Mechanistic Study of a Re-Catalyzed Monoalkylation of Phenols

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Mechanistic Study of a Re-Catalyzed Monoalkylation of Phenols Dan Lehnherr,* Xiao Wang,* Feng Peng, Mikhail Reibarkh, Mark Weisel, and Kevin M. Maloney Department of Process Research and Development, Merck & Co., Inc., Rahway, New Jersey 07065, United States

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S Supporting Information *

ABSTRACT: A mechanistic study of a rhenium catalyzed monoalkylation of phenols is described. Reaction kinetics reveals a zero-order dependence on both alkene and phenol and a half order dependence on catalyst. Isotopic labeling studies, competition experiments, kinetic isotope effects, and Hammett analysis together afford experimental data consistent with a reversible C−H activation step and an irreversible hydrometalation process. The turnover-limiting step is identified as catalyst deaggregation. NMR studies of binary mixtures of catalyst and a single substrate (alkene or phenol) as well as those of reaction mixtures identify potential intermediates and off-cycle species. Despite the numerous Re complexes formed in these mixtures, the overall reaction is both high yielding and highly selective for monoalkylation of phenols.

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INTRODUCTION Efficient introduction of a single alkyl group onto an arene remains a challenging transformation. For example, Friedel− Crafts alkylations generally provide regioisomeric mixtures in addition to overalkylated products.1 Olefin hydroarylation reactions can effectively control the regioisomeric site of alkylation on arenes, although controlling for branched versus linear products (Markovnikov versus anti-Markovnikov addition) is challenging and can require impractical directing groups.2 Methods to obtain the linear hydroarylation product are well-documented;3 however, the identification of branch selective methods remain significantly more elusive.4 A rhenium-catalyzed alkylation of phenols with terminal alkenes was initially reported by Takai and co-workers (Scheme 1).5,6 This reaction provides exclusively the

converts commodity feedstocks to pharmaceutically relevant intermediates.

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POTENTIAL REACTION MECHANISMS Several potential reaction mechanisms were initially considered for this transformation, and our experiments were designed to probe for evidence that would either support or refute these postulated mechanisms. Alkylation of electron rich arenes immediately draws attention to the possibility of a Friedel− Crafts type mechanism (electrophilic aromatic substitution) whereby a cationic intermediate derived from the alkene is trapped by the electron-rich arene (Scheme 2A). Alternatively, a Re-catalyzed C−H activation processes may be operational and such reactions have been previously reported.8,9 The hydroxyl group could act as a directing group for metalation of the arene via a C−H activation-type mechanism, with an alkene hydrometalation process occurring either after or prior to the C−H activation step (Scheme 2B,C, respectively) and in either case followed by a reductive coupling step. The hydrometalation event could be a concerted 1,2-metal hydride insertion into the alkene or alternatively proceed via a 2-step process in which hydrogen atom transfer (HAT) to the olefin provides a radical pair that subsequently recombines to afford a net hydrometalation;10 a subsequent reductive coupling could afford product. Scheme 2D illustrates an alternative cyclometalation pathway in which a phenol bound to the Re catalyst could serve as a template for the alkene to annulate the arene via a dearomatization-rearomatization pathway, followed by either reductive elimination or protodemetalation. This

Scheme 1. Re-Catalyzed Regioselective Markovnikov Monoalkylation of Mequinol

monoalkylation product, is highly regioselective, and results in sole observation of the branched product. This result is in contrast to other methods of functionalizing phenols that afford mixtures of mono- and bis-alkylated products.7 As part of our efforts to obtain an efficient synthesis of orthomonoalkylated phenols for the development of active pharmaceutical ingredients, we sought to leverage this Recatalyzed reaction as well as to gain understanding of the underlying reaction mechanism. Herein we report a detailed mechanistic investigation of this reaction that successfully © XXXX American Chemical Society

Special Issue: The Roles of Organometallic Chemistry in Pharmaceutical Research and Development Received: July 31, 2018

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DOI: 10.1021/acs.organomet.8b00543 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2. Selected Potential Mechanisms Prior to Mechanistic Study

cyclometalation pathway would bear a resemblance to literature-proposed two-π-component cyclization processes involving rhenium carbonyl catalysts.6,11

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Scheme 3. Competition Reaction Under Re-Catalyzed Alkylation Conditions

ELECTRONIC EFFECTS

While a variety of phenols and alkenes are competent substrates in the reaction shown in Scheme 1,5,6 we chose to use mequinol (1) and 1-octene (2) as model substrates to study the generation of monoalkylated phenol 3. Due to both the solvent and the alkene having boiling points below the reaction temperature of 140 °C, some of the mechanistic studies focused on the closely related system that employs mesitylene and 1-dodecene (4), both of which have higher boiling points, to generate phenol 5.12 If a Friedel−Crafts-type mechanism were operational (Scheme 2A), then the electrophilic cation intermediate derived from the alkene should be trapped by arenes, such as 1,4-dimethoxybenzene, with comparable sterics and electronics to that of mequinol. In a competitive alkylation reaction using a 1:1 molar mixture of mequinol (1) and 1,4dimethoxybenzene (6) subjected to the typical Re-catalysis conditions in Scheme 1 but with 3 equiv of 1-octene, we failed to observe any alkylation of dimethoxybenzene; instead, only alkylation of mequinol was observed, along with recovery of 1,4-dimethoxybenzene (Scheme 3). This result was inconsistent with a Friedel−Crafts-type mechanism and also highlights that even if the phenol were required to generate the active catalyst and necessary intermediates a hydroxyl group must be attached to the arene in order for that arene to undergo alkylation. Consistent with preserving the operating

mechanism, these competitive alkylation reactions did not afford any bis-alkylated products. Furthermore, if the reaction involves a Friedel−Crafts type mechanism, then the rate of alkylation should be proportional to the electron density of the arene across a substrate series. The relative initial rate of product formation was measured for para-substituted phenols (R = F, MeO, Me) in order to probe this hypothesis (Figure 1). Alkylation of fluorophenol (8) was observed to be the fastest, followed by methoxyphenol (1), and finally methylphenol (6). Attempts to correlate these rates with Hammett parameters are shown in Figure 1. While no correlation is observed with the σpara parameters (Figure 1, right) that would reflect correlation with regards to the general electron density of the ring, the σmeta values do correlate with the rates of alkylation (Figure 1, left). The σmeta correlation illustrates that the rate increases with more electron withdrawing groups related to the site of alkylation, with fluoro being the most electron withdrawing in this series, followed by methoxy then methyl. This trend is inconsistent with a Friedel−Crafts type mechanism that would show increasing rates as the electron density increase on the carbon meta to the substituent being varied. B


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Organometallics

Re−π-allyl complex)13 or the vinyl C−D bonds.14 The observations shown in Scheme 4 are consistent with an irreversible hydrometalation process. If the mechanism involves C−H activation of the arene, then the C−H cleavage can be in one of three categories: (1) reversible, thus not turnover-limiting, (2) irreversible and turnover-limiting, or (3) irreversible but not turnover-limiting. This aspect was probed using deuterated-mequinol in the Recatalyzed alkylation (Scheme 5). To assess the former scenario, Scheme 5. Experiments Illustrating Reversibility of C−H Activation and Lack of a Large (≥3) Primary Kinetic Isotope Effect Figure 1. Relative initial rate of product formation measured after the induction period plotted versus: (left) σmeta value with R2 = 0.93 (correlation), and (right) σpara with R2 = 0.26 (lack of correlation).

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KINETIC ISOTOPE EFFECT AND DEUTERIUM LABELLING STUDIES We utilized a series of deuterium-labeled substrates in the Recatalyzed alkylation of mequinol in order to gain insight into the C−H bond-breaking events, including potential reversibility (Scheme 4). Using 1-octene labeled with deuterium(s) at Scheme 4. Deuterium-Labelling Experiments Using Various Alkene Substrates

we utilized 2,3,5,6-d4-1 and terminated the reaction at partial conversion (ca. 50% conversion) to recover the unreacted starting material and measure its deuterium content by NMR spectroscopy (Scheme 5A). If C−D cleavage is reversible, we expect depleted deuterium content in the recovered starting material relative to its original content, particularly at the site of alkylation (ortho to the hydroxyl group), for which the hydroxyl group from mequinol may serve as a potential proton source (vide infra). Indeed, the recovered mequinol was observed to have significant loss in deuterium content ortho to the hydroxyl group (ca. 26% H incorporation) relative to the loss observed at the meta positions (ca. 7% H incorporation), consistent with reversible C−H activation prior to the turnover-limiting step. The product isolated from that reaction mixture clearly illustrates similar levels of depletion at the

either the terminal (C1) or internal (C2) alkene position, or even at the allylic position (C3), all resulted in exclusive transfer of deuterium(s) to the product without scrambling or depletion of the deuterium content. Specifically, using 1,1-d2oct-1-ene (1,1-d2-2) with two deuteriums at the terminal alkene position provided product 1,1-d2-3, while products associated with loss of one or both deuteriums (i.e., 1-d1-3 or 1,1-d0-3) were not observed. Similarly, subjecting 2-d1-oct-1ene (2-d1-2) to the reaction conditions in the presence of mequinol provided product (2-d1-3) exclusively, without observing products associated with scrambling or loss of deuterium content (e.g., 2-d0-3). Using 3,3-d2-oct-1-ene (3,3d2-2) in the reaction provided exclusively 3,3-d2-3. All of these observations are consistent with a reaction pathway that does not involve reversible cleavage of the allylic C−D bonds (e.g., C


