Mechanistic Studies of Alkene Isomerization Catalyzed by CCC-Pincer

Jan 9, 2014 - Spring Melody M. Knapp, Sarah E. Shaner, Daniel Kim, Dimitar Y. Shopov, Jennifer A. Tendler,. David M. Pudalov, and Anthony R. Chianese*...
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Mechanistic Studies of Alkene Isomerization Catalyzed by CCCPincer Complexes of Iridium Spring Melody M. Knapp, Sarah E. Shaner, Daniel Kim, Dimitar Y. Shopov, Jennifer A. Tendler, David M. Pudalov, and Anthony R. Chianese* Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, United States S Supporting Information *

ABSTRACT: Iridium complexes containing CCC-pincer m-phenylene-bridged N-heterocyclic carbene ligands were examined as catalysts for alkene isomerization. Complexes containing either mesityl or adamantyl side groups were found to catalyze the isomerization of a number of alkenes to the internal isomers, including 1-octene, vinylcyclohexane, and allylbenzene. Mechanistic studies indicate a surprising dichotomy, apparently caused by ligand steric effects. For the mesityl-substituted catalyst, several lines of evidence provide strong support for isomerization via an iridium allyl hydride intermediate: (1) H−D crossover experiments indicate that 1,3-hydrogen migration is exclusively intramolecular, (2) the catalyst resting state, a π-allyl hydride species, was isolated and serves as a kinetically competent catalyst, (3) NMR experiments indicate that the π-allyl hydride resting state undergoes reversible C−H reductive elimination that is rapid relative to catalytic turnover, and (4) kinetic studies indicate that the isomerization reaction is first order in substrate and catalyst, consistent with turnover-limiting ligand substitution. H−D crossover experiments for alkene isomerization catalyzed by the adamantyl-substituted complex show selectivity for a 1,3deuterium shift, as well as the intermolecular transfer of hydrogen. These results are consistent with an insertion/elimination mechanism proceeding selectively through a secondary metal−alkyl or with a π-allyl-type mechanism with an unknown pathway for intermolecular hydrogen crossover.

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INTRODUCTION Selective C−H functionalization of alkanes is of great interest because it has the potential to provide access to valuable commodity chemicals, such as alkenes, alcohols, and amines, from inexpensive petroleum feedstocks.1 In particular, the dehydrogenation of linear alkanes through C−H bond activation can be used to selectively generate α-olefins, a crucial step in alkane metathesis reactions.1a,f The iridiumcontaining PCP-pincer complex (tBu4PCP)IrH2 is an exceptional catalyst for the dehydrogenation of alkanes, achieving over 1000 turnovers in the transfer dehydrogenation of cyclooctane;2 this complex also provided the first reported example of catalyzed acceptorless dehydrogenation of a linear alkane.3 Further work with the related (tBu4PCP)IrH4, (iPr4PCP)IrH4, (tBu3MePCP)IrH4, and (tBu2Me2PCP)IrH4 complexes showed that the highest rates and greatest turnover numbers for alkane dehydrogenation could be achieved with (tBu3MePCP)IrH4, indicating that the rate of dehydrogenation © 2014 American Chemical Society

and catalyst lifetime are sensitive to the steric bulk of the substituents on the phosphines.4 These PCP-pincer catalysts show excellent kinetic selectivity for α-olefins, but this selectivity is degraded over time by competing catalysis of alkene isomerization.4,5 We recently investigated alkane dehydrogenation using iridium complexes containing monoanionic CCC-pincer ligands, where the phosphorus-donor fragments are replaced with N-heterocyclic carbenes (NHCs; Figure 1).6 The iridium complexes containing the bulky aryl Nsubstituents mesityl (1-Mes) and 3,5-di-tert-butylphenyl (1dtbp) formed active catalysts for alkane dehydrogenation on activation with sodium tert-butoxide and were also found to catalyze the isomerization of the resulting alkenes. Interestingly, iridium complexes containing the bulky alkyl substituents adamantyl (1-Ad) and tert-butyl (1-tBu) did not catalyze Received: August 6, 2013 Published: January 9, 2014 473

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kinetically generating the desired E isomers by the sterically encumbered nature of the η3-allyl intermediate.20 As we have previously reported, both 1-Mes and 1-Ad are active alkene isomerization catalysts on activation with NaOtBu.6b Of the two, 1-Ad was a more efficient alkene isomerization catalyst, giving a TON of 487 after 30 min at 60 °C for the isomerization of 1-octene. In comparison, a reaction catalyzed by 1-Mes required nearly 24 h at 100 °C to reach a TON of 435. Notably, 1-Mes catalyzes the dehydrogenation of alkanes, while 1-Ad does not.6 As catalysis of alkene isomerization limits the selectivity for α-olefins in alkane dehydrogenation, the rational design of selective alkane dehydrogenation catalysts will benefit from an understanding of the available mechanisms for alkene isomerization. Recently, Goldman, Brookhart, and co-workers described an extensive study of the mechanism of alkene isomerization catalyzed by PCP-iridium complexes and concluded that a π-allyl-type mechanism is operative in those systems.5b Herein we report an investigation of the mechanism of alkene isomerization catalyzed by 1-Ad and 1-Mes. The studies conducted on 1Mes include H−D scrambling and crossover experiments, which indicate that 1,3-hydrogen migration is exclusively intramolecular. A π-allyl hydride species, believed to be the catalyst resting state, was observed spectroscopically during catalysis, was isolated, and serves as a kinetically competent catalyst. EXSY indicates that this intermediate undergoes fast, reversible C−H reductive elimination. Finally, kinetic studies indicate that the isomerization reaction is first order in substrate and catalyst, consistent with turnover-limiting ligand substitution. These studies strongly support the conclusion that 1Mes catalyzes isomerization via the π-allyl mechanism. Although limited solubility prevented a complete study on 1Ad, H−D scrambling and crossover experiments suggest that it catalyzes isomerization either via an insertion/elimination mechanism proceeding selectively through a secondary metal−alkyl or via a π-allyl-type mechanism with an unknown pathway for intermolecular hydrogen crossover.

