Î²-Alkyl Elimination: Fundamental Principles and Some Applications

Review pubs.acs.org/CR

β‑Alkyl Elimination: Fundamental Principles and Some Applications Matthew E. O’Reilly, Saikat Dutta, and Adam S. Veige* Department of Chemistry, Center for Catalysis, University of Florida, Gainesville, Florida 32611, United States ABSTRACT: This review describes organometallic compounds and materials that are capable of mediating a rarely encountered but fundamentally important reaction: β-alkyl elimination at the metal−Cα−Cβ−R moiety, in which an alkyl group attached to the Cβ atom is transferred to the metal or to a coordinated substrate. The objectives of this review are to provide a cohesive fundamental understanding of β-alkyl-elimination reactions and to highlight its applications in olefin polymerization, alkane hydrogenolysis, depolymerization of branched polymers, ring-opening polymerization of cycloalkanes, and other useful organic reactions. To provide a coherent understanding of the β-alkyl elimination reaction, special attention is given to conditions and strategies used to facilitate β-alkyl-elimination/ transfer events in metal-catalyzed olefin polymerization, which provide the well-studied examples. 5.4. β-Alkyl Elimination from Iridium, Rhodium, and Cobalt Complexes 5.5. β-Alkyl Elimination from Organoruthenium Complexes 5.6. β-Alkyl Elimination from Organoniobium Complexes 6. Ring-Opening Polymerization (ROP) of Strained Cycloalkanes via β-Alkyl Elimination 6.1. ROP via β-Alkyl Elimination by Metallocene Catalysts 6.2. ROP via β-Alkyl Elimination by Palladium Catalysts 7. β-Alkyl Elimination Applied to Organic Synthesis 8. β-Alkyl Elimination by Solid-Supported and Homogeneous Catalysts for Alkane Hydrogenolysis and Depolymerization 8.1. Hydrogenolysis of Alkanes by Solid-Supported Catalysts 8.2. Depolymerization by solid-supported catalysts 8.3. Depolymerization by Homogeneous Catalysts 9. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction and Scope 2. Lessons from β-Hydride Elimination 3. β-Me Elimination within Metallocene Olefin Polymerization Catalysts 3.1. Chain-Termination by β-Me Elimination during Propylene Polymerization 3.2. Sterically Directed β-Me Elimination over βH Elimination 3.3. Agostic Interactions during β-Me Elimination 3.4. Direct β-Me Transfer to Coordinated Monomer 3.5. Ligand Steric Influence over Catalyst Activity and Polymerization/Oligomerization 3.6. Ligand Steric Influence over β-Me Transfer to Metal and Coordinated Monomer 3.7. Chain Sterics on β-Me Elimination 3.8. Counteranion Influence over β-Me Elimination Reactions 3.9. Metal Influence over β-Me/β-H Elimination Selectivity 3.10. β-Me Elimination from Non-Group IV Metallocene Complexes 3.11. Substrate Effect on β-Alkyl Elimination 3.12. Designed Metallocene Complexes for Competitive β-Alkyl and β-Hydride Transfer during Propylene Polymerization 4. β-Alkyl Transfer Processes from Early Transition Metal Complexes 5. β-Alkyl Transfer Processes in Middle and Late Transition Metal Complexes 5.1. β-Alkyl and β-Arene Elimination from Organoplatinum Complexes 5.2. β-Alkyl Elimination from Organopalladium Complexes 5.3. β-Alkyl Elimination from Organonickel Complexes

© 2016 American Chemical Society

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1. INTRODUCTION AND SCOPE The 1,2-insertion of olefins into metal−alkyl and metal-hydride bonds and their microscopic reverse, β-elimination (Scheme 1; left), continue to garner significant attention since they are

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Received: January 20, 2016 Published: July 1, 2016 8105

DOI: 10.1021/acs.chemrev.6b00054 Chem. Rev. 2016, 116, 8105−8145

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Scheme 1. Left: General Schematic for 1,2 Insertion and De-insertion (β-Elimination). Right: Catalytic Cycle for Olefin Polymerization Showing Two Chain-Termination Steps: (1) BHE To Form a Metal-Hydride and (2) BHT to Coordinated Olefin

interrogate the reaction are more difficult to devise. Selectively cleaving C−C bonds in preference to C−H bonds remains a challenging topic in organometallic chemistry.4,47 However, the occurrence of β-alkyl elimination is prevalent during propylene polymerization with sterically crowded metallocene catalysts,48−56 and is a key step for alkane/polymer hydrogenolysis.57−67 Thus, a thorough mechanistic understanding of β-alkyl elimination in chain-termination/-transfer events is highly relevant. This review describes β-alkyl transfer reactions in organometallic compounds, primarily in the solution phase, and will focus on the role of β-alkyl elimination as a key step in a number of important processes, including chain-transfer/ termination in olefin polymerization and oligomerization (Section 3), ring-opening polymerization of cycloalkanes (Section 5), organic synthesis (Section 5), and hydrogenolysis of long-chain alkanes and polymers by heterogeneous catalysts (Section 7). The structure and bonding features of the precatalyst and proposed transition states involved in β-alkyl transfer steps are emphasized in Section 3 to provide a fundamental backdrop to the review. The trend displayed by d0 cyclopentadienyl group 4 metal complexes toward favoring βalkyl over β-hydride transfer is an area of particular focus. The features of β-alkyl elimination in solid-state “depolymerization” are especially underscored to deduce the factors responsible for such processes, with the goal of extending the strategy to constructing well-defined homogeneous catalysts for polymer hydrogenolysis. Relevant theoretical studies to understand metal−alkyl bond rupture during β-alkyl elimination in preference to β-hydride elimination are also discussed. Identification of the fundamental differences in these processes will be key to development of next generation synthetic strategies involving β-alkyl elimination.

fundamental steps in olefin polymerization, as well as stoichiometric and catalytic reactions involving organometallic species in solution and the solid-state.1 While β-alkyl elimination reactions are rarely observed in comparison to insertion reactions and β-H elimination, there is still an enormous wealth of literature on β-alkyl elimination transformation. Consequently, β-alkyl reactions have become colloquial, particularly within the propylene polymerization and organic synthesis communities, such that authors may have overlooked the significance of those results. Remarkably, a concise overview of β-alkyl elimination transformation that merges the fundamental understanding of this reaction among the various fields is missing. Rather than providing a limitless listing of reactions involving a β-alkyl elimination, we decided to provide the reader with a comprehensive understanding of βalkyl elimination from well-studied organometallic complexes and to highlight applications in olefin polymerization, alkane hydrogenolysis, depolymerization of branched polymers, ringopening polymerization of cycloalkanes, and other useful organic reactions. Consequently, the reader will find this review centers on thoroughly examined β-alkyl elimination from metallocene olefin polymerization catalysts. In contrast, examples of β-alkyl elimination in organic synthesis are quite numerous but do not add much to the current conceptual understanding. The scope of this section provides a brief overview of the different reaction-types rather than a complete listing. Reviewed previously and therefore not the focus of this review are β-alkyl or aryl eliminations from M−X−Cβ−R species (where X = O and N).2−13 During metal-catalyzed olefin polymerization, the β-elimination reaction is a critical chain-termination step that controls the polymer length (Scheme 1; right). For efficient olefin polymerization catalysis, it is generally accepted that, in the absence of added chain-transfer agents, the most relevant modes for chain-transfer/termination are via β-hydrogen elimination (BHE) to the metal and direct β-hydrogen transfer (BHT) to the coordinated olefin (Scheme 1; right).14 Motivation to study these fundamental reactions stems from the need to control chain length in olefin polymerization and selective olefin oligomerization. Theoretical and experimental interrogation of β-hydrogen elimination involving early15−25 and late26−44 transition metal complexes is reasonably well-understood. Surprisingly, the principles governing β-alkyl elimination are not well-established, especially considering that the microscopic reverse, olefin insertion into a metal−alkyl bond, is such a well-studied transformation.45,46 One reason for the lack of fundamental studies is that breaking C−C bonds in the presence of other reactive C−H bonds is a rare event, and systems designed to

2. LESSONS FROM β-HYDRIDE ELIMINATION Any meaningful discussion of β-alkyl elimination requires an introduction to β-hydride elimination, especially for cases when both β-hydride and β-alkyl are operable within the same complex. To further highlight their prevalence, β-H eliminations have been reported for a large number of both early68−70 and late71−74-transition metal complexes, in particular within the context of olefin polymerization/oligomerization and hydrovinylation reactions.75−80 Some key requirements for β-hydride elimination are (1) an open coordination site adjacent to the migrating hydride ligand, (2) alignment of the β-hydrogen atoms with the metal orbital (hence, the importance of steric interactions between the metal fragment and alkyl chain), and (3) a β-hydride agostic interaction with the metal center (Figure 1).16,81,82 Subtle factors such as the 8106


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metal complexes. Despite the drastic BDE differences, late transition metal complexes mediate β-alkyl elimination, but some caveats exist and will be discussed.

3. β-ME ELIMINATION WITHIN METALLOCENE OLEFIN POLYMERIZATION CATALYSTS

Figure 1. β-C−H bond agostic interaction.

