σ-Bond Metathesis: A 30-Year Retrospective - ACS Publications

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σ‑Bond Metathesis: A 30-Year Retrospective Rory Waterman* Department of Chemistry, University of Vermont, Burlington, Vermont 05405, United States ABSTRACT: A review of σ-bond metathesis is presented using Watson’s 1983 observation of degenerative methyl ligand exchange at metallocene compounds as a starting point. This review has two major parts. The first presents identifying features and reactivity characteristics of this pivotal mechanistic step for high oxidation state metals, which are given with some historical context, though not in historical order for clarity of discussion. The second part presents several selected examples of the exploits of this transformation with a particular focus on catalytic bond forming reactions.

1. INTRODUCTION Students of organometallic chemistry learn of fundamental reaction steps at transition metals that frequently include oxidative addition, reductive elimination, and migratory insertion reactions, among others. One of these steps is σbond metathesis, a reaction particular to d0 and d0f n metal compounds that results in ligand exchange, metal−element bond formation and cleavage, and element−element bond formation and cleavage (eq 1). Conceptually similar to its more famous kin olefin metathesis, this is a simpler molecular dance,1 the exchange of substrate σ bonds with those at a metal. LnM−Z + X−Y → LnM−X + Y−Z

centered transition state to explain the kinetic data collected for methane activation.2 It would be foolhardy to consider this year some anniversary of σ-bond metathesis. Doubtlessly, Watson’s contributions in 1983 initiated a squall of activity in early-transition-metal and fblock metal chemistry toward C−H bond activation, yet the activation of C−H bonds at d0 metal compounds was known for years prior to that communication.4 Moreover, σ-bond metathesis is a broad mechanistic step that impacts a variety of bond activation and formation reactions, and the roots of the mechanistic understanding of this transformation are deeper than that 1983 work. Even the name “σ-bond metathesis” would not be proposed by Bercaw until four years later, and this was coined largely out of necessity for teaching about the transformation.5 Regardless, that seminal discovery does provide a convenient milestone at which to pause and consider a pivotal reaction type in organometallic chemistry.

(1)

Regardless of its apparent simplicity, σ-bond metathesis is a powerful transformation. Strong chemical bonds have been cleaved, unique and valuable products have been synthesized, and productive catalytic chemistry has been built around this single step. This review is prompted by the 30th anniversary of a report often cited as the prompt for aggressive efforts to identify, study, and apply this reaction type in organometallic synthesis and catalysis. Of course, it should be noted the reaction step and a good deal of understanding about it, though not named at that time, certainly existed prior to that date. In 1983, Watson reported that for lutetium and yttrium metallocenes degenerative alkyl ligand exchange is not only possible but facile (eq 2).2 This reactivity was well illustrated

2. NATURE OF σ-BOND METATHESIS In a basic sense, σ-bond metathesis is the concerted exchange of a metal−ligand σ-bond with one of an incoming substrate where the reaction proceeds via a [2σ + 2σ] cycloaddition of a metal−ligand bond with that of a substrate (Figure 1). As this is

Figure 1. σ-bond metathesis transition from the reaction in eq 1 drawn as a [2σ + 2σ] cycloaddition. The currently accepted kitelike transition state, consistent with theoretical study, is depicted in Figure 3

a concerted process, the formal cycloaddition step is a transition state rather than an intermediate. Thus, it would be anticipated that all occupied orbitals are engaged in bonding throughout the course of the reaction. There are a variety of consequences of such a transition state. Principally, reactions proceeding via σ-bond metathesis should favor substrates that possess more favorable orbitals to engage in continuous bonding, despite the obvious geometric constraints of a four-

with methane, but in that report, exchange of a methyl ligand with benzene solvent was also observed. Ironically, Watson reported similar C−H activation reactivity at lutenocene methyl and hydride complexes for a variety of substrates, including pyridine, a class of substrates that will feature later in this review.3 What perhaps distinguishes the former communication from the latter in an historical context regarding σ-bond metathesis is that Watson proposed the possibility of a four© XXXX American Chemical Society

Received: July 30, 2013

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Table 1. Activation Entropy and Kinetic Isotope Data for Selected Examples of σ-Bond Metathesis of Various E−H Bonds bond

reacn

ΔS⧧/eu

H/D KIE

ref

As−H C−Hb C−Hd C−Hd,e H−H P−H Si−H Sn−H

+ Ph2AsH → (N3N)ZrH + Ph4As2 Cp*2ScCH3 + CH2CHPh → Cp*2ScCHCHPh + CH4 Cp*2Th(κ2-CH2C(CH3)2CH2) + CH4 → Cp*2ThCH3(CH2tBu) Cp*2ScCH2C(CH3)3 + CH4 → Cp*2ScCH3 + CMe4 Cp*2Th(CH2tBu)OtBu + H2 → Cp*2Th(H)OtBu + CH3CH2tBu (N3N)ZrPHPh + PhPH2 → (N3N)ZrH + (PhPH)2 CpCp*HfCl(SiH2Ph) + PhSiH3 → CpCp*Hf(H)Cl + (PhSiH2)2 CpCp*HfHCl + Mes2SnH2 → CpCp*Hf(SnHMes2)Cl +H2

−32(2) −36c n/a −36(1) −50.8(7) −36(2) −21(6) −42(1)

n/a 2.8(2) 6(2) 5.2(1) 2.5(4) 3.1(5) 2.9(2) 1.2(1)

9 7 10 11 12 13 14 15

(N3N)ZrAsPh2a

a

N3N = N(CH2CH2NSiMe3)33−. bsp2-hybridized C−H substrate bond. cAverage value for multiple substrates. dsp3-hybridized C−H substrate bond. Intramolecular competition.

e

of the entropy of activation in σ-bond metathesis as had been done in the examples provided. The entropy of activation is a telling feature for this reaction. In 2002, Neale and Tilley identified two different mechanistic steps in the reaction of stannanes with group 4 mixed-ring metallocenes. Formation of hafnocene stannyl complexes by Sn−H activation appears to proceed by σ-bond metathesis, whereas hafnocene-catalyzed tin−tin bond formation appears to occur by α-stannylene elimination (Chart 1).15 The latter

centered transition state. Additionally, continuous bonding should lead to conserved stereochemistry in reactions with, for example, chiral substrates. Historically, this transformation is particular to metal compounds with a d0 electron count, and observation of σbond metathesis is typified at early-transition-metal and lanthanide metallocene compounds in high oxidation states. Therefore, these metal compounds often represent the best studied examples of σ-bond metathesis reactions. In some ways, σ-bond metathesis has always been defined by what it is not, a sequence of oxidative addition and reductive elimination. Of course, there are exceptions. Heyduk and co-workers have observed oxidative addition, reductive elimination, and other redox reactivity at group 4 metals by application of redoxnoninnocent ancillary ligands.6 The exchange of ligands per eq 1 might be expected to proceed via oxidative addition followed by reductive elimination, except that metal compounds without an accessible +2 oxidation state engage in such reactivity. As a result, σ-bond metathesis was distinguished as almost an alternative to that more classical oxidative addition/reductive elimination sequence in the earlier literature dedicated to this reaction.7 2.1. Activation Parameters and Isotope Effects. It is the study of early-transition-metal and lanthanide metallocene complexes that dominated the field and provides the fundamental underpinnings for the current understanding of σ-bond metathesis. Experimental evidence for σ-bond metathesis includes the observation of significant, negative values for entropy of activation for a variety of reactions. Table 1 presents a selection of ΔS⧧ values; this is not a complete list of such data; rather, it is selected examples by various elements. It would be expected that the ΔS⧧ values are negative from a biomolecular process (metal complex plus incoming substrate), consistent with the second-order kinetics commonly observed. Because these data were most frequently obtained in aprotic, nonpolar solvents, solvation effects in the transition state were eliminated as a cause for the observed values. Given the large magnitude of the ΔS⧧ values, it was further surmised that there was significant loss of vibrational and rotational freedom in the transition state. This hypothesis is supported by the observation of substantial, negative values for entropy of activation in firstorder σ-bond metathesis reactions (e.g., cyclometalation). Thus, the σ-bond metathesis transition state is often referred to as “highly ordered” as a defining characteristic. Rothwell drew interesting analogies between the ΔS⧧ values for the then burgeoning σ-bond metathesis mechanism and those of ligand cyclometalation reactions by oxidative addition as well as organic ring-closing reactions.8 The implicit suggestion was that manipulation of steric factors could be used to take advantage

