Iron-Catalyzed Amide Formation from the Dehydrogenative Coupling

May 2, 2017 - Organometallics , 2017, 36 (10), pp 2020–2025 ... N[CH2CH2(PiPr2)]2) selectively catalyzes the dehydrogenative intermolecular coupling...
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Iron-Catalyzed Amide Formation from the Dehydrogenative Coupling of Alcohols and Secondary Amines Elizabeth M. Lane,† Katherine B. Uttley,‡ Nilay Hazari,§ and Wesley Bernskoetter*,‡ †

Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States Department of Chemistry, The University of Missouri, Columbia, Missouri 65211, United States § Department of Chemistry, Yale University, New Haven, Connecticut 06511, United States ‡

S Supporting Information *

ABSTRACT: The five-coordinate iron(II) hydride complex (iPrPNP)Fe(H)(CO) (iPrPNP = N[CH2CH2(PiPr2)]2) selectively catalyzes the dehydrogenative intermolecular coupling of alcohols and secondary amines to form tertiary amides. This is the most productive base-metal catalyst for dehydrogenative amidation reported to date, in some cases achieving up to 600 turnovers. The catalyst works well for sterically undemanding amines and alcohols or cyclic substrates and is particularly effective in the synthesis of formamides from methanol. However, the catalyst performance declines rapidly with the incorporation of large substituents on the amine or alcohol substrate. Variable-temperature NMR spectroscopic studies suggest that the catalyst resting state is an off-cycle iron(II) methoxide species, (iPrPN(H)P)Fe(H)(OCH3)(CO), resulting from addition of methanol across the Fe−N bond of (iPrPNP)Fe(H)(CO). This reversibly formed iron(II) methoxide complex is favored at mild temperatures but eliminates methanol upon heating.

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INTRODUCTION Amides are key functional groups in organic synthesis, biochemistry, and pharmaceuticals.1 For example, it is estimated that approximately 25% of drug molecules contain amide moieties.2 Current methods for large-scale amide synthesis typically involve the reaction of carboxylic acids or more commonly their activated derivatives (such as acid chlorides, acid anhydrides, and acyl azides)3−5 with stoichiometric amounts of amines, along with additional inorganic or organic promoters. As a result, these procedures are not atom efficient, generate considerable chemical waste, and use relatively expensive starting materials.1 The dehydrogenative coupling of alcohols and amines to form amides (with dihydrogen as the only byproduct; Scheme 1) offers a more direct and environmentally benign alternative to obtaining these materials. Although coinage metals such as silver6 and gold7,8 have demonstrated some effectiveness as heterogeneous catalysts for dehydrogenative amide synthesis, these systems normally require high catalyst loadings, harsh reaction conditions, and long reaction times or have limited substrate scopes. Homogenous transition-metal catalysts incorporating rhenium,9 rhodium,10 and (most extensively) ruthenium have shown greater activity.11−28 The most prominent example, a ruthenium-pincer catalyst reported in 2007 by Milstein and coworkers, achieved turnover numbers (TONs) of nearly 1000 for the dehydrogenative coupling of primary amines with sterically accessible alcohols.12 This spurred the development of a host of other precious-metal catalysts for dehydrogenative amide synthesis, but all suffered from the limitations of requiring an exogenous base, the need for a stoichiometric hydrogen acceptor, low productivity (TONs < 100), or a © XXXX American Chemical Society

combination of these factors. Notably, most existing catalysts perform best with primary amine substrates, with only a few ruthenium catalysts demonstrating any compatibility with secondary amine substrates.16,17,19,21,23−25,27 To date, only two of these ruthenium catalysts have achieved TONs greater than 25 for the formation of tertiary amides under base-free and/or hydrogen-acceptor-free conditions (Figure 1).21,24 Furthermore, the development of promoters incorporating cheaper, less toxic metals is lacking, as only two base-metal catalysts (one copper and one manganese) have been described for this dehydrogenative amidation.29,30 While these recent studies demonstrate some promise of base-metal-mediated coupling of amines and alcohols, both exhibit limited tertiary amide production (TONs < 50). Herein we report the ability of the five-coordinate iron(II) complex (iPrPNP)Fe(H)(CO) (1; iPr PNP = N[CH2CH2(PiPr2)]2) to catalyze the dehydrogenation of alcohols in the presence of secondary amines to selectively produce amides in the absence of hydrogen acceptors or base promoters (Figure 1). Complex 1 is the most productive base-metal catalyst yet developed for intermolecular dehydrogenative amide synthesis and gives TONs that exceed those previously described for base or precious metals with respect to tertiary amide formation.31

