Platinum-Catalyzed CâH Functionalization - Chemical Reviews (ACS

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Platinum-Catalyzed C−H Functionalization Jay A. Labinger* Beckman Institute, California Institute of Technology, Pasadena, California 91125, United States ABSTRACT: Recent developments in C−H bond activation and functionalization by Pt complexes are surveyed. Topics include the following: fundamental mechanistic investigations of C−H activation; stoichiometric intra- and intermolecular C−H activation; reactions of dioxygen with Pt(II) complexes that may be relevant to substrate oxygenation; and both stoichiometric and catalytic formation of C−O, C−C, and C−B bonds via C−H activation. Current interests and trends are discussed, both in the context of historical work and to forecast future directions and opportunities for the field.

CONTENTS 1. Introduction: Background and Scope 2. C−H Activation at Pt 2.1. Mechanistic Investigations 2.2. Intramolecular C−H Activation 2.3. Intermolecular C−H Activation 3. C−H Functionalization 3.1. Oxygenation 3.1.1. Reactions of O2 with Pt(II) Species 3.1.2. C−O Bond Formation 3.2. C−C Bond Formation 3.3. C−B Bond Formation 3.4. H/D Exchange 4. Outlook Author Information Corresponding Author Notes Biography References

While Shilov and his co-workers in the (then) Soviet Union continued active research into this and related chemistry over the next couple of decades, it was not until the late 1980s that their work attracted widespread interest. By then the generality of facile C−H activation at transition-metal centers had been well-established, but most such reactions appeared unsuited to any practical scheme for converting alkanes to more valuable products. Most of the obvious nonoxidative routes (carbonylation, dehydrogenation, etc.) are thermodynamically disfavored at the low temperatures where organometallic chemistry is likely to operate, while nearly all of the complexes shown to activate C−H bonds are incompatible with oxidizing conditions. Because the Shilov chemistry stood out as a clear exception to those limitations,4 the next 2 decades featured extensive work aimed at both understanding the detailed mechanism of C−H functionalization by Pt and exploiting that understanding for designing useful functionalization schemes, including potentially economically viable routes for conversion of alkanesespecially methaneas well as selective transformations applicable to problems in synthesis of complex organic molecules. More recently, however, the prevalence of Pt in C−H activation reports appears to have fallen off significantly. Why? I suspect (I have not attempted any systematic investigation to support these conjectures) that the main interest in studying C−H activation has shifted somewhat away from fundamental work toward more practical applications, with a substantially larger component of investigators coming from the organic− synthetic side, instead of the organometallic−inorganic− mechanistic side, than in the previous period. That would entail a decreased interest in Pt as a logical consequence, for two reasons. First, the obvious one of economics: Pt is expensive. There has been a pronounced turn to the cheaper “earth-abundant” first-row transition metals in recent years, for C−H functionalization as well as in other spheres of organotransition-metal chemistry. Second, there is the common

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1. INTRODUCTION: BACKGROUND AND SCOPE Platinum has played a central role in the evolution of C−H activation chemistry by soluble species over the last halfcentury, in both fundamental understanding and approaches to actual functionalization. A number of early examples of intramolecular C−H activationligand cyclometalation involved Pt complexes;1 more significantly, the first really well-defined example of intermolecular C−H activation by a transition-metal complex was the H/D exchange of arenes (as well as a few cases of nonaromatic C−H bonds, such as cyclohexane and the benzylic position of toluene) catalyzed by Pt(II) in aqueous acetic acid, reported by Garnett and Hodges in 1967.2 H/D exchange might be considered a rather trivial form of C−H activation, but that work was extended to true functionalization, several years later, by the addition of Pt(IV): solutions of [PtCl4]2− and [PtCl6]2− in water or aqueous acetic acid oxidize alkanes to a mixture of alcohols and alkyl chlorides at temperatures around 120 °C, a combination named the “Shilov system” after its discoverer.3 © 2016 American Chemical Society

Special Issue: CH Activation Received: August 29, 2016 Published: October 21, 2016 8483

DOI: 10.1021/acs.chemrev.6b00583 Chem. Rev. 2017, 117, 8483−8496

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peroxo complexes of platinum group metals, includes a (very small) number of examples involving Pt itself.19 Still another entire issue, Vol. 793 of the Journal of Organometallic Chemistry, is dedicated as a memorial to Alex Shilov, who died in 2014. It features a number of personal perspectives/reminiscences on Shilov and his work,20−24 along with additional reviews and original research articles, including a review of recent advances in C−H activation by Ni, Pd, and Pt.25 The main body of this review is divided into two sections, the first dealing with stoichiometric C−H activation reactions both intra- and intermolecularthat lead only to C−Pt bonds and the other discussing “true” functionalizations (both stoichiometric and catalytic) that result in new C−X bonds.