Figure 1. Previously reported iridium complexes containing CCCpincer ligands.

alkane dehydrogenation but were highly active catalysts for alkene isomerization.6b While competing alkene isomerization limits the selectivity of an alkane dehydrogenation catalyst for the formation of αolefins, there are a number of applications in which a highly active and selective alkene isomerization catalyst is desirable. For example, alkene isomerization is an important step in the Shell Higher Olefins Process,7 the synthesis of linear amines from internal olefins via hydroaminomethylation,8 and the industrial synthesis of terpene compounds.9 Additionally, the facile preparation of (E)-alkene isomers of allylbenzene derivatives is of interest to the fragrance and flavoring industries.10 Many different catalysts have been developed to facilitate alkene isomerization, including those containing Ti,11 Cr,12 Fe,13 Co,14 Ni,14a,15 Rh,13a,14a,16 Ru,17 Ir,5b,18 Pd,13a,19 and Pt.20 There are two commonly accepted mechanisms of catalyzed alkene isomerizationthe π-allyl mechanism and the alkyl mechanism (Scheme 1).21 The π-allyl mechanism starts with coordination of an olefin to a low-valent metal, followed by oxidative addition of the allylic C−H bond to form an η3-allyl complex with a metal hydride ligand. That metal hydride undergoes reductive elimination into the terminal position to generate the isomerized alkene via an overall 1,3-hydride shift. As shown in Scheme 1b, the alkyl mechanism starts with a metal hydride complex. Migratory insertion of the coordinated alkene into the metal hydride forms either a primary or a secondary metal alkyl complex. In the example shown, the alkene inserts to form a secondary metal alkyl complex. Finally, β-H elimination from the internal carbon results in alkene isomerization via a formal 1,3-hydride shift. Of the two mechanisms presented here, the π-allyl mechanism has been proposed to be the pathway with a greater intrinsic bias toward

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RESULTS AND DISCUSSION Isomerization of Alkenes. We previously described the transfer dehydrogenation of n-octane with the sacrificial hydrogen acceptor norbornene using the precatalysts 1-Mes and 1-Ad, which were activated with NaOtBu at 150 °C.6b Transfer dehydrogenation of n-octane with 1-Mes occurred with 12 turnovers in 20 h, while 1-Ad showed no activity. When 1-hexene was used as the sacrificial hydrogen acceptor

Scheme 1. Two Mechanisms Proposed for Olefin Isomerization: (a) π-Allyl Mechanism; (b) Alkyl Mechanism

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Table 1. Alkene Isomerization Results using 1a

a Reactions performed using 1.0 mmol of alkene, 2.0 μmol of catalyst, and 20 μmol of NaOtBu in 2 mL of toluene. Conversions were determined at 24 h by 1H NMR. bT/C/I is the ratio of trans-2-alkene isomer to cis-2-alkene isomer to the remaining internal isomers (3-alkene or 4-alkene, if possible). The T/C/I ratios given were observed at 24 h. T/C/I ratios observed over the course of the reaction are provided in the Supporting Information. cSee ref 6b.

contributing to the decreased isomerization activity of both catalysts toward allyl phenyl ether. Isomerizations conducted using 1-Ad showed a slight preference for the thermodynamic product, generating phenyl propenyl ether in a 1:5.2 E:Z ratio, while 1-Mes produced a 1:1.4 E:Z ratio of products. Previous investigations of the isomerization of allyl phenyl ether under thermodynamic control show that (Z)-phenyl propenyl ether is exclusively formed.26 This same preference for the Z isomer has been observed with other catalysts,20 where a thermodynamic mixture of products was generated. Given that neither catalyst generated the thermodynamic mixture of products and that the E:Z ratio remained constant throughout the reactions, the ratio of products formed is likely due to kinetic selectivity of the catalysts. Vinylcyclohexane was isomerized readily to ethylidenecyclohexane by both 1-Mes and 1-Ad (Table 1, entries 5 and 6), with no further conversion of ethylidenecyclohexane to 1-ethyl1-cyclohexene or other cyclohexene isomers. In this case, isomerization with 1-Mes was faster, which may be due to reduced steric repulsion between the coordinated alkene and the bulky N substituents on 1-Mes in comparison to 1-Ad (see Figure S2 in the Supporting Information for time-course plots of isomerization). The catalysts both gave complete conversion of allylbenzene in 24 h at 100 °C (Table 1, entries 7 and 8). As shown in Figure 2, 1-Ad gave faster conversion, with 267 turnovers in the first 15 min of the reaction, while 1-Mes gave only 149 turnovers in the same time period. The E:Z ratio of β-

under the same reaction conditions, no dehydrogenation was observed. Instead, 1-Mes catalyzed the isomerization of 1hexene to its internal isomers, giving 730 turnovers in 60 min. Alkene isomerization studies were also conducted using 1octene as the substrate. At 100 °C and 0.2 mol % of catalyst with 2 mol % of NaOtBu, 1-Mes required 24 h to reach 97% conversion of 1-octene into its internal isomers. In comparison, 1-Ad required only 30 min at 60 °C to reach the same level of conversion. When all 1-octene was consumed, 1-Ad continued to isomerize the 2-octene product into the 3-octene and 4octene internal isomers, eventually approaching the thermodynamic ratio of linear octene isomers. In contrast, after 24 h at 100 °C, the reaction catalyzed by 1-Mes did not reach a thermodynamic ratio of octene isomers, indicating that 1-Ad is more reactive toward both terminal and internal alkenes. To determine the effect of different functional groups and substrate size on catalyst activity, further alkene isomerization studies using 1-Ad and 1-Mes and the alkenes allyl alcohol, vinylcyclohexane, allyl phenyl ether, and allylbenzene were conducted (Table 1). In comparison to that of 1-octene, both catalysts displayed minimal activity toward the isomerization of allyl alcohol. When the reaction was catalyzed by 1-Ad at 80 °C, propionaldehyde was generated with 5% yield after 24 h. No conversion was observed when 1-Mes was used. Allyl alcohol is known to isomerize to the enol and then isomerize (or tautomerize) further to form propionaldehyde.17b,c,22 A number of iridium catalysts,23 including the pincer complexes Ir(CF3PCP)24 and Ir(tBu4POCOP),25 have been shown to either catalytically or stoichiometrically decarbonylate aldehydes. Decarbonylation of propionaldehyde may occur with 1-Ad and 1-Mes, contributing to the very low activity of these catalysts toward allyl alcohol. No attempt was made to determine the fate of the iridium in our reactions employing the allyl alcohol. Both catalysts were more active for isomerization of other substrates; therefore, reactions with allyl alcohol were not pursued further. Both complexes catalyze the isomerization of allyl phenyl ether (Table 1, entries 3 and 4), although these reactions were much slower and required higher temperatures in comparison to the other alkenes investigated. Theoretical investigations with other catalysts have indicated that the oxygen of allyl ethers or allyl alcohols coordinates to the catalyst during isomerization.17b Competitive coordination of the ether over the alkene would inhibit alkene isomerization and may be

Figure 2. Isomerization of allylbenzene catalyzed by 1-Mes (◆) or 1Ad (■) at 100 °C. The reaction was performed with 1.0 mmol of alkene, 2.0 μmol of catalyst, and 20 μmol of NaOtBu in 2 mL of toluene. 475