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RESULTS AND DISCUSSION Catalytic Studies. Several independent research endeavors (including within our laboratories) have reported the successful Received: April 6, 2017

A

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Organometallics Scheme 1. Metal-Promoted Pathways for Dehydrogenative Amidation

Table 1. Dehydrogenative Amidation of Methanol Catalyzed by 1a

Figure 1. Efficient (a) ruthenium and (b) iron catalysts for dehydrogenative coupling of alcohols and 2° amines.

application of iron-pincer catalysts of the type (RPNP)Fe(H)(CO) (R = iPr, Cy) for the dehydrogenation of alcohols to esters,32 ketones,32 or carbon dioxide.33,34 Recently, we also described the use of 1 in the catalytic base-free hydrogenation of amides,35 which suggests it may have the potential to catalyze the dehydrogenative formation of amides. Due to the success of 4-formylmorpholine as a substrate in prior amide hydrogenation experiments,35 investigations into the microscopic reverse reaction began with attempts to couple morpholine and methanol in the presence of 1. An initial experiment was performed using a 4:1 amine:alcohol ratio and a 0.1 mol % loading of 1 in 10 mL of dioxane at 80 °C for 8 h. Despite the lack of optimization, this reaction yielded a promising TON of 297 for the formation of 4-formylmorpholine. Using this reaction as a benchmark, the solvent, reaction volume, time, temperature, and substrate ratio were all varied to generate an optimized set of reaction conditions (Figures S1− S5 in the Supporting Information). Using these optimized conditions (illustrated in Table 1), a TON of 503 was observed for the formation of 4-formylmorpholine (entry 3), which rivals the highest TONs reported for ruthenium catalysts in dehydrogenative amidation using secondary amines.11−28 The TON was confirmed by NMR analysis of the 4-formylmorpholine product and by collection of the dihydrogen produced (86% of expected H2 was collected). Encouraged by the activity of 1 with traditionally more difficult secondary amines, the substrate scope of dehydrogenative amidation was further explored (Table 1). Overall, 1 is extremely effective at amide production using cyclic secondary amines (entries 1−3), with a maximum TON of 600 observed in the formylation of 1,2,3,4tetrahydroisoquinoline. However, the catalytic activity appears

a Reaction conditions: 3 μmol of catalyst (0.1 mol %), 3 mmol of alcohol, and 12 mmol of amine in 5 mL of THF at 80 °C for 8 h. Each entry is an average of two trials, and the TON was determined by GC analysis of amide production unless otherwise indicated. bDetermined by 1H NMR spectroscopy. cOnly one trial.

strongly correlated to the size of the substituents on the amine. For example, we compared the sterically restricted cyclic amines to substrates with even small linear substituents, such as ethyl, which shows a significant drop in TON down to approximately 200 (entry 4). This effect is further demonstrated as the amine substituents increase in size (entries 5−7), with only trace conversion observed for the phenyl and isopropyl derivatives. Attempts to generate diamides starting from cyclic amides (entries 8 and 9) were also unsuccessful, although previous studies from our laboratories suggest that this may be due to a competing 1,2-addition reaction between secondary amides and 1.35 Initial studies into extending the B

