C H Activation of Hydrocarbon Substrates Enabled by Lewis ...

Research Proposal – Spring 2019

C–H Activation of Hydrocarbon Substrates Enabled by Lewis-Acid Catalyzed NO Bending in Re-Nitrosyl Complexes for the Application

of Hydromethylation of Olefins

Advisor: Professor Shannon S. Stahl


Scheme 1. Selected examples of methane functionalization.

a) Tilley: σ-Bond Metathesis for Hydromethylation of Methane

b) Bergman: Oxidative Addition of Methane

Sc CH3 CH4+ +CH3Sc

ReLMe3P PMe3

CH4+ hv, – LRe

Me3PPMe3

ReMe3PMe3P

HCH3

c) This work: σ-Complex-Assisted-Metathesis Hydromethylation of Methane

R5 R5R5

ReNO

LA

PR3

HCH3

CH4+ +CH3Re

PR3NCH3O

+ LA

NO bending

R = H, Me; L = PMe3, CO

R = alkyl, aryl; LA = Lewis Acid

Scheme 2. Redox non-innocent properties of the nitrosyl ligand.

N

M

ON

M

O

N

M

O

b) Coordination Modes of Nitrosyl Ligand c) Redox Noninnocence of Nitrosyl Ligand

NO+ NO–NO•

Nitroxyl1e – donor

Nitric Oxide2e– donor

Nitrosonium3e– donor

LnM N O LnM NO

Linear, M(n) Bent, M(n+2)

a) Catalytic Nitrosyl Effect in Alkene Hydrogenations by Berke and Coworkers

LnReI N O LnReIII NO

Linear Bent

R1 R2

H2

LA LA23 °Cneat R1 R2

LA = Lewis Acid

Background and Significance. Catalytic C–H activation is a cornerstone of synthetic organic chemistry. A key application is the

functionalization of hydrocarbon feedstocks for uses ranging from medicinal to industrial chemistry.1–3 An ideal hydrocarbon feedstock is natural gas– a cheap, readily available natural resource primarily composed of methane (70-90%). Despite its abundance, the direct use of methane as a feedstock remains one of the most challenging obstacles due to its low intrinsic reactivity.4 This difficulty arises from the properties of methane: its large ionization potential (~12.5 eV)5,6 hinders its proclivity toward homo- or heterolytic C–H bond cleavage (BDE = 105 kcal/mol, pKa = 50),7 and the absence of a dipole moment, small polarizability, and low proton and electron affinity render it relatively inert to reactivity.8 The low intrinsic reactivity of methane, coupled with the higher reactivity of methane-functionalized products, highlights a central challenge to the selective functionalization of inert C–H bonds.

Despite these challenges, examples of catalytic methane functionalization by C–C bond formation and C–O bond formation have been developed.9 Hydromethylation of olefins using methane poses a particularly attractive atom economical route for the synthesis of higher order alkanes. Catalytic hydromethylation of propene to isobutane has been demonstrated by the Tilley lab10 using methane by employing a Cp*2ScMe catalyst through σ-bond metathesis, albeit with only 3 TON (Scheme 1a). To increase catalyst reactivity, Tilley employed more open ansa-Me2Si(C5Me4)2ScCH3, which allowed for faster activation of methane and hydromethylation of other olefins (cyclopentene, cis-butene, isobutylene, and propene), but the presence of side-products arising from competing pathways often accompanied the hydromethylated product. 11 Thus, the identity of a selective catalyst for hydromethylation of olefins using methane remains an unsolved problem.

Fundamental studies on C–H activation have helped identify promising organometallic systems for selective C–H functionalization. Cyclopentadienylrhenium (CpRe) complexes have been shown to be remarkably active catalysts for selective C–H activation of a variety of hydrocarbon substrates under mild conditions.12,13 Bergman and coworkers studied stoichiometric C–H bond activation selectivities of CpRe complexes and were able to show preferential methane activation in solvent-level cyclohexane to yield isolable CpRe methyl hydrides (Scheme 1b).14 The intermolecular C–H bond selectivities mirrored the thermodynamic stabilities of the corresponding C–H activated products.15 Methane activation was also demonstrated by Jones et. al in catalytic H/D exchange of alkanes in d6-benzene using CpRe(PPh3)2H2, where preferential binding of methane was observed over both ethane and cyclopentane.16,17 The majority of these CpRe complexes bear phosphine and CO ligands, achieving C–H activation by opening a coordination site through photolysis of a bound ligand or through thermal activation. However, these modes of catalyst activation are limited in their tunability. Therefore, alternative methods of catalyst activation could allow for more selective and tunable modes of activation.