3.1. Chain-Termination by β-Me Elimination during Propylene Polymerization

alkyl chain length, can also affect β-hydride agostic interactions. For instance, calculations by Brintzinger et al. indicate that a longer alkyl chain Zr−(CH2)2CH3 enhances β-C−H agostic interactions either via an inductive effect or via hyperconjugation, yielding a lower energy barrier for β-H elimination (12 kcal/mol) relative to the Zr−CH2CH3 fragment (18 kcal/ mol).83,84 Imparting selectivity for the analogous β-alkyl elimination over β-hydride elimination reaction is a challenging feat because it involves cleavage of a weaker but less accessible C−C bond.4,47 Teuben and co-workers present a computational investigation into the factors influencing the selectivity of a competitive β-Me/β-H migration.85 For the theoretical species X2MRq (X = Cl, M = Hf, Ti when q = +1, Zr, Nb when q = 0), the optimization and single-point energy calculations reveal a thermodynamic preference for β-Me elimination over β-H elimination. The preference at first seems counterintuitive, given that metal-hydride bonds are generally stronger than metal−alkyl bonds.45 However, the presence of stronger bonds between a d0 transition metal and the methyl group provides additional stability for the metal−alkyl species. The origin of the additional bonding interaction comes from an orbital centered on the CH3 weakly π-donating to the LUMO of the d0 of the X2M metal fragment (Figure 2a) and is analogous to the hyperconjugation effect encountered in organic molecules. Their calculations predict, not surprisingly, that a more electrophilic metal ion enhances the −CH3 π-donation, thus increasing the stability of M−CH3 over M−H species. Conversely, a more electron rich metal ion will favor M−H formation. For example, using the more electron-donating Cp* ligand in Cp*2MX2 {M = Hf, Zr; X = H, CH3; Cp* = permethylcylopentadienide},86 metal-hydrides are more thermodynamically favorable than metal-alkyls according to bond dissociation energies (BDE).87 Despite Zr−H and Hf−H species being more thermodynamically favorable than the corresponding metal−alkyl species, the difference between them is less than 10 kcal/mol, whereas, with later transition metals, this difference becomes much larger (>15 kcal/mol).87 For example, in (PEt3)2PtX2 (X = H, CH3), the difference in Pt−X bond dissociation energies is over 25 kcal/mol. Considering this, the most likely metal ions to exhibit β-alkyl events would be d0 early transition metals. Indeed, many of the examples of β-alkyl elimination come from early transition

Production of commercial polymers by metallocene-catalyzed oligomerization of lower olefins (e.g., ethylene, propylene) (Scheme 1) is an important industrial process, in which β-H elimination/transfer from a growing polymer chain to the metal center is a prevalent chain-termination event.88,89 In contrast, by employing sterically crowded catalysts of the form (Cp*)2M {Cp* = η5-C5Me5; M = Zr, Hf), β-Me elimination becomes the dominant chain-termination pathway in propylene polymerization.48−56 For instance, propylene polymerization with Cp*CpZrCl2/MAO proceeds with 80% β-H transfer; however, the same reaction with Cp*2ZrCl2/MAO occurs via 91% β-Me transfer,52 as evidenced by the presence of gem-dimethyl endgroups, which can be easily identified by NMR spectroscopy (Scheme 2). Scheme 2. Resulting Polymers via β-Me Chain-Transfer Termination

3.2. Sterically Directed β-Me Elimination over β-H Elimination

The first observation of β-Me elimination was the decomposition of trineopentylaluminum to trimethylaluminum and isobutene reported in 1960.90 About 22 years later, β-Me elimination from the metallocene complex (Cp*)2LuCH2CH(CH3)2 (1) was observed. The decomposition products are isobutane, (Cp*)2LuCH3 (2), (Cp*)2LuCH3CHCH2 (3), and (Cp*)2LuCH3C(Me)CH2 (4), among others (Scheme 3).91,92 A notable feature of this process is that β-alkyl and β-H elimination are competitive events. The authors also noted that the β-alkyl elimination step is an endergonic process, and requires that isobutane be released (C−H bond activation) to drive the process by removing olefin from the system. Later, Marks and co-workers prepared cationic complexes [Cp*2MMe(THT)][B(C6F4)4] (7: M = Z; 8: M= Hf) (THT =

Figure 2. (a) Interaction of CH3 π-orbitals with the LUMO of the metal fragment; (b) Schematic representation of the β-Me versus β-H elimination. 8107


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Scheme 3. Decomposition of 1 To Yield Isobutene and 2−6 via Competitive β-H and β-Me Elimination Steps

Figure 3. Left: structure of catalysts 7 and 8. Right: propylene oligomer products from catalysts 7 and 8.

from the Cp* ligand, thereby elevating the barrier to hydride transfer. By contrast, transition state A for β-Me elimination experiences less steric interaction with the Cp* ligand. Hence, the selectivity for β-Me over β-H elimination in propylene oligomerization at Cp*2M catalysts can be solely attributed to the lower steric interactions in transition state A vs transition state B. The following sections discuss other factors involved in stabilizing the transition state for β-Me elimination.

tetrahydrothiophene) (Figure 3) as active catalysts for the oligomerization of propene. They obtained products consistent with β-Me instead of β-H elimination.53 Teuben and coworkers proposed that, while β-H elimination as the chaintransfer pathway is more thermodynamically preferred,50 β-Me elimination from [Cp*2M−CH2−CH(CH3)−R] is kinetically preferred as a consequence of the sterically crowded metallocene catalysts. A key factor for favoring β-Me over β-H elimination as proposed by Teuben is the presence of sterically encumbering alkyl groups attached to Cp* ligands. Figure 4 depicts two

3.3. Agostic Interactions during β-Me Elimination

Agostic interactions in β-H eliminations play an important role in lowering the transition-state barrier (Figure 1).16,81,82 Similarly, the analogous γ-agostic interactions facilitate β-Me elimination. In a deuterium-labeling experiment using (CpRn)2Zr(μ-CH3B(C6F5)3)(CH2C(CH3)2CD3) {CpRn = C5H5 (d3-9), C5Me5 (d3-10), C5Me4H (d3-11), ethylenebis(indenyl) (d3-12), 1,2-(SiMe2)2(η5-3,5-C5H(CHMe2)2)(η5C5H3) (d3-13)}, Bercaw and co-workers observe that the βCH3 group forms a stronger γ-agostic interaction than the βCD3 group, thereby preferring β-CH3 elimination (Scheme 4A). A kinetic isotope effect (KIE) of ∼1.4 for β-CH3/β-CD3 elimination in the products was observed regardless of the ligand (Scheme 4).93 A more important observation is that the KIE of 1.4 is consistent with previous KIE studies probing the α-agostic interaction during olefin insertion into M−R bonds.94−97 Hence, agostic interactions contribute equally to β-Me elimination and its microscopic reverse, olefin insertion, suggesting that the transition state is halfway along the reaction coordinate (Scheme 4b).

Figure 4. Teuben model for β-Me elimination. Proposed transition states A and B for β-elimination with Cp*2MR catalysts showing steric congestion (arcs between the green highlighted areas) that disfavor B.

possible transition states for β-Me (A) and β-H elimination (B). In both cases, the transferring group aligns perpendicular to the Cp*(centroid)−Zr−Cp*(centroid), allowing the σC−C or σC−H bond to overlap with the metal d-orbital. However, to achieve the prerequisite geometry for β-H elimination (B), the adjacent methyl group experiences a significant steric repulsion 8108


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Scheme 4. (a) Kinetic Isotope Effect (KIE) Probing of γ-Agostic Interaction during β-Me Elimination with Zirconocene Complexes 9−13. (b) Depiction Showing α- and γ-Agostic Interactions (red half arrow) Assisting Isobutylene Insertion and βMe Elimination Steps, Respectively

Having established via KIE experiments that α- and γ-agostic interactions contribute equally to olefin insertion and β-Me elimination steps, it is possible to extrapolate the role of γagostic interactions during β-Me elimination by evaluating αagostic interactions during the microscopic reverse reaction, olefin insertion. Computations by Brintzinger and co-workers provide some clarity to this transformation.98 The authors conclude that α-agostic interactions stabilize the tilting of the −CH3 fragment toward the olefin prior to insertion (Figure 5A). As evident by the truncated orbital diagram, the tilting of

elimination steps employ during their respective transformations. Another remarkable similarity to the β-H termination step is the ability of the β-Me group to undergo direct transfer to a coordinated monomer. Baird and coworkers99 proposed this mechanism in propylene polymerization with the cationic zirconocene complex [Cp*2ZrMe(μMe)B(C6F5)3] (14). The predominant chain termination for 14 is β-Me elimination during propylene polymerizations, which are usually performed at high propylene pressures. However, in a low temperature experiment trapping the [Cp*2ZrCH2CH(Me)-P]+ {P = polymer chain} intermediate and removing excess propylene, the attached polymer chain terminates by β-H elimination upon warming. This result at first seems contradictory to the present understanding of β-Me elimination by a sterically crowded metallocene catalyst, since the presence of free monomer would not influence this process. In a reevaluation of the mechanism, the authors propose that a second chain-termination mechanism occurs, where a β-Me group of a growing polymer chain transfers directly to a coordinated propene monomer instead of forming a Zr−Me intermediate (Scheme 5). This transfer mechanism is similar to direct β-H transfer (BHT) to a monomer (Scheme 1).14,53,100−106 Even in the present β-Me transfer (BMT) mechanism, steric interactions between the polymer chain and Cp* ligand serve an important role in aligning the β-methyl in an appropriate position for transfer to the coordinated monomer. However, because of ambiguities of the nature of the polymeric end-groups obtained from different experiments, a relation between the rates of the chain transfer and the monomer concentrations cannot be established. Nevertheless, these results suggest that a second mechanism, β-Me transfer (BMT) to coordinated olefin, in addition to β-Me elimination (BME) is operable as a chain-termination step and likely competes with BHE.99 In addition, for steric and electronic reasons, a probable transition state for BMT (Scheme 5B) may be favorable due to less ring strain, and C−C bond formation is thermodynamically preferred over a weaker Zr−C bond.