Chart 1. Two Proposed Transition States, α-Stannylene Elimination (Left) and σ-Bond Metathesis (Right), Associated with Sn−H Bond Activation at Mixed-Ring Metallocene Compounds of Hafnium and Zirconium

mechanism is a somewhat unusual but increasingly general transformation that effectively yields low-valent fragments. The two reactions are similar in that they are both characterized by highly ordered transition states and are known to affect maingroup-element dehydrocoupling.16 Thus, distinguishing between these mechanistic steps is of interest to that community and an example where experimental values are known for both reaction types with the same catalyst and substrate element is informative. For steps identified as each σ-bond metathesis and α-stannylene elimination, ΔS⧧ values were measured by Neale and Tilley. In the σ-bond metathesis reaction, ΔS⧧ = −42(1) eu was measured (Table 1),15 while for α-stannylene elimination, ΔS⧧ = −15(1) eu was measured.17 The difference between these values is striking and suggests that the magnitude of the ΔS⧧ value could be useful in distinguishing mechanistic steps. Of course, nearly 20 years had transpired since the degenerative methyl ligand exchange was observed (our milestone) to identify a scenario in which a viable competitive step with σbond metathesis may be present to compare these values. That absence illustrates why the absolute magnitude of ΔS⧧ itself has not become a common benchmark for σ-bond metathesis. The second commonly defining mechanistic feature of σbond metathesis is a substantial primary H/D kinetic isotope effect (Table 1). It is reasonable to anticipate a substantial isotope effect in reactions that break and form E−H bonds. Indeed, the values observed are often rather large, on the basis of the high degree of E−H cleavage in the transition state. However, the shape of the transition state may also play a B

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distribution in the transition state and transition state energy, but there is not as compelling a trend as that observed for lanthanide complexes.19 Minimally, this is an area of curiosity where experiment and theory do not meet, and it would appear that more experimental work is needed to better compare to the computational analysis of lanthanides or greater theoretical study is required, such as a “computational Hammett study”, of reported experimental systems. There is the reasonable possibility that bond activation via σbond metathesis may be preempted by E−H bond coordination (e.g., an agostic interaction or a σ complex). The presence of a vacant coordination site at the reactive metal may allow for interaction with the σ-bonding electrons of a substrate prior to organization of the σ-bond metathesis transition state. Agostic interactions and σ-complexes, well studied for C−H bonds, are known to be important in a variety of bond activation processes.25 There are certainly instances where formation of a σ-complex prior to E−H activation results in more complex kinetic relationships, particularly with respect to isotope effects.26 While most of the examples in Table 1 have substantial H/D kinetic isotope effects, there is one outlier. Activation of stannane Sn−H bonds via σ-bond metathesis results in an unexpectedly low kinetic isotope effect of 1.2(1) (Table 1). The authors attributed this low value to exactly that phenomenon: precoordination of the Sn−H bond to the hafnium prior to activation by σ-bond metathesis.15 Interestingly, speculation regarding the potential importance of σ complexes in these reactions surfaced shortly after the reaction was named,8 but the only instance of such an interaction manifesting itself in kinetic data appears to be the Sn−H example provided by Tilley.15 It is important to clarify this point from the more extensive experimental study of σcomplexes in bond activation by 1,2-addition reactions at d0 metal−ligand multiply bonded compounds.27 From a computational standpoint, σ-complexes are certainly feasible. For example, adducts between silane and the series of lanthanocene hydrides are computationally accessible, but the dissociation energies are quite small, calculated to be less than 10 kcal mol−1 in most instances.18c Examples for transition metals from groups 3 and 4 have also been calculated to be stable.28,29 2.2. Structure of the Transition State. These features have led to the proposed four-centered transition state for σbond metathesis (Figure 3). Of particular note is the position of the elements, the two disposed α versus the one β with respect to the metal, as the arrangement of elements is pivotal. The transition state is often thought of as a [2σ + 2σ] cycloaddition, which would be a symmetry-forbidden process,30 but the availability of vacant metal orbitals relaxes this symmetry requirement. Calculated transition states for σ-bond metathesis have revealed a strong interaction between the metal and the element in the β-position of the transition state.18,21,28,29,31 Thus, the transition state is distinctly distorted from a true [2 + 2] cycloaddition. Early examples of this transition state are notably drawn as squares, but evidence is much more consistent with a so-called kitelike transition state (Figure 3). This basic [2σ + 2σ] cycloaddition transition state was cautiously proposed by Watson, though not named, to explain the observations with lutetium,2 but it had been considered earlier still. Brintzinger had used extended Hückel calculations to identify a reaction pathway for the hydrogenolysis of zirconocene alkyl complexes (eq 3).32

pivotal role in the magnitude of the kinetic isotope effect. A series of lanthanocene hydride complexes in exchange with H2, SiH4, and CH4 have been studied computationally, and for formation of hydrogen (i.e., Si or C α-to the metal center), the E−H−H (E = H, Si, or C) angle varies from approximately 148 to 174° in the transition state.18 A transfer of hydrogen that is increasingly linear is expected to have a greater isotope effect. In these and subsequent studies by Eisenstein, the transfer of hydrogen from the incoming substrate to the metal substituent is referred to as a proton transfer.19 This notion harkens back to an early proposal for the mechanism of hydrogenation of zirconocene alkyl complexes by a type of heterolytic hydrogen cleavage that predates both the naming of this reaction and Watson’s discovery.20 However, in these more recent reports, the term proton transfer arises from a charge delocalization around the transition state that is greater for more polarized lanthanide metal complexes than for transition-metal or transition-metal-like compounds.18b The terms proton transfer and protonolysis are suggestive of broader reactivity than σbond metathesis itself. For example, proton transfer can occur by σ-bond metathesis, but not all σ-bond metathesis reactions are proton transfer reactions (cf. section 2.4). The examples noted here have all the features of σ-bond metathesis, and the computations show significant cationic charge at the transferred hydrogen atom. Thus, these examples can be considered σbond metathesis reactions that proceed via proton transfer. Charge distribution has been an important consideration in some computational analyses of these reactions. Analysis of charge in the σ-bond metathesis transition state has shown that greater charge separation along the C−H−C vector for degenerative methyl ligand exchange correlates with a lower activation barrier for the family of lanthocene complexes.19 This is suggested to be akin to a proton transfer event between two lanthanide methyl ligands (i.e., Cδ−−Hδ+−Cδ−; Figure 2).18b,19

Figure 2. Transition state for calculated degenerative methyl ligand exchange at lanthanocene, illustrating charge distribution between elements.

