Kinetics and Mechanism of Isocyanide-Promoted Carbene Insertion

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Kinetics and Mechanism of Isocyanide-Promoted Carbene Insertion into the Aryl Substituent of an N‑Heterocyclic Carbene Ligand in Ruthenium-Based Metathesis Catalysts Justin R. Griffiths, Elan J. Hofman, Jerome B. Keister,* and Steven T. Diver* Department of Chemistry, University at Buffalo, the State University of New York, Buffalo, New York 14260-3000, United States S Supporting Information *

ABSTRACT: In situ IR spectroscopy was used to study the kinetics of addition of L = alkyl and aryl isocyanides to the Grubbs secondgeneration carbene complex Ru(H 2 IMes)(CHPh)(PCy 3 )Cl 2 (H2IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene), which triggers carbene insertion into an aromatic ring of the N-heterocyclic carbene supporting ligand, forming Ru{1-mesityl-3-(7′-Ph-2′,4′,6′trimethylcycloheptatrienyl)-4,5-dihydroimidazol-2-ylidene}L2(PCy3)Cl2. The rate law was determined to be first order in isocyanide concentration and first order in carbene complex concentration. For various isocyanides CNR the rate increases as R = tertbutyl ≪ cyclohexyl < n-octyl < CH2Ph ≈ CH2CO2Me ≈ CH2SO2C6H4-4-Me < C6H4-4-OMe < C6H4-4-Cl. The proposed mechanism involves reversible addition of isocyanide followed by rate-determining, irreversible carbene insertion and subsequent, rapid addition of the second isocyanide. The carbene insertion is accelerated by the electrophilicity of the carbene, which is enhanced due to ligand binding by isocyanides with lower σ-donor/ π-acceptor ratios.

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INTRODUCTION The development of functional group tolerant ruthenium-based alkene metathesis catalysts (Figure 1) has led to numerous applications in organic and materials synthesis.1 In 1999, the Grubbs group reported the “second-generation” ruthenium carbene complex 3, bearing an N-heterocyclic carbene (NHC) ligand, 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (H2IMes).2 Due to the special properties of the NHC ligand, complex 3 was found to be a superior catalyst for alkene metathesis in comparison to its predecessor, benzylidene 1.3 Use of 3 in organic synthesis applications led to an expanded scope of both the alkene and ene-yne metathesis reactions. Recently, phosphine-free catalysts 4,4 5,5 and 66 have been developed, which has provided a wider repertoire of reactive alkenes in cross-metathesis applications. Catalysts 3 and 4 are the most widely used ruthenium carbenes in organic synthesis applications and are commercially available. Generally, the N-heterocyclic carbene ligand of the catalysts 2−6 plays an important role as a supporting ligand in metathesis reactions.7 The N-heterocyclic carbene is stabilizing and electron-donating. The strong σ-donating properties of the N-heterocyclic carbene contribute electron density to the metal, which serves to stabilize the reactive intermediates in alkene metathesis. In the crystal structure of the H2IMes-liganded ruthenium carbene, one mesityl group drapes over the carbene.8 Molecular orbital interactions between the carbene fragment and the aromatic ring are evident in the second-generation Grubbs complex and are strengthened on binding of a strong π-acceptor ligand, such as CO or an isocyanide.9 © XXXX American Chemical Society

The close proximity of the aromatic ring and the electrophilic carbene could lead to unwanted reactions between propagating metal alkylidenes and the aromatic ligand. For instance, the Grubbs group has identified a benzylic CH insertion product as a minor byproduct in the synthesis of complex 3.10 Interaction between the electrophilic carbene and the aromatic ring of the N-heterocyclic carbene supporting ligand could lead to carbene insertion into the aromatic ring system. A ligand-promoted insertion of a carbene into an aromatic tetraaza macrocycle was reported previously by Floriani et al.11 We previously reported a novel carbene insertion reaction involving the Grubbs’ second-generation complex 3, which is initiated by coordination of carbon monoxide or isocyanide ligands (Scheme 1a).12 The full paper12b described a variety of different ruthenium carbene complexes such as 2−4, which undergo the ligand-promoted Buchner reaction, and studied the ability of different added ligands to trigger the reaction. The earliest work studied the effect of carbon monoxide, but our more comprehensive study described other ligands that gave this unique chemistry, including phosphites and isocyanides. The resulting coordination complexes were structurally characterized. On binding, we speculated that the π-acceptor ligand dramatically changed the electronic properties of the metal, causing the carbene to become electrophilic. A mechanism was proposed,12a and subsequent DFT investigation by Cavallo13 lent support to the proposed mechanism and found an Received: May 3, 2017

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Figure 1. Grubbs’ ruthenium carbene complexes 1−6.

Scheme 1. Buchner Carbene Insertion Promoted by Carbon Monoxide or Isocyanides12,18

unexpected intermediate. Though the Buchner reaction was found to be very fast for CO and isocyanides, no detailed kinetic data were previously obtained for the ligand-promoted Buchner reaction. The Buchner reaction has emerged as a useful tool to stop metathesis reactions and to aid in the removal of the organometallic product. The second-generation Grubbs complex 3 may still be present at the conclusion of a metathesis reaction, and other ruthenium carbenes may be present as well. It is useful to quench these intermediates so that undesired reactions (e.g., polymerization) do not occur during product isolation. The addition of a ligand such as an isocyanide leads to the Buchner reaction, rapidly destroying metathesis-active carbene species. As such, the ligand-promoted Buchner reaction provides a useful method to quench metathesis activity and to remove ruthenium from the product mixture when the promoting ligand is a polar potassium isocyanoacetate14 or a silica gel supported isocyanide.15 Grela and co-workers found that amine-containing isocyanide ligands are also useful as reagents for the removal of ruthenium.16 Though it is known to be fast on a practical level, is the quench fast enough to arrest reactive metal carbene intermediates? The ligand-promoted Buchner reaction is also important because it both provides insight into a fundamental aspect of

carbene reactivity and may provide a decomposition route for ruthenium carbenes during alkene metathesis. Ethylene is produced during the alkene metathesis of 1-alkenes and is frequently used to promote difficult ene-yne metatheses using 3 and other carbenes.17 Despite the fact that ethylene has donor− acceptor properties similar to those of CO, it does not induce the Buchner insertion. Do the steric properties of the π-acceptor ligand influence its ability to promote the Buchner insertion? On the basis of these considerations, we were interested in studying the rate of the Buchner reaction using different isocyanide π-acceptor ligands. Since the ligand properties of the isocyanide are key in flipping the carbene reactivity, we focused our investigation on the isocyanide ligands. We wanted to evaluate the key ligand attributes which affect the rate of this process. In this contribution, we report a study of the kinetics of the insertion and propose a mechanism for the net process shown in Scheme 1b.