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Scheme 2. Expected Products of the Deuterium Scrambling Experiment with d2-Allylbiphenyl: (a) Sole Product of the π-Allyl Mechanism; (b) Some Products of the Alkyl Mechanism

deuterium label in the allylic position may undergo a net 1,3migration or a net 1,2-migration. In practice, catalysts operating via the alkyl mechanism typically give extensive scrambling of deuterium labels due to reversible alkene insertion.13a,29 Labeling studies employing a substrate deuterated in the allylic position, such as allyl-1,1-d2 alcohol29 or 3-ethyl-1pentene-3-d1,13c have been used previously to examine the mechanism of isomerization of other alkene isomerization catalysts. Because of the excellent activity of both 1-Mes and 1Ad toward allylbenzene, as well as the good selectivity of both catalysts for the trans product, 4-(2-propenyl-1,1-d2)-1,1′biphenyl (henceforth called d2-allylbiphenyl) was used as the substrate of choice. If isomerization proceeds via the π-allyl mechanism, d2-allylbiphenyl is expected to isomerize exclusively via a 1,3-deuterium shift into 4-(1-propenyl-1,3-d2)-1,1′biphenyl (henceforth called 1,3-d 2 -1-propenylbiphenyl) (Scheme 2a). A mixture of deuterated 1-propenylbiphenyl isomerization products is likely if isomerization occurs via the alkyl mechanism, because isomerization can occur via both a 1,2-deuterium shift and a 1,3-deuterium shift (Scheme 2b). In a typical hydrogen−deuterium scrambling experiment, d2allylbiphenyl was combined with 0.8 mol % of the precatalyst 1Ad and 8 mol % of NaOtBu, and the isomerization reaction was monitored via NMR spectroscopy at 80 °C (Figure 3). Under these conditions, d2-allylbiphenyl was completely converted into a mixture of 1-propenylbiphenyl isomers within 2 h. Following the reaction by 1H NMR spectroscopy, the peaks

methylstyrene products generated by both catalysts remains essentially constant at 95:5 E:Z throughout the reaction (see Table S2 in the Supporting Information). Thermodynamic studies have previously been conducted on the isomerization of allylbenzene to its β-methylstyrene isomers: the thermodynamic mixture at 100 °C is composed of 0.3% allylbenzene, 4.9% cis-β-methylstyrene, and 94.9% trans-β-methylstyrene,27 indicating that both catalysts are forming the thermodynamic ratio of β-methylstyrene products.28 Because of the high activity of both 1-Mes and 1-Ad toward allylbenzene isomerization and the predominant formation of a single product, further mechanistic studies conducted with both catalysts were done using allylbenzene and its derivatives. H/D Scrambling and Crossover Experiments with 1Ad. Hydrogen−deuterium scrambling studies and hydrogen− deuterium crossover experiments are known to generate different and distinguishable products, depending on the mechanism of isomerization; therefore, these experiments serve as an initial indicator of which of the two accepted isomerization mechanisms is in operation.13c,29 The hydride shift that occurs during alkene isomerization following the πallyl mechanism is completely intramolecular, and isomerization occurs via a 1,3-hydride shift.13c The alkyl mechanism, in contrast, starts with a metal hydride complex; thus, hydride transfer is intermolecular. Additionally, this mechanism may involve insertion of the alkene into the metal hydride via both Markovnikov and anti-Markovnikov addition, so that a 476

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Figure 3. (a) Isomerization of d2-allylbiphenyl to d2-1-propenylbiphenyl products catalyzed by 1-Ad. The reaction was performed with 0.5 M d2allylbiphenyl, 4 mM (0.8 mol %) catalyst, and 40 mM NaOtBu in 0.44 mL of d6-benzene. 13C NMR spectra were collected at room temperature after 98% conversion was observed by 1H NMR spectroscopy. (b) 13C{1H} NMR spectrum with C−H NOE enhancement. (c) Quantitative 13C{1H} NMR spectrum of the propenyl carbons (C1, C2, and C3) for the products of the isomerization reaction catalyzed by 1-Ad. The heights of C1 are equivalent in both spectra.

Figure 4. 13C NMR of the products of the alkene isomerization reaction with 1-Ad and (a) allylbenzene, (b) d2-allylbiphenyl, and (c) a 1:1 mixture of allylbenzene and d2-allylbiphenyl.

corresponding to d2-allylbiphenyl at 6.00 and 5.14 ppm disappeared, and the product peaks appeared at 6.20 and 1.80 ppm. No signal was observed at 6.45 ppm, indicating complete deuteration of the internal vinylic position (C1). No deuterium was incorporated into the vinylic C2 carbon, indicating that all H−D scrambling is occurring via a 1,3deuterium shift, which is consistent with a π-allyl mechanism. However, the 1H NMR resonance corresponding to the methyl group at 1.80 ppm appeared as a mixture of a doublet and a doublet of 1:1:1 triplets, indicating the product contains a mixture of CH2D and CH3 at the C3 position. 13C NMR

spectroscopy confirmed that a mixture of products formed, where overlapping methyl carbon peaks are observed between 17.4 and 18.7 ppm (Figure 3). These signals correspond to four different products based on the C−D splitting pattern. The most downfield signal is a singlet, indicating complete protiation of the methylene carbon (−CH3). A 1:1:1 triplet, 1:2:3:2:1 quintet, and 1:3:6:7:6:3:1 septet are also present, corresponding to single (−CH2D), double (−CHD2), and complete (−CD3) deuteration of the methylene carbon. This product distribution requires H−D crossover between substrates, which is indicative of the alkyl mechanism. To 477

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Figure 5. 13C NMR spectra of the products of the alkene isomerization reaction with 1-Mes and (a) allylbenzene, (b) d2-allylbiphenyl, and (c) a 1:1 mixture of allylbenzene and d2-allylbiphenyl.

between iridium allyl hydride intermediates, occurring by an unknown, possibly base-mediated31 pathway. We are aware of only one example of an alkene isomerization catalyst where both H−D crossover between substrates occurred and a selective 1,3-deuterium shift was observed.30c To explain their results, the authors of that study proposed that catalysis proceeded via a modified π-allyl mechanism, where the pathway for crossover was not specified. Attempts were made to isolate and characterize the catalyst resting state to gain a better understanding of the mechanism of the reaction. In these attempts, a hydride species was observed in the 1H NMR spectrum. However, this species was not well characterized; therefore, it is unknown if it is a resting state of the catalytic cycle. Additionally, the catalyst was not completely soluble for the entirety of the reaction, despite the low catalyst loading. These experimental challenges prohibited a more thorough kinetic characterization of the reaction using 1-Ad; thus, it is difficult to gain a more thorough understanding of the mechanism by which this catalyst operates. H/D Scrambling and Crossover Experiments with 1Mes. The same H/D scrambling experiments were conducted with the catalyst 1-Mes. In these experiments, d2-allylbiphenyl was completely converted into d2-1-propenylbiphenyl within 15 h at 80 °C, as indicated by the disappearance of the reactant peaks at 6.00 and 5.14 ppm and the appearance of the product peaks at 6.19 and 1.80 ppm in the 1H NMR spectrum. Again, no peak was observed at 6.45 ppm, indicating complete deuteration of the internal vinyl position (C1). The positions of each deuterium in the product were confirmed using 13C NMR spectroscopy, where the peaks at 131.3 and 18.6 ppm both appeared as 1:1:1 triplets, indicating that each carbon contains one deuterium (Figure 5b). No deuterium incorporation was observed for the peak at 125.6 ppm, indicating that an exclusive 1,3-deuterium shift occurred, consistent with the π-allyl mechanism. The intramolecular nature of hydrogen transfer catalyzed by 1-Mes was confirmed with a hydrogen−deuterium crossover experiment. Equimolar amounts of allylbenzene and d2allylbiphenyl were added to a solution containing 0.4 mol % of the precatalyst 1-Mes and 4 mol % of NaOtBu, and the isomerization reaction was monitored via NMR. No deuterium crossover between allylbenzene and d2-allylbiphenyl occurred,

