Article Cite This: J. Am. Chem. Soc. 2017, 139, 14707-14713
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Sequential Visible-Light Photoactivation and Palladium Catalysis Enabling Enantioselective [4+2] Cycloadditions Miao-Miao Li,†,§ Yi Wei,†,§ Jie Liu,† Hong-Wei Chen,† Liang-Qiu Lu,*,†,‡ and Wen-Jing Xiao†,¶ †
Hubei International Scientific and Technological Cooperation Base of Pesticide and Green Synthesis, Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, 152 Luoyu Road, Wuhan, Hubei 430079, People’s Republic of China ‡ State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China ¶ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, People’s Republic of China S Supporting Information *
ABSTRACT: Catalytic asymmetric cycloadditions of reactive ketene intermediates provide new opportunities for the production of chiral heterocyclic molecules. Though known for over 100 years, ketenes still remain underexplored in the field of transition-metal (TM)-catalyzed asymmetric cycloadditions because (1) ketenes, as highly electron-deficient species, are possibly unstable to lowvalence TMs (i.e., decarbonylation or aggregation) and (2) the conventional thermal synthesis of ketenes from acyl chlorides and amines may be incompatible with TM catalysis (i.e., reactive acyl chloride and amine hydrochloride byproducts). Herein, we detail the unprecedented asymmetric [4+2] cycloaddition of vinyl benzoxazinanones with a variety of ketene intermediates via sequential visible-light photoactivation and palladium catalysis. It is well demonstrated that the traceless and transient generation of ketenes from α-diazoketones through visible-light-induced Wolff rearrangement is important for the success of present cycloaddition. Furthermore, chiral palladium catalysts with a new, chiral hybrid P, S ligand enable asymmetric cycloaddition with high reaction selectivity and enantiocontrol.
1. INTRODUCTION Ketenes are a family of versatile and reactive intermediates that are extensively applied to synthetic chemistry, materials chemistry, and chemical biology.1 More than 10 named reactions are related to ketene species, such as Arndt−Eistert homologation, Danheiser benzannulation, and Staudinger cycloaddition.2 In particular, in the early 2000s, the strategies of asymmetric nucleophilic (Nu) catalysis,3 Lewis acid (LA) catalysis,4 and bifunctional asymmetric catalysis merging Nu/LA catalysis5 were elegantly developed for enantioselective ketene cycloadditions.1d,6 In sharp contrast, asymmetric transition metal (TM) catalysis remains underexploited in this field,7 although this strategy has great potential to produce structurally diverse heterocycles due to its distinct feedstock (Figure 1, top). This underuse can be ascribed to one or two of the following reasons. Ketenes, as highly electron-deficient species, can be susceptive to low-valence TMs, tending to decarbonylate or aggregate.7b,8 Additional difficulties of TM-catalyzed ketene cycloadditions result from the use of acyl chlorides and amines in the conventional thermal synthesis of ketenes. First, most monosubstituted ketenes are highly reactive and unisolatable and, thus, are prepared and used in situ.9 The accompanying byproduct, amine·HCl, may be harmful to the TM-catalyzed organic transformations.10 © 2017 American Chemical Society
Second, some disubstituted ketenes are relatively stable and can be purified through vacuum distillation; however, this method limits the functional-group tolerance of ketenes in order to reduce ketene aggregation. Third, the deacidification of acyl chlorides to generate ketenes is a relatively slow process,9 especially for disubstituted ketenes, and thus, unreacted but still reactive acyl chlorides will react with lowvalence TM catalysts.11 Visible-light photoactivation has demonstrated a powerful ability to create unique reaction manifolds in organic synthesis that are generally unavailable or difficult to obtain using conventional thermal approaches.12 The photolytic Wolff rearrangement of α-diazoketones is a photochemical transformation enabling the efficient, traceless synthesis of ketenes under mild conditions.13 This process uses only light as a clean reagent and energy, accompanied by the release of harmless nitrogen gas. Although known for over 60 years,14 the potential of this process has not been fully exploited in catalytic asymmetric ketene cycloadditions. To the best of our knowledge, there was only one isolated reaction from Lectka’s group, who used a UV-light-induced Wolff rearrangement to study the Received: August 6, 2017 Published: September 27, 2017 14707
DOI: 10.1021/jacs.7b08310 J. Am. Chem. Soc. 2017, 139, 14707−14713
Article
Journal of the American Chemical Society
Figure 1. Enantioselective cycloadditions of ketenes with vinyl benzoxazinanones via sequential visible-light photoactivation and palladium catalysis. Ts: tosyl. Figure 2. Design blueprint: exploiting the Pd-catalyzed asymmetric [4+2] cycloaddition with the traceless, in situ photogeneration of ketene intermediates. [Pd]*: chiral palladium species; AAA: asymmetric allylic alkylation.