This proposal marries well with the general observation of polarized bonding for organolanthanide compounds (i.e., Lnδ+−Cδ−). In earlier computational work on scandium systems by Steigerwald and Goddard, however, a nonpolar transition state was found.21 Charge distribution has been less important in experimental studies. In early work by Bercaw and co-workers, there was little effect on the rate of σ-bond metathesis from changes to the electron-donating or -withdrawing character of substrates in reactions at scandium compounds.7 Rothwell identified a similarly weak relationship between substitution and rate for cyclometalation reactions of aryloxide ligands at zirconium with benzyl leaving groups22 as well as for hydrogenolysis of substituted aryl ligands at tantalum.23 In fact, a recent study by Sadow uses the observation of a significant ρ value in the Hammett analysis of N−Si bond formation by magnesium catalysts, in part, to dismiss σ-bond metathesis as the operant mechanism.24 From computational studies of transition-metal compounds, there appears to be some link between charge C

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computationally, and an elegant example of comparative σbond metathesis reactions for different E−E′ bonds in the same substrate (E = Si, C; E′ = H, E) with lanthanocene hydride comes from Eisenstein and co-workers.34 An internal competition between different transition states in a σ-bond metathesis pathway is an important consideration. For example, the reaction of a metal hydride with silane can proceed by two possible paths, one in which silicon is in the α-position and a second in which silicon is in the β-position (Scheme 2). The

Figure 3. Example of a currently accepted, kitelike σ-bond metathesis transition state per eq 1

Scheme 2. Reaction of a Metal Hydride with Silane Can Proceed with Silicon in the α-Position (Top) to Yield a Metal Silyl Product or with Silicon in the β-Position to Yield Hydride Ligand Exchange (Bottom)

These calculations were undertaken to reconcile significant differences in the mechanism of hydrogenolysis proposed by both Bercaw and Schwartz. The former proposal involved cyclometalation of the metallocene ligand, related to what would later become known as a tuck-in complex, through reductive elimination and oxidative addition of hydrogen,33 and the latter proposal involved polarization of H2 via linear coordination to the metal center and transfer of H+ to the alkyl ligand (Chart 2).20b Brintzinger identified an intermediate σ Chart 2. Key Intermediates in the Two Proposed Activation of H2 by Schwartz (Left) and Bercaw (Right) at Zirconocene Alkyl Compounds per Eq 3

former path yields ligand exchange of a hydride for a silyl ligand, and the latter path affords degenerative hydride ligand exchange. The aforementioned study made that comparison as well as provided a direct comparison between Si−H and C−H bond activation for the substrate H3SiCH3 at the same metal complex (Figure 4). There were several interesting results from those studies. In all cases, σ-complexes preceded the σ-bond metathesis transition state. The degenerative hydride exchange is vastly more favorable for Si−H than for C−H bonds, which places the group 14 element in the β-position of the transition state (Figure 4, right). Additionally, the ligand exchange of hydride for silyl has a lower barrier than the ligand exchange of hydride for alkyl where the overall reactions have approximately the same driving force (Figure 4, left).34 The general conclusion from the study is that silanes should undergo facile hydride or silyl exchange with a metal hydride, whereas the only energetically feasible path for alkyl complexes is C−H activation to form an alkyl. Though these results were obtained for lanthanum, it has been suggested that these general energy profiles are metal invariant, but steric factors may be important.35 The corollary to that conclusion, which is not trivial, is that all Si−H activation processes should be more facile than C−H activation processes via σ-bond metathesis. Therefore, organosilanes should be expected to react exclusively at Si−H bonds. That inferred conclusion is consistent with the copious literature on silane reactivity with metal compounds.36 Other studies have revealed that the nature of the silane substitution is important in yielding silyl versus hydride compounds, suggesting that these reactions are highly tunable.37 In the comparative study noted above, the activation energy calculated for the degenerative hydride ligand exchange with an alkane, a reaction that proceeds with carbon assuming the βposition of the σ-bond metathesis transition state, is great and approximately 3−7-fold greater than any other process.34 Those results are consistent with early experimental observations that bond formation to carbon does not occur, particularly C−C bond formation via σ-bond metathesis.38 The general conclusion is that carbon does not assume the β-position of the

complex of H2 in the computations en route to a four-centered transition state for both the degenerative hydride ligand exchange as well as alkyl ligand cleavage (Scheme 1).32 Scheme 1. Calculated Pathway for Hydride Ligand Exchange at Cp2Zr(H)2 Adapted from Brintzinger’s Computational Results32,a

a

Note that the highest energy point (center structure) corresponds to the kitelike transition state later identified specifically for σ-bond metathesis and that hydrogen σ complexes are important for both approach and loss of H2.

Those calculations revealed an interaction of the hydrogen atom in the β-position with zirconium, consistent with later analyses.28d Schwartz later revised mechanistic proposals of a charge-separated heterolytic cleavage of hydrogen to a fourcentered transition state,20b consistent with Brintzinger’s computational model, for the cleavage of zirconium−carbon bonds by hydrogen to give alkanes and zirconium hydride products.20a 2.3. Considerations That Affect Reactivity. There are distinct consequences arising from the high degree of interaction between the metal and the element in the βposition of the transition state. This interaction has been proposed to be a stabilizing factor for the transition state.18,28e,29 Thus, elements that can better accommodate orbital overlap with the metal center have a lower energy transition state and are more facile substrates for σ-bond metathesis reactions. This relationship has been explored D

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Figure 4. Simplified energy profiles (kcal mol−1) calculated for the reaction of Cp2LaH with MeSiH3 adapted from work by Eisenstein and coworkers.34

σ-bond metathesis transition state under reasonable conditions. The exact reason for this difficulty is unclear. It has been suggested that charge buildup in the transition state is less well stabilized with carbon in the β-position.34 There is also a prevailing indication in the literature that orbital overlap is much better for the carbon in the α-position of the transition state. This argument is perhaps more chemically intuitive. Carbon in any position of the σ-bond metathesis transition state would be five-coordinate, but a stabilized transition state also appears to require an additional orbital interaction between metal and the β-element. That interaction with the metal may be too inefficient for carbon to provide an energetically accessible transformation. The irony of carbon’s reticence to assume the β-position of the transition state is that σ-bond metathesis was identified by virtue of the C−H activation of methane, a noteworthy chemical challenge,39 but the reaction then appeared unsuitable for C−C bond formation thereafter. In reviews that consider the topic of C−H bond activation, σ-bond metathesis is given its due,8,39,40 but it might be considered a weaker contributor toward alkane functionalization than other routes. Thus, the study of σ-bond metathesis in applied chemical transformations frequently has followed from instances where that mechanistic step has been applied to elements other than carbon. While those pursuits certainly argue strongly for the value of σ-bond metathesis, the following examples of C−C bond forming catalysis that features σ-bond metathesis as a key mechanistic step further buttress that argument. Though carbon is essentially absent from the β-position, the nature of the carbon atom in the α-position is important. Again, the most complete study of these kinds of relationships comes from Bercaw and co-workers, who reported a strong effect on rate based on the hybridization at the carbon atom participating in the σ-bond metathesis step. The relative rates of reaction for C−H bonds followed this trend: sp > sp2 > sp3.7 Because these and related studies show little to no substituent effect (vide supra), the effect on rate was attributed to hybridization. The rationale for the observation is that the atoms with greater s character have better orbital overlap, lowering the energy of the transition state relative to atoms with lower s character and therefore poorer orbital overlap. This rationale should result in the reverse trend for the reaction of an sp3-hybridized C−H bond with various M−C (sp, sp2, sp3) substrates, which has not been observed presumably due to thermodynamic factors. For sp3-hybridized carbon atoms, however, primary C−H bonds react more quickly than secondary C−H bonds. This trend is