Recently, coordination of Lewis acids to nitrosyl (NO) ligands bound to Re has been shown to effect efficient alkene hydrogenation by modulating the coordination geometry of the nitrosyl


Scheme 3. Synthesis of CpxRe(NO)(CH3)(PR3) complexes.

RePR3ON

CH3

Re2(CO)10 + R

R R

RR H

150-210 °CRe

COOCCO

R5

NO+BF4–

Re+COON

CO

R5

BF4–

PhIO MeCN

Re+NCMeON

CO

R5

BF4–PR3

2-butanonereflux

Re+PR3ON

CO

R5

BF4–1. LiEt3BH2. BH3•THF

R5

ligand (Scheme 2a). Bending of the nitrosyl ligand opens a coordination site for binding in concert with a formal 2e– oxidation of Re, enhancing the reactivity of the complex.18–21 Despite being a prototypical example of a non-innocent ligand,6 NO has been underutilized for cooperative catalysis.22,23 The redox non-innocent nitrosyl ligand can support multiple coordination modes, including a linear NO+ ligand as a 3e– donor or a bent NO– as a 1e– donor (Scheme 2b), which formally oxidizes the metal by 2e– in concert with the opening of a vacant coordination site (Scheme 2c).

Given the limited examples taking advantage of the catalytic nitrosyl effect, the redox non-innocence of the nitrosyl ligand represents an untapped reservoir that can have major implications in the development of future catalyst systems. Moreover, the addition of Lewis acids to change the coordination mode of the bound nitrosyl ligand as well as alter the electronics of the metal center offers a unique handle to open a coordination site and tune the metal complex for desired reactivity. Given the precedence for selective C–H activation by CpRe complexes bearing CO ligands, incorporation of a comparably π-acidic nitrosyl ligand would likely maintain this selectivity while also providing a handle for altering the coordination environment and electronic structure of Re in tandem with a judicious choice of Lewis acid. The correlation of the steric and electronic effects of ligand environment on the metal center with C–H bond selectivities could add to fundamental understandings of C–H activation for the improvement and development of future catalyst systems. With this in mind, a cooperative CpRe-nitrosyl/Lewis acid catalyst system capable of selective C–H activation will be developed for the application of hydromethylation of simple alkenes using methane as a feedstock (Scheme 1d). Specific Aims:

I. Synthesize CpxRe(NO)(CH3)(PR3) complexes and correlate the electronic and steric parameters ofthe ligand framework and the effect of Lewis acids on NO bending.

II. Elucidate the C–H bond selectivities as a function of ligand framework and Lewis acid electronicand steric parameters and evaluate the Re-NO/LA catalyst systems for stoichiometric C–H activation to identify a catalyst system capable of selective methane activation.

III. Using the knowledge gained in Aims 1 and 2, apply a selective Re-nitrosyl/Lewis Acid catalystsystem toward stoichiometric and catalytic hydromethylation of simple olefins.

Experimental Design and Methods. Specific Aim 1. Synthesize Re-NO complexes and explore the effect of Lewis acids on NO bending.

To investigate the effect of different ligand environments and Lewis acids (LA) on the ability to induce NO bending, a variety of Re-NO complexes of the formula CpxRe(NO)(CH3)(PR3) (Cpx = derivatized Cp ligands and analogs; R = alkyl, aryl) will be synthesized and characterized according to literature precedent (Scheme 3).24–27 Cyclopentadienyl derivatives including Cp, Cp* (Cp* = pentamethylcyclopentadienyl), Tp (Tp = trispyrazolylborate), and indenyl ligands will be employed in combination with a variety of trialkyl- and triarylphosphine ligands. Their effect on the nitrosyl ligand geometry will be characterized by measurement and correlation of v(NO) by IR spectroscopy and Re-N-O bond angles derived from X-ray crystallographic structures. Decreased MNO bond angle and lengthened N–O bonds correlate with decreasing v(NO) stretching frequency, and thus can provide initial insight into the general geometry of the nitrosyl ligand.28 This can inform us generally on the electronic environment of the complex. Increased electron density on the metal typically increases the contribution of MàNO π backbonding, which strengthens the M–N bond while weakening the N–O bond for a lower v(NO) stretching frequency and decreased MNO bond angle.

