently information, in its own way, but neither is particularly effective came to be universally known, is a highly versatile platform for in terms of explanatory power and pointing the way forward. the generalized reaction of eq 1. Examples of A−B, in addition The 1960s saw the beginnings of determined efforts toward to those already mentioned, include organic halides such as systematic, reactivity-based organization, well represented by MeI, metal halides such as SnCl4, metal hydrides such as R3SiH, Collman’s 1968 Accounts of Chemical Research article4 “Patterns etc.4 of Organometallic Reactions Related to Homogeneous I III Catalysis”, in which he identified electron count and Ir Cl(CO)(PPh32 )+−→ A B Ir ClAB(CO)(PPh32 ) (1) coordinative unsaturation as key concepts and described important reactivity patterns such as migratory insertion and, Since this reaction involves a net formal oxidation from Ir(I) especially, oxidative addition. This approach increasingly took to Ir(III) accompanied by increases in both coordination hold, culminating in the seminal 1980 text by Collman and number (4 to 6) and electron count (16 to 18), the term fi Hegedus,5 which clearly demonstrated its pedagogical oxidative addition seems obvious and logical; the rst person to 9 fi strengths. The great utility of reactivity- and mechanism- use it in this context though (so far as I have been able to nd) was not Vaska, but rather Collman, in a 1965 paper on the based thinking, which has been demonstrated in all aspects of 10 organotransition metal chemistryboth textbook and frontier related chemistry of Ru(CO)3(PPh3)2 (eq 2). Note that these sciencecan fairly be said to have all started with oxidative are not perfect analogues: while the formal oxidation state does addition. increase by 2 units, from Ru(0) to Ru(II), the starting This tutorial begins with the basic concept of oxidative compound is 5-coordinate, 18-electron, with one of the original addition and issues concerning its definition and scope. It then CO ligands being lost at some point. That raises the question even at this extremely early point in the historyof what a focuses on one particular class of oxidative addition, reactions “ ” of alkyl and aryl halides, emphasizing the mechanistic variety pattern really entails. Is stoichiometric similarity what matters, observed even for stoichiometrically similar transformations, and concludes with the relevance of mechanistic considerations Received: June 29, 2015 for some practical applications. It is not intended as a Published: October 26, 2015 © 2015 American Chemical Society 4784 DOI: 10.1021/acs.organomet.5b00565 Organometallics 2015, 34, 4784−4795 Organometallics Tutorial − ν and if so, how much variability should we allow? Or should O2 parameters the O O bond length and OO of the O2 ’ mechanistic similarity be the main criterion? To what extent adduct of Vaska s compound to those of free O2 and its anions. can we infer one from the other? These are important issues As Table 2 shows, the IrIII−peroxo picture looks by far the that will recur throughout the discussion. best.13 0 Ru (CO)332 (PPh )+− A B Table 2. Bond Parameters for Several O2 Species II →+Ru AB(CO)232 (PPh ) CO (2) ̂ ν −1 species rOO,A OO,cm ’ In retrospect, it is clear that Vaska s compound was the ideal dioxygen, O2 1.21 1556 − starting point for studying oxidative addition. We can change superoxide, O2 1.33 1145 2− ligands virtually at will: Cl to other X-type ligands, such as peroxide, O2 1.49 820 halides and pseudohalides, and PPh3 to other L-type ligands, IrCl(O2(CO)(PPh3)2 1.47 850 usually a tertiary phosphine or arsine.11 That flexibility provides ready access to examining electronic and steric effects on the kinetics and (in some cases) thermodynamics of the reaction. Monitoring kinetics is likewise convenient, using either visible As these are apparently “real” oxidations, at least by the ν or infrared spectroscopy. For the former, the starting material is criterion of CO, we can expect certain trends in thermody- yellow, while just about every oxidative adduct is colorless; the namic favorability, with the trends in kinetics quite possibly latter takes advantage of the fact that CO stretching bands in but not necessarilyrunning in parallel. These are mostly metal carbonyls are strong, sharp, and highly sensitive to the borne out in experience. For example, the equilibrium constants electronic environment at the metal center, reflecting the for formation of the H2 adduct of IrCl(CO)L2 follow the degree of back-bonding into the CO π* orbitals. fi sequence L = PPh3 <P(n-Bu)3 > P(cyclohexyl)3: the rst That last feature helps sheds light on an important question: inequality reflects the greater basicity/electron donating power are these reactions truly oxidative, or is this only in a formal of trialkyl- vs triarylphosphines, while the second reflects steric sense? (We do not normally think of H as an oxidizing agent, 2 crowding, which is more pronounced for the 6-coordinate but all one-electron ligands are conventionally treated as anions ff for the determination of formal oxidation state, including H−.) product than the 4-coordinate reactant. The e ect of varying Table 1 shows ν values for several examples of eq 1 the X ligand is generally less predictable, perhaps because the CO effects of electronegativity and π-donor ability can operate in Table 1. CO Stretching Frequencies for IrClAB(CO)(PPh ) opposing senses. 3 2 Comparing complexes of different metals, especially when − ν −1 A B CO,cm the differences extend across periodic groups and electronic none 1969 configurations, is complicated by effects of net charge, H2 1983 preference for 4- vs 5-coordination, etc. However, trends O2 2015 within more or less isostructural complexes from the same HCl 2045 periodic group are usually reliable. In particular, there is a CH3I 2047 general tendency for higher oxidation states to become Cl2 2075 increasingly favored on descending within a group of the periodic table (compare, for example, the M(VIII) species fi (including O2, which does not exactly t the model; we will FeO4, which is unknown, RuO4, a metastable, uncontrollably return to that shortly). Everything else being equal (which it powerful oxidant, and OsO4, a stable and useful reagent for ff strictly is not, since the geometries of the products di er: H2 organic oxidations), and that applies to oxidative additions: fi and O2 add in a cis con guration and the rest in trans), a higher thermodynamically for certain and often kinetically as well. For ν ff CO value indicates a higher e ective oxidation state. example, the H2 adducts of Rh complexes RhCl(CO)L2 are Note that the strong oxidant Cl2 gives the greatest increase in much less stable than those of Ir analogues; likewise, the rate of ν CO, while H2 appears to result in only a small (but real, and addition of RX is considerably slower for Rh than for Ir, an positive) change, as we might have expected. Actually the latter effect of which we will see in the very last section. − is somewhat misleading: metal hydrogen stretching vibrations This trend bears a good deal of responsibility for yet another − −1 fall in the same frequency range, typically 2000 2200 cm , generalization, that the best homogeneous catalysts are found and if symmetry permits, observed peaks represent mixed −1 among the second-row transition metals Ru, Rh and Pd in vibrations, so that the band at 1983 cm is not a pure CO particular.