general within the set of metal complexes that react with C−H bonds and has been attributed to steric factors.8,39,40 Though there have been few studies examining the regioselectivity of σ-bond metathesis, trends from early work are telling. Bercaw and co-workers measured the distribution of tolyl isomers formed by reaction of Cp*2ScMe with toluene (Scheme 3).7 There were three key observations: First, a nearly Scheme 3. Distribution of Toluene C−H Activation Products and Relative Product Ratio at Cp*2ScMe under Thermodynamic Control, 36 h Reaction Time at 115 °C7

statistical distribution of meta and para products was formed under both kinetic and thermodynamic conditions. This distribution is inconsistent with a classic electrophilic aromatic substitution or other processes that involves attack of scandium on π orbitals. Second, the o-tolyl product was formed relatively slowly in comparison to the other isomers and displayed lower thermodynamic stability, factors that were attributed to steric pressure from the methyl substituent. Third, kinetic benzylic C−H bond activation to form Cp*2ScCH2Ph was facile, but this compound was thermodynamically unstable and decomposed to scandium aryl products, which alludes to the relative strength of the metal−element bond formed.7 The final observation is different than a “leaving group” effect, where the relative metal−element bond dissociation energy governs the relative rate at which the product is formed. A good example of this phenomenon is the cyclometalation of aryloxide ligands at tantalum(V) centers that were shown to displace a benzyl ligand more quickly than a methyl ligand.8 The difference in rates in that example correlated with the relative M−C bond energies. This is, of course, a challenging analysis, as steric factors must be omitted for a true comparison. The identity of the metal is an important consideration in these transformations. A comparison between the original observations of Watson and Bercaw shows that yttrium is much E

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likely to be difficult, and this is reflected by the lack of such distinction consistently in the literature. There are limiting scenarios in which protonolysis is unique from σ-bond metathesis. Direct addition of H+ to the metal followed by transfer to another ligand (i.e., reductive elimination) is possible, but this scenario, where the metal has available electron density, is unrelated to this review. More germane, a ligand may possess a basic lone pair of electrons that can be protonated directly without interaction of the metal−ligand σbond. For the purposes of this review, these reactions will be considered protonolysis and not covered. Reactions that are termed protonolysis but still appear to proceed via a [2σ + 2σ] cycloaddition step are included. The distinction between these potential transformations has been made by the authors of the original studies.

more active than lutetium, which is more active than scandium with respect to methyl ligand exchange.2,7 From theoretical studies across the lanthanides, there have been consistent trends of metal versus transition state energy.18 An empirical experimental observation is that transition metals which engage in facile intermolecular C−H bond activation come from the more electropositive group 3 metallocenes (e.g., Cp′2MX), while neutral group 4 metallocenes (e.g., Cp′2MX2) do not engage in such reactivity (Cp′ = C5R5−, R = H, alkyl). The idea that relatively electropositive metal centers engage in more facile σ-bond metathesis was recognized early. For example, a titanium hydride was calculated to have a higher barrier to σ-bond metathesis than an analogous scandium hydride, but the isoelectronic titanium hydride cation was shown to have the smallest activation barrier of the set.21 A potent experimental example of this increased reactivity toward σ-bond metathesis for electrophilic compounds is demonstrated by Cp*2ZrH+, which undergoes H/D exchange with benzened6,41 whereas Cp*2Zr(H)2 is unreactive with benezene-d6.42 Furthermore, the experimental observation that Kaminsky-type catalysts can engage in H/D exchange with methane was proposed to proceed via σ-bond metathesis through a theoretical study.43 The increased reactivity of the zirconocene cation noted above was discovered in investigations of olefin polymerization reactions, where electrophilic complexes often exhibit greater rates of polymerization. That those polymerization reactions proceed via a [2σ + 2π] transition state while σ-bond metathesis is a [2σ + 2σ] transition gave rise to the hypothesis that the reactivity of identical metals toward σ-bond metathesis might be enhanced by increasing the degree of electrophilicity at the metal,44 an idea supported by theoretical study.43,45 Indeed, mechanistic analysis by Sadow and Tilley showed that cationic and zwitterionic hafnocene complexes CpCp*HfRX (R = H, Me, SiMes2H; Mes = 2,4,6-Me3C6H2; X = anionic ligand or weakly coordinating anion) exhibited substantially greater reactivity for σ-bond metathesis for a variety of bonds.44,46 Those studies clearly illustrated greater reactivity for complexes that possess greater cationic character. To restate, the more that X− is dissociated from CpCp*HfR+, the more rapidly σ-bond metathesis occurs. 2.4. Possible Four-Centered Reactions. There are a variety of four-centered reactions that are possible at the d0 and d0fn metal compounds implicated in σ-bond metathesis. One example is 1,2-addition reactions across M−X multiple bonds. While these are chemically interesting transformations that have been implicated in important C−H bond activation reactions,8,39,40 they fundamentally violate the definition of σbond metathesis in that these proceed via a [2σ + 2π] cycloaddition step and are therefore outside the scope of this review. There is an important distinction to be made between σbond metathesis and protonolysis reactions. The former reaction type suggests the reaction of nonpolar bonds, while the latter is reminiscent of hydrogen cation transfer. In truth, these two reaction types certainly converge at early transition metals and any reasonable experimental distinction can become blurred. Any element−hydrogen activation step would have a substantial primary isotope effect if E−H cleavage were important at the transition state. Likewise, any transfer of hydrogen that proceeds via an ordered transition state would have concomitantly high entropy of activation values. Clearly, an experimental distinction between these transformations is

3. EXAMPLES OF σ-BOND METATHESIS IN CATALYSIS For the purposes of this review, a set of examples of catalytic reactions where σ-bond metathesis plays a pivotal role is presented. This is not meant to be an inclusive list. Rather, these examples of this mechanistic step in action illustrate three important features: (1) knowledge of σ-bond metathesis as a mechanistic step has the enhanced understanding of important chemical processes already under study, (2) novel catalytic chemistry can be built around σ-bond metathesis as a key mechanistic step, and (3) despite the observation that carbon does not readily assume the β-position of the σ-bond metathesis transition state, productive C−C and C−E bond formation has nevertheless been realized through σ-bond metathesis. This is not to suggest that copious interesting examples of stoichiometric transformations dependent on σ-bond metathesis do not exist. One example of a stoichiometric process that has high synthetic utility featuring σ-bond metathesis is the synthesis of organic heterocyclic products from zirconacyclopentadiene compounds.25 Though the C−C bond-forming process is an oxidative coupling, the liberation of these heterocycles with main-group electrophiles has been proposed to proceed via ordered, four-centered transition states (eq 4).47

3.1. Olefin Polymerization. The most immediate application of identifying σ-bond metathesis as a general reaction type for d0 metal centers was the potential impact of that understanding on alkene polymerization catalysis. There are a variety of possible C−H activation reactions during the course of a polymerization reaction. While β-hydrogen elimination is arguably the most prevalent, σ-bond metathesis may a significant role for certain reactions. Interestingly, σ-bond metathesis provided an additional dimension for the empirical observation that cationic group 4 metallocenes are better polymerization catalysts than the isoelectronic group 3 metallocenes. For most alkenes, except ethylene and propene, scandocene alkyl and aryl derivatives do not undergo insertion; rather, σ-bond metathesis occurs to result in loss of alkane ligand with formation of an alkenyl ligand at scandium (eq 5).7,48 Likewise, terminal alkynes also engage in C−H activation