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RESULTS In situ IR spectroscopy was used to monitor the net Buchner reaction via the relatively strong NC stretches in the 2300−1900 cm−1 region. Treatment of a solution of complex 3 with 2.0 equiv of 4-chlorophenyl isocyanide resulted in a rapid color change from red to light orange-yellow due to 7a. B

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Figure 2. IR spectra during reaction of 4-chlorophenyl isocyanide (0.02 M) with 3 (0.01 M), at 0 °C.

where x is the concentration of isocyanide consumed at time t, A0 is the concentration of isocyanide at time 0, and B0 is the concentration of complex 3 at time 0. The linearity (Figure 4)

Figure 2 shows the IR spectral changes during the reaction. Disappearance of the absorption at 2129 cm−1 due to free isocyanide is accompanied by increases in absorptions at 2060 and 2005 cm−1 due to the Buchner insertion product. No peaks attributable to intermediates are observed. The isosbestic point at 2124 cm−1 indicates that only reactants and a single product are present at any time. Since we are monitoring the reaction via the change in absorbance of the isocyanide reactant, pseudo-first-order isocyanide conditions (large excess of isocyanide) cannot be used. Therefore, data analysis was done at a stoichiometric 2:1 molar ratio of isocyanide to carbene complex. Because of the 2:1 reaction stoichiometry, the disappearance of the isocyanide is twice as fast as the decrease in concentration of 3. Under stoichiometric conditions, plots of [CNR]−1 vs time are linear (Figure 3), indicating that the reaction is second order overall,

Figure 4. First-order plot for 4-chlorophenyl isocyanide with 3 at 0 °C, 0.01 M isocyanide, and 0.01 M 3.

indicates that the rate law is first order in complex 3 as well as isocyanide. At 0 °C the second-order rate constant is 2.8(0.3) M−1 s−1. The rate was unaffected by the presence of excess PCy3. At 0.1 M PCy3, 0.01 M 3, and 0.02 M 4-chlorophenyl isocyanide the second-order rate constant was 2.7 M−1 s−1, the same as that above within experimental error. This is as expected from the kinetics of dissociative phosphine exchange for 3 and other complexes, previously reported by Grubbs et al.3a On the basis of their data, the extrapolated initial rate of PCy3 dissociation (0.01 M Ru complex) at 0 ◦C is 1 × 10−8 M s−1, in comparison to the rate of 5.6 × 10−4 M s−1 for this Buchner reaction. Therefore, phosphine dissociation does not precede the Buchner reaction. Activation parameters were determined from measurements at temperatures from 0 to −30 °C. An Eyring plot yielded ΔH⧧ as +34.6(2.9) kJ/mol (+8.27 kcal/mol) and ΔS⧧ as −109(11) J/(K mol) (−26.1 eu). The negative entropy of activation is as expected for a second-order process. In contrast, ΔS⧧ for PCy3 dissociation from 3 is +54 J/(K mol) (+13 eu).3a The observed activation parameters support an associative process as the ratelimiting step. Effects of Isocyanide Substituents. Having determined the rate law for 4-chlorophenyl isocyanide, we next wanted to compare the rates for various isocyanide substituents.

Figure 3. Second-order plot for 4-chlorophenyl isocyanide with 3 at 0 °C, 0.02 M isocyanide, and 0.01 M 3.

either first order in [CNR] and first order in ruthenium carbene or second order in [CNR]. To establish the order in [3], a 1:1 molar ratio was used. In this case, the integrated rate equation for a rate law first order in each reactant is given by ⎧ (2B0 − x) ⎫ ⎧ 2B ⎫ ⎬ = (2B0 − A 0)kt + ln⎨ 0 ⎬ ln⎨ ⎩ (A 0 − x ) ⎭ ⎩ A0 ⎭ C

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Organometallics Scheme 2. Buchner Insertion Involving the Hoveyda Complex 412b

Figure 5. Cyclohexyl isocyanide promoted Buchner reaction of 3, shown as absorbance vs time and tracked using in situ IR. IR bands: cyclohexyl isocyanide at 2145 cm−1, intermediate 7g at 2100 cm−1, and product 8g at 2185 cm−1. Conditions: 0.01 M 3, 0.2 M CyNC, CH2Cl2, 25 °C.

low yield of the tris(isocyanide) was obtained at the 2:1 (CNR to Ru complex) stoichiometry used in the kinetic experiments. Figure 5 shows the change in absorbances at 2145 cm−1 due to cyclohexyl isocyanide, at 2100 cm−1 due to the bis(isocyanide) Buchner insertion product 7g, and at 2185 cm−1 due to the tris(isocyanide) complex 8g (3:1 isocyanide to Ru molar ratio) during a reaction at 25 °C. Similar results were obtained for n-octyl isocyanide (data not shown). Ligand substitution of isocyanide for PCy3 prior to the Buchner insertion must also be considered as a possible mechanism for formation of 8. Grubbs et al. previously reported the kinetics of dissociative phosphine exchange for 3 and other complexes.3a On the basis of their data, the extrapolated initial rate of PCy3 dissociation (0.01 M Ru complex) at 25 °C is 7 × 10−7 M s−1 and at 0 °C, 1 × 10−8 M s−1. The slowest measured isocyanide-promoted Buchner reactions had initial rates of 2.4 × 10−5 M s−1 (cyclohexyl isocyanide) at 25 ◦C and 8.2 × 10−6 M s−1 (n-octyl isocyanide) at 0 °C. Therefore, in all these cases the initial rate of the Buchner reaction is at least 10 times faster than the limiting rate for PCy3 dissociation. This is in accord with the kinetic data above in the presence and absence of added PCy3. However, higher temperatures should favor the dissociative PCy3 process over the associative isocyanide-promoted reaction, and so there may be conditions under which the rate of Buchner reaction with 3 will be slower than isocyanide substitution for PCy3; in such a case we expect the reaction with a second equivalent of isocyanide would generate a Buchner insertion, to be followed by coordination of a third isocyanide to give the same tris(isocyanide) product.

