Selective N Functionalization of Methane and Ethane to Aminated

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Selective N Functionalization of Methane and Ethane to Aminated Derivatives by Main-Group-Directed C−H Activation Niles Jensen Gunsalus,†,§ Sae Hume Park,†,§ Brian G. Hashiguchi,† Anjaneyulu Koppaka,† Stacey J. Smith,‡ Daniel H. Ess,*,‡ and Roy A. Periana*,‡ †

The Scripps Energy and Materials Center, Department of Chemistry, The Scripps Research Institute, Jupiter, Florida 33453, United States ‡ Department of Chemistry and Biochemistry, Brigham Young University (BYU), Provo, Utah 84602, United States

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S Supporting Information *

ABSTRACT: To our knowledge, there is no precedent for the direct conversion of light alkanes to aminated products by CH activation. Previous molecular systems reported for alkane amination operate by radical or nitrene-type reactions which generally result in unintended overfunctionalization. Here we disclose the first examples of direct conversion of methane and ethane to aminated products by C−H activation facilitated by electrophilic, main-group complexes dissolved in N-acids. Alkane conversion products were achieved with selectivities up to >95% and yields up to 65% based on added main-group electrophile. Experimental studies and DFT calculations support a C−H activation mechanism to generate a metal−alkyl intermediate that undergoes N-functionalization. be achieved via generation of “protected”, electron-deficient products, thereby minimizing overoxidation to products such as imines, which is a general problem in previously reported light alkane amination reactions.11−13 A key challenge in designing an electrophilic system for CHA/MRF reactions for N-functionalization of alkanes is preventing inhibition of the electrophilic metal center with nitrogen-containing reagents that are typically basic and coordinating as well as susceptible to overoxidation. Fortunately, N-acids with a broad range of acidities can be generated through the introduction of one or two electronwithdrawing groups on nitrogen. When two hydrogens on NH3 are replaced with trifluoromethanesulfonyl groups (CF3SO2), the corresponding N-acid, bis(trifluoromethanesulfonyl)imide (HNTf2), is a stronger protic N-acid than trifluoroacetic acid (HTFA). This commercially available, low-melting (mp 60 °C) N-acid is believed to be one of the strongest nitrogen acids known with a pKa of ∼−10.14 We reasoned that use of this N-acid as a neat solvent could allow generation of highly electrophilic metal species that could react with alkanes by electrophilic CH activation. Given previous success with use of the stoichiometric reaction of Tl(III) for CH activation and oxy-functionalization

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mines are widely utilized, high-value-added feedstocks used in the production of industrial, household, and medical products.1 As shown in Figure 1A, significant reductions in chemical waste, CO2 generation, and energy use along with substantial cost savings could be realized by direct reaction of the parent alkane (dashed lines), rather than by the indirect processes (solid lines). One approach to direct C−H functionalization that has been found to be quite efficient and selective begins with electrophilic C−H activation (CHA) followed by metal− alkyl functionalization (MRF) to an upgraded alkane product. This CHA/MRF reaction strategy has been demonstrated for the selective conversion of alkanes to oxygenates, boranes, and silanes. 2 Approaches based on CHA have not been demonstrated for direct conversion of light alkanes to Nfunctionalized products (Figure 1B). Previous amination reactions have used reactive precursors, such as azides3−5 or amides and amines with oxidants,6 to form metal−nitrene species that function by direct C−H bond insertion.7 Amination has also been reported with nitrogen radicals8 and organo nitrenoids.9,10 A major advantage of the CHA/MRF approach for alkane amination (Figure 1C) is that the reactivity is controlled by the metal reagent rather than the reacting nitrogen species, allowing for potential utilization of a diverse variety of nitrogen reagents. Additionally, because the CHA mechanism is electrophilic in nature, high selectivity can © XXXX American Chemical Society

Received: April 16, 2019

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DOI: 10.1021/acs.organomet.9b00246 Organometallics XXXX, XXX, XXX−XXX

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of the postreaction mixture can be seen in the Supporting Information. It is plausible that this Tl(TFA)3/HNTf2 system operated by CHA of the alkane, in agreement with previous investigations of Tl(TFA)3 with alkanes in HTFA.15 However, as no Tl-alkyl intermediates expected from CHA could be observed, we moved to examine the reaction mechanism in further detail. We were interested in the origin of the five different products, particularly with the 1,2-difunctionalized products (Tf2N− CH2CH2−NTf2, TFA-CH2CH2−TFA, and Tf2N−CH2CH2− TFA), and whether they formed in parallel with EtNTf2 and EtTFA or were the result of subsequent reaction of these monofunctionalized products. A time course of the reaction shows a constant ratio of all five products, suggesting that they are all formed, in parallel, from a common intermediate. We hypothesized that this intermediate could be the putative species EtTlX2 (where X = NTf2, TFA), which would be expected from Tl(III)-mediated electrophilic CHA of ethane (Scheme 1, blue pathway).