F

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studied and reveals all the trappings of σ-bond metathesis, including a large negative entropy of activation and substantial primary kinetic isotope effect (Table 1).14 These observations led to the hypothesis that two distinct steps occurring by σbond metathesis are important in the catalytic cycle. Silanes are metalated by Si−H activation followed by Si−Si bond formation (Scheme 4).

at scandocene complexes, resulting in terminal alkynyl ligands, whereas internal alkynes undergo insertion reactions. Early ab initio computations on the reaction of acetylene with Cl2ScH, as a model for the scandocene complexes, revealed energetic barriers for C−H activation processes via σ-bond metathesis higher than those for insertion of alkyne.31 These results were echoed for isoelectronic cationic zirconium complexes in later computational studies, but it was found that the insertion reaction was even more favorable in comparison to σ-bond metathesis for the zirconium systems.28a,b Those computational studies are consistent with the trend that alkene insertion at cationic zirconocene compounds is tremendously facile, certainly more so than for scandocene complexes. However, the significant disparity between the computational and theoretic study on scandium regarding insertion versus σbond metathesis is concerning, though exactly the same substituents on all species were not used in both works. For certain termination reactions, σ-bond metathesis may be important. Added reagents such as organosilanes49 as well as heterocycles such as thiophene50 may engage in E−H activation via σ-bond metathesis at d0 and d0fn catalysts. 3.2. Dehydropolymerization and Dehydrocoupling. After the initial discovery that Si−Si bonds can be formed via metal-catalyzed hydrogen loss (e.g., dehydrocoupling or dehydropolymerization) by Wilkinson’s catalyst,51 there was limited activity in that field until Harrod’s report of silane dehydropolymerization using titanocene catalysts in 1985.52 The interest in polysilanes stems from their unusual electronic properties in comparison to polyolefins, including σ-conjugation.53 These molecules continue to be important for a variety of applications, and investigations of these materials have expanded to heavier germanium congeners in search of similar properties.54 Harrod’s 1985 report initiated a surge of interest in group 4 metallocene catalysts for the dehydropolymerization of silanes. The two principal reasons for the heightened interest in these molecules in comparison to other catalysts was the substantial activity these compounds displayed and the fact that the many group 4 metallocenes engage in dehydropolymerizations without competitive silane redistribution. The exploits of the investigators engaged in this work is engaging and the subject of several reviews.55 Over the course of those studies, several different mechanistic possibilities were entertained.16,55 Tilley and co-workers identified evidence for σ-bond metathesis in the dehydropolymerization of silanes by group 4 metallocenes through a series of studies,55c and this has become the most widely accepted mechanism for silane dehydrocoupling by d0 metal catalysts.16,55 Unlike carbon, silicon readily assumes either the α- or β-position of the σ-bond metathesis without a significant energetic penalty, an empirical observation that has also been established through theoretical study.34 The principal systems for which σ-bond metathesis was initially proposed were mixed-ring zirconocene and hafnocene complexes. The modeling of individual steps in the catalysis provided a framework for understanding the mechanism. It was observed that d0 metal hydride compounds react readily with hydrosilanes to afford hydrogen and the metal silyl derivative.14,56 This is not a unique observation, and such a strategy is commonplace for the preparation of metal silyl compounds.36 Likewise, it was shown that isolated metal silyl compounds react readily with silanes to afford disilane (i.e., Si−Si bond formation) and the metal hydride.14,56 The latter step was well

Scheme 4. Catalytic Cycle for the Dehydropolymerization of Silanes by Group 4 Metallocene Catalysts as Proposed by Tilley55c

A catalytic cycle based on σ-bond metathesis explains a number of features of the catalysis. Sterically encumbered monomers should be disfavored in the relatively crowded transition state, an observation that is verified in experimental studies. The overall behavior of the polymerization is that of a stepwise condensation-type chain growth, which is well modeled with σ-bond metathesis steps. Finally, the inability to produce very high molecular weight polymers is also explained mechanistically with σ-bond metathesis. As the polymer chains grow, the entropic penalty for back-biting at the copious number of Si−H bonds along the chain diminishes, and formation of rings is, as expected, observed regularly in silane dehydropolymerization reactions.55 Consistent with a σ-bond metathesis mechanism, it was demonstrated that more electrophilic d0 metallocene compounds, zwitterionic and cationic, dehydrocouple silanes with greater rates than do the neutral analogues (Figure 5).44,46a As

Figure 5. Examples of neutral, zwitterionic, and cationic hafnium compounds that exhibited greater reactivity with respect to σ-bond metathesis with increasing cationic character at hafnium.44

with the neutral compounds, kinetic data supported σ-bond metathesis as the operant mechanistic steps. Catalysis with cationic compounds was shown to be quantitatively faster than that for the neutral or zwitterionic analogues. Regardless of this accelerated rate, the molecular weights of the resultant polysilane products were lower than those for neutral catalysts, where the reduced molecular weight of the products was attributed to redistribution. The increased reactivity of the cationic compounds toward σ-bond metathesis allowed for more facile Si−C cleavage, as evidenced by the observation of silane redistribution products.44 Indeed, one significant advantage of group 4 metallocenes as catalysts for this G

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heterodehydrocoupling product. A good example of this selectivity is the heterodehydrocoupling of silanes and phosphines to afford silylphosphines, as originally reported by Harrod and co-workers for titanocene catalysts.62 A related example by Waterman and co-workers demonstrated similar heterodehydrocoupling of phosphines with either silanes or germanes using triamidoamine-supported zirconium catalysts.63 In general, silicon is considered to have a relatively low barrier to assuming the β-position of the σ-bond metathesis transition state, though less study has been conducted on phosphorus as a participant in σ-bond metathesis.28d,34 These comparisons, though not complete, suggest that a transition state in which silicon rather than phosphorus is in the β-position would be preferred. Equally important to the selectivity in these dehydrocoupling reactions is that a particular element, in this instance phosphorus, preferentially assumes the α-position of the transition state (Figure 6).

transformation is that redistribution is generally not observed for the neutral metallocene catalysts.55 Thus, faster catalysis was observed, but this catalysis was a combination of productive dehydrocoupling and unproductive redistribution.44,46a These observations suggest that increased reactivity toward σ-bond metathesis must be tempered to prevent competitive bond cleavage reactions. Other elements are readily dehydrocoupled by earlytransition-metal catalysts, including tin57 and phosphorus, following from the original work with organosilanes.58 Stannane dehydrocoupling was pursed with catalysts similar to those that had given polysilanes,57 and high-molecularweight polystannanes were obtained with these metallocene catalysts.59 The early working hypothesis regarding the mechanism followed from silane dehydrocoupling. However, study by Neale and Tilley has uncovered an alternative mechanism α-stannylene elimination, which appears to be operant (section 2.1 and Chart 1) .15,17,60 Phosphine dehydrocoupling has been known since the mid1990s,61 and the prevalence of terminal phosphinidene ligands in reactions with group 4 metallocene catalysts has led to the suggestion that these reactions occur by 1,2-addition rather than σ-bond metathesis.58a However, kinetic data and isotope effect measurements for triamidoamine-supported zirconium complexes are consistent with σ-bond metathesis as the P−P bond forming step in dehydrocoupling catalysis affected by those compounds (Table 1), and a catalytic cycle incorporating these steps has been proposed (Scheme 5).13,58a

Figure 6. A σ-bond metathesis transition state that would favor phosphorus−silicon heterodehydrcoupling by placing silicon in the βposition.

