Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 10293−10298
pubs.acs.org/journal/ascecg
Clean Oxidation of (Hetero)benzylic Csp3−H Bonds with Molecular Oxygen Kai-Jian Liu,† Zhen-Hong Duan,† Xiu-Ling Zeng,† Meng Sun,‡ Zilong Tang,‡ Si Jiang,§ Zhong Cao,§ and Wei-Min He*,† †
Department of Chemistry, Hunan University of Science and Engineering, Yangzi Tang Road No. 130, Yongzhou 425100, China School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Taoyuan Road No. 9, Xiangtan 411201, China § Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, Changsha University of Science and Technology, Wanjiali Road No. 960, Changsha 410114, China
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‡
S Supporting Information *
ABSTRACT: By employing atmospheric dioxygen as the sole oxidant, an eco-friendly protocol for the clean preparation of various aromatic (cyclic) ketones through phosphinatepromoted oxidation of benzylic Csp3−H bonds under base-, metal-, additive-, organic-solvent-free conditions has been developed.
KEYWORDS: Base free, Metal free, Dioxygen, Dioxygenation, Aromatic ketone
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Scheme 1. Oxidation of Benzylic Csp3−H Bonds
INTRODUCTION (Hetero)aromatic ketones are widely present in natural products, biologically active compounds, and artificial drugs.1−4 They also act as useful building blocks and versatile synthetic precursors in organic synthesis and material sciences.5−8 As a consequence, much effort and energy have been dedicated to developing various methodologies for the preparation of such motifs. Among the known protocols, the benzylic Csp3−H bond oxidation reaction is one of the most efficient methods, given the inexpensiveness, abundance, and accessibility of alkyl aromatics as industrial materials.9,10 Traditionally, (super)stoichiometric amounts of hazardous oxidants (e.g., high-valent metal salts, halides, and peroxides) were employed for these chemical transformations, generally generating large amounts of chemical waste and unwanted byproducts (Scheme 1a).11−17 Therefore, developing an ecofriendly and general protocol for the oxidation of alkyl (hetero)aromatics using safe and environment friendly oxidants is highly appealing. Compared to conventional inorganic or organic oxidants, molecular oxygen is a perfect oxidant because of its environmental benignancy, low cost, and natural abundance.18−39 Over the past decades, significant achievements have been obtained in the oxidation of alkyl aromatics with dioxygen catalyzed by transition metal−ligand complex, uncommercialized metal-supported catalyst, or activated carbon catalyst (Scheme 1b).40−51 Nevertheless, those protocols suffer from limitations of requirement of catalyst, tedious procedures, and meagre substrate generality, and few © 2019 American Chemical Society
methods are available for the direct oxidation of heterobenzylic Csp3−H bonds to construct heteroaryl ketones and dioxygenation products. Due to the low threshold of transition-metal Received: January 1, 2019 Revised: May 24, 2019 Published: May 28, 2019 10293
DOI: 10.1021/acssuschemeng.9b00002 ACS Sustainable Chem. Eng. 2019, 7, 10293−10298
Research Article
ACS Sustainable Chemistry & Engineering
