Alkene Insertions into a Ru–PR2 Bond - Organometallics (ACS

Dec 1, 2016 - Department of Chemistry, University of Victoria, P.O. Box 1700 Stn CSC, Victoria, British Columbia, Canada V8W 2Y2. ‡ Max Planck Insti...
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Alkene Insertions into a Ru−PR2 Bond Krista M. E. Burton,† Dimitrios A. Pantazis,‡ Roman G. Belli,† Robert McDonald,§ and Lisa Rosenberg*,† †

Department of Chemistry, University of Victoria, P.O. Box 1700 Stn CSC, Victoria, British Columbia, Canada V8W 2Y2 Max Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36, D-45470 Mülheim an der Ruhr, Germany § X-ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 ‡

S Supporting Information *

ABSTRACT: An unusually broad series of discrete alkene insertion reactions has provided the opportunity to examine the mechanism(s) of this fundamental carbon−heteroatom bondforming process. Ethylene, electron-rich and electron-poor (activated) alkenes all react with the Ru−P double bond in Ru(η5-indenyl)(PCy2)(PPh3) to form κ2-ruthenaphosphacyclobutanes. Thermal decomposition of these metallacycles in solution, via alkene deinsertion and β-hydride elimination, is particularly favored for electron-rich alkenes, and hydride-containing decomposition products are implicit intermediates in the observed isomerization of 1-hexene. Kinetic studies, including a Hammett analysis of the insertion reactions of para-substituted styrenes, suggest that two distinct inner-sphere pathways operate for the insertion of electron-rich versus activated alkenes. DFT analyses have identified one pathway involving simple cycloaddition via a four-centered transition state and another that proceeds through an η2-alkene intermediate. Such an intermediate was observed spectroscopically during formation of the ethylene metallacycle, but not for substituted alkenes. We propose that “pre-polarized”, activated alkenes participate in direct cycloaddition, while ratedetermining η2-adduct formation is necessary for the activation of electron-rich alkenes toward migratory insertion into the Ru−P bond.

■

Scheme 15

INTRODUCTION The insertion of alkenes into metal−ligand bonds is a fundamental organometallic transformation that can be critical to catalytic processes involving these important chemical feedstocks. Mechanisms of alkene insertion into M−C and M−H bonds are fairly well-defined, since a reasonable number of examples exist for which the insertion has been directly observed and studied.1 However, examples of alkene insertions into M−E bonds where E is a heteroatom are rare,1,2 despite the increasing importance of catalytic heterofunctionalization of unsaturated substrates in the synthesis of value-added molecules.3 Of these, alkene insertions into M−O and M−N bonds have received the most attention, in the context of catalytic alkene oxidation, alkoxidation, and amination chemistries.4 We have found just a handful of explicit examples of the insertion of alkenes or alkynes into metal−phosphido bonds (M−PR2) (Schemes 1−4). These span main-group and transition-metal systems but are notably limited to activated unsaturated substrates. The insertion of an alkyne at a Mg bis(phosphido) complex (Scheme 1) models proposed intermediates in the formation of group 2 phosphacyclopentadienides5a,b and also provides evidence for this critical insertion step in the calcium-catalyzed hydrophosphination of a wider range of butadiynes by diphenylphosphine.5c Similarly, the © 2016 American Chemical Society

Scheme 27

complexes shown in Scheme 4 are models for a Pt phosphido hydride complex implicated as a key intermediate in catalytic alkene hydrophosphination;6a,b these reactions provided evidence for the possible importance of acrylonitrile insertion Received: September 26, 2016 Published: December 1, 2016 3970

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Organometallics Scheme 38

This metallaphosphacyclobutane moiety persists through coordination of the incoming primary phosphine substrate prior to protonolysis to regenerate a lanthanum phosphido intermediate. Such “residual” M−P interactions are clearly an important feature in systems where coordinative unsaturation results from alkene/alkyne insertion; the DFT model of a proposed catalytic intermediate resulting from alkyne insertion into a Ca−PPh2 bond shows a similar Ca−P interaction that restores octahedral geometry at Ca2+ (Scheme 7),5c but no comparable Nb−P interaction is observed in the structure calculated for an 18e alkyne insertion product analogous to those illustrated in Scheme 3.8

Scheme 46a

Scheme 7

into the Pt−PR2 bond in this catalysis, although subsequent studies provided strong evidence for the importance of an alternative mechanism involving outer-sphere nucleophilic attack of the phosphido ligand at this activated alkene.6c,d The examples in Schemes 1 and 4 highlight the general relevance of metal phosphido complexes to a growing interest in metal-catalyzed hydrophosphination of unsaturated organic molecules by primary or secondary phosphines.3e,f,9 Many examples of this catalysis are proposed to involve M−PR2 intermediates, which may then go on to react with alkene/ alkyne by inner- or outer-sphere processes.9f,10 Several computational studies of hydrophosphination catalyst systems have examined migratory insertions of carbon multiple bonds into the M−PR2 bond. For example, the insertion of acetylene into the Pd−PMe2 bond in Pd(η2-CHCH)(PH3)(PMe2)H11 (Scheme 5) models putative intermediates in the

We previously reported the reactions of the coordinatively unsaturated ruthenium phosphido complex 114b with both simple and activated alkenes and alkynes to give metallaphosphacyclobutane (3)14d and metallaphosphacyclobutene (4)14e products, respectively (Scheme 8). The nonpolar Ru−C Scheme 8

Scheme 5

Pd-catalyzed hydrophosphination of alkynes.12 This work highlights an interesting dependence of the barrier to Pd−P insertion on the rotational conformation of the phosphido ligand lone pair with respect to the Pd−P bond and the incoming alkyne and shows that the ground state for the product of alkyne insertion into the Pd−P bond (at least in silico, in the absence of other L donors) is a κ2-metallaphosphacyclobutene complex. Similarly, a computational study examining the migratory insertion of η2-bound alkene into the La−P bond of a proposed phosphido intermediate in the lanthanocene-catalyzed intramolecular hydrophosphination of α,ω-pentenylphosphine found that the lanthanum-bound alkylphosphine resulting from insertion shows a weak La−P interaction (Scheme 6).13

bond in these complexes is relatively stable toward protonolysis (unlike the Ca and La systems described above), although we have demonstrated a stepwise alkene hydrophosphination cycle based on 3 using [NEt3H]+ or stronger organic acids for this step.14g These clean alkene/alkyne addition reactions provide an unusual opportunity to examine the mechanism(s) of insertion into a metal−phosphido bond. The wide scope of the alkene insertion reactions giving 3 is particularly intriguing; it ranges from the strongly activated acrylonitrile to simple alkenes such as ethylene and 1-hexene and even the electronrich ethyl vinyl ether.15 We previously obtained strong evidence for the concerted nature of the reaction of ethylene with 1.14d We have now examined more closely that reaction and those of a range of terminal alkenes. The studies presented below point to two possible inner-sphere pathways for the formation of the metallacycles 3; one involves a single, concerted cycloaddition at the Ru−P double bond in 1, and the other occurs via an η2alkene intermediate and “traditional” migratory insertion. We propose the electronic character of the reacting alkene determines which pathway dominates for this Ru system.