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removed from the dehydrogenation site (entries 4 and 5). The absence of catalysis in the presence of 2,2,2-trifluoroethanol (entry 6) is likely due to its relative acidity and its electronwithdrawing substituent. Interestingly, methanol far outperforms the other alcohols explored. When this observation is combined with the results on the effect of sterics on the amine substrate, it indicates that there is a strong steric effect at both sides of the product carbonyl moiety. To some extent these results are unsurprising, as the large substrate groups would potentially interfere with metal-catalyzed dehydrogenation steps in which the substrates or intermediates must pass the four sizable isopropyl substituents of the pincer ligand. Although the overall substrate scope exhibited by 1 remains quite sterically limited, the high productivity observed for small substrates suggests that the inherent activity of the iron center is significant and could perhaps be leveraged by further attenuation of the ancillary ligand environment. Mechanistic Considerations. Recent investigations of transition-metal-catalyzed dehydrogenative amidation have nearly all proposed the same general pathway for amide formation (Scheme 1).12,14,26 Each begins with the dehydrogenation of the alcohol to the corresponding aldehyde followed by trapping with either amine or an additional 1 equiv of alcohol to generate a hemiaminal or hemiacetal intermediate, respectively. Dehydrogenation of an intermediate hemiaminal would directly afford an amide, while oxidation of the hemiacetal would yield an ester which could then produce amide via a transamidation process. However, the exact mechanisms for dehydrogenative amidation are likely catalyst and system dependent. For example, many ruthenium catalysts employ base additives to achieve optimum performance, which can potentially serve many different roles. These include enhancing coordination of the alcohol to the metal,15 facilitating amine to aldehyde proton transfer, obviating hemiaminal formation,26 and promoting ester transamidation.19 Prior studies within our laboratories demonstrate that 1 is quite capable of promoting ester formation, achieving a TON of 107 over just 12 min for the methanol to methyl formate conversion under conditions similar to those used in this work for amidation.33 In addition, amidation experiments using catalyst 1 and morpholine with methyl formate in place of alcohol yielded substantial quantities of 4-formylmorpholine (Figure 2).37 These observations suggest that the lower pathway illustrated in Scheme 1 is at least viable for 1-catalyzed amidation. However, further mechanistic experiments substituting benzaldehyde for alcohol during the amidation process also afforded the corresponding amide product (Figure 2), which validates the hemiaminal-dependent path for amide production.37 Since these preliminary investigations indicate that both of the Scheme 1 pathways are competent for 1, it is clear that a more exacting and extensive mechanistic undertaking will be required to elucidate the precise path of 1-catalyzed amidation.

scope of amines to include primary amine substrates have indicated a propensity for 1 to further substitute the initially formed formamides to yield ureas (among other products). Explorations into optimizing and controlling the selectivity of this process will be addressed in future work. Despite the aforementioned steric limitations on the amine substrates, the ability of 1 to catalyze the production of a range of formamides using methanol is significant, as small alcohols have to overcome a larger activation energy for dehydrogenation, which makes them incompatible with many catalysts.36 Additionally, formamides are highly desirable products due to both their role as organic intermediates and their high biological activities, making them integral to many pharmaceuticals.27 Most of the few existing methods for using methanol as a “green” C1 building block in place of hazardous formylating reagents in formamide syntheses require harsh conditions, exhibit long reaction times, and show limited scope.27 Of the many previously reported catalysts for amide formation from alcohols and amines, only three (two ruthenium-based and one manganese-based) were at all capable of formamide production, and then only with low TONs (≤50).23,27,30 The success of 1 in dehydrogenating a challenging substrate such as methanol motivated investigation into the alcohol substrate scope for amidation. Using morpholine as the amine substrate, the catalytic activity of 1 was examined with a range of primary alcohols (Table 2). Similar to the observations with Table 2. Alcohol Dehydrogenation in the Presence of Morpholine Catalyzed by 1a

entry

alcohol

TON

1 2 3 4 5 6

CH3OH CH3CH2OH CH3(CH2)5OH C6H5(CH2)2OH C6H5CH2OH CF3CH2OH

503 50 13 10 10 0b

Reaction conditions: 3 μmol of catalyst (0.1 mol %), 3 mmol of alcohol, and 12 mmol of amine in 5 mL of THF at 80 °C for 8 h. Each entry is an average of two trials, and the TON was determined by GC analysis of amide production unless otherwise indicated. bDetermined by 1H NMR spectroscopy. a

amines, the performance of 1 is highly sensitive to the steric profile of the alcohol (entries 1−3). This effect on catalytic activity remains even when the larger substituent is slightly

Figure 2. Iron-catalyzed coupling of benzaldehyde and methyl formate with morpholine. C

