Dehydrogenation of Alkanes and Aliphatic Groups by Pincer-Ligated

Review pubs.acs.org/CR

Dehydrogenation of Alkanes and Aliphatic Groups by Pincer-Ligated Metal Complexes Akshai Kumar,*,‡ Tariq M. Bhatti,† and Alan S. Goldman*,† †

Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States ‡ Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati-781039, Assam, India ABSTRACT: The alkyl group is the most common component of organic molecules and the most difficult to selectively functionalize. The development of catalysts for dehydrogenation of alkyl groups to give the corresponding olefins could open almost unlimited avenues to functionalization. Homogeneous systems, or more generally systems based on molecular (including solid-supported) catalysts, probably offer the greatest potential for regio- and chemoselective dehydrogenation of alkyl groups and alkanes. The greatest progress to date in this area has been achieved with pincer-ligated transitionmetal-based catalysts; this and related chemistry are the subject of this review. Chemists are still far from achieving the most obvious and perhaps most attractive goal in this area, the dehydrogenation of simple alkanes to yield alkenes (specifically monoenes) with high yield and selectivity. Greater progress has been made with tandem catalysis and related approaches in which the initial dehydrogenated product undergoes a desirable secondary reaction. Also reviewed is the substantial progress that has been made in the closely related area of dehydrogenation of alkyl groups of substrates containing heteroatoms.

CONTENTS 1. Introduction 1.1. Pincer Complexes 1.2. Alkane Dehydrogenation 2. Alkane Dehydrogenation by Pincer−Iridium Complexes 2.1. Initial Reports of Pincer−Iridium-Catalyzed Alkane Dehydrogenation 2.2. Scope of Pincer−Iridium-Catalyzed Alkane Dehydrogenation 2.3. Mechanism and “Loss of Regioselectivity″: Double Bond Isomerization by Pincer−Ir Dehydrogenation Catalysts 2.4. Solid/Gas-Phase Systems and the Use of Ethylene and Propylene 2.5. Fluorinated Pincer−Iridium Dehydrogenation Catalysts 2.6. PC(sp3)P Pincer−Iridium Catalysts 2.7. PBP Pincer−Iridium Catalysts 2.8. Non-Phosphine-Based Pincer−Iridium Complexes 3. Dehydrogenation of Alkanes by Pincer Complexes of Metals Other Than Iridium 3.1. Alkane Dehydrogenation by Pincer−Ruthenium Complexes 3.2. Alkane Dehydrogenation by Pincer−Osmium Complexes 3.3. Alkane Dehydrogenation by Pincer−Rhodium Complexes (and Comparison with Pincer−Iridium Complexes)

© 2017 American Chemical Society

4. Tandem Reactions Involving Alkane Dehydrogenation 4.1. Alkane Metathesis and Alkane−Alkene Coupling Reactions 4.1.1. Alkane Metathesis 4.1.2. Alkane−Alkene Coupling 4.2. Aromatics via Dehydroaromatization, Alkyl Group Cross-Metathesis, Cyclodimerization, and Aryl−Alkyl Coupling Reactions 4.3. Silylation and Borylation of Alkanes via Dehydrogenation 5. Dehydrogenation of Substrates Containing Heteroatoms 5.1. Reactions of Pincer−Iridium Complexes with Ethers 5.2. Enones by α,β-Dehydrogenation of Ketones 5.3. Dehydrogenation of Amine C−C Linkages 6. Outlook Associated Content Special Issue Paper Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

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Received: May 5, 2017 Published: September 27, 2017 12357

DOI: 10.1021/acs.chemrev.7b00247 Chem. Rev. 2017, 117, 12357−12384

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1. INTRODUCTION

compounded further if regioselectivity is desired, as in the case of producing 1-alkenes which are intermediates for a wide range of reactions. Such terminally selective dehydrogenation requires cleavage of the strongest bond of the molecule (the terminal C−H bond) to give the least thermodynamically favored isomer. Selective dehydrogenation at other specific positions of linear alkanes is even more challenging due to the chemical similarity of nonterminal positions. Thus, the ability to effect the controlled dehydrogenation of alkanes or alkyl groups to give the corresponding monoenes is a prominent challenge of modern organic chemistry and catalysis.8−10 Offering some hope to counter the discouraging considerations noted above, in the early 1980s, a number of examples were found of transition-metal-based fragments that underwent regioselective oxidative addition of the terminal C−H bonds of n-alkanes.11−25 About the same time, the initial examples of alkane dehydrogenation using well-defined homogeneous transition-metal complexes were reported;26−31 these seemed all the more intriguing in the context of the regioselectivity observed in the “simple” oxidative addition systems. The first example of a transition-metal-mediated alkane dehydrogenation was reported by Crabtree in 1979. Reaction of a cationic Ir(III) complex (1) with 3,3-dimethyl-1-butene (tertbutylethylene, TBE) in refluxing 1,2-dichloroethane containing either cyclopentane or cyclooctane (COA) was found to yield the corresponding cyclopentadienyl (2) and cyclooctadiene (3) iridium complexes, respectively (eq 1).26−28 Later, Felkin and

1.1. Pincer Complexes

The chemistry of pincer-ligated metal complexes has seen explosive growth since Moulton and Shaw’s seminal 1976 report of a wide range of complexes of transition metals (including Rh, Ir, Ni, Pd, and Pt) bearing the ligand now typically abbreviated as tBu4PCP (or just tBuPCP).1

“Pincer complexes” have found applications promoting stoichiometric and catalytic transformations relevant to fuels, commodity chemicals, and fine chemicals. The complexes themselves often have luminescent behavior, a property which lends itself to use in optoelectronics.2−4 The term “pincer ligand”, coined by Van Koten5 in 1989, initially referred to ligands with a central anionic carbon and two flanking, datively bound units that rigidly enforce a meridional geometry about a metal center. This definition has relaxed such that it is now often applied to any ligand that chelates a metal center in a mertridentate configuration. With regard to catalyst design, this class of compounds is attractive for the high thermal stability afforded by the tridentate coordination mode, steric properties which may inhibit undesired dimerization or oligomerization while allowing small molecules access to the metal center, and their modularity, which makes it possible to synthesize many permutations of the three coordinating groups, allowing almost unlimited ligand tuning. These features make pincer ligands particularly suitable as ancillary ligands for catalysts for the dehydrogenation of alkanes and alkyl groups, the topic of the present review. Since the first report of their activity in this context by Kaska and Jensen in 1996,6 pincer ligands have played a dominant role in the development of catalysts for such dehydrogenations. This review will extend to dehydrogenations beyond those of alkanes to other aliphatic groups as well. It will be focused, however, on “unactivated” C−H bonds, i.e., substrates in which the activity or the thermodynamics of dehydrogenation are not dramatically different from those of the corresponding alkanes.

co-workers demonstrated the use of L2ReH7 (L = PPh3 and PEt2Ph) to dehydrogenate alkanes in the presence of TBE, leading to the formation of complexes of the corresponding unsaturated hydrocarbons.29−31 The dehydrogenation of alkanes is a highly endothermic process unless coupled with a hydrogenation reaction (“transfer dehydrogenation”, eq 2). Since the initial report by Crabtree,26

1.2. Alkane Dehydrogenation

Alkanes, our must abundant organic resource, are notoriously unamenable to controlled chemical transformations. In contrast, olefins are key chemical intermediates that may be derivatized to alcohols, amines, alkyl halides, alkyl boronates, alkylsilanes, and alkylphosphines, cleaved into ketones, aldehydes, or carboxylic acids, oligomerized to higher alkenes with various end uses, transformed through pericyclic reactions, cross-coupled with other sp, sp2, and even sp3 carbon centers, dimerized using metathesis catalysts, or enchained into polymers with myriad commercial applications. Therefore, an efficient and selective method to dehydrogenate alkanes or alkyl groups to the corresponding mono-olefins would be of tremendous value. This is, however, a daunting goal indeed.7 In the simple example of the dehydrogenation of a cycloalkane, the cycloalkene product will have C−H bonds that are much weaker and generally more reactive than those of the starting cycloalkane. Thus, the challenge of converting an alkane to an alkene typically encompasses the significant difficulty of stopping at the initially formed alkene. The challenge is

the use of TBE as a sacrificial acceptor has continued with great success. TBE cannot undergo double bond isomerization, and the high degree of steric bulk mitigates catalyst inhibition due to olefin binding. Moreover, TBE may be regenerated from its hydrogenated product, “TBA” (neohexane), thus in principle allowing recycling and practical applications in larger systems.32 While all alkane dehydrogenations are highly endothermic, there is significant variability among them. For example, dehydrogenation enthalpies are ca. 23 kcal/mol for COA, ca. 28 kcal/mol for unstrained cycloalkanes and for n-alkanes at 12358


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coordinate bisphosphino d8 fragment). Fragment 8 then dehydrogenates 1 equiv of alkane, leading to H2Rh(PMe3)2Cl (9). Photoliberated CO then displaces dihydrogen from complex 9, presumably via CO addition product 10, to regenerate 7 (and thermodynamically drive the acceptorless dehydrogenation). Recent work in this direction from the Beller group uses a controlled light source along with additives such as bipyridine, CO2, and ethylformate to obtain improved turnover numbers (TONs) in acceptorless photocatalytic alkane dehydrogenation using 7 by mitigating its decomposition.49−51 The three-coordinate d8 fragment Rh(PMe3)2Cl (8) was thus shown to be highly active toward alkanes, but in the work discussed above, it was only accessed photochemically. The Goldman group later found that carbonyl complex 7 as well as several other complexes, Rh(PMe3)2Cl(L) (L = PiPr3, 11a; L = PCy3, 11b; L = PMe3, 11c), was highly active for the thermochemical transfer dehydrogenation of alkanes, but only in the presence of a H2 atmosphere.52,53 It was proposed that H2 adds to these four-coordinate species to induce dissociation of L, to give H2Rh(PMe3)2Cl (9), which is then dehydrogenated by the acceptor to yield the active three-coordinate 14-electron species 8 (Figure 1, shown for L = CO). Use of H2, however, is problematic, as simple hydrogenation (addition of H2) of the acceptor occurs, in addition to the cycle shown in Figure 1, wasting several equivalents independently of those that accept hydrogen from the alkane substrate. This led to the investigation of [Rh(PMe3)2(μ-Cl)]2 (12) as a catalyst precursor, but like the mononuclear species, the rhodium centers in this dimer are four-coordinate and inactive in the absence of H2. In an effort to thwart dimerization, the Goldman group investigated the pincer “analogue” of this species, (Me4PCP)RhH2, but this was found to be inactive in either the presence or absence of H2 (and thus the first attempt to use pincer complexes for alkane dehydrogenation was a complete failure!).54 Calculations by Krogh-Jespersen explained the poor activity of this species on the basis of a comparison of Rh(PH3)2Cl with Rh(PH3)2Ph (as a model for the pincer complex) which showed that replacing a chloride ligand with a phenyl group greatly disfavored the thermodynamics of addition of H2 or C−H bonds to the three-coordinate Rh center, or, in other words, resulted in greatly reduced M−H or M−R bond strengths in the species [M]RH (M = Rh(PMe3)2Ph or (Me4PCP)Rh; R = hydrocarbyl or H).55 This effect was surprising in that the phenyl group, being more electrondonating than chloride, might have been expected to favor rather than strongly disfavoroxidative addition. This apparent incongruity was explained years later in a systematic computational study showing that the more electron-donating ligands in the plane of the H−H or C−H addition greatly weaken the resulting M−H or M−C bonds due to their stronger trans influence.56

internal positions, and ca. 30 kcal/mol for the terminal position of n-alkanes.33 In view of the relatively low value for COA and the equivalence of its C−H bonds, COA is a convenient “model alkane, and the COA/TBE couple has been used as a “benchmark reaction” in the evaluation of new dehydrogenation catalysts. However, the idiosyncrasies of this systemthe low enthalpy of dehydrogenation, the symmetry of COA, and the inability of TBE to strongly coordinate to the metal center or to undergo double bond isomerizationmean that care must be taken in extrapolating from this system to other aliphatic groups. Conclusions drawn from studies of the COA/ TBE system will very possibly not extend to acyclic alkanes (or even to unstrained cycloalkanes). In addition to the greater enthalpy of dehydrogenation of linear alkanes in comparison to that of COA, linear olefinic products may bind to the active metal center much more strongly than cyclooctene (COE), either in an η2-fashion or as an allyl group. Crabtree investigated the use of a base in combination with [IrH2(Me2CO)2(PR3)2][SbF6] (4) (R = p-FC6H4, cyclohexyl) to catalyze COA/TBE transfer dehydrogenation.34 Deprotonation combined with loss of acetone can afford the neutral threecoordinate IrH(PR3)2 (R = p-FC6H4, cyclohexyl) or, more likely, a precursor thereof. This species bears a strong relationship to pincer-iridium complexes such as (PCP)Ir, critical to much of the work discussed in this review. Subsequently, Felkin reported that polyhydride systems based on (PR3 ) 2 IrH 5 (R = p-FC 6 H4 , iPr) acted as catalyst precursors,35,36 likely also via three-coordinate d8 species (PR3)2IrH.37 Crabtree also reported that photochemical alkane dehydrogenation was catalyzed by H2Ir(PR3)2(η2-OC(O)CF3) (5) (R = p-FC6H4, cyclohexyl).38,39 Dehydrogenation was proposed to proceed through Ir(PR3)2(η1-OC(O)CF3) (6), which is also a three-coordinate d8 complex with mutually trans phosphine groups, analogous to (PCP)Ir. Building upon these reports of thermal and photochemical alkane C−H activation, Eisenberg reported the photochemical carbonylation of alkanes by Rh(PPh3)2(CO)Cl,40,41 which was improved upon by Tanaka using trans-Rh(PMe3)2(CO)Cl (7). The groups of Tanaka,42−45 Saito,46,47 and Goldman48 then independently reported that 7 could effect photochemical dehydrogenation of alkanes with unprecedentedly high turnover numbers. The Goldman group demonstrated that the role of light was to dissociate CO from 7 (Figure 1) to generate the catalytically active complex Rh(PMe3)2Cl (8) (another three-