Figure 1. (A) Comparison of current, multistep, indirect processes (solid lines) to hypothetical, direct routes (dashed lines) for the conversion of ethane to ethylenediamine. (B) Known C−H activation/functionalization reactions of methane to oxygenates, boranes, and silanes. Such functionalization mechanisms are unknown for converting methane to amines. (C) Desired metal-mediated electrophilic C−H activation to a metal−alkyl intermediate (M-R) followed by nitrogen functionalization.

Scheme 1. Proposed Tl(III)-Mediated CHA of Ethane To Generate EtTlX2 That Undergoes MR Functionalization via Two Parallel Pathways To Give Mono (EtX)- and Difunctionalized Products (XCH2CH2X), Where X Is TFA or NTf2a

of alkanes,15 we reasoned that this would be a good focal point for our initial investigations for alkane amination in HNTf2. Reactions with alkanes were carried out using molten HNTf2 containing stoichiometric amounts of TlIIIX3 salts. Our initial investigations involved attempts at preparing the previously undescribed bis(trifluoromethanesulfonyl)imide salt of Tl(III), Tl(NTf2)3. Unfortunately, all our attempts at preparing this Tl(III) salt resulted in a Tl(III) salt that was contaminated by oxy anions. Given the difficulty of preparing pure Tl(NTf2)3, we conducted alkane amination studies with 300 mM of Tl(TFA)3 in neat HNTf2, given the ease of preparation16 of Tl(TFA)3. The reaction between Tl(TFA)3 and 500 psig of methane was found to proceed at 180 °C to give two products, methyl bis(trifluoromethanesulfonyl)imide (MeNTf2) and methyl trifluoroacetate (MeTFA), in a combined yield of 54% based on added Tl(TFA)3(Table S1, entry 1). The selectivity of the reaction for MeNTf2:MeTFA was found to be 11.5:1. The 1H NMR spectrum of the postreaction mixture can be found in the Supporting Information. Control studies indicate that, in the presence of Tl(TFA)3, MeNTf2 and MeTFA do not interconvert in HNTf2 at 180 °C. In addition to methane, Tl(TFA)3 was also found to react with ethane in HNTf2. Reactions with ethane were found to proceed rapidly at both low temperatures (80 °C) and low pressures (reactivity observed as low as 1 atm of ethane pressure). A typical reaction was carried out with 500 psig of ethane at 80 °C and resulted in a total product yield of 65% based on added Tl(TFA)3 after just 1 h (Table S1, entry 2). Five products were observed in the reaction mixture, EtNTf2 and EtTFA as well as three products resulting from 1,2difunctionalization of ethane, Tf2N−CH2CH2−NTf2, TFACH2CH2−TFA, and Tf2N−CH2CH2−TFA. All five products were confirmed via spiking with independently synthesized samples. Control studies show that these products are stable in the presence or absence of Tl(TFA)3. The 1H NMR spectrum

a

As shown, treatment of Et2TlX with TlX3 generates the same EtTlX2 species by alkyl transfer that generates the identical products.

To examine this in more detail, we sought to independently synthesize and study whether this species would generate the same five products in the same ratio as would be expected if it were the common intermediate. Given the known instability of monoalkyl thallium species, we sought to generate EtTlX2 in situ by known, facile alkyl transfer reactions17−19 (Scheme 1, red pathway). Et2Tl(TFA) is stable in neat HNTf2 at 80 °C. However, upon addition of Et2Tl(TFA) to a neat HNTf2 solution containing excess Tl(TFA)3 at 80 °C (see the Supporting Information for full experimental details), immediate and quantitative conversion of the Et2Tl(TFA) was observed along with essentially quantitative formation of the identical five products in a ratio very similar to that of the reaction of Tl(TFA)3/HNTf2 with ethane (see Table S1, entries 2 and 3). This provides very strong mechanistic evidence for the intermediacy of EtTlX2 in this CHA reaction of Tl(TFA)3 in HNTf2 with ethane. To study the reaction N-functionalization mechanism without complications from oxy anions that generate Ofunctionalized products, the reaction of HgII(NTf2)2 (which is isoelectronic with TlIII(NTf2)3) with methane was examined in HNTf2. Hg(NTf2)2 is a known compound that can be synthesized,20 free of all oxy anions. Reactions of HgII(NTf2)2 in Tf2NH were carried out at 180 °C with 1000 psig of CH4 B