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between free and iron-activated methanol. At −80 °C, two distinct peaks for the bound and free 13CH3 fragment were observed in the 13C{1H} spectrum and a sharp resonance for 2 was seen at 93.2 ppm in the 31P NMR spectrum (Figure S2 in the Supporting Information). A proton-coupled 13C spectrum showed that the labeled carbon is still attached to three protons, supporting the presence of an iron-methoxy species (Figure S2). The exact position of the 31P{1H} NMR peak for 2 depends on both the temperature and the number of equivalents of methanol, due to the equilibrium described above. The formation of 2 via deprotonation of methanol is likely an off-cycle reaction that competes with catalytic amidation. As mentioned above and illustrated in Scheme 1, the dehydrogenation of an alcohol to an intermediate aldehyde has been implicated in most related amidation processes.12,14,26 Our prior experimental and computational studies of 1-catalyzed methanol dehydrogenation have found that this reaction proceeds via a concerted transfer of the hydrogenation atoms associated with the methanol O−H and C−H bonds. Dehydrogenation pathways involving intermediates analogous to complex 2 were found to be significantly higher in energy.33 Thus, while formation of 2 may have some kinetic relevance to catalytic amidation, it is likely not directly on its pathway and the high-temperature catalytic conditions presumably minimize the significance of 2 in amide formation.

During the course of the mechanistic and catalytic experiments described above, it was noted that the reaction solution reversibly changed color. When the catalytic experiments were assembled, the solution of 1 and amine immediately changed from deep red to yellow-orange upon addition of the alcohol substrate at room temperature. When the solution was heated during catalysis, the color would quickly revert back to a shade of deep red resembling its original hue. In order to ascertain any mechanistic implications this color change might entail, in situ NMR spectroscopic studies using morpholine and methanol were performed. Multiple experiments showed that the color change and corresponding changes to the 31P and 1H NMR spectra were exclusively dependent on the addition of alcohol (Figure S1 in the Supporting Information), whereas treatment of 1 with only morpholine produced no observable reaction. The reaction of 1 with methanol was examined using variable-temperature NMR spectroscopy, which indicated an equilibrium between 1 and the iron-methoxy species (iPrPN(H)P)Fe(H)(OCH3)(CO) (2) formed by the addition of methanol across the Fe−N bond. At all observed temperatures between 22 and 70 °C, only one 31P NMR resonance was seen, although its chemical shift changes with temperature (Figure 3). This is indicative of a rapid equilibration between 1 and 2

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CONCLUDING REMARKS The five-coordinate iron complex 1 exhibits the highest productivity yet reported for base-metal-catalyzed intermolecular dehydrogenative coupling of alcohols and amines to amides. Despite its notable success in producing formamides, the primary drawback of 1 is the steric limitation of its alcohol and amine substrate scope, which will likely obviate more widespread use in synthetic methodology. However, this ironmediated amidation provides an intriguing preference for secondary amines, which is a complement to the leading precious-metal catalysts, which work best with primary amines. In fact, the TONs of up to 600 for tertiary amide production far exceed the performance of any catalyst to date. The native activity and divergent selectivity of 1-catalyzed dehydrogenative amidation strongly motivate further investigation into the mechanism of this process as well as catalyst development. Perhaps by alteration of the steric environment about the catalyst, the substrate scope for amidation can be widened beyond methanol and relatively small amines. Likewise, understanding the mechanistic origins of the preference for 1 to amidate secondary amines could further expand transitionmetal-mediated synthetic methods in this area. Both of these endeavors are the current subjects of investigation in our laboratories.

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Figure 3. Variable-temperature 31P NMR spectra indicating an equilibrium between 1 and 2 in the presence of 6 equiv of methanol. The sample was dissolved in benzene-d6.

EXPERIMENTAL DETAILS

General Considerations. All manipulations were carried out under a nitrogen or argon atmosphere using standard Schlenk, vacuum, cannula, or glovebox techniques. Catalysts 1 and (iPrPNHP)Fe(H)(HCO2)(CO) (1-formate) were prepared as previously described.38 Amide products (for gas chromatography response factors) that were not commercially available were prepared using previously reported procedures.39,40 All other chemicals were purchased from Aldrich, Fisher, Strem, Synthonix, Oakwood Chemicals, VWR, or Cambridge Isotope Laboratories. Liquid amine and alcohol substrates were dried over calcium hydride or sodium hydride, purified by vacuum transfer or distillation, and stored over 3 Å

on the time scale of the NMR experiment, and the position of the resonance represents an average value for the chemical shifts of the two species present in solution. The change in the 31 P NMR chemical shift suggests that 1 is favored at high temperatures and 2 is preferred at lower temperatures, as expected for a bimolecular to unimolecular reaction. The identity of 2 was confirmed using 13CH3OH labeling and lowtemperature NMR studies, which slowed the exchange rate D