2. ALKANE DEHYDROGENATION BY PINCER−IRIDIUM COMPLEXES 2.1. Initial Reports of Pincer−Iridium-Catalyzed Alkane Dehydrogenation

The weakness of the M−H and M−C bonds formed upon H− H or C−H addition to (Me4PCP)Rh, relative to the catalytically active fragment Rh(PMe3)2Cl, was calculated to be offset by the substitution of a third-row metal center, Ir, for second-row Rh.55 These computational results are in accord with the

Figure 1. Photochemical acceptorless dehydrogenation of alkanes (blue arrows) and thermal transfer dehydrogenation (red arrows) catalyzed by Rh(PMe3)2Cl(CO) (7) under a hydrogen atmosphere (hydrogen-catalyzed). Black arrows represent reaction steps common to both photochemical and thermal systems. 12359


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finding by Kaska and Jensen that a PCP pincer-ligated iridium complex, (tBu4PCP)IrH2 (13-H2), was extremely effective for alkane transfer dehydrogenation, far more so than the Rh analogues.6 This key discovery laid the groundwork for the development and the subsequent predominance of pincerligated catalysts, particularly iridium-based, in the area of organometallic-catalyzed alkane dehydrogenation.10,57−60 Complex 13-H2 exhibited excellent stability and activity at temperatures as high as 200 °C.6 The dehydrogenation of COA catalyzed by 13-H2 was performed with periodic addition of TBE (increments of 0.4 M), and the reaction proceeded at a rate of 12 turnovers (TOs)/min, giving up to 1000 TOs. Importantly, it can be seen that not only is 13 a very active catalyst, it is also very robust even at high temperature. It might be considered in this context that when comparing complexes of second-row metals with those of third-row analogues, even if the former have intrinsically similar or even greater catalytic activity, the latter will tend to have higher stability.57−62 The high thermal stability of 13-H2 prompted Kaska, Jensen, and Goldman to investigate the dehydrogenation of alkanes in the absence of any acceptor, a highly endothermic reaction that would necessarily require high temperature. 13-H2 (1.0 mM) indeed catalyzed acceptorless dehydrogenation of cyclodecane, affording 360 TOs in 24 h at 200 °C.55 Subsequently, the Goldman group synthesized a precursor of the isopropylsubstituted analogue, (iPr4PCP)IrH4 (14-H4), which was found to be even more reactive as a catalyst for acceptorless cyclodecane dehydrogenation. The dehydrogenation of cyclodecane was followed in part with secondary dehydrogenation and isomerization to give trans,trans-1,5-cyclodecadiene; this was in turn followed by a Cope rearrangement to give trans-1,2divinylcyclohexane, which acted as a hydrogen acceptor to complete a catalytic cycle for the isomerization of cyclodecane to diethylcyclohexane (Figure 2).

It should be noted that the dihydride, tetrahydride, and olefin complexes of pincer−iridium fragments have been used as precatalysts for alkane dehydrogenation, as have the corresponding iridium hydrido chlorides plus base.10,57−60 All observations indicate that all of these approaches generate a common intermediate, i.e., the three-coordinate d8 (R4PCP)Ir, as the catalytically active species. (Accordingly, in this review, rather than referencing the catalyst precursor that was used in the experiment under discussion, we will often simply refer to the pincer−metal fragment as the catalyst.) 2.2. Scope of Pincer−Iridium-Catalyzed Alkane Dehydrogenation

Subsequent years have seen the development of numerous relatives or analogues of (tBu4 PCP)Ir (13) for alkane dehydrogenation (Figure 3).10,57−60 Variations include changes in the backbone aryl group,66−71 substitution of the CH2 linkers,72−80 and modifications of the ligating atoms.81−85 In addition to alkane dehydrogenation, the resulting catalysts have found utility in the dehydrogenation of several other substrates (see section 5),51,77,84−88 and as will be seen in the later part of this review, dehydrogenation has been exploited in tandem with secondary reactions, some involving secondary catalysts, giving rise to a range of hydrocarbon transformations, including alkane metathesis,10,57−60,76,81,89−92 alkyl group cross-metathesis,93 alkane dehydroaromatization,75,94,95 alkane oligomerization,96 alkane−alkene coupling,97−99 alkyl−aryl coupling,100,101 cocyclization with ethylene,102,103 and alkane borylation84 and silylation.104 With an objective of designing catalysts with even greater thermal stability, Haenel, Kaska, and co-workers reported the synthesis of (tBu4Anthraphos)IrH2 (33-H2) and its use for the acceptorless dehydrogenation of alkanes;71 the complex was found to tolerate temperatures up to 250 °C. The Goldman group synthesized the adamantyl-substituted species (Ad4PCP)IrH4 (15-H4), which is sterically similar to but more stable than the (tBu4PCP)Ir analogue, and was, accordingly, particularly effective for acceptorless alkane dehydrogenation (a reaction which intrinsically requires high temperature to overcome the enthalpic barrier).105 By replacing the CH2 linkers in the PCP backbone with oxygen linkers, the Brookhart72−74 and Jensen79 groups independently synthesized numerous derivatives of (R4POCOP)Ir (23−28). While (tBu4POCOP)IrH2 (23-H2) has been shown to have greater activity than (tBu4PCP)IrH4 (13H4) for COA/TBE dehydrogenation, the latter is more active for the transfer dehydrogenation of linear alkanes.73 Goldman and co-workers conducted a systematic investigation to study the effect of steric factors on catalytic efficiency.81 Methyl-for-tBu-substituted analogues of the parent complex (tBu4PCP)IrH4 (13-H4) were synthesized, specifically (tBu3MePCP)IrH4 (16-H4) and (tBuMePCPtBuMe)IrH4 (17-H4). Density functional theory (DFT) studies indicated that the first Me-for-tBu substitution would have a significantly favorable effect in lowering the overall energy barrier for the n-alkane/1alkene transfer dehydrogenation cycle. The effect of additional substitutions was less pronounced largely due to cancellation of the effect on olefin binding (which inhibits catalysis) and the favorable effect on the barrier of the reaction of the three-

Figure 2. Net isomerization of cyclodecane catalyzed by (iPr4PCP)Ir via (i) dehydrogenation, (ii) Cope rearrangement, and (iii) hydrogenation of the rearranged product.

This offered an early example of the possibility of combining dehydrogenation with secondary reactions, and in particular doing so in a way that required neither a net hydrogen acceptor nor the extrusion of H2 from solution. The term “hydrogenborrowing”, later coined by Williams,63 may be applied to describe this isomerization. The isopropyl-substituted pincer complex was also found in the absence of an acceptorto afford the first example of catalytic dehydrogenation of n-alkanes.64 Goldman and Jensen then demonstrated that both 13-H4 and the isopropyl analogue 14-H4 exhibited high kinetic selectivity for the formation of 1octene in the transfer dehydrogenation of n-octane at 150 °C (eq 3).65 Unfortunately, this remarkably regioselective dehydrogenation is accompanied by catalytic isomerization, which limited the yields of 1-octene to under 100 mM. 12360


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Figure 3. Some variations of pincer-ligated iridium complexes that have been investigated for alkane dehydrogenation.

Table 1. n-Alkane/TBE Transfer Dehydrogenation by Pincer−Iridium Complexes: Varying Steric Crowding at Different TBE Concentrations

coordinate species with the alkane. These predictions were complemented by experimental studies on n-octane transfer dehydrogenation using TBE or norbornene (NBE) which showed that 16 was more active than either 13 or 17 (see entry 1, Table 1). However, it was also reported that 17 tended to form dinuclear clusters, and hence, it was difficult to determine whether dimer formation, rather than intrinsically lower activity, was the source of the lower turnover numbers.81 An indication that dimer formation was indeed the determining factor was derived from later experiments with a very high concentration of TBE (4.1 M, at 200 °C), which would be expected to push any equilibrium toward the monomer. Under such conditions (entry 2, Table 1), 17 exhibited greater catalytic activity than 16.106,107

Huang and co-workers recently reported very high activity for transfer dehydrogenation by the sulfur-substituted catalyst (iPr4PSCOP)Ir (31).77 An initial rate of 2900 TOs/h was observed for COA (3.9 M)/TBE (3.9 M) transfer dehydrogenation with a solution of (iPr4PSCOP)IrHCl (1.3 mM, plus 2.0 mM NaOtBu to eliminate HCl) at 200 °C. Under the same conditions with (tBu4PCP)IrH2 (13-H2), transfer dehydrogenation proceeds more slowly, 1200 TOs/h, but is faster with (tBu4POCOP)IrH2 (23-H2) (6900 TOs/h).73 Catalyst 23, however, yields only a maximum conversion of about 62% as the reaction levels off after 6 h, whereas with iPr4PSCOP complex 31 complete consumption of TBE is observed after about 8 h. In contrast to the excellent performance of 31, the much more crowded (tBu4PSCOP)Ir (1.3 mM) gave an initial 12361


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rate of 410 TOs/h with a final TON of 106 in 8 h. Other derivatives, (tBu2PSCOPR2)Ir (R = Cy, Et), were also found to be significantly less effective than 31.78 n-Octane transfer dehydrogenation with TBE (0.5 M) was reported to be more effective with (iPr4PSCOP)Ir (31) (1400 TOs/h) than with either (tBu4PCP)IrH2 (13-H2) (820 TOs/h) or (tBu4POCOP)IrH2 (23-H2) (220 TOs/h).77 At comparable turnovers, the regioselectivity for 1-octene obtained with 31 was very similar (30%, TON of ca. 115) to that obtained using 13 (only trace amounts of 1-octene were obtained with 23-H2). Yamamoto reported the synthesis of a series of tricyclic pincer−iridium complexes (7−6−7-R4PCP)Ir (R = iPr (36), Ph (37), cyclohexyl, and 3,5-xylyl; Figure 3 and Table 2) isolated

enhance the thermal stability of the catalyst by maintaining tridentate coordination, the flexible backbone could favor catalytic activity. High rates and turnover numbers were indeed obtained. (7−6−7-Ph4PCP)Ir (37) was apparently the most effective for COA/TBE transfer dehydrogenation, affording, for example, 4100 TOs after 24 h at 200 °C. Detailed structural and computational studies supported the hypothesis that the framework afforded conformational freedom that allowed the high activity. It is noteworthy that this appears to be the first PCP-type pincer ligand with PPh2 groups successfully applied to alkane dehydrogenation; perhaps cyclometalation is more likely to occur with less constrained frameworks, although this remains to be determined. While phenyl-substituted 37 was insufficiently soluble in n-octane for catalysis, complex 36 demonstrated good activity for n-octane/NBE transfer dehydrogenation at 150 °C.80

Table 2. (7−6−7-R4PCP)Ir (36, R = iPr; 37, R = Ph) (0.5 mM) Catalyzed COA (4.6 M)/TBE (3.1 M) Transfer Dehydrogenation at Various Temperatures

2.3. Mechanism and “Loss of Regioselectivity″: Double Bond Isomerization by Pincer−Ir Dehydrogenation Catalysts

Figure 4 depicts generalized catalytic cycles proposed108 for pincer−iridium-catalyzed n-alkane/alkene transfer dehydrogenation, illustrated with butane/TBE, along with the associated isomerization109,110 of the α-olefin product. Hydrogenation of TBE by dihydride complex 49 yields the catalytically active 14electron three-coordinate Ir(I) species 51. Alkane C−H addition to 51 and subsequent β-hydride elimination result in olefin formation. Though the general features of the mechanism are probably very similar for various catalysts, there are distinct differences between catalysts concerning the nature of the resting states and rate-determining steps. Sterically crowded catalysts such as (tBu4PCP)Ir (13), especially in the absence (or under very low concentrations) of strongly binding alkenes such as α-olefin favor the corresponding dihydride (49) as the resting state. In the presence of significant concentrations of olefin, the resting state can be Ir−olefin complexes such as 57, 58, or 59. Bulky

as the corresponding hydrido chlorides and their use as catalysts (activated with 2 equiv of NaOtBu) for transfer dehydrogenation (COA/TBE and n-octane/TBE).80 The authors considered that while the fused-ring geometry could

Figure 4. Pincer−iridium-catalyzed n-alkane/TBE transfer dehydrogenation and associated α-olefin isomerization: mechanistic cycles. 12362


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Figure 5. Isomerization of 1-octene catalyzed by (tBu4PCP)Ir (13) at 125 °C.109 Isomerization rates in n-octane and p-xylene solvent were found to be identical, arguing against an important role for a small (unobserved) concentration of (tBuPCP)IrH2.