Additional electronic and steric parameters of the cyclopentadienyl derivatives/analogs and phosphines will be evaluated and correlated to the v(NO) stretching frequency and MNO bond angle. Recently, Rovis et. al correlated reactivity and selectivity of CpxRh(III) complexes in C–H functionalization reactions by parameterizing a variety of electronic and steric properties.29 Thus, with the intention of


Scheme 4. Possible pathways of C–H activation of alkyl and aryl substrates by CpxRe(NO)(CH3)(PR3).

RePR3N

CH3

ReNH3C

O

LA

PR3

ReI d6, 18 e− ReIII d4, 16 e−

O

LA R' H

ReNH3C

O

LA

PR3

HR'

ReIII d4, 18 e−

ReNH3C

O

LA

PR3

HR'Formation of

σ−Complex

σ−Complex-Assisted-

Metathesis

ReNR'

O

LA

PR3

ReIII d4, 16 e−

CH4

ReNH3C

O

LA

PR3

HR'

Oxidative Addition

ReV d2, 18 e−

RePR3N

R'ReI d6, 18 e−

O

LA

Solv

ReSolv

NR'

O

LA

PR3

ReIII d4, 18 e−

Reductive Elimination

NO Bending

R = alkyl, arylR' = alkyl, arylLA = Lewis acidSolv = solvent

CH4

correlating the tabulated parameters to propensity for NO bending upon Lewis acid addition, the Cpx ligands and phosphines will be parameterized according to their effect on v(NO) stretching frequency and MNO bond angle, 31P chemical shift,30 and Tolman cone angles.29,31–34 A relationship between σ donor strength of the phosphines, represented by the dual parameters EB and CB as tabulated by Drago and coworkers,35 will also be established.

Then, the effect of Lewis acids, including boron compounds, boron/hydrosilane systems, and metal salts, will be added to probe the effect on the nitrosyl ligand geometry by measurement of v(NO) by IR spectroscopy and Re-N-O bond angles from X-ray crystallography. Coordination of a Lewis acid to the Lewis basic ONO withdraws electron density from the nitrosyl ligand inducing increased π backbonding from the metal to the π* of NO. This increased π backbonding reduces the bond order and consequently decreases the v(NO) stretching frequency and Re-N-O bond angle to bent geometry.28,36–38 The identity of the Lewis acid has implications on the degree of bending, which ultimately influences the reactivity of the Re-NO complex. Upon Lewis acid addition, the Re-NO/LA complexes will be re-characterized and parameterized as described for CpxRe(NO)(CH3)(PR3) complexes in addition to Lewis acid strength as dictated by the Gutmann-Beckett method.39,40 Thus, a relationship between steric and electronic parameters of both the ligand and the Lewis acid additive can be established with respect to C–H bond activation and selectivities. Specific Aim 2. Evaluate the selectivity and efficacy of Re-NO/LA catalyst systems for stoichiometric C–H Activation.

Next, the CpxRe(NO)(CH3)(PR3)/LA catalyst systems will be investigated in their ability to selectively activate aryl, alkenyl, methyl, 1°, 2°, and 3° C–H bonds. Stoichiometric addition of CpxRe-NO complexes to solvent-level C–H substrate will be carried out before Lewis acid addition, with the bound Lewis acid CpxRe-NO complexes (if isolable), and with concomitant addition of Lewis acid for generation of the bent nitrosyl in situ.18,20,22,23 Thus, the C–H selectivities can be evaluated for the bound and unbound CpxRe-NO complexes in addition to the effect of LA coordination prior to interaction with C–H substrate.

In principle, C–H activation is likely to occur through one of two major pathways: oxidative addition or σ-complex assisted metathesis (Scheme 4).1,9,41 Coordination of the σ C–H bond of a hydrocarbon substrate often precedes C–H activation, and the formation of metal alkyl/aryl σ-complexes have been shown to play a paramount role in dictating C–H bond selectivities.1,42–47 Furthermore, formation of σ-complex is a requirement for the σ-complex assisted metathesis pathway to occur. Oxidative addition of C–H substrate to CpxReIII(NO-LA)(CH3)(PR3) will generate an 18e– CpxReV alkyl/aryl hydride. Isolable ReV complexes are uncommon,48–51 thus if this pathway were operative, it would most likely undergo rapid reductive elimination of CH4 to yield CpxReIII(NO-LA)(R’)(PR3) (R’ = bound C–H substrate). A σ-complex assisted metathesis pathway would access the corresponding C–H activated CpxReIII(NO-LA)(R’)(PR3) complex without proceeding through a high oxidation state ReV intermediate. Alternatively, it is possible for phosphine dissociation to occur prior to C–H activation. Depending on whether LA coordination is reversible,19 nitrosyl bending may be reversible, and thus the identity and strength of the LA will influence the mechanism of C–H activation. In situ monitoring of the v(NO) during C–H activation can lead to


insights on the nature of reversible bending of the nitrosyl ligand and probe its role in promoting C–H activation.