DOI: 10.1021/acs.organomet.9b00246 Organometallics XXXX, XXX, XXX−XXX

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M06/def2-TZVP DFT estimate for the reaction free energy of C−H activation shows that the formation of (Tf2N)HgII−CH3 is thermodynamically favorable with ΔG = −8.1 kcal/mol (Figure 3) and predicts that the dominant form is the ionized

(Figure 2A and Table S1, entry 4). Two methyl products were selectively generated in 15% total yield based on HgII(NTf2)2.

Figure 2. 1H NMR stack plot of the postreaction mixtures for (A) methane with Hg(NTf2)2 in Tf2NH (the inset shows the molecular structure of (Tf2N)HgII(CH3)), (B) reaction of 13CH4 with HgII(NTf2)2 in Tf2NH, and (C) reaction of methane with Hg(N(SO2F)2)2 in (SO2F)2NH (the inset shows the molecular structure of MeHgN(SO2F)2).

The minor product, ∼5% yield, was found to be MeNTf2. Importantly, consistent with a C−H activation mechanism to generate M-R intermediates, the other major product in the reaction mixture, generated in ∼10% yield, was identified as (Tf 2N)Hg II−CH3 by comparison to an independently synthesized sample. The molecular structure of this intermediate, as determined by X-ray diffraction, can be seen in the inset of Figure 2A. Reactions with 99% 13CH4 (Figure 2B and Table S1, entry 5) confirmed that both MeNTf2 and (Tf2N)HgII(CH3) were derived from 13CH4. Control studies indicated that MeNTf2 is “protected” from further reaction with HgII(NTf2)2 in Tf2NH at 180 °C. To examine the MRF, N-functionalization reaction step from the M-R intermediate in more detail, we examined functionalization of independently synthesized (Tf2N)HgII− CH3 in Tf2NH at 180 °C in the absence of methane. After 1 h, 1 H NMR indicated that ∼29% of the (Tf2N)HgII−CH3 underwent reductive N-functionalization to give MeNTf2 with ∼61% remaining unreacted (Table S1, entry 6). Analysis of the gas phase by GC/MS indicated that trace amounts of methane were generated, suggesting that protonolysis of the HgII−methyl intermediate (the microscopic reverse of C−H activation of methane) is slower than N-functionalization. This suggests that under the same reaction conditions with methane that formation of the Hg−methyl intermediate is likely largely irreversible. To test this, we examined the reaction of CH4 in neat Tf2ND with HgII(NTf2)2. This reaction showed no detectable (±5% D-incorporation detection limit) deuterium incorporation into the gas-phase CH4. The reaction was also found to proceed in HN(SO2F)2 with the corresponding, previously characterized Hg(NSO2F)2 salt.20 Using this less sterically hindered nitrogen acid with a comparable acidity to HNTf2, the reaction yield with methane was found to increase to 48% with the observation of MeN(SO2F)2 (45%) and MeHg(N(SO2F)2) (3%) (Figure 2C and Table S1, entry 7). Consistent with the observation of (Tf2N)HgII−CH3 as a stable intermediate in the reaction of Hg(NTf2)2 with CH4, the

Figure 3. (A) M06 DFT 3D representations of Hg(NTf2)2, TS1, and TS2. (B) Gibbs energy landscape outline of Hg(NTf2)2-mediated functionalization of methane to MeNTf2 through (Tf2N)HgCH3 and examples of higher energy reaction pathways in comparison to TS1.