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°C for 24 h (Scheme 6). The recovered dimethoxybenzene was analyzed by NMR spectroscopy and revealed partial depletion

remaining ortho position (ca. 23% H incorporation), while the sites meta to the hydroxyl group reveal clear differences. The position ortho to the site of alkylation shows 10% H incorporation, comparable to levels observed in the recovered starting material. In contrast, the other site meta to the OH (para to the site of alkylation) reveals 23% H incorporation, significantly greater than the 7% observed in the recovered starting material. This observation is consistent with a process involving reversible C−H activation para to the site of alkylation; however, the catalyst appears to be unable to promote subsequent alkylation to the bis-alkylated product, which results in the selectivity for the monoalkylation product. Summarizing, reversible C−H activation occurs preferentially at the position ortho to the OH on the arene (Scheme 5A) where the depletion of deuterium is significantly greater at the ortho-position versus the meta-position on the recovered mequinol. The observation that the alkylation product obtained in the Scheme 5A has a higher degree of deuterium depletion at the sites meta to OH relative to those found in the recovered mequinol implies that the (undesired) C−H cleavage at the meta-site occurs preferentially on the monoalkylation product than on the starting material (mequinol). While C−H cleavage does occur to a small degree at the sites meta to the OH (Scheme 5A), alkylation is not observed meta on the OH on either the starting material or the monoalkylated product, suggesting that the alkylation regioselectivity is not arising solely from the C−H activation step but mostly due to the subsequent step(s). When reacting 2,3,5,6-d4-mequinol with 1-octene as shown in Scheme 5A,B, the newly formed methyl group in the product is a mixture of CH2D and CH3 groups as observed by quantitative 13C NMR, consistent with phenol OH and the arene C−H taking part in a reversible exchange process. This hypothesis is solidified when using OD-d1-mequinol with 1octene and observing deuterium incorporation in the newly formed methyl group in the product (Scheme 5C). We emphasize that the methyl group obtained in these experiment are exclusively CH3 or CH2D groups, and no evidence of generating a CD2H group was observed; these results are consistent with an irreversible hydrometalation process. The reversibility of the arene C−H cleavage event implies it is not the turnover-limiting step, and we solidified this conclusion using proof by contradiction logic. If C−H cleavage were turnover-limiting, then a large primary kinetic isotope effect (KIE) should be observed (typically greater than 3) in alkylation rates for protio vs deutero arene analogs when carried out in parallel experiments (Scheme 5D). Initial rates reveal a KIE of 1.39, which is inconsistent with C−H cleavage being turnover-limiting. A value slightly greater than one could be due to an equilibrium isotope effect, in which there is a preference of the resting state of the catalyst to be bound to mequinol over the d4-mequinol. Returning to the question of why 1,4-dimethoxybenzene does not get alkylated by 1-octene under typical reaction conditions (Scheme 2), we considered two potential reasons. Either the C−H activation necessitates an OH to be present on the arene. Alternatively, the C−H activation occurs, but one of the subsequent steps has an energy barrier too high to allow alkylation to occur. Toward discerning between the two scenarios, we carried out an analogous competitive alkylation process as shown in Scheme 2 but using a 1:1 mixture of mequinol and d4-1,4,-dimethoxybenzene in the presence of excess 1-octene with 3 mol % of Re2(CO)10 in toluene at 140

Scheme 6. Probing Necessity of a Hydroxyl Group for C−H Activation into the Arene

of its deuterium content, namely, ca. 10% H incorporation. This observation is consistent with reversible C−H activation into the arene and the alkylation selectivity arising from a subsequent step in the catalytic cycle. It should be emphasized that the deuterium depletion could be proceeding via a different mechanism or catalytic intermediate than that necessary for productive alkylation of the phenol. To provide further mechanistic insight, we performed several additional experiments with deuterated substrates (Scheme 7). KIE experiments using alkenes with one or more deuterium(s) at the C1-, C2-, or C3-position of 1-octene support the mechanism shown in Scheme 2B and disprove the one in Scheme 2C. Using 3,3-d2-oct-1-ene (3,3-d2-2) in a competition experiment with unlabeled oct-1-ene (2, 5 equiv of each alkene) did not afford an appreciable KIE (KIE = 1.18) as determined by quantitative 13C NMR analysis of the product mixture at full conversion (Scheme 7C). This result invalidates mechanisms proceeding via Re−π-allyl intermediates.13 A similar experiment using 2-d1-oct-1-ene (2-d1-2) once again results in the absence of a KIE (KIE = 1.01, Scheme 7B). The use of 1,1-d 2 -oct-1-ene (1,1-d 2 -2) in a one-pot intermolecular competition experiments afforded a small normal KIE of 1.50, as measured via NMR product ratios; this KIE was determined to be even smaller in independent parallel rate experiments (KIE = 1.22, Scheme 7A). These data are inconsistent with hydrometalation being turnover-limiting, as the rehybridization from sp2 to sp3 should afford an inverse secondary KIE (KIE < 1) and this was not observed. The lack of an inverse secondary KIE when using either 2d1-2 or 2,3,5,6-d4-1 indicates that C−C bond formation is not turnover-limiting. One should expect an inverse secondary KIE with H versus D isotopologues that have the isotope directly attached to the carbon undergoing rehybridization from sp2 to sp3, namely, the arene C−H ortho to the hydroxyl (Scheme 5D) and for alkenes with deuterium atom at C2 (Scheme 7B). Experimentally, neither of these two experiments revealed an inverse secondary KIE: the former gave a KIE of 1.39 and the later experiment gave KIE of 1.01. While rhenium carbonyl complexes are known to be able to cleave vinyl C−H bonds,14 the lack of a large primary KIE (>3) in any of the reactions in Scheme 6 with deuterated analogs of 1-octene rule out any C− H bond breaking events occurring on the alkene at C1-, C2-, and C3-positions in the turnover-limiting step; these results are consistent with mechanisms shown in Scheme 2B.

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KINETICS In order to gain further insight into the mechanism and its turnover-limiting step, we obtained kinetic data for the alkylation reaction in Scheme 1. We sought to use NMR spectroscopy to monitor the reaction as it would simultaD


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Organometallics Scheme 7. Determination of KIEs Using Various Deuterium-Labelled Alkenes

neously provide an opportunity to gain structural information into potential intermediates, the catalyst resting state, and offcycle species. The high temperature of this reaction together with the temperature limitation of NMR probes precluded us from carrying reactions at the typical 140−150 °C range. We were however able to use 1H NMR spectroscopy to monitor a representative reaction mixture at 130 °C containing mequinol (1 equiv), 1-octene (1.5 equiv), and Re2(CO)10 (2.5 mol %) in d8-toluene in a flame-sealed NMR tube. We observed an overall zero-order reaction profile as the concentration of product increased linearly. Consumption of each substrate (mequinol and 1-octene) revealed a linear decrease in their temporal concentration profiles, consistent with zero-order dependency. These observations imply one of two scenarios: (scenario 1) Both substrates are bound to the catalyst in its resting state prior to the turnover-limiting step. (scenario 2) One substrate is bound to the catalyst in its resting state, and the interaction with the second substrate does not occur until after the turnover-limiting transition state. In scenario 2, the turnover-limiting step could include ligand dissociation or catalyst reorganization (e.g., deaggregation of the catalyst or oxidative addition/reductive elimination events). One additional observation from Figure 2 is the non-zero x-intercept in the product formation linear fit which suggests an induction period, consistent with precatalyst Re2(CO)10 losing one or more CO ligands to coordinate substrate(s). If CO dissociation is required for precatalyst Re2(CO)10 to achieve the active catalyst, then CO would act as an inhibitor in the reaction, which should result in an inverse rate dependency in CO concentration. Indeed, upon comparing reactions with a finite headspace volume (closed vessel system) versus pseudoinfinite headspace (open vessel system), two clear observations were made. The closed reaction system, having higher CO concentration, had a slower rate of product formation and a longer induction period compared to the open reaction systems, which should have lower concentrations of CO. This comparison is illustrated in the time course data shown in the Supporting Information for reactions carried out at 140 °C using 1-dodecene (bp 214 °C) in mesitylene (bp 163 °C) instead of 1-octene (bp 122 °C) in toluene (bp 110

Figure 2. Temporal concentration profiles of 1-octene (2), mequinol (1), and alkylated product (3) monitored by 1H NMR recorded at 130 °C in the presence of 2.5 mol % Re2(CO)10 in d8-toluene.