generalization that third-row transition metals are mainly good for mechanistic investigation of stoichiometric transformations, whereas their second-row analogs work better in actual catalytic applications, as a consequence of too-strong metal−ligand bonds in the former. Of course this statement is by no means universally validthe replacement of the Rh-based Monsanto process by Ir-based Cativa as the preferred technology for methanol carbonylation is just one counterexamplebut I expect that a quantitative survey (which, again, I have not carried out) of fundamental mechanistic vs practical synthetic studies of catalytic C−H functionalization would find more Pt in the former group and more Pd in the latter. The last study of C−H activation by Pt in this journal (which, tellingly, was titled “Mechanistic Aspects of C−H Activation by Pt Complexes”) was the outstandingly thorough review by Lersch and Tilset in 2005;5 it featured nearly 350 references, even with the explicit exclusion of papers dealing with the (still) very common topic of intramolecular activation. In contrast, a related 2010 review6 found only 50−60 papers subsequent to the Lersch/Tilset effort; the level of activity in the 5+ years since then has been much the same (even including intramolecular activation, which comprises a significant fraction of the published papers). This review will concentrate on post-2010 developments, with discussion of earlier work largely limited to comparisons and placing newer results in context. Despite the word “catalyzed” in the title, I include findings in stoichiometric C−H activation that may be relevant to mechanistic understanding, along with true catalytic functionalization. For convenience I list here some recent reviews (not all limited to Pt) that may provide useful leads to earlier references: A perspective on C−H functionalization includes historical background, general principles, and a large number of illustrative examples.7 An editorial on the same topic highlights very recent (post-2014) work in the same area.8 A review on molecular catalysts for methane functionalization includes brief historical overviews of the Shilov and Catalytica systems.9 Two reviews examine reactions of C−H bonds in water10 and protic media,11 respectively. Two reviews cover C−H functionalization mediated by Pt complexes in general12 and, more specifically, by Pt complexes of Tp (trispyrazolylborate) and nacnac ligands.13 A review examining the roles of “true” and “masked” threecoordinate T-shaped Pt(II) intermediates in a variety of reactions includes an extensive survey of C−H activations.14 (“Masked” intermediates are those in which a fourth coordination site is occupied by a weakly bonded solvent or counteranion ormore significantly for this subjectan agostic C−H bond.) Reviews of reactions of O2 with Pd and Pt complexes specifically,15 and with late-transition-metal complexes in general,16 include findings relevant to the overall process of C−H functionalization, if not necessarily to the actual C−H activation step itself. (See also section 3.1.1 below.) A related review covers oxidative M−C functionalization by O2 and H2O2, using Pd and Pt.17 The last two of these reviews are in an issue (Vol. 45, issue 6) of Accounts of Chemical Research devoted entirely to the topic of C−H functionalization. Another entire issue of a journal, Vol. 40, issue 4, of Chemical Society Reviews, focuses on the theme of C−H functionalization in organic synthesis, introduced by guest editors active in the field.18 One review therein, on C−H oxidation by oxo and

2. C−H ACTIVATION AT PT 2.1. Mechanistic Investigations

Interest in the intimate mechanism of C−H activation at Pt(II) goes all the way back to the earliest Shilov work. Starting with coordination of a C−H bond to form a σ-complex, two limiting cases may be considered for going forward: a stepwise or redox route, consisting of oxidative addition to form a Pt(IV) alkyl hydride followed by proton loss, and a nonredox route, where proton is lost directly from the Pt(II) σ-complex (Scheme 1). A Scheme 1

variation of the latter involves interaction between the H and a departing ligand, sometimes termed σ-complex-assisted metathesis (σ-CAM). Distinguishing among these is not straightforward, especially for the Shilov system itself; note, for example, that observation of a Pt(IV) hydride does not necessarily prove that it is an actual intermediate in the C−H activation process, since the oxidative addition step may be (and often is) reversible, as indicated in Scheme 1. Accordingly, a good deal of the historical work has been computational, while experimental studies have often focused on the microscopic reverse reaction, protonolysis. That holds true for recent work as well. Vidossich et al. carried out an ab initio molecular dynamics study of methane activation by the dominant Shilov species PtCl2(H2O)2 in aqueous solution, giving results much in agreement with previous studies: the cis isomer of the σcomplex, cis-PtCl2(H2O)(η2-CH4), is more stable than the trans isomer; the barrier to C−H cleavage leading to PtCl2(H2O)H(CH3) is low, around 6 kcal/mol; and the resulting Pt−H is quite acidic (pKa ∼ −5) and is readily lost to solutionso readily, in fact, that they prefer to describe the intermediate as protonated Pt(II) instead of a Pt(IV) hydride.26 Prince and Cundari describe methane activation at the cationic N-heterocyclic carbene (NHC) ligand-substituted Pt(II) complex 1 (Scheme 2), as studied by DFT. Several subtle variants of nonredox routes, including the so-called oxidative hydrogen migration (OHM, a close relative of σ8484


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Scheme 3a

Scheme 2

a

Reproduced with permission from ref 31. Copyright 2012 Elsevier B.V.

CAM), were all found to exhibit higher barriers than the redox (oxidative addition/reductive elimination) pathway, with C−H oxidative addition being the rate-determing step (RDS). For the neutral dimethoxy analog of 1, the barriers to both redox and nonredox routes are closer to one another, but higher than for the cationic case.27 An assessment of the impact of relativistic effects, such as spin−orbit coupling and outer-core correlation, on calculations for C−H activation for heavy elements such as Pt and Ir concluded that there is indeed a contributionincluding these terms consistently gives higher calculated barriersbut the magnitude is only on the order of 1 kcal/mol or less.28 Protonolysis of L2PtMe2 complexes by triflic acid has been examined by DFT, in an attempt to distinguish preferences for direct protonation at the Pt−C bond (SE2) or stepwise protonation at Pt followed by reductive elimination (SE(ox)) the microscopic reverses of the nonredox and redox C−H activation pathways, respectivelyas a function of spectator ligand L. The concerted SE2 route was found to be preferred for softer ligands, such as phosphines or cyclooctadiene, whereas there was not such a clear trend for harder nitrogen-centered ligands. In all cases, the final loss of methane involves associative displacement by solvent (usually acetonitrile) or triflate anion.29 A companion study on (Et3P)2Pt(η1-benzyl)2, with a variety of meta- and para-substituted benzyl groups, used both kinetics and DFT calculations to assess the substituent effect on the various steps in the protonolysis sequence, but no general implications for C−H activation were deduced from the findings.30