Scheme 6

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RESULTS AND DISCUSSION Synthesis and Characterization of Metallacyclobutanes. Since our initial report of P−C bond formation via the apparent [2 + 2] cycloaddition reactions of both activated (acrylonitrile) and simple alkenes (1-hexene and ethylene) at the Ru−P double bond in Ru(η5-indenyl)(PCy2)(PPh3) (1),14d we have developed a “one-pot” procedure for the preparation of these and other ruthenaphosphacyclobutane complexes directly from the secondary phosphine precursor 2 (Scheme 9).14g We Scheme 9 Figure 1. Molecular structure of syn-[Ru(η 5 -indenyl)(κ 2 PhCHCH2PCy2)(PPh3)] (syn-3b). Atoms are represented by Gaussian ellipsoids at the 30% probability level. Selected interatomic distances (Å) and bond angles (deg) (C* denotes the centroid of the ring defined by C(7A)−C(1)−C(2)−C(3)−C(3A)): Ru−P(1) = 2.2927(4), Ru−P(2) = 2.2830(4), Ru−C(12) = 2.1869(13), Ru−C* = 1.959, C(11)−C(12) = 1.5407(18), P(1)−C(11) = 1.8336(13); P(1)−Ru−P(2) = 97.154(13), P(1)−Ru−C(12) = 69.90(3), P(2)− Ru−C(12) = 88.96(4), P(1)−Ru−C* = 130.9, P(2)−Ru−C* = 125.4, C(12)−Ru−C* = 127.0, Ru−P(1)−C(11) = 89.04(4), P(1)−C(11)− C(12) = 98.72(9), Ru−C(12)−C(11) = 101.30(8). Indenyl crystallographic slip distortion18 Δ = d[Ru−C(7A),C(3A)] − d[Ru− C(1),C(3)] = 0.151.

have used this higher-yielding method to make the previously characterized metallacycles 3a−d;16 for this work we also prepared the new analogue 3e, derived from propene, and the previously partially characterized ethyl vinyl ether metallacycle 3f, using this direct route. As described below, this has given us the opportunity to carefully examine the thermal stability and reactivity of these complexes. The new metallacycles 3e,f gave spectra similar to those reported previously for 3a−d (see the Supporting Information). Particularly diagnostic are the 31P{1H} NMR signals for the PCy2 portion of the four-membered rings, which appear at high field (e.g., δ(31P) for syn-3e in d6-benzene −10.8 ppm (−PCy2−) and 60.0 ppm (PPh3)). For the substituted substrates (b−f), absolute regioselectivity of cycloaddition gives products containing the substituent R on the carbon α to Ru in the metallacycle. Complexes 3b−e contain exclusively the syn diastereomer, in which the substituent R lies on the same side of the metallacycle as the indenyl ligand at Ru, while the vinyl ether product, 3f, is inevitably isolated as a 1:1 mixture of the syn and anti isomers. Single-crystal X-ray diffraction confirmed the solid-state structures of complexes syn-3b and syn-3e (Figures 1 and 2); neither structure shows unusual or unexpected features. Both complexes adopt rotational conformations in their solid-state structures that place the strongly σ donating α-alkyl carbon of the metallacycle anti to the C6 ring of the indenyl. We, and others, have previously noted this conformational preference of the η5-indenyl ligand.17 Solution Behavior of Metallacycles 3. Our previous room-temperature NMR studies and characterization of 3a−d gave no evidence for the reversibility of alkene insertion,14d but we have now more thoroughly investigated the thermal stability of complexes 3a−f in solution, by heating samples in d8-toluene at 60 °C. The complexes undergo variable amounts of alkene deinsertion to complex 1 under these conditions, as diagnosed primarily by the observation of 31P{1H} NMR signals due to complex 5, which results from irreversible orthometalation of the PPh3 ligand in 1 (Scheme 10).14b,19 For complexes 3d−f (formed from particularly electron rich alkenes with R = Bun, Me, OEt) the intermediacy of 1 is observed visually; the golden color of the metallacycle darkens to the blue-green associated with complex 1 within hours and then eventually lightens again to the golden orange of 5. Transient 31P{1H} NMR signals for 1 are observed for these complexes and for 3b (R = Ph). For

Figure 2. Molecular structure of syn-[Ru(η 5 -indenyl)(κ 2 MeCHCH2PCy2)(PPh3)] (syn-3e). Atoms are represented by Gaussian ellipsoids at the 30% probability level. Selected interatomic distances (Å) and bond angles (deg) (C* denotes the centroid of the ring defined by C(7A)−C(1)−C(2)−C(3)−C(3A)): Ru−P(1) = 2.2732(4), Ru−P(2) = 2.2900(4), Ru−C(12) = 2.1817(16), Ru−C* = 1.948, C(11)−C(12) = 1.552(2), P(1)−C(11) = 1.8270(16); P(1)− Ru−P(2) = 97.844(14), P(1)−Ru−C(12) = 68.37(4), P(2)−Ru− C(12) = 89.44(4), P(1)−Ru−C* = 130.3, P(2)−Ru−C* = 125.9, C(12)−Ru−C* = 126.9, Ru−P(1)−C(11) = 90.11(5), P(1)−C(11)− C(12) = 95.29(10), Ru−C(12)−C(11) = 101.52(9). Indenyl crystallographic slip distortion18 Δ = d[Ru−C(7A),C(3A)] − d[Ru− C(1),C(3)] = 0.115.