Figure 5. (a) Calculated structural changes to the M−CH3 tilting angle during olefin insertion/deinsertion. (b) Reorientation of the CH3 σ-orbital toward the olefin (c) agostic interaction stabilizing the −CH3 tilting and olefin insertion.

the −CH3 effectively reduces the σ-bonding to the metal (Figure 5B), and this enthalpically difficult transformation is remediated by the α-agostic interaction (Figure 5C). The computational results conclude that the α-agostic interaction can provide ∼23 kcal/mol of stabilization during the tilting process. Also, the agostic interaction provides additional electron stabilization to the Zr-ion during the −CH3 insertion step, which would otherwise involve a 14-electron complex. 3.4. Direct β-Me Transfer to Coordinated Monomer

3.5. Ligand Steric Influence over Catalyst Activity and Polymerization/Oligomerization

The previous discussion highlights the similar agostic interactions that olefin insertion and β-hydride and β-methyl

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Scheme 5. Two Modes of β-Me Migration from a Growing Polymer Chain to (a) a Metal Center and (b) a Metal-Coordinated Monomer during Propylene Polymerization Catalyzed by [Cp*2ZrMe(μ-Me)B(C6F5)3] (14)

Cp* ligand also increases the resulting polymer molecular weight,107,108 due to (1) the higher transition state barrier for chain termination (β-Me elimination) from the increased steric hindrance around the metal and (2) the decreased metal acidity when bound to the π-basic alkyl substituted Cp* ligand.109 However, too much steric bulk can shut down polymerization and favor oligomerization. For example, Okuda et al. effectively employed this strategy to selective oligomerize propylene into 4-methyl-1-pentene (Scheme 6 and Table 1).110 Increasing the

Table 1. Three Main Products (A: 4-methyl-1-pentene, B: 4,6-dimethyl-1-heptene, C: oligomers) Obtained by Dimerization/Oligomerization Catalysts 15−22 Catalyst 15 16 17 18 19 20 21 22

Scheme 6. (Top) Hafnocene Catalyst with Varying R1 and R2 Substituents 15−22. (Bottom) Proposed Cycle for Selective Dimerization of Propenea

(H, Me) (Me, Me) (Et, Me) (nBu, Me) (iBu, Me) (Et, Et) (nBu, nBu) (iBu, iBu)

A (%)

B (%)

C (%)

23.9 31.1 34.3 40.2 41.5 41.9 54.2 58.5

19.4 15.4 16.0 18.2 19.7 18.6 18.1 16.6

48.8 24.2 22.3 23.0 21.3 21.4 11.1 9.3

repulsion between the alkyl substituents of the cyclopentadienyls and the growing oligomer chain places the β-Me groups into an energetically favorable arrangement for facile elimination, in agreement with the model originally proposed by Teuben.50 3.6. Ligand Steric Influence over β-Me Transfer to Metal and Coordinated Monomer

In addition to controlling the selectivity of β-H and β-Me elimination reactions, the proper placement of sterics on the metallocene ring can (1) control the tacticity of the resulting polymer and (2) influence the mode of chain transfer of β-H and β-Me elimination. An example of the latter case occurs during the β-hydride termination step by dimethylsilyl-bridged zirconocene complexes (CH3)2Si(benz[e]indenyl)2ZrCl2 (23) and (CH3)2Si(2-methylbenzyl[e]indenyl)2ZrCl2 (24) (Figure 6). Whereas complex 23 undergoes both BHE to form Zr−H and BHT to coordinated olefins, for complex 24, the α-Me substituents attached to the ligand actually block chain

a

The three major products are highlighted in blue boxes.

steric congestion within hafnocene polymerization catalysts (η5C5Me4R1)(η5-C5Me4R2)HfCl2/MAO blocks propylene insertion at the metal center (propagation)110 and facilitates β-Me elimination (chain termination) in the following order (R1, R2): 15 (Me,H) < 16 (Me,Me) < 17 (Et,Et) < 18 (nBu,Me) ≈ 19 (tBu,Me) ≈ 20 (Et,Et) < 21 (nBu,nBu) ≈ 22 (tBu,tBu). The increased bulkiness of the Cp* ligand causes an improved selectivity for 4-methyl-1-pentene over other oligomeric products by slowing propagation relative to the β-Me chaintermination step. This model further supports that steric

Figure 6. Effect of methyl-substitution on β-H transfer to coordinated propylene. The lateral extension angle is indicated by the dashed red lines. 8110


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Figure 7. Steric effects of an alkyl chain (R3 = CH3 and H) on the rate of β-Me and β-H elimination.

Scheme 7. β-Me Elimination from [Cp*2Zr(CH2CMe3)]+ and [Cp2Zr(CH2CMe3)]+

Me transfer to a coordinated olefin (rate of β-Me exhibits a dependence on propylene pressure). However, increasing steric congestion even further, complexes 21 and 22 (R1/R2 = n Bu/nBu and iBu/ iBu) shut down the direct BMT to monomer, and only BME can occur at the metal center.

termination by β-H transfer (BHT) directly to a coordinated monomer, allowing only BHE to occur.111 The transition-state model for a β-H transfer to a coordinated propylene monomer as presented in Figure 6 suggests that the β-H transfer in 23 and 24 requires an open coordination site with a lateral extension angle of more than 180°. The methyl substituents in the α-position to each bridgehead cause a severe narrowing of the lateral extension (Figure 6: red lines), effectively blocking BHT chain-termination reaction channels. There are advantages for shutting down BHT to monomer. When only BHE occurs, increasing the olefin concentration accelerates the propagation rate relative to termination resulting in high molecular weight polymer. In contrast, with BHT mechanism operable, increasing the pressure/concentration of olefin has negligible effects on the polymer chain length since the rate of both termination and propagation steps increase with higher olefin concentration. Okuda et al. also observed the mechanism of β-Me elimination change by varying the steric congestion of the hafnocene polymerization catalyst (η5-C5Me4R1)(η5-C5Me4R2)HfCl2 (Scheme 6).110 The predominant β-Me elimination mechanism observed with small alkyl groups, 15−20 (R1/R2 = H/Me, Me/Me, Et/Me, nBu/Me, tBu/Me, Et/Et), is direct β-

3.7. Chain Sterics on β-Me Elimination

In addition to probing the steric effects by varying the alkylation on Cp ligands, Okuda et al. were able to observe the influence of chain sterics (R3 = H, CH3) on the selectivity for βMe/β-H elimination (Figure 7).110 Analysis of the relative rates (kβ‑Me and kβ‑H) in propylene oligomerization with hafnocene catalysts suggests that the growing chain with a secondary carbon in the γ-position influences the selectivity for β-Me elimination. Similar to the Cp ring substituents R1 and R2, larger δ-substituents on R3 promote β-Me elimination. These results confirm that the steric interaction between the polymer chain and Cp* ligand plays an important role in favoring β-Me over β-H elimination. 3.8. Counteranion Influence over β-Me Elimination Reactions

At this point, we have discussed the importance of employing sterically bulky Cp ligands to promote β-Me elimination over 8111


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Baird.24,117 The ΔG‡ values for the complexes are similar, but it is evident that more polar solvents lower the overall free energy barrier by 2 kcal/mol. Evaluation of ΔH‡ and ΔS‡ components reveals that the more polar solvent lowers ΔH‡ by ∼5 kcal/mol, but this is offset by a more ordered ΔS‡ in the transition state. The authors proposed that the solvent assists the dissociation of [BMe(C6F5)3]− anion from the metal coordination sphere during the intramolecular, nucleophilic displacement process, as shown in Scheme 8. The proposed model is consistent with lowering of ΔH‡ with increasing solvent polarity (toluene-d8 to C6D5Cl), but also causing a more ordered transition state.

the competing β-H transfer process during propylene oligomerization and polymerization reactions. In addition, bulky ligands can influence oligomer/polymer length by modulating the activation energy barriers for propagation and chain termination. Another subtle factor affecting the propagation and chain-termination rates is the counteranion.112,113 In the presence of potentially coordinating counteranions, the additional large bulky Cp* ligands can enhance β-Me elimination. For instance, the cationic neopentyl complex [Cp*2Zr(CH2CMe3)]X (10: X = MeB(C6F5)3−) undergoes isobutene elimination instantaneously at −75 °C whereas less crowded complex 9 (Cp = C 5H 5, X = MeB(C6F5)3−) exhibits reversible β-Me elimination at room temperature.114 Further emphasizing the influence of the counteranion on the β-alkyl elimination event, replacing MeB(C6F5)3− anion with [B(C6F5)4]− in complex 9 (Scheme 7) causes rapid β-alkyl elimination to occur at −78 °C. Hence, the stronger donor ability of the counteranion suppresses βelimination. Likewise, adding THF to the in situ generated complex 10 provides the solvent adduct 10-THF and suppresses β-Me elimination, which only occurs at room temperature. For complex 10, the larger substituents of the Cp* ligand weaken the anion coordination to the Zr-ion. The weakened anionic interaction is evident in the Zr···CH3BR3 distance in the crystal structures (Figure 8) of analogous Zr−Me

3.9. Metal Influence over β-Me/β-H Elimination Selectivity

The propylene polymerization results presented in Table 3118,55 reveal that the metal ion has some influence over β-Me/β-H elimination selectivity. The higher β-Me/β-H exhibited by Hf as compared to Zr is possibly due to the slightly smaller covalent radius for Hf, which results in a shorter Cp*−Hf distance and better steric crowding in transition state B (Figure 4). 3.10. β-Me Elimination from Non-Group IV Metallocene Complexes

