Letter pubs.acs.org/acscatalysis
Iron-Catalyzed Reductive Cyclization of o‑Nitrostyrenes Using Phenylsilane as the Terminal Reductant Michael Shevlin,†,‡ Xinyu Guan,‡ and Tom G. Driver*,‡,§ †
Department of Process Research & Development, Merck & Co., Inc., 126 East Lincoln Avenue, Rahway, New Jersey 07065, United States ‡ Department of Chemistry, University at Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States § Institute of Next Generation Matter Transformation, College of Chemical Engineering, Huaqiao University, 668 Jimei Boulevard, Xiamen, Fujian 361021, People’s Republic of China S Supporting Information *
ABSTRACT: Using microscale high-throughput experimentation, an efficient, earth-abundant iron phenanthroline complex was discovered to catalyze the reductive cyclization of orthonitrostyrenes into indoles via nitrosoarene reactive intermediates. This method requires only 1 mol % of Fe(OAc)2 and 1 mol % of 4,7-(MeO)2phen and uses phenylsilane as a convenient terminal reductant. The scope and limitations of the method were illustrated with 21 examples, and an investigation into the kinetics of the reaction revealed first-order behavior in catalyst and silane and zero-order behavior with respect to nitrostyrene. KEYWORDS: iron, catalysis, indole, nitro, nitroso, silane
R
Scheme 1. Identifying Mild Reductive Cyclization Conditions To Convert o-Nitrostyrenes into Indoles
eduction of stable and readily available nitroarenes can access reactive intermediates that can trigger the formation of carbon−nitrogen bonds.1 Transforming orthonitrostyrenes into indoles via nitroso reactive intermediates has been pursued because of the ubiquitous nature of this Nheterocycle in natural- and synthetic molecules.2 This reductive cyclization is typically triggered using stoichiometric amounts of a reductant such as phosphite,3 a Grignard reagent,4 iron,5 zinc,5a,b titanium(III),6 a diborane reagent,7 or the combination of a palladium catalyst and carbon monoxide.8 The harsh reaction conditionshigh temperatures, high pressures, strongly acidic-, basic- or toxic reagentslimit their applicability for the synthesis of functionalized, complex molecules. To overcome these limitations, we hypothesized that the oxophilicity of first-row transition metals might be exploited using a low-valent or hydridic p-block compounds as oxygenatom acceptors to result in mild oxygen-atom transfer catalysis (Scheme 1).9 The cost, abundance, and greater number of accessible oxidation states of first-row transition metals in comparison to precious metals has spurred significant research interest.10 Recently, these catalysts have demonstrated broad utility in a range of reductive transformations,11 and an ironcatalyzed hydroamination of alkenes using nitroarenes has recently been reported by Baran and co-workers.12 This latter reaction, however, requires high catalyst loading and a second reduction step to deoxygenate the N-hydroxylamine intermediate to produce the desired product. Given the wide range of potential combinations of catalyst and reducing agent, we anticipated that microscale high-throughput experimentation could be employed to rapidly identify the optimal conditions to © XXXX American Chemical Society
mildly transform ortho-nitrostyrenes into indoles via nitrosoarene reactive intermediates.13,14 ortho-Nitrostilbene 1a was used as the substrate for our search for a transition-metal catalyst and a mild deoxygenating reagent that would produce 2-phenylindole 2a (Figure 1). A wide variety of reducing agents, low-valent metal compounds, and main-group reducing agents were examined in combination with Fe-, Co-, and Ni-catalysts ligated with either 1,10phenanthroline (phen) or 1,1′-bis(diphenylphosphino)ferrocene (dppf). The analogous Pd-catalysts were included as control experiments, and a reaction temperature of 80 °C Received: June 12, 2017 Revised: July 17, 2017
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DOI: 10.1021/acscatal.7b01915 ACS Catal. 2017, 7, 5518−5522
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ACS Catalysis Table 1. Optimization of the Reductive Cyclization
a
entry
MXn
ligand
mol %
silane
2a, %a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Fe(Cl)2 Fe(OAc)2 Fe(OAc)2 ... Fe(OAc)2 Fe(OAc)2 Fe(OAc)2 Fe(OAc)2 Fe(OAc)2 Fe(OAc)2 Fe(OAc)2 Fe(OAc)2 Fe(OAc)2 Fe(OAc)2 Fe(OAc)2 Fe(OAc)2
phen phen ... ... dppe dtbpy TMEDA L1 L2 phen phen L2 phen phen phen phen
10 10 10 ... 10 10 10 10 10 2 1 0.5 1 1 1 1
PhSiH3 PhSiH3 PhSiH3 PhSiH3 PhSiH3 PhSiH3 PhSiH3 PhSiH3 PhSiH3 PhSiH3 PhSiH3 PhSiH3 Ph2SiH2 Et3SiH PHMS (EtO)3SiH
57 75 46 0 73 72 52 81 99 83 94 87b 44 1 32 76
Yield determined by quantitative HPLC analysis. bIsolated yield.