sp2 C−H bond of the imine substituent R, which would give a five-membered ring. The latter would normally be thought to be preferred, but there is also precedent for favoring an endoover an exo-metallacycle (i.e., with the CN double bond inside rather than outside the chelate ring) in imine cyclometalations. The alternative possibility of C−F activation, which was found to dominate in related systems, would also give an endo, five-membered ring; thus, the preference for C− H activation here is somewhat unexpected. In a similar sequence, Pt(IV) complex 4 converts to 5 upon refluxing in toluene, presumably also via C−C reductive elimination to give intermediate A, followed by C−H activation and loss of methane; unlike Scheme 3, though, here the C−H activation is exclusively at the sp2 ortho-position of the tolyl group. In contrast, when SMe2 is replaced by PPh3 to give 4′ (accompanied by isomerization), the same treatment gives a mixture of products 5′ and 6′ derived from sp2- and sp3-C−H activation, respectively, somewhat favoring the latter (Scheme 4).32,33 Clearly a significant spectator ligand effect must operate Scheme 4a

2.2. Intramolecular C−H Activation

As remarked earlier, this class of reaction includes some of the earliest work in the field. It is likely that much of the current interest derives from the potential relevance to Pd-catalyzed directed C−H functionalization, an area in which there have been many important advances of late. While catalytic examples involving Pt are rare, Pt chemistry does offer advantages for systematic investigation of mechanistic features. I include here only a selection of examples that delineate the range of recent cyclometalations. There are also several reports of stoichiometric C−C bond formation mediated by C−H activation at Pt, most of which are considered below in section 3.2. The groups of Crespo and Love have extensively explored cyclometalation of imines and other N-centered ligands, with particular interest in the factors determining the regioselectivity of C−H activation. An example that illustrates both regioselectivity issues and C−C bond formation starts with imino-phenyl−Pt(IV) complex 2, which slowly converts at room temperature in solution to imino-benzyl−Pt(II) complex 3 (Scheme 3).31 The probable mechanism involves C(phenyl)−C(methyl) reductive elimination, followed by C−H activation at the resulting benzylic position and subsequent loss of methane, although alternative possibilities could not be ruled out. In this sequence, activation of the sp3 C−H bond to form a six-membered metallacycle must take place instead of an ortho-

a

Reproduced with permission from ref 32. Copyright 2012 American Chemical Society.

along with the other factorsaliphatic vs aromatic, ring size, endo vs exothat control the course of these cyclometalations. Again, it should be emphasized that these reactions, complex as they may be, are not necessarily curiosities of only academic interest; they may well carry potential implications for catalytic functionalization, if not by Pt, than by Pd and other metals. Among other explorations of reactivity preference, Pt(II) complexes of a bidentate amine-imine ligand (8) with a dangling biphenyl group, generated by C−C reductive elimination from an aryl−Pt(IV) precursor (7), undergo competing C−H activation of the proximal or distal arene ring to give five- or seven-membered metallacycles (9, 10), respectively (Scheme 5). A detailed study of kinetics led to 8485


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Scheme 5a

Scheme 7a

a


a


understanding of the factors controlling product preference, including interconversion between the isomeric intermediates that are precursors to specific products.34−36 Some of the various five- and seven-membered metallacycles have been examined as possible antitumor agents,37 as well as for luminescence properties potentially relevant to OLED applications.38 The pyridylquinoline−Pt(II) complex 11 undergoes “rollover” cyclometalation at room temperature; under certain conditions, 12 can be protonated at the free N without disturbing the Pt−methyl bond, but when the DMSO ligand is replaced with PPh3, “retro-rollover” takes place spontaneously (Scheme 6).39 Several even more complex structures have been found to result from multiple C−H activation steps, such as the dinuclear Pt2 complex formed by double metalation of the central benzene ring in a multidentate phosphine-amide ligand (Scheme 7).40 C−H activation at a methyl position of both xylylphosphine ligands in L2Pt(II) complexes gives rise to doubly cyclometalated species with either cis or trans Pt−C bonds, depending on the exact ligand. These “metallabicycles” undergo very interesting further chemistry, such as protonation to cationic species with agostic methyl groups, characterized crystallographically as well as by NMR (Scheme 8).41 Metallabicycles resulting from metalation of two phenylpyridine-type ligands at the same Pt center could not be obtained by the typical thermally induced reactions, but they

Scheme 8a

a

Reproduced with permission from ref 41. Copyright 2015 John Wiley & Sons.

were readily generated photochemically (Scheme 9). Mechanistic studies revealed that C−H activation proceeds via an oxidative addition route, to give Pt(IV)−H intermediates (detected by NMR), and that no radical species participate;

Scheme 6a

a

Reproduced with permission from ref 39. Copyright 2013 American Chemical Society. 8486


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Scheme 9a

such reaction) and X is weakly coordinating HOB(C6F5)3 or triflate, to give a mixture of isomeric anisyl complexes, primarily ortho (Scheme 13). A kinetic isotope effect (KIE) of 3.6 was Scheme 13a

a


a

computational studies support reaction from a triplet MLCT excited state (Scheme 10).42

measured, suggesting C−H activation is the RDS; the virtual absence of H/D scrambling or any other evidence for reversibility indicates that the isomeric preference is kinetic, not thermodynamic. The effect of the (NN) ligand appears to be structural, probably a consequence of chelate ring size (sixmembered works; five-membered does not), since there is no corresponding trend in electronic properties, as assessed from the IR of the respective carbonyl complexes.45 The cyclometalated T-shaped NHC complex 13 activates benzene reversibly: in neat benzene at 120 °C, the equilibrium favors the phenyl complex 14 by around 4:1, while heating 14 in heptane effects complete reversion to 13 (Scheme 14).46


Scheme 10a

a


Examples that exhibit aliphatic C−H activation include a phosphite complex (Scheme 11)43 and a T-shaped NHC complex (Scheme 12);44 the former results in a less-common six-membered metallacyclic ring.