obtain data more reflective of the relative concentrations of these products, a quantitative 13C{1H} NMR spectrum was collected to eliminate any NOE contribution to the 13C NMR signal (Figure 3c). To confirm that H−D crossover between substrates occurs with 1-Ad as the catalyst, a reaction was conducted using equimolar amounts of allylbenzene and d2-allylbiphenyl with 0.4 mol % of precatalyst 1-Ad and 4 mol % of NaOtBu, and the isomerization reaction was monitored via 1H and 13C NMR spectroscopy. As would be expected on the basis of the above observations, H−D crossover between the substrates was observed, although the catalyst maintained its selectivity for a 1,3-deuterium shift. As seen in Figure 4, the β-methylstyrene product was partially deuterated at the methyl position, and no deuterium was incorporated into any of the other alkenyl carbons. The 1-propenylbiphenyl product shows complete deuteration at the internal alkene position (C1) and partial protiation at the methyl (C3) position. The results of the crossover experiment catalyzed by 1-Ad provide evidence that is not fully consistent with what is typically observed for either the π-allyl mechanism or the alkyl mechanism and does not allow us to definitively determine the mechanism of isomerization. There are several examples in the literature of catalysts that operate via the alkyl mechanism; typically, deuterium labeling studies generated products of a 1,2-deuterium shift and a 1,3-deuterium shift with H−D crossover between substrates.13a,19d,29,30 Catalysts that operate via the π-allyl mechanism generated exclusively the products of a 1,3-deuterium shift13c without H−D crossover between substrates.5b The observed product distribution during isomerization with 1-Ad may be rationalized with modifications to either mechanism. If the alkyl mechanism is operative here, the selectivity for 1,3-deuterium migration over 1,2-migration may be explained by selective insertion of the alkene substrate into Ir−H (or Ir−D) to give a secondary metal alkyl (Scheme 2b, left pathway), with negligible insertion to give a primary metal alkyl (Scheme 2b, right pathway). Alternatively, the selectivity of 1-Ad for the product with a 1,3-deuterium shift is consistent with the π-allyl mechanism, although this mechanism does not produce the intermolecular hydrogen crossover observed. In this case, the observed crossover may be due to H−D exchange 478

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was repeated using 2-Mes (0.5 mM) as the catalyst, and the isomerization reaction occurred at the same rate. These kinetic results, combined with the observation of only 2-Mes via NMR during isomerization, indicate that 2-Mes is kinetically competent as a catalytic intermediate. Further kinetic studies were done using 2-Mes, which showed that the isomerization reaction has first-order dependence on both [allylbenzene] (Figure 8) and [2-Mes] (Figure 9), giving a second-order rate constant (k) of 0.177 ± 0.009 M−1 s−1 (rate = k[allylbenzene][2-Mes]).32 Proposed Mechanism of Isomerization with 1-Mes. On the basis of the results of the studies done with 1-Mes and 2-Mes above, the mechanism of isomerization shown in Scheme 5 is proposed: (a) 1-Mes undergoes dehydrohalogenation by NaO t Bu or NaOMe to generate an Ir(I) intermediate, which reacts with 1-alkene to generate the η2-1alkene complex; (b) fast oxidative addition of an allylic C−H bond occurs, followed by rotation of the η3-allyl ligand to generate the resting state of the catalyst, (c) 2-Mes is produced; (d) reductive elimination generates the η2-2-alkene complex; (e) 2-alkene is substituted for 1-alkene, starting the cycle again. The parts of the catalytic cycle most strongly supported by experimental evidence are steps c−e, as follows: (c) the identity of 2-Mes was confirmed by NMR spectroscopy and X-ray crystallography studies and is confirmed to be a part of the catalytic cycle by showing chemical and kinetic competence in comparison to the precatalyst 1-Mes; (d) exchange spectroscopy shows a fast equilibrium between the resting state and an η2-2-alkene complex, which is likely an intermediate before the turnover-limiting step of the reaction; (e) the first-order dependence of the reaction on both the alkene and the catalyst indicates that the alkene substitution step proceeds via associative ligand displacement and is likely the turnoverlimiting step of the reaction. It should be noted that these studies were conducted using benzylic alkenes; thus, the mechanism of isomerization may be different for isomerization of alkyl-substituted alkenes. A similar mechanism was proposed by Goldman and coworkers for alkyl-substituted alkene isomerization catalyzed by (tBu4PCP)IrH2 and (tBu4POCOP)IrH2.5b In contrast to our observations with 1-Mes, the PCP-Ir systems have an η2-1alkene resting state similar to (a) in Scheme 5. A similar π-allyl intermediate (c) was synthesized independently, however this intermediate was only observed spectroscopically at low temperatures and was found to convert to the catalyst resting state upon warming. DFT calculations confirmed that alkene isomerization proceeds via π-allyl intermediates in the PCP-Ir systems. Interestingly, the calculations show that the π-allyl intermediate is preferentially formed by dissociation of the η2bound alkene to give a σ complex of the allylic C−H bond, which then undergoes oxidative addition. Our data do not distinguish between this mechanism and a direct addition of the allylic C−H bond without alkene dissociation.

and the only products observed were the normal isomerization products, as shown in the 13C NMR spectrum (Figure 5c). These results suggest that 1-Mes catalyzes alkene isomerization following the π-allyl mechanism. Observation of an Ir(III) η3-Allyl Hydride Catalyst Resting State. The H/D scrambling experiments and crossover experiments indicate that alkene isomerization with 1-Mes likely proceeds via the π-allyl mechanism. Treatment of 1-Mes with allylbenzene (20 equiv) and 10 equiv of NaOtBu or NaOMe in benzene at 80 °C generated an Ir(III) η3-allyl hydride species (2-Mes) cleanly (Scheme 4). The 1H NMR Scheme 4. Synthesis of the η3-Allyl Complex 2-Mes

spectrum for 2-Mes includes a hydride resonance at −14.7 ppm and four signals for the cinnamyl ligand at 5.01, 3.56, 2.05, and 1.30 ppm. Additionally, six separate signals are observed for the mesityl methyl groups, reflecting the loss of Cs symmetry upon formation of the allyl complex. This species is generated quickly and is observed throughout the catalytic reaction. No other iridium species is observed in the 1H NMR spectrum. An exchange spectroscopy (EXSY) experiment conducted at 25 °C reveals that the hydride ligand exchanges with the terminal CH2 hydrogens on the allyl ligand. This is consistent with a rapid equilibrium between the η3-allyl and the η2-alkenyl complexes, where the alkene is at the internal position (Figure 6). No exchange was observed between the hydride and the

Figure 6. Equilibrium between the η3-allyl complex 2-Mes catalyst resting state and the internal η2-alkenyl complex.