Figure 6. Low-temperature generation of an η3-allyl hydride complex from (tBu4POCOP)IrH2 and allene.

pathway” was calculated to proceed via initial oxidative addition of the allylic C−H bond of an α-olefin to the 14-electron threecoordinate iridium species (51) to generate an η1-allyl hydride (54). Complex 54 then undergoes coordination of the double bond to give an η3-allyl hydride (55) which then undergoes the reverse reaction, an η3−η1 transformation, in the reverse sense, to yield 56, which undergoes C−H elimination to give internal olefin (Figure 4).109 Complementary evidence in support of the allyl pathway was obtained by low-temperature NMR experiments involving the addition of allene to (tBu4POCOP)IrH2 (23-H2) at −88 °C, which resulted in the generation of (tBu4POCOP)Ir(η3-allyl)H (58) along with (tBu4POCOP)Ir(propenyl)H (58′) (Figure 6). Both 58 and 58′ were found to isomerize to give (tBu4POCOP)Ir(η2-propene) (59).109 The group of Chianese has also reported the involvement of an η3-allyl pathway in alkene isomerization reactions catalyzed by CCC pincer−iridium complex 42.111 Subsequent studies highlighted the perils of extrapolating conclusions from one pincer−Ir complex to even a closely related one. 110 In contrast to both ( tBu 4 PCP)Ir and (tBu4POCOP)Ir, (iPr4PCP)Ir was found to catalyze isomerization via both hydride addition and η3-allyl pathways under typical conditions. Under “hydrogen-poor” conditions (i.e., in the presence of high concentrations of active hydrogen acceptors such as propene and ethylene), however, steady-state concentrations of dihydride are negligible, and only then is the hydride pathway effectively nonoperative.110

complexes such as 13 are found to form an iridium(III) hydrido vinyl complex (59) with TBE,108 while sterically less demanding complexes tend to form an Ir(I)−TBE π-complex (58). 72 The α-olefin product resulting from n-alkane dehydrogenation binds more strongly than TBE (as either 58 or 59) to give 57. In general, high acceptor concentrations and the use of sterically less hindered acceptors would be expected to retard the activity of a catalyst whose resting state is an olefin complex, while similar conditions would promote the rates of a catalyst with a dihydride resting state (49).76 The isomerization of α-olefins, particularly by systems in which metal hydrides are known to be present, is generally expected to proceed via a “hydride addition pathway”. In the present case, an initial 2,1-addition of the Ir−H bond of dihydride 49 across the double bond of the α-olefin results in formation of a 2-alkyl hydride (53) which can yield an internal olefin via 3,2-β-hydride elimination. In the cases of the relatively bulky species (tBu4PCP)Ir (13) and (tBu4POCOP)Ir (23), however, a detailed experimental and computational study of n-alkane/TBE transfer dehydrogenation by KroghJespersen, Brookhart, and Goldman led to some unexpected conclusions.109 The major resting state was observed to be the 1-octene complex. The hydride pathway would therefore require that a concentration of dihydride too small to be detected would be the active species. This small concentration would vary with the alkane concentration as per the steady state indicated in Figure 5. However, the rate of olefin isomerization was found to be identical in p-xylene and n-octane solvent. Complemented by labeling studies and kinetics, this strongly argued against the operation of the hydride addition pathway for isomerization by these complexes. The experimentally based conclusion that olefin isomerization by 13 and 23 proceeds via an η3-allyl pathway was supported by DFT calculations. Unexpectedly, the “allyl

2.4. Solid/Gas-Phase Systems and the Use of Ethylene and Propylene

Goldman and co-workers have recently reported the transfer dehydrogenation of gas-phase light alkanes with ethylene and propene at 200−240 °C catalyzed by solid-phase (iPr4PCP)Ir12363


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(C2H4) (14-C2H4).110,112,113 Under these conditions, remarkably high rates and turnover numbers were obtained. Most surprisingly, these systems afforded yields of α-olefins much greater than those obtained in solution-phase experiments.110,112,113 This finding seems quite counterintuitive considering that selectivity is often regarded as the strong suit of homogeneous catalysis. The terminal regioselectivity afforded by the solid-phase catalyst indicates that it is acting as a molecular catalyst. Indeed, comparison with solution-phase kinetic studies, combined with the results of DFT calculations, supports the conclusion that in the solid phase the catalyst behaves essentially no differently than in solution. As noted above, (iPr4PCP)Ir can effect isomerization via either hydride or allyl pathways, while DFT calculations predict that isomerization via the allyl pathway is slower, relative to alkane dehydrogenation, than is the case for (tBu4PCP)Ir. The conditions of the solid/gas-phase catalysis strongly disfavor the amount of dihydride present at any time, specifically a low concentration of alkane (gas phase vs solution phase) and a relatively high concentration of highly active, sterically unhindered, hydrogen acceptor (either ethylene or propene) (Figure 7). On the basis of these considerations, the

(tBuMePCPtBuMe)Ir (17).107 With 14-C2H4 (7.0 mM) as the catalyst precursor and 5.1 M TBE, the yield of piperylene (1.2 M) was ca. 100-fold greater than yields obtained with the use of bulky catalysts such as (tBu4PCP)Ir (13).107 2.5. Fluorinated Pincer−Iridium Dehydrogenation Catalysts

The electronic properties of the pincer−iridium center have been modulated by introducing methoxy (18, 19, and 26),66,73 dimethylamino (20),67 pentafluorophenyl (28),73 and ester (21)67 functionalities to the arene backbone of PCP-type pincer ligands. A presumably much greater effect has been explored by Roddick, who has obtained very promising results using bis(trifluoromethyl)phosphino groups as in 22.82,114−116 The transfer dehydrogenation of COA/TBE (1:1) was catalyzed at 200 °C by (CF3PCP)Ir(η4-COD) (60; 1.3 mM) (eq 4), although rates (ca. 40 TOs/h) were significantly slower than those obtained with either (tBu4PCP)Ir (1200 TOs/h) or (tBu4POCOP)Ir (900 TOs/h).73

The use of lower concentrations of TBE (1.3 M) with (CF3PCP)Ir(η4-COD) (60a) (1.3 mM) allowed a faster rate (155 TOs/h) of COA dehydrogenation.82,114−116 As previously noted, the suppression of reaction rates and yields by higher concentrations of TBE dates back to the earliest pincer−metalcatalyzed alkane dehydrogenations.6 Roddick found that (CF3PCP)Ir(η4-isoprene) (60b) was obtained as a major species when (CF3PCP)Ir(η4-COD) (60a) was treated with COA/TBE (1:1) at room temperature.82,114−116 (60b was independently synthesized by the treatment of (CF3PCP)Ir(H)(Cl)(η2-C2H4) (61) with excess isoprene in the presence of triethylamine; eq 4.) This led to the consideration that isoprene was the major contaminant of the commercially obtained TBE that was used in these reactions. Repeating the transfer dehydrogenation of COA catalyzed by 60 with high-purity TBE (3.9 M) at 200 °C resulted in an improved rate (136 TOs/h).82,114−116 It appears likely that the decreased rates that frequently result from increased concentrations of TBE are due to a combination of effects, including both the presence of impurities (isoprene or isopentene) and inhibition by the TBE itself due to its binding to the reactive fragment.73,117 Quite interestingly, catalytic activity of the CF3PCP complexes was uninhibited by the presence of a N2 atmosphere or by the addition of water. This observation contrasts sharply with the behavior of the nonfluorinated, more electron-rich, complexes such as (tBu4PCP)IrH2 (13-H2) which readily form dinitrogen complexes118,119 and oxidatively add water.120 These studies hold important promise for the development of catalytic systems that do not require exclusion of species such as N2 or water (or even perhaps for systems that require their inclusion). Although rates were not as high, the productivity of 60 (1.3 mM) for catalytic transfer dehydrogenation of COA/TBE (1:1)

Figure 7. Catalytic cycle for pentane/ethylene or pentane/propylene transfer dehydrogenation.

relative rates of isomerization and dehydrogenation under the solid/gas-phase conditions could be predicted from the solution-phase results to give high yields of α-olefin (1-pentene for most of the studies under these conditions). Prior to the work with (iPr4PCP)Ir and ethylene or propene as the acceptor, yields of α-olefin achieved with transfer dehydrogenation catalysts had never reached 100 mM; the best result was 97 mM 1-octene from n-octane/1-decene (0.5 M) transfer dehydrogenation catalyzed by (tBu4PCP)Ir (13; 1 mM) at 150 °C.65 Using ethylene (4 atm) as the hydrogen acceptor, catalysis at 200 °C by (iPr4PCP)Ir(C2H4) (14-C2H4) in n-octane solution afforded 250 mM 1-octene.110,112,113 While this value far surpassed the previous record, under solid/gas-phase conditions (240 °C), the transfer dehydrogenation of npentane with ethylene (4 atm), catalyzed by 14-C2H4, resulted in an even greater yield of α-olefin; after thermolysis and condensation of all species present in the gas phase, the concentration of 1-pentene was found to be 520 mM.110,112,113 Dienes are useful chemical intermediates in the production of synthetic rubbers, adhesives, and resins. Brookhart found that 1,3-pentadiene (piperylene) could be obtained in good yield, up to ca. 40%, from n-pentane using (iPr4Anthraphos)Ir (34) as the catalyst and propene as the acceptor.103 Piperylene yields were limited by Diels−Alder cyclization with the hydrogen acceptor. However, the Diels−Alder products may be catalytically dehydroaromatized to the commercially valuable building blocks toluene and xylene. The Goldman group subsequently reported very high yields of piperylene in the transfer dehydrogenation of n-pentane at high concentrations of TBE using sterically uncrowded species (iPr4PCP)Ir (14) or 12364


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Table 3. Comparison of Final TONs in COA (3.9 M)/TBE (3.9 M) Transfer Dehydrogenation Catalyzed by Various Pincer− Iridium Catalysts at 200 °C for 40 h

at 200 °C (660 TOs after 58 h)82,114−116 was even greater than that of (tBu4PCP)Ir (13) (230 TOs in 20 h).73 The catalytic activity of 60 did, however, decrease with time; 19F NMR experiments revealed the formation of free (CF3PCP)H, indicative of decomposition rather than loss of catalytic activity solely due to product inhibition. Again illustrating the difficulty of extrapolating from dehydrogenation of COA to n-alkanes, low activity (81 TOs in 48 h) was observed for n-octane/TBE (0.2 M) transfer dehydrogenation catalyzed by 60 at 150 °C.82,114−116 Catalyst 60 was also less effective for the acceptorless dehydrogenation of cyclodecane (92 TOs in 24 h)82,114−116 than various (R4PCP)Ir precursors such as (pOMe-iPr4PCP)IrH4 (19-H4) (2100 TOs).66,105 The Wendt group recently reported the synthesis of an electron-poor pincer−iridium complex, [(mCF3)2-tBu4POCOP]IrHCl (28b-HCl), an m-CF3-disubstituted derivative of (tBu4POCOP)IrHCl (23-HCl).70 This complex gave very high turnover numbers for COA/TBE (1:1) transfer dehydrogenation (Table 3). These numbers may be compared with those of the parent (tBu4POCOP)IrH2 (23-H2) catalyst, which gives lower conversions (but higher initial rates). paraSubstituted derivatives 28a and 28c gave yields that were comparable (entries 4−6, Table 3).70,73 In general, it is far from straightforward to predict the effect on catalytic activity of varying electronic factors within the general “(PCP)Ir” class of catalysts (i.e., including POCOP and related pincers). The factors that favor the kinetics and thermodynamics of C−H addition, and possibly β-H elimination, will often favor the binding of olefin to the 14electron fragment;56,66,121 these effects will therefore tend to cancel out, generally in fairly unpredictable ways. The conclusions based on cyclooctane dehydrogenation are often not applicable to linear alkanes, and both the nature and the concentration of the acceptor will influence not only absolute rates, but also relative rates, depending upon how the variations influence the strength of binding to the olefin acceptor. In general, within the “(PCP)Ir” class of catalysts, variation in stability, rather than in the actual level of activity, seems to be more important in determining the productivity at typical reaction times and temperatures. Perhaps most surprisingly, relatively subtle electronic effects can even greatly influence regioselectivity.122