Isocyanides are useful for quenching metathesis reactions, and knowledge of relative rates will guide in the choice of quenching ligand. Reactions of 3 with 4-methoxyphenyl isocyanide, toluenesulfonylmethyl isocyanide (TosMIC), methyl isocyanoacetate, benzyl isocyanide, n-octyl isocyanide, cyclohexyl isocyanide, and tert-butyl isocyanide were kinetically evaluated. For all isocyanides the initial product of the reaction is the bis(isocyanide) Buchner insertion product 7 (Scheme 1b). At 0 °C with alkyl isocyanides, or with aryl isocyanides at all temperatures, 7 is the only product observed. However, for slower Buchner reactions or at higher reaction temperatures, additional ligand substitution by unreacted isocyanide occurs. This converts the initially formed bis(isocyanide) product 7 into the tris(isocyanide) complex 8 and releases free PCy3. Tris(isocyanide) Buchner products have been previously prepared by reaction of Hoveyda−Blechert carbene 4 with 3 equiv of 4-chlorophenyl isocyanide (Scheme 2).12b The product exists as a mixture of conformational (cycloheptatrienyl ring) isomers, and the coordination geometry has been established by X-ray crystallography. A third isocyanide ligand displaced the coordinating ether ligand present in the chelate 4. For alkyl isocyanides, the initial bis(isocyanide) intermediate 7 can be observed. For cyclohexyl isocyanide, the disappearance of free isocyanide was accompanied by the appearance of two new absorptions at 2062 and 2100 cm−1, due to the expected bis(isocyanide) Buchner insertion product. However, this complex is unstable and, with time, the tris(isocyanide) complex 8 is formed and free tricyclohexylphosphine is observed in the 31P NMR spectrum of the product mixture. We were unable to isolate and fully characterize the bis(isocyanide) complex, and a D

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factor into the faster rate of the Buchner reaction; on the basis of activation parameters reported by Grubbs et al.,3a the rate constant of 1.5 × 10−10 s−1 at −78 °C for PPh3 dissociative substitution would predict a half-life of 8 × 107 min. Clearly, the rate of isocyanide-promoted Buchner insertion is faster than that of PPh3 ligand dissociation.

Table 1. Rate Constants for Reaction of 3 with Isocyanides CNR isocyanide CNC6H4-4-Cl

CNC6H4-4-OMe CNCH2SO2C6H4-4-Me CNCH2CO2Me CNCH2Ph CN(CH2)7CH3 CN-cyclohexyl

rate constant (M−1 s−1)

temp (°C)

2.8(0.3) 1.18(0.10) 0.82(0.15) 0.37(0.07) 1.42(0.12) 0.67(0.03) 0.63(0.08) 0.57(0.04) 0.041(0.001) 0.381(0.006) 0.121(0.005)

0.0 −10.0 −20.0 −30.0 0.0 0.0 0.0 0.0 0.0 25.0 25.0

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DISCUSSION Mechanism of Ligand-Induced Buchner Insertion. Metal-catalyzed diazoalkane decompositions in the presence of arenes and alkenes have shown synthetic utility for the preparation of both cycloheptatrienes and cyclopropanes.19 Metal carbenoid additions to carbon−carbon π bonds typically involve very electrophilic carbenes. Ruthenium carbenes used in alkene metathesis are much less electrophilic and typically do not form cyclopropanation products. The mechanism of the ligand-induced Buchner reaction observed in the Grubbs second-generation ligand environment is believed to follow a cyclopropanation pathway due to an electrophilic carbene. DFT calculations have been used to probe the energetics of CO-induced Buchner insertion. Cavallo et al. calculated the energies of intermediates in CO-promoted Buchner insertion for Ru(H2IMes)Cl2(CH2)L (L = CO, PMe3).13 They also calculated free energies for coordination of L1 = CO, PMe3 and other Lewis bases to Ru(H2IMes)Cl2(CH2)L2 (L2 = PMe3, py, PF3, CO). For example, ΔG for coordination to Ru(H2IMes)Cl2(CH2)(PMe3) ranges from −17.7 kcal/mol for methyl isocyanide to +3.6 kcal/mol for PMe3. For phosphine binding, steric effects obviously would be much more significant for the PCy3 analogue, which would result in less favorable coordination free energies. Cavallo’s DFT calculations suggest that the binding of the π-acceptor ligand occurs at the open apical position, trans to the benzylidene moiety. The calculations also led to the proposal that the role of the π acid is not due to lowering the energy barrier for carbene attack on the mesityl ring but instead serves to stabilize the metallacycle 13 (Scheme 3) prior to opening to give the norcaradienyl ring expansion product. Comparisons of rates for Buchner insertions of 3 induced by reactions with isosteric 4-chlorophenyl and 4-methoxyphenyl isocyanides and nearly isoelectronic octyl and cyclohexyl isocyanides demonstrate that both electronic and steric properties of the isocyanide affect the rate. The origin of the steric effect is obvious from the crystal structure8 of 3, as the vacant coordination site is highly congested, sandwiched between a