Extensive work characterizing Re σ-alkane complexes has been done through NMR and IR spectroscopy, and the equilibrium between the σ-alkane complexes and the corresponding oxidative addition product has been studied for various CpxRe complexes and analogs.52–55 With regards to the sterics of the ligand framework, studies by George and coworkers found increased alkane displacement from Re alkane σ-complexes for substituted cyclopentadienyl derivatives (pentamethyl and pentaphenyl) over the Cp analog.56 This was attributed to destabilizing interactions between the steric bulk of the ligand and the coordinated alkane. George and coworkers also found substantially different lifetimes of LRe(CO)2(alkane) σ-complexes bearing different cyclopentadienyl analogs (L = Cp, Tp, and Tp* = tris(3,5-dimethyl-pyrazol-1-yl)borate) in the order of Tp* << Tp < Cp.57 Taken together, these studies imply increased stability of theRe alkane σ-complexes with decreased steric bulk on the cyclopentadienyl ligand. Since σ-complex formation will most likely precede C–H activation, whatever the mechanism, we anticipate that the degree and rate of C–H activation will increase with decreased steric bulk of the cyclopentadienyl ligand, and thus expect Cp to be the most competent ligand in this chemistry.

To that end, similar studies have probed the equilibrium between σ-complexation and formal C–H activation. George and coworkers quantified the ratio of Re alkane σ-complexes and Re alkyl hydrides in solution, finding increased amounts of oxidative addition products observed for Cp- over Cp*-Re complexes. This is similar to the increased amounts of CpRe methane oxidative addition products over the corresponding Cp* analogs observed by Bergman and coworkers. 14,15 Ball and coworkers found higher degrees of oxidative addition observed for the [CpRe(CO)(PF3)], which demonstrated an equilibrium between the σ-complex and C–H oxidative product, while [CpRe(CO)2] favored exclusive complexation of alkanes.58 Conflicting reports exist as to relative σ-donor abilities and π-acidities of CO and PF3,59–62 but it is clear that the ligand environment of CpxRe complexes are essential for dictating C–H bond activation vs. complexation. Due to the high π-acidity of NO and increased Re®NO π donation upon nitrosyl bending, we anticipate strongly σ-donating phosphines with decreased π-acidity would most likely stabilize the electron deficient metal center. Similar trends on the effect of the steric bulk of the phosphine to those established for the Cpx ligands may indicate that decreased steric bulk will help favor σ-complex formation and C–H activation as well.

Given the tunability of Re-nitrosyl/Lewis acid co-catalysis, the π-acidity can be controlled to maximize reactivity as there appears to be a delicate balance of complexation vs. C–H activation with regards to the supporting ligands of the CpxRe complex. Through systematic exchange of the ligand framework on Re, we hope to elucidate the principles that govern this equilibrium. Therefore, similar studies on σ-complexation vs. C–H activation of hydrocarbons using low-temperature NMR spectroscopy and time-resolved IR spectroscopy monitoring v(NO) will be carried out. The results of these studies will be correlated with the parameterization of the ligands and Lewis acid of choice in order to add to the fundamental understanding of C–H complexation vs. activation. By evaluating the library of CpxRe-NO/LA catalyst systems, we hope to identify one or more systems capable of selective methane activation. Specific Aim 3. Apply Re-nitrosyl/Lewis Acid catalyst system toward Hydromethylation of Olefins.

Little precedent exists for hydromethylation due to the challenging nature of methane activation.11,63 However, with the identification of a CpxRe-NO/LA system capable of selective C–H activation of CH4 described in Aims 1 and 2, we hope to accomplish the hydromethylation of simple alkenes. Stoichiometric hydromethylation of ethylene will first be investigated using the CpxRe-NO/LA catalyst system. Ideally, the CpxRe-NO/LA catalyst system would preferentially bind methane over ethylene, however, it could be possible to modulate the partial pressures of the two gases to bias binding of methane over ethylene if binding is competitive.64,65 Isobutylene would also be employed to investigate the impact of sterics on the system, as well as propylene to probe the regioselectivity. Once conditions have been established to enact stoichiometric hydromethylation, conditions capable of catalytic hydromethylation of simple alkenes will be developed.