°C) in which 5 mol % Re2(CO)10 was used to amplify the effect of CO on reaction rate. Initial rate kinetic data measured after the induction period were obtained at 150 °C to examine the rate of product formation as a function of catalyst loading. Varying the total catalyst loading (0.625 mol %, 1.25 mol % and 2.50 mol %) revealed a half order dependence (Figure 3). This fractional order dependency is consistent with the catalyst resting state having to deaggregate prior to the turnover-limiting transition state. Several likely scenarios exist, particularly since dinuclear (Re2Ln) and tetranuclear (Re4Ln) rhenium carbonyl complexes have been reported.15,16 The resting state catalyst could be dinuclear (ReLn)2 implying that the turnover-limiting step would feature a mononuclear form (ReLn). In an alternate scenario, the catalyst resting state could be tetranuclear (ReLn)4 implying that the turnover-limiting step would feature a dinuclear form (ReLn)2. Given that cleavage of Re−Re bonds has been reported to be mediated both by thermal and photochemical methods,17 it is not unreasonable to propose deaggregation via cleavage of a Re−Re bond under the reaction conditions presented herein. In fact, heating a 1:1 E


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Organometallics

were detected as well as other internal alkene isomers (cisand/or trans-2c and 2d) with a ratio of 1/2a/2b/(2c + 2d) = 1:0.37:0.28:0.52 (Scheme 8). Cleavage of the terminal vinyl C−H bond in terminal alkenes has previously been reported under photolysis conditions via the formation of complex 10, (R = Bu, δH of −14.7 ppm).14a Analysis of the hydridic 1H NMR region (ca. −5 to −20 ppm) reveals signals at δH of −14.4 and −17.31 ppm, each corresponding to less than 1% of the reaction mixture (relative to 1-octene). DOSY characterization reveals both hydrides correspond to diffusion coefficients D = 8.0 × 10−10 m2/s compared to 17.9 × 10−10 m2/s for the olefins, consistent with the hydride complexes being larger than 1-octene. The signal at δH −14.4 ppm is consistent with formation of bridging hydride complex 11, analogous to complex 10,14a which may be associated with the alkene isomerization described above. The signal at δH −17.3 ppm is characteristic of bridging hydrides in cyclic Re clusters, most likely that of cyclic trimer [Re(μ2-H)(CO)4]3 (12).18 Interestingly, 1H NMR spectra of reaction mixtures do not contain a signal at −14.4 ppm associated with bridging hydride complex 11, and alkene isomerization of 1-octene is minimal. Since we did not observe scrambling or depletion of deuterium content in products obtained when using deuterium-labeled alkenes at the C1-, C2-, or C3-position in Scheme 4, we propose the isomerization processes in Scheme 8 are not relevant to the on-cycle behavior for the alkylation of mequinol (vide inf ra). Toward gaining insight into the mass balance of the alkene isomerization process in Scheme 8, we carried out prolonged heating of a binary mixture of 1-octene and Re2(CO)10 for 24 h at 150 °C and used an external standard (1,2,4-trichlorobenzene) to determine the sum of octene isomers both at T = 0 and 24 h. The sum of all octene isomers at 24 h accounts for quantitative recovery relative to the initial 1-octene content, allowing us to rule out an alkene polymerization process as a significant decomposition pathway. Next we shifted our attention toward understanding how mequinol interacts with Re2(CO)10. Independent reaction of either methanol or phenol with Re2(CO)10 in the presence of trimethylamine-N-oxide, an oxidant used to remove CO ligands from metal carbonyls,19 provides bridged oxide (CO)nRe−(μ2-OR)(μ2-H)−Re(CO)n or their related dehydrogenated forms (CO)nRe−(μ2-OR)m−Re(CO)n (m = 2 or 3, R = Ph, Me, H).15 Tetrameric Re clusters as well as linear oligomers have been characterized in the literature by both NMR spectroscopy and X-ray crystallography.16 It is also known that photolysis of ethereal solutions of Re2(CO)10 in the presence of H2O results in quantitative formation of tetrameric Re4(CO)12(OH)4 via a H3Re3(CO)12 intermediate (12).16q−s,17c By analogy, we wondered if the phenol in our reaction could participate in a similar process to generate either dimeric (CO) 4 Re(μ 2 -OAr) 2 Re(CO) 4 or tetrameric Re4(CO)12(OAr)4. Using a combination of 1D and 2D NMR spectroscopy including DOSY NMR, we observed five different compounds were formed when characterizing a 4:1 mixture of mequinol

Figure 3. Effect of total catalyst concentration on the initial rate of product formation in the alkylation of mequinol using 1-octene at 150 °C in toluene.

mixture of isotopically labeled 185Re2(CO)10 and 187Re2(CO)10 in octane for 16 h at 150 °C provides nearly complete conversion to the crossover product 185Re187Re(CO)10 which is consistent with cleavage of the Re−Re bond.17a

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NMR STUDY OF BINARY MIXTURES We were interested in gaining further insight into potential intermediates and the resting state of the catalyst; however, due to experimental limitations in obtaining detailed 2D NMR at 130 °C, as well as signal broadening in the 1H NMR data at those temperatures, we relied on data acquisition at (or around) room temperature on reaction mixtures as well as binary mixtures. We prepared binary mixtures of Re2(CO)10 (1 equiv) with either 1-octene (4 equiv) or mequinol (4 equiv) in d8-toluene solutions and heated those samples to 150 °C for 2 h, followed by cooling to 25 °C for analysis via NMR spectroscopy (Scheme 8 and Scheme 9). The binary mixture of Scheme 8. Experiment Probing Binary Reactivity of 1Octene and Re2(CO)10

1-octene and Re2(CO)10 revealed that olefin isomerization occurs; both cis- and trans-2-octene (2a and 2b, respectively)

Scheme 9. Experiments Probing Binary Reactivity of Mequinol and Re2(CO)10

F


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Figure 4. 1H NMR of 4:1 mequinol/Re2(CO)10 mixture in d8-toluene at 25 °C after heating for 2 h at 150 °C.

Table 1. 1H and 13C NMR Chemical Shift (ppm) of the Mequinol Ligands of Complexes 14−16 and Mequinol (1) in d8Toluene along with the Diffusion Coefficient Obtained from DOSY Spectrum at 25 °C assignment C-1 C-2 C-3 C-4 C-5 H-2 H-3 H-5

complex 14 (2:1 ligand ratio)c

complex 15

mequinol (1)

154.6 114.5 118.3 161.7 54.7

159.7 85.6 83.0 159.8 54.7

149.9 114.7 115.9 153.7 54.8

6.77 4.28 7.47 5.97 3.36

6.65a

4.13 (dd, 4.9, 3.2 Hz)

6.64 (d, 9.0 Hz)

6.97 (d, 9.0 Hz)

5.07 (dd, 4.9, 3.2 Hz)

6.56 (d, 9.0 Hz)

3.31 (s)

3.31 (s)

3.35 (s)

(d, 9.0 Hz) (d, 7.7 Hz) (d, 9.0 Hz) (d, 7.7 Hz) (s); 2.58 (s)

−10.7 (s)b

hydride D (× 10−10 m2/s)

complex 16

154.0; 134.1 114.5; 85.6 119.5; 85.0 159.3; 149.4 54.9; 56.9

4.80

5.98

6.92

11.5

COSY NMR experiments reveal this signal is coupled to the doublet at δH 6.97 but due to being obstructed by signals associated with mequinol, the coupling constant could not be measured. bWhile the signal at δH −17.3 ppm has a comparable diffusion coefficient (D) as the signals assigned to complex 16 according to the DOSY spectrum, this signal was assigned to complex 12. cSee Scheme 9 for the atom numbering definition. a

and Re2(CO)10 in d8-toluene that had been heated to 150 °C prior to its characterization at, or near, 25 °C (Scheme 9 and Figure 4) due to instrumental limitations precluding characterization at 150 °C. While the equilibrium distribution and composition observed at 25 °C may differ significantly from those present under elevated reaction temperature, these experiments nonetheless provided valuable structural information. We observed two metal hydrides complexes devoid of mequinol ligands (12 and 13), as well as three Re complexes containing mequinol ligands (14−16) based on NMR analysis (Table 1), in addition to the majority of the mequinol remaining nonligated to Re. 1 H NMR revealed a signal at δH −5.61 ppm, characteristic of a terminal rhenium hydride complex, specifically HRe(CO)5

(13);20 additionally, a signal characteristic of a bridging hydride from a cyclic Re clusters was observed at δH −17.3 ppm, likely that of cyclic trimer 12, [Re(μ2-H)(CO)4]3, also observed in the binary mixture experiment with 1-octene and Re2(CO)10.18 DOSY NMR experiments are in agreement with the signal at δH −17.3 ppm being associated with a structure of significantly larger size than that of the complex associated with the signal at −5.6 ppm (HRe(CO)5, 13). HMBC NMR analysis reveals H−C coupling between each of the above Rehydride signals to a set of CO ligands. Specifically, the signal at δH of −5.6 ppm is correlated to two 13C signals, δC 182.3 (2JCH = 7.4 Hz) and 182.7 (2JCH = 7.5 Hz) ppm, consistent with the two inequivalent CO ligands in HRe(CO)5. The signal at δH of −17.3 ppm is correlated to two 13C signals, δC 179.3 (2JCH < 2 G