After establishing that the additions of alkyl isocyanides cause the same Buchner insertions in all cases, we determined the rates (Table 1). For those reactions which gave a mixture of bis- and tris(isocyanide) products, we used the initial rates (where the contribution of the ligand substitution was small) and assumed the same rate law to determine rate constants. At 0 °C, the rate constants decreased in the order 4-chlorophenyl isocyanide (2.8(0.2) M−1 s−1) > 4-methoxyphenyl isocyanide (1.42(0.12) M−1 s−1) > TsCH2NC (0.67(0.08) M−1 s−1) ≈ methyl isocyanoacetate (0.63(0.08) M−1 s−1) ≈ benzyl isocyanide (0.57(0.04) M−1 s−1) > n-octyl isocyanide (0.041(0.001) M−1 s−1). At this temperature more hindered alkyl isocyanides reacted too slowly for convenient kinetic analysis; therefore, we used 25 °C for comparison. At 25 °C, the rate constants decreased as n-octyl isocyanide (0.381(0.006) M−1 s−1) > cyclohexyl isocyanide (0.121(0.005) M−1 s−1) > tert-butyl isocyanide (slow). Clearly, both steric and electronic properties affect the rate of reaction. The rate is accelerated for electron-withdrawing substituents and retarded by increasing steric bulk. Effect of Variation in PR3 for Grubbs’ Complexes. In order to determine whether the phosphine ligand influences carbene insertion, the triphenylphosphine analogue of 3, Ru(H2IMes)(CHPh)(PPh3)Cl2, was studied for comparison. Triphenylphosphine is a weaker σ donor than the more electron rich Cy3P present in Grubbs complex 3. The reaction with 4-chlorophenyl isocyanide was too fast to be measured (less than 1 min) even at −78 °C. This result does not address the question of steric vs electronic differentiation, since PPh3 is both smaller and less electron donating than PCy3. Although PPh3 dissociation is faster than PCy3 dissociation, this does not

Scheme 3. Proposed Mechanism for CNR-Promoted Buchner Insertiona

a

Intermediate 13 was proposed by Cavallo et al. on the basis of DFT calculations.13 E

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Organometallics mesityl group and a cyclohexyl group up to a distance of ca. 7 Å from the ruthenium atom. The minimum distance between a methyl hydrogen of the mesityl and the 4-methylene hydrogen of the nearest cyclohexyl group is 2.4 Å, a distance within van der Waals contact. Accommodation of an isocyanide requires these groups to separate such that, in the crystal structure of Buchner product 7a,12b the mesityl methyl group is 3.24 Å from the closest atom of the 4-chlorophenyl group. Clearly the apical coordination site is constrained and most accessible to slender ligands. The origin of the electronic effect is less clear. Typical Hammett parameter sets related to inductive effects rank from electron donating to electron withdrawing: alkyl < benzyl < aryl < CH2CO2R. Figure 6 is a Hammett plot of ln k

Figure 7. Plot of ln k for CNR vs δ for the equatorial carbonyls of Cr(CO)5(CNR). R: 1, 4-ClC6H5; 2, 4-MeOC6H5; 3, CH2Ts; 4, CH2CO2Me; 5, Bn; 6, octyl. The trend line does not include 3 (CNCH2Ts).

thickness of CH2Ts, access to the apical coordination site is sterically impeded, resulting in a decelerated Buchner insertion. Therefore, we suggest that steric effects make the reaction slower for CNCH2Ts than is predicted on the basis of electronic effects, as revealed by the δCO values.

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MECHANISM The experimentally determined rate law is consistent with three mechanisms: (a) rate-limiting and irreversible addition of one isocyanide, with fast follow-up steps (b) rate-limiting addition of one isocyanide with concerted benzylidene addition to the mesityl group and fast follow-up steps (c) reversible coordination of one isocyanide, followed by rate-limiting benzylidene addition to the mesityl group, with fast follow-up steps All three would be expected to display decreasing rates with increasingly sterically hindered isocyanides, as is observed. However, mechanisms a and b would be expected to display decreasing rates for less nucleophilic isocyanides, the opposite of the observed behavior for aryl vs primary alkyl and methyl isocyanoacetate vs primary alkyl. This suggests that the electrophilicity of the carbene, as influenced by prior isocyanide coordination, is the determining factor in the rate-determining step. Thus, we favor mechanism c (Scheme 3). Addition of isocyanide is proposed to be reversible; most probably addition is trans to the carbene, forming an 18-electron ruthenium carbene species 12. In this case, the observed second-order rate constant for the reaction contains the equilibrium constant k12/k21 for isocyanide coordination (which must be small, since no intermediate complex is observable in the IR spectrum) and the rate-limiting constant k23. The equilibrium constant should be proportional to the isocyanide ligand properties: it would increase with increased nucleophilicity of CNR and decrease with increasing steric hindrance. However, the step k23 should be relatively insensitive to CNR steric bulk and strongly affected by alkylidene electrophilicity resulting from the trans isocyanide. Due to the π back-bonding to the isocyanide, the ruthenium center no longer is able to stabilize the π orbital of the carbenic carbon of the benzylidene. This lack of stabilization weakens the ruthenium−carbene bond, making the carbene more electrophilic.24 Cyclopropanation of the proximal mesityl moiety of the N-heterocyclic carbene generates 16-electron norcaradiene intermediate 14, which is followed by a second isocyanide addition and electrocyclic ring opening to

Figure 6. Plot of ln k for CNR vs σi for R: 1, 4-ClC6H5; 2, 4-MeOC6H5; 3, CH2Ts; 4, CH2CO2Me; 5, Bn; 6, octyl. The plot shows no correlation between Hammett σi and the rate of Buchner insertion.

for Buchner insertion vs σi.20 Scatter can be seen in the plot, and no correlation between these parameters is evident. Other Taft parameters such as σp do not show improved correlations. A property which may be more relevant to the Buchner insertion is the σ-donor to π-acceptor ratio of the isocyanide as a ligand to a transition metal. Ample precedent exists for use of the 13C chemical shifts of the CO ligands of Ni(CO)3L or Cr(CO)5L complexes as a measure of this ratio for L.21 Better σ donor/π acceptor ligands L cause deshielding of the carbonyl ligands, resulting in downfield chemical shifts. Two previous studies have reported Cr(CO)5(isocyanide) properties vs 13C NMR shifts.22 A significant compilation of isocyanides of varying electronic and steric demand can be found in the work of Figueroa et al.22c Figure 7 shows the correlation of ln k for Buchner insertion vs chemical shift for the equatorial carbonyls of Cr(CO)5(CNR).23,21 An increase in reaction rate correlates with more upfield chemical shifts. Except for CNCH2Ts, the correlation is good. The exception for CNCH2Ts is surprising. Hammett parameters for inductive effects for CH2Ts and CH2CO2Me are quite similar, as are the rate constants for Buchner insertion, but the chemical shifts for Cr(CO)5(CNCH2Ts) and Cr(CO)5(CNCH2CO2Me) are very different. The steric properties are difficult to quantify, but clearly CNCH2Ts should be larger than CNCH2CO2Me. Molecular modeling of isocyanides in conformations such that the substituents fill the minimum distance perpendicular to the NC vector reveals the following van der Waals thicknesses: 4-ClC6H5 ≈ 4-MeOC6H5, 3.4 Å; CH2CO2Me = Bn = octyl, 4.2 Å; cyclohexyl, 5.25 Å; CH2Ts, 5.4 Å. Due to the greater F