Scheme 10

complexes 3a,c the alkene deinsertion is sufficiently slow, relative to orthometalation, that no blue or green color is observed (nor any NMR signals due to complex 1); even after weeks of heating, these samples show just 5−10% conversion to 3972

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epimerization to give thermodynamic syn:anti ratios of 1:1 (∼24 h) and 1:4 (∼4 mo), respectively.22 However, heating syn-3d,e (R = Bun, Me) gave, in addition to anti-3, a variety of other ruthenium phosphine-containing products, as determined by 31P{1H} NMR (Scheme 11). These include apparent E and Z isomers of the vinylphosphine hydride complexes 6d,e, as well as trace amounts of additional hydride-containing species that may arise from β-H elimination from the pendant Bun and Me groups on the metallacycles in 3d,e, respectively.23 Additional new singlets apparently correspond to Ru(η5indenyl)(η3-CH2CHCH(Y))(L), where L = PPh3, PCy2H and Y = H, Prn, resulting from allylic C−H abstraction from the alkene followed by phosphine dissociation.24 1H NMR indicates that isomerization of the excess free 1-hexene is occurring,25 and at least four ruthenium hydride signals are observed for the reactions of both 3d and 3e. When metallacycle 3f (R = OEt) is heated to 60 °C in the presence of excess ethyl vinyl ether, we also observe signals attributed to the product 6f resulting from possible β-hydride elimination. The much simpler 31P{1H} NMR spectrum in this case shows more clearly than for 3d,e that the anti isomer is more persistent than the syn isomer under these conditions, but it is not clear whether this arises from selective decomposition or represents an actual thermal isomer distribution. Kinetic Analysis of Alkene Cycloaddition at the RuP Bond in 1. Even in the absence of alkene substrate, complex 1 exhibits a rich chemistry in solution (Scheme 12, left). At room temperature, the phosphido complex is in rapid equilibrium with the phosphaalkene hydride complex 7, apparently via reversible β-hydride elimination.14b Our reactivity studies of 1 all indicate that complex 7, which comprises 10% of the 31 1 P{ H} NMR signal intensity for samples of 1 at room temperature, is unreactive toward other reagents on the time scale of its conversion to 1.14c−e As described above, complex 1 also slowly and irreversibly isomerizes to the orthometalated complex 5. The first-order rate constant, kom, for this 1,2-C−H addition reaction in toluene at room temperature is on the order of 10−6 s−1. While competing orthometalation is a concern for long-term solution stability of 1, this rate constant is 3 orders of magnitude smaller than those measured for the alkene insertion reactions examined in this work (kadd; Table 1). At the alkene concentrations used in the kinetic studies described below, complex 5 was not observed, and the formation of 7 is a minor pathway that did not not affect the measurement of kadd (see the Supporting Information). We monitored the disappearance of dark blue 1 at λmax 590 nm under pseudo-first-order conditions, using 150−600 equiv of alkene substrate (for details see the Supporting Information) to obtain second-order rate constants, kadd, for the addition of alkenes to 1 (Table 1).26 Acrylonitrile and ethylene reacted too quickly for us to collect UV data; therefore we have estimated a

complex 5. For all complexes except 3c additional minor 31 1 P{ H} NMR signals appear that have been assigned as the vinylphosphine hydride complex Ru(η 5 -indenyl)H(PCy2CH2CH(R))(PPh3), 6 (vide infra; Scheme 11).20 Scheme 11

Because of these competing orthometalation and apparent βhydride elimination reactions, we were unable to measure equilibrium constants (K = kadd/krev; Scheme 12) for the Scheme 12

formation of the metallacycles 3 from 1 from these experiments; however the fact that we do not observe any 1 during thermolysis of 3a,c does suggest that the equilibrium lies heavily toward the metallacycles in these cases, while for the other systems, the equilibrium constants are smaller. We have assumed that the syn:anti diastereomer ratios we observe for the metallacycles isolated from the one-pot syntheses represent the kinetic product distribution.21 We attempted to establish thermodynamic syn:anti diastereomer ratios for the metallacycles by heating samples of 3b−f in the presence of excess (typically 50 equiv) alkene, to inhibit competing deinsertion and subsequent orthometalation. For syn-3b (R = Ph) and syn-3c (R = CN), we did observe

Table 1. Room-Temperature Rate Constants and Activation Parameters for the Addition of Selected Terminal Alkenes, RCH CH2, to [Ru(η5-indenyl)(PCy2)(PPh3)] (1)a R CN, H Ph Bun OEt

kadd/10−3 M−1 s−1

ΔH⧧, kcal/mol

ΔS⧧, eu

ΔG⧧(295 K), kcal/mol

Ea, kcal/mol

−50 ± 19 −50 ± 17 −53 ± 17

18c 20 ± 1 21 ± 1 20 ± 1

7.4 ± 0.2 7.6 ± 0.2 6.2 ± 0.7

b

500 5.7 ± 0.2 2.5 ± 0.1 6.1 ± 0.2

6.7 ± 0.2 7.2 ± 0.2 5.7 ± 0.5

a

Reactions carried out under pseudo-first-order conditions in toluene. bEstimated minimum kobs determined from rate = kobs[alkene], where rate = 1/30 s (average time for injection and mixing of the alkene) and [alkene] = 0.06 M. cMaximum value, based on estimated minimum kadd. 3973

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Organometallics minimum value for kadd on the basis of an average alkene injection and mixing time of ∼30 s. Variable-temperature measurements gave the activation parameters shown in Table 1; of note are the relatively large, negative values for ΔS⧧, which are consistent with the associative nature of the reaction. The room-temperature rate constants decrease both with increased steric bulk (e.g., compare kadd for the two-electron-rich R: small OEt and bulkier Bun) and with greater electron-donating ability of the alkene substituent (e.g., compare kadd for the two small R: withdrawing CN and donating OEt). We have put some effort into understanding how both electron-rich and electron-poor alkenes react with 1 to give metallacyclic products 3. We previously showed that the reaction of ethylene with complex 1 occurs via a concerted mechanism, based on our observation of the conservation of stereochemistry during the reactions of cis- and trans-d2ethylene with 1.14d Although it seems reasonable that a similar, presumably inner-sphere, mechanism could be in effect for the reactions of 1-hexene, propene, and ethyl vinyl ether, it is possible that, despite the established Lewis acidity at Ru in this system,14b,c,f the reactions of the more activated alkenes, styrene and acrylonitrile, could be occurring via an outer-sphere, stepwise Michael addition pathway (Scheme 13). As described

Figure 3. Hammett plot for the reaction of para-substituted styrenes with 1. Except for X = OCH3, where an error bar is included, errors are smaller than the data points shown.

our attempts to obtain evidence for the importance of putative zwitterionic intermediates in the reactions of electron-poor alkenes were unsuccessful. For example, we added acrylonitrile and 4-bromostyrene (activated alkenes) and propene (an electron-rich control) to 1 in the presence of electrophilic trapping reagents (e.g., Scheme 14). When these were