internal allyl hydrogens. Treatment of 1-Mes with 20 equiv of trans-β-methylstyrene and 10 equiv of NaOtBu in benzene at 80 °C also generated 2-Mes, indicating that the π-allyl complex can be formed using either the terminal alkene reagents or the internal alkene products. The solution structure of the π-allyl complex was confirmed in the solid state using X-ray crystallography (Figure 7). An ORTEP diagram of 2-Mes shows that the terminal allylic carbon is trans to Caryl in the CCC-pincer backbone. The allyl ligand extends over the pincer backbone and away from the bulky mesityl groups. Although it was not observed in the crystal structure, the hydride ligand is likely positioned cis to the terminal allyl carbon, and cis to each pincer carbon. Kinetics of Catalytic Isomerization with 1-Mes. A kinetic study of the isomerization of allylbenzene was conducted using either 1-Mes or 2-Mes as the catalyst with d8-toluene as the NMR solvent. Addition of allylbenzene (100 mM) to 1-Mes (0.5 mM) and NaOtBu (5 mM) generated βmethylstyrene with 98% yield in 12.8 h at 100 °C. The reaction

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SUMMARY The CCC-pincer iridium complexes 1-Mes and 1-Ad were investigated as catalysts for alkene isomerization. Both complexes are active alkene isomerization catalysts for a number of substrates, with the exception of allyl alcohol. The mechanism of isomerization of both catalysts was investigated using H−D scrambling and crossover experiments. Further mechanistic studies were conducted with 1-Mes, for which the catalyst resting state was synthesized and isolated. It was 479

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Figure 7. Crystal structure of 2-Mes, showing 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected metric data (bond lengths in Å and angles in deg): Ir(1)−C(8), 2.042(4); Ir(1)−C(5), 1.982(4); Ir(1)−C(2), 2.019(5); Ir(1)−C(44), 2.270(5); Ir(1)−C(45), 2.172(5); Ir(1)−C(46), 2.303(6); C(2)−Ir(1)−C(5), 78.00(18); C(2)−Ir(1)−C(8), 155.03(17); C(5)−Ir(1)−C(8), 77.35(18); C(5)−Ir(1)−C(46), 112.7(2); C(5)−Ir(1)−C(44), 177.82(19).

Figure 8. First-order kinetics plot for the isomerization of allylbenzene to β-methylstyrene catalyzed by 2-Mes at 100 °C. The reaction was performed with 0.996 M allylbenzene and 0.25−0.99 mM (0.25−1.0 mol %) catalyst in 1.0 mL of d8-toluene: (a) 0.5 mol % of 2-Mes, kobs = (8.35 ± 0.04) × 10−5 s−1; (b) 0.5 mol % of 2-Mes, kobs = (8.47 ± 0.05) × 10−5 s−1; (c) 1.0 mol % of 2-Mes, kobs = (18.47 ± 0.13) × 10−5 s−1; (d) 0.25 mol % of 2-Mes, kobs = (2.84 ± 0.02) × 10−5 s−1. Data were plotted to 4 half-lives or 12.8 h, whichever came first. activated alumina, using an MBraun Solvent Purification System. Flash chromatography using solvent gradients was performed using a Combiflash RF system. 1-Mes,6b 1-Ad,6b and allylbiphenyl33 were prepared as previously described. Procedures for the preparation of d2allylbiphenyl can be found in the Supporting Information. No isomerization reactions were conducted in the absence of ambient light. cis-β-Methylstyrene was stored at −32 °C in the dark until use. Instrumentation. Nuclear magnetic resonance spectra were recorded on a Bruker 400 MHz (1H, 400 MHz; 13C, 100 MHz) spectrometer and referenced to the residual solvent resonance (δ in parts per million and J in Hz). Elemental analyses were performed by Robertson Microlit, Madison, NJ.

determined that the size of the pincer side group drastically affected the mechanism of isomerization: the mesitylsubstituted catalyst proceeds via the π-allyl mechanism, while the adamantyl-substituted catalyst operates via a distinct mechanism that is not unambiguously determined by the available data.

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EXPERIMENTAL SECTION

Materials and Methods. Unless stated otherwise, all manipulations were carried out in either an Ar-filled MBraun Labmaster 130 glovebox or on a Schlenk line using Ar. Solvents were generally purified by sparging with argon and passing through columns of 480

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Synthesis of the Iridium Complex 2-Mes with β-Methylstyrene. 1-Mes (2 mg, 2.4 μmol) and sodium tert-butoxide (2.3 mg, 24 μmol) were combined with 1 mL of C6D6 in an oven-dried vial in the glovebox. β-Methylstyrene (6.4 μL, 49 μmol) was added to the reaction mixture, which was then heated to 75 °C with stirring for 1.5 h. The reaction mixture was cooled to room temperature and then transferred to a J. Young tube. The 1H NMR spectrum obtained contains peaks corresponding solely to 2-Mes (as prepared with allylbenzene) and β-methylstyrene. Isomerization of Allylbenzene with 1-Mes and 1-Ad. In an argon-filled glovebox, a vial was charged with a stirbar, the iridium catalyst (1.96 μmol), allylbenzene (132 μL, 996 μmol), sodium tertbutoxide (1.9 mg, 19.7 μmol), and 2.0 mL of toluene. The vial was capped and heated in the glovebox while the contents were stirred. At the time points indicated, aliquots were removed for analysis. A 50 μL aliquot was transferred to 700 μL of CDCl3. The progress of the reaction was monitored by 1H NMR spectroscopy by observing the disappearance of the allylbenzene resonances at 5.82 (m, 1H), 4.93 (m, 2H), and 3.17 ppm (d, 2H) and the appearance of the βmethylstyrene resonances at 6.26 (m, 1H), 5.98 (m, 1H), and 1.66 ppm (d, 3H). Separately, a 50 μL aliquot was added to 1.5 mL of ethyl acetate, filtered, and analyzed by GC-MS to determine the ratios of internal isomers formed. Isomerization of Vinylcyclohexane with 1-Mes and 1-Ad. In an argon-filled glovebox, a vial was charged with a stirbar, the iridium catalyst (1.96 μmol), vinylcyclohexane (137 μL, 1.0 mmol), sodium tert-butoxide (1.9 mg, 19.7 μmol), and 2.0 mL of toluene. The vial was capped and heated in the glovebox while the contents were stirred. At the time points indicated, aliquots were removed for analysis. A 50 μL aliquot was transferred to 700 μL of CDCl3. The progress of the reaction was monitored by 1H NMR spectroscopy by observing the disappearance of the vinylcyclohexane resonance at 5.85 ppm (m, 1H) and the appearance of the ethylidenecyclohexane resonance at 5.20 ppm (q, 1H). Separately, a 50 μL aliquot was added to 1.5 mL of ethyl acetate, filtered, and analyzed by GC-MS. Isomerization of Allyl Phenyl Ether with 1-Mes and 1-Ad. In an argon-filled glovebox, a vial was charged with a stirbar, the iridium catalyst (1.96 μmol), allyl phenyl ether (137 μL, 998 μmol), sodium tert-butoxide (1.9 mg, 19.7 μmol), and 2.0 mL of toluene. The vial was capped and heated in the glovebox while the contents were stirred. At the time points indicated, aliquots were removed for analysis. A 50 μL aliquot was transferred to 700 μL of CDCl3. The progress of the reaction was monitored by 1H NMR spectroscopy by observing the disappearance of the allyl phenyl ether resonance at 4.64 ppm (dt, 2H) and the appearance of the cis-1-propenyloxybenzene resonance at 1.85 ppm (d, 3H) and the trans-1-propenyloxybenzene resonance at 1.79 ppm (d, 3H). Separately, a 50 μL aliquot was added to 1.5 mL of ethyl acetate, filtered, and analyzed by GC-MS to determine the ratios of internal isomers formed. General Procedure for Allylbenzene Isomerization with 2Mes. In an argon-filled glovebox, 2-Mes (1.9 mg, 0.002 mmol) was

Figure 9. Second-order kinetics plot of the observed first-order rate constant vs catalyst concentration for the isomerization of allylbenzene to β-methylstyrene catalyzed by 2-Mes at 100 °C. The plot gives a second-order rate constant of 0.177 ± 0.009 M−1 s−1.