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Organometallics Hz) and 181.2 (2JCH < 2 Hz), consistent with the two inequivalent CO ligands in cyclic trimer 12, [Re(μ2− H)(CO)4]3. The three complexes that contain mequinol as a ligand, tentatively assigned as 14−16 (see Supporting Information). Complexes 14 and 15 are not associated with hydridic signals based on DOSY NMR, whereas complex 16 has the same diffusion coefficient as hydridic signals at δH −10.7 ppm and −17.3 ppm (either by coincidence or due to the complex truly having a hydride). Since the signal at δH −17.3 ppm was also observed in mixtures not containing mequinol, namely, the mixture of 1-octene and Re2(CO)10 described above and assigned to trimeric [HRe(CO)4]3 (12), we do not believe this signal is associated with a mequinol derived complex. Complex 16 is associated with a bridging rhenium hydride signal appears (δH −10.72 ppm) which is correlated to four 13C signals, δC 187.4, 184.5, 184.0, 180.6 ppm (all with 2JCH < 2 Hz), which are presumably CO ligands bound to Re. While the structures of 14−16 could not be conclusively determined, tentative proposals are shown in the Supporting Information along with a more detailed discussion. Complex 15 is the second most abundant of these complexes and features one type of mequinol ligand whose NMR signals are minimally perturbed compared to nonligated mequinol (1). We propose the mequinol is oxygen bound to the Re instead of C-bound which would strongly perturb the 1H and 13C chemical shifts and coupling constants associated with mequinol.15,21 In contrast, the most abundant complex (14) features two types of chemically inequivalent mequinol ligands with a 2:1 ligand stoichiometry, implying a total of at least three mequinol ligands on the complex. The major set of mequinol ligand signals in complex 14 are quite similar to that found in complex 15, suggesting similar oxygen-bound coordination,15 while the minor set of mequinol ligand signals of 14 differ significantly as evidenced by the dramatic upfield shift in the 13C signals signal for C2 and C3 of the arene ring (see Scheme 9 for the atom numbering definition), namely ca. δC 84 ppm in 15 versus δC 115 ppm in mequinol, 1. The chemical shift of these types of mequinol ligands is suggestive of a cationic Re complexed with the arene via the π-system in a symmetrical fashion (e.g., η5- or η6-binding, or rapidly equilibrating η2-binding relative to the NMR time scale). This type of Re-arene interaction provides increased electron density, which not only results in the dramatic upfield shift in the 13C signals signal for C2 and C3, but also in a decreased magnitude of the 3JHH coupling for the hydrogens at those ring positions, while increasing magnitude of the 4JHH meta coupling through Re. The minor ligand in complex 14 displays chemical shift and coupling constant effects (ca. δC2/C3 85 ppm, 3JH2H3 = 7.7 Hz) resembling those found in complex 16 (ca. δC2/C3 84 ppm, 3JH2H3 = 4.9 Hz) that features similar Re− arene π-system interaction, in contrast to the unligated mequinol (ca. δC2/C3 115 ppm, 3JH2H3 = 9.0 Hz).21 Rhenium complexes featuring an η5- and/or η6-bound phenolate or phenol have been characterized in the literature both by NMR spectroscopy and X-ray crystallography, and display related chemical shift effects (see the Supporting Information for complexes displaying similar chemical shift and coupling constant effects to those observed with 14 and 16 upon η5and/or η6-binding to Re).21 On the basis of the diffusion coefficient of each Re complex in the mixture from Scheme 8 and that smaller diffusion coefficients are associated with larger molecular weight compounds, we propose that complex 14 has

the largest molecular weight, followed by 15 and then 16 which should have a molecular weight similar to that of 12 (898 g/mol) since they have comparable diffusion coefficients, and finally HRe(CO)5 (13, 327 g/mol, D = 13.4 × 10−10 m2/ s). Since no alkylation of mequinol occurs when reactions are carried out at 115 °C, we compared spectral data of 4:1 mixtures of mequinol and Re2(CO)10 that were obtained via either heating to 115 or 150 °C, and the only difference was that complex 16 was not observed in the 115 °C treated sample. This observation could suggest that compound 16 is derived from a catalytically relevant intermediate. When a reaction mixture (1 equiv of mequinol, 1.5 equiv of 1-octene, and 30 mol % of Re2(CO)10 in d8-toluene) was heated to partial conversion at 130 °C and subsequently analyzed at 25 °C by NMR spectroscopy, we observed complexes that were previously identified in binary mixtures. Specifically, the reaction mixture contained HRe(CO)5 (13), complexes 14 and 15, and [HRe(CO)4]3 (12), while no signals associated with complex 11 were observed. Only the hydride at −10.7 ppm associated with complex 16 was observed, while the remaining expected signals for 16 were obscured by other resonances from 1-octene. No evidence of alkene coordination to the Re was observed, consistent with no complexation occurring before the turnover-limiting step. Analysis of the NMR data of a reaction mixture at 130 °C did reveal the presence of HRe(CO)5 (13) and [HRe(CO)4]3 (12) as well as signals associated with mequinol ligands different than unligated mequinol (1); unfortunately, due to signal broadening, limited sensitivity, and lack of 2D NMR data we were not able to identify their structure. At this point all the data is consistent with a mechanism outlined in Scheme 2B; while we already discussed reasons for ruling out Scheme 2A,C above, we turn the discussion as to why many variations based on Scheme 2D can be ruled out. If the mechanism in Scheme 2D was operational, then the lack of a primary KIE when using d4-1 versus 1 implies that the third step could not be turnover-limiting; similarly, the lack of an inverse KIE when using either d4-1 or 1,1,-d2-2 versus 2 rule out the second step as the turnover-limiting (consistent with the lack of evidence for a Re complex containing an alkene when analyzing reaction mixtures by NMR). If the fourth step in Scheme 2D was turnover-limiting, then reaction mixtures would contain a buildup of an intermediate that has the new C−C bond between the octyl side chain and the arene carbon ortho to the OH, which we did not observe. While a positive order in 1-octene would be expected if the first step Scheme 2D was turnover-limiting (unless the turnover-limiting step requires ligand dissociation from the Re complex prior to alkene coordination). While we cannot completely rule out all variations of Scheme 2D from operating, we do have evidence for specific steps of the catalytic cycle associated with Scheme 2B (e.g., reversibility in the C−H activation), unlike that for Scheme 2D. Additionally, specific examples from the substrate scope (vide infra) showcase examples for which alkylation can occur para instead of ortho to the OH when using gemdisubstituted alkenes for the alkylation of phenol,5a ruling out the possibility of a cyclometalation pathway illustrated in Scheme 2D.

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PROPOSED REACTION MECHANISM Arriving at a detailed catalytic cycle with structures for each intermediate is rendered difficult due to the fact that (1) H


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Organometallics

Subsequent reductive coupling from 23 generates product 3. The fact that alkylated product 3 was not observed when radical inhibitors such as TEMPO or ABNO were present (10 mol %) when attempting the Re-catalyzed alkylation of mequinol with 1-octene at 140 °C in toluene for 24 h is consistent with the proposed hydrometalation proceeding via a radical pathway.23 This net hydroarylation reaction features catalyst deaggreation as the turnover-limiting step which is in contrast with Rhcatalyzed hydroarylation reactions in which the reductive elimination is turnover-limiting and C−H activation is reversible,24 also in contrast to numerous Pd and Rh C−H activation functionalization methods that feature C−H cleavage as the turnover-limiting step.25 Finally we note the induction period observed in this Re-catalysis is associated with CO dissociation from precatalyst Re2(CO)10 to open one or more coordination sites in order to bind mequinol (18) and generate the resting state of the catalyst. Several subtle details of this Re-catalysis require elaboration, specifically aspects of potential alkene isomerization and the role of the OH on the arene, both of which are discussed below. Scheme 11 illustrates how alkene isomerization and migration may occur in reactions where mequinol is absent,

numerous Re complexes were observed both in mixtures derived from the reaction and those from binary component experiments; (2) not all Re complexes could be structurally determined (e.g., complexes 14−16), and (3) signal broadening and limited sensitivity in the spectral data collected on reaction mixtures at 130 °C made structural assignment and correlation to the signals observed at 25 °C challenging. Nonetheless, we propose a catalytic cycle shown in Scheme 10 consistent with the collection of experimental evidence Scheme 10. Proposed Catalytic Cycle for the Re-Catalyzed Alkylation of Phenols

Scheme 11. Rationalization for Alkene Isomerization in the Absence of Mequinol

presented herein. From the isotopic labeling experiments, it is clear that C−H activation is reversible, and the net hydrometalation process on the alkene is irreversible.22 The NMR spectroscopic characterization of binary mixtures as well as a reaction mixture highlight that complexation of mequinol to Re is feasible before the turnover-limiting step. On the basis of the zero-order overall kinetics of the reaction and the fractional order in catalyst, we suggest that C−H activation of mequinol occurs first, directed by the OH group preferentially to the ortho-position, followed by the turnover-limiting step of deaggregating catalyst resting state 19. Subsequent to the turnover-limiting step, a net hydrometalation process occurs. If hydrometalation was proceeding via direct insertion into the alkene, then sterics would predict formation of the linear alkylation product, which is not observed. In contrast, the observed branched selectivity in the net hydroarylation process is consistent a radical rebound mechanism proceeding via a radical pair (Scheme 2B, bottom pathway) in which generation of a 2° akyl radical is preferred over a 1° one. Net hydrometalation of alkenes via a radical pair mechanism is known for HMn(CO)5,10b,c as well as other metal carbonyl hydride complexes.10d Hydrogen atom transfer to the C1position of the terminal alkene affords a radical pair consisting of 21 and 22. Subsequent geminate recombination of the radical pair provides the net hydroarylation product 23, a Re complex with both an octyl and an arene as ligands (23).

that is, the binary mixture of 1-octene and Re2(CO)10 shown in Scheme 8. In the absence of mequniol to act as a ligand on Re, thermal homolysis of Re2(CO)10 generates •Re(CO)5 (24) which could promote isomerization via intermediate 25 via a hydrogen atom transfer mechanism (Scheme 11A). Repeating the process starting from 2-octene 2a or 2b could lead to 3octene 2c. In contrast, when mequinol is present, minimal Re2(CO)10 is present since the catalyst resting state features mequinol ligated to Re, thus attenuating isomerization pathway shown in Scheme 11A. Instead, mequinol-ligated rhenium hydride (26) can be added to 1-octene to provide a radical pair consisting of 27 and 28 (Scheme 11B) that can either undergo subsequent radical recombination to the hydrometalated intermediate (29) or subsequent hydrogen atom abstraction to regenerate an alkene, such as 1-octene or isomeric 2-octene with either trans- or cis-geometry (2a or 2b, respectively). Since no evidence of reversible hydrometalation was observed in Scheme 4, we conclude that the ligand(s) attached to the Re (either CO and/or mequinol depending on the reaction condition), may influence the reversibility or selectivity of either reaching hydrometalated intermediate 29 or regenerI


Article

Organometallics ation of an alkene. While β-hydride elimination is known occur with many transition metals (e.g., Pd, Rh, etc.), we note that this process is known to be disfavored for Re.26 Additionally, if the reductive coupling from intermediate 29 to generate product 3 is fast relative to β-hydride elimination back to octene derivatives, then we do not expect to observe evidence of reversibility in the hydrometalation (including alkene isomerization). Finally, we turned our attention to several interesting examples from the substrate scope initially reported by Takai5a and rationalize the unexpected product outcome. Scheme 12

consistent with HAT to the alkene affording an alkyl radical that if stabilized has a lifetime long enough to allow for the rhenium in intermediate 22 to migrate to the sterically favored para-position on the arene prior to radical recombination (intermediate 23) and subsequent reductive coupling to the alkylated product (e.g., 34 and 37). Finally, since C−H activation occurs preferentially but not exclusively at the position ortho to the OH (see Scheme 5A); thus the exclusive ortho-regioselectivity for alkylation when using 1-octene must arise from a step subsequent to the C−H activation, likely related to the catalyst deaggreation step in which the barrier for meta-rhenated-arene is energetically destabilized relative to its ortho-isomer.