mechanism of the asymmetric [2+2] cycloaddition of acetyl chlorides with imino esters.9 Herein, by utilizing enantioselective cycloaddition reactions of α-diazoketones and vinyl benzoxazinanones as examples, we develop the concept of sequential visible-light photoactivation and palladium catalysis to realize the TM-catalyzed asymmetric cycloaddition of reactive ketene intermediates (Figure 1, bottom). Reaction Design. Our previous studies on visible-lightdriven photochemical synthesis15 and TM-catalyzed asymmetric cycloadditions16 led us to speculate that the synthetic potential of the classic photolytic Wolff rearrangement could be further exploited by merging these two seemingly distinct research fields. Figure 2 illustrates our design blueprint of the visible-light-driven, Pd-catalyzed asymmetric photochemical [4+2] cycloaddition of vinyl benzoxazinanone 1a and α-diazoketone 2. We hypothesized that the visible-light photoactivation of diazo compound 2 would promote Wolff rearrangement to smoothly form the reactive ketene intermediate 3.17 Meanwhile, the chiral Pd(0) catalyst reacts with vinyl benzoxazinanone 1a to generate the Pd-containing 1,4-dipolar intermediate C through oxidative addition/decarboxylation processes.18 Following this, we anticipate that the nucleophilic addition of tosyl amide anion to ketene 3 would smoothly proceed with the formation of a new zwitterionic intermediate D. Finally, an intramolecular Pd-catalyzed AAA reaction of the enolate component would occur and furnish the biologically related chiral quinolinone 4,19 and the next palladium catalysis cycle would begin. While the design plan in Figure 2 is attractive in theory, two challenges remain to substantiate this idea. First, we recognize the mechanistic requirement that α-diazoketone 2 must be chemically inert to palladium species in this photochemical cycloaddition, though α-diazo compounds are known to be reactive to many TMs,20 including palladium,21 forming metal carbenes. Second, the asymmetric construction of contiguous stereocenters, including a chiral quaternary carbon, is
another formidable task, although this structural feature is common in many natural products, pharmaceuticals, and agricultural agents.22 We believe that the rational choice and tuning of chiral ligands can help address these challenges.
2. RESULTS AND DISCUSSION Condition Optimization. We first examined the feasibility of the Pd-catalyzed asymmetric [4+2] cycloaddition of prepared ketene 3a to vinyl benzoxazinanone 1a in dichloromethane (DCM). As depicted in Table 1, commercially available chiral P-ligands L1 (Trost’s ligand)23a and L2 (You’s ligand)23b and chiral P,S ligand L3, developed by our group,16c were tested for this reaction due to their established catalytic capacity and excellent enantiocontrol.18b,16b We observed that ligand L1 failed to promote this transformation (Table 1, entry 1) and ligand L2 succeeded in affording the desired quinolinone product 4aa in high yield but with low stereoselectivity (entry 2: 91% yield, 87:13 e.r. and 63:37 d.r.). Delightedly, the reaction performed with ligand L3 provided an excellent yield along with greatly improved enantio- and diastereoselectivity (entry 3: 99% yield, 93:7 e.r. and 87:13 d.r.). Implementing the reaction at 0 °C further improved the enantiocontrol (entry 4: 96% yield, 95:5 e.r. and 91:9 d.r.). Other reaction parameters, such as solvent effects and substrate ratios (Tables S1 and S2 in the Supporting Information), were studied, and DCM and 1:2 ratios of 1a to 3a were identified as the best choices. This impressive result prompted us to further modify the electronic and steric properties of the chiral hybrid P,S ligand L3. In the field of asymmetric catalysis, the trans effect is widely used to guide the design and optimization of chiral hybrid ligands.24 Given that the sulfide component is 14708
DOI: 10.1021/jacs.7b08310 J. Am. Chem. Soc. 2017, 139, 14707−14713
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Journal of the American Chemical Society Table 1. Selected Condition Optimizationa