Scheme 14a

Scheme 11a

a


a


Normally, aryl−Pt(II) complexes are considerably more thermodynamically stable than their alkyl counterparts; here the latter is favored by steric effects as well as some contribution from an agostic interaction, which is absent in the former. [Pt(bipy)Me]+ reacts in the gas phase with benzene and toluene to give the aryl cations [Pt(bipy)Ar]+ (with a small amount of competing activation at the benzylic position of toluene). Extensive H/D scrambling is observed in labeling studies, consistent with computational results supporting a redox (oxidative addition/reductive elimination) mechanism competing with the nonredox metathesis-like mechanism, whereas the Ni and Pd analogs follow only the latter.47 Nonexchange activationsreactions of C−H bonds with Pt−X rather than Pt−R speciesare more relevant to Shilovlike actual functionalization. Pincer complex 15 activates benzene (or toluene) in the presence of base to give a phenyl complex; several similar transformations have been reported previously. Here, however, in the absence of base but with added KB(C6F5)4 to replace triflate with a more weakly coordinating anion, complex 16, protonated at the central N of

a

Scheme 12

a


2.3. Intermolecular C−H Activation

Several reports examine metathesis-like activations of arene C− H bonds with R−Pt(II), a class of reaction that has long been a mainstay of mechanistic investigations. Anisole reacts with (NN)PtMeX complexes, where (NN) is dipyridylketone or dipyridylamine (but not bipyridine; that complex exhibits no 8487


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Scheme 17a

the pincer ligand, is obtained instead (Scheme 15). This activation thus essentially amounts to addition of the C−H Scheme 15a

a Reproduced with permission from ref 50. Copyright 2013 Royal Society of Chemistry.

phenanthroline analog gives no such reactivity), all suggest that TFE can probably still be considered effectively inert for most uses. Several reports are concerned with C−H activation at Pt(0) centers. Noting predictions that the ability of (PP)Pt(0) to effect C−H oxidative addition should depend on the P−Pt−P bite angle, with medium values expected to be favorable, the authors generated the six-membered ring species 17 and found that, on reduction to Pt(0), benzene or toluene added reversibly to give a reasonably stable Pt(II) aryl hydride, which rather surprisingly reacted further (slowly at room temperature, faster at 80 °C) to biaryls and dimeric Pt hydride 18 (Scheme 18). DFT calculations suggest that the latter step

a

Reproduced with permission from ref 48. Copyright 2013 Royal Society of Chemistry.

bond across the central Pt−N bond of the (noninnocent) pincer ligand, described in the paper as a sort of frustrated Lewis pair.48 A highly sterically crowded (diimine)dihydroxo Pt(II) complex reacts with benzene to give the corresponding mono- and diphenyl complexes plus water (Scheme 16). The Scheme 16a

Scheme 18a

a Reproduced with permission from ref 51. Copyright 2012 Royal Society of Chemistry. a Reproduced with permission from ref 49. Copyright 2013 Royal Society of Chemistry.

proceeds via a second oxidative addition of arene to give a Pt(IV) intermediate, (PP)PtH2Ar2, followed by C−C reductive elimination.51 More recent calculations on methane activation by a variety of d10 LM and L2M centers (where M is a group 9, 10, or 11 metal), including L2Pt(0), suggest that it is bite angle flexibility, rather than the actual bite angle value itself, that governs reactivity.52 A rare example of a stable Pt(II) alkyl hydride, anionic complex 19, was prepared by the stepwise route of Scheme 19, not by C−H activation; DFT calculations predict that oxidative addition of C−H bonds to (as yet hypothetical) [(PCN)Pt]− should be kinetically and thermodynamically favorable.53

unusual kinetics and other observations suggest that most of the C−H activation actually is catalyzed heterogeneously by Pt(0), formed during heating, although a low level of homogeneous activation does appear to take place in dilute solutions.49 Trifluoroethanol (TFE) has long been a popular solvent choice for these sorts of investigations: it is a good polar solvent but a weakly coordinating ligand, and C−H activation of the solvent has not posed any significant complication, probably owing to the strong deactivating effect of the adjacent electronwithdrawing groups. However, TFE has now been shown to react under more extreme conditions: it is oxidized by O2 in the presence of a Pt(II) complex and sulfuric acid to trifluoroethyl trifluoroacetate, along with small amounts of trifluoroacetaldehyde and its acetal (Scheme 17).50 The high temperature and O2 pressures required for this catalytic oxidation, along with the special nature of the supporting dppz ligand (the simple

3. C−H FUNCTIONALIZATION A good number of papers report converting C−H bonds to something else; although the majority are not catalytic, there may be opportunities for synthesis of complex organic molecules, in addition to gaining fundamental knowledge potentially applicable to future practical processes. These are surveyed according to the nature of the new bonds formed. 8488