While group 4 metallocene compounds are suitable candidates as polymerization catalysts; metallocene compounds of other early transition metals, such as bis(cyclopentadienyl)scandium hydrides, alkyls, and related compounds, are also excellent alternatives.119−121 Gas phase experiments reveal that Sc metal ions are capable of mediating C−H and C−C bond cleavage via β-Me elimination of butane.122 Initial investigations by Bercaw and co-workers employed the permethylscandocene complex, Cp*2ScR (30, R = H, CH3) for mechanistic studies because the mononuclear 14-electron complex is similar to cationic group 4 metallocenes. Complex 30 polymerizes ethylene, but undergoes only a single insertion with α-olefins prior to σ-bond metathesis with vinylic C−H bonds (Scheme 9).119 In fact, the Sc−R bond of 30 readily reacts with C−H bonds,120,123 posing a challenge for olefin polymerization. Employing organoscandium derivatives OpScH(PMe3) (31) {Op = (η5-C5Me4)2SiMe2)} and [DpScH]2 (32) {Dp = (η5C5H3tBu)2SiMe2)} with tied-back Cp ligands provides a more open metal coordination sphere that curtails unproductive C− H bond activation (Scheme 10). Indeed, complexes 31 and 32 are viable catalysts for the dimerization of α-olefins and cyclization of dienes.124 Using a less sterically crowded ligand, [(Cp*SiNR)(PMe3)Sc(μ-H)]2 (33) {Cp*SiNR = (η5-C5Me4)SiMe2(η1-NCMe3)} polymerizes α-olefins.125 Although no βalkyl elimination was observed during propylene polymerization using complex 33, complex 32 can catalyze the ringopening of methylenecyclobutane and methylenecyclopropane via insertion/β-alkyl elimination, thus proving that β-alkyl elimination is kinetically feasible in the more sterically crowded system (Scheme 11).126 To examine competitive β-H, β-CH3, and β-R (R = higher alkyl) elimination processes, reacting complex 31-PMe3 with isobutylene affords a variety of products, including scandium containing complexes OpSc(CH3)(PMe3) (34) and OpSc(CHPMe2) (35) (Scheme 12).127 The authors propose β-CH3 elimination to give propene as a critical step that accounts for the observed distribution of products. In an experiment to test the relative rates of β-Me elimination vs β-H elimination, treating in situ generated OpSc(CH2CHMe2) (PMe3) (36) with alkyne provides a means

Figure 8. Zr···CH3BR3 distances for complexes 25 and 26.

complexes, [Cp′2Zr(Me)][μ-MeB(C6F5)3] (25: Cp′ = 1,2Me2C5H3) and [Cp*2Zr(Me)][μ-MeB(C6F5)3] (26). The Zr··· CH3BR3 distance increases from 2.569(3) Å in 25 to 2.640(7) Å in 26.115 Another important factor is that the highly alkylated Cp* may also provide better electronic stabilization to the lowcoordinate 14-electron Zr ion. Ion pair dissociation is an important factor in determining the true activation energy parameters for β-Me elimination. Table 2 contains the activation energy barriers for β-Me elimination for zirconocene and hafnocene complexes 9, 10, 25, 28, and 29 (Scheme 8), undertaken by Marks116 and Table 2. Activation Energy Barrier for β-Me Elimination from Cp′2M(Np)(μ-Me)B(C6F5)3 (9, 10, 25, 28, and 29) Metallocene 25 28 29 29 9 9 10

Solvent toluened8 toluened8 toluened8 C6D5Cl toluened8 C6D5Cl CD2Cl2

ΔH‡ (kcal/mol)

ΔS‡ (cal/mol)

ΔG‡ (0 °C) (kcal/mol)

22.5 ± 0.9

4.3 ± 3.3

21.2 ± 0.2

17.3 ± 0.9

−11.9 ± 3.4

20.7 ± 0.2

21.4 ± 1.1

8.0 ± 4.0

19.2 ± 1.1

16.5 ± 5 22.3 ± 0.9

−7.0 ± 18 8.5 ± 3.0

18.4 ± 5 20.0 ± 0.9

18.8 ± 0.9 11.2 ± 0.4

0.6 ± 3.2 −12.7 ± 2.0

18.6 ± 0.9 14.7 ± 0.7 8112


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Scheme 8. β-Me Elimination from Cp′2M(Np)(μ-Me)B(C6F5)3 (9, 10, 25, 28, and 29)

Table 3. Percentage of Different Chain-Transfer Mechanisms in Polypropylene Samplesa,55,118

Scheme 11. Reversible Branching of Dienes Involving Insertion/β-Alkyl Elimination Catalyzed by 32

Metallocene

Temp

β-H

β-Me

At-tr

Cp2ZrCl2 Cp2ZrCl2 Cp2HfCl2 Cp*2ZrCl2 Cp*2ZrCl2 Cp*2ZrCl2 Cp*2HfCl2 Cp*2HfCl2

50 0 50 50 0 −40 50 0

100 100 100 7.9 7.1

91.1 81.8

2.0 2.0

98 62.7

1 11.1 100 Traces 35.3

a Calculated from the relative intensities of different end-groups: β-H = vinylidene, β-Me = alkyl, Al-tr = (isobutyl-allyl)/2.

elimination. However, C−H bond activation will likely become a problem due to several factors: (1) although steric congestion of Cp*2Sc(H) (30) hinders olefin coordination, the ionic radius of Sc3+ is actually larger than Zr4+ and Hf4+ in corresponding olefin polymerization catalysts Cp*2ZrX2 and Cp*2HfX2, suggesting that the poorer electrophilicity of Sc vs cationic Zr and Hf is detrimental to olefin coordination; and (2) the strongly polarized Sc−R bond causes the R-group to be very nucleophilic or “Grignard-like”, thus lowering the barrier to σbond metathesis. The competition between β-Me elimination and C−H bond activation is also evident in other non-group IV metallocene complexes. Evans et al. described the formation of an unusual bridging planar trimethylenemethane (TMM) dianion (41) from neopentyl precursor intermediate (40) via sequential βalkyl elimination and C−H bond activations (Scheme 14).128 Formation of the unusual TMM complex 41 requires β-Me elimination followed by consecutive C−H activation in a cascade reaction to convert a neopentyl group to a trimethylenemethane dianion. Apparently, the puckering of TMM ligands (μ−η3:η3-C(CH2)3) upon metalation results in pyramidal arrangement of the central carbon. Moreover, these results provide the third example of β-alkyl elimination from neutral metallocene complexes including Sc and Lu. In addition, there is a striking resemblance between Samarocene (Sm) and Scandocene (Sc) complexes that show competitive βMe elimination and C−H bond activation processes. It is also interesting to note that Cp*2Sm(CH2CMe3) (40) and Cp*2Lu(CH2CHMe2) (1) undergo β-Me elimination, but the corresponding [Cp*2Sc(CH2CMe3)]123 does not. Lastly, metallocene complexes of yttrium have shown mild potential for β-alkyl elimination. Cp*2YH2 (42) can mediate ring-opening β-alkyl elimination of methylenecyclobutane to afford complex 43 (Scheme 15).129

Scheme 9. Insertion of Propene into a Sc−R Bond (R = H, CH3) Followed by C−H Bond Activation

Scheme 10. Reactivity of 31, 32, and 33 toward α-Olefins

to trap the β-elimination product (Scheme 13). The majority was β-H elimination product 37, and the β-Me elimination product 38 accounted for less than 5%. Finally, a scandium derivative [(Op)Sc-CH2CH(CH2CH3)2] (39) exhibits no βCH2CH3 elimination in oligomerization of 2-ethyl-1-butene. These results confirm that scandocene oligomerization catalysts are capable of mediating β-Me elimination similar to their group 4 counterparts. With that said, for complexes 31 and 32, β-H elimination remains the dominant chain-termination pathway, which agrees with Teuben’s model, and more bulky ligand sterics are needed to increase the competiveness of β-Me

3.11. Substrate Effect on β-Alkyl Elimination

The choice of olefin also has a significant effect on β-alkyl elimination. For instance, while the predominant mechanism of polypropylene chain termination by (Cp*)2MCl2 (M = Zr, Hf)/MAO is β-Me elimination, the chain-termination mecha8113


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Scheme 12. Reaction Products from Isobutene Insertion into (Op)ScH(PMe3) (31-PMe3) {Op = (η5-C5Me4)2SiMe2}

Scheme 13. Trapping Experiment to Monitor β-Me and β-H Elimination from an in Situ Generated OpSc(CH2CHMe2) (PMe3) (36) {Op = (η5-C5Me4)2SiMe2)}

Scheme 14. Sequential β-Alkyl Elimination from 40 for the Formation of Unusual Sm Complex 41 with a Trimethylenemethane (TMM) Ligand

In the case of cyclopolymerization of 2-methyl-1,5-hexadiene (MHD), β-Me elimination is the exclusive pathway for chaintransfer with [Cp*2ZrMe][(μ-Me)B(C6F5)3] (26) as the catalyst (Scheme 16).131 The polymerization of 1,1-disubstituted α-olefins is rare, but the copolymerization of isobutylene and ethylene was achieved using the electrophilic 12-electron Ti complex, [Me2Si(C5Me4)(NC10H19)]TiMe+ (44), which has a relatively unhindered coordination sphere.132 The polymerization reactions achieved up to 45% isobutylene monomer incorporation with substantial alternating monomer units in the polymer structure. The polymer contained exclusively vinylidene end-groups arising from β-Me elimination from the terminal isobutylene (IB) unit during the chain-termination step (Scheme 17). Furthermore, increasing the IB content lowers the copolymer molecular weight, suggesting an

Scheme 15. Ring Opening of Methylenecyclobutene by 42

nism reverts back to β-H elimination for the polymerization of the larger 1-butene,51 and β-ethyl elimination is not observed. A subsequent end-group analysis of 1-butene polymerization with rac-(dimethylsilyl)bis(4,5,6,7-tetrahydro-l-indenyl)zirconium and MAO as cocatalyst showed that multiple end-groups arise via rearrangement (chain-walking) and β-H transfer, while βMe elimination is observed in Cl−). The corresponding platinum(IV) methylcyclopropane complexes (103−104) were also prepared, but only complex 104 with coordinatively labile CF3CO2− anion undergoes ringopening β-alkyl elimination (Scheme 29).153 For complex 104, β-alkyl elimination occurs at a significantly slower rate than the coordinatively unsaturated PtII analogues, due to a large energetic penalty to create an open coordination site cis to the methylcyclopropyl ligand. In addition, an unfavorable electronic environment is created after β-alkyl elimination when two Pt−C occupy positions trans to each other.