Such selectivity relates to the ground state stability of a phosphido (PR2−) ligand over a silyl ligand (SiR3−) at the metal. Chemical intuition would suggest that a phosphine would formally protonate a silyl ligand and the increased stability of the resultant phosphido ligand would be based on a degree of ligand to metal π-donation from the phosphorus lone pair.16 Between these two factors, the increased stability of the metal−phosphido complex and the lower energy of the transition state with silicon in the β-position, heterodehydrocoupling would be favored. In general, this is a good strategy for the synthesis of E−E′ bonds with early-transition-metal catalysts, and several other examples have been realized, most commonly with silicon.16,55b,64 3.3. Heterofunctionalization. Though only a limited set of compounds readily activate C−H bonds via σ-bond metathesis, this mechanistic step does allow for the activation of a range of other E−H bonds (vide supra). As such, a number of d0 and d0fn complexes engage in catalytic E−H addition to alkenes, alkynes, and other unsaturated substrates, the so-called heterofunctionalization reactions, which developed independently but appeared over a time frame similar to that for dehydrocoupling.65 Classical systems that engage in this kind of catalysis, which are either implicated or experimentally/ theoretically demonstrated to engage in σ-bond metathesis as part of the heterofunctionalization catalysis, include lanthanides, early transition metals, some actinides, and more recently early main-group metals, particularly those from group 2.66 The set of catalytic heterofunctionalization reactions in which these metals engages is vast, including hydrogenation,67 hydroamination,66,67e,68 hydrophosphination,66,67e,69 hydrosilation,67e hydrothiolation,70 and other related transformations. Studies of virtually all of these reactions were initiated with metallocene derivatives for early-metal or f-block catalysts and were followed with non-metallocene catalysts. In an early mechanistic investigation of the catalytic hydrogenation of 1-hexene by lanthanide compounds, Marks

Scheme 5. Catalytic Cycle for the Dehydrocoupling of Phosphines by Triamidoamine Zirconium Catalysts as Proposed by Waterman13

Similarly, the reaction of secondary arsines with triamidoamine-supported zirconium catalysts offered activation parameters consistent with those for phosphines, which suggested σbond metathesis for arsine dehydrocoupling (Table 1). Unfortunately, H/D exchange with the ancillary ligand in that catalysis was too fast to allow for a measurement of a kinetic isotope effect.9 As dehydrocoupling catalysis advanced, it was discovered that early-transition-metal catalysts can capitalize on the energetics of the σ-bond metathesis transition state to afford selectivity in reactions that form bonds between different elements or heterodehydrocoupling. For two elements that have different propensities to assume the β-position of the σ-bond metathesis transition state, the difference in energy between the two possible transition states can aid in selectivity for the H

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and co-workers established a first-order dependence on hydrogen that was attributed to turnover-limiting hydrogenolysis of the alkene-insertion product (Scheme 6).71 That

Scheme 7. Proposed Catalytic Cycle for the Hydrophosphination of Carbodiimides by TriamidoamineSupported Zirconium Compoundsa

Scheme 6. Catalytic Cycle for the Hydrogenation of Terminal Alkenes Where Hydrogenolysis via σ-Bond Metathesis Is Turnover Limiting on the Basis of the Steps Proposed for Terminal Alkene Substrates by Marks71

a

Both the P−H activation and product liberation via cyclometalation were suggested to proceed via σ-bond metathesis.75

activation. Among the possible mechanistic steps, σ-bond metathesis has been a crucial step in several catalytic processes. 4.1. Pyridine Functionalization. The pyridine functionality remains ubiquitous in bioactive molecules as well as materials, and pyridines are viable precursors to an array of other heterocycles via conventional synthetic methods.76 Thus, facile routes to substituted pyridines continue to remain attractive. In 1989, Taylor and Jordan reported the catalytic alkylation of α-picoline by the net insertion of alkene into the ortho C−H bond using Cp2ZrMe+ as a catalyst (Scheme 8).77

observation led the investigators to invoke a four-centered transition state consistent with their own prior work in bond activation as well as that of others (vide supra). The focus of the study, however, was an understanding of the catalytic reaction and not specifically elucidating the hydrogenolysis step. This may partially be due to the fact that the dependence on hydrogen was variable with the nature of alkene, where the hydrogenation of cyclohexene proceeded with a first-order dependence on alkene rather than hydrogen. That observation did not change the authors’ interpretation of the hydrogenation step, but it prompted the hypothesis that alkene insertion was turnover limiting for bulkier substrates.71 In a later study on related systems, kinetic isotope effect data were collected for several catalysts with values of 1.5−2.3 being found, consistent with σ-bond metathesis.72 These lanthanide catalysts are not exclusively bound to hydrogen transfer via σ-bond metathesis. A theoretical study on lanthanide-catalyzed intramolecular hydrophosphination catalysis, discovered by Marks and co-workers,73 demonstrated that hydrogen transfer from phosphines does not proceed via σbond metathesis, though an ordered transition state consistent with experimental data was calculated.74 However, for zirconium complexes that catalyze the intermolecular hydrophosphination of terminal alkynes and carbodiimides, P−H activation would appear to proceed via σ-bond metathesis.75 In prior work with those compounds, kinetic data support P−H bond activation by σ-bond metathesis.13 During the hydrophosphination catalysis, product liberation does not occur via direct reaction with phosphine substrate; rather, there is an ancillary ligand cyclometalation step, presumed to also occur by σ-bond metathesis though not established by further study, followed by subsequent P−H addition, similar to that in the dehydrocoupling catalysis (Scheme 7).75 These two examples demonstrate that the observation of E−H activation at a d0 or d0fn metal is not itself sufficient to implicate σ-bond metathesis. However, it is likely that a number of catalytic heterofuctionalization reactions catalyzed by these metals do involve σ-bond metathesis steps.

Scheme 8. Catalytic Alkylation of α-Picoline with a Zirconocene Catalyst (Cp′ = C5H5−, C5Me5−) Proposed by Jordan77

In that work, the strategy that substrate coordination would lead to more facile C−H activation (i.e., cyclometalation) was applied, and it was reasonably anticipated that any cyclometalated Zr−C bond could be subject to insertion of an alkene. Pivotal to the catalysis was the addition of hydrogen. In the absence of hydrogen, there was no catalysis and the reaction stopped after insertion of alkene into the Zr−C bond. Indeed, reaction of the isolated alkene insertion product with α-picoline did not liberate 2-methyl-6-isopropylpyridine. The added hydrogen affords Zr−C cleavage, and C−H activation at αpicoline was proposed to occur upon displacement of the more sterically encumbered product.77 Hydrogen, in this reaction, is not a reagent; rather, it is a cocatalyst that facilitates product liberation. The catalytic cycle proposed initially has been borne out in a theoretical study that provides some explanation for the

4. CATALYTIC BOND FORMATION VIA C−H ACTIVATION There is still tremendous interest in catalytic reactions that result in productive synthetic chemistry via C−H bond I