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Scheme 19a

Scheme 20a

a


3.1. Oxygenation

The promise of useful Pt-mediated conversion of C−H to C− O dates all the way back to the original Shilov work, with the Catalytica system (bpymPtCl2 + SO3; bpym = bipyrimidine) for converting methane to methyl bisulfate being the most successfulthough still not practicalaccomplishment to date. A key hurdle is to be able to use the most practical oxidant, O2, which is usually not capable of effecting a needed oxidation step directly. There are indirect ways to make O2 the terminal oxidantthe oxidant for the R−Pt(II) intermediate in the original Shilov mechanism, Pt(IV), can be replaced by Cu(II)/O2 with some success (see below); the related Catalytica mechanism uses SO3 as oxidant, and the byproduct SO2 can be separately reoxidized using (very!) old technologybut in neither case has the overall integrated process proven viable. Accordingly, a good deal of work over the last couple of decades has been devoted to getting alkyl−Pt(II) species to react with O2. Usually such transformations, even when successful, do not complete an actual C−H to C−O conversion, as the species under study was not obtained via C− H activation, but the fundamental knowledge about what is needed to make this step work could help in the eventual design of a system that can carry out the complete cycle. Several reviews of this field were cited in the Introduction above; this section will briefly highlight more recent work, along with other approaches to C−H oxygenation. 3.1.1. Reactions of O2 with Pt(II) Species. The mechanism of the aerobic oxidation of methyl−Pt(II) complexes of the ambidentate ligand dipyridylmethanesulfonate (dpms), a system introduced by Vedernikov around 10 years ago, has been examined both experimentally54 and by DFT.55 The key feature of the dpms ligand is its ability to shift from the bidentate mode of coordination, preferred for square-planar Pt(II), to tridentate, thereby enthalpically stabilizing octahedral Pt(IV) without incurring the entropic penalty of coordination of a free solvent molecule or other ligand. The reaction is believed to proceed via O2 adduct 20, which is protonated to Pt(IV)−hydroperoxide 21, that in turn oxidizes a second Pt(II) center to give 2 equiv of the R−Pt(IV) product 22 (Scheme 20). The pH dependence of the reaction can be explained in terms of an equilibrium between aquo and hydroxo species (labeled a in the scheme). Competing formation of dimethyl− Pt(IV) species appears to take place via SN2 attack by Pt(II) on MePt(IV), as has been proposed in other systems; plausible routes involving Pt(III) are less likely, according to computational findings. Closely similar behaviorboth the aerobic oxidation and competing methyl transferis exhibited by methyl−Pt(II) complexes of two other ambidentate ligands: bis(3,5-dimethylpyrazol-1-yl)acetate (23)56 and (methoxy)(methyl)bis(2-pyridyl)borate (24)57 (Scheme 21). The latter system is further complicated by methyl migration from B to Pt.

a


Scheme 21

Direct insertion of O2 into a methyl−Pt(II) [or −Pd(II)] bond can be achieved under photoirradiation (Scheme 22); the Scheme 22a

a


unsubstituted terpyridine analog does not react with O2, suggesting that steric crowding from the 6,6′ substituents weakens the metal−carbon bond. Originally, this chemistry had been proposed to involve metal-complex-sensitized formation of singlet oxygen, but mechanistic studies suggest instead an excited-state triplet dinuclear M−M bonded intermediate.58 3.1.2. C−O Bond Formation. Much of the work here is aimed at further elucidating and/or improving upon the Shilov and Catalytica systems. A general review of oxidative functionalization of alkanes by dioxygen59 includes a brief discussion of the Cu-modified Shilov system, wherein Pt(IV) is replaced by Cu(II) as oxidant, the latter in turn being regenerated by O2a combination earlier found to give a moderate number of turnovers for hydroxylation of watersoluble alkane derivatives.60 The author of the review suggests that the mechanism must involve Pt(0) and peroxo intermediates, as Cu(II) is thermodynamically incapable of oxidizing (inorganic) Pt(II) to Pt(IV). However, that argument is clearly invalid, as it has been previously established that Cu(II) oxidizes the key Shilov intermediate, R−Pt(II), even faster than Pt(IV) does.61 The “obvious” mechanism that one 8489


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not efficiently oxidize the Pt(II) precursor to the intermediate, a reaction that would remove the C−H activating species from the system. That requirement is satisfied by Cu(II) (see Scheme 23 above) and originally was thought to be valid for the Catalytica system as well. However, new results show the inorganic Pt(II) species is in fact oxidized to Pt(IV) under catalytic conditions, at a rate comparable to the catalytic functionalization of methane, an observation that at first seems inconsistent with the good stability and high turnover numbers observed for the system. Several explanations were explored, including possible C−H activation by Pt(IV) itself; the correct interpretation appears to be based on a “self-repair” process whereby the Pt(IV) species thus generated react extremely rapidly with the R−Pt(II) intermediates. Note that this situation is completely analogous to the original Shilov system, in which [PtCl 6 ] 2− rapidly oxidizes [RPtCl 3 ] 2− . DFT calculations suggest that a redox reaction between bpymPtClMe and a five-coordinate Pt(IV) species, [bpymPtCl2(H2SO4)]+, would have an activation barrier around 6 kcal/mol, consistent with the proposed mechanism. This conclusion suggests that rapid oxidation of the C−H activating species can be compatible with an effective functionalization system, so long as the relative rates of the various steps are mutually appropriate; in particular, the selfrepair step must be very fast.65 The Catalytica system has also been applied to ethane; it gives clean formation of two products, EtOSO3H and HO3SOCH2CH2SO3H (isethionic acid, ITA). Interestingly, the reaction is considerably faster than the corresponding methane functionalization, proceeding at a comparable rate at substantially lower temperatures (160 vs 220 °C). The reaction is believed to proceed via ethylene (Scheme 25), as supported by both experimental and computational findings; most importantly, very fast β-H elimination in the ethyl−Pt(II) intermediate means that C−H activation becomes the RDS for ethane, whereas it is oxidation of Me−Pt(II) for methane. That difference accounts for both the higher activity and the

would derive from the basic Shilov system (Scheme 23) is compatible with all existing evidence. Scheme 23