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• The rate is accelerated by electron-withdrawing isocyanide substituents and retarded by increasing steric bulk. • Our observations led to a proposed mechanism which involves reversible coordination of a single isocyanide ligand, followed by rate-determining carbene migration to the mesityl group. • A single isocyanide binding is likely to be sufficient to trigger the Buchner reaction. Because isocyanides are powerful quenching reagents for metathesis reactions and have seen use as metal scavengers, these data have important implications for selection of a quenching agent and for future materials synthesis. For the fastest quenching rates, aromatic isocyanides offer the best choice. In addition, many aromatic isocyanides are commercially available. For sequesteration of metal through multivalency, an alkyl isocyanide may offer a better choice due to the potential for three binding ligands. Synthesis and evaluation of new materials as quenching and scavenging agents are the focus of continuing studies in our laboratories.

provide the cycloheptatrienyl product 7. Cavallo’s ipso metallocyclic intermediate 1313 lies between 12 and 14. Strongly σ donating ligands are not adequate to trigger the Buchner insertion. The ligand-induced Buchner reaction requires a ligand that possesses strong σ-donor and π-acceptor properties. σ donation results in metal binding but is not itself sufficient to cause the Buchner insertion. For instance, pyridine can add to the metal, as seen in the 18-electron pyridine solvate 6, but pyridine does not cause a Buchner reaction. The presence of σ donors that are not π acceptors induces a different decomposition pathway which is mechanistically distinct from the Buchner reaction. Grubbs et al. reported that decomposition of the methylidene complex 8 in the presence of excess pyridine forms MePCy3+ and Ru(H2IMes)(py)3Cl2.25 More recently, Fogg et al. have studied Lewis base promoted decomposition in greater detail.26 The initial binding at the open apical position of the ruthenium carbene complex requires a small ligand. In this study, sterically hindered alkyl isocyanides gave slower Buchner reactions in comparison to linear (i.e., normal) alkyl isocyanides. It may be inferred that highly congested aromatic isocyanides will react more slowly that the aryl isocyanides studied here due to poor initial binding to access intermediate 12. Previously, a similar trend was observed for phosphites. The key switching step that alters the electronic nature of the carbene occurs at the stage of intermediate 12 in Scheme 3. The steric size of the binding ligand and its σ-donor/π-acceptor properties are key in inducing the Buchner insertion. Another important factor influencing the rate and effectiveness of the Buchner reaction is the electronic environment of the metal. Our previous study found that electron-rich metal carbenes such as Fischer carbenes do not undergo the Buchner reaction, whereas π conjugation (such as that found in vinyl carbene 2) is permitted.12b In addition, the nature of the carbene ligand coordinated trans to the NHC ligand is significant, as it contributes or withdraws electron density from the metal, which modulates or increases chemical reactivity toward the Buchner reaction. The interplay of the combined electronic effects arising from the carbene substituent combined with that of the trans donor ligand (such as Cy3P) requires further study. This kinetic study was focused on the most commonly used Grubbs carbenes, but new metal carbene complexes continue to be discovered or developed to perform specific duties. Future studies on these ruthenium carbenes and postulated intermediates in alkene and ene-yne metathesis are ongoing in these laboratories.

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EXPERIMENTAL SECTION

General Information. Unless otherwise stated, reactions were conducted with oven-dried glassware under an atmosphere of nitrogen. Solvent (dichloromethane) was passed through alumina (Anhydrous Engineering solvent purifier) and stored under nitrogen. The Grubbs second-generation catalyst 3 and Hoveyda-II complex 4 were obtained from Materia Inc. (Pasadena, CA) and used as received. Ru(H2IMes)(CHPh)(PPh3)Cl2 was also synthesized from 3 according to the literature. All isocyanides were either purchased (4-chlorophenyl isocyanide, cyclohexyl isocyanide, benzyl isocyanide, TsCH2NC) or synthesized by previously reported literature procedures (4-methoxyphenyl isocyanide,27 octyl isocyanide,28 methyl isocyanoacetate14) and purified prior to use by distillation or column chromatography with EtOAc/hexanes. All ruthenium carbene and isocyanide stock solutions were stored under nitrogen and used immediately after preparation. Flash chromatography was carried out on untreated silica gel 60 from Sorbtech Technologies Inc. (230−400 mesh) under air pressure. Thinlayer chromatography (TLC) was performed on glass-backed silica plates (F254, 250 μm thickness, EMD Millipore) and visualized with UV light, phosphomolybdic acid, iodine, or potassium permanganate. IR kinetics were obtained using a ReactIR iC10 instrument equipped with a K4 conduit and a SiComp sensor (2.5 cm × 10 cm) running iC IR software. Preparative TLC was carried out using glass-backed silica plates (F254, 1 mm thickness) and visualized with UV light. 1H NMR spectra were recorded at 300, 400, or 500 MHz, proton-decoupled 13C NMR spectra were recorded at 75, 100, or 125 MHz using Varian Mercury 300, Inova 400, and Inova 500 instruments, and protondecoupled 31P NMR spectra were recorded at 121 MHz on a Varian Inova 400 spectometer. 1H NMR chemical shifts are reported in ppm relative to the solvent used (chloroform-d, 1H 7.26 ppm, 13C 77 ppm; benzene-d6, 1H 7.15 ppm, 13C 128.06 ppm; dichloromethane-d2, 1H 7.15 ppm), and 31P was referenced using H3PO4 as an external reference. Infrared spectra were recorded using a PerkinElmer Spectrum Two FTIR-ATR instrument. Mass analysis was performed on a Bruker SolariX 12T FTMS instrument using electrospray ionization with acetonitrile or dichloromethane as solvent. Kinetic Procedures: Buchner Reaction. Buchner insertion kinetics were tracked using a ReactIR iC10 instrument equipped with a K4 conduit and a SiComp sensor (2.5 cm × 10 cm) running iC IR software. The disappearance of isocyanide was tracked using the NC stretch in the region 2300−2000 cm−1. Temperature control was achieved using a water bath regulated by a Hake C1 thermostat. Reactions were performed using a 50 mL oven-dried flask with an internally threaded glass adapter sealed with a PTFE bushing and Viton O-ring around the probe. The flask was equipped with a magnetic stirrer, sealed with a rubber septum, and cooled under a nitrogen atmosphere.