Scheme 12. Rationalizing the Alkylation Products Observed when Using Gem-Disubstituted Alkenes

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CONCLUSIONS The mechanism of a rhenium-catalyzed monoalkylation of phenols using terminal alkenes was studied. The results suggest that the reaction does not proceed via a Friedel−Crafts type pathway but instead proceeds via a reversible C−H activation, then a net hydroarylation process. The branched hydroarylation product is accounted by the hydrometalation process proceeding via a radical rebound mechanism of hydrogen atom transfer to the alkene, followed by geminate recombination of the radical pair. The turnover-limiting step is proposed to be catalyst deaggregation of the resting state dimer that contains mequinol ligands followed by the irreversible hydrometalation 2-step process and subsequent reductive coupling affords selectively the mono-ortho-alkylated product. We propose the resting state of the catalyst contains mequinol ligands as supported by the kinetics and various NMR experiments. Interestingly, the kinetic dependence on catalyst was found to be half-order, indicating that the resting state of the catalysts is a dimeric form of that found in the turnover-limiting transition state; presumably, cleavage of this dimer necessitates the high temperatures required for the transformation. This requirement for high temperature is the major drawback of this highly selective and high yielding reaction. The mechanistic insight obtained from this study suggests designing catalysts able to undergo deaggregation at lower temperatures could improve catalysis. Figure 1 illustrates that the rate of alkylation is dependent on the substituent on the phenol. This is rationalized due to the phenol derivative being a ligand on the Re complex undergoing the turnover-limiting deaggregation step; thus, the electronics of the phenol could influence the energy barrier of this process. The data supporting that phenol is a ligand on the Re complex in the turnover-limiting is based on the kinetics displaying zero-order in phenol and the observation that the arene C−H activation is reversible (Scheme 5A). Deaggregation of the active catalyst may be influenced through innovative ligand design or heterobimetallic systems to modulate the strength of the bond associated with deaggreation process.27 We hope this work will inspire and provide direction for future research to address current limitations of this remarkably selective and convergent catalytic method to access ortho-monoalkylated phenols.

illustrates how the use of phenol with different alkenes can generate either ortho- or para-functionalization depending on the alkene. While the use of 1-octene provides orthofunctionalization of phenol to generate 33 in 76% yield, para-alkylation product 34 (and 4% of the ortho-product, not shown) is obtained when using closely related 2,2-disubsutited alkene 35. We rationalize this explanation based on our mechanistic proposal that HAT to the alkene generates an alkyl radical intermediate, the latter example generates a radical (3° radical) that is significantly more stabilized compared to the former example (2° radical). The more stable the radical, the less reactive it should be; therefore, providing more time for intermediate Ar−[Re]• (22) to undergo potential rearrangement (Re-migration from the ortho to the para-site relative to the OH), in turn allowing the formation of the sterically favored para-product instead of the ortho-product. Similarly, HAT to diene 36 would produce a stabilized allylic radical that can react via the sterically more accessible C4carbon instead of the C2-carbon to generate product 37. The formation of para-substituted products 34 and 37 clearly enable us to rule out Scheme 2D since cyclometalation process would proceed via a closed transition state only able to generate ortho-susbstitution products, decisively placing the proposed mechanism in Scheme 10 as the operating mechanism. The role of the OH on the arene clearly facilitates the C−H activation at the ortho-position (see recovered mequinol in Scheme 5A), yet the examples from Scheme 12 clearly highlight that alkylation is not solely directed to the orthoposition, since para-alkylation products can be obtained when using gem-disubstituted alkenes. These observations are

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EXPERIMENTAL SECTION

General Experimental Details. Unless otherwise noted, all reactions were performed in an N2-filled glovebox using 10 mL microwave vials (Biotage Microwave Reaction Vials, part number 351521) or 30 mL microwave vials (Biotage Microwave Reaction Vials, part number 354833) equipped with a magnetic stir bar (Biotage Stirbars, part number 355543) and sealed using a Biotage J


Article

Organometallics Microwave Cap with Septum (part number 352298) with the aid of a Biotage microwave vial crimper (part number 353671). Reaction vials were heated by being placed inside a heated metal block (IKA Dry Block Heater DB 4.5, part number 0004469000, for round-bottom tubes (15/16 mm), pore size: Ø 17.5 mm, no. of holes: 12, hole depth: 48.4 mm, block dimensions (width × depth × height): 95 × 76 × 51 mm). Heating of reaction mixtures was performed using a temperature-controlled hot plate equipped with stirring and a thermocouple. Evaporation and concentration in vacuo was done using variable vacuum via a vacuum controller (ca. 400−40 mmHg). Column chromatography was done using a Teledyne ISCO CombiFlash Rf+ chromatography system using prepacked single-use silica packed cartridges (RediSep Rf Gold Normal-Phase Silica, 20− 40 μm average particle size, 60 Å average pore size). Materials. Reagents were purchased (reagent-grade) from commercial suppliers and used without further purification, unless otherwise described. Anhydrous solvents (toluene and mesitylene) were obtained from Sigma-Aldrich as part of their Sure/Seal bottles product line. 1-Octene (98%) was purchased from Acros; 1-dodecene (≥99.0%, GC) was purchased from Sigma-Aldrich, Re2(CO)10 (sublimed) was purchased from Acros. 1,1-d2-Oct-1-ene (98% purity, > 99% d2-content) was purchased from Cambridge Isotope Laboratories. 3,3-d3-Oct-1-ene (98% purity) and 2,3-d2-4-methoxyphenol (≥98.0% purity) were purchased from ALSACHIM. 2-d1Oct-1-ene (>97% purity, 99.5% d1-content) was purchased from CDN Isotopes. 2,3,5,6-d4-4-Methoxyphenol was purchased from CDN Isotopes (98% purity, >99% d4-content) as light beige solid and repurified by column chromatography (silica gel, gradient 0−20% EtOAc in hexanes to afford a white solid, ≥99% purity by NMR, >99% d4-content by NMR) prior to using. Instrumentation. Proton nuclear magnetic resonance (1H NMR) spectra and carbon nuclear magnetic resonance (13C NMR) spectra were recorded at 25 °C (unless stated otherwise). The NMR spectrometer used for the KIE measurements was a Bruker AVANCE III HD 600 MHz NMR spectrometer equipped with a 5 mm BBO CryoProbe Prodigy. The NMR spectrometers used for the characterization of the Re complexes and binary mixtures were a Bruker AVANCE III HD 600 MHz NMR spectrometer and Bruker AVANCE III HD 500 MHz equipped with a 5 mm BBO SmartProbe. The NMR spectrometer for reaction time course data at 130 °C was the Varian VNMRS 600 MHz NMR spectrometer equipped with a 5 mm OneProbe. All other NMR characterization performed on a Bruker AVANCE III HD 500 MHz equipped with a 5 mm BBO probe. Chemical shifts for protons are reported in parts per million downfield from tetramethylsilane and are referenced to residual proton of the NMR solvent according to values reported in the literature.28 Chemical shifts for carbon are reported in parts per million downfield from tetramethylsilane and are referenced to the carbon resonances of the NMR solvent. The solvent peak was for samples in CDCl3 was referenced to 7.26 ppm for 1H and 77.0 ppm for 13C, while for samples in CD2Cl2, it was referenced to 5.32 ppm for 1H and 54.0 ppm for 13C. Analysis of reaction mixtures and quantification of starting material and product was done using HPLC or NMR analysis. NMR KIE Measurements. The ratio of protonated versus deuterated carbon was determined by quantitative 13C NMR by using inverse gated decoupling with calibrated 90 deg pulse, and d1 was set to 120 s (d1 > 10 T1, where d1 is the total recycling delay and T1 is the longitudinal (or spin−lattice) relaxation time decay constant), and the transmitter frequency was place at the middle of the protonated and deuterated carbon. Typical Procedure for the Synthesis of 3.5a In an N2-filled glovebox, a 10 mL microwave vial was equipped with a magnetic stir bar and charged with Re2(CO)10 (0.020 g, 0.031 mmol), followed by 4-methoxphenol (0.155 g, 1.25 mmol), followed by addition of toluene (0.625 mL), and finally 1-octene (0.297 mL, 0.212 g, 1.87 mmol) was added to the mixture. The vial was sealed using a microwave cap with the aid of a microwave vial crimper. The sealed microwave vial was placed into a 140 °C preheated metal block. The reaction mixture was aged for 24 h accompanied by rotary stirring