Scheme 26. Thermolytic Rearrangement of cis(Silylmethyl)Pt(II) Complexes 84 Depicting Agostic Interactions with PPh3 and CH2SiMe3 That Stabilize 3Coordinate Intermediate 85 Prior to β-Me Transfer

withdrawing aryl substituents (ArR: R = 4-Me2N (86) < 4OMe (87) < 4-CH3 (88) ≈ 3-CH3 (89) < 4-tBu (90) < 4-F (91) ≈ H (92) < 4-CF3 (93) ≈ 3-CF3 (94) < 2-CH3 (95) < 2CF3 (96) ≪ ArR = C6F5 (97)).149 The electron-withdrawing groups stabilize the anionic charge build-up on the aromatic ring during the elimination step. Also, note that 95 (ArR: R = 2-CH3) is substantially faster than 89 (ArR: R = 3-CH3) and 90 (ArR: R = 4-tBu). An explanation is that steric congestion 8118


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Scheme 27. Thermolytic Rearrangement of cis-(Silylmethyl)Pt(II) Complexes (86−97) Involving β-Ar Transfer

Scheme 28. Ring-Strain Induced β-Alkyl Elimination from Platinum(II) (Methycyclopropyl) Complexes, 97−102

Scheme 29. Ring-Strain Induced β-Alkyl Elimination from Platinum(IV) (Methycyclopropyl) Complexes (103−104)

Extending this ring-opening β-alkyl elimination to a larger ring system, Flood and co-workers used platinum(methylcyclobutyl) complexes, (PMe3)2PtCl(R) (105) and (dmpe)PtCl(CH 2 C(CH 3 )(CH 2 ) 2 CH 2 )(106) {R = (1methylcyclobutyl)methyl}. Complexes 105 and 106 undergo β-alkyl elimination at 140 °C to yield 2-methylpenta-1,4-diene (Scheme 30A).154,155 The ability of the phosphine ligand to dissociate plays an important role in this transformation; the reaction is complete for complex 105 in 95% microstructure B (Scheme 53). Both Rossi and Marks observed that Ti(CH2Ph)4 and 25 can copolymerize ethylene and methylenecyclobutane to give the ring-opened microstructure C as the predominant structure. Given that a d0 metal is unlikely to undergo oxidative addition, Marks proposed a different mechanism from Rossi’s, instead invoking C−C bond cleavage via β-alkyl elimination. Expanding the scope from their initial study, Marks and coworkers explored different catalysts and substrates for ROZP. Scheme 54 depicts the diverse polymer/oligomer products obtained by matching a substrate with the appropriate catalyst. For example, the homopolymerization of methylenecyclobutane (MCB) to form a ring-opened polymer is achieved using only catalyst [(1,2-Me2Cp)2ZrMe][(μ-Me)B(C6F5)3] (25). Other zirconocene catalysts result in ill-defined polymers with (1,2Me2Cp)2ZrMe+ ≫ (Me5Cp)2ZrMe+), and is unaffected by the counterion.187 In contrast, for the polymerization of methylenecyclopropane (MCP), the catalyst yielding the highest selectivity for ringopened polymer D is the lutidinium catalyst 155, which is inert toward MCB ring-opening polymerization. Furthermore, 8126


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either ring-opening β-alkyl elimination or “zipping-up” to occur. In the case of the lanthanide-catalyzed reactions, β-H elimination intervenes before “zipping-up” propagation begins. The major difference between the copolymerization of ethylene with methylcyclobutane (MCB) and methylcyclopropane (MCP), where the ring-opened microstructure is the major unit, is that, for the former, zirconium catalysts (25−27 and 152−154) are active and, for the latter, lanthanide catalysts (155−157) and 158 are active. Interestingly, the more opened coordination sphere of the zirconium catalyst, 158, is more active than the lanthanide catalysts toward incorporating ringopened MCP into the polymer chain (Scheme 54), with a reactivity order of 158 > 155 > 156 > 157.187 In addition to ring-size effects, Marks and co-workers explored the effect of substituents attached to the methylenecyclopropane for ROZP with early transition metal catalysts. The homopolymerization of 7-methylenebicyclo[4.1.0]heptane by catalyst 26 (Scheme 55) affords ring-unopened polymer I,

smallest propensity to incorporate 7-methylenebicyclo[4.1.0]heptane relative to ethylene.192 In contrast to the previous examples, metallocene catalysts 25−27 and 152−159 do not polymerize 2-phenyl-1-methylenecyclopropane (Scheme 56). However, catalysts 26, 155, 156, and [Cp*2YH]2 (160) are capable of copolymerizing 2phenyl-1-methylenecyclopropane and ethylene.193 The copolymerization results in two distinct β-elimination sequences, where the C−C(Ph) bond or the C−C(H)2 bond is cleaved. For the zirconocene catalyst 26, a higher amount of 2-phenyl-1methylenecyclopropane is incorporated into the copolymer and microstructure L is exclusively formed. With the larger metallocene catalysts 155, 156, and 160, a lower amount of 2-phenyl-1-methylenecyclopropane incorporates and a mixture of microstructures L and M forms. In fact, the size of the metal ion (ionic radius) plays an important role in the selectivity of the β-elimination step. The smaller Zr catalyst 26 prefers microstructure L as a result of steric congestion, but with increasing metal ion size (Lu < Sm), microstructure M becomes more favorable due to η6-coordination of the phenyl ring.194,195

Scheme 55. Homopolymerization of 7Methylenebicyclo[4.1.0]heptane and Copolymerization with Ethylene by Catalysts 26, 155, 158, and 159

6.2. ROP via β-Alkyl Elimination by Palladium Catalysts

Although it is generally accepted that metallocene catalysts exhibit lower barriers to β-alkyl elimination compared to later transition metal complexes, only late transition metal catalysts can homopolymerize 2-phenyl-1-methylenecyclopropane. Osakada and co-workers reported the exclusive ring-opening homopolymerization by diamine supported Pd catalysts 161− 166 (Scheme 57).196 In this case, the mechanism is quite different from previous examples. For example, the insertion occurs at the terminal end (i.e., 2,1-insertion), producing a quaternary palladium alkyl fragment that undergoes a ringopening β-alkyl elimination to form an allyl intermediate. Labeling studies show that bond cleavage occurs exclusively at the H2C−C(H)Ph bond, and subsequent monomer insertion occurs at the Pd−CHPh carbon. It is interesting to note that NiCl2(allyl) will also homopolymerize 2-phenyl-1-methylenecyclopropane but the cyclopropane ring remains intact, further highlighting the importance of the metal ion during this process.197 In contrast to early transition metal catalysts, an advantage of using late transition metal catalysts is the ability to copolymerize olefins and CO without poisoning the catalyst. Complex 167 catalyzes the ring-opening copolymerization of exomethylenecycloalkane and CO to afford polyketones (N) with alternating monomer units (Scheme 58: top).198 Further investigation into this copolymerization by Osakada using N,N

while copolymerization with ethylene results in a ring-opened structure that effectively terminates the polymer chain J. Moving to more sterically opened catalysts 155, 158, and 159, copolymerization with ethylene gives ring-opened microstructure K. However, the ratio of incorporation of 7methylenebicyclo[4.1.0]heptane into the copolymer is very low. This is particularly evident with catalyst 155, which has the

Scheme 56. Copolymerization of M with Ethylene by 26, 155, 156, and 160

8127


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Scheme 57. Ring-Opening Homopolymierization of 2-Phenyl-1-methylenecyclopropane by Catalysts 161−166

Scheme 58. Copolymerization of Cyclopropane with CO

Scheme 59. Copolymerization of Cyclopropane with CO

bidentate Pd catalysts 168 and 169 demonstrated that monosubstituted 2-phenyl-1-methylenecyclopropylene can also be copolymerized.199 In contrast to the previous homopolymerization of 2-phenyl-1-methylenecyclopropane by Pd catalysts, monomer insertion proceeds by 1,2-insertion, and subsequently β-alkyl elimination can cleave the C−C(H)Ph or the C−CH2 units to yield microstructures O and P, respectively.199 The relative distributions of microstructures O and P vary between 50 and 70% O and 50−30% P

depending on ligand choice, the cocatalyst (e.g., NaB(C5F5)4, NaBF4), and the solvent.200 Palladium catalysts (161, 163, 165, and 168−180) also catalyze the copolymerization of 7-methylenebicyclo[4.1.0]heptane201 and 2-alkoxy-1-methylenecyclopropane202 with CO (Scheme 59). Under these conditions, the catalysts are living, and in the case of 7-methylenebicyclo[4.1.0]heptane, optically active polymer P forms, using a chiral bisoxazoline Pd complex.203 8128


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Scheme 60. Polymerization of 1,5-Hexadiene and Copolymerization with Propylene by 181 To Produce Polymer with RingClosing R Units and Ring-Opened S Units

Scheme 61. Possible Ring-Opening Functionalization Products from Methylenecyclopropane via β-Alkyl Elimination (A−D) and Oxidative Addition (C−E)