Mechanistic Consideration. We performed a series of experiments to elucidate the mechanism of this asymmetric photoreaction (Figure 3a−d). First, the UV−vis absorption spectrum of α-diazoketone 2a was detected (red line, Figure 3a), confirming that this species absorbs visible light from about 410 to 500 nm from blue LEDs (blue line, Figure 3a). Second, successive light/dark experiments with 0.5 h time intervals were performed, and the result illustrated in Figure 3b implies that the Pd-catalyzed thermal [4+2] cycloaddition is a faster process than the photoconversion of diazo compound 2a. If not, the ketene 3a remaining after the light period would continue to react with vinyl benzoxazinanone 1a during the dark period, and an obvious increase in quinolinone 4aa would be observed. Third, the whole reaction profile was monitored by analyzing the 1H NMR spectra of the reaction mixture. As depicted in Figure 3c, the ketene intermediate 3a was detected until the photochemical [4+2] cycloaddition was nearly complete, and interestingly, this species gradually disappeared again. This phenomenon is consistent with the conclusion of a rapid Pd-catalyzed cycloaddition drawn from the light/dark experiments and implied that ketenes may decompose in the presence of Pd catalysts. Then, several decomposition experiments were performed by exposing α-diazoketone 2a and ketene 3a to Pd(0/II) species or blue light. As shown in Figure 3d, we found that (1) Pd(II)/L7-catalyzed diazo decomposition is much slower than the photolysis process and the Pd-catalyzed cycloaddition; more importantly, chiral ligand L7 seems to be helpful to slow this unfavorable process; and (2) ketene intermediate 3a is indeed unstable to Pd(0) catalysts, which is why it appeared and then soon disappeared, as shown in Figure 3c. These mechanistic studies can explain the cause of the good chemical selectivity in this transformation, although both diazo and ketene compounds decompose in the presence of Pd(0/II) catalysts. Additionally, these results help to explain why this photochemical approach to transiently generate ketenes in situ usually produces quinolinone products in higher yields than the direct use of prepared ketenes (vide inf ra). Furthermore, we proposed possible stereocontrol modes to rationalize the asymmetric induction based on the established configuration of a chiral Pd(II) complex.16c As illustrated in Figure 3e, the intramolecular AAA reaction occurs on the sulfide side due to the trans effect.24,25 Compared with a phenyl group, a methyl group is believed to produce relatively less steric repulsion with the back π-allyl-Pd component; thus, mode I seemed to be more favorable than mode II. Importantly, the stereochemical result of product 4aa is consistent with this speculation. Following this, we conducted two cycloaddition reactions using prepared ketene 3a in the presence of Et3N·HCl (Figure 3f, left) or using acyl chloride 5a and Et3N to form ketene 3a in situ (Figure 3f, right). As a result, quinolinone 4aa was produced in very low yield. We speculate that Et3N·HCl, the byproduct of the in situ generation of ketene from acyl chloride 5a, reduces the nucleophilicity of the tosyl amide anion (C) or enolate (D) via protonation. These experiments clearly demonstrate the advantage of the traceless, in situ photogeneration of ketenes. Substrate Scope. Having identified the optimized conditions and finished the related mechanism studies, we turned our attention to demonstrating the generality of asymmetric photochemical [4+2] cycloadditions. To compare the reaction efficiency and selectivity, both the Pd-catalyzed cycloaddition reactions of vinyl benzoxazinanones with prepared ketenes and α-diazoketones were performed adopting the optimized
a
Conditions A (entries 1−9): 1a (0.2 mmol), 3a (0.4 mmol), Pd2(dba)3·CHCl3 (5 mol %), L1−L8 (11 mol %) in anhydrous DCM (2 mL). Conditions B (entries 10−12): 1a (0.1 mmol), 2a (0.2 mmol), Pd2(dba)3·CHCl3 (5 mol %), L7 (11 mol %) in anhydrous DCM (1.5 mL). Yields are for isolated products. The e.r. and d.r. values were determined by chiral HPLC analysis on a chiral stationary phase. b3 W blue LED bulb (light intensity = 2.8 mW/cm2). c 6 W blue LEDs (light intensity = 32.8 mW/cm2). dba: dibenzylideneacetone; rt: room temperature.