changing the identity of ligand on the reaction outcome (entries 5−9). No change in yield was observed when dppe or dtbpy were used, but a diminished yield of 2a was observed using TMEDA as the ligand. Substituted phenanthrolines were next examined (entries 8 and 9): a modest increase in the yield was obtained using the sterically encumbered 2,9-dimethyl1,10-phenanthroline, and a nearly quantitative yield of 2a was obtained if the electron-rich 4,7-dimethoxy-1,10-phenanthroline was used as the ligand. Next, we examined if the loading of the iron and phenanthroline could be reduced (entries 10−12). To our delight, we found that synthetically useful yields of 2phenylindole was formed using as little as 0.5 mol % of Fe(OAc)2 and 4,7-(MeO)2phen (entry 12). Finally, the effect of changing the identity of the reductant on the reaction outcome was examined (entries 13−16). Although triethoxysilane could be used in place of phenylsilane without attenuating the yield of 2a too much, employing diphenylsilane, triethylsilane, or the environmentally friendly polymethylhydrosiloxane resulted in significantly reduced indole formation. Using these optimal conditions, the effects of changing the substituent on the nitroarene on the reaction was surveyed (Table 2). We found that both electron-withdrawing and electron-donating substituents were well tolerated at the 3position or 4-position to provide indoles 2a−2j in high yield (entries 1−10). Next the effect of increasing the steric environment was examined. Irrespective of whether an additional ortho-substituent was placed next to the nitro group or next to the alkenyl substituent indoles 2k and 2l were smoothly formed (entries 11 and 12). We next investigated the influence of changing the identity of the ortho-alkenyl substituent on indole formation (Table 3). We found that changing the electronic nature of the β-aryl substituent or replacing it with a β-alkyl group did not attenuate the yield of the reductive cyclization of nitrostyrene 1 (entries 1−5). Substrates bearing α-phenyl or α-methyl groups were also efficiently converted to 3-substituted indoles using the optimal conditions (entries 6 and 7). In contrast, submission of
Figure 1. Initial high-throughput experimentation survey. Conditions: 10 μmol of 1a, 10 mol % of Pd(OAc)2, FeCl2, CoCl2 or NiCl2, 10 mol % of phen or dppf, 3 equiv of reducing agent, DMA or PhCF3, 80 °C, 18 h. Size of circles indicate yield of 2a as determined using quantitative HPLC analysis.
was chosen to select for mild conditions. In addition to the desired 2-phenylindole 2a, N-hydroxyindole 3a and aniline 4a were observed under many conditions.15,16 While the best reaction outcome was obtained with B2pin2 and Pd(OAc)2, promising yields of 2-phenylindole were observed using either FeCl2 or CoCl2 and dithionite-, borane-, or silane-reductant. We were delighted to observe that pairing PhSiH3 with 10 mol % of FeCl2 and phen provided an encouraging yield of 2a of 69%. With this lead hit, a more focused optimization of the reaction parameters was performed using high-throughput experimentation (Table 1).17 While the combination of FeCl2 and phenanthroline discovered in the initial screen provided a decent yield in DMA, a more convenient ethereal solvent was sought to ease the workup and purification of the reaction mixture. A solvent screen revealed that DME caused the yield of 2a to diminish (entry 1).17 Increasing the solubility of the catalyst by changing iron’s counterion to acetate restored the yield of the reductive cyclization (entry 2). In the absence of a supporting ligand, the yield was significantly attenuated, and no indole was observed if iron was excluded from the reaction mixture (entries 3 and 4). Next, we examined the effect of 5519
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ACS Catalysis Table 2. Scope of the Fe-Catalyzed Reductive Cyclization
a
entry
#
R1
R2
1 2 3 4 5 6 7 8 9 10 11 12
a b c d e f g h i j k l
H H H H H H H H H H H Me
H H H H H H H F3 C Me MeO H H
R3
R4
H H F3C H MeO2C H Cl H F H Me H MeO H H H H H H H −CHCH−CHCH− H H
yield, %a 96 90 98 96 88 86 97 82 96 80 98 78
Isolated after silica gel chromatography.