Synthesis of the Iridium Complex 2-Mes with Allylbenzene. 1-Mes (24.7 mg, 0.030 mmol) and sodium methoxide (17.7 mg, 0.328 mmol) were combined with 5 mL of benzene in an oven-dried scintillation vial in the glovebox. Allylbenzene (80.4 μL, 0.607 mmol) was added to the reaction mixture, which was then heated to 80 °C with stirring for 1 h. The reaction mixture was cooled to room temperature and then filtered through a 0.45 μm PTFE Whatman filter to obtain a clear yellow solution. The volatiles were removed under reduced pressure to obtain a yellow solid. The solid was extracted with 20 mL of pentane, and the solution was filtered. The solid was washed with additional pentane until the filtrate became clear. The pentane was removed under vacuum to yield a gummy yellow solid. Benzene was added to dissolve the solid, and then this solution was placed in the freezer until solid. Sublimation of the benzene yielded the product as a powdery yellow solid. Yield: 15.3 mg, 0.018 mmol, 59%. 1H NMR (C6D6): δ 8.02 (d, 3JHH = 8.5 Hz, 1H), 7.85 (d, 3JHH = 8.2 Hz, 1H), 7.74 (d, 3JHH = 3.1 Hz, 1H), 7.72 (d, 3JHH = 3.5 Hz, 1H), 7.36 (t, 3JHH = 7.8 Hz, 1H), 7.07 (t, 3JHH = 8.0 Hz, 1H), 7.02 (t, 3JHH = 8.0 Hz, 1H), 6.93 (t, 3JHH = 7.6 Hz, 1H), 6.89 (t, 3JHH = 7.6 Hz, 1H), 6.82−6.97 (m, 1H), 6.78 (s, 1H), 6.74 (s, 1H), 6.71−6.68 (m, 2H), 6.68−6.64 (m, 2H), 6.64−6.57 (m, 4H), 5.01 (td, 3JHH =10.2 Hz, 7.2 Hz, 1H), 3.56 (d, 3JHH =10.2 Hz, 1H), 2.19 (s, 3H), 2.17 (s, 3H), 2.05 (d, 3JHH = 7.2 Hz, 1H), 2.04 (s, 3H), 2.03 (s, 3H), 2.02 (s, 3H), 1.94 (s, 3H), 1.30 (d, 3JHH = 10.9 Hz, 1H), −14.75 (s, 1H) 13C NMR (C6D6): δ 183.0, 179.6, 156.7, 145.5, 144.5, 139.1, 138.9, 138.3, 138.1, 137.0, 136.9, 135.8, 135.3, 133.2, 132.8, 132.7, 132.6, 130.3, 130.2, 129.3, 129.2, 127.9, 126.7, 123.4, 123.3, 123.0, 122.5, 122.4, 119.8, 111.6, 111.0, 110.4, 110.0, 108.4, 107.8, 93.6, 55.5, 21.0, 20.9, 20.1, 20.0, 19.7, 18.5, 18.1. Anal. Calcd for C47H43N4Ir: C, 65.94; H, 5.06; N, 6.54. Found: C, 65.72; H, 5.08; N, 6.39.

Scheme 5. Proposed Mechanism of Isomerization using 1-Mes or 2-Mes as the Catalysta