This Pt(II)−Cu(II) modified Shilov system was found to effect selective catalytic hydroxylation of aliphatic amines.62 In the presence of 1 equiv of acid, to keep the amines in protonated form, substrates such as N-alkylpyrrolidines were oxidized to the corresponding monoalcohols (Scheme 24) in Scheme 24a

a

Adapted with permission from ref 62. Copyright 2015 American Chemical Society.

good yieldsa few exceeding 100% based on Cu, reflecting some reoxidation of Cu(I) (reactions were performed under air). The hydroxylation proceeded with moderate to good selectivity for the terminal position, depending on chain length: selectivity for the 1- vs 2-position was >20:1 for R = ethyl, decreasing to around 2:1 for R = pentyl. This trend was attributed to the deactivating effect of the electron-withdrawing ammonium substituent, which is attenuated as it moves further from the reacting C−H positions. Protonation of the amine plays two additional key roles: making the substrate soluble in the aqueous medium and preventing deactivation of the Pt(II) species by amine coordination. The oxidation of methane by O2, catalyzed by Pt(II) combined with either Fe(II) or the polyoxometalate H5PMo10V2O40 as relay oxidant, was used as a model system for the design and optimization of a microfluidic reactor; up to 25 turnovers to methanol were achieved, with formic acid as the main byproduct.63 The use of higher temperatures combined with added chloride and/or ionic liquids was attempted as a means of stabilizing the Shilov system against decomposition to Pt metal, as assessed by measuring the extent of H/D exchange in methane; no actual oxidation products were reported.64 Their analysis does not appear to take any account of the wellestablished fact that Pt(0) is also capable of catalyzing H/D exchange, however. A combined experimental−computational mechanistic study on the Catalytica system addresses an important challenge in Shilov-based approaches: any oxidant chosen to replace Pt(IV) must be capable of oxidizing the R−Pt(II) intermediate very efficiently, since the latter’s formation (by electrophilic activation) is highly reversible. However, that oxidant must

Scheme 25a

a


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an alkyl- or arylating reagent, followed by C−C reductive elimination, or (2) insertion of an alkene or alkyne into the Pt− C bond initially generated by C−H activation. Several studies relevant to the first have been reported, although only one represents true catalytic functionalization. Crespo and coworkers found that an aryl C−Pt bond formed by intermolecular C−H activation of an arene solvent undergoes C−C reductive elimination with a aryl C−Pt bond (derived from cyclometalation of an imine ligand, but by C−Cl not C− H activation); subsequent remote C−H activation leads to a seven-membered cyclometalated biphenyl derivative. Although there are several plausible sequences of events, computational studies suggest the most probable pathway is that shown (for benzene; toluene and xylene were also examined) in Scheme 28.71

observation of much less H/D exchange of unreacted alkane with ethane than with methane.66 A heterogenized version of the Catalytica system was generated by incorporating PtCl2 into a triazine-based polymeric framework (Scheme 26); the resulting solid catalyst Scheme 26a

Scheme 28a

a


gave methane conversion performances comparable to the homogeneous catalyst, and it could be successfully recycled a number of times.67 The solid catalyst has been characterized by a battery of techniques, including solid-state 195Pt NMR, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy.68 Somewhat better activitybut poorer stabilitywas observed for catalysts prepared by supporting Pt(II) on a high surface area nitrogen-doped carbon, obtained by pyrolysis of biomaterials such as lobster shells.69 The possibility of oxygenating a Pt−aryl bond by pyridine Noxidea sort of organometallic Baeyer−Villiger reactionwas examined computationally. The effect of changing substituents X (Scheme 27) was predicted to be similar to that in organic analogs: reaction is favored by electron-withdrawing groups on the oxidant and by electron-donating groups on the migrating aryl. All calculated barriers were found to be rather high, however.70

a


The relative preference for sp2−sp3 vs sp3−sp3 C−C reductive elimination was examined by means of thermolysis of a series of (dimethyl)(metallacyclic aryl)−Pt(IV) complexes. Cationic imine complex 25, obtained by treating the corresponding neutral bromide with Ag+, undergoes exclusive

3.2. C−C Bond Formation

Construction of a C−C bond following C−H activation basically follows one of two routes: (1) installation of a second Pt−C bond, either via a second C−H activation or by means of Scheme 27a

a

Reproduced with permission from ref 70. Copyright 2011 American Chemical Society. 8491