closer to the reaction site of the intramolecular AAA process (trans effect: P > S), a series of new chiral P,S ligands (L4−L8) with different sulfide components were synthesized and evaluated (entries 5−9). An obvious improvement in the enantioselectivity was observed for the reaction using bromosubstituted chiral ligand L7 (entry 8: 93% yield, 98:2 e.r. and 94:6 d.r.). In contrast, chiral ligand L8, which has a steric isopropyl group at the ortho position, was erosive to the reaction efficiency (entry 9: 53% yield, 93:7 e.r. and 92:8 d.r.). With the optimal palladium catalytic system in hand, we examined the feasibility of the visible-light-induced asymmetric [4+2] cycloaddition of vinyl benzoxazinanone 1a and α-diazoketone 2a. In accordance with the reaction design, irradiation by a 3 W blue LED bulb in the laboratory at room temperature provided product 4aa in moderate yield with excellent stereocontrol (entry 10: 60% yield, 96:4 e.r. and 93:7 d.r.). In addition, the photoreaction using 6 W LEDs produced 4aa with greatly improved reaction efficiency (entry 11: 99% yield, 96:4 e.r. and 93:7 d.r.). No product 4aa was observed when the light was carefully excluded, proving that light is necessary for this cycloaddition reaction (entry 12). 14709
DOI: 10.1021/jacs.7b08310 J. Am. Chem. Soc. 2017, 139, 14707−14713
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Journal of the American Chemical Society
Figure 3. Mechanistic investigations. (a) UV−vis absorption spectrum of α-diazoketone 2a in DCM and emission spectrum of 6 W blue LEDs. (b) Successive light/dark experiments of the model reaction. (c) Reaction profiles of the model reaction. (d) Decomposition experiments of α-diazoketone 2a and ketene 3a. (e) Proposed asymmetric induction modes. (f) Control experiments with ketene 3a or acyl chloride 5a.
component was evaluated for this asymmetric photoreaction. First, a variety of aryl α-diazoketones having different substitution patterns were proven to be applicable, giving the corresponding products in excellent yields with high levels of asymmetric induction (4ab−4ag, 97−99% yields, 94:6−97:3 e.r. and 90:10 −>95:5 d.r.). Notably, compared with thermal processes of the prepared ketenes, we confirm that higher reaction efficiency and the same level of enantiocontrol can be generally achieved in the
conditions in Table 1, entries 8 and 11. All the results are summarized in Table 2. With respect to vinyl benzoxazinanones in photochemical reactions, variations in the electronic properties and positions of the aromatic moiety have little effect on the reaction efficiency and stereocontrol, producing structurally diverse quinolinones in high yields with excellent enantio- and diastereoselectivities (4aa−4ha, 94−99% yields, 95:5−97:3 e.r. and 92:8 −>95:5 d.r.). Next, the dimensionality of the α-diazoketone 14710
DOI: 10.1021/jacs.7b08310 J. Am. Chem. Soc. 2017, 139, 14707−14713
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Journal of the American Chemical Society Table 2. Substrate Scope for the Asymmetric Pd-Catalyzed Photochemical [4 + 2] Cycloadditionsa
a
Unless otherwise noted, the photochemical reactions were conducted under the conditions described in Table 1, entry 11. bReactions performed with prepared ketenes under the conditions described in Table 1, entry 8. c5 equiv of α-diazoketones were used. Yields are for isolated products. The e.r. and d.r. values were determined by chiral HPLC analysis on a chiral stationary phase.
1,2-diphenyl α-diazoketone were also amenable to this transformation, affording the enantioenriched quinolinones 4ah−4al in 80−99% yields with up to 98:2 e.r. and 90:10 d.r.