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necessity of hydrogen. In those studies,78 it was shown that the C−H activation steps occur by σ-bond metathesis. Importantly, it was demonstrated that the hydrogen cleavage of the Zr−C bond proceeded with a significantly lower barrier than that for cleavage with a second equivalent of α-picoline.78 That idea fueled the possibility of cleavage with other substrates via σbond metathesis, but these reactions were less successful than those with hydrogen.79 In the time since, however, there have been numerous advances in this catalysis made by Jordan and co-workers, including expansion of substrate and scope,79,80 stereoselective alkene insertion,81 and the activation of sp3hybridized C−H bonds.82 Other transition metals engage in related reactivity. One interesting example of alkyne coupling married with C−H activation by σ-bond metathesis has been reported with nonmetallocene hafnium complexes by Mashima (eq 6) in

coupling to substrates such as phenanthridine, bipyridine, and isoquinoline are suggestive of some generality to the reactivity and the potential for broader synthetic utility. 4.2. Alkyne Coupling Reactions. Rare-earth metals have become ubiquitous in catalytic reactions that result in C−C bond formation using terminal alkyne substrates. An early example of this transformation was contemporary with Jordan’s work with pyridine substitution. Heeres and Teuben reported the dimerization of terminal alkynes using Cp*2MCH2SiMe3 (M = Y, La, Ce) as catalysts (Chart 3).91 Of course, such Chart 3. Selected Examples of C−C Bond Forming Reactions with Terminal Alkynes as Substrates

coupling reactions were known for late transition metals for almost two decades prior. In addition to the discovery of new catalysts for transformation, Teuben also proposed that C−H activation in the catalysis occurs via σ-bond metathesis. Zhou and co-workers have been expanding on this reactivity since the early 2000s. One particularly noteworthy example of that work was the dimerization of carbazole-substituted terminal alkynes to afford enynes that exhibited remarkable properties as single light emitters in organic light-emitting devices.92 Other examples of coupling reactions include the reaction of terminal alkynes with carbodiimides that has been accomplished with early-transition-metal catalysts reported by Erker,93 actinide catalysts reported by Eisen,94 and rare-earth-element catalysts reported by Komeyama and Takaki and later by Hou (Chart 3).95 A less common functionalization was realized by Hou and co-workers, who have demonstrated the rare-earthmetal-catalyzed coupling of alkynes with carbodiimides (Chart 3).96 In all examples, these transformations are proposed to engage in C−H activation via σ-bond metathesis. These reactions then proceed via insertion of unsaturated substrate and liberation by C−H activation. In this regard, these transformations are highly similar to the pyridine functionalization reactions (vide supra), and Scheme 8 might be considered a general catalytic cycle where the need for exogenous hydrogen exists only for substrates that cannot be emancipated from the metal by a metathetical reaction with substrate. 4.3. Hydromethylation. A related transformation that utilizes C−H activation by σ-bond metathesis is the hydromethylation of propene, as reported by Sadow and Tilley in 2003.11 This reaction is remarkable for several reasons. Catalytic functionalization of methane is still rare and of high interest.39 The scandocene catalyst is highly selective for hydromethylation over β-elimination or olefin oligomerization products. The proposed catalytic cycle was determined through analysis of stoichiometric steps, including the C−H activation of methane at Cp*2ScCH2C(CH3)3 as well as by direct analysis of the catalytic reaction. On the basis of those data, the reaction was proposed to proceed via insertion of alkene into the Sc−C

2010.83 In this instance, activation of an sp3-hybridized C−H bond occurs followed by alkyne coupling that is facilitated by added B(C6F5)3.83 While this reactivity implies that catalytic C−H activation via σ-bond metathesis for any number of heteroatom-functionalized organic substrates may be accessible, this is not true. There are examples of other catalytic functionalization reactions with amine substrates using tantalum catalysts that engage in C−H activation via an elimination route rather than σ-bond metathesis.84 For rare-earth elements, related insertion chemistry into activated ortho C−H bonds of pyridine was reported by Teuben in 1994.85 Expansion of rare-earth-element-catalyzed pyridine functionalization to a broader group of alkenes was realized by Hou some 7 years later, which suggests challenges still remain in this kind of chemistry.86 More recently, Hou and co-workers have demonstrated double benzylic functionalization with a related yttrium catalyst.87 These systems appear to proceed by a mechanism similar to that proposed by Jordan on the basis of experimental observations as well as theoretical studies.88 These catalysts are still likely to provide new catalytic reactions. One example to support this notion is Oyamada and Hou’s catalytic functionalization of anisoles with rare-earth halfsandwich catalysts that is proposed to proceed by a mechanism similar to that for pyridine functionalization.89 A recent development from Diaconescu and co-workers is that group 3 metal complexes undergo C−C bond formation following an analogous C−H activation, presumably by σ-bond metathesis (eq 7).90 Though these are stoichiometric trans-

formations, the observation of similar reactivity with other rings, including benzoquinoline and acridine, as well as J

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bond with subsequent product elimination by metathesis with methane (Scheme 9).11 The selectivity for the secondary Scheme 9. Mechanism Proposed by Sadow and Tilley for the Hydromethylation of Alkenes by Scandocene Catalysts11

Figure 7. Transition state most likely to provide selective C−Si bond formation during methane dehydrosilation catalysis.

While these reactions are limited in both turnover number and frequency, the set of reactions that provide organic products from methane is still starkly limited, and activation of methane remains a tremendous challenge.39 The transformations suggest that other, more facile routes may be identified with further investigation, and the successful dehydrosilylation of anisoles supports that hypothesis.100 4.5. Alkane Metathesis. High-oxidation-state metals supported on silica or alumina exhibit remarkable reactivity toward alkanes, including oxidation, hydrogenolysis, and metathesis.101 For example, Dufaud and Basset reported the decomposition of polyethylene to low-molecular-weight oligomers with hydrogen using a silica-supported zirconium catalyst.102 In this study, a single-site zirconium hydride was identified, (SiO)3ZrH, from catalyst preparation. In 2007, Thieuleux, Basset, and co-workers reported that silica−aluminasupported zirconium catalysts were competent for the homoligation of propane (eq 8).103 In this study, two different zirconium species, mono- and dihydrides, were identified on the surface.

product was rationalized in a later theoretical study by a lower activation barrier for that insertion over other reaction pathways, which also supported the proposed mechanism based on calculated energetic barriers to various possible reactions.35 Kinetic data strongly support σ-bond metathesis. For example, the reaction of Cp*2ScCH2CMe3 with methane and methane-d4 yields a kinetic isotope effect of 10.2, which is certainly telling. Perhaps even more interesting, Sadow and Tilley measured the kinetic isotope effect for an intermolecular competition in the reaction of Cp*2ScCH2CMe3 with CH2D2 to give a value of 5.2(1) (Table 1). Though a relatively low enthalpy of activation was measured, ΔH⧧ = 11.4(1) kcal mol−1, the kinetic isotope effect is attributed to a high degree of C−H bond cleavage in the transition state.11 It was later found that the ansa-scandocene derivative Me2Si(C5Me4)2ScCH2CH(CH2CH3)2 has much greater activity in hydromethylation of alkenes than Cp*2ScMe, up to 2 orders of magnitude increase in rate.97 Part of this increased reactivity stems from accelerated rates of methane C−H bond activation. However, the increased reactivity did come at a cost for selectivity. Isobutylene and 2-methyl-1-pentene byproducts were observed with isobutane in the reaction of methane with propene in the presence of the ansa-scandocene catalyst.97 The ansa effect in this catalysis was studied theoretically, and those studies demonstrated that the increased rate of reactivity is due to a combination of steric and electronic factors.98 These studies also revealed that the formation of byproducts in the catalysis with propene was due to the reduced barriers for competitive processes based on the ancillary ligand.98 4.4. Methane Dehydrosilyation. A related functionalization of methane was also achieved with scandocene catalysts. In methane hydrosilation, the heterodehydrocoupling of methane with an organosilane occurs.99 This is a more challenging transformation than other heterodehydrocoupling reactions, due to the difficulty in breaking the methane C−H bond. In these reactions, good selectivity for methane functionalization was observed with limited silane dehydrocoupling or other competitive processes. The limitation imposed by sluggish C− H activation is substantial, resulting in only 7−8 turnovers over the course of 1 week. The selectivity of the process most likely arises from silicon assuming the β-position of the σ-bond metathesis transition state, which would allow for C−Si bond formation (Figure 7). This transition state would require generation of the scandocene methyl derivative. The difficulty of C−H bond cleavage helps explain the observation of Cp*2ScH formation during the catalysis.99