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Scheme 31a

loss of ethane to give Pt(II) metallacycle 26, whereas the analogous complex with a saturated amine chelate ligand, 27, leads to methane instead, presumably via sp2−sp3 C−C reductive elimination followed by aryl C−H activation to give 28 (Scheme 29). DFT calculations suggest that the sp2−sp3 C− Scheme 29a

a


a


C coupling mode is inherently preferred, but it is disfavored in the mer geometry of 25; the more rigid unsaturated ligand prevents it from assuming a fac geometry as in 27.72 Arylation of naphthalene and other arenes by diaryliodonium salts is catalyzed by Pt(II), an extension of earlier work on catalysis by Pd(II). Yields range from around 20 to 85%, with a strong regiopreference for isomer B (Scheme 30); mechanistic

that of benzene; given the relative concentrations, it is clearly not the case that diethylbenzene is formed by a consecutive route. Rather, once ethylbenzene is formed at a Pt center, a second ethylene must coordinate and react at a rate competitive with dissociation from the catalyst.74 Effects of the supporting ligand have also been investigated. Replacing bipy with dipyridylmethane to give a six-membered chelate ring (Scheme 32, E = CH2) increased both activity and

Scheme 30a

Scheme 32a

a


studies suggest that the first step is oxidative addition of Ar2I+ to give Ar−Pt(IV), followed by intermolecular C−H activation and C−C reductive elimination, with the last being the RDS. In contrast, with Pd the oxidation step is rate-limiting, and the opposite regioselectivity is observed.73 Further investigations on the use of Pt in catalytic hydroarylation of alkenes have been reported by the Gunnoe and Goldberg groups. Cationic complexes [(4,4′-di-t-Bu-2,2′bipyridyl)PtPhL]+ (L = THF, CH3CN, NC5F5), in benzene solution under ethylene, catalyze formation of ethylbenzene and diethylbenzenes at 100 °C. Yields of 90% (based on ethylene as the limiting reagent) were achieved, corresponding to around 100 turnovers; about 20% of the products are diethylbenzenes, the isomers of which are obtained in comparable amounts (the exact ratios of products depend somewhat on L). Mechanistic studies, including kinetics, isotope labeling, and stoichiometric benzene activation in the absence of ethylene, combined with DFT calculations, implicate the mechanism shown in Scheme 31 (or a variant thereof, in which the catalyst resting state labeled as 4 is off-cycle), with C−H activation (k4) as the RDS. The rate of alkylation of free ethylbenzene was measured to be around 3 times slower than

a


catalyst longevity, while the other modifications shown gave poorer performance in activity and/or longevity.75 Hydrophenylation of α-olefins can be tuned to favor Markovnikov or anti-Markovnikov regioselectivity by substituents on unsymmetrical pyridyl-pyrrolide ligands (Scheme 33); precatalyst 29, methylated on the pyrrolide ring, gives an A:B ratio of 13:84 in the hydrophenylation of propylene, whereas the unmethylated analog 30 gives nearly equal amounts of the isomers; methylation on the pyridyl ring (31) gave mostly 2methylstyrene instead, but with considerably lower overall activity.76 One paper reports C−C bond construction in the cyclization of (o-isopropylphenyl)phenyl acetylene to a substituted indene via aliphatic C−H activation (Scheme 34). PtCl2 was found to be the most effective catalyst (ligands such as phosphines or diimines reduced yields considerably), with added Cu salts providing some further improvement; the best combination, using CuBr as the additive, gave 82% yield. Two reasonable 8492


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Scheme 33a

Scheme 35a

a


°C (Scheme 36). More interestingly, the reaction works well at a wide range of aromatic C−H bonds, including sterically

a


Scheme 36a

a

Scheme 34

a


hindered ones such as mesitylene, which gives (2,4,6trimethylphenyl)Bpin as the sole product in 72% yield. With most other metal catalysts, including Ir, such crowded positions are unreactive: mesitylene is borylated by Ir at a benzylic position. Benzenes with fluoro and other electron-withdrawing substituents work well too, as do a variety of aromatic heterocycles. The large measured KIE, 4.8, suggests that C− H activation is the RDS.79 A quite different Pt complex, pincer complex 32, is an effective precatalyst, in combination with an additive such as n-BuLi, for borylation of fluoroarenes (Scheme 37). The regiopreference for positions ortho to F (as well as the failure of toluene and anisole to react at all) indicates a strong promoting electronic effect, again in contrast to Ir catalysis, which tends to exhibit steric hindrance.80

a


mechanisms are shown in Scheme 34: electrophilic activation of the tertiary C−H bond of the isopropyl group, followed by C− Pt addition across the triple bond and protonolysis (A, top), or transfer of a hydride to the α-carbon of the triple bond followed by C−C reductive elimination (B, bottom). The absence of any H/D scrambling in a crossover experiment requires that the activated H remains with the same Pt center, favoring the second mechanism; that conclusion is also suggested by the absence of any cyclized product from the methyl- or ethylphenyl analogs, as hydride transfer would effectively generate an intermediate with carbocationic character, which could require the added stabilization available with the isopropyl group.77 The reverse process, catalytic C−C bond cleavage, was observed in the reaction of a Pt(II) complex with a strained tricyclic spiro(norbornene-cyclopropane) compound, which undergoes rearrangement to pentahydroindene in high yield (Scheme 35). Deuterium-labeling studies indicate that the reaction is initiated by C−H activation at a cyclopropane site.78

3.4. H/D Exchange

While not strictly a functionalization, this behavior is frequently used to assess C−H activation reactivity, as in the (somewhat questionable?) example cited above (section 3.1.2). A number of diimine−Pt(II) complexes were tested as catalysts for H/D exchange between benzene and RCO2D, where the aryl groups on the diimine were variously substituted with alkyls or halides in the 2,6- or 3,5- positions (Scheme 38).81 No clear correlation with electron density could be observed, in contrast to earlier studies on stoichiometric C−H activation of benzene by [(NN)PtMe(solvent)]+ complexes,82 but there is an apparent steric effect, and not the one that might have been expected: 2,6-subsituted aryls, especially those with halide substituents, show marked acceleration. While no definitive explanation is available, it was suggested that steric crowding might inhibit replacement of one coordinated benzene by another (and/or by solvent), a process known to be associative in related systems, such that multiple exchange would be favored; indeed, substantially more multiply exchanged isotopologs were found for Ar = 2,6-C6H3Cl2 than for the 3,5-isomer.