So far we have reviewed ring-opening β-alkyl elimination of strained cyclic substrates that produce ring-opened polymer. Yet another extension is to use linear diene substrates that form cyclic intermediates that undergo ring-opening β-alkyl eliminations to produce new polymer structures. Coates and Hustad reported such a system using titanium catalyst 181 for the polymerization of 1,5-hexadiene (Scheme 60).204 The resulting polymer contained a mixture of methylene-1,3-cyclopentane (R; 63%) and 3-vinyl tetramethylene (S; 37%) units. The cyclic R structure forms when 1,2-insertion of 1,5-hexadiene forms a stable 5-member ring. In contrast, the 2,1-insertion followed by

cyclization forms a 4-member ring that immediately results in the ring-opening β-alkyl elimination product S. The copolymerization of 1,5-hexadiene with propylene produced a polymer composed of predominantly polypropropylene (87− 96%) with incorporation of both R and S units. Similar R and S substructures were also observed during the polymerization of 1,5-hexadiene using V(acac)3/Al(C2H5)2Cl as catalyst.205 8129


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Scheme 62. Ring-Opening Functionalization of Methylenecyclopropane Reactions Involving β-Carbon Migration

Scheme 63. Examples of Ring-Expansion β-Alkyl Transfer Reactions To Form 5-, 6-, 7-, and 8-Member Rings

7. β-ALKYL ELIMINATION APPLIED TO ORGANIC SYNTHESIS Growing applications of ring-opening β-alkyl elimination reactions provide new synthetic methodologies to obtain organic products. For example, β-alkyl elimination reactions involving M−O α−Cβ−R and M−N α−Cβ−R have been extensively used in organic synthesis,2−13 which is outside the scope of this review. The corresponding β-alkyl elimination reactions from a M−Cα−Cβ−R moiety commonly feature

methylenecyclopropane and larger ring systems and have been recently reviewed.5,206−208 Hence, a brief overview of such reactions is presented. Scheme 61 depicts the two plausible mechanisms for C−C bond cleavage of methylenecyclopropane: oxidative addition and β-alkyl transfer. In the latter case, the selectivity is determined by insertion of an alkene into the M−A bond (1,2-insertion vs 2,1-insertion) followed by ringopening β-alkyl elimination depicted in the blue and red arrows in Scheme 61. Depending on the symmetry of the 8130


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Scheme 64. Aromaticity Driven β-Me Elimination

methylenecyclopropane, a total of four different products can be obtained A−D. Also note that oxidative addition can produce similar products C and D, which makes the mechanism difficult to determine. Overall, the 1,2-insertion pathway tends to be more prevalent.5 Scheme 62 depicts the different functionalities installed into a methylenecyclopropane by hydrosilylation (A),209−212 hydroacylation (B),213 hydrostannation (C),211,214 hydroamination (D), 215−221 hydroborylation (E), 211 hydrocarbonation (F),222−225 cyanosilation (G),226 isomerization (H),210,227 hydroalkoxylation (I),86,228 silaboration (J),229−231 and diboration (K)232 with concomitant ring-opening β-alkyl elimination. In addition to installing new functionalities, β-alkyl migration can be used to construct larger ring systems in ring-expansion β-alkyl transfer reactions. Scheme 63 depicts three examples of 5-, 6-, 7-, and 8-member rings constructed from cyclopropyl and cyclobutane rings.5,233−239 In cases A and B, oxidative addition of cyclobutanone precedes β-alkyl elimination to construct the larger ring system, whereas, in case C, C−H bond activation is the initial step. In addition to ring strain, aromaticity driven β-Me elimination can be effectively employed to dealkylate steroid derivatives240−242 and other cyclohexyl compounds (Scheme 64).243−245

Scheme 65. Proposed Surface Reaction Mechanism for the TIBA Ligands on an Aluminum Surface Undergoing β-H (600 K) Elimination

8.1. Hydrogenolysis of Alkanes by Solid-Supported Catalysts

Hydroprocessing of petrochemical feedstocks and diesel-based hydrocarbons for high-value chemicals involves hydrogenolysis of saturated hydrocarbons at high temperatures using supported metal particles (Ni, Pt, Rh, and Re), and involves β-alkyl transfers as the key strategy for C−C bond cleavage.246,247 The high temperatures required and the low selectivity led to further investigations for alternative metal catalysts capable of low-temperature hydrogenolysis of alkanes (neopentane, isobutane, propane). The supported zirconium hydride [(SiO)ZrH] (182) derived from alkyl species [( SiO)ZrNp3] (183) (Np = neopentyl) is highly electrophilic due to its d0 configuration and electronic unsaturation compared to cyclopentadienyl stabilized 16-electron zirconium hydrides. Supported 182 is exceptionally reactive for C3−C5 alkane hydrogenolysis.58 A proposed mechanism for the neopentane hydrogenolysis also requires C−H activation through σ bond metathesis at the zirconium hydride (Scheme 66). The major advantage of the zirconium-surface hydride is the strong zirconium−oxygen bond (Zr−O ∼ 2.4 Å),59 which provides a strong grafting of the metal to the support, thus reducing the possibility of bimolecular deactivation by site isolation, an inherent complication in molecular catalysts. To understand the role of silica-supported catalysts for cleaving C−C bonds, Basset and co-workers investigated the

8. β-ALKYL ELIMINATION BY SOLID-SUPPORTED AND HOMOGENEOUS CATALYSTS FOR ALKANE HYDROGENOLYSIS AND DEPOLYMERIZATION Chemical vapor deposition of aluminum films using triisobutylaluminum (TIBA) as a metal precursor results in simultaneous β-H and β-alkyl eliminations at 600 K (Scheme 65).57 The methyl group left on the surface presumably decomposes further via α-H elimination, but the key feature is the effective diffusion of isobutyl groups on the Al(111) surface with or without attachment to Al atoms. 8131


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Scheme 66. Hydrogenolysis of Neopentane by SilicaSupported Zirconium Hydride

Scheme 67. Proposed Mechanism of Alkane Hydrogenolysis Based on the Catalytically-Active Species, (a) Zr Monohydride (182) and (b) Zr Dihydride (184)

and butane.60 It is envisaged that surface Hf species follow a mechanism similar to that proposed for the Zr-derivative,59 based on the analysis of the product mixtures that contain propane and methane resulting from combined secondary C− H activation and β-Me elimination. Somewhat different results for the hydrogenolysis of neopentane are observed with the silica-supported titanium hydride (SiO)3TiH (187).61 The methane-to-ethane ratios in hydrogenolysis are 1:1 for Ti and 3:1 for Zr and Hf, indicating the possibility of a carbon skeletal rearrangement of the alkyl ligands attached to the metal center, allowing access to pathway B to yield a higher percentage of ethane (Scheme 70). The 1:1 methane:ethane ratio indicates that both pathways occur on a similar time scale. Further support for this proposal is formation of isobutene and nbutane at early reaction times. The major difference in the case of 187 is perhaps that the metal−alkyl-olefin species have longer lifetimes, thereby permitting reinsertion of olefin into the metal−carbon bond. In addition, the higher ethane ratio suggests that β-Et elimination is also occurring. In contrast to silica-supported Group IV metal-hydrides (Ti, Zr, Hf) that produce a distribution containing higher alkanes (ethane, propane, 2-methylpropane, etc.) from neopentane, low-temperature hydrogenolysis of alkanes by tantalum hydride supported on silica (SiO)2TaIIIH (188) produces 100% methane.252 Because the production of methane from ethane cannot be explained by a β-alkyl transfer mechanism, another process must occur for Ta−H. Further support for a different mechanism comes from the hydrogenolysis of 2,2-dimethylbutane (Np-Me) to yield neopentane as the major product. Evidence of C−C cleavage without β-alkyl transfer implies that a four-centered transition state σ-bond metathesis mechanism occurs (Scheme 71). More recently, DFT calculations analyzing the potential energy surface of a MCM-41 supported Ta−H suggest a different mechanism for the hydrogenolysis of alkanes.253,254 While a mixture of [(SiO)2TaIIIH] (188) and [( SiO)2TaV(H)3] (189) is observed experimentally,255 the lowest energy profile for propane hydrogenolysis indicates that TaV 189 is the active species. These studies indicate that the C−C bond cleavage step is likely an α-Me elimination to form a

structural features of the catalyst at the molecular level. The process involves a well-defined (SiO)ZrNp3 (183) surface complex that transforms to a mixture of species, namely monohydride (Si−O)3-ZrH (182) and (=Si)H2, as confirmed by Zr K-edge X-ray absorption fine structure (EXAFS) and 29Si solid-state NMR spectroscopy.59 The formation of 182 requires the rupture of Si−O−Si bonds in 183 to form two additional Zr−OSi bonds and (Si)H2. While Basset and co-workers59 proposed that the active species for group(IV) hydrogenolysis catalysts are the surface monohydrides, the actual active species is difficult to confirm without well-defined homogeneous models. Lunin proposed an alternative model based on DFT calculations indicating silicasupported Zr dihydrides (184) are the active species.248 Using 1 H DQ solid-state NMR, Basset and co-workers determined that 20−30% of the hydrides on the surface are Zrdihydrides,249 and their DFT calculations indicate a substantially lower thermodynamic barrier for alkane hydrogenolysis. In Basset’s model, C−C bond cleavage to produce a metal− olefin−alkyl intermediate is a very endergonic step (Scheme 67a). However, if the extruded olefin directly inserts into Zr−H to form a Zr−dialkyl intermediate (Scheme 67b), the enthalpic penalties are avoided. Nevertheless, the discrepancies do not detract from the importance of β-alkyl elimination in either mechanism. In the absence of hydrogen gas, silica supported 184 can also catalyze the homologation of propane gas into C1−C10 hydrocarbons (Scheme 68).250 Notably, the formation of methane, ethane, and C4 and C5 products as highlighted can only arise from β-Me elimination. Quite interestingly, the corresponding reaction using a silica supported CrIII species (185) yields predominantly propene via propane dehydrogenation (72%), while the β-Me elimination pathway, yielding ethane, ethylene, and methane, accounts for minor products ( [Hf]s−H (191) > [Ta]s−H (192) > [W]s−H (193) in terms of conversion and C1−C3 product selectivity (Table 5).256 [W]s−H (193) also gives a higher percentage of C3 products (Table 5), similar to [Ta]s−H (192), rather than Group IV catalysts 190 and 191, suggesting an α-alkyl transfer mechanism 8133