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CONCLUSIONS In conclusion, the rate of isocyanide-promoted Buchner reactions depends on a balance of σ-donor/π-acceptor properties of the isocyanide ligand. Aryl and alkyl isocyanides were studied, where aryl isocyanides gave the fastest Buchner reactions. The reaction is an associative process where the initial binding of isocyanide occurs, showing first-order rate dependence on isocyanide and the Ru carbene complex. Aryl isocyanides yielded products with two isocyanide ligands. Slender alkyl isocyanides, on the other hand, gave an additional substitution reaction, replacing the tricyclohexylphosphine ligand, giving a tris(isocyanide) coordination complex. • Isocyanide-induced Buchner carbene insertion proceeds with a rate law first order in isocyanide and first-order in carbene complex. G

DOI: 10.1021/acs.organomet.7b00342 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Complex 8d.

General Kinetic Procedure. Reactions were carried out by transferring a desired amount of the isocyanide stock solution (2.8 mL) using a gastight syringe. The solution was then equilibrated at the desired temperature (0−25 °C) for 10 min under a nitrogen atmosphere. Then a volume of ruthenium carbene stock solution (0.2 mL) was injected into the reaction flask (total volume 3 mL). Consumption of isocyanide was followed in situ by the decreasing IR absorbance of the isocyanide functional group. All reactions were run in duplicate using the same stock solutions; additional runs were also performed using new stock solutions in order to assess the reproducibility of the experiments. The obtained data were treated with error limits at the 95% confidence interval using Microsoft Excel. Confirmation of the product was achieved by addition of mesitylene (0.33 equiv) with CDCl3, and the product mixture was analyzed by 1 H NMR. Varying the isocyanide stock solutions and multiple runs gave the kinetic data reported in Tables S1−S3 in the Supporting Information. Characterization of Buchner Insertion Products. In general, the isocyanide-promoted Buchner reactions were clean and afforded the insertion products 7 and 8 cleanly. In most cases, the complexes could be purified by flash chromatography, and purity was assessed in all cases by 1H NMR. Over time, acquisition of carbon-13 data showed evidence of decomposition occurring in solution over a period of hours. Evidence of decomposition precluded satisfactory combustion analysis for new complexes except for 7b and 8d. Complex 7a.

Rf = 0.13 (30% EtOAc/1% NEt3/hexanes). The purity was >93% based on 1H NMR. 1H NMR (300 MHz, CD2Cl2, ppm): δ 7.54 (d, 3 J = 7.4 Hz, 2 H, aromatic), 7.18−7.43 (m, 16 H, aromatic), 7.09 (t, 3 J = 6.9 Hz1 H, aromatic), 7.01 (t, 3J = 6.9 Hz, 2 H, aromatic), 6.75 (s, 1 H, mesityl CH), 6.70 (s, 1 H, mesityl CH), 5.70 (s, 1 H, vinylic CHCHT), 5.62 (s, 1 H, vinylic CHCHT), and 5.00−4.70 (m, 3 H, benzylic methine and CH2Ph), 4.53 (s, 2 H, CH2Ph), 4.32 (br s, 2 H, CH2Ph), 3.64−3.55 (m, 2 H, NCH2CH2N), 3.42−3.37 (m, 1 H, NCH2CH2N), 3.14−3.09 (m, 1 H, NCH2CH2N), 2.44 (s, 6 H, mesityl CH3), 2.13 (s, 3 H, CH3), 2.12 (s, 3 H, CH3), 2.06 (s, 3 H, CH3), 1.76 (s, 3 H, CH3). IR (CH2Cl2): 2144 and 2200 cm−1. Low-resolution MS (ESI+, m/z): molecular ion calculated for M+ − Cl [C52H53N5RuCl]+ 884.30, found 884.31. Anal. Calcd for C52H53N5Cl2Ru: C, 67.89; H, 5.81; N, 7.61; Cl, 7.71. Found: C, 67.55; H, 5.95; N, 7.25; Cl, 7.40. Complex 8f.

The product was isolated as a bright yellow solid (23 mg, 64% yield after column chromatography (10−30% EtOAc/hexanes). Rf = 0.19 (30% EtOAc/hexanes). The purity was >97% based on 1H NMR. 1H NMR (500 MHz, CDCl3, ppm): δ 7.51 (d, 3J = 7.5 Hz, 2 H, o-phenyl CH), 7.23 (t, 3J = 7.0 Hz, 2 H, m-phenyl CH), 7.20−7.17 (m, 1 H, p-phenyl CH), 6.89 (s, 2 H, mesityl CH), 5.75 (s, 1 H, vinylic CHCHT), 5.63 (s, 1 H, vinylic CHCHT), 4.83 (s, 1 H, benzylic methine), 3.64− 3.50 (m, 6 H), 3.39−3.32 (m, 1 H), 3.11−3.02 (m, 3 H), 2.43 (s, 3 H, mesityl CH3), 2.42, (s, 3 H, mesityl CH3), 2.24 (s, 3 H, mesityl CH3), 2.15 (s, 3 H, vinylic CH3), 2.06 (s, 3 H, vinylic CH3), 1.82 (s, 3 H, vinylic CH3), 1.73 (pentet, 3J = 7.7 Hz, 4 H, aliphatic), 1.55 (br s, 3 H, aliphatic), 1.43−1.33 (m, 6 H, aliphatic), 1.32−1.18 (m, 26 H, aliphatic), 0.89−0.85 (m, 9 H, alkyl CH3). IR (CH2Cl2): 2125 and 2145 (s) cm−1. Low-resolution MS (ESI+, m/z): molecular ion calculated for M+ − Cl [C55H83N5RuCl]+ 950.54, found 950.54. Due to the instability of this compound, combustion analysis was not obtained. Complex 8g.