2.6. PC(sp3)P Pincer−Iridium Catalysts

The Brookhart group recently investigated (RtriptycenePC(sp3)P)Ir(η2-C2H4) (48) for the transfer dehydrogenation of alkanes (Scheme 1).69 The bridgehead sp3 metalated carbon of Scheme 1. PC(sp3)P−Iridium Catalysts

the (RtriptycenePC(sp3)P)Ir unit is strongly σ-donating, while the framework precludes α- and β-hydrogen elimination pathways.123−126 For the transfer dehydrogenation of n-octane (6 M TBE) at 200 °C, complex 48a gave very high initial rates (2400 TOs/h) and turnover numbers (6000 TOs, based on TBA, after 10 h). Even at 100 °C, transfer dehydrogenation of n-octane by 48a proceeded to give complete conversion of 0.5 M TBE.69 For COA/TBE (1:1) transfer dehydrogenation at 200 °C, 48a (1.3 mM) afforded 2600 TOs after 4 h. Complexes with cyclohexyl (48b) or cyclopentyl (48c) groups in place of the phosphino iPr groups were far less effective than 48a (35 and 40 TOs, respectively, after 24 h).69 It seems likely this is due to cyclometalation of the more extensive (but not much more sterically demanding) cycloalkyl groups. Following the success of Brookhart and Goldman67,92 in anchoring pincer−iridium complexes with polar substituents, such as (p-Me2N-tBu4PCP)Ir (20), onto solid supports such as γ-alumina, the Brookhart group tested the use of 48d for heterogeneous transfer dehydrogenation of alkanes.69 Under homogeneous solution-phase conditions, catalysts 48a (1 mM) and 48d (1 mM) gave comparable product yields for COA/ TBE (1:1) transfer dehydrogenation at 200 °C (2040 and 2800 TOs, respectively, after 20 h). With various types of γ-alumina screened as supports, virtually all (>99%) catalytic activity of 48a was lost. This observation is in accord with earlier reports by Brookhart and Goldman67,92 which showed the deactivation by alumina of PCP−iridium catalysts lacking polar substituents. However, addition of neutral or low-soda γ-alumina to a 12365


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(RNCOPtBu2)IrHCl, were synthesized, 47a−c (Figure 3 and Scheme 3), where R = H, Me, and tBu. respectively (Figure 3).

solution of 48d resulted in loss of the solution’s orange color, which was acquired by the alumina. The resulting mixture functioned as a heterogeneous system, affording TONs over 50% greater than those obtained in solution. With basic or acidic alumina, by contrast, >90% of the catalytic activity of 48d was lost (although this still compares favorably with 48a, which completely lost activity in the presence of these types of alumina).69 Wendt has reported potential (PC(sp 3 )P)Ir catalyst precursors 63-HCl and 64-HCl (Scheme 1).127,128 Their effectiveness as catalysts for COA/TBE transfer dehydrogenation was found to be quite low, which was attributed to low thermal stability. The contrast between these complexes and the triptycene-based (PC(sp3)P)Ir catalysts reported by Brookhart (48), which were found to be particularly stable, is certainly noteworthy. The instability of 63 and 64 may be attributable to C(sp3)−H elimination, which is generally much more favorable than C(sp2)−H elimination; in the case of 48, however, elimination may be prevented by the rigidity of the triptycene framework.

Scheme 3. NCOP and CCC Pincer−Iridium Catalysts

The complexes were investigated for transfer dehydrogenation of COA and n-octane with 0.5 M TBE at 150 °C.84 The initial rate of COA transfer dehydrogenation obtained with 47a (1100 TOs/h) was remarkably greater than that obtained with 47b (12 TOs/h) or 47c (6 TOs/h), which was attributed to steric effects. For n-alkanes which have a reduced steric demand relative to COA, 47b and 47c showed dehydrogenation activity of the same order of magnitude as (although still less than) that of 47a. For instance, in the transfer dehydrogenation of noctane/TBE (0.5 M) at 150 °C, 47a gave 325 TOs/h, while initial rates of 84 and 60 TOs/h were obtained with 47b and 47c, respectively.84 The acceptorless dehydrogenation of alkanes by bis(Nheterocyclic carbene)aryl pincer (CCC)−iridium complexes (40−45; Figure 3 and Scheme 3) has been extensively studied by Chianese and co-workers.111,133−135 Complexes 42, 43, and 44 (0.5 mM) gave TONs of 103, 84, and 35, respectively, after 22 h of the acceptorless dehydrogenation of cyclooctane (bp 150 °C). In reactions catalyzed by 42, addition of COE (equivalent to a TON of 100) had a negligible effect on catalytic activity.134 Use of cyclodecane, as a higher boiling point (201 °C) substrate/solvent, did not result in increased turnover numbers, with 102 TOs obtained with 42 after 22 h. These studies indicated that turnover numbers are limited by catalyst decomposition, rather than inhibition by product. (For comparison, the best catalytic system reported for acceptorless cyclodecane dehydrogenation is based on (p-OMe-iPr4PCP)IrH4 (19-H4; 1 mM), which gave 2100 TOs in 24 h.66) The catalytic activity of 42 was not influenced by the presence of a N2 atmosphere. However, under an atmosphere of air, the activity of these catalysts was negligible.111,133−135 In the acceptorless dehydrogenation of n-undecane (bp 196 °C), catalysts 42 and 43 afforded about 50 TOs.111,133−135 Catalyst 44, which showed activity lower than that of 42 or 43 for acceptorless dehydrogenation of cycloalkanes, gave 97 turnovers in 22 h for the acceptorless dehydrogenation of nundecane. The values are comparable to those of the most efficient catalytic systems based on (PCP)Ir catalysts. Only internal undecenes were observed in these reactions, which is to be expected as the Chianese group has shown that the CCC pincer-based iridium complexes are capable of catalyzing alkene isomerization.111,133−135 The suggestion of a new chapter in dehydrogenation by pincer−iridium complexes was recently raised with a report by Nishiyama136 that n-octane reacts with (dmPhebox)Ir(OAc)2(H2O) (38) in the presence of K2CO3 at 160 °C to give (dmPhebox)Ir(OAc)(n-octyl) (65).136 Goldberg and co-workers shortly thereafter reported that, at 200 °C, 38 effects the stoichiometric dehydrogenation of n-octane to give (dmPhebox)Ir(OAc)(H) (66) and octenes (eq 5).131 Independent experiments with 65 confirmed its intermediacy in this reaction. At early reaction times (3 h, ca. 30% conversion), the

2.7. PBP Pincer−Iridium Catalysts

Yamashita and Tanoue have reported the synthesis of pincer− iridium complexes in which the central coordinating atom is boron, such as (iPr4PBP)IrHCl (39-HCl) and (iPr4PBP)Ir(C2H4) (39-C2H4) (Scheme 2).129 These complexes were tested for Scheme 2. PBP−Iridium Catalysts

COA/TBE (1:1) transfer dehydrogenation at temperatures ranging from 160 to 220 °C. Very modest dehydrogenation activity was observed. The highest yield of dehydrogenation product was about 43 TOs, which was obtained with 39-HCl (1.3 mM) and LiTMP (lithium 2,2,6,6-tetramethylpiperidide, 2 mM) as the activating base at 220 °C.129 Ozerov recently reported modest COA/TBE transfer dehydrogenation activity with iridium complexes supported by a bis[(dialkylphosphino)aryl]boryl (iPr4PBP) pincer ligand (62).130 The boryl group may play a “noninnocent” role in C− H activation as indicated by the NMR spectrum of (iPr4PBP)IrPhH (62-PhH), which suggests a boryl−hydride interaction (Scheme 2).130 2.8. Non-Phosphine-Based Pincer−Iridium Complexes

In addition to the many phosphine-based pincer−iridium systems, there have been numerous reports on alkane dehydrogenation catalyzed by non-phosphine-based pincer− iridium systems with motifs such as AsCAs,85 NCN,131,132 and CCC.111,133−135 The synthesis of an arsine analogue of POCOP catalysts, (tBu4AsOCOAs)IrH2 (46-H2), and its use in catalytic transfer dehydrogenation of COA was reported by Jensen in 2014.85 COA/TBE (1:1) transfer dehydrogenation with 1 mM (tBu4AsOCOAs)IrH2 (46-H2) at 200 °C gave about 300 TOs after 30 min, which is about 3−4 times less than that of (tBu4POCOP)IrH2 (23-H2) under the same conditions.73,77,85 Huang and co-workers recently reported the synthesis of “RNCOPtBu” ligand precursors which include an aryl group and phosphinite and pyridine arms, where R is an ortho-substituent on the pyridine unit. Iridium hydrido chloride complexes, 12366


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O2 at the high temperatures required for the dehydrogenation of eq 5. Goldman and Goldberg, however, very recently reported that both the C−H addition step and, especially, the β-H elimination step of eq 5 can be catalyzed by Na+ or other simple Lewis acids; the resulting reduction of the temperature required for eq 5 is promising with respect to realization of the catalytic cycle of Figure 8.140

3. DEHYDROGENATION OF ALKANES BY PINCER COMPLEXES OF METALS OTHER THAN IRIDIUM 3.1. Alkane Dehydrogenation by Pincer−Ruthenium Complexes

While the dehydrogenation of alcohols and other polar substrates by pincer−ruthenium and even pincer−iron complexes has been extensively developed and investigated,141−145 only recently has alkane dehydrogenation by pincer complexes of group 8 metals been explored. In 2011, the Roddick group reported investigations of (CF3PCP)Ru complexes for alkane dehydrogenation. Addition of a H2 atmosphere to a solution of (CF3PCP)Ru(COD)H (70) at 80 °C did not yield the expected (CF3PCP)RuH(H2)2, which the investigators had hoped would serve as a precatalyst.146,147 Instead the reaction led to the loss of COD and the formation of dimeric ruthenium complex 71. Upon treatment with excess COD, dimer 71 reverted back to 70 (eq 7). The use of COD

major octene product was 1-octene, although the product mixture was predominantly internal olefins when the reaction was complete in 120 h. The mechanism of n-octane dehydrogenation mediated by (dmPhebox)Ir(OAc)2(H2O) (38) appears to involve a C−H activation (concerted metalation deprotonation, CMD137,138) at the Ir(III) center, in contrast to the phosphine-based pincer−iridium systems discussed above where an Ir(I) species is presumed to be responsible for the C−H activation.139 Note that the very different mechanisms both give high selectivity for the terminal position of n-alkanes. In accord with the proposed intermediacy of only Ir(III) species, the alkane dehydrogenation of eq 5 was found not to be inhibited by the presence of N2, water, or α-olefin. Indeed, addition of water (ca. 120 equiv) resulted in a 33% increase in the yield of the stoichiometric noctane dehydrogenation products.37 In a subsequent report, the Goldberg group found that (dmPhebox)Ir(OAc)(H) (66) reacted quantitatively with molecular oxygen to give (dmPhebox)Ir(OAc)2(H2O) (38) (eq 6), the species that initially underwent reaction with n(dmPhebox)Ir(OAc)(H) (66) + O2 + HOAc dm

→ ( Phebox)Ir(OAc)2 (H 2O) (38)

complex 70 (12 mM) as a precatalyst for acceptorless dehydrogenation of COA afforded only three turnovers (36 mM) in 10 min, and the catalyst was fully transformed to inactive dimeric 71 (eq 7). When 70 (6 mM) was used for COA/TBE (1:1) transfer dehydrogenation, only 18 turnovers (54 mM) were obtained, with quantitative conversion of 70 to 71. Lowering the catalyst concentration and raising the temperature resulted in better performance as might be expected from a catalyst that is deactivated by dimerization. Using 1.25 mM 70, COA/TBE (1:1) transfer dehydrogenation was catalyzed to give 164 TOs (205 mM) in 3 h at 150 °C and 186 TOs (230 mM) in 30 min at 200 °C. As was observed with the iridium catalyst (CF3PCP)Ir(COD) (60),82,114−116 the activity of (CF3PCP)Ru(COD)H (70) was found to be unaffected by the presence of N2 or 100 equiv of H2O. Remarkably, the reaction rate and final productivity of the ruthenium analogue were unaffected even by an O2 atmosphere.146,147 For dehydrogenation catalyzed by PCP-type pincer complexes of group 8 metals, Roddick proposed that the active species is likely to be a d6 four-coordinate metal(II) hydride (Figure 9),146,147 in contrast with the d8 three-coordinate M(I) species that are generally believed to effect C−H activation in pincer−Ir or pincer−Rh chemistry. Huang and co-workers have recently synthesized and investigated a series of pincer−ruthenium complexes (72−76; Figure 10) for the transfer dehydrogenation of alkanes.148 The complex (iPr4POCOP)Ru(NBD)H (73; NBD = norborna-

(6)

octane (eq 5).132 The sum of eqs 5 and 6 constitutes an overall dehydrogenation of n-octane with O2 as the hydrogen acceptor, in principle catalyzed by 38 (Figure 8). The use of O2 as the acceptor is obviously economically very attractive as compared with the use of a sacrificial olefin. Unfortunately, however, whereas the oxidation of eq 6 proceeded cleanly at ambient temperature, 38 underwent decomposition in the presence of