Scheme 13 Scheme 14

in the Introduction, this mechanism was shown by Glueck to be in effect for some Pt hydrophosphination systems, and subsequently it has been invoked to explain the activity of other late metals in the catalytic hydrophosphination of activated alkenes.27 Such outer-sphere nucleophilic attack by metal−phosphido ligands at electrophilic carbon centers is also important in cross-coupling reactions of secondary phosphines catalyzed by late-metal phosphido complexes, including Ru systems.28 To gauge the impact of the electronic character of the alkene on metallacycle formation, we carried out a Hammett analysis of the reactions of 1 with a series of para-substituted styrenes. The curved plot in Figure 3 indicates a change in the nature of the rate-determining step (RDS) for this series of alkenes as they vary from electron rich to electron poor. The rate of metallacycle formation is insensitive to electron-releasing character of the para substituent in the more electron rich styrenes (0 < ρ < 1 region),29 but the rate increases significantly with increasing electron-withdrawing ability of the para substituent in the electron-poor styrenes (ρ > 1 region). These observations suggest that polarization of the alkene is important in the RDS for the electron-poor styrenes, while increasing electron richness (or an opposite polarization) is not important in the RDS for electron-rich styrenes. The results from the Hammett study could be rationalized by a switch in mechanism along the series of alkenes studied, from a concerted, inner-sphere mechanism for the electron-rich alkenes to the stepwise, outer-sphere mechanism shown in Scheme 13 for the polarized, electron-poor alkenes, which should be accelerated for more activated substrates. However

benzophenone or benzaldehyde, no products other than the metallacycles 3 were observed. Addition of triethylammonium chloride to the reaction mixtures gave a mixture of 3 and the secondary phosphine complex 2, which results from interception of the extremely nucleophilic phosphido ligand in 1 by the proton.14b Thus these reactions provided no evidence for the formation of zwitterionic intermediates. We further investigated the possible importance of zwitterionic intermediates for these reactions by measuring rate constants for the cycloaddition of both the electron-rich pMe styrene and the electron-poor p-Br styrene in solvents of variable polarity (Table 2). We anticipated that the rate Table 2. Pseudo-First-Order Rate Constants for the Reactions of Selected Styrenes with 1 in Different Solvents kobs/10−2 min−1 solvent

ε

toluene fluorobenzene THF

2.32 5.42 7.32

a

ENT b

p-Br styrene

p-Me styrene

0.099 0.194 0.207

15.2 ± 1.0 14.8 ± 0.3 9.7 ± 0.1

4.7 ± 0.1 4.8 ± 0.2 3.45 ± 0.03

a

Dielectric constant.31 bNormalized Dimroth−Reichardt polarization parameter.32

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Organometallics constant for cycloaddition of p-Br styrene would vary with the solvent polarity if an outer-sphere, Michael-type mechanism was in effect, while that for the p-Me styrene cycloaddition would be unaffected by different solvents. However, neither kobs value is affected by a change from toluene to fluorobenzene, and both kobs values decrease slightly in the more polar THF. Overall, these experiments indicate that neither alkene reacts via a mechanism in which the rate-determining transition state has undergone a large change in polarization relative to the ground state structure that precedes it in the reaction trajectory. These results do not provide evidence for the formation of zwitterionic intermediates, although they also do not preclude the possibility that such intermediates might be forming.30 Computational Studies of the Formation of Ethylene Metallacycle 3a. We have identified three possible trajectories for the reaction of ethylene with 1 to generate complex 3a, using DFT (Figure 4). One is direct, with a single four-centered

Figure 5. View along the η5-indenyl−Ru bond showing rotational isomers placing the C6 ring either anti or syn to the new Ru−C bond of the metallacycle 3a.

C5−Ru bond. It is connected with the starting phosphido complex 1 via a distinct η2-ethylene adduct, anti-8a, and two distinct transition states, anti-TS2 and -TS3 (blue lines in Figure 4). A slightly higher energy conformer, syn-3a, is connected with 1 via syn-8a and syn-TS1 and -TS2 (green lines in Figure 4). We have also located a third conformer of 3a (not shown), which is also syn but differs in the relative conformations around the two P−Cy bonds in the metallacycle. We have not yet identified a syn conformer for the transition state structure TS1 associated with the concerted cycloaddition pathway. If the barriers to rotation around the Ru−C5 bond were low relative to the barriers between minima in these trajectories, then we could assume that facile conformational equilibrations would effectively generate a Boltzmann distribution of conformers for each minimum and transition state, for which the lowest energies would define the overall barriers throughout the trajectory. For example, for the 1,2-insertion pathways shown in Figure 4, we would anticipate a barrier of 15.0 kcal/ mol for formation of intermediate 3a and a barrier of 25.3 kcal/ mol for the rate-determining 1,2-insertion step (i.e., the syn pathway), which would ultimately produce the more stable anti product. However, examination of the minima we have identified so far suggests that this assumption is probably not valid. Rotations of the indenyl ligand are “geared” to some degree with rotations of the PPh3 ligand and, regardless of the relative conformations of the P−Cy bonds, it seems highly unlikely that the C6 ring of the indenyl ligand would be able to “pass over” the PCy2 fragment in a complete 360° rotation. Calculations for the simpler system in which the η5-indenyl ligand is replaced with η5-Cp give a comparable energy for TS1 in the concerted pathway (43.4 kcal/mol), and energies for the two-step insertion pathway resemble those for the syn pathway (green lines) shown in Figure 4 (TS2, 16.8 kcal/mol; η2 adduct, −4.5 kcal/mol; TS3, 23.6 kcal/mol; product metallacycle, −10.5 kcal/mol). Although collectively these calculations seem to suggest that a “direct” cycloaddition of ethylene at 1 is disfavored relative to a 1,2-insertion pathway via an η2-ethylene intermediate, we are reluctant to rely heavily on the calculated relative energies, given the uncertainties arising from these conformational issues. Experimental support for a 1,2-insertion mechanism in the reaction of ethylene with 1 comes from the fact that we observe the η2-ethylene intermediate 8a at low temperature, both by visual inspection and by 31P{1H} NMR. Addition of ethylene to a solution of complex 1 in d8-toluene at −80 °C causes a color change from dark blue to deep red, similar to other L adducts of 1 we have prepared.14b,c,f Low-temperature 31P{1H} NMR of this red solution (Figure 6, bottom) shows two new signals at 55.3 ppm (ω1/2 = 40 Hz) and 27.6 ppm (ω1/2 = 148 Hz) in addition to signals for the cycloaddition product 3a and small amounts of 1 and the phosphaalkene complex 7. We attribute the new signals to complex 8a. For other six-coordinate L adducts of 1, very low or nonexistent 2JPP coupling between

Figure 4. Relative Gibbs free energy diagram (298.15 K, kcal/mol) for the reaction of ethylene with complex 1 as determined by DFT (PBED3BJ/ZORA). [Ru] = η5-indenyl.