Key to both of these catalytic reactions, which are highly related, is the activation of alkane C−H bonds at a zirconium hydride. Several theoretical studies have identified σ-bond metathesis as the most facile route for C−H activation.101,104 In experimental studies, Casty, Hall, and co-workers found a large negative entropy of activation, ΔS⧧ = −27(3) eu, associated with the H/D exchange of D2 with methane using a silicasupported zirconium catalyst.105 These investigators attempted to measure a kinetic isotope effect for H2 and D2 exchange but found rates were too high with hydrogen to obtain a reliable rate,105 which at least suggests a substantial primary isotope effect. Thieuleux, Basset, and co-workers identified that the zirconium dihydride sites react with C−H bonds more quickly, a feature attributed to the increased electrophilicity of those sites.106 A σ-bond metathesis activation of C−H bonds is proposed to be the initial step to yield zirconium alkyl compounds, where C−C bond forming and breaking steps are proposed to occur via insertion and elimination steps.101,104e Thus, the observation that carbon is reticent to assume the βposition of a σ-bond metathesis transition state is unchallenged by these reactions that clearly make and break C−C bonds. Related reactivity as well as conversion of methane to higher alkanes has been reported with supported tantalum catalysts.101 For reactions that both catalysts perform, the tantalum systems give better performance metrics than do the zirconium systems. The tantalum systems largely activate C−H bonds via 1,2addition reactions across unsaturated metal−carbon bonds. In those systems, however, initial activation of alkanes at a K

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remains a theoretical construct, and features that experimentally distinguish σ-CAM from other mechanistic steps including σbond metathesis have yet to be identified.

tantalum hydride as well as alkane liberation are likely to occur via σ-bond metathesis.101,104e The superior performance of the tantalum catalyst is likely due to the greater reactivity of the alkylidene and alkylidyne intermediates as compared to the zirconium alkyl intermediates engaging solely in σ-bond metathesis.

6. CONCLUDING REMARKS After more than 30 years, σ-bond metathesis continues to be a vibrant reaction type in organometallic chemistry, responsible for the cleavage of strong bonds as well as the synthesis of interesting and valuable molecules. It also remains a workhorse step in now routine transformations to value-added molecules. Substantial effort in both experiment and theory have contributed to an advanced understanding of the features of σ-bond metathesis. The study of σ-bond metathesis has fueled growth in catalysis using high-oxidation-state metals. Advances in early-metal and f-block-metal reaction chemistry continue due to the advantages of σ-bond metathesis and despite the limitations that σ-bond metathesis imposes (e.g., no carbon in the β-position of the transition state). Though many implications may be made from the copious work on σ-bond metathesis, there are four historical trends that have emerged in this review. First, the study of a variety of element−hydrogen bond activation processes has substantially influenced the understanding of σ-bond metathesis, even though the identification and early experimental parameters were found for metal compounds that participate in C−H bond activation. Reactions such as heterofunctionalization and dehydrocoupling have given opportunities to test parameters that would not necessarily be apparent solely from the study of C−H activation such as the identity of the element in the βposition of the transition state. Second, the understanding of σbond metathesis has been a marriage of theory and experiment. While the results of each undertaking have not always been in lockstep, theoretical probing of the transition state, guided by experimental observation, has yielded great understanding. Third, there is a delicate balance with σ-bond metathesis that can be upset to unproductive reactivity. There are now examples where well-defined reactivity is undercut by catalysts that are overactive for bond cleavage. Fourth, σ-bond metathesis remains a potent route to C−H activation and a promising route for C−H bond functionalization. Application of a mechanistic understanding of σ-bond metathesis has resulted in a clear progression in the discovery of systems that better activate as well as functionalize C−H bonds. Finally, the historical trends suggest that additional, interesting discoveries await where σ-bond metathesis is a pivotal step.

5. EVOLUTION OF σ-BOND METATHESIS TO LATE TRANSITION METALS Shortly after the general establishment of σ-bond metathesis as a new reaction type for high-oxidation-state metals, it was suggested that late-transition-metal complexes could undergo hydrogen activation via a four-centered transition state, akin to that of σ-bond metathesis.107 In more recent years, evidence for σ-bond metathesis at non-d0 and late-transition-metal complexes has mounted. Detailed theoretical studies across metal systems have revealed a continuum of bond activation and formation processes at metal centers where this reactivity appears to be bookended by oxidative addition/reductive elimination and σ-bond metathesis.28d,108 Hartwig and co-workers identified a set of data consistent with a four-centered transition state for the exchange of methyl and hydrogen between CpRu(PPh3)2Me and HBcat (eq 9, cat

= catechol).109 The investigators were able to eliminate the possibility of ring slippage or phosphine dissociation that might precede an oxidative addition step. The substantial kinetic isotope effect kH/D = 1.62(13) is consistent with a high degree of B−H bond breaking in the transition state, as is true for classic examples of σ-bond metathesis (Table 1).109 Throughout the 1990s, an increasing number of systems afforded experimental evidence of reactivity consistent with σbond metathesis. A variety of metals were implicated, including iron, tungsten, and rhodium, and a common feature in these examples was that the substrate was a borane.28c This persistence of late metals reacting with boranes in an apparent four-center transition state was rationalized by the ability of the vacant orbital at boron able to overlap with the metal center and thus lower the transition state energy,28c similar to the rationale for lower barriers to reaction of silicon in comparison to those of carbon in σ-bond metathesis for traditional d0 systems.34 Indeed, this reactivity with boranes was eventually termed “boron-assisted” σ-bond metathesis.110 In examining the borylation of alkanes as well as hydrogenation and H/D exchange, Perutz and Sabo-Etienne identified a series of features for metals with d4 through d8 electron counts involved in those reactions. If the metal oxidation state remained unchanged throughout the course of a multistep process where σ complexes are stable, Perutz and Sabo-Etienne termed such reactions σ-CAM (complex-assisted metathesis) as a unique mechanistic path from oxidative addition/reductive elimination and σ-bond metathesis.108 The identification of σ-CAM explains the observation of apparent σbond metathesis at late transition metals. Since this discovery, numerous examples of σ-CAM-implicated reactions have been identified, which have been the subject of thorough review.28d,29,108 Identification of σ-CAM as a reaction step

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest. L

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Biography

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Rory Waterman is an associate professor of chemistry at the University of Vermont. He earned undergraduate and graduate degrees at the Universities of Rochester and Chicago, respectively, and completed postdoctoral work at the University of California, Berkeley, as a Miller Fellow. He has won an NSF CAREER award (2008), an Alfred P. Sloan Research Fellowship (2009), a Cottrell Scholar Award (2009), and a Humboldt Research Fellowship for Experienced Researchers (2013). His current research interests include inorganic and organometallic chemistry to address problems in synthesis, catalysis, materials, and energy.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation through awards CHE-0747612 and CHE1265608 as well as the Alexander von Humboldt Foundation.

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