Figure 8. Hypothetical catalytic cycle for dehydrogenation of n-octane with O2 as the hydrogen acceptor. 12367


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the uncertainty of the thermodynamic data, or perhaps impurities may have acted as hydrogen acceptors.148 3.2. Alkane Dehydrogenation by Pincer−Osmium Complexes

Following their reports of dehydrogenation by (CF3PCP)Ru complexes,146,147 the Roddick group investigated osmium analogues.149 (CF3PCP)Os(COD)H (78) (1.2 mM) was inactive for COA/TBE (1:1) transfer dehydrogenation at 150 °C. Increasing the temperature to 200 °C, however, afforded an initial rate of 1500 TOs/h. This is slightly lower than that of the Ru analogue (CF3PCP)Ru(COD)H (70),146,147 but 78 demonstrated greater thermal stability; 610 TOs of cyclooctene were obtained after 8 h, whereas the Ru analogue gave a maximum TON of ∼350 after 20 min. With time, the catalyst (CF3PCP)Os(COD)H (78) decomposed to an inactive species, characterized by NMR spectroscopy as (CF3PCP)Os(COD)X (79), where X was proposed to be −CH2CH2tBu. As noted above, both (CF3PCP)Ir(COD) (60) and (CF3PCP)Ru(COD)H (70) were found to be stable to N2 and water, while 70 was stable even under an atmosphere of O2. (CF3PCP)Os(COD)H (78) showed no significant change in activity when COA/TBE (1:1) transfer dehydrogenation was performed at 200 °C with added water or under N2.149 The initial reaction rate was also unaffected by an O2 atmosphere (200 Torr), but catalytic activity decreased significantly within 1 h. 19F NMR analysis of the reaction mixture indicated the conversion of 78 to (CF3PCP)Os(CO)2H (80). On the basis of this observation and several control experiments, the authors suggested that the effect of O2 was indirect: trace amounts of acyclic alkanes were converted to aldehyde, which underwent decarbonylation by 78 to give the inactive complex 80.149 Complex 78 was inactive at 150 °C for the acceptorless dehydrogenation of COA.149 However, at 190 °C, 78 (1 mM) catalyzed acceptorless dehydrogenation of cyclodecane to give 125 TOs after 1 h; this is comparable to the activity of the widely studied (tBu4PCP)IrH2 (13-H2).55

Figure 9. Outline of the proposed cycle for alkane dehydrogenation catalyzed by (PCP)M complexes (M = Ru, Os).

diene) (1.0 mM) was first tested for the transfer dehydrogenation of COA at 200 °C with varying amounts of TBE (0.1−0.8 M) as the acceptor. At lower TBE concentrations (0.1−0.3 M), full conversion of TBE to TBA was observed, while higher [TBE] gave lower TONs; the maximum was achieved with 0.35 M TBE (306 mM). The use of highly purified TBE did not have a great effect on catalytic activity.148 (iPr4PCP)Ru(NBD)H (72; 1 mM) gave an initial rate of COA/TBE (0.35 M) transfer dehydrogenation of 1130 TOs/h at 200 °C, but only 260 TOs after 24 h. Complex 74 initially gave 420 TOs/h with 320 TOs after 24 h, which represented a small improvement over 73.148 Surprisingly, the chloride complex (iPr4POCOP)Ru(NBD)Cl (76) was found to be as effective as a precatalyst as the hydride analogue (74), giving an initial dehydrogenation rate of 390 TOs/h and a final yield of 240 TOs in 24 h. The PSCOP complex (75) showed very poor activity, giving only 8 TOs after 4 h. Various additives such as N2, ethyl acetate, diethyl ether, and acetone were well tolerated by catalyst 74 for COA/TBE transfer dehydrogenation.148 It may be that pincer−Ru catalysts are generally more tolerant of various non-hydrocarbon species than the iridium-based catalysts, although, unlike the (CF3PCP)Ru catalyst discussed above, the activity of 74 was completely inhibited by air. For transfer dehydrogenation of n-octane/TBE (0.4 M), the pincer−ruthenium catalysts (72−75) gave very low turnovers even at 200 °C.148 Acceptorless dehydrogenation of COA (boiling point 149 °C) catalyzed by 74 (2.5 mM) gave 39 TOs after 12 h. Surprisingly, it was reported that comparable yields were obtained in a sealed vessel at 200 °C; catalyst 74 (1.0 mM) in 2.7 mL of COA gave 29 and 36 TOs in 10 and 50 mL sealed tubes, respectively, after 1 h. Published values for the enthalpy of COA dehydrogenation are in the range of 23−24 kcal/mol.33 If it is assumed that COA and COE have equal entropies of formation, the lower enthalpy value corresponds to ΔG = 8.2 kcal/mol and an equilibrium constant of approximately 1.6 × 10−4 atm for COA dehydrogenation at 200 °C. The formation of 29 mM COE in a 10 mL vessel, assuming formation of an equimolar quantity of H2 (predominantly in the gas phase), suggests a value of K that is about 1.6 × 10−3 atm. This relatively modest apparent discrepancy may be attributable to

3.3. Alkane Dehydrogenation by Pincer−Rhodium Complexes (and Comparison with Pincer−Iridium Complexes)

As discussed in the Introduction, Rh complexes of PCP-type pincers are far less active as dehydrogenation catalysts than their Ir congeners. For example, (tBuPCP)RhH2 (81-H2) gives a negligible rate of 1.8 TOs/h for COA/TBE (0.2 M) transfer dehydrogenation at 200 °C.6 This sharp contrast with the high catalytic activity of the fragment (PMe3)2RhCl42−48,52,53 was ultimately explained in terms of the strong trans influence exerted by the PCP aryl metal-bound carbon, relative to chloride, which disfavors the formation of Rh−H or Rh−C bonds and therefore the corresponding additions to give Rh(III).56 Accordingly, Brookhart and Goldman investigated pincer−Rh and pincer−Ir analogues with weak-trans-influence groups at the pincer central position (Scheme 4). In this context, the carbazolide-based PNP ligand (carb-PNP) and the corresponding iridium and rhodium ethylene complexes (83-

Figure 10. Pincer−ruthenium complexes studied by the Huang group for alkane dehydrogenation. 12368


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Scheme 4

Figure 11. Stoichiometric hydrogenation of TBE by (carb-PNP)M (M = Rh, Ir).

Figure 12. Catalytic hydrogenation of TBE by (carb-PNP)M (M = Rh, Ir).

C2H4 and 84-C2H4) were synthesized;62,150 the sp2-hybridized nitrogen of this PNP ligand is much less σ-donating than the sp2-hybridized coordinating carbon of PCP ligands. Iridium complex 83-C2H4 reacted with H2 to give 83-H2; this is the carb-PNP analogue of complexes (RPCP)IrH2, which are such important intermediates in alkane transfer dehydrogenation. However, neither 83-H2 nor any other potential catalyst precursor based on fragment 83 proved active for alkane transfer dehydrogenation. DFT calculations revealed that, as a result of the weak-trans-influence coordinating N atom, the thermodynamics of dehydrogenating 83-H2 to give the 14electron Ir(I) fragment were much more unfavorable than those of the analogous reaction with (RPCP)IrH2; as a result, olefin hydrogenation was apparently the (very) slow step for transfer dehydrogenation. 83-H2 did not react with TBE or propylene even at 100 °C (Figure 11). Ethylene, typically the most reactive hydrogen acceptor (although it often coordinates with reactive fragments and strongly inhibits catalysis), underwent coordination with (RPCP)IrH2, but even then, the reaction to give ethane and 83-C2H4 was quite slow even at 75 °C.150 In contrast with the Ir analogue, (carb-PNP)RhH2 (84-H2) reacted with ethylene at room temperature to give ethane plus 84-C2H4 (Figure 11). Even TBE was hydrogenated within a few hours at 80 °C; thus, at least the hydrogenation part of a potential transfer hydrogenation cycle is fast with 84-H2. As noted above, PCP−rhodium complexes fail to effect alkane transfer dehydrogenation because the M(I)/M(III) thermodynamics too strongly favor the M(I) state. As hoped, in the case of the weaker trans-influence pincer ligand carb-PNP, this is apparently not a problem; 84-H2 is a fairly active catalyst for COA/TBE (1:1) transfer dehydrogenation, with initial rates at 200 °C over 500 TOs/h.62 Thus, the comparison of the carb-PNP and RPCP complexes of Ir and Rh strongly supports the conclusion that was drawn

on the basis of a comparison of (RPCP)M and ML2Cl complexes (M = Rh, Ir). Namely, for effective alkane dehydrogenation by pincer−M complexes, in the case of Ir a strong-trans-influence coordinating group is desired to disfavor the M(III) oxidation state, while in the case of Rh a weak-transinfluence coordinating group is needed to favor the same oxidation state. The (carb-PNP)M studies offered other insights into mechanistic aspects of alkane dehydrogenation. Quite surprisingly, while iridium complex 83-H2 did not stoichiometrically hydrogenate olefins at room temperature, not even at 100 °C with TBE, nor did it catalyze alkane transfer dehydrogenation, it proved to be active for catalytic hydrogenation of the same olefins at room temperature (Figure 12). Labeling and DFT studies both indicated that insertion of olefin into an Ir−H bond of 83-H2 was facile. Alkane transfer dehydrogenation, at least via the “standard” pathway of Figure 4, would require elimination from the resulting alkyl hydride; thermodynamically, this Ir(III)−Ir(I) reaction step is very unfavorable for 83. However, under an atmosphere of H2, the iridium(III) alkyl hydride undergoes addition of H2 and then elimination of alkane to regenerate the dihydride (Figure 13). Thus, instead of proceeding via the thermodynamically prohibitive Ir(I) intermediate, the cycle proceeds via H2 addition to Ir(III).150

Figure 13. Calculated (DFT) cycle for hydrogenation of ethylene by (carb-PNP)Ir (83). 12369


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Figure 14. Comparison of the reaction of TBE with (carb-PNP)MH2 for M = Rh (84) and Ir (83).

The DFT calculations indicate that this proceeds via two transitions states (TSs), one for H2 addition and one for subsequent C−H elimination; both TSs are essentially Ir(V) in character. The calculations also suggest that the TSs are connected by an Ir(V) intermediate which was calculated to be only 0.7 kcal/mol below the TS for H2 addition and was higher in free energy than the TS. While the nature or even the existence of such an intermediate can be called into question, the kinetics of the reaction are only affected by the nature of the TSs, which are clearly the products of H2 addition to the iridium(III) alkyl hydride (most importantly) and are strongly Ir(V) in character.150 The fact that iridium complex 83-H2 catalyzes olefin hydrogenation might seem to suggest that it could be effective for dehydrogenation, particularly since transfer of hydrogen from alkane to Ir(I) fragment 83 is calculated to be very favorable. However, transfer dehydrogenation is not the microscopic reverse of simple hydrogenation. The key intermediate for transfer dehydrogenation (via the conventional cycle of Figure 4) is the 14-electron pincer−M(I) species, which is not accessed in the 83-catalyzed hydrogenation (Figures 13 and 14). Hydrogenation is, however, obviously the reverse of acceptorless dehydrogenation. A complex that catalyzes hydrogenationunder the conditions that thermodynamically permit acceptorless dehydrogenationmust therefore be an acceptorless dehydrogenation catalyst. However, under such conditions, which include an extremely low concentration of H2, hydrogenation by 83 would be extremely slow since the rate is first order150 in [H2]. Catalyzing acceptorless dehydrogenation via the reverse of the highoxidation-state cycle of Figures 13 and 14 would require that the TSs for addition to Ir(III) (C−H addition to the dihydride and H2 addition to the alkyl hydride) should be significantly more favorable than is the case for 83. Finally, an additional unanticipated result emerged from the (carb-PNP)Rh studies. The rate-determining step in the dehydrogenation of alkanes by 84 was calculated to be the formation of an agostic Rh−(C−H) interaction; on the basis of microscopic reversibility, the rate-determining step for olefin hydrogenation is dissociation of the same agostic interaction. The hydrogenation direction is perhaps easiest to consider. It was calculated that olefin insertion leads directly to the agostic complex 84-(alkyl)(H); this step is reversible, while loss of the agostic interaction is irreversible and followed by rapid elimination of alkane (Figure 15). This is primarily a consequence of the coordination geometry of the agostic product resulting from olefin insertion in which the strongtrans-influence Rh-bound alkyl carbon is positioned trans to the terminal hydride ligand. Only after cleavage of the agostic interaction can the complex adopt the much lower energy “Y”

Figure 15. Schematic energy diagram illustrating that breaking or making the agostic bond of (carb-PNP)Rh(H)(alkyl) (84-(alkyl)(H)) is rate-determining for alkane dehydrogenation or olefin hydrogenation.

geometry (∠N−Rh−H = 139°; ∠N−Rh−C = 149°); it can then undergo facile reductive C−H elimination. This appears to be the first time that such a step has been proposed as ratedetermining either for hydrogenation or for dehydrogenation.62 A Rh complex of a pincer ligand with an even weaker transinfluence coordinating group (4,5-bis(diisopropylphosphino)9,9-dimethyl-9H-xanthene),151,152 82 (Scheme 4), was also found to catalyze COA/TBE transfer dehydrogenation. Although the catalytic activity of this cationic species is quite low, it is still much greater than that of the much more electron-rich (PCP)Rh complexes. It appears that the much weaker electron donation from the POP oxygen favors the formation of Rh−H and Rh−C bonds and thus the Rh(III) state, as discussed above. Indeed, it was proposed that the very low activity was attributable to the unfavorable thermodynamics of H2 loss from 82-H2 and commensurately difficult olefin hydrogenation. Thus, the (carb-PNP) ligand appears to engender the most favorable balance between Rh(I) and Rh(III) states (i.e., the thermodynamics of H2 addition to the Rh(I) species and H2 addition to olefin are comparable), while POP favors Rh(III) too strongly, and PCP too strongly favors Rh(I).