[Hg−CH3]+[NTf2]− structure. The DFT calculations identified an accessible C−H activation barrier via TS1 of 31.8 kcal/ mol that is consistent with the 180 °C temperature and 500− 1000 psig pressure range of methane. This relatively low barrier for methane results from the high electrophilicity of the HgII center in Hg(NTf2)2, which allows the very weakly basic [NTf2]− to induce deprotonation of the coordinated CH bond. The CHA transition state is several kcal/mol lower in energy in comparison to alternative transition states such as direct Nfunctionalization, proton-coupled electron transfer (PCET), and Hg−N bond homolysis (Figure 3B). The higher energy Hg−N bond homolysis and PCET energy are consistent with the observation that addition of O2 had no effect on the rate or selectivity of the reactions of CH4 with Hg(NTf2)2 in neat HNTf2. We also identified the M-R nitrogen functionalization transition state TS2, which has a reaction step barrier of 29.5 kcal/mol relative to [Hg−CH3]+[NTf2]−. Reaction via TS2 leads to MeNTf2 and two-electron reduction of HgII to Hg0 that is slightly endergonic by 1.0 kcal/mol. However, the overall reaction is exothermic from reaction of Hg0 either with excess HgII to generate (HgI(NTf2))2 (−32.3 kcal/mol exergonic) or with another Hg0 atom (heat of condensation −13.1 kcal/mol exergonic). The exergonic C−H activation to generate [Hg−CH3]+[NTf2]− and lower barrier for functionalization of this species are consistent with both the lack of deuterium incorporation into methane for reactions run in Tf2ND and the observation of [Hg−CH3]+[NTf2]− as a metastable intermediate. C

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(2) Gunsalus, N. J.; Koppaka, A.; Park, S. H.; Bischof, S. M.; Hashiguchi, B. G.; Periana, R. A. Homogeneous Functionalization of Methane. Chem. Rev. 2017, 117 (13), 8521−8573. (3) King, E. R.; Hennessy, E. T.; Betley, T. A. Catalytic C-H Bond Amination from High-Spin Iron Imido Complexes. J. Am. Chem. Soc. 2011, 133 (13), 4917−4923. (4) Nguyen, Q.; Sun, K.; Driver, T. G. Rh2(II)-Catalyzed Intramolecular Aliphatic C-H Bond Amination Reactions Using Aryl Azides as the N-Atom Source. J. Am. Chem. Soc. 2012, 134 (17), 7262−7265. (5) Sloan, M. F.; Renfrow, W. B.; Breslow, D. S. Thermal Reactions of Sulfonyl Azides with Aliphatic Hydrocarbons. Tetrahedron Lett. 1964, 5 (39−4), 2905−2909. (6) Espino, C. G.; Du Bois, J. A Rh-catalyzed C-H insertion reaction for the oxidative conversion of carbamates to oxazolidinones. Angew. Chem., Int. Ed. 2001, 40 (3), 598−600. (7) Park, Y.; Kim, Y.; Chang, S. Transition Metal-Catalyzed C-H Amination: Scope, Mechanism and Applications. Chem. Rev. 2017, 117 (13), 9247−9301. (8) Taniguchi, Y.; Kitamura, T.; Fujiwara, Y.; Horie, S.; Takaki, K. Copper(II) catalyzed reaction of gaseous alkanes with amine Noxides. Catal. Today 1997, 36 (1), 85−89. (9) Ochiai, M.; Miyamoto, K.; Kaneaki, T.; Hayashi, S.; Nakanishi, W. Highly Regioselective Amination of Unactivated Alkanes by Hypervalent Sulfonylimino-λ3-Bromane. Science 2011, 332 (6028), 448−451. (10) Breslow, D. S.; Edwards, E. I.; Linsay, E. C.; Omura, H. Insertion of Sulfonylnitrenes into Carbon-Hydrogen Bonds of Saturated-Hydrocarbons - Acid-Catalyzed Thermolysis of N-Alkyl Sulfonamides. J. Am. Chem. Soc. 1976, 98 (14), 4268−4275. (11) Michos, D.; Sassano, C. A.; Krajnik, P.; Crabtree, R. H. Amination of Methane and Ethane by Mercury Photosensitization in the Presence of Ammonia. Angew. Chem., Int. Ed. Engl. 1993, 32 (10), 1491−1492. (12) Krajnik, P.; Michos, D.; Crabtree, R. H. Amination of Methane Higher Alkanes and Alkenes by Mercury Photosensitization in the Presence of Ammonia. New J. Chem. 1993, 17 (12), 805−813. (13) Filthaus, M.; Schwertmann, L.; Neuhaus, P.; Seidel, R. W.; Oppel, I. M.; Bettinger, H. F. C-H Bond Amination by Photochemically Generated Transient Borylnitrenes at Room Temperature: A Combined Experimental and Theoretical Investigation of the Insertion Mechanism and Influence of Substituents. Organometallics 2012, 31 (10), 3894−3903. (14) Foropoulos, J.; Desmarteau, D. D. Synthesis, Properties, and Reactions of Bis((Trifluoromethyl)Sulfonyl) Imide (CF3SO2)2NH. Inorg. Chem. 1984, 23 (23), 3720−3723. (15) Hashiguchi, B. G.; Konnick, M. M.; Bischof, S. M.; Gustafson, S. J.; Devarajan, D.; Gunsalus, N.; Ess, D. H.; Periana, R. A. MainGroup Compounds Selectively Oxidize Mixtures of Methane, Ethane and Propane to Alcohol Esters. Science 2014, 343 (6176), 1232− 1237. (16) Mckillop, A.; Hunt, J. D.; Zelesko, M. J.; Fowler, J. S.; Taylor, E. C.; Mcgilliv, G.; Kienzle, F. Thallium in Organic Synthesis. XXII. Electrophilic Aromatic Thallation Using Thallium(III) Trifluoroacetate - Simple Synthesis of Aromatic Iodides. J. Am. Chem. Soc. 1971, 93 (19), 4841−4844. (17) Kochi, J. K. Electron-Transfer Mechanisms for Organometallic Intermediates in Catalytic Reactions. Acc. Chem. Res. 1974, 7 (10), 351−360. (18) Kurosawa, H.; Okawara, R. Preparation and Properties of Monoalkylthallium Derivatives. J. Organomet. Chem. 1967, 10 (2), 211−217. (19) Ingold, C. K. Organo-Metal Substitutions. Helv. Chim. Acta 1964, 47 (5), 1191−1203. (20) Janke, C. J.; Tortorelli, L. J.; Burn, J. L. E.; Tucker, C. A.; Woods, C. Synthesis, Characterization and Reactivity of PhosphineBridged and Arsine-Bridged Dirhodium Complexes Containing Bridging Pyrazolate and Triazolate Ligands. Inorg. Chem. 1986, 25 (25), 4597−4602.