providing the best result (entry 5). Increasing the promoter loading had little influence on the oxidation (entry 11), whereas a lower yield was observed when the promoter loading was reduced (entry 12). Replacing dioxygen with air diminished the yield of 2a (entry 13). Elevating the reaction temperature did not improve the oxidation outcome (entry 14), whereas a decrease in the yield of 2a was detected when the temperature was dropped to 120 °C (entry 15). As a blank experiment, no oxidation reaction occurred when the reaction was conducted in the absence of phosphinate promoter (entry 16). With the optimal reaction conditions in hand (Table 1, entry 5), the substrate scope and limitations of the aerobic oxidation were investigated, where various alkyl aromatics were subjected to the standard conditions, and these results are summarized in Table 2. First, ethylbenzenes modified with either electron-rich
residual tolerance for pharmaceuticals, it is highly desirable to synthesize them under transition-metal-free conditions. Recently, Xu, Tan, and Wang reported the KOtBu-promoted oxidation of benzylic methylenes to diarylketones with dioxygen under metal-free conditions. Unfortunately, the reaction substrate scope was relatively limited,t and it only worked with diarylmethanes (Scheme 1c).52 Therefore, developing an eco-friendly and general protocol for the oxidation of alkyl (hetero)aromatics and diarylmethanes using O2 as the sole oxidant would be of great synthetic value in organic chemistry and the pharmaceutical industry. In 2012, Xiang’s group reported the Mn(II)/Co(II)/ PhP(O)HOnBu-cocatalyzed oxidative dimerization of styrenes with dioxygen as the oxidant.53 To the best of our knowledge, there have been no reports of alkyl phenylphosphinatepromoted oxidation of alkyl arenes using O2 as the sole oxidant. In line with our ongoing interest in green chemistry,54−60 herein, we report an eco-friendly protocol for the clean preparation of diverse aromatic ketones through the aerobic oxidation of a series of (hetero)benzylic methylenes with atmospheric dioxygen as the oxidant under metal-, base-, additive-, and organic-solvent-free conditions (Scheme 1d). This developed catalytic system is general and efficient with remarkable functional-group compatibility even toward 1,4diacetylbenzene and anthracenedione.
Table 2. Reaction Scopea
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RESULTS AND DISCUSSION Initially, oxidation of ethylbenzene 1a with atmospheric dioxygen was performed in the presence of 3 equiv of methyl phenylphosphinate [PhP(O)H(OMe)] at 130 °C. To our delight, the expected acetophenone 2a was formed in 32% GCMS yield (Table 1, entry 1). As anticipated, the oxidation efficiency was found to be strongly dependent on the nature of the promoter (entries 2−10). Among all of the promoters analyzed, we found that phenylphosphinate motifs promoted the desired transformation, with n-octyl phenylphosphinate Table 1. Optimization of Reaction Conditionsa
entry
promoter
temp.
yieldb(%)
1 2 3 4 5 6 7 8 9 10 11 12 13c 14 15 16
PhP(O)H(OMe) (3 equiv) PhP(O)H(OEt) (3 equiv) PhP(O)H(On-Pr) (3 equiv) PhP(O)H(On-penty) (3 equiv) PhP(O)H(On-octyl) (3 equiv) PhP(O)H(On-decyl) (3 equiv) PhP(O)(OEt)2 (3 equiv) P(OEt)3 (3 equiv) P(O)Et3 (3 equiv) PEt3 (3 equiv) PhP(O)H(On-octyl) (4 equiv) PhP(O)H(On-octyl) (2 equiv) PhP(O)H(On-octyl) (3 equiv) PhP(O)H(On-octyl) (3 equiv) PhP(O)H(On-octyl) (3 equiv)
130 °C 130 °C 130 °C 130 °C 130 °C 130 °C 130 °C 130 °C 130 °C 130 °C 130 °C 130 °C 130 °C 140 °C 120 °C 130 °C
32 41 47 86 90 88 N.R. N.R. N.R. N.R. 90 81 41 89 80 N.D.