4. TANDEM REACTIONS INVOLVING ALKANE DEHYDROGENATION 4.1. Alkane Metathesis and Alkane−Alkene Coupling Reactions

Global demand for liquid fuels from nonconventional sources is expected to increase throughout this century.153−155 Most of this growth will be based on diesel fuel; demand will be particularly great for heavy (C9−C19) less branched alkanes 12370


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Figure 16. Alkane metathesis by the Brookhart−Goldman tandem system.

and slow steps obtained with 13 or 23.76 Indeed, under the same conditions, the hybrid catalyst 29 was about 3-fold more active for AM than either 13 or 23. In agreement with the underlying hypothesis involving more comparable rates of alkane dehydrogenation and olefin hydrogenation, the resting state of 29 was found to be a mixture of 29-H2 and 29-olefin species. The sterically less hindered hybrid (tBu2PCOPiPr2)Ir (32) afforded rates 4 times faster than those of (tBu4PCOP)Ir (29); however, the even less crowded (iPr4PCOP)Ir (30) outpaced 29 by only 2-fold. (tBu4POCOP)Ir (23) and various PCOP catalyst (29, 30, and 32) complexes show very poor selectivity toward formation of n-decane from n-hexane in AM,76,81,89 whereas (tBu4PCP)IrH2 (13) gave much better, though still modest, selectivity. The low selectivity of the POCOP and PCOP catalysts in AM was originally assumed to be attributable to isomerization of the presumed intermediate 1-hexene, which upon olefin metathesis would be expected to give a mixture of chain lengths in accord with observation.109,110 Brookhart and Goldman, however, have shown that the different selectivity of the POCOP and PCOP catalysts relative to 13 can in fact be attributed to different selectivity for n-alkane dehydrogenation.122 Whereas (tBu4PCP)Ir (13) (as well as (iPr4PCP)Ir (14)) is highly selective for the dehydrogenation at the terminal position of n-alkanes, POCOP and PCOP catalysts appear to show slight selectivity for production of internal olefins. This would account for the very poor selectivity for C2n−2 n-alkane from Cn n-alkane exhibited by the POCOP and PCOP catalysts in AM, perhaps further exacerbated by olefin isomerization (which presumably adversely affects the selectivity of AM by all the catalysts).76,81,89 Scott, Brookhart, and Goldman have extended tandemcatalyzed alkane metathesis to the metathesis of cycloalkanes, specifically the strained-ring species COA and cyclodecane. Conversions of COA to higher cycloalkanes, C16, C24, C32, and C40, in amounts decreasing with increasing carbon number, were observed using 13, 23, or (tBu3MePCP)Ir (16) in combination with 85.90 In addition, depending upon the nature of the pincer catalyst and the amount of acceptor initially added, varying amounts (up to ca. 30%) of higher polymers were obtained; these were part of a bimodal distribution and independent of the distribution of C 8n oligomers. Smaller quantities of ring-contraction products were also observed (e.g., an up to 5.6% yield of cycloheptane from COA), presumed to be formed via double bond isomerization of carbene intermediates. Comparable results were obtained with cyclodecane.90

which have excellent combustion properties for diesel. Moreover, compared with the cyclic and aromatic components in petroleum-derived diesel fuel, these alkanes are much less prone to give rise to air-borne particulates156 which are estimated to be responsible for ca. 3 million premature deaths per year globally.157,158 In this context, Fischer−Tropsch (F− T) catalysis159−163 appears to hold much promise. The potential sources of carbon for syngas for F−T catalysis range from natural gas to atmospheric CO2, and the products are exclusively linear hydrocarbons. However, light n-alkanes comprise a substantial fraction of the ultimate mix of products from the F−T process; these are not useful as diesel fuel and generally not very desirable. Light alkanes also constitute a major fraction of global natural gas and oil reserves.164 Thus, the ability to convert light alkanes to heavies, and to interconvert alkanes more generally, potentially has great societal value. 4.1.1. Alkane Metathesis. Alkane metathesis (AM) by a heterogeneous catalytic system (Pt/alumina mixed with tungsten oxide on silica) was first reported in 1973 by Burnett and Hughes.165 Basset and Coperet later extensively developed and investigated surface-supported organometallic alkane metathesis catalysts.166−170 However, these heterogeneous systems were all very unselective. In 2006, Goldman, Brookhart, and co-workers reported the first homogeneous system for alkane metathesis,89 comprising pincer−iridium alkane dehydrogenation catalysts operating in tandem with Schrock’s MoF12 olefin metathesis catalyst (85) as shown in Figure 16. Whereas the heterogeneous Basset−Coperet system converts linear alkanes to a mixture of linear and branched alkanes,165−170 the Brookhart−Goldman system affords exclusively n-alkanes.89 High selectivity for C2n−2 n-alkane plus ethane from Cn n-alkane, however, still remains elusive. There have been several reviews on these earlier studies on homogeneous alkane metathesis.10,57−59 Despite showing similar rates of alkane metathesis, the resting states of (tBu4PCP)Ir (13) and (tBu4POCOP)Ir (23) catalysts were found to be quite different under typical AM conditions, specifically, (tBu4PCP)IrH2 and (tBu4POCOP)Ir(olefin), respectively.76,109 This is indicative of fast alkane dehydrogenation followed by a slow, rate-determining, hydrogenation segment for the cycle catalyzed by 13, and the reverse for 23. Goldman and co-workers therefore hypothesized that the “hybrid” phosphine/phosphinite catalyst (tBu4PCOP)Ir (29) would afford an overall faster rate, as the rate of each segment of the cycle would be intermediate between those of the fast 12371


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4.1.2. Alkane−Alkene Coupling. An ideal upgrading of light alkanes would result in complete conversion to heavier products without the generation of lighter hydrocarbons (such as ethane in the case of alkane metathesis76,81,89). Goldman and co-workers have shown that one way to achieve this is via a pincer−iridium-catalyzed dehydrogenation of alkanes and subsequent dimerization of the resulting olefin, followed by transfer hydrogenation of the heavier olefin product (Figure 17a).96,171 Unlike AM, this reaction is endothermic and

maintaining a low steady-state concentration of 1-hexene, the authors conducted the catalysis with slow addition of 1-hexene via a syringe pump. Indeed, TONs and cooperativity (91%) were thereby greatly increased. Additionally, Bercaw and Labinger also obtained C14 products from n-heptane, using TBE (which is not dimerized by 86) as the hydrogen acceptor. This may be viewed in terms of Figure 17a, but with hydrogenation of TBE instead of loss of H2.97−99 4.2. Aromatics via Dehydroaromatization, Alkyl Group Cross-Metathesis, Cyclodimerization, and Aryl−Alkyl Coupling Reactions

Aromatics are key chemical building blocks. Benzene, toluene, and xylene (BTX), which comprise three of the “seven basic building blocks of the chemical industry”,173 are in large part byproducts of gasoline refining. As demand for gasoline slowly declines while demand for chemicals continuously increases, new methods for BTX production are in great demand.174 Linear alkylbenzenes (LABs) are produced in very large quantities as precursors for surfactants with high detersive power.175,176 Encouraged by the observation of benzene produced in the transfer dehydrogenation of hexane catalyzed by (pOMe-iPr4PCP)IrH4 (19-H4) with high amounts of TBE or NBE acceptor,66 Goldman and Brookhart investigated various pincer−iridium complexes for the dehydroaromatization of nalkanes.75,94,177 (tBu4PCP)IrH4 (13-H4) and (tBu4POCOP)IrH4 (23-H4) were essentially completely inactive for these reactions, but sterically less hindered pincer−iridium catalysts, including (iPr4PCP)Ir (14), (iPr4Anthraphos)Ir (34), and (iPr4PCOP)Ir (30), gave promising results. In particular, the mixed phosphine−phosphinite complex 30 gave about 0.67 M (44% yield) benzene starting from 1.53 M n-hexane and 6.12 M TBE after 120 h at 165 °C (Scheme 5). The dehydroaromatization reactions proceed via pincer− iridium-catalyzed sequential dehydrogenation of alkanes to form conjugated trienes, which undergo electrocyclization to give cyclohexadienes, which are further dehydrogenated to aromatics.75,94,177 For alkanes higher than C6, significant amounts of dialkylbenzenes were produced (o-alkyltoluenes in particular). For instance, in the dehydroaromatization of noctane (1.4 M) catalyzed by 30 (5 mM) at 165 °C, with 5.6 M TBE, a 7:1 mixture of o-xylene and ethylbenzene was obtained with a combined yield of 86% after 118 h. When n-dodecane was used, o-pentyltoluene was the major dehydroaromatization product (ca. 21%), along with the unexpected formation of benzene in significant amounts (ca. 17%) (Scheme 6).75,95

Figure 17. Schemes to upgrade hydrocarbons via tandem alkane dehydrogenation/olefin dimerization: (a) alkane dimerization, (b) alkane−olefin coupling.

requires expulsion of H2. In independent work, Bercaw, Labinger, and co-workers considered that alkane−alkene mixtures, which are typical of light hydrocarbon feeds, could be upgraded via a tandem catalytic system comprising an alkane transfer dehydrogenation catalyst and an alkene oligomerization catalyst.97−99 This coupling of hydrocarbons is exothermic and would not require expulsion of H2 (Figure 17b). Following Schrock’s172 success with Cp*TaCl2(C2H4) (86), which selectively dimerized α-olefins, Bercaw and Labinger explored the use of pincer−iridium dehydrogenation catalysts in tandem with 86 (Figure 18).97−99 Iridium catalysts (tBu4POCOP)IrH2 (23-H2), (tBu4PCP)IrH2 (13-H2), and (iPr4PCP)IrH4 (14-H4) were investigated. Among these, it was found that 23-H2 was not active for n-heptane/1-hexene coupling; this may be attributable to its low regioselectivity122 of dehydrogenation for the formation of α-olefins. Cocatalyzed coupling of 1-hexene with n-heptane was achieved with (tBu4PCP)IrH4 (13-H4) (10 mM) and 86 (16 mM) at 125 °C, to give both C13 and C14 alkenes with 35% cooperativity (cooperativity was defined as the fraction of dehydrogenation product, i.e., 1-heptene, incorporated into C13 and C14 alkenes).97−99 The lack of C13 and C14 alkane product indicates an incomplete catalytic cycle (Figure 18), attributable to the poor hydrogen-accepting ability of the 1,1-disubstituted alkenes. The major products derived from 1-hexene, however, were n-hexane and, particularly at early reaction times, C12 dimers. Concluding that cooperativity would be enhanced by

Figure 18. Bercaw and Labinger’s alkane−olefin coupling cocatalyzed by pincer−Ir complexes and 86. 12372


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Scheme 5

Scheme 6

Scheme 7

Scheme 8

hexene and 3-ethylcyclohexene. The mixture was then subject to dehydrogenation at 400 °C using Pt/Al2O3, which proceeded in ca. 90% yield to give p-xylene and ethylbenzene (8.5:1). The reaction was also conducted in a single vessel, with (iPr4Anthraphos)Ir as the catalyst, under 600 psi of ethylene at 250 °C for 192 h. A 10% yield of aromatic products was obtained. These reactions have also been extended to the synthesis of piperylene and toluene via pincer−iridiumcatalyzed disproportionation of pentene and a tandem Diels− Alder cyclization with ethylene.103 Schrock and co-workers have reported a new route to nalkylarenes, wherein ethylbenzene and n-alkane are subject to a pincer−iridium dehydrogenation catalyst operating in tandem with various olefin metathesis catalysts (Scheme 8).93 The net reaction, alkyl group cross-metathesis (AGCM), is analogous to alkane metathesis and thus does not require olefinic acceptors. Only mono-n-alkylbenzene products were observed. As with nalkane metathesis, a broad distribution of chain lengths was observed with the POCOP−iridium and PCOP−iridium complexes investigated, consistent with the metathesis of styrene with internal olefins, but with (tBu4PCP)Ir (13), the reaction with n-octane gave excellent selectivity for 1-phenyl-