Calculation of the analogous C−H activation transition state for reaction of the C−H bonds of MeNTf2 with Hg(NTf2)2 showed a free energy barrier of 41 kcal/mol. This is consistent with the high observed reaction selectivity for formation of MeNTf2 from CH4 and C−H activation by an electrophilic substitution process with the electron-withdrawing NTf2 group providing “protection” of methyl C−H bonds of MeNTf2 against overfunctionalization. In conclusion, we have shown that highly electrophilic, main-group oxidants based on Tl(III) and Hg(II) can be used to efficiently and selectively convert light alkanes to nitrogenfunctionalized products. Importantly, experimental and theoretical work suggests that these alkane functionalization processes proceed via a C−H activation, M-R N-functionalization mechanism that is previously unprecedented for light alkanes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00246. Experimental details including protocols and data for pressure reactions, compound synthesis and characterization data, mechanistic investigations, computational data, and methodology and crystallographic information concerning the MeHg intermediates (PDF) Accession Codes

CCDC 1810843−1810844 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for D.H.E.: [email protected]. *E-mail for R.A.P.: [email protected]. ORCID

Niles Jensen Gunsalus: 0000-0003-3219-2011 Anjaneyulu Koppaka: 0000-0002-8884-2215 Daniel H. Ess: 0000-0001-5689-9762 Roy A. Periana: 0000-0001-7838-257X Author Contributions §

N.J.G. and S.H.P. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Hyconix Inc. for support of this work at ScrippsFlorida and the opportunity to publish this work. D.H.E. acknowledges support of this work by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Catalysis Science Program, under Award # DE-SC0018329.

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REFERENCES

(1) Maxwell, G. R., Synthetic Nitrogen Products: A Practical Guide to the Products and Processes. In Synthetic Nitrogen Products: A Practical Guide to the Products and Processes [Online]; Kluwer Academic/Plenum Publishers: New York, 2005. D

DOI: 10.1021/acs.organomet.9b00246 Organometallics XXXX, XXX, XXX−XXX