3.3. C−B Bond Formation

Most previous catalysts for arene borylation have been Ir-based; two recent reports show that Pt can be used as well and may offer advantages in certain cases. NHC−Pt(0) catalysts were found to effect borylation of benzene by bis(pinacolato)diboron (B2pin2; HBpin does not work) in good yield at 100 8493


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Scheme 37a

a

Reproduced with permission from ref 80. Copyright 2015 Royal Society of Chemistry.

Ir (section 3.3); hydroxylation of amines with good regioselectivity by a modified Shilov system (section 3.1.2); efficient hydroarylation of unsaturated hydrocarbons (section 3.2). All in all, recent contributions in the field of Pt-catalyzed C− H activation might fairly be said to be more impressive in terms of quality than quantityinvestigators appear to be picking their targets more selectivelyand future developments may be anticipated to continue along the same route.

Scheme 38

One issue that should be kept in mind, when H/D exchange of arenes is employed as a surrogate for C−H activation in general, is the possibility of acid catalysis instead of (or in addition to) metal catalysis. Even though the test medium may seem to be too weakly acidic to promote metal-free exchange, as with the carboxylic acids used in the above example, coordination of the acid to the metal can generate a much more (Brønsted) acidic reagent than the free acid itself, one that might well be capable of effecting aromatic H/D exchange, without the need to invoke any C−H activation by the metal. Distinguishing these alternatives may not be straightforward. Gunnoe and co-workers have addressed this problem by means of a combined experimental and computational study and found that for a variety of main-group and transition-metal species, including Pt(NHC)(OTf)2, catalysis by proton appears to be the preferred mechanism.83 Whether that conclusion applies to the above study (Scheme 38) is far from clear; the observation of ligand-dependent isotopolog distribution may suggest that it does not.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest. Biography Jay Labinger is administrator of the Beckman Institute and faculty associate in chemistry at Caltech. He received his Ph.D. in chemistry from Harvard University and had subsequent positions at Princeton, Notre Dame, Occidental Petroleum, and ARCO before joining Caltech in 1986. His research interests include organometallic chemistry, energy-related catalysis, and literary and cultural aspects of science.

REFERENCES (1) For an example, see the following: Cheney, A. J.; Mann, B. E.; Shaw, B. L.; Slade, R. M. Intramolecular Platinum−Carbon Bond Formation Promoted by Steric Hindrance. J. Chem. Soc. D 1970, 1176−1177. (2) Garnett, J. L.; Hodges, R. J. Homogeneous Metal-Catalyzed Exchange of Aromatic Compounds. A New General Isotopic Hydrogen Labeling Procedure. J. Am. Chem. Soc. 1967, 89, 4546− 4547. (3) Gol’dshleger, N. F.; Eskova, V. V.; Shilov, A. E.; Shteinman, A. A. Alkane Reactions in Solutions of Chloride Complexes of Platinum. Zh. Fiz. Khim. 1972, 46, 1353. (4) Labinger, J. A. Selective Alkane Oxidation: Hot and Cold Approaches to a Hot Problem. J. Mol. Catal. A: Chem. 2004, 220, 27− 35. (5) Lersch, M.; Tilset, M. Mechanistic Aspects of C-H Activation by Pt Complexes. Chem. Rev. 2005, 105, 2471−2526. (6) Labinger, J. A.; Bercaw, J. E. The Role of Higher Oxidation State Species in Platinum-Mediated C−H Bond Activation and Functionalization. In Higher Oxidation State Organopalladium and Platinum Chemistry; Canty, A. J., Ed., Springer: Heidelberg, 2011; pp 29−60. (7) Hartwig, J. F. Evolution of C−H Bond Functionalization from Methane to Methodology. J. Am. Chem. Soc. 2016, 138, 2−24. (8) Davies, H. M. L.; Morton, D. Recent Advances in C−H Functionalization. J. Org. Chem. 2016, 81, 343−350. (9) Gunsalus, N. J.; Konnick, M. M.; Hashiguchi, B. G.; Periana, R. A. Discrete Molecular Catalysts for Methane Functionalization. Isr. J. Chem. 2014, 54, 1467−1480.

4. OUTLOOK Despite all the developments and important insights that have been generated in the more than 30 years since the Shilov discoverycontinuing right up to the presentthere still does not appear to be a clear path for exploiting C−H activation at Pt for the sorts of large-scale applications that have been envisioned, especially an economically viable methane conversion process. Many important advances have been reportednew reaction media, ingenious ligands for promoting oxidation by O2, etc.but at least one practical difficulty always seems to remain unsolved. On the other hand, there is nothing that indicates anything inherently wrong with the approach, and it may well be that the magic combination of metal complex, oxidant, reaction conditions, and process design is out there waiting to be discovered. In the meantime, though, there have been many impressive, if more modest, accomplishments in this area. Besides the major role of Pt chemistry as a platform for generating mechanistic understanding, which has been well-documented in these pages, a number of findings suggest Pt-based C−H catalytic functionalization may ultimately prove more useful for smaller-scale organic syntheses than large-scale conversion schemes. Just to highlight a few: Pt-catalyzed arene borylation, with quite different selectivity patterns than previously studied 8494


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