transition state (TS1, orange lines in Figure 4) in which the RuPCy2 and CC double-bond characters are largely preserved. The other two involve the formation of an η2C2H4 adduct, complex 8a, in which the Ru−P double bond has been disrupted and the geometry at P has changed from planar to pyramidal (blue and green lines in Figure 4). This intermediate undergoes 1,2-insertion of the coordinated alkene into the Ru−P bond. In these and subsequent calculations for the substituted alkenes acrylonitrile and ethyl vinyl ether (vide infra) we have found no evidence for indenyl ring slippage from η5 to η3 hapticity.33 We have also searched for outer-sphere trajectories that might lead to zwitterionic intermediates, but no such species were located even upon inclusion of a continuum dielectric model that might help stabilize charge separation.34 The trajectories shown in Figure 4 illustrate a challenge in modeling this η5-indenyl system; calculated energies are highly sensitive to conformational variations, especially those associated with rotation of the indenyl ligand around the Ru−η5-indenyl bond (Figure 5). Thus, in the lowest energy conformer of the product 3a, the C6 ring of the indenyl ligand lies anti to the α-carbon of the metallacycle, with respect to the 3975

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proceed at −80 °C (only signals due to unreacted 1 and isomer 7 were observed). Even when we monitored the samples as the probe temperature was slowly raised from −80 °C to room temperature (which sent the reactions to completion), we observed only signals due to 1/7 and metallacyclic product 3. These results could point to the destabilization of a putative η2 intermediate for the substituted alkenes, relative to a subsequent transition state for 1,2-insertion (i.e., an insertion pathway in which the formation of alkene adduct is ratedetermining), or they could indicate that the direct cycloaddition trajectory shown in Figure 4 is more relevant or accessible in the reactions of substituted alkenes. In fact the operation of one or the other of the two insertion pathways shown in Figure 7, depending on the electronic

Figure 6. 31P{1H} NMR (202.46 MHz, d8-toluene) of the reaction of 1 with ethylene showing the formation of intermediate 8a at −80 °C and its subsequent conversion to the metallacyclic product 3a when the sample is warmed to room temperature. Signals due to the phosphaalkene complex 7 are observed when the sample is at −80 °C, due to its slowed isomerization to 1. (Low-field signals for planar P in 1 and 7 are not shown.) Signals due to an unidentified impurity are marked with asterisks.

PPh3 and the PCy2 ligands in their room-temperature spectra is diagnostic of the pyramidal geometry at PCy2,35 but at −80 °C such fine structure (or the absence thereof) is not apparent for 8a. The 31P chemical shift for the PPh3 ligand in these adducts is routinely seen at 50−55 ppm; therefore the relatively sharp signal at 55.3 ppm is assigned to the PPh3 ligand in 8a. The chemical shift observed for the pyramidal PCy2 ligand varies quite widely for such L adducts and seems to shift downfield as a function of where the equilibrium lies between adduct and (complex 1 + free L). The shift of 56.9 ppm for the CO adduct,14b which forms irreversibly, probably represents a “real” δ value for pyramidal Ru−PCy2, while the shift of 82.9 ppm for the pyridine adduct,14c which is quite unstable relative to 1 plus free pyridine and cannot be isolated, represents a significant contribution of the planar PCy2 in 1 (δ276 ppm) to the average solution structure. In this context, the signal at 27.6 ppm, which must be due to the PCy2 ligand, has an unexpectedly high field shift. This, along with the breadth of the peak, may indicate an equilibrium between 8a and the cycloadduct 3a, for which δ(PCy2) is −11 ppm. When it was warmed to room temperature, the sample converted completely and cleanly to the bright yellow cycloadduct 3a (Figure 6, top). Our ability to observe the transient formation of 8a suggests that the subsequent 1,2-insertion step in this pathway is rate determining, as suggested by the syn trajectory shown in Figure 4. Relevance of the Ethylene Computational Results to the Reactions of Substituted Alkenes with 1. When we monitored reactions of substituted alkenes H2CCH(R) with 1 by low-temperature 31P{1H} NMR, we did not observe any signals due to η2-alkene adducts 8. For acrylonitrile, the reaction was complete (signals due only to metallacycle 3c) by the time the reagents (precooled to −80 °C) were mixed and the NMR tube was placed in the precooled spectrometer. For the additions of 4-bromostyrene, propene, and ethyl vinyl ether to 1 under comparable conditions, the reactions did not

Figure 7. Energy profiles illustrating the distinct inner-sphere mechanisms that may be operating for activated versus nonactivated terminal alkenes in their reaction with complex 1.

character of the substituted alkene, provides a possible explanation for the apparent change in rate-determining step that is suggested by the Hammett plot in Figure 3. Thus, a direct cycloaddition pathway (blue lines in Figure 7) could be favored for activated alkenes, which are effectively “prepolarized” in such a way that facilitates alignment of the electrophilic alkene carbon with the nucleophilic phosphorus in this four-centered transition state. The barrier to this process would be lowered with increasing electron-withdrawing ability of the alkene substituent, consistent with the right-hand portion of the Hammett plot (Figure 3). On the other hand, coordination to Ru may be necessary to “activate” the simple or electron-rich alkenes for intramolecular nucleophilic attack by the phosphido ligand, leading to migratory insertion (red lines in Figure 7). A similar mechanism appears to be operating for ethylene, for which the insertion step is rate determining. For electron-rich terminal alkenes, though, the considerable steric encumbrance at Ru, especially as provided by the PPh3 ligand and the indenyl C6 ring, would cause the barrier to η2adduct formation to become rate determining, as shown in Figure 7, and the rate to be more sensitive to steric rather than electronic character of the alkene substituent. This is consistent with the left-hand portion of the Hammett plot. We modeled computationally the reactions of 1 with acrylonitrile and ethyl vinyl ether, as the most extreme examples of electron-poor and electron-rich alkenes examined in this study, respectively. We followed only the formation of 3976

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reactions of the resulting metallacycles 3 are especially pronounced for the electron-rich analogues and represent classic, fundamental processes in organometallic chemistry that have not previously been observed for phosphido insertion products. Despite issues arising from the conformational variability of the η5-indenyl−Ru and P−Cy bonds, DFT analysis points to the migratory insertion of ethylene via an η2 adduct of 1 to give metallacycle 3a, and this intermediate has been identified by low-temperature 31P{1H} NMR. Computational studies also suggest an alternate, concerted cycloaddition route for the formation of metallacycles 3 may be possible; the operation of such a pathway for activated alkenes, while the migratory insertion pathway dominates for simple and electron rich alkenes, is consistent with the divergent behaviors of alkenes of differing electronic character illustrated by a range of kinetic studies and observations.