prior to being cooled to room temperature. Column chromatography (silica gel, gradient 0−20% EtOAc in hexanes) afforded 3 (0.285 g, 97%). 1H NMR (600 MHz, CD2Cl2) δ 6.72 (d, J = 3.0 Hz, 1H), 6.68 (d, J = 8.6 Hz, 1H), 6.59 (dd, J = 8.6, 3.0 Hz, 1H), 4.62 (s, 1H), 3.02 (hept, J = 7.0 Hz, 1H), 1.62 (dddd, J = 13.2, 9.9, 7.5, 5.1 Hz, 1H), 1.57−1.50 (m, 1H), 1.34−1.22 (m, 8H), 1.20 (d, J = 6.9 Hz, 3H), 0.87 (t, J = 7.0 Hz, 3H). 13C{1H} NMR (151 MHz, CD2Cl2) δ 154.5, 147.6, 135.5, 116.3, 113.7, 111.5, 56.1, 37.7, 33.1, 32.4, 30.0, 28.2, 23.2, 21.3, 14.4. Spectral characterization is consistent with that reported in ref 5a. Typical Procedure for the Synthesis of 5.5b In an N2-filled glovebox, a 10 mL microwave vial was equipped with a magnetic stir bar and charged with Re2(CO)10 (0.020 g, 0.031 mmol), followed by 4-methoxyphenol (0.155 g, 1.25 mmol), followed by addition of toluene (0.625 mL), and finally 1-dodecene (0.416 mL, 0.315 g, 1.87 mmol) was added to the mixture. The vial was sealed using a microwave cap with the aid of a microwave vial crimper. The sealed microwave vial was placed into a 140 °C preheated metal block. The reaction mixture was aged for 24 h accompanied by rotary stirring prior to being cooled to room temperature. Column chromatography (silica gel, gradient 0−20% EtOAc in hexanes) afforded 5 (0.330 g, 90%). δ 1H NMR (600 MHz, CD2Cl2) δ 6.71 (d, J = 3.0 Hz, 1H), 6.68 (d, J = 8.6 Hz, 1H), 6.59 (dd, J = 8.6, 3.0 Hz, 1H), 4.56 (s, 1H), 3.73 (s, 3H), 3.01 (hept, J = 7.0 Hz, 1H), 1.65−1.57 (m, 1H), 1.57− 1.49 (m, 1H), 1.33−1.22 (m, 16H), 1.20 (d, J = 6.9 Hz, 3H), 0.88 (t, J = 7.0 Hz, 3H). 13C{1H} NMR (151 MHz, CD2Cl2) δ 154.5, 147.6, 135.5, 116.3, 113.6, 111.4, 56.1, 37.7, 33.1, 32.5, 30.4, 30.2, 30.2, 30.2, 29.9, 28.3, 23.3, 21.3, 14.5. Spectral Data of Deuterated Starting Materials. Spectra Data for 1,1-d2-2. 1H NMR (500 MHz, CD2Cl2) δ 5.85−5.80 (m, 1H), 2.06 (q, J = 7.1 Hz, 2H), 1.44−1.36 (m, 2H), 1.35−1.25 (m, 6H), 0.90 (t, J = 7.0 Hz, 3H). 13C{1H} NMR (126 MHz, CD2Cl2) δ 139.7, 113.9 (p, J = 23.7 Hz), 34.4, 32.429.7, 29.5, 23.3, 14.5. Spectral Data for 2-d1-2. 1H NMR (500 MHz, CD2Cl2): δ 5.01− 4.98 (m, 1H), 4.94−4.92 (m, 1H), 2.05 (t, J = 7.2 Hz, 2H), 1.44− 1.24 (m, 8H), 0.90 (t, J = Hz, 3H). 13C{1H} NMR (125 MHz, CD2Cl2): δ 139.61 (t, J = 23.1 Hz), 114.31, 34.38, 32.43, 29.63, 29.53, 23.32, 14.51. Spectral Data for 3,3-d2-2. 1H NMR (500 MHz, CD2Cl2): δ 5.83 (dd, J = 10.2, 17.2 Hz, 1H), 5.00 (d, J = 17.2 Hz, 1H), 4.93 (d, J = 10.2, Hz, 1H), 1.41−1.24 (m, 8H), 0.90 (t, J = 6.1 Hz, 3H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 139.88, 114.51, 33.75 (p, J = 19.2 Hz), 29.50, 29.47, 23.32, 14.51. Spectral Data for 2,3,5,6-d4-1. 1H NMR (500 MHz, CD2Cl2): δ 5.48 (br s, 1H), 3.76 (s, 3H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 154.12, 150.10, 116.23 (t, J = 24.2 Hz), 115.07 (t, J = 24.3 Hz), 56.32. Synthesis of OD-d1-Mequinol (OD-d1-1). To a vial containing mequinol (1, 2.10 g, 16.9 mmol) was added d4-methanol (>99.8% d4content, 15 mL) at 25 °C, and the mixture was attached to a rotary evaporator and the vial partially submerged in a 40 °C water bath. The mixture was mixed for 5 min via spinning the sample with the aid of the rotary evaporator without applying vacuum, followed by removal of the solvent in vacuo. This H−D exchange cycle was repeated a total of five times to obtain OD-d1-1 (2.1 g, quantitative) as a white solid. 1H NMR (500 MHz, CD2Cl2): δ 6.84−6.77 (m, 4H), 3.77 (s, 3H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 154.14, 150.16, 116.53, 115.49. Synthesis of 2,3,5,6-d4-6. To a 30 mL microwave vial containing a PTFE stirbar was added 2,3,5,6-d4-mequinol (2.40 g, 18.7 mmol) followed by MeCN (20 mL), Cs2CO3 (6.10 g, 18.7 mmol), and finally MeI (5.83 mL, 13.3 g, 13.3 mmol) at 25 °C. The vial was sealed using a microwave cap with the aid of a microwave vial crimper. The sealed microwave vial was placed into a metal block and heated to 55 °C for 30 h accompanied by rotary stirring. The mixture was cooled to rt and diluted with CH2Cl2, the organic phase washed with H2O (2×), dried over Na2SO4, filtered, and the solvent removed in vacuo. Column chromatography (silica gel, gradient 0−30% EtOAc in hexanes) afforded 2,3,5,6-d4-6 (2.11 g, 79%). 1H NMR (500 MHz, CD2Cl2): δ 3.75 (s, 6H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 154.32, 114.76 (t, J = 24.3 Hz), 56.16. K


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Organometallics

integration of the CH and CD signals at δ 32.4 (1.00C) and 32.0 (0.99C), respectively. KIE Using 3,3-d2-Oct-1-ene. The ratio of 3 to 3,3-d2-3 was determined to be 1.18 via quantitative 13C{1H} NMR (151 MHz, CD2Cl2) via integration of the CH2 and CD2 signals at δ 37.7 (1.00C) and 36.8 (0.85C), respectively. Reaction of OD-d1-1 with 1-Octene. In an N2-filled glovebox, a 10 mL microwave vial was equipped with a magnetic stir bar and charged with Re2(CO)10 (0.023 g, 0.035 mmol), followed by OD-d1-1 (0.157 g, 1.26 mmol) and toluene (0.628 mL); finally, 1-octene (0.295 mL, 0.211 g, 1.88 mmol) was added to the mixture. The vial was sealed using a microwave cap with the aid of a microwave vial crimper. The sealed microwave vial was placed into a 140 °C preheated metal block. The reaction mixture was aged for 24 h accompanied by rotary stirring prior to being cooled to room temperature. Column chromatography (silica gel, gradient 0−20% EtOAc in hexanes) afforded dn-3 (mixture of deuterium label content, see Scheme 5C). The mixture was characterized by quantitative 13 C{1H} NMR (151 MHz, CD2Cl2) via integration of the CH2 and CD2 signals at δ 20.8 (s, 1.00C) and 20.5 (t, J = 19.2 Hz, 0.19C), respectively; additionally, we did not observe evidence for a CD2H group (lack of pentet around 20 ppm).