All reactions were carried out in a 5 mL round-bottom flask in the presence of 1 (0.6 mmol), PhP(O)H(On-octyl) (1.8 mmol), O2 balloon, 130 °C. The reaction time is usually 15 h; isolated yields are reported. a
Conditions: 1a (0.3 mmol), promoter, 130 °C, O2 balloon, 15 h. b Yield was determined by GC-MS. cAtmospheric air was used instead of O2. a
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DOI: 10.1021/acssuschemeng.9b00002 ACS Sustainable Chem. Eng. 2019, 7, 10293−10298
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ACS Sustainable Chemistry & Engineering or electron-poor substituents at the para position of the phenyl ring underwent the oxidation smoothly to afford the expected oxygenation products in good to excellent yields (2a−2p). Higher steric hindrance of the substituent of ethylbenzene substrate indicated reduced reactivity; hence, a relatively lower yield of oxidation product was obtained (2b vs 2q−2r). We were pleased to find that the oxidation conditions could tolerate sensitive functional groups, such as base-labile silyl ester group (2f) and oxidizable sensitive aldehyde (2m) groups. In previous reported oxidation reactions it was hard to construct dioxygenated products because of the electronwithdrawing effect of the carbonyl groups on the monoketones which are less reactive than their dialkyl arene precursors. Pleasingly, the expected double- oxidation product (2s) was obtained smoothly in 90% yield under the standard reaction conditions. Both 1-ethylnaphthalene and 2-ethylnaphthalene smoothly underwent the oxidation process to form the products 2t and 2u in 92% and 84% yield, respectively. It should be noted that O-, S-, and N-containing heteroaromatic motifs were first applied under the base- and metal-free conditions to yield the desired products (2v−2x) with good yields. Delightfully, a series of β-substituted alkyl benzenes could be oxidized to generate the desired ketones in good yield. The alkyl chain lengths and isomeric structures did not significantly affect the reaction outcome (2y−2aa). Furthermore, (haloalkyl)benzenes could be converted exclusively into the desired haloalkylketones (2ab−2ad). Notably, substituted diarylmethanes were also found to be suitable reaction partners, and the corresponding oxidation products were isolated in good yields (2ae−2aj). We were pleased to find that various (hetero)aromatic hydrocarbons bearing a methylene moiety in the (hetero)cyclic framework underwent oxidation to furnish the aromatic cyclic ketones in good yields (2ak− 2ap), thus further enhancing the reaction scope of the developed transformation. Moreover, the anthracene-9,10dione (2aq) could be selectively prepared from the readily available 9,10-dihydroanthracene raw material. The limitation of the present reaction was also realized. Prolonging the reaction time to 25 h, no double-oxidation product was detected in the oxidation of 2,3-dihydro-1H-indene and 1,4dihydronaphthalene. Treatment of methylbenzene under the optimized reaction conditions could not deliver any oxidation product. When 4-ethylbenzonitrile, 1-ethyl-4-nitrobenzene, 3ethyl-1H-indole, or 2-ethyl-1H-pyrrole was employed as the substrate, no oxidation reaction occurred. Ethylbenzenes bearing easily oxidized groups, such as methyl sulfide (1ar) and alkene (1as), are not suitable as substrates in the current oxidation reaction. Both the easily oxidized and the ethyl groups were oxidized and gave the double-oxidation products (2ar and 2s′) in moderate to good yields. To prove the practicality of the developed aerobic oxidation reaction, a scale-up reaction of p-diethylbenzene (1s,8 mmol) was carried out under the standard reaction conditions (Scheme 2a). Pleasingly, the metal-free oxidation provided the desired p-diacetylbenzene 2s in 89% yield, showing a promising application potential for pharmaceutical industry scale-up. The late-stage structural modification of drugs is an important strategy for pharmaceutical research. Thus, three drug derivatives were submitted to the standard conditions (Scheme 2b). All of them could furnish the corresponding ketones (2at−2av). Considering that this type of aerobic oxidation of ethylbenzene may involve the α-phenylethanol (3a) intermediate,
Scheme 2. Gram-Scale Synthesis and Late-Stage Structural Modification
we attempted to oxidize 3a under standard conditions, and an 88% yield of 2a was detected (Scheme 3a). This result proved that α-phenylethanol is the likely reaction intermediate for the developed oxidation. The addition of radical scavenger (0.1 or 2 equiv of TEMPO or BHT) to the aerobic oxidation led to no conversion of ethylbenzene, indicating that a radical pathway might be involved in the present oxidation reaction (Scheme 3b). When anhydrous TBHP or H2O2 was applied as the sole Scheme 3. Control Experiments
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ACS Sustainable Chemistry & Engineering Scheme 4. Plausible Reaction Mechanism
conditions and easy construction of dioxygenation products; (e) the present oxidation reaction could be easily scaled up; (f) the general and metal-free oxidation reaction allows for production of pharmaceutical and bioactive polycyclic aromatic ketones, xanthenone, and dihydroquinolinone from easily available starting materials. The cleanness and simplicity of the present reaction will make it attractive in the pharmaceutical industry.