One of the main challenges of the above dehydroaromatization reactions is presented by the use of multiple equivalents of TBE. Although recent studies have shown that TBE can effectively be regenerated from TBA,32 from an industrial point of view, the use of propene and especially ethylene178−181 is probably preferred. It is noteworthy in this context that Goldman and Brookhart reported good dehydroaromatization yields using propene as the hydrogen acceptor.75,94,177 The Brookhart group has extended these studies to the use of ethylene in the dual role as an acceptor and as a dienophile for the synthesis of piperylene, toluene, and p-xylene.102,103 Considering that 1-hexene is obtained from ethylene trimerization, Brookhart has demonstrated that p-xylene can be synthesized from ethylene as the sole feedstock (Scheme 7). The disproportionation of 1-hexene was catalyzed by 0.04 mol % pincer−iridium catalysts at 180 °C to give the thermodynamically favored diene, 2,4-hexadiene. (iPr4Anthraphos)Ir (34) proved to be particularly efficient (222 TOs/h); as in the case of dehydroaromatization, sterically hindered catalysts (tBu4PCP)Ir (13) and (tBu4POCOP)Ir (23) were essentially inactive. Subsequent Diels−Alder dimerization of 2,4-hexadiene with ethylene (ca. 600 psi) at 250 °C gave about 96% conversion to an 8:1 mixture of 3,6-dimethylcyclo12373


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Scheme 9

Scheme 10

(alkenyl benzenes) was far lower, e.g., 100 mM after 72 h. These results reflect not only the high compatibility of the catalysts, but also a striking synergy of the pincer−iridium and zeolite catalysts operating in tandem. The zeolite catalyst, by effecting cyclization, appears to both mitigate the inhibition of the pincer−iridium catalyst by linear alkenes and dienes and prevent the back-reaction of the released H2 with alkenylbenzenes.100,101 (tBu4POCOP)Ir (23, 2 mM) in tandem with H-SSZ-25 zeolite gave even better results than the tBu4PCP analogue, with 5.4 M cyclized products formed after 24 h. The sterically less hindered (iPr4PCP)Ir (14) gave yields that were lower (1.8 M after 72 h), but still quite high relative to the yields of olefin resulting from acceptorless dehydrogenation in the absence of zeolite.100,101 Attempts to extend this catalysis to intermolecular aryl−alkyl coupling have thus far not been successful.100,101

octane (up to a 12:1 ratio of 1-phenyloctane to 1-phenylheptane). The highest yields were obtained with tungsten mono(aryl oxide) pyrrolide93 complexes; these were also found to be exceptionally effective for AM (n-octane self-metathesis). It should be noted in this context that commercial linear alkylbenzenes (LABs) are actually derivatives of n-alkanes with aryl groups located anywhere on the chain except at the terminal position, reflecting their synthesis by Friedel−Crafts alkylation of benzene. n-Alkylbenzenes, as produced via AGCM, have potentially desirable properties such as greater thermal stability, but they have not been explored commercially due to a historical lack of any economically practical access. The AGCM reaction of n-octane and ethylbenzene cocatalyzed by (tBu4PCP)Ir (13) and W(NAr′)(C3H6)(pyr)(OHIPT) (87) (Ar′ = 2,6-Me2C6H3; OHIPT = 2,6-(2,4,6iPr3C6H2)2C6H3O) at 180 °C gave high yields of 1-phenyloctane (350 mM). A 17:1 ratio of 1-phenyloctane to tetradecane (the product of AM, undesired in this case) was obtained.93 Starting with n-alkylbenzenes, Goldman and co-workers have demonstrated that pincer−iridium dehydrogenation catalysts can operate in tandem with zeolites as arene alkylation catalysts, to effect a net intramolecular coupling of aryl and alkyl C−H bonds with the loss of H2; the reactions were conducted without a hydrogen acceptor (Scheme 9).100,101 Refluxing npentylbenzene solutions of pincer−iridium catalysts in the presence of zeolite led to the formation of 1-methyl-1,2,3,4tetrahydronaphthalene. Dehydrogenation and skeletal isomerization result in formation of 1-methylnaphthalene, 2methylnaphthalene, and 2-methyl-1,2,3,4-tetrahydronaphthalene.100,101 The reaction of n-pentylbenzene (5.8 M) cocatalyzed by (tBu4PCP)IrH2 (13-H2) (2 mM) and H-SSZ-25 zeolite (0.3 g/ 2.7 mL) at 205 °C gave 4.5 M (78%) cyclized product after 72 h.100,101 Under the same conditions, in the presence of the zeolite, but in the absence of pincer−Ir catalyst, cyclized product formed at only ca. 5% of the rate as in the tandem system, to give only 360 mM (6%) after 72 h. Conversely, in the absence of zeolite, catalysis by 13 did not give any cyclized product and the total yield of dehydrogenated products

4.3. Silylation and Borylation of Alkanes via Dehydrogenation

The conversion of alkanes to higher value hydrocarbons, including olefins, aromatics, or even higher value alkanes, has great potential applicability as discussed above. However, given the great chemical versatility of alkenes, the ability to produce olefinic intermediates from alkanes suggests intriguing possibilities for transformations to many products other than hydrocarbons. Two such possibilities were realized by Huang, using a one-pot, two-step protocol, for alkane silylation and for alkane borylation. Interestingly, catalysts that achieve these same transformations directly (i.e., not via tandem catalysis) have been extensively developed, and reviewed, by Hartwig.9,182−184 Linear alkylsilanes find wide application as silicon rubbers, molding products, and utility coatings. The Huang group has developed a direct and selective catalytic system to transform alkanes to n-alkylsilanes.104 Dehydrogenation of n-alkane by pincer−iridium catalyst (iPr4PSCOP)Ir (31) to give a mixture of terminal and internal olefins, was followed by addition of a pincer−iron complex (88 or 89) that catalyzed both isomerization of internal olefin to α-olefin and hydrosilylation to give linear alkylsilanes (Scheme 10).104 12374


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Scheme 11

Independent experiments showed that (iPr4PSCOP)IrHCl (0.5 mol %), with NaHBEt3 (1 mol %) as the activator, in an nhexane solution of trans-3-octene and bis(trimethylsiloxy)methylsilane at room temperature for 12 h did not yield primary alkylsilane;104 the reaction resulted in only about 6% branched silane, while 93% of the trans-3-octene remained unreacted. Independently, in the absence of iridium complex, a 5 mol % concentration of the pincer−iron catalyst precursor 88 or 89 (and 10 mol % NaHBEt3) gave an about 86% yield of the linear alkylsilane (presumably proceeding via prior isomerization to 1-octene).104 The Huang group then conducted one-pot alkane dehydrogenation/isomerization/hydrosilylation (Scheme 11).104 A solution of n-octane/TBE (0.25 M) containing 31-HCl (2.5 mM) and NaOtBu (2.5 mM) was heated at 200 °C for 10 min. This was followed by addition of pincer−Fe catalyst 88 or 89 (25 mM) and NaHBEt3 (50 mM) along with the silane (0.25 M) at room temperature. The highest yield of linear alkylsilane, 83%, was obtained from the reaction of n-octane with diethylsilane. The same protocol was employed by Huang for the conversion of alkanes to linear alkylboronate esters.104 Precursor 31-HCl (2.5 mM) was used to catalyze transfer dehydrogenation of n-octane with TBE (0.25 M) at 200 °C to give octenes. A different pincer−iron catalyst, 90 (25 mM), was then added to the reaction mixture; it catalyzed isomerization− hydroboration of the octenes at room temperature, to give a ca. 95% yield of linear alkylboronate ester.104

Figure 19. Catalytic dehydrogenation of THF by (tBu4PCP)IrH2 (13H2).

Tetrahydrofuran, isochroman, coumaran, 2-methyl-2,3-dihydrobenzofuran, dioxane, and 2-methyltetrahydrofuran were dehydrogenated in good to moderate yields in the presence of 0.1−5 mol % 31. Dehydrogenation of tetrahydrofuran was much more complete in this example than in the earlier study by Kaska and Jensen, and furan was obtained as the major product (72%) along with only trace (2%) amounts of 2,3-dihydrofuran.77 The thioether tetrahydrothiophene was converted to thiophene in only poor (5%) yields; this was attributed to the strong coordination of sulfur to iridium, inhibiting catalysis (Figure 20). Unexpected reactivity was observed from the dehydrogenation of 2,3-dihydrobenzofuran (coumaran): trace amounts of the biaryl species 2,2′-bibenzofuran were formed, apparently via dehydrogenative cross-coupling of the initial product, benzofuran (Figure 20). This biaryl was the major product (37% yield) when 3 mol % 31 and 2 equiv of TBE were used. Brookhart found that transfer dehydrogenation of cyclic ethers could be efficiently catalyzed even under solvent-free conditions.185 With only 0.2 mol % (iPr4Anthraphos)IrHCl (34HCl) (activated in situ by sodium tert-butoxide) and stoichiometric amounts of TBE inside sealed vials at 120 °C, dehydrogenated products were obtained, in 65−90% yields after 15 h, from tetrahydrofuran, 1,4-dioxane, N-methylmorpholine, dihydrobenzofuran, and isochroman. Comparable yields were obtained under the same conditions using (iPr4triptycenePC(sp3)P)IrHCl (48a-HCl) or (iPr4PCP)IrHCl (14-HCl). Building on their previous success using ethylene as an economical, easily separable hydrogen acceptor,93,94 the Brookhart group subsequently reported on its use for transfer dehydrogenation of cyclic ethers.185 Among the three catalysts tested, 48a-HCl, activated with NaOtBu, was particularly effective with ethylene as the hydrogen acceptor, giving activity comparable to that obtained with TBE. Dehydrogenation of acyclic ethers was challenged by strong product inhibition, resulting in catalyst productivity approximately 1/10 of that with cyclic ether substrates. Diethyl ether, di-n-propyl ether, ethyl tert-butyl ether, and trimethylsiloxyethane were dehydrogenated in modest yields with 0.2 mol % 14-HCl, 34-HCl, or 48a-HCl (activated by NaOtBu) using TBE as the hydrogen acceptor. Increasing the loading to 2 mol % 48a-HCl led to an up to 55% yield with trimethylsiloxyethane as the substrate. Substituting TBE with 1 atm of

5. DEHYDROGENATION OF SUBSTRATES CONTAINING HETEROATOMS 5.1. Reactions of Pincer−Iridium Complexes with Ethers

Shortly after the initial report of catalytic cyclooctane dehydrogenation by (tBuPCP)IrH2 (13-H2),6 Kaska and Jensen used the same complex to catalyze the transfer dehydrogenation of tetrahydrofuran using TBE as the hydrogen acceptor.86 Although this reaction is enthalpically favorable, only partial hydrogenation of TBE occurred even at 200 °C. There appeared to be high selectivity for 2,3-dihydrofuran, although some 2,5-dihydrofuran formed as well, and a substantial quantity of furan, presumably a secondary dehydrogenation product, was observed. Further investigations using pincer complexes for dehydrogenation of THF were not reported for over a decade afterward (Figure 19). The development of highly active catalysts supported on new pincer scaffolds motivated renewed studies on the transfer dehydrogenation of ethers. In 2014, Huang and co-workers reported that (iPr4PSCOP)Ir (31) effected the transfer dehydrogenation of a series of cyclic ethers with a stoichiometric amount of TBE at 120 °C in p-xylene solvent.77 12375


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Figure 20. Catalytic dehydrogenation of various cyclic ethers by (iPr4PSCOP)IrHCl (31-HCl).

Figure 21. Stoichiometric and catalytic dehydrogenation of various ketones by (tBuPCP)IrH2 (13-H2).

Blocking the pathway to dehydroaromatization with the choice of 3,3-dimethylcyclohexanone as the substrate enabled the desired catalytic cycle, forming the α,β-dehydrogenated product in 85% yield (17 turnovers) in 10 h at 120 °C. (pMeO-tBuPCP)Ir (18) was more efficient, affording a 99% yield in the same reaction. Dehydrogenation of cycloheptanone by 13 afforded the expected tropone, which underwent C−H activation to form a four-membered metallacycle. However, in contrast to the stability of the iridafuran formed from 13 and 3pentanone, this underwent an unusual further reaction to form a tricyclic tropone dimer as the major product. In independent experiments with tropone, in the absence of cycloheptanone and norbornene, formation of the same tricyclic dimer was catalyzed by 13-H2 in good yield. Thus, the scope of successful catalytic ketone dehydrogenation was extremely limited. Study of deactivation mechanisms, however, yielded insight into features incompatible with selective transfer α,β-dehydrogenation of ketones by pincer− iridium species, and the reaction with cycloheptanone suggested that dehydrogenation could be driven by subsequent reaction of the dehydrogenated ketone.202

ethylene markedly reduced the performance of complexes 34 and 14, while results with 48a were relatively unaffected.185 5.2. Enones by α,β-Dehydrogenation of Ketones

The enduring utility of reactions such as the aldol condensation, eliminations, or allylic oxidations notwithstanding, the preparation of enones continues to inspire new synthetic methods. Outside the scope of this review, significant and practical advances in homogeneous aerobic oxidation of ketones have been achieved in the laboratories of Stahl186−193 and others.194−198 These often lead to aromatized products, such as phenols and anilines, with substitution patterns complementary to those easily achieved by electrophilic aromatic substitution. There have been very limited reports of catalytic α,β-dehydrogenation of ketones by (non-pincer) transition-metal complexes.199−201 The application of pincer−iridium complexes to the transfer α,β-dehydrogenation of ketones is limited to a 2011 report by Goldman and co-workers.202 In this study, (tBu4PCP)Ir (13) was found to stoichiometrically dehydrogenate 3-pentanone under ambient conditions to form an iridafuran; further heating only led to rearrangement to a more stable iridafuran. Such fivemembered metallacycles have also been observed in the reaction of α,β-unsaturated ketones with ruthenium and osmium Xantphos complexes (Figure 21).203 Metallacycle formation was avoided in the dehydrogenation of cyclohexanone, but product inhibition resulted from O−H activation subsequent to dehydroaromatization to give phenol.