Reaction of Mequinol with 1,1-d2-Oct-1-ene. In an N2-filled glovebox, a 10 mL microwave vial was equipped with a magnetic stir bar and charged with Re2(CO)10 (0.020 g, 0.031 mmol), followed by 4-methoxyphenol (0.155 g, 1.25 mmol) and then toluene (0.625 mL); finally, 1,1-d2-oct-1-ene (0.297 mL, 0.212 g, 1.87 mmol) was added to the mixture. The vial was sealed using a microwave cap with the aid of a microwave vial crimper. The sealed microwave vial was placed into a 140 °C preheated metal block. The reaction mixture was aged for 24 h accompanied by rotary stirring prior to being cooled to room temperature. Column chromatography (silica gel, gradient 0−20% EtOAc in hexanes) afforded 1,1-d2-3 (0.280 g, 94%). 1H NMR (600 MHz, CD2Cl2) δ 6.81 (d, J = 2.9 Hz, 1H), 6.73 (d, J = 8.6 Hz, 1H), 6.65 (dd, J = 8.6, 2.9 Hz, 1H), 3.80 (s, 3H), 3.11 (q, J = 7.2 Hz, 1H), 1.74−1.63 (m, 1H), 1.63−1.55 (m, 1H), 1.42−1.24 (m, 8H), 1.22 (d, J = 7.2 Hz, 1H), 0.98−0.91 (m, 3H). 13C{1H} NMR (151 MHz, CD2Cl2) δ 154.4, 148.0, 136.0, 116.6, 114.0, 111.7, 56.4, 37.8, 33.0, 32.6, 30.2, 28.4, 23.4, 20.9 (p, J = 29.2 Hz), 14.6. Reaction of Mequinol with 2-d2-Oct-1-ene. In an N2-filled glovebox, a 10 mL microwave vial was equipped with a magnetic stir bar and charged with Re2(CO)10 (0.020 g, 0.031 mmol), followed by 4-methoxyphenol (0.155 g, 1.25 mmol) and toluene (0.625 mL); finally, 2-d1-oct-1-ene (0.297 mL, 0.212 g, 1.87 mmol) was added to the mixture. The vial was sealed using a microwave cap with the aid of a microwave vial crimper. The sealed microwave vial was placed into a 140 °C preheated metal block. The reaction mixture was aged for 24 h accompanied by rotary stirring prior to being cooled to room temperature. Column chromatography (silica gel, gradient 0−20% EtOAc in hexanes) afforded 2-d1-3 (0.281 g, 95%). 1H NMR (600 MHz, CD2Cl2): δ 6.72 (d, J = 3.0 Hz, 1H), 6.68 (d, J = 8.6 Hz, 1H), 6.59 (dd, J = 8.6, 3.0 Hz, 1H), 4.68 (br, 1H), 3.74 (s, 3H), 1.64−1.57 (m, 1H), 1.56−1.50 (m, 1H), 1.34−1.20 (m, 8H), 1.19 (s, 3H), 0.87 (t, J = 7.0 Hz, 3H). 13C{1H} NMR (151 MHz, CD2Cl2) δ 154.5, 147.6, 135.5, 116.3, 113.7, 111.5, 56.1, 37.6, 32.4, 30.0, 28.2, 23.2, 21.2, 14.4. Reaction of Mequinol with 3,3-d2-Oct-1-ene. In an N2-filled glovebox, a 10 mL microwave vial was equipped with a magnetic stir bar and charged with Re2(CO)10 (0.020 g, 0.031 mmol), followed by 4-methoxyphenol (0.155 g, 1.25 mmol) and toluene (0.625 mL); finally, 3,3-d2-oct-1-ene (0.297 mL, 0.212 g, 1.87 mmol) was added to the mixture. The vial was sealed using a microwave cap with the aid of a microwave vial crimper. The sealed microwave vial was placed into a 140 °C preheated metal block. The reaction mixture was aged for 24 h accompanied by rotary stirring prior to being cooled to room temperature. Column chromatography (silica gel, gradient 0−20% EtOAc in hexanes) afforded 3,3-d2-3 (0.278 g, 94%). 1H NMR (600 MHz, CD2Cl2) δ 6.62 (d, J = 3.1 Hz, 1H), 6.59 (d, J = 8.6 Hz, 1H), 6.50 (dd, J = 8.6, 3.1 Hz, 1H), 3.65 (s, 3H), 2.92 (q, J = 6.9 Hz, 1H), 1.24−1.12 (m, 8H), 1.11 (d, J = 6.9 Hz, 3H), 0.78 (t, J = 7.0 Hz, 3H). 13 C{1H} NMR (151 MHz, CD2Cl2) δ 154.5, 147.7, 135.6, 116.3, 113.7, 111.4, 56.1, 36.8 (p, J = 19.1 Hz), 32.9, 32.4, 30.0, 28.0, 23.2, 21.3, 14.4. General Procedure for Determining the KIE for 1-Octene versus dn-Oct-1-ene (Competition Setting). In an N2-filled glovebox, a 10 mL microwave vial was equipped with a magnetic stir bar and charged with Re2(CO)10 (0.020 g, 0.031 mmol), followed by 4-methoxyphenol (0.155 g, 1.25 mmol) and toluene (1.25 mL); finally, 1-octene (0.701 g, 6.24 mmol) and dn-oct-1-ene (6.24 mmol, either 1,1-d2-oct-1-ene, 2-d1-oct-1-ene, or 3,3-d3-oct-1-ene) were added to the mixture. The vial was sealed using a microwave cap with the aid of a microwave vial crimper. The sealed microwave vial was placed into a 140 °C preheated metal block. The reaction mixture was aged for 24 h accompanied by rotary stirring prior to being cooled to room temperature. Column chromatography (silica gel, gradient 0−20% EtOAc in hexanes) afforded a mixture of 3 and dn-3. KIE Using 1,1-d2-Oct-1-ene. The ratio of 3 to 1,1-d2-3 was determined to be 1.50 via quantitative 13C{1H} NMR (151 MHz, CD2Cl2) via integration of the CH2 and CD2 signals at δ 21.3 (1.00C) and 20.9 (0.66C), respectively. KIE Using 2-d1-Oct-1-ene. The ratio of 3 to 2-d1-3 was determined to be 1.01 via quantitative 13C{1H} NMR (151 MHz, CD2Cl2) via

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00543. Additional experimental details and NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.L.). *E-mail: [email protected] (X.W.). ORCID

Dan Lehnherr: 0000-0001-8392-1208 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to David Hesk for coordinating procurement of various isotopically labelled alkenes used in this manuscript. We thank Huaming Sheng, Ryan Cohen, Yu-hong Lam, Karthik Narsimhan, and Alexei Buevich (all at Merck & Co., Inc.) for their contributions to this project. We acknowledge Louis-Charles Campeau, Rebecca Ruck, Artis Klapars, Jennifer Obligacion, Patrick Fier, and Heather Johnson (all at Merck & Co., Inc.) for useful feedback.

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ABBREVIATIONS ABNO = 9-azabicyclo[3.3.1]nonane N-oxyl; EtOAc = ethyl acetate; HPLC = high-pressure liquid chromatography; LC = liquid chromatography; NA = not available; ND = not detected; PTFE = polytetrafluoroethylene; rt = room temperature (ca. 25 °C); TEMPO = 2,2,6,6-tetramethylpiperidine 1oxyl; v/v = volume per volume percent

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REFERENCES

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ization of a New Benzoylrhenate Intermediate and Selective OrthoAcetylation of (η6-Arene) tricarbonylchromium Complexes. Organometallics 1999, 18, 2786−2790. (d) Geng, X.; Wang, C. RheniumCatalyzed [4 + 1] Annulation of Azobenzenes and Aldehydes via Isolable Cyclic Rhenium(I) Complexes. Org. Lett. 2015, 17, 2434− 2437. (e) Sun, R.; Wang, T.; Zhang, S.; Chu, X.; Zhu, B. Synthesis, structures and catalytic activity of cyclometalated rhenium complexes. RSC Adv. 2017, 7, 17063−17070. (10) (a) Crossley, S. W. M.; Obradors, C.; Martinez, R. M.; Shenvi, R. A. Mn-, Fe-, and Co-Catalyzed Radical Hydrofunctionalizations of Olefins. Chem. Rev. 2016, 116, 8912−9000. (b) Wassink, B.; Thomas, M. J.; Wright, S. C.; Gillis, D. J.; Baird, M. C. Mechanisms of the hydrometalation (insertion) and stoichiometric hydrogenation reactions of conjugated dienes effected by manganese pentacarbonyl hydride: processes involving the radical pair mechanism. J. Am. Chem. Soc. 1987, 109, 1995−2002. (c) Sweany, R. L.; Halpern, J. Hydrogenation of α-methylstyrene by hydridopentacarbonylmanganese (I). Evidence for a free-radical mechanism. J. Am. Chem. Soc. 1977, 99, 8335−8337. (d) Shackleton, T. A.; Baird, M. C. The radical pair mechanism in hydrometalation and stoichiometric hydrogenation reactions of dicarbonyl(cyclopentadienyl)iron hydride η5-C5H5Fe(CO)2H with conjugated dienes. Organometallics 1989, 8, 2225− 2232. (e) Choi, J.; Tang, L.; Norton, J. R. Kinetics of Hydrogen Atom Transfer from (η5-C5H5)Cr(CO)3H to Various Olefins: Influence of Olefin Structure. J. Am. Chem. Soc. 2007, 129, 234−240. (f) Baird, M. C. Seventeen-electron metal-centered radicals. Chem. Rev. 1988, 88, 1217−1227. (g) Green, S. A.; Matos, J. L. M.; Yagi, A.; Shenvi, R. A. Branch-Selective Hydroarylation: Iodoarene−Olefin Cross-Coupling. J. Am. Chem. Soc. 2016, 138, 12779−12782. (h) Kuo, J. L.; Hartung, J.; Han, A.; Norton, J. R. Direct Generation of Oxygen-Stabilized Radicals by H• Transfer from Transition Metal Hydrides. J. Am. Chem. Soc. 2015, 137, 1036−1039. (i) Estes, D. P.; Norton, J. R.; Jockusch, S.; Sattler, W. Mechanisms by which Alkynes React with CpCr(CO)3H. Application to Radical Cyclization. J. Am. Chem. Soc. 2012, 134, 15512−15518. (j) Sweany, R. L.; Halpern, J. Hydrogenation of alpha-methylstyrene by hydridopentacarbonylmanganese (I). Evidence for free-radical mechanism. J. Am. Chem. Soc. 1977, 99, 8335−8337. (k) Choi, J.; Tang, L.; Norton, J. R. Kinetics of Hydrogen Atom Transfer from (η5-C5H5)Cr(CO)3H to Various Olefins: Influence of Olefin Structure. J. Am. Chem. Soc. 2007, 129, 234−240. (l) Smith, D. M.; Pulling, M. E.; Norton, J. R. Tin-Free and Catalytic Radical Cyclization. J. Am. Chem. Soc. 2007, 129, 770−771. (11) Kuninobu, Y.; Kawata, A.; Takai, K. Rhenium-Catalyzed Insertion of Terminal Acetylenes into a C−H Bond of Active Methylene Compounds. Org. Lett. 2005, 7, 4823−4825. (12) Boiling points of the components are 214 °C for 1-dodecene, 163 °C for mesitylene, 122 °C for 1-octene, and 110 °C for toluene. (13) Rhenium-π-allyl complexes have previously been reported; see (a) Casey, C. P.; Yi, C. S. Synthesis of cationic (π-allyl)rhenium complexes and their reactions with carbon nucleophiles. Organometallics 1990, 9, 2413−2414. (b) Casey, C. P.; Yi, C. S. Acidcatalyzed isomerization and deuterium exchange of rhenium alkene complexes via in-place rotation of an agostic alkylrhenium cation. Organometallics 1991, 10, 33−35. (c) Wilberger, R.; Piotrowski, H.; Mayer, P.; Lorenz, I.-P. Synthesis of the dimeric μ2-O aminoalkoxy complexes of rhenium [(CO)3Re(μ2−O∩NH2)]2 and their condensation products with acetone [(CO)3Re(μ2−O∩NCMe2)]2. Inorg. Chem. Commun. 2002, 5, 897−902. (d) Casey, C. P.; Boller, T. M.; Samec, J. S. M.; Reinert-Nash, J. R. Quantitative Determination of the Regioselectivity of Nucleophilic Addition to η3-Propargyl Rhenium Complexes and Direct Observation of an Equilibrium between η3-Propargyl Rhenium Complexes and Rhenacyclobutenes. Organometallics 2009, 28, 123−131. (14) Cleavage of vinylic C−H bonds by rhenium carbonyl catalysts has been reported; see (a) Nubel, P. O.; Brown, T. L. Photolysis of dirhenium decacarbonyl in the presence of simple olefins. Thermal reactivity of (μ-hydrido)(μ-alkenyl)dirhenium octacarbonyl compounds. J. Am. Chem. Soc. 1984, 106, 644−652. (b) Sarakha, M.; Cozzi, M.; Ferraudi, G. Mechanism of (μ-H)(μ-alkenyl)Re2(CO)8 N