oxidant under nitrogen atmosphere, only a trace of acetophenone was observed (Scheme 3c). These results suggested that molecular oxygen played a critical role in the oxidation reaction. By using 18O2 as the sole oxidant, oxidation of 1a afforded the 18O-labeled 2a, which confirmed that the oxygen atom of carbonyl originated from atmospheric dioxygen (Scheme 3d). When the reaction was performed in the presence of 2 equiv of H218O, no 18O-2a was detected (Scheme 3e). An intermolecular kinetic isotope effect (KIE) of 1.2 was obtained in a 1:1 mixture of ethylbenzene 1a and ethylbenzene-D10 (1a-D10) (Scheme 3f), suggesting that cleavage of the methylene sp3 C−H bond might not be involved in the rate-limiting step in the present oxidation reaction. Furthermore, decomposition of n-octyl phenylphosphinate affording phenylphosphinic acid and di-n-octyl phenylphosphonate was observed by using 31P NMR experiments (Figures S1 and S2, Supporting Information). On the basis of the above-mentioned observations and previous reports,42,52 a possible mechanistic pathway for the developed oxidation reaction was proposed in Scheme 4. First, the phosphorus-center radical B was generated in situ from the oxidation of alkyl phenylphosphinate A with molecular oxygen. Second, the radical B abstracted a hydrogen atom from the benzylic C−H bond of ethylbenzene 1 to form the radical intermediate C, which was trapped by molecular oxygen to deliver a peroxide radical intermediate D. Then the intermediate D abstracted a proton from alkyl phenylphosphinate A to yield a molecule of E, which underwent βH cleavage and elimination of a molecule of water to generate the desired product 2. Compound E may also be converted to the benzyl alcohol F (detected by GC-MS), which was followed by dioxygen oxidation, affording the target ketone 2.
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
General Procedure for the Synthesis of Ketones 2. A mixture of ethylbenzene 1 (0.6 mmol) and n-octyl phenylphosphinate (457 mg, 1.8 mmol) was added to a 5 mL round-bottom flask with an O2 balloon at room temperature; then the contents were stirred at 130 °C. The reaction typically took 15 h. The progress of the reaction was monitored by TLC or GC-MS. Upon completion, the reaction was cooled down to room temperature. The resultant residue was purified by silica gel column chromatography to afford the desired product 2. S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00002. 1 H and 13C NMR spectra of compounds 2 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Wei-Min He: 0000-0002-9481-6697
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Notes
The authors declare no competing financial interest.
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CONCLUSIONS In conclusion, a facile and eco-friendly protocol for the preparation of various (hetero)aromatic ketones has been developed. In the presence of 3 equiv of PhP(O)H(On-octyl), 35 examples of (hetero)arylalkyl ketones, 6 examples of diaryl ketones, and 7 examples of aromatic cyclic ketones were efficiently prepared with dioxygen as the sole oxidant at 130 °C under metal-, base-, additive-, and organic-solvent-free conditions. The principal advantages of this process are outlined as follows: (a) the abundance and easy accessibility of raw materials; (b) water as the sole side product; (c) phosphinate has a dual role as promoter and reaction medium; (d) the oxidation reaction proceeded with excellent functionalgroup tolerance, as proved by the fact that base- and oxidantsensitive groups were intact under the optimal reaction
ACKNOWLEDGMENTS We are grateful for financial support from the Hunan Provincial Natural Science Foundation of China (Nos. 2019JJ40090 and 2019JJ20008).
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DOI: 10.1021/acssuschemeng.9b00002 ACS Sustainable Chem. Eng. 2019, 7, 10293−10298
Research Article
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DOI: 10.1021/acssuschemeng.9b00002 ACS Sustainable Chem. Eng. 2019, 7, 10293−10298