the syn diastereomers (OEt and CN groups syn to indenyl with respect to the plane of the product metallacycle). We also tracked only the conformations in which CC addition occurs anti with respect to the C6 ring of the indenyl (see Figure 5), because for these substrates addition via the syn conformer is disfavored or impossible due to steric interactions. In both cases, we identified intermediates and transition states consistent with the two types of inner-sphere trajectory previously established computationally for ethylene (Figure 4). However, as shown in Table 3, the calculated relative Table 3. Relative Gibbs Free Energies (298.15 K, kcal/mol) for Transition States and Intermediates in the Reactions of Acrylonitrile and Ethyl Vinyl Ether with 1a alkene

TS1

TS2

8

TS3

syn-3

acrylonitrile ethyl vinyl ether

37.4 36.8

20.2 24.4

0.0 9.0

22.1 33.5

−22.6 −9.7

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a

Structure labels correspond to transition states and intermediates shown in Figure 4 for ethylene. The reference energy (0 kcal/mol) refers to complex 1 plus alkene.

EXPERIMENTAL SECTION

General Considerations. Unless otherwise specified, all procedures were carried out under nitrogen in a glovebox or using conventional Schlenk techniques. Solvents were degassed and then dried by passing through columns of activated alumina, except for fluorobenzene (distilled from P2O5, degassed by three freeze−pump− thaw cycles) and THF (distilled from Na/benzophenone, stored over Na/benzophenone, degassed by three freeze−pump−thaw cycles, and vacuum-transferred before use). Deuterated solvents (CIL) were dried over Na/benzophenone (d8-toluene, d6-benzene) or calcium hydride (d1-chloroform), degassed by three freeze−pump−thaw cycles, and vacuum-transferred before use. Unless otherwise specified, reagents were purchased from Sigma-Aldrich Canada or Praxair Canada and used as received or predried and degassed using established procedures. Pyridine hydrochloride was recrystallized before use. 2,6Lutidine hydrochloride,36 Ru(η5-indenyl)Cl(PHCy2)PPh3 (2),14a and metallacycles 3a−d14g were prepared using literature procedures. NMR spectra were recorded at ambient temperature, unless otherwise noted, on a Bruker AVANCE 500 spectrometer (500.13 or 500.27 MHz for 1H, 125.77 MHz for 13C, and 202.46 MHz for 31P) or on a Bruker AVANCE 300 spectrometer (121.49 or 121.55 MHz for 31P). Chemical shifts are reported in ppm and referenced to residual protonated solvent peaks: 7.16 ppm (C6D5H), 7.09 ppm (C7D7H) for 1H; 128.39 ppm (C6D6), 137.86 ppm (C7D8) for 13C. All 1 H and 13C chemical shifts are reported relative to tetramethylsilane (TMS), and 31P shifts are relative to 85% H3PO4(aq). UV−vis spectra and kinetic data were acquired using a Varian Cary-5 or Varian Cary100 UV−vis−NIR spectrophotometer. Microanalysis was performed by Canadian Microanalytical Service Ltd. (Delta, BC, Canada). Crystallographic Details. Crystals of syn-3b were grown by layering acetonitrile onto a dichloromethane solution of the compound, while crystals of syn-3e were grown by layering toluene onto an acetonitrile solution of the compound. Suitable crystals were selected, coated with a thin layer of Paratone-N, and then mounted on a glass fiber. Diffraction data for syn-3b were obtained using a Bruker PLATFORM diffractometer equipped with an APEX II CCD area detector,37 while for syn-3e the data were obtained using a Bruker D8 diffractometer/APEX II detector combination. In both cases the diffraction measurements were made with the crystals cooled to −100 °C under a cold nitrogen gas stream, and Mo Kα radiation (λ = 0.71073 Å) was used. For both compounds the structures were determined using a Patterson search for heavy atoms followed by structure expansion, as implemented in the DIRDIF-200838 program system. The structure refinements were completed using the leastsquares refinement program SHELXL-2014.39 For syn-3e, the C1S− C7S (Ph−CH3) distance within the inversion-disordered solvent toluene molecule was assigned a target value of 1.50(1) Å during refinement, while the phenyl ring carbons of this solvent molecule were refined as a regular hexagon with a C−C bond distance of 1.39 Å. For both structures, hydrogen atoms were generated in idealized positions based on the sp2 or sp3 geometries of their attached carbons

energies for these trajectories are quite similar to those for ethylene, suggesting the η2-adduct pathway should be favored for both acrylonitrile and ethyl vinyl ether. Obviously this does not support our proposal of distinct pathways for the two different types of alkenes. These values do reflect some experimental observations (the acrylonitrile metallacycle 3c is much more stable relative to 1 plus free alkene than is the ethyl vinyl ether metallacycle 3f, and the barrier to addition of ethyl vinyl ether is at least 2 kcal/mol higher than that for acrylonitrile (calculated for the η2-adduct pathway but not for the direct cycloaddition pathway)), but we remain dubious as to their overall reliability in predicting barriers, given the uncertainty surrounding the conformational landscape at this crowded Ru indenyl complex (vide supra). One final piece of experimental evidence supports the possibility of divergent pathways for addition of alkenes to 1. The kinetic syn:anti diastereomer ratios obtained for complexes 3 illustrate different sensitivities to the bulk of the alkene substituent for the two types of alkene. In particular, of the two least bulky alkenes, electron-poor acrylonitrile gives >99% syn diastereomer, while electron-rich ethyl vinyl ether gives a 50:50 syn:anti mixture. When the substituent bulk is increased, for example for p-Br styrene and 1-hexene, respectively, selectivity for the syn isomers increases in both cases to >99%. Nevertheless, assuming that our experimentally observed syn:anti ratios do represent a kinetic distribution of isomers (vide supra), with the stereochemistry at the carbon α to Ru being set during the rate-determining step, the trend in observed diastereomer ratios does seem to suggest the ratedetermining step in these cycloaddition reactions is not the same for both electron-poor and electron-rich alkenes.