The data for samples prepared were identical with spectroscopic data previously reported in the literature.4 Methine protons in the cycloheptatriene substructure are particularly diagnostic. Select 1H NMR (300 MHz, CDCl3, ppm): δ 5.61 (s, 1 H, vinylic CHCHT), 5.41 (s, 1 H, vinylic CHCHT) and 5.04 (s, 1 H, benzylic methine). IR (CH2Cl2): 2120 (s), 2060 and 2015 (s) cm−1. Complex 7b.

The product was isolated as an orange solid (50 mg, 70% yield) after recrystallization from CH2Cl2/pentane. The purity was >98% based on 1 H NMR. 1H NMR (500 MHz, CDCl3, ppm): δ 7.49 (d, 3J = 8.0 Hz, 2 H, o-phenyl CH), 7.30 (d, 3J = 8.5 Hz, 2 H, m-Ar CH), 7.21 (t, 3J = 7.5 Hz, 2 H, m-phenyl CH), 7.16 (d, 3J = 7.0 Hz, 1 H, p-phenyl CH), 6.95 (d, 3J = 8.5 Hz, 2 H, m-Ar CH), 6.83−6.78 (m, 4 H, o-Ar CH), 6.64 (s, 1 H, mesityl CH), 6.60 (s, 1 H, mesityl CH), 5.94 (s, 1 H, vinylic CHCHT), 5.41 (s, 1 H, vinylic CHCHT), 5.12 (s, 1 H, benzylic methine), 3.82 (s, 6 H, OCH3), 3.55−3.38 (m, 3 H, NCH2CH2N), 3.18 (m, 1 H, NCH2CH2N), 2.53 (s, 3 H, mesityl CH3), 2.49 (s, 3 H, mesityl CH3), 2.22 (s, 3 H, vinyl CH3), 2.12 (m, 6 H, vinyl CH3), 2.00−1.85 (m, 6 H), 1.67−1.5 (m, 19 H), 1.47 (s, 3 H), 1.21 (m, 4 H), 0.94 (m, 6 H). IR (CH2Cl2): 2070, 2130 (s), and 2040 (s) cm−1. Low-resolution MS (ESI+, m/z): molecular ion calculated for M+ − Cl [C62H79N4O2PRuCl]+ 1079.47, found 1079.50. Anal. Calcd for C62H79Cl2N4O2PRu: C, 66.77; H, 7.14; N, 5.02; Cl, 6.36. Found: C, 66.41; H, 7.10; N, 5.04; Cl, 6.25.

The product was isolated as a yellow solid (10 mg, 33% yield) by column chromatography (5% EtOAc/3% NEt3/hexanes). Rf = 0.31 (30% EtOAc/1% NEt3/hexanes). The purity was >97% based on 1H NMR. 1H NMR (500 MHz, CDCl3, ppm): δ 7.54 (d, 3J = 6.5 Hz, 2 H, o-phenyl CH), 7.24 (t, 3J = 7.0 Hz, 2 H, m-phenyl CH), 7.19−7.17 (m, 1 H, p-phenyl CH), 6.90 (s, 1 H, mesityl CH), 6.88 (s, 1 H, mesityl CH), 5.75 (s, 1 H, vinylic CHCHT), 5.62 (s, 1 H, vinylic CHCHT), 4.94 (s, 1 H, benzylic methine), 3.84−3.79 (m, 1 H, cyclohexyl CH), 3.58−3.49 (m, 2 H, NCH2CH2N), 3.37−3.30 (m, 1 H, NCH2CH2N), 3.08−3.04 (m, 1 H, NCH2CH2N), 2.43 (s, 3 H, mesityl CH3), 2.41 (s, 3 H, mesityl CH3), 2.25 (s, 3 H, mesityl CH3), 2.14 (s, 3 H, vinylic CH3), 2.05 (s, 3 H, vinylic CH3), 1.99−1.93 (m, 3 H, H

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Organometallics aliphatic), 1.83−1.40 (m, 20 H, aliphatic), 1.27 (m, 10 H, aliphatic). IR (CH2Cl2): 2100, 2135, and 2185 cm−1. Low-resolution MS (ESI+, m/z): molecular ion calculated for M+ − Cl [C49H65N5RuCl]+ 860.40, found 860.40. Due to the instability of this compound, combustion analysis was not obtained. Complex 7c.

assistance and Materia, Inc. (Pasadena, CA), for supplying Grubbs’ catalyst.

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(1) (a) Buchmeiser, M. R. Chem. Rev. 2000, 100, 1565−1604. (b) Fuerstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012−3043. (c) Frenzel, U.; Nuyken, O. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2895−2916. (d) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592−4633. (e) Diver, S. T.; Giessert, A. J. Chem. Rev. 2004, 104, 1317−1382. (f) Grubbs, R. H. Tetrahedron 2004, 60, 7117−7140. (g) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4490−4527. (h) Deshmukh, P. H.; Blechert, S. Dalton Trans. 2007, 2479−2491. (i) Nicolaou, K. C.; Snyder, S. A. Vignette 1: The Olefin Metathesis Reaction in Complex Molecule Construction. In Handbook of Metathesis; Wiley-VCH: Weinheim, Germany, 2008; pp 323−337. (2) (a) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann, W. A. Angew. Chem., Int. Ed. 1998, 37, 2490−2493. (b) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953−956. (3) (a) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 6543−6554. (b) Sanford, M. S.; Ulman, M.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 749−750. (4) (a) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168−8179. (b) Gessler, S.; Randl, S.; Blechert, S. Tetrahedron Lett. 2000, 41, 9973−9976. (5) Grela, K.; Harutyunyan, S.; Michrowska, A. Angew. Chem., Int. Ed. 2002, 41, 4038−4040. (6) Sanford, M. S.; Love, J. A.; Grubbs, R. H. Organometallics 2001, 20, 5314−5318. (7) (a) Huang, J.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18, 5375−5380. (b) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674−2678. (c) Huang, J.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P.; Capps, K. B.; Bauer, A.; Hoff, C. D. Inorg. Chem. 2000, 39, 1042−1045. (d) DiezGonzalez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251, 874−883. (e) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122− 3172. (8) Lehman, S. E., Jr.; Wagener, K. B. Organometallics 2005, 24, 1477−1482. (9) Fernández, I.; Lugan, N.; Lavigne, G. Organometallics 2012, 31, 1155−1160. (10) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.; Scholl, M.; Choi, T.-L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 2546−2558. (11) Klose, A.; Solari, E.; Floriani, C.; Geremia, S.; Randaccio, L. Angew. Chem., Int. Ed. 1998, 37, 148−150. (12) (a) Galan, B. R.; Gembicky, M.; Dominiak, P. M.; Keister, J. B.; Diver, S. T. J. Am. Chem. Soc. 2005, 127, 15702−15703. (b) Galan, B. R.; Pitak, M.; Gembicky, M.; Keister, J. B.; Diver, S. T. J. Am. Chem. Soc. 2009, 131, 6822−6832. (13) Poater, A.; Ragone, F.; Correa, A.; Cavallo, L. J. Am. Chem. Soc. 2009, 131, 9000−9006. (14) Galan, B. R.; Kalbarczyk, K. P.; Szczepankiewicz, S.; Keister, J. B.; Diver, S. T. Org. Lett. 2007, 9, 1203−1206. (15) French, J. M.; Caras, C. A.; Diver, S. T. Org. Lett. 2013, 15, 5416−5419. (16) Szczepaniak, G.; Urbaniak, K.; Wierzbicka, C.; Kosiński, K.; Skowerski, K.; Grela, K. ChemSusChem 2015, 8, 4139−4148. (17) Mori, M.; Sakakibara, N.; Kinoshita, A. J. Org. Chem. 1998, 63, 6082−6083. (18) Griffiths, J. R.; Keister, J. B.; Diver, S. T. J. Am. Chem. Soc. 2016, 138, 5380−5391. (19) (a) Anciaux, A. J.; Demonceau, A.; Noels, A. F.; Hubert, A. J.; Warin, R.; Teyssie, P. J. Org. Chem. 1981, 46, 873−6. (b) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic Methods for Organic Synthesis with Diazo Compounds: From Cyclopropanes to Ylides; WileyInterscience: New York, 1998. (20) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165−95. (21) Bodner, G. M. Inorg. Chem. 1975, 14, 2694−9.