Table 3. Effect of o-Alkenyl Identity on N-Heterocycle Formation
time to imply a zero-order dependence of the rate on the concentration of the substrate (Figure 2b). These observations suggest that the reaction of catalyst with silane is turnover limiting and that the reduction of the nitro-group and subsequent cyclization of the nitrosoarene are rapid. While indole formation could occur through several potential mechanisms, our data suggest that the reductive cyclization of nitrostyrenes occurs via an iron hydride catalytic intermediate (Scheme 2). Reduction of [4,7-(MeO)2phen]Fe(OAc)2 5 produces the reactive iron hydride 6. In contrast to the (boxmi)Fe(κ2-OAc)-catalyzed reduction of ketones using (EtO)2MeSiH,11e,f no induction period was observed. We attribute the absence of this induction to the coordinately less saturated nature of 5 that enables σ-bond metathesis with the silane to be more facile in comparison to (boxmi)Fe(κ2-OAc), which requires a slow reduction of the acetate to ethoxide by silane to produce the active catalyst.11f Coordination of nitrostyrene 1 with iron hydride 6 produces complex 7.18 Hydride reduction of the nitro group produces 8 (κ1- or κ2coordinated),19,20 which fragments to produce iron hydroxide 9 and nitrosostyrene 10.21 Rate-limiting reduction of 9 with silane regenerates the iron hydride and produces siloxane and H2. Electrocyclization of nitrosostyrene 10 followed by proton elimination produces N-hydroxyindole 3,22 which is reduced by silane to form indole. Several experiments were performed to test the veracity of our proposed catalytic cycle. First, significant effervescence was observed during larger-scale reactions, and the identity of the gas byproduct was established to be H2 using 1H NMR spectroscopy. Second, an intermediate was observed when HPLC was used to monitor the reaction progress (Scheme 3). Its identity was confirmed by independent synthesis to be Nhydroxyindole 3a. While partial reduction (49%) to indole 2a was observed upon exposure of 3a to 1 mol % of Fe(OAc)2 and 4,7-(MeO)2phen, complete conversion required both the iron catalyst and phenylsilane reductant. No reduction of 3a was observed upon exposure to only phenylsilane. Third, several control experiments were performed to differentiate between radical and two-electron processes: the addition of radical traps, cyclooctene, 1,4-cyclohexadiene, or BHT, to the reaction mixture did not affect the outcome. We interpret these results to suggest that either free radical reactive intermediates are not
a
Isolated after silica gel chromatography. bQuantitative conversion to the aniline observed.
ortho-aryl substituted nitroarenes to reaction conditions were not successful (entry 9). Instead of the desired N-heterocycle, reduction to aniline was observed. To establish a preliminary understanding of the mechanism of the Fe-catalyzed reductive cyclization, a series of experiments were performed to quantitatively measure the effect of changing the concentration of the reagents on the rate of the reaction. Initial rate studies revealed the first order behavior of the catalyst and phenylsilane reductant (Figure 2a). Using a large excess of the silane, detailed kinetic profiling of the reaction progress revealed a linear consumption of nitrostyrene 1a with 5520
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ACS Catalysis
Scheme 2. Potential Mechanism for the Fe-Catalyzed Reductive Cyclization of o-Nitrostyrenes
Scheme 3. Control Experiments
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Figure 2. (a) Initial rate studies for the reaction of nitrostyrene 1a with PhSiH3. (b) Kinetic profiles for the reaction of nitrostyrene 1a in the presence of a large excess of PhSiH3.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b01915. Experimental procedures, kinetic, spectroscopic and analytical data (PDF)
formed in the reductive cyclization or do not escape the coordination sphere of the catalyst. In conclusion, high-throughput experimentation was used to discover the optimal reaction conditions that employ an earth abundant iron phenanthroline catalyst and phenylsilane to transform ortho-nitrostyrenes into indoles. Our investigations suggest this reductive cyclization occurs through an iron hydride-mediated reduction of nitrostyrene to generate a reactive nitroso intermediate, which cyclizes to produce the N-heterocycle after subsequent Fe-catalyzed reduction of the N-hydroxyindole intermediate by phenylsilane. The turnoverlimiting step is regeneration of the iron hydride with phenylsilane. Our future experiments will be aimed at providing more clarity into the mechanism, and to exploit the mechanistic insight into the development of new Fe-catalyzed reactions to produce N-heterocycles.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Michael Shevlin: 0000-0003-2566-5095 Tom G. Driver: 0000-0001-7001-342X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the National Science Foundation CHE1564959, the University of Illinois at Chicago, Office of the Vice-Chancellor of Research, Huaqiao University, Xiamen and the Fujian Hundred Talents Plan for their generous financial support. We thank Dr. Jon Jurica (Merck & Co., Inc. 5521
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ACS Catalysis
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Kenilworth, NJ, U.S.A.) for assistance with instrumentation for the kinetic studies and Mr. Furong Sun (UIUC) for highresolution mass spectrometry data.
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