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CONCLUSIONS The studies described here demonstrate an unusually rich array of chemistry stemming from the seemingly simple addition of alkenes to the planar phosphido complex 1. Relative to other discrete examples of the insertion of alkenes into metal− phosphido bonds, this system stands out for allowing the formation of metallacycles incorporating simple and electronrich alkenes, as well as the more common activated substrates. Competing thermal deinsertion and β-hydride elimination 3977

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Monitoring Reactions of 1 with Alkenes at Low T by 31P{1H} NMR. General Procedure for Gaseous Alkenes. The headspace above a d8-toluene solution of 1 (8−12 mg, 0.01−0.02 mmol, 0.6 mL) in a J. Young tube was evacuated and back-filled with reagent gas (1 atm). The tube was sealed and cooled to −80 °C in a dry ice/acetone bath. The dark blue sample with a thin yellow top layer was inverted to mix the reagents and then quickly placed in the precooled (193−200 K) spectrometer. Ethylene. The sample rapidly turned dark red upon inversion (×1). Three phosphine-containing species were observed in solution NMR (see Figure 6). (The complexity of the 1H NMR spectrum prevented full characterization of the η2 adduct 8a.) When the sample was removed from the cooled instrument and warmed to room temperature, the color changed from dark red to a clear, bright yellow, indicative of full conversion to 3a, which was confirmed spectroscopically. Propene. Spectra were obtained after 10 and 20 min at the initial temperature of 200 K, then once after warming to 210 and 220 K, and every 8 min over 1 h at 230 K. Slow conversion of 1 to 3e was observed at 230 K. General Procedure for Liquid Alkenes. A dark blue d8-toluene solution of 1 (8−15 mg, 0.01−0.02 mmol, 0.6 mL) was placed in an NMR tube with a septum cap. A reference spectrum was obtained, and then both instrument and sample were cooled to 193 K and the instrument was retuned and shimmed. The sample was removed from the instrument, and cooled (4 °C) alkene (8−105 μL, ∼10 equiv) was injected via gastight syringe. The sample was inverted three times to mix and quickly returned to the cooled instrument. Acrylonitrile. The first spectrum (acquired over 7 min) showed that complete conversion of 1 to 3c had already occurred. 4-Bromostyrene. Spectra were obtained after 10 and 20 min at the initial temperature of 200 K and then every 7 min over 50 min at 240 K and every 7 min over 75 min at 260 K. Slow conversion of 1 to pBr-3b was observed at 260 K. Ethyl Vinyl Ether. Spectra were obtained every 15 min for 1 h each at 193, 200, 210, 220, and 230 K. At 230 K, slow conversion of 1 to 3f was observed. Computational Details. All calculations were performed with the ORCA program package.41 Optimizations were carried out without constraints using the PBE functional42 with D3BJ dispersion corrections43 and the zero-order regular approximation (ZORA) Hamiltonian44 to include scalar relativistic effects. All-electron ZORArecontracted basis sets were used as provided by ORCA,45 based on the def2-TZVP(-f) Karlsruhe basis sets.46a Completely decontracted def2-TZVP/J auxiliary basis sets were employed for the resolution of the identity approximation.46b Tight integration grids (Grid6 in ORCA nomenclature) and tight SCF convergence criteria were used. Analytic frequencies were computed for all optimized structures to verify the nature of the stationary points depending on the number of imaginary modes (minima versus transition states) and obtain thermochemical corrections in order to compute the final Gibbs free energies. All reported transition states have a single imaginary frequency.

and given isotropic displacement parameters 120% of the Ueqs for their parent C atoms. One-Pot Syntheses of Metallacycles 3. The general procedure for this reaction was described previously.14g Unsaturated substrate was added either by gastight syringe (liquids) or by placing the degassed mixture under 1 atm of gaseous reagents. Reaction mixtures were worked up as described below. 31P{1H} NMR data for all metallacycles are given in Table S1 in the Supporting Information, while Tables S2 and S3 give 1H and 13C NMR data, respectively, for 3e,f. [Ru(η5-indenyl)(κ2-H3CCHCH2PCy2)(PPh3)] (3e). Reagents: 2 (304 mg, 0.427 mmol); KOBut (57 mg, 0.51 mmol); toluene (20 mL); propene (1 atm). The solution was filtered through Celite, and the solvent was removed under vacuum to give an orange powder, which was washed with acetonitrile (3 × 5 mL) and dried under vacuum to give 3e as a bright yellow-orange powder (257 mg, 0.38 mmol, 91% yield). 1H (NOESY) and 31P{1H} NMR spectra indicate the exclusive formation of syn-3d (see the Supporting Information). A portion of the crude product (71 mg, 0.10 mmol) was recrystallized from methylene chloride (∼1 mL) by slow layer diffusion of acetonitrile (∼5 mL) (42 mg, 0.059 mmol, 59% yield) Anal. Calcd for C45.75H54.5Cl0.5P2Ru (formula includes 0.25 equiv of CH2Cl2 and 0.5 equiv of C7H8; see the Supporting Information): C, 69.98; H, 7.00. Found: C, 69.70; H, 7.11. Mp 131 °C dec. [Ru(η5-indenyl)(κ2-EtOCHCH2PCy2)(PPh3)] (3f). Reagents: 2 (349 mg, 0.490 mmol); KOBut (69 mg, 0.62 mmol); toluene (10 mL); ethyl vinyl ether (0.14 mL, 1.5 mmol). The solvent was removed under vacuum to give an orange residue, which was extracted with hexanes (5 mL) and filtered through Celite to remove solid impurities. The filtrate was cooled to −23 °C for 1 day, which gave an oily black precipitate. Further filtering through a Hirsch funnel gave a clear orange solution, from which orange crystals of 3f (260 mg, 0.35 mmol, 71%) were obtained by slow evaporation. The 31P{1H} NMR spectrum indicated the formation of a 1:1 mixture of syn- and anti3d, in agreement with spectroscopic characterization previously reported.14d Anal. Calcd for C43H52P2ORu: C, 69.06; H, 7.01. Found: C, 69.05; H, 7.23. Mp: 128−131 °C dec. Monitoring Thermal Decomposition of Metallacycles 3a−f. Flame-sealed NMR samples of each metallacycle (5−9 mg) in C7D8 (0.7 mL) were prepared, and initial NMR spectra (300 MHz 1H and 121 MHz 31P{1H}) of the yellow-orange solutions were acquired. The samples were placed in a 60 °C (±5 °C) oil bath, and further spectra (1H and 31P{1H}, 300 MHz) were obtained after varying amounts of time. Relative amounts of phosphorus-containing species in solution were estimated from 31P{1H} NMR integrations.40 Monitoring Attempted Thermal Epimerization of Metallacycles 3b−f. These samples were prepared and monitored as described above for the thermal decomposition experiments, except that each sample also included additional amounts of the relevant alkene (for styrene, acrylonitrile, 1-hexene, and ethyl vinyl ether used 40−50 μL to obtain 50 equiv; for propene condensed ∼7 equiv using constant volume bulb). Attempts To Trap Zwitterionic Intermediates. Reactions of Acrylonitrile and 4-Bromostyrene with 1. A solution of one of three trapping agents (benzophenone, benzaldehyde, or triethylamine hydrochloride; 1−3 equiv) and alkene (1−3 equiv) in d6-benzene (0.3 mL) was added to a solution of 1 (10 mg, 0.014 mmol) in d6benzene (0.3 mL) in an NMR tube. Reaction of Propene with 1. Propene (1 atm) was added to the evacuated headspace of a J. Young tube containing a solution of 1 (10 mg, 0.014 mmol) and trapping agent (benzophenone or benzaldehyde; 1−3 equiv) in d6-benzene (0.6 mL). Formation of metallacycle products was observed in all cases, with no new species visible by 31P{1H} NMR. When triethylamine hydrochloride was used as the trapping agent, formation of the secondary phosphine complex [RuCl(η5-indenyl)(PHCy2)(PPh3) (2) was observed concurrent with metallacycle formation. Control solutions of 1 demonstrated no reactivity with either benzophenone or benzaldehyde.