5.3. Dehydrogenation of Amine C−C Linkages

The dehydrogenation of primary and secondary amines typically occurs across the C−N bond to give rise to imines204 and nitriles.205 Subsequent reactivity may lead to C−C bond cleavage and catalyst decomposition by formation of stable iridium isonitrile adducts.206 Under transfer dehydrogenation 12376


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conditions, the imine products may undergo non-metalmediated condensations.207 Tertiary amines have been reported to undergo transfer dehydrogenation to give enamines, which are highly versatile intermediates. This may offer an alternative route to enamines, complementary to condensations of amines with aldehydes or ketones. Dehydrogenation of ethylamines catalyzed by (tBuPCP)IrH2 (13-H2) with 2 equiv of TBE in p-xylene at 90 °C affords mono- and bisvinylamines (eq 8).88 Wendt’s

Scheme 12. Acceptorless Dehydrogenation of NHeterocycles by PNP Pincer Complexes of Co and Fe (Shown for Quinolone)a

a

Under the same conditions, 1,2,3,4-tetrahydronaphthalene or 1,2dihydronaphthalene was not dehydrogenated.

ation steps are therefore well precedented for first-row-metal catalysts, namely, dehydrogenation of C−X (X = N or O) units, presumably via an outer sphere mechanism.213 Accordingly, even the thermodynamically “easy” dehydrogenation of a related hydrocarbon, 1,2-dihydronaphthalene, was not catalyzed by either of these first-row-metal PNP complexes.211,212 Nonetheless, with the N-heterocycle substrates, C−C linkages are dehydrogenated and C−C double bonds are formed. The fact that this proceeds indirectly (via isomerization) is not promising in terms of prospects for alkane dehydrogenation by such catalysts, but conversely, it suggests new avenues to the important goal of C−C dehydrogenation of functionalized molecules.

cyclohexane-backbone complex 64 performed similarly to 13 in the α,β-dehydrogenation of triethylamine.128 Remarkably, some unstable enamines that decompose within hours upon isolation, such as N,N-divinylethylamine, were stabilized in solution with 13 for up to 2 months. The mechanism of this stabilization has not been explored, but taken together with the mild conditions and reagents used to form these enamines, this method may provide entry to further synthetic connections. For example, enamines obtained from α,β-dehydrogenation of tetrahydrophenanthrenes and tetrahydrophenanthrolines have been coupled with electrophiles such as aromatic aldehydes208 and ethyl trifluoropyruvate.209 (iPr4PSCOP)Ir (31), discussed above as an effective catalyst for the transfer dehydrogenation of cyclic ethers (Figure 20), was also found to be effective for the transfer dehydrogenation of N-heterocycles incorporating both secondary and tertiary nitrogen atoms.77 In general, these reactions required higher catalyst loading (typically 5%), and in some cases higher temperature, than dehydrogenation of the cyclic ethers. For example, N-alkylpyrrolidines were dehydrogenated to give Nalkylpyrroles in good yield at 150 °C (eq 9).77

6. OUTLOOK Alkane dehydrogenation by pincer complexes came to the fore 20 years ago with reports of catalysis by (PCP)Ir precursors, including acceptorless dehydrogenation and, in particular, the formation of α-olefins from n-alkanes. The ability to effect alkane dehydrogenation with this class of complexes or closely related iridium complexes has steadily developed over the years. However, despite such progress, we are still far from achieving useful, high-yield conversions of alkanes to simple olefins, and farther still from the high-yield conversion of n-alkanes to αolefins. Fortunately, the inability to obtain high yields of olefin with high selectivity does not preclude valuable applications of this catalysis. Several important breakthroughs have been made on the basis of (PCP)Ir-catalyzed dehydrogenation of n-alkanes followed by secondary reactions. The secondary reactions may be catalyzed in tandem with the dehydrogenation (e.g., olefin metathesis, to achieve alkane metathesis), or the secondary reactions may be dehydrogenations themselves, followed by another step (e.g., electrocyclization of trienes or Diels−Alder addition to dienes). Other desirable coupled-reaction systems, such as alkane oligomerization, or alkene−alkane or arene− alkane coupling, also seem attainable. To our knowledge, pincer-ligated catalysts for hydrocarbon conversions have not yet seen any large-scale industrial application. This does not appear to be attributable to any one particular consideration. For example, turnover frequencies are already at very acceptable levels in many cases. Systems that require hydrogen acceptors are economical if the hydrogen acceptor is sufficiently cheap or if it can be easily recycled. Acceptorless systems, however, are generally preferable. Such systems may involve intrinsically acceptorless reactions such as alkane metathesis, or systems that evolve hydrogen (the value of which can certainly increase the economic appeal). Results from batch processes on the benchtop scale indicate that

Beyond possible applications to organic synthesis, research into aliphatic dehydrogenation and dehydroaromatization of cyclic amines has also been motivated by the development of homogeneous catalysts for dehydrogenation of liquid organic hydrogen carriers (LOHCs). With this in mind, Jensen and coworkers evaluated the performance of 13-H2, 23-H2, and (iPrPCP)IrH2 (14-H2) as catalysts for the acceptorless dehydrogenation of N-ethylperhydrocarbazole (EPHC).210 Each catalyst was competent at 200 °C, in all cases consuming the entire charge of neat EPHC at 200 °C using 1 mol % catalyst loadings without any observed product inhibition. Activity was, however, markedly higher with 23-H2, which gave an initial turnover frequency (TOF) of ca. 50 TOs/h versus 5 TOs/h for 13-H2 and 10 TOs/h for 14-H2. Lowering the temperature to 150 °C still allowed for potentially useful turnover frequencies of 3 TOs/h. Jones has reported the acceptorless dehydrogenation of various N-heterocycles by pincer complexes of first-row metals iron (91)211 and cobalt (92)212 (Scheme 12). These reactions appear to proceed via initial dehydrogenation of C−N linkages followed by isomerization to form C−C double bonds, and then another C−N dehydrogenation. The direct dehydrogen12377


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metal-catalyzed dehydrogenations to expand and to be enriched by the many developments that are currently impacting C−H activation and catalysis.

catalyst stability is satisfactory, but perhaps the biggest obstacle to application on the commodity or fuel scale is the lack of proof to date that this will translate to robustness under realworld plant conditions. Of course, the likelihood of reaching at least pilot-scale implementation increases with the potential profitability of the transformation. Thus, we anticipate that the first large-scale applications would involve relatively high value products such as p-xylene or aromatics of even higher value, and systems amenable to simple engineering (including minimal separations and recycling). Pincer−iridium-catalyzed dehydrogenation of HCCH linkages has been extended to molecules with heteroatoms, such as ethers or tertiary amines. The scope of such reactions is fairly narrow, however; it is typically limited by coordination of the heteroatoms, and particularly by binding of dehydrogenated functionalized species to the catalyst. However, there has been noteworthy success with the dehydrogenation of alkanes in tandem with secondary coupling to silanes and boranes. Thus, (PCP)Ir-type catalysts, and related catalysts based on low-valent fragments more generally, continue to show great promise for the conversion of alkanes to other hydrocarbons (including to other alkanes) or the conversions of non-alkane hydrocarbons (e.g., Schrock’s styrene/n-alkane cross-metathesis). We expect that tandem or sequential reaction systems will continue to be developed, and we anticipate many more breakthroughs in this area. For the reactions of functionalized substrates, however, or secondary reactions with polar or oxidizing reagents, the potential scope of such catalysts seems more constrained. To expand the scope of useful aliphatic dehydrogenations beyond hydrocarbon substrates, and especially to expand the scope of hydrocarbon conversions to give heteroatomfunctionalized products, less electron-rich complexes seem most promising. Pincer catalysts are showing promise in this context as well. Of the many attractive traits of pincer ligands, none are obviously specific to electron-rich complexes. Complexes bearing electron-poor pincer ligands, or even “conventional” pincer ligands coordinating high-oxidationstate complexes, are far more tolerant of functional groups and even oxygen than (PCP)Ir-type dehydrogenation catalysts. If at any point it seemed as if the dehydrogenation of alkanes or alkyl groups required low-oxidation-state catalysts, that misconception has been strongly refuted in recent years. The development of new catalysts that are tolerant of oxidants and polar groups more generally will likely therefore open major new avenues for catalytic dehydrogenation-based hydrocarbon conversion in the near future. Finally, we note that the results to date in this area have been achieved with a surprisingly limited range of metals. As the range of reported pincer complexes rapidly broadens, we expect that the scope of catalysts for aliphatic dehydrogenations will commensurately expand. The paradigmatic pathway of C−H addition to a low-valent species followed by β-H elimination may continue to be dominated by the platinum metals, perhaps even just the four metals that have thus far been successful (Ru, Os, Rh, Ir). For other modes of C−H activation, however, the range of metals is certainly less limited; these modes include concerted metalation deprotonation, σ-bond metathesis, oxidative hydrogen migration, σ-complex-assisted metathesis, electrophilic activation (these categories overlap substantially), and others.25 In view of the properties of pincer ligands that have already proven so useful, and the great versatility of olefins (functionalized and otherwise), we expect the scope of pincer−

ASSOCIATED CONTENT Special Issue Paper

This paper is an additional review for Chem. Rev. 2017, 117, issue 13, “CH Activation”.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Alan S. Goldman: 0000-0002-2774-710X Notes

The authors declare no competing financial interest. Biographies Akshai Kumar grew up in Mangalore, India, and obtained his B.Sc. in 2002 from Mangalore University. From the same university in 2004, he received his Masters degree in inorganic chemistry. He pursued his doctoral studies under the supervision of Prof. Ashoka G. Samuelson at the Indian Institute of Science, Bangalore, where he was awarded a Ph.D. degree in 2009. After a postdoctoral stint in the same laboratory, in October 2010, he joined the group of Prof. Andreas Terfort at Goethe University, Frankfurt, Germany, as a postdoctoral fellow. In 2012, he extended his postdoctoral activities and worked in the Goldman group at Rutgers, The State University of New Jersey, New Brunswick. In 2015, he was appointed as Assistant Professor in Chemistry at the Indian Institute of Technology Guwahati. Currently his research emphasis is on pincer−metal-catalyzed C−H and C−F activation reactions for the synthesis of fuel chemicals and heteroatomdoped π-conjugated organic materials. Tariq M. Bhatti was born in Nassau, Bahamas, and grew up in Maryland. After earning his B.S. in chemistry from the University of Maryland in 2009, he spent some time in industry. From 2013 to 2015, he was a chemist with W.R. Grace & Co. in Columbia, MD, where he worked on the development and commercialization of Ziegler−Natta catalysts and UNIPOL process technologies. He is currently a graduate student at Rutgers, The State University of New Jersey, studying reactions of organometallic pincer complexes in the laboratory of Alan Goldman. Alan S. Goldman received his B.A. from Columbia University, where he worked in the laboratory of Walter Klemperer studying polyoxometalates. In the laboratory of David Tyler at Columbia, he studied organometallic photochemistry and 19-valence-electron complexes, receiving his Ph.D. in 1985. As an IBM Postdoctoral Fellow in the laboratory of Jack Halpern at the University of Chicago, he then studied the catalytic chemistry of iridium polyhydride complexes. In 1987, he joined the faculty at Rutgers, The State University of New Jersey, where he is now Distinguished Professor of Chemistry. His research group is focused on the development and study of transition-metal-based complexes for the catalytic reactions of small molecules and related fundamental chemistry, with an emphasis on transformations relevant to the global energy system.

ACKNOWLEDGMENTS A.K. thanks the Science and Engineering Research Board, Department of Science & Technology (Grant DST-SERB YSS/ 12378


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2015/000573), and Board of Research in Nuclear Sciences, Department of Atomic Energy (Grant BRNS-DAE 37(2)/20/ 13/2016). T.M.B. and A.S.G. gratefully acknowledge the National Science Foundation CCI Center for Enabling New Technologies through Catalysis (CENTC) (Grant CHE1205189).

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Dehydrogenation of Alkanes and Aliphatic Groups by Pincer-Ligated

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