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Organometallics

Hernández-Valdés, D.; Gotzmann, C.; Fox, T.; Spingler, B.; Alberto, R. A Mixed-Ring Sandwich Complex from Unexpected Ring Contraction in [Re(η6-C6H5Br)(η6-C6R6)](PF6). Inorg. Chem. 2017, 56, 6297−6301. (c) Green, M. L. H.; O’Hare, D. Synthesis of η-arene rhenium compounds using rhenium atoms. Crystal structures of [Re2(η-C6H6)(η- C6H8)2(μ-H)2] and [Re(η-C6H6)(η-C6H8)H]. J. Chem. Soc., Dalton Trans. 1987, 403−410. (d) Alberto, R. From oxo to carbonyl and arene complexes; A journey through technetium chemistry. J. Organomet. Chem. 2018, 869, 264−269. (e) Chordia, M. D.; Smith, P. L.; Meiere, S. H.; Sabat, M.; Harman, W. D. A Facile Diels−Alder Reaction with Benzene: Synthesis of the Bicyclo[2.2.2]octene Skeleton Promoted by Rhenium. J. Am. Chem. Soc. 2001, 123, 10756−10757. (f) Honig, E. D.; Meng, Q.-J.; Robinson, W. T.; Williard, P. G.; Sweigart, D. A. Nucleophilic addition to coordinated cycloheptatriene: a mechanistic and structural study. Organometallics 1985, 4, 871−877. (g) Trifonova, E. A.; Perekalin, D. S.; Lyssenko, K.A.; Kudinov, A. R. Synthesis and structures of cationic bis(arene) rhenium complexes. J. J. Organomet. Chem. 2013, 727, 60−63. (h) Reynolds, M. A.; Guzei, I. A.; Angelici, R. J. Re2(CO)10-Mediated Carbon−Hydrogen and Carbon−Sulfur Bond Cleavage of Dibenzothiophene and 2,5-Dimethylthiophene. Organometallics 2001, 20, 1071−1078. (i) Smith, P. L.; Keane, J. M.; Shankman, S. E.; Chordia, M. D.; Harman, W. D. Michael Addition Reactions with η2Coordinated Anisoles: Controlling the Stereochemistry of the Para and Benzylic Carbons. J. Am. Chem. Soc. 2004, 126, 15543−15551. (j) Rosini, G. P.; Jones, W. D. An unorthodox reaction of indene with ReH7(PPh3)2: the competitive formation of an η5-indanyl ligand versus an η5-indenyl ligand. J. Am. Chem. Soc. 1993, 115, 965−974. (k) Gutierrez, A.; Wilkinson, G.; Hussain-Bates, B.; Hursthouse, M. B. t-Butylimido and t-butylimido oxo aryls of rhenium. X-ray crystal structures of Re(NBut)2(2,6-Me2C6H3)2, Re(NBut)(2,4,6-Me3C6H2)(C27H32), Re(NBut)3(2,4,6-Me3C6H2), [(ButN)Br(2,4,6-Me3C6H2)Re(μ-NBut)(μ-O)Re(OC6H2Me2CH2)(NBut)], [Re(NBut)(O)Ar(μO)]2, Ar = 2,6-C6H3 and 2,4,6-C6H2. Polyhedron 1990, 9, 2081−2096. (l) Higgitt, C. L.; Hugo Klahn, A.; Moore, M. H.; Oelckers, B.; Partridge, M. G.; Perutz, R. N. Structure and dynamics of the η2hexafluorobenzenecomplexes [Re(η5-C5H4R) (CO)2(η2-C6F6)](R = H or Me) and[Rh(η5-C5Me5)(PMe3)(η2-C6F6)]. J. Chem. Soc., Dalton Trans. 1997, 1269−1280. (22) Re-hydrides have been shown to hydrometalate alkynes; see (a) Dubeck, M.; Schell, R. A. The Addition of Dicyclopentadienylrhenium Hydride to Some Acetylenes. Inorg. Chem. 1964, 3, 1757−1760. (b) Wilford, J. B.; Stone, F. G. A. Chemistry of the Metal Carbonyls. XXVIII. Addition of Rhenium Pentacarbonyl Hydride to Fluoroolefins. Inorg. Chem. 1965, 4, 93−97. (23) Trapping of radicals derived from Mn- and Re-carbonyl complexes by TEMPO, quinones, and tetracyanoethylene has been reported; see (a) Jaitner, P.; Huber, W.; Hunter, G.; Scheidsteger, O. Synthesis and X-ray Structure of (2,2,6,6-Tetramethylpiperidinyl-1oxo-O,N)tricarbonyl-manganese(0); ″Side-on″ Coordination of the nitroxyl radical to Manganese. J. Organomet. Chem. 1983, 259, C1− C5. (b) Foster, T.; Chen, K. S.; Wan, J. K. S. An esr study of the reactions of decacarbonyldimanganese, trimethyltinpentacarbonylmanganese and decacarbonyldirhenium with quinones and αdiketones. J. Organomet. Chem. 1980, 184, 113−124. (c) Krusic, P. J.; Stoklosa, H.; Manzer, L. E.; Meakin, P. Electron spin resonance study of the oxidative homolytic cleavage of metal-metal and metalcarbon bonds. J. Am. Chem. Soc. 1975, 97, 667−669. (24) For leading references on Rh-catalyzed hydroarylation and mechanistic studies thereof, see (a) Lenges, C. P.; Brookhart, M. Addition of Olefins to Aromatic Ketones Catalyzed by Rh(I) Olefin Complexes. J. Am. Chem. Soc. 1999, 121, 6616−6623. (b) Kakiuchi, F.; Ohtaki, H.; Sonoda, M.; Chatani, N.; Murai, S. Mechanistic Study of the Ru(H)2(CO)(PPh3)3-Catalyzed Addition of C−H Bonds in Aromatic Esters to Olefins. Chem. Lett. 2001, 30, 918−919. (25) Gomez-Gallego, M.; Sierra, M. A. Kinetic Isotope Effects in the Study of Organometallic Reaction Mechanisms. Chem. Rev. 2011, 111, 4857−4963.

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Article

Organometallics (26) (a) Kiel, W. A.; Lin, G.-Y.; Gladysz, J. A. Unprecedented regiospecificity and stereospecificity in reactions of Ph3C+PF6− with rhenium alkyls of the formula (η-C5H5)Re(NO)(PPh3)(CH2R). J. Am. Chem. Soc. 1980, 102, 3299−3301. (b) Romão, C. C. Rhenium: Organometallic Chemistry. Encycl. Inorg. Chem., 2006, DOI: 10.1002/0470862106.ia205. (27) The intermetallic bond of Mn2(CO)10 has been reported to be weaker than that in Re2(CO)10. Heterobimetallic complex (CO)5Re− Mn(CO)10 has an unusually strong bond, although ligand substitution has been shown to result in being able to modulate the hemolytic Re− Mn bond dissociation energy in either direction (stronger or weaker). For details, see (a) Coville, N. J.; Leins, A. E. MnRe(CO)10: A review of the chemical and physical properties of a simple heterobimetallic non-bridged dimer. J. Cluster Sci. 1993, 4, 185−230. (b) Goodman, J. L.; Peters, K. S.; Vaida, V. The determination of the manganesemanganese bond strength in Mn2(CO)10 using pulsed time-resolved photoacoustic calorimetry. Organometallics 1986, 5, 815−816. (c) Rheingold, A. L.; Meckstroth, W. K.; Ridge, D. P. Crystal and molecular structure of rhenium manganese decacarbonyl, ReMn(CO)10, containing an unexpectedly short rhenium-manganese bond. Inorg. Chem. 1986, 25, 3706−3707. (d) Flitcroft, N.; Huggins, D. K.; Kaesz, H. D. Infrared Spectra of (CO)5Mn-Re(CO)5 and the Carbonyls of Manganese, Technetium, and Rhenium; Assignment of CO and M = C Stretching Absorptions. Inorg. Chem. 1964, 3, 1123−1130. (28) (a) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities. J. Org. Chem. 1997, 62, 7512−7515. (b) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. NMR Chemical Shifts of Trace Impurities: Common Laboratory Solvents, Organics, and Gases in Deuterated Solvents Relevant to Organometallic Chemists. Organometallics 2010, 29, 2176−2179.

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