Pd-catalyzed asymmetric photocycloadditions. In addition to methyl-substituted aryl α-diazoketones, analogous substrates with other alkyl groups (i.e., ethyl, n-butyl, isopropyl, and benzyl) and 14711
DOI: 10.1021/jacs.7b08310 J. Am. Chem. Soc. 2017, 139, 14707−14713
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Journal of the American Chemical Society To further show the advantage of the in situ photogeneration of ketenes, α-diazoketones bearing significant functional groups, such as allyl, alkynyl, and ether groups, were examined under the standard photoreaction conditions. Delightedly, these reactions proceeded well and delivered the highly functionalized quinolinones 4am−4ao in high yields with satisfied stereocontrol. In addition, the cyclic α-diazoketone 2p can be utilized as an effective substrate for this photocycloaddition, producing chiral quinolinone 4ap bearing a spiroquaternary carbon in 99% yield with 94:6 e.r. and >95:5 d.r. Surprisingly, the scope of the α-diazoketones can be successfully extended to monosubstituted α-diazoketones. For example, asymmetric [4+2] photocycloadditions using monosubstituted α-diazoketones with methyl, isopropyl, tert-butyl, cyclopropyl, or cyclohexenyl groups proceeded well; quinolinone products with two vicinal tertiary-carbon stereogenic centers were achieved in good yields with high enantioselectivities (4aq−4au: 75−95% yields, 92:8−95:5 e.r. and >95:5 d.r.). The single crystal of product 4ar was obtained, and thus, the absolute configurations of these products were clearly established (see details in the Supporting Information), showing the anti-conformation of the isopropyl group relative to the vinyl group.26 It is worth noting that monosubstituted ketenes are usually not stable enough to be purified. We demonstrated that by treating propionyl chloride with Et3N to form ketene in situ, the Pd-catalyzed thermal [4+2] cycloaddition of vinyl benzoxazinanone 1a failed to produce the corresponding quinolinone 4aq. Demonstrations of Synthetic Utility. We implemented a gram-scale flow reaction with vinyl benzoxazinanone 1a and α-diazoketone 2a as the substrates to demonstrate the synthetic practicability of this asymmetric photoreaction. Delightedly, the reaction directly using sunlight with a largely reduced catalyst loading proceeded very well, producing the enantioenriched quinolinone 4aa in a comparable result with that in the laboratory (Figure 4).
Figure 5. Synthetic transformations of quinolinone products. Mes: mesityl.
3. CONCLUSION In summary, we have described the first Pd-catalyzed enantioselective [4+2] cycloaddition of vinyl benzoxazinanones with a wide variety of photogenerated ketenes. Structurally diverse chiral quinolinones were produced with high reaction efficiency and stereocontrol under extremely mild conditions. This study not only highlighted the substantial practicality of the visible-light-driven photoconversion of α-diazoketones but also demonstrated the unique reactivity of the resulting ketenes in Pd-catalyzed asymmetric cycloadditions. We anticipate that the sequential strategy of merging visible-light photoactivation and TM catalysis will open new avenues for the development of asymmetric ketene cycloadditions as well as asymmetric photochemical syntheses.
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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/jacs.7b08310. Experimental procedures; spectral data (PDF) Crystallographic data (CIF) Crystallographic data (CIF)
Figure 4. Sunlight-driven gram-scale reaction performed in a flow reactor.
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Additionally, selected transformations were carried out to show the ease of further modifying the quinolinone products (Figure 5). For example, the ring-closing metathesis (RCM) reaction of 4am successfully delivered partly hydrogenated phenanthridin-6(5H)-one 6 in 88% yield (Figure 5a).27 The sequential reduction of 4aa using diisobutylaluminum hydride (DIBAL-H) and triethylsilane provided chiral tetrahydroquinoline 8 in 68% yield (Figure 5b and c).28 The reduction/ allylation operation of 4aa was carried out to afford chiral tetrahydroquinoline 9 in 71% yield (Figure 5b and d).28 The absolute configuration of product 9 was undoubtedly established through single-crystal X-ray diffraction analysis, which implies the anti-configuration of phenyl group with a vinyl or allyl group.26 Furthermore, the tosyl group of quinolinone 4aa can be easily removed by treatment with magnesium chips in dry menthol (Figure 5e: 10, 85% yield).28
AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Liang-Qiu Lu: 0000-0003-2177-4729 Wen-Jing Xiao: 0000-0002-9318-6021 Author Contributions §
M.-M. Li and Y. Wei contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Nos. 21472057, 21572074, 21772052, and 21772053), the Program of Introducing Talents of Discipline to Universities of China (111 Program, B17019), the Foundation for the Author 14712
DOI: 10.1021/jacs.7b08310 J. Am. Chem. Soc. 2017, 139, 14707−14713
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of National Excellent Doctoral Dissertation of PR China (No. 201422), and the Natural Science Foundation of Hubei Province (2015CFA033 and 2017AH047) for support of this research. This paper is dedicated to Professor David MacMillan on the occasion of his 50th birthday.
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DOI: 10.1021/jacs.7b08310 J. Am. Chem. Soc. 2017, 139, 14707−14713