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Scheme 70. Ti-Surface Hydride Catalyzed Hydrogenolysis of Alkanes Involving Skeletal Isomerization-Driven Product Formation

[Ti−H]SiO2, the catalyst activity remains the same, but the selectivity for the C1−C4 products is enhanced from 33% to 56%. The lower activity of {[Ti−H]SiO2 + SiO2−Al2O3‑(500)} compared to [Ti−H]SiO2‑Al2O3 supports the assertion that the alumina directly interacts with the active site to improve the catalyst’s activity. One hypothesis for the enhanced activity is that the more electrophilic silica−alumina support stabilizes the Ti-π-bonded olefin-alkyl intermediate better after β-alkyl elimination (Scheme 73).259

Scheme 71. Alkane Hydrogenolysis with Tantalum Hydride Supported on Silica Catalyst 188

8.2. Depolymerization by solid-supported catalysts

In 2014, the worldwide production of polymers reached 311 million tons,260 with an estimated 50% of the polymer used for single-use disposable applications such as packaging or consumer disposables. From the total global polymer production, approximately 25% will be disposed in landfills. Significant effort has been directed toward engineering processes to (1) reuse the polymer directly, (2) recycle the polymer into the chemical feedstock, or (3) recuperate the energy content by incineration. The challenge associated with polymer degradation into monomers or other valuable chemicals depends largely on the polymer’s chemical structure and functionalities. Polymers containing oxygen or sulfur linkage atoms can be degraded with the appropriate chemical reagents/catalyst or radiation.261−263 Even polymers containing a linking CC moiety such as diene-based polymer (PB, natural rubber) can be degraded into oligomers using Grubbs catalyst via metathetical depolymerization.264−266 However, aliphatic polymers such as polyethylene (PE) and polypropylene (PP), containing linking C−C single bonds, account for the majority of all polymers produced. Their depolymerization into valuable chemical commodities is not developed, since most synthetic polyhydrocarbons are inert due to their (1) carbon-backbone with few accessible active sites for chemical transformation and (2) the large enthalpic penalty of depolymerization.

Scheme 72. DFT Investigated Mechanism of Propane Hydrogenolysis by 189

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Table 5. Conversion and Product Distribution at Steady-State for the Hydrogenolysis of Butane over Different Silica−Alumina Supported Metal-Hydride Catalysts

Scheme 73. Hydrogenolysis of Neopentyl Titanium on a Silica−Alumina Surface Resulting in Surface Titanium Hydridesa

The more electrophilic Ti-center on SiO2−Al2O3 stabilizes the Ti−alkyl−olefin intermediate resulting from β-carbon transfer (C−C cleavage) during hydrogenolysis of alkane. a

The circularized utilization of nonrenewable petroleumderived olefinic substrates, where components from depolymerization can be reused, repolymerized, or used as fuels, is an incredibly challenging task.268 An ideal solution would be controlled depolymerization of branched polymers into linear polymers and low-molecular weight alkanes under mild reaction conditions (Figure 10). The branched polyolefin can be repurposed into linear polyolefin while providing valuable low-molecular weight hydrocarbons. Developing this technology can be a key solution to recycling polymers. Selective scission of C−C bonds at branching sites in polymers or further polyolefin degradation can potentially be achieved via β-alkyl elimination at transition metal catalysts (Scheme 74). However, depolymerization to yield an olefin from polyolefins is endothermic. To successfully drive β-alkyl elimination, the alkene must be removed from the system to drive the equilibrium. H2 as reactant in the presence of a suitable catalyst displaces the thermodynamic equilibrium through the hydrogenation of olefins (eq 2, Scheme 75). In a

Table 6. Hydrogenolysis of Paraffin Wax: Comparison between Surface Titanium Hydrides Supported on Silica and Silica−Alumina Conversion (wt %) C1−C2 selectivity (wt %) C3−C4 selectivity (wt %) C5−C9 selectivity (wt %) C10−C22 selectivity (wt %)

[Ti−H]silica(500)

[Ti−H]silica−alumina(500)

30 24 9 28 39

100 6 7 24 63

The first spectroscopic (electron spin resonance, ESR) detection of polyolefin degradation promoted by elemental sulfur under high H2 pressure was discovered more than a decade ago,267 Currently, the thermal and catalytic pyrolysis process is a more practical method of recycling polymers. This process overcomes the otherwise challenging thermodynamic barriers of depolymerization into monomers or oligomers at the cost of increased energy input. 8135


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Figure 10. Depolymerization of branched polymer chain, P = polymer chain.

Scheme 74. β-Alkyl Transfer: P = Polymer Chain

(C18 to C50) catalyzed by (SiO)3ZrH provides 84% conversion to lower alkanes (C1 to C9). DFT calculations modeling the depolymerization by a zirconium monohydride grafted on a silica surface were performed. Using propane as a model substrate, β-alkyl transfer at the zirconium center immobilized on a (100) silica surface is the rate-limiting step with the activation energy 29 kcal/mol.269 Interestingly, zirconium attached to a (111) silica surface yields a higher β-alkyl elimination barrier of 41 kcal/mol. These results highlight the sensitivity of the catalytic activity to the support surface. In addition, the DFT mechanistic study also concluded that the olefin does not remain coordinated to the metal center immediately after β-alkyl elimination. This is overall favorable for depolymerizaion, since reinsertion of the polymer chain with an olefinic double bond into the Zr−C bond becomes less favorable. The corresponding silica supported Hf hydride containing a mixture of [(SiO)3Hf(H)] (186) and [(SiO)2Hf(H)2] (199) is capable of polymerizing ethylene and isobutene, and depolymerizing polyethylene, polypropylene, and polyisobutene under H2.66 The relative catalytic activity between 186 and 199 during depolymerization could not be determined. Hence, a particular challenge with grafted group 4 metals on silica supports is understanding the nature of the active species. Further development of homogeneous models capable of depolymerizing a range of polymers would provide insight into the supported active species, if not enhance depolymerization due to easier mass transport. It is also possible that there are multiple active sites on the silica support. Such a scenario could be further elucidated by homogeneous models.

successful depolymerization process, polyolefins composed mainly of C−C σ bonds transform into diesel range fuels and ultimately into methane/ethane. Scheme 75. Hydrogenolysis of High-Molecular Weight Straight Chain Alkanes

The first report of low temperature hydrogenolysis of paraffinic polymers via β-alkyl elimination as the rate-limiting step demonstrated the effectiveness of silica−alumina supported zirconium monohydrides 182 and 198 containing active (SiO)3ZrH units (Scheme 76).64 The structure of 198 explains the further electrophilicity enhancement of the zirconium center when an aluminum hydride group (solid support) is in close proximity to the zirconium, and thus imparts even more electrophilicity in comparison to the same Zr compound on pure silica. Product distribution from a degradation reaction of low-molecular weight polyethylene

Scheme 76. Proposed Structure of Zr−H Supported on Silica-Alumina

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Scheme 77. Proposed Reaction Pathway Involving β-Alkyl Elimination for Chain-Cleavage

8.3. Depolymerization by Homogeneous Catalysts

the same: (1) an open coordination site adjacent to the M−R species is a prerequisite, and (2) γ-agostic interactions assist in the C−C bond cleaving step by stabilizing the alkyl fragment as it reorients toward the metal. Competing β-hydride elimination, which requires similar agostic interactions and in most cases is kinetically and thermodynamically preferred, is one of the largest challenges to exploiting β-alkyl elimination in synthesis. This is particularly true for late transition metal complexes, which have a larger M−H vs M−C bond energy difference {ΔHM‑H − ΔHM‑C > 15 kcal/mol} than early transition metals.87 Kinetically, the calculated activation energies needed for β-H elimination in Pt−R species are at least 20 kcal/mol lower than the Ea for the corresponding β-alkyl eliminations.157 Hence, driving forces such as ring strain or aromatization have been used to favor βalkyl elimination. Nevertheless, there are many brilliant examples that employ ring-opening β-alkyl elimination reactions in polymer and organic synthesis. In particular, late transition metal complexes rival their metallocene counterparts as catalysts for ring-opening polymerization reactions, and the functional group tolerance of the late transition metal complexes makes them ideal candidates for copolymerization with polar substrates and organic synthesis. However, a future challenge for organometallic researchers is the design of ancillary ligands for middle-to-late transition metal complexes to enhance the competiveness of β-alkyl elimination without the need for “other driving forces”. To achieve these goals, much can be learned from unassisted β-Me elimination from early transition metallocene complexes. Group 4 metallocene complexes featuring sterically crowded cyclopentadienyl ligands are exemplary models for β-alkyl elimination. Cationic Cp*2ZrR+ and Cp*2HfR+ complexes, which polymerize or oligomerize propene, undergo β-Me elimination as the predominant chain-termination pathway over β-H elimination. According to Teuben’s model, sterically large Cp* ligands force the β-Me to lie adjacent to the metal ion.50 Again, there is some remarkable similarity between β-H and βMe elimination events. For instance, β-Me elimination can undergo methyl group transfer either to a metal (BME) or directly to the coordinated monomer (BMT).24 BME and BMT are analogous transfer modes documented for β-H termination steps. A new area of focus is the design of ligands to control the modes of β-Me elimination. Okuda and Zambelli show that ligand design can influence the mode of βelimination to favor either metal or monomer transfer.110,111 In addition, other factors, including counteranion, solvent polarity, coordinating ligand, and chain-length, all affect the βalkyl elimination reaction and subsequent selectivity. Discovery and development of solid-state hydrides of group 4 metals for the hydrogenolysis of alkanes and polymers highlights the continued importance of β-alkyl elimination reactions and their potential applications. In particular, the depolymerization of polyolefin- and polydiene-based polymers has attracted considerable interest. We have critically discussed