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molecular sieves. Solid substrates were purified by sublimation, followed by recrystallization (if necessary). Bulk solvents were dried and deoxygenated using literature procedures.41 NMR solvents were dried over 3 Å molecular sieves and then used without further manipulation or were dried over sodium and then vacuum-transferred prior to use. Hydrogen and carbon dioxide were purchased from Airgas and were used as received. 1H, 13C, and 31P NMR spectra were recorded on Bruker 300 MHz Avance II+, 300 MHz DRX, 500 MHz DRX, and 600 MHz spectrometers at ambient temperature, unless otherwise noted. Chemical shifts are reported in ppm; J values are given in Hz. 1H and 13C chemical shifts are referenced to residual solvent signals; 31P chemical shifts are referenced to an external standard of H3PO4. Probe temperatures were calibrated using ethylene glycol and methanol as previously described.42 Gas chromatography was performed on a Thermofisher Scientific Trace 1300 Series gas chromatograph with an FID using helium as the carrier gas. General Procedure for the Formation of Complex 2. In a drybox, 5 mg of complex 1 was dissolved in either C6D6 or d8-THF in a 20 mL scintillation vial and then transferred to a J. Young tube. The tube was then sealed, removed from the glovebox, and degassed, and 6 equiv of methanol or 13C-labeled methanol were added via calibrated gas bulb. Select NMR data for 2 (at −80 °C): 1H NMR (d8-THF, 300 MHz) −24.3 (t, 1H, J = 52, Fe−H); 13C{1H} NMR (d8-THF, 300 MHz) 59.58 (s, Fe−O-CH3); 13C NMR (d8-THF, 300 MHz) 59.57 (q, J = 132, Fe-O-CH3); 31P{1H} NMR (d8-THF, 300 MHz) 93.21 (d, J = 32). General Methods for Catalytic Alcohol Dehydrogenation in the Presence of Amines. In a drybox, a 100 mL Schlenk tube was loaded with 5 mL of tetrahydrofuran (THF), 12 mmol of amine, 3 μmol of catalyst 1, and 3 mmol of alcohol and then sealed. It was immediately placed in an oil bath preheated to 80 °C, and the contents were stirred for 8 h. It was then cooled in an ice bath for 30 min prior to analysis. For analysis using NMR spectroscopy, 100 μL of the reaction solution were placed in an NMR tube with 395 μL of CDCl3 and 5 μL of mesitylene standard and an NMR delay time of 60 s was used. For analysis by GC, 100 μL of the reaction solution were diluted to 1 mL with THF and a mesitylene standard (0.024 or 0.0024 M after final dilution) was added. General Procedure for H2 Collection Studies. In a drybox, a 100 mL Schlenk flask was loaded with 2.5 mL of THF, 6 mmol of morpholine, 1.5 μmol of catalyst 1, and 1.5 mmol of methanol. The flask was fitted with a reflux condenser and an adapter with a stopcock, removed from the glovebox, and connected to a gas buret setup that had been pre-sparged with N2. The connecting hoses/trap were subjected to two evacuation/refill cycles, the trap was cooled in a dry ice/acetone bath, and a small amount of vacuum was used to reset the starting water volume. The Schlenk flask was lowered into an oil bath (preheated to 80 °C), and the system was allowed to equilibrate for 3.5 min, after which a small amount of vacuum was used to restore the water volume in the buret (with the connection to the reflux setup closed). The stopcock was then opened, the reaction was allowed to proceed for 8 h, and the change in the water level in the gas buret was used to determine the TON. This procedure for determining TON has been previously reported.38

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Nilay Hazari: 0000-0001-8337-198X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support by Brown University, the Curators of the University of Missouri, and the National Science Foundation (CHE-1350047). N.H. and W.B. are fellows of the Alfred P. Sloan Foundation.

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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.7b00258. Additional experimental data and select NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

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