The sample decomposed upon attempted isolation. IR (CH2Cl2): 2030, 2090, and 2130 (s) cm−1. Due to the instability of this compound, additional spectroscopic properties were not measured and combustion analysis was not obtained. Complex 8e.

The product was isolated as a yellow solid (10 mg, 35% yield) by column chromatography (5% MeOH/1% NEt3/CH2Cl2). Rf = 0.5 in 30% MeOH/CH2Cl2. The purity was >85% based on 1H NMR. 1 H NMR (500 MHz, CDCl3, ppm): δ 7.49 (d, 3J = 7.0 Hz, 2 H, o-phenyl CH), 7.24 (t, 3J = 7.5 Hz, 2 H, m-phenyl CH), 7.20−7.17 (m, 1 H, p-phenyl CH), 6.88 (s, 1 H, mesityl CH), 6.87 (s, 1 H, mesityl CH) 5.75 (s, 1 H, vinylic CHCHT), 5.63 (s, 1 H, vinylic CHCHT), 4.80 (s, 1 H, benzylic methine), 6.62−4.58 (m, 4 H, CH2CO2Me), 3.98 (br s, 2 H, CH2CO2Me), 3.79−3.73 (m, 9 H, CO2CH3), 3.59−3.56 (m, 2 H, NCH2CH2N), 3.41−3.37 (m, 1 H, NCH2CH2N), 3.12−3.08 (m, 1 H, NCH2CH2N), 2.45 (s, 3 H, mesityl CH3), 2.43 (s, 3 H, mesityl CH3), 2.52 (s, 3 H, mesityl CH3), 2.14 (s, 3 H, vinylic CH3), 2.08 (s, 3 H, vinylic CH3), 1.82 (s, 3 H, vinylic CH3). IR (CH2Cl2): 2100 (s), 2120, and 2165 cm−1. Due to the instability of this compound, neither mass spectrometry or combustion analysis was obtained.

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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.7b00342. Details of the kinetic derivations, tables of data, and characterization of (OC)5Cr(isocyanide) complexes (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.B.K.: keister@buffalo.edu. *E-mail for S.T.D.: diver@buffalo.edu. ORCID

Jerome B. Keister: 0000-0001-5376-9399 Steven T. Diver: 0000-0003-2840-6726 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF (CHE-601206, to S.T.D. and J.B.K.). The authors thank Synthia Gratia for technical I

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Organometallics (22) (a) Connor, J. A.; Jones, E. M.; McEwen, G. K.; Lloyd, M. K.; McCleverty, J. A. J. Chem. Soc., Dalton Trans. 1972, 1246−53. (b) Johnston, R. F. J. Coord. Chem. 1994, 31, 57−66. (c) Carpenter, A. E.; Mokhtarzadeh, C. C.; Ripatti, D. S.; Havrylyuk, I.; Kamezawa, R.; Moore, C. E.; Rheingold, A. L.; Figueroa, J. S. Inorg. Chem. 2015, 54, 2936−2944. (23) Chemical shifts were determined in d6-benzene. A very similar correlation is found for the trans carbonyl resonances, but it has been previously concluded that the cis carbonyl shifts are more consistent with other measures of σ-donor/π-acceptor character. See ref 21. (24) Klose, A.; Solari, E.; Hesschenbrouck, J.; Floriani, C.; Re, N.; Geremia, S.; Randaccio, L. Organometallics 1999, 18, 360−372. (25) Hong, S. H.; Wenzel, A. G.; Salguero, T. T.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2007, 129, 7961−7968. (26) (a) Lummiss, J. A. M.; McClennan, W. L.; McDonald, R.; Fogg, D. E. Organometallics 2014, 33, 6738−6741. (b) McClennan, W. L.; Rufh, S. A.; Lummiss, J. A. M.; Fogg, D. E. J. Am. Chem. Soc. 2016, 138, 14668−14677. (27) Dewanji, A.; Mueck-Lichtenfeld, C.; Bergander, K.; Daniliuc, C. G.; Studer, A. Chem. - Eur. J. 2015, 21, 12295−12298. (28) Lei, J.; Wu, X.; Zhu, Q. Org. Lett. 2015, 17, 2322−2325.

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DOI: 10.1021/acs.organomet.7b00342 Organometallics XXXX, XXX, XXX−XXX