Homogenous Cp2ZrHCl (200) is an effective reagent for mediating chain-scission reactions of polybutadiene (PB), polyisoprene, and poly(styrene-co-butadiene). The mechanism proceeds via olefin insertion into the hydride followed by βalkyl elimination (Scheme 77).65 However, 200 is not regenerated to continue a catalytic cycle. The overall molecular weight after chain-scission can be effectively controlled by the initial concentration of 200. Remarkably, product analysis retains a similar PDI to the initial polymer, affirming the wellcontrolled nature of the chain-scission. Using iBu3Al as a chaintransfer reagent allows catalytic regeneration of 200, but a Al/ Zr ratio >30 is overall detrimental to catalytic chain-scission. This report of polymer chain-scission with an organometallic hydrozirconating agent paves the way for new organometallic compounds for depolymerization. Addition of TEMPO to (Me6TREN)CuCl2 {Me6TREN = tridimethylaminoethylamine} (201) in situ generates a CuI species that catalyzes low temperature depolymerization of Nisopropylacrylamide (NIPAM) based polymers.67 The C−C bond cleavage step occurs through an effective β-alkyl elimination step. (Scheme 78). Scheme 78. Mechanism Proposed for TEMPO-Induced CuIICentered Depolymerization

9. SUMMARY AND OUTLOOK We have presented an extensive overview of (1) homogeneous metal complexes capable of β-alkyl and β-arene elimination, (2) catalytic applications for polymer and organic synthesis, and (3) solid supported hydrides of group 4 metals for alkane/polymer hydrogenolysis. Particular attention was paid to the mechanism for β-alkyl elimination, and when data permitted, its selectivity over competing β-hydride elimination. For simplicity, homogeneous systems that undergo β-alkyl elimination can be separated into two categories: early transition metallocene complexes and middle-to-late transition metal complexes. For both, however, the basic framework for β-alkyl elimination is 8137


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(2) Ruhland, K. Transition-Metal-Mediated Cleavage and Activation of C-C Single Bonds. Eur. J. Org. Chem. 2012, 2012, 2683−2706. (3) Murakami, M.; Matsuda, T. Metal-Catalysed Cleavage of CarbonCarbon Bonds. Chem. Commun. 2011, 47, 1100−1105. (4) Jun, C. H. Transition Metal-Catalyzed Carbon-Carbon Bond Activation. Chem. Soc. Rev. 2004, 33, 610−618. (5) Miura, M.; Satoh, T. In Palladium in Organic Synthesis; Tsuji, J., Ed.; Springer: Berlin Heidelberg, 2005; Vol. 14, pp 1−20. (6) Matsuda, T. In Cleavage of Carbon-Carbon Single Bonds by Transition Metals; Wiley-VCH Verlag GmbH & Co. KGaA: 2015; pp 35−88. (7) Matsuda, T. In Cleavage of Carbon-Carbon Single Bonds by Transition Metals; Wiley-VCH Verlag GmbH & Co. KGaA: 2015; pp 89−118. (8) Murakami, M.; Ishida, N. In Cleavage of Carbon-Carbon Single Bonds by Transition Metals; Wiley-VCH Verlag GmbH & Co. KGaA: 2015; pp 1−34. (9) Murakami, M.; Ishida, N. In Cleavage of Carbon-Carbon Single Bonds by Transition Metals; Wiley-VCH Verlag GmbH & Co. KGaA: 2015; pp 221−252. (10) Murakami, M.; Ishida, N. In Cleavage of Carbon-Carbon Single Bonds by Transition Metals; Wiley-VCH Verlag GmbH & Co. KGaA: 2015; pp 253−272. (11) Satoh, T.; Miura, M. In Cleavage of Carbon-Carbon Single Bonds by Transition Metals; Wiley-VCH Verlag GmbH & Co. KGaA: 2015; pp 119−164. (12) Tobisu, M. In Cleavage of Carbon-Carbon Single Bonds by Transition Metals; Wiley-VCH Verlag GmbH & Co. KGaA: 2015; pp 193−220. (13) Yorimitsu, H. In Cleavage of Carbon-Carbon Single Bonds by Transition Metals; Wiley-VCH Verlag GmbH & Co. KGaA: 2015; pp 165−192. (14) Talarico, G.; Budzelaar, P. H. M. Variability of Chain Transfer to Monomer Step in Olefin Polymerization. Organometallics 2008, 27, 4098−4107. (15) Chisholm, M. H.; Eichhorn, B. W.; Huffman, J. C. TungstenCarbon, Carbon-Carbon, and Carbon-Hydrogen Bond Activations in the Chemistry of W2r2(OR′)4(W≡W) Complexes. 3. Competitive αHydrogen Versus β-Hydrogen Eliminations and Ethylene-to-Acetylene Hydrogen Transfers. Organometallics 1989, 8, 80−89. (16) Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. Ethylene Insertion and Beta-Hydrogen Elimination for the Permethylscandocene Alkyl Complexes: A Syudy of the Chain Propagation and Termination Steps in Ziegler-Natta Polymerization of Ethylene. J. Am. Chem. Soc. 1990, 112, 1566−1577. (17) Garr, C. D.; Finke, R. G. Radical Cage Effects in Adocobinamide (Axial-Base-Off Coenzyme-B12: A Simple Method for Trapping [Ado..Coii] Radical Pairs, a New Beta-H Elimination Product from the Radical Pair and Measurement of an Unprecedented Large CageRecombination Efficiency Factor, Fc ≥ 0.94. J. Am. Chem. Soc. 1992, 114, 10440−10445. (18) Alelyunas, Y. W.; Guo, Z. Y.; Lapointe, R. E.; Jordan, R. F. Structure and Reactivity of (C5H4Me)2ZR(CH2CH2R) (CH3CN)n+ Complexes: Competition between Insertion and Beta-H-Elimination. Organometallics 1993, 12, 544−553. (19) Wang, S. Y. S.; Abboud, K. A.; Boncella, J. M. Ligand-Induced Beta-H Transfer in W(VI) Dialkyl Complexes. J. Am. Chem. Soc. 1997, 119, 11990−11991. (20) Tsai, Y. C.; Johnson, M. J. A.; Mindiola, D. J.; Cummins, C. C.; Klooster, W. T.; Koetzle, T. F. A Cyclometalated Resting State for a Reactive Molybdenum Amide: Favorable Consequences of BetaHydrogen Elimination Including Reductive Cleavage, Coupling, and Complexation. J. Am. Chem. Soc. 1999, 121, 10426−10427. (21) Komiya, S.; Yasuda, T.; Fukuoka, A.; Hirano, M. Synthesis of Hydridoplatinum-Molybdenum (or Tungsten) Heterodinuclear Complexes by Beta-Hydrogen Elimination of (dppe) EtPt-MCp(CO)(3). Selective Hydride Transfer from Pt to Mo (or W). J. Mol. Catal. A: Chem. 2000, 159, 63−70.

supported metal-hydrides to better understand their design features in hopes of translating that information to the creation of homogeneous counterparts, an area open for investigation. We hope that the past and recent findings on β-alkyl transfer reactions presented in this review demonstrate the fundamental importance of β-alkyl reactions and will inspire new synthetic applications. So far, ring-expansion reactions, ring-opening polymerizations, and alkane and polymer hydrogenolysis serve as prime evidence of the promising potential of β-alkyl elimination reactions.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]fl.edu. Notes

The authors declare no competing financial interest. Biographies Matthew E. O’Reilly received his B.S. in Chemistry from Lee University in 2008. He worked briefly as an REU researcher at the University of Strasbourg with Prof. Pierre Braunstein, before moving to the University of Florida, where he obtained his Ph.D. in Inorganic Chemistry in 2013 with Prof. Adam Veige. He then worked as a postdoctoral researcher at the University of Virginia in the Center for Catalytic Hydrocarbon Functionalization and is currently a postdoctoral fellow with Prof. Surendranath at the Massachusetts Institute of Technology. His predominant interests include homogeneous and immobilized transition metal catalysts and electrocatalysis. Saikat Dutta is senior postdoctoral researcher at the Catalysis Center for Energy Innovation (US DOE EFRC) at the University of Delaware, USA. He received a Ph.D. in Inorganic Chemistry from Indian Institute of Science, Bangalore. After a couple of postdoctoral research experiences in the area of polymerization catalysis and biomass chemistry, Saikat joined the research group of Prof. Adam Veige as a Nehru Fulbright Postdoctoral Fellow in 2012−2013 at the University of Florida, USA. His research experience focuses on the chemistry of biomass and design of porous materials for their electrochemical and catalytic applications. His current research includes discovering fundamental and applied aspects of the chemistry of biorenewables. Adam Veige is Professor of Chemistry at the University of Florida and the current Director of the UF Center for Catalysis. Dr. Veige obtained his Ph.D. in 2003 at Cornell University and did postdoctoral studies at the Massachusetts Institute of Technology. Dr. Veige’s research focuses on the synthesis of highly active catalysts for creating value-added products, including the polymerization of olefins and alkynes to create cyclic polymers. A specific area of focus centers on the development and application of trianionic pincer ligands in organometallic chemistry. Other areas of current interest include the synthesis of in-chain metallopolymers using iClick technology invented by the Veige group, and delivering cytotoxic metal ions to cancer cells via aptamers.

ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation CHE-1265993 (A.S.V.). REFERENCES (1) Crabtree, R. H. The Organometallc Chemistry of the Transition Metals, 5th ed.; Wiley: 2005. 8138


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