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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.6b00757.

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Tabulated NMR data and spectra, crystallographic data, kinetic details and data, computational data (PDF) Calculated structures (ZIP) Crystallographic data (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for L.R.: [email protected]. 3978

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H.; Huffman, J. C.; Wu, G.; Mindiola, D. J. J. Am. Chem. Soc. 2006, 128, 13575−13585. (11) Ananikov, V. P.; Makarov, A. V.; Beletskaya, I. P. Chem. - Eur. J. 2011, 17, 12623−12630. These studies are discussed in the broader context of generic features of alkene or alkyne insertion into metal− heteroatom bonds in ref 2. (12) Kazankova, M. A.; Efimova, I. V.; Kochetkov, A. N.; Afanas’ev, V. V.; Beletskaya, I. P.; Dixneuf, P. H. Synlett 2001, 497−500. (13) Motta, A.; Fragala, I. L.; Marks, T. J. Organometallics 2005, 24, 4995−5003. (14) (a) Derrah, E. J.; Marlinga, J. C.; Mitra, D.; Friesen, D. M.; Hall, S. A.; McDonald, R.; Rosenberg, L. Organometallics 2005, 24, 5817− 5827. (b) Derrah, E. J.; Pantazis, D. A.; McDonald, R.; Rosenberg, L. Organometallics 2007, 26, 1473−1482. (c) Derrah, E. J.; Giesbrecht, K. E.; McDonald, R.; Rosenberg, L. Organometallics 2008, 27, 5025− 5032. (d) Derrah, E. J.; Pantazis, D. A.; McDonald, R.; Rosenberg, L. Angew. Chem., Int. Ed. 2010, 49, 3367−3370. (e) Derrah, E. J.; McDonald, R.; Rosenberg, L. Chem. Commun. 2010, 46, 4592−4594. (f) Hoyle, M. A. M.; Pantazis, D. A.; Burton, H. M.; McDonald, R.; Rosenberg, L. Organometallics 2011, 30, 6458−6465. (g) Belli, R. G.; Burton, K. M. E.; Rufh, S. A.; McDonald, R.; Rosenberg, L. Organometallics 2015, 34, 5637−5646. (15) Examples of hydrophosphination resulting from the addition of simple or electron-rich alkenes to discrete metal phosphido complexes are rare. (a) Geer, A. M.; Serrano, A. L.; de Bruin, B.; Ciriano, M. A.; Tejel, C. Angew. Chem., Int. Ed. 2015, 54, 472−475. (b) Ganushevich, Y. S.; Miluykov, V. A.; Polyancev, F. M.; Latypov, S. K.; Lönnecke, P.; Hey-Hawkins, E.; Yakhvarov, D. G.; Sinyashin, O. G., 2013, 32, 3914− 3919. (16) Reference 14d reports the synthesis and full spectroscopic characterization of complexes 3a,c,d and the generation of complex 3f in solution, along with its partial characterization by 31P and 1H NMR. The synthesis and full spectroscopic characterization of complex 3b is reported in ref 14g. (17) See ref 14a and references therein. (18) Faller, J. W.; Crabtree, R. H.; Habib, A. Organometallics 1985, 4, 929−935. Typically a slip distortion of less than 0.25 Å indicates η5indenyl coordination. (19) Further evidence for the reversibility of alkene insertion in this system comes from NMR-scale experiments in which 10 equiv of acrylonitrile was added to metallacycles 3b,e (R = Ph, Me): no reaction was observed over 48 h at room temperature, but within 1 h of heating to 60 °C 10−33% of the original metallacycle had converted to 3c, as determined by 31P{1H} NMR. See the Supporting Information. (20) Among these thermolysis samples, this product was most pronounced for the ethylene metallacycle 3a. A 1H/31P HMBC NMR spectrum showed correlation of 31P signals attributed to 6a with a triplet at −16.2 ppm and of one of the signals with complex multiplets in the vinylic region (4.9−5.4 ppm) (see the Supporting Information). (21) Evidence that the 1:1 diastereomer ratio observed for 3f isolated from the one-pot synthesis is the kinetic distribution for this metallacycle comes from monitoring the addition of ethyl vinyl ether to complex 1 by 31P{1H} NMR at room temperature; even the earliest time points show the formation of both syn- and anti-3f in a 1:1 ratio. See the Supporting Information. (22) The sample containing 3b and excess styrene showed trace amounts of the vinylphosphine product 6b. (23) The “endocyclic” β-H elimination is reminiscent of a proposed decomposition pathway for metallacyclobutane intermediates in Rucatalyzed olefin metathesis: Janse van Rensburg, W.; Steynberg, P. J.; Meyer, W. H.; Kirk, M. M.; Forman, G. S. J. Am. Chem. Soc. 2004, 126, 14332−14333. (24) Recent work in our group on the Cp* analogues of this Ru system indicate that this allylic abstraction process dominates in in the reaction of 1-hexene with Ru(η5-Cp*)(PR2)(PPh3): Yang, J.; LangisBarsetti, S.; McDonald, R.; Rosenberg, L. Manuscript in preparation. (25) Recent leading references for the isomerization of alkenes mediated by Ru hydride complexes: (a) Yue, C. J.; Liu, Y.; He, R. J.

Dimitrios A. Pantazis: 0000-0002-2146-9065 Lisa Rosenberg: 0000-0003-4917-184X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSERC of Canada (Discovery Grant to L.R., CGS-M, PGS-D to R.G.B.) and the University of Victoria (Graduate Fellowships to K.M.E.B. and R.G.B.) for funding.

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DOI: 10.1021/acs.organomet.6b00757 Organometallics 2016, 35, 3970−3980

Article

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DOI: 10.1021/acs.organomet.6b00757 Organometallics 2016, 35, 3970−3980