A proposed catalytic cycle is depicted in Scheme 5. Lewis acid coordination to the Lewis basic ONO atom of CpReI(NO)(CH3)(PR3) induces nitrosyl bending in concert with a formal 2e– oxidation of the metal


to generate a 16e– coordinatively unsaturated CpReIII complex.18–21 As a middle transition metal, Re tends to favor coordinatively saturated 18e– complexes, however, it is possible that this is a reversible step dependent Lewis acid coordination. This would allow for transient generation of a coordinatively unsaturated complex that can be trapped by alkene binding to regenerate an 18e– complex. Alkene coordination is followed by alkene insertion into the Re–CH3 to generate a Re–alkyl and open a coordination site for methane binding.66–72 Discrete formation of an 18e– CpReIII-methane σ-complex52,73,74 precedes σ-complex-assisted-metathesis75–80 to release hydromethylated product and regenerate the CpReIII(NO-LA)(CH3)(PR3) complex. The reversibility of Lewis acid binding, nitrosyl ligand bending, and the possibility for phosphine dissociation or ring slippage could allow other pathways for hydromethylation to occur.

Deleterious pathways such as β-hydride elimination and oligomerization must be avoided for efficient hydromethylation, as identified by Tilley and coworkers.10,11,81,82 As suggested by the differences in reactivity seen in his Cp* bound catalyst in comparison to the ansa-Cp derivative, the steric environment of the catalyst plays an essential role in dictating the possibility of these pathways. Follow-up computational studies identified the key problem as favorable alkene coordination following alkene insertion into the Sc–Me bond, suggesting prevention of olefin coordination would be helpful in allowing for CH4 activation.81 Increased steric bulk on the olefin, such as in the case of isobutylene, can bias selectivity exclusively for the hydromethylated product, as shown in the sole formation of neopentane using the ansa-CpSc derived complex. Thus, it is clear that there exists a balance between allowing for methane coordination and olefin insertion while preventing undesired excessive olefin binding or polymerization. Modulation of the steric environment of the Re complex and the relative pressures of methane and ethylene will be carried out to address this issue. A more open ligand environment would facilitate alkene binding and insertion but risk turning on oligomerization or β-hydride elimination pathways. Precedence exists for Re-catalyzed polymerization, however, the majority of these complexes are suggested to proceed through an alkylidene intermediate and are not similar in structure to the CpRe complexes proposed.68,83–86

If β-hydride elimination is reversible, it would allow for re-entry in the catalytic cycle.87Additionally, β-hydride elimination requires an open coordination site adjacent to the bound alkane,88 meaning controlling the amount of vacant coordination sites by discouraging phosphine dissociation or avoiding ring slippage89–91 may actually be more favorable. Different cyclopentadienyl ligands exhibit different propensities toward ring slippage, as demonstrated in the indenyl effect,92–97 and increased steric bulk on phosphines tends to promote dissociation.98–100 A balance must be struck between discouraging phosphine dissociation while maintaining a more sterically bulky environment to prevent oligomerization. Given the handles by which the sterics and the electronics of the catalyst system can be tuned through ligand modification and Lewis acid addition, we will bias favorable reactivity over these deleterious pathways. Concluding Remarks.

The proposed research would add to the fundamental understanding of the impact of the sterics and electronics of the ligand environment on the metal and how that can influence C–H bond activation selectivities. Further, the exploitation of the redox non-innocent nitrosyl ligand remains a relatively underexplored phenomenon, and thus, the systematic study of the influence on ligand environment and

Scheme 5. Proposed catalytic cycle of the hydromethylation of simple alkenes with CpRe(NO)(PR3)(CH3)/LA: NO bending promotes alkene coordination, followed by insertion into the Re–CH3 bond. Formation of a methane σ-complex is followed by σ-complex-assisted metathesis yielding product.

RePR3N

CH3

ReNH3C

LA

O

LA

PR3

ReNH3C

O

LA

PR3

ReNO

LA

PR3

ReNO

LA

PR3

HCH3

ReNO

LA

PR3

HCH3

NO Bending

Alkene Coordination

AlkeneInsertion

Formation of Methane σ−Complex

σ−Complex-Assisted-

Metathesis

ReI d6, 18 e−

ReIII d4, 16 e−

ReIII d4, 18 e−

ReIII d4, 16 e−

ReIII d4, 18 e−

O

CH3

H

CH4


Lewis acid addition in inducing nitrosyl bending could provide the tools necessary for the development of future catalyst systems capable of taking advantage of this unique ligand cooperativity. Lastly, tackling the challenge of methane activation through hydromethylation of alkenes could pave the way for new and improved methane functionalization reactions.


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