a

Mesityl groups and the pincer backbone are omitted for clarity. 481

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dissolved in 1.0 mL of d8-toluene to form a 2.2 mM stock solution. In a J. Young tube, allylbenzene (13.2 μL, 0.1 mmol) and d8-toluene (537− 874 μL) were added to the 2-Mes stock solution (113−450 μL, 0.25− 1.0 mol %). The J. Young tube was inserted into a preheated and preshimmed NMR set to 100 °C. The progress of the reaction was monitored by 1H NMR spectroscopy by observing the disappearance of the allylbenzene resonances at 5.82 (m, 1H), 4.93 (m, 2H), and 3.17 ppm (d, 2H) and the appearance of the β-methylstyrene resonances at 6.26 (m, 1H), 5.98 (m, 1H), and 1.66 ppm (d, 3H). Isomerization of Allylbiphenyl or d2-Allylbiphenyl with 1Mes and 1-Ad. In an argon-filled glovebox, a vial was charged with a stirbar, the iridium catalyst (1.98 μmol), d2-allylbiphenyl (55.5 μL, 253 μmol), sodium tert-butoxide (1.9 mg, 20 μmol), and 0.44 mL of C6D6. The vial was capped and heated in the glovebox while the contents were stirred, and aliquots were removed periodically for analysis. A 50 μL aliquot was transferred to 700 μL of C6D6 in an NMR tube. The progress of the reaction was monitored by 1H NMR spectroscopy by observing the disappearance of the allylbiphenyl resonances at 5.85 (m, 1H), 4.97 (m, 2H), and 3.20 ppm (d, 2H) and the appearance of the 1-propenylbiphenyl resonances at 6.34 (m, 1H), 6.05 (m, 1H), and 1.69 ppm (d, 3H). The spectroscopic data are in agreement with reported values.34 When d2-allylbiphenyl was used, no peaks were observed at 3.20 or 6.34 ppm. Isomerization with 1-Mes generated 1,3-d2-1-propenylbiphenyl: 1H NMR (C6D6) δ 7.52−7.42 (m, 4H), 7.30−7.20 (m, 4H), 7.17−7.12 (m, 1H), 6.07 (br t, 3JHH = 7.1 Hz, 1H), 1.68 (d1:1:1t, 3JHH = 6.7 Hz, 2JDH = 2.0 Hz, 2H); 13C NMR (C6D6) δ 141.4, 140.0, 137.4, 131.0 (1:1:1t, 1JCD = 23.1 Hz), 129.1, 127.6, 127.4, 127.3, 126.8, 125.5, 18.3 (1:1:1t, 1JCD = 19.4 Hz). Isomerization with 1-Ad generated 1,3-d2-1-propenylbiphenyl, 1-d1-1propenylbiphenyl, and 1,3,3-d3-1-propenylbiphenyl. The 1H and 13C NMR peaks observed were the same for each compound except for those listed here. 1-d1-1-propenylbiphenyl: 1H NMR (C6D6) δ 1.68 (d, 3 JHH = 6.6 Hz, 2H); 13C NMR (C6D6): δ 18.6 (s). 1,3,3-d3-1propenylbiphenyl: 13C NMR (C6D6) δ 18.0 (1:2:3:2:1quintet, 1JCD = 19.1 Hz). General Procedure for Allylbiphenyl or d2-Allylbiphenyl Isomerization with 2-Mes. In an argon-filled glovebox, 2-Mes (1.9 mg, 0.002 mmol) was dissolved in 1.0 mL of d8-toluene to form a 2.2 mM stock solution. In a J. Young tube, either allylbiphenyl or d2allylbiphenyl (20.4 mg, 0.10 mmol) was combined with 0.23 mL of the 2-Mes stock solution (0.51 μmol, 0.5 mol %) and d8-toluene to make a 1.0 mL solution. The J. Young tube was inserted into a preheated and preshimmed NMR spectrometer set to 100 °C. The progress of the reaction was monitored by 1H NMR spectroscopy by observing the disappearance of the allylbiphenyl resonances at 5.85 (m, 1H), 4.97 (m, 2H), and 3.20 ppm (d, 2H) and the appearance of the 4propenylbiphenyl resonances at 6.34 (m, 1H), 6.05 (m, 1H), and 1.69 ppm (d, 3H). The spectroscopic data are in agreement with reported values.34 When d2-allylbiphenyl was used, no peaks were observed at 3.20 or 6.34 ppm. Crossover Experiments. In an argon-filled glovebox, a vial was charged with a stirbar, the iridium catalyst (1.98 μmol), allylbenzene (33.5 μL, 253 μmol), d2-allylbiphenyl (55.5 μL, 253 μmol), sodium tert-butoxide (1.9 mg, 20 μmol), and 0.44 mL of C6D6. The vial was capped and heated in the glovebox while the contents were stirred, and aliquots were removed periodically for analysis. A 50 μL aliquot was transferred to 700 μL of C6D6 in an NMR tube. The progress of the reaction was monitored by 1H NMR spectroscopy by observing the disappearance of the allylbenzene resonances at 5.82 (m, 1H), 4.93 (m, 2H), and 3.17 ppm (d, 2H) and the d2-allylbiphenyl resonances at 5.85 (m, 1H) and 4.97 ppm (m, 2H) and the appearance of the βmethylstyrene resonances at 6.26 (m, 1H), 5.98 (m, 1H), and 1.66 ppm (d, 3H) and the 4-propenylbiphenyl resonances at 6.34 (m, 1H), 6.05 (m, 1H), and 1.69 ppm (d, 3H). At the end of the reaction, a 13C NMR spectrum was acquired, and the position of the deuteriums in the products was determined by observing the β-methylstyrene resonances at 131.6, 125.5, and 18.5 ppm and the 4-propenylbiphenyl resonances at 131.0, 125.4, and 18.3 ppm. Isomerization with 1-Mes generated the normal products β-methylstyrene and 1,3-d2-1propenylbiphenyl. The products observed from isomerization with

1-Ad include 1,3-d2-1-propenylbiphenyl, 1-d1-1-propenylbiphenyl, and 3-d1-β-methylstyrene. The 1H and 13C NMR peaks observed for 3-d1β-methylstyrene were the same as the resonances observed with βmethylstyrene except for those listed here. 3-d1-β-methylstyrene: 1H NMR (C6D6) δ 1.65 (d1:1:1t, 3JHH = 6.5 Hz, 2JDH = 2.0 Hz, 2H); 13C NMR (C6D6) δ 18.2 (1:1:1t, 1JCD = 19.2 Hz). Isomerization of cis-β-Methylstyrene with 1-Ad. In an argonfilled glovebox, a vial was charged with a stirbar, 1-Ad (2.04 μmol), cisβ-methylstyrene (33.1 μL, 255 μmol), sodium tert-butoxide (1.96 mg, 20.3 μmol), and 0.44 mL of d6-benzene. The vial was capped and heated in the glovebox at 80 °C while the contents were stirred. At 15, 30, and 45 min and 1, 2, and 24 h, an 80 μL aliquot was transferred to 450 μL of d6-benzene. The progress of the reaction was monitored by 1 H NMR spectroscopy by observing the disappearance of the cis-βmethylstyrene resonance at 1.81 ppm (dd, 3H) and the appearance of the trans-β-methylstyrene resonance at 1.75 ppm (dd, 3H). Control Isomerization of cis-β-Methylstyrene. In an argonfilled glovebox, a vial was charged with a stirbar, cis-β-methylstyrene (33.1 μL, 255 μmol), sodium tert-butoxide (1.96 mg, 20.3 μmol), and 0.44 mL of d6-benzene. The vial was capped and heated in the glovebox at 80 °C while the contents were stirred. At 15, 30, and 45 min and 1, 2, and 24 h, an 80 μL aliquot was transferred to 450 μL of d6-benzene. The progress of the reaction was monitored by 1H NMR spectroscopy by observing the disappearance of the cis-β-methylstyrene resonance at 1.81 ppm (dd, 3H) and the appearance of the transβ-methylstyrene resonance at 1.75 ppm (dd, 3H). Even after 24 h, no isomerization was observed. Competition Isomerization of cis-β-Methylstyrene and Allylbenzene or d2-Allylbiphenyl with 1-Ad. In an argon-filled glovebox, a vial was charged with a stirbar, 1-Ad (2.04 μmol), cis-βmethylstyrene (48.9 μL, 377 μmol), allylbenzene (33.8 μL, 255 μmol) or d2-allylbiphenyl (55.5 μL, 255 μmol), sodium tert-butoxide (1.96 mg, 20.3 μmol), and 0.44 mL of d6-benzene. The vial was capped and heated in the glovebox at 80 °C while the contents were stirred. At 0, 15, 30, and 45 min and 1, 2, and 24 h, an 80 μL aliquot was transferred to 450 μL of d6-benzene. The progress of the reaction was monitored by 1H NMR spectroscopy by observing the disappearance of the allylbenzene resonance at 3.28 ppm (d, 2H) and the cis-βmethylstyrene resonance at 1.81 ppm (dd, 3H) and the appearance of the trans-β-methylstyrene resonance at 1.75 ppm (dd, 3H). EXSY Experiments. In an argon-filled glovebox, a vial was charged with a stirbar, 1-Mes (8.0 mg, 9.8 μmol), allylbenzene (26.4 μL, 199 μmol), sodium tert-butoxide (9.4 mg, 97.8 μmol), and 2.0 mL of benzene. The reaction mixture was heated to 80 °C with stirring for 1 h, and then the solvent was removed under vacuum. The yellow solid was dissolved in C6D6 and added to a J. Young tube. Formation of the 2-Mes was confirmed by 1H NMR, and then a 2D NOESY experiment was conducted at 25 °C. X-ray Structure Determination of 2-Mes. X-ray-quality crystals were grown by adding 2-Mes (synthesized using NaOtBu) to pentane until it was just dissolved and then cooling to −32 °C. After 2 months, X-ray-quality crystals were present in the sample. Structure determinations were performed on an Oxford Diffraction Gemini-R diffractometer, using Cu Kα radiation. Single crystals were mounted on Hampton Research Cryoloops using Paratone-N oil. Unit cell determination, data collection and reduction, and absorption correction were performed using the CrysAlisPro software package.35 Direct methods structure solution was accomplished using SIR92,36 and full-matrix least-squares refinement was carried out using CRYSTALS.37 All non-hydrogen atoms were refined anisotropically. Unless otherwise noted, hydrogen atoms were placed in calculated positions, and their positions were initially refined using distance and angle restraints. All hydrogen positions were fixed in place for the final refinement cycles. Highly disordered solvent was present in the unit cell; correction for this residual density was performed using the option SQUEEZE in the program package PLATON.38 A total of 213 electrons per unit cell were removed, from a total potential solventaccessible void of 1396 Å3. In the early stages of refinement, minor disorder was observed in the allyl fragment. Attempts to include this 482

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1969, 91, 7553−7554. (c) Casey, C. P.; Cyr, C. R. J. Am. Chem. Soc. 1973, 95, 2248−2253. (d) Mayer, M.; Welther, A.; von Wangelin, A. J. ChemCatChem 2011, 3, 1567−1571. (14) (a) Roos, L.; Orchin, M. J. Am. Chem. Soc. 1965, 87, 5502− 5504. (b) Casey, C. P.; Cyr, C. R. J. Am. Chem. Soc. 1971, 93, 1280− 1282. (c) Casey, C. P.; Cyr, C. R. J. Am. Chem. Soc. 1973, 95, 2240− 2247. (d) Taylor, P.; Orchin, M. J. Am. Chem. Soc. 1971, 93, 6504− 6506. (15) (a) Tolman, C. A. J. Am. Chem. Soc. 1972, 94, 2994−2999. (b) Lim, H. J.; Smith, C. R.; RajanBabu, T. V. J. Org. Chem. 2009, 74, 4565−4572. (16) Morrill, T. C.; D’Souza, C. A. Organometallics 2003, 22, 1626− 1629. (17) (a) Sharma, S. K.; Srivastava, V. K.; Jasra, R. V. J. Mol. Catal. A: Chem. 2006, 245, 200−209. (b) Varela-Á lvarez, A.; Sordo, J. A.; Piedra, E.; Nebra, N.; Cadierno, V.; Gimeno, J. Chem. Eur. J. 2011, 17, 10583−10599. (c) Larsen, C. R.; Grotjahn, D. B. J. Am. Chem. Soc. 2012, 134, 10357−10360. (d) Dı ́az-Á lvarez, A. E.; Crochet, P.; Cadierno, V. Tetrahedron 2012, 68, 2611−2620. (18) Baxendale, I. R.; Lee, A. L.; Ley, S. V. Synlett 2001, 2004−2004. (19) (a) Yu, J.; Gaunt, M. J.; Spencer, J. B. J. Org. Chem. 2002, 67, 4627−4629. (b) Kim, I. S.; Dong, G. R.; Jung, Y. H. J. Org. Chem. 2007, 72, 5424−5426. (c) Canovese, L.; Santo, C.; Visentin, F. Organometallics 2008, 27, 3577−3581. (d) Spallek, M. J.; Stockinger, S.; Goddard, R.; Trapp, O. Adv. Synth. Catal. 2012, 354, 1466−1480. (20) Scarso, A.; Colladon, M.; Sgarbossa, P.; Santo, C.; Michelin, R. A.; Strukul, G. Organometallics 2010, 29, 1487−1497. (21) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 5th ed.; Wiley: Hoboken, NJ, 2009; pp 229−231. (22) Bergens, S. H.; Bosnich, B. J. Am. Chem. Soc. 1991, 113, 958− 967. (23) (a) Olsen, E. P. K.; Madsen, R. Chem. Eur. J. 2012, 18, 16023− 16029. (b) Iwai, T.; Fujihara, T.; Tsuji, Y. Chem. Commun. 2008, 6215−6217. (24) Adams, J. J.; Arulsamy, N.; Roddick, D. M. Organometallics 2012, 31, 1439−1447. (25) Polukeev, A. V.; Petrovskii, P. V.; Peregudov, A. S.; Ezernitskaya, M. G.; Koridze, A. A. Organometallics 2013, 32, 1000−1015. (26) Taskinen, E. J. Chem. Soc., Perkin Trans. 2 2001, 1824−1834. (27) Taskinen, E.; Lindholm, N. J. Phys. Org. Chem. 1994, 7, 256− 258. (28) On the suggestion of a reviewer, we ran a competition experiment between β-methylstyrene and allylbenzene using 1-Ad as the catalyst to determine whether the ratio of products is due to kinetic or thermodynamic selectivity. This experiment shows that the product distribution is due to kinetic selectivity of the reaction. See Figure S3 in the Supporting Information for the time-course conversion data. (29) Courchay, F. C.; Sworen, J. C.; Ghiviriga, I.; Abboud, K. A.; Wagener, K. B. Organometallics 2006, 25, 6074−6086. (30) (a) Bingham, D.; Webster, D. E.; Wells, P. B. J. Chem. Soc., Dalton. Trans. 1974, 1514−1518. (b) Bingham, D.; Webster, D. E.; Wells, P. B. J. Chem. Soc., Dalton. Trans. 1974, 1519−1521. (c) Harrod, J. F.; Chalk, A. J. J. Am. Chem. Soc. 1966, 88, 3491−3497. (31) (a) Erdogan, G.; Grotjahn, D. B. J. Am. Chem. Soc. 2009, 131, 10354−10355. (b) Hanessian, S.; Giroux, S.; Larsson, A. Org. Lett. 2006, 8, 5481−5484. (32) KIE experiments were attempted, but these experiments were complicated by early nonlinear data for the protiated substrates, while deuterated substrates consistently gave linear data. Because of the differing amounts of linearity in the kinetics plot for each substrate, we were not able to make any conclusive statements about these studies. (33) Gerbino, D. C.; Mandolesi, S. D.; Schmalz, H.-G.; Podestá, J. C. Eur. J. Org. Chem. 2009, 3964−3972. (34) Stieber, F.; Grether, U.; Waldmann, H. Chem. Eur. J. 2003, 9, 3270−3281. (35) Xcalibur CCD system, CrysAlisPro Software system, Version 1.171.32; Oxford Diffraction Ltd., 2007.

disorder in the model did not produce improved results. The iridiumbound hydride was not located in the difference map.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

A CIF file and text, tables, and figures giving crystallographic data for 2-Mes, complete experimental procedures and NMR spectra for the synthesis of d2-allylbiphenyl, time course plots for the isomerization of vinylcyclohexane and allylphenyl ether, time course data for the E:Z ratio of products observed during isomerization of allyl phenyl ether and allylbenzene, time course data for competition experiments on cis-β-methylstyrene and allylbenzene isomerization catalyzed by 1-Ad, detailed NMR assignments for 2-Mes, and full 1H and 13C NMR spectra from the isomerization reaction with allylbiphenyl. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

*E-mail for A.R.C.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (Grant No. CHE1057792) for financial support. REFERENCES

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