Cycloisomerization of Conjugated Allenones into Furans under Mild

Letter Cite This: Org. Lett. 2019, 21, 5552−5555

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Cycloisomerization of Conjugated Allenones into Furans under Mild Conditions Catalyzed by Ligandless Au Nanoparticles Leandros Zorba, Marios Kidonakis, Iakovos Saridakis, and Manolis Stratakis* Department of Chemistry, University of Crete, Voutes, 71003 Heraklion, Greece

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S Supporting Information *

ABSTRACT: Au nanoparticles supported on TiO2 (1 mol %) catalyze the quantitative cycloisomerization of conjugated allenones into furans under very mild conditions. The reaction rate is accelerated by adding acetic acid (1 equiv), but the acid does not participate in the protodeauration step as in the corresponding Au(III)-catalyzed transformation. The process is purely heterogeneous, allowing thus the recycling and reuse of the catalyst effectively in several runs.

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for the cyclization to occur. The role of acid is to protodemetalate the cyclized intermediate, avoiding its sidedimerization reactions.14 Waser’s group captured the aurated cyclized intermediate with an alkynylbenziodoxolone reagent, forming 3-alkynyl furans.15 Finally, the Au(I)-catalyzed cascade cycloisomerization/ring-opening reaction of cyclopropyl allenyl ketones by alcohols or ketones was more recently reported, providing access to polysubstituted furans.16 Au(0) nanoparticles have been used in the past as rather unexpected catalysts in cycloisomerization reactions where the activation of a triple bond initiates nucleophilic attack by a carbon nucleophile (Scheme 1),17 but also in intermolecular addition reactions on alkynes by other types of nucleophiles.18 The origins of these reactivities possibly arise from the interaction of the substrates with the corners and edges of Au nanoparticles, in which the low-coordinated Au atoms have an

he cycloisomerization of conjugated allenones is a straightforward method for the synthesis of furans,1 which are valuable intermediates2 in organic synthesis. This transformation can occur by flash vacuum thermolysis, but the conditions are extremely harsh (>520 °C).3 Marshall’s group presented in 1990 the first example of a metal-catalyzed allenone to furan isomerization under milder conditions (100 °C) using as catalysts either (Ph3P)3RhCl (10 mol %) or AgBF4 (20%).4,5 Hashmi observed that in the presence of Pd(II) catalysts, terminal allenyl ketones form primarily dimeric products and that the anticipated furans are formed as minor ones.6,7 It was proposed that after an intramolecular oxypalladation on the terminal carbon of the double bond, a step mechanistically similar to the metal-catalyzed Marshall’s cycloisomerization,4,8 the intermediate couples with an additional molecule of allenyl ketone faster than undergoing reductive elimination. Later, it was reported that the cycloisomerization is also feasible under catalysis by Cu(I), In(III), or Sn(II),9 observing in the case of thio, acyloxy, seleno, phosphatyloxy, halo, and sulfonyloxy-substituted allenones a synthetically useful 1,2-migration/cycloisomerization pathway. On the other hand, in the presence of a cationic Fe-complex10a or under noncatalyzed forcing conditions,10b aryl allenyl ketones isomerize into arylidene indanones, instead of to furans. Ιonic gold compounds and complexes were also proven as highly reactive catalysts under homogeneous conditions. In the presence of AuCl3 (1 mol %), allenyl ketones react rapidly yielding the corresponding furans,11 but additional dimeric side products are also formed from the coupling between the reactant and the ortho-position of the produced furans. Gevorgyan’s group reported that in addition to Au(III), complexes of Au(I) are also efficient cycloisomerization catalysts.9,12 In fact, regiodivergent products may be obtained in the case of halo-substituted allenones switching from Au(III) to Au(I) or changing the solvent. Che and co-workers found that a Au(III)-porphyrin (1 mol %) is also an excellent cycloisomerization catalyst in acetone as solvent at 60 °C.13 The addition of a protic acid (CF3COOH) is highly essential © 2019 American Chemical Society

Scheme 1. Selected Cycloisomerization Reactions Catalyzed by Au Nanoparticles Supported on Metal Oxides

Received: May 30, 2019 Published: July 8, 2019 5552

DOI: 10.1021/acs.orglett.9b01869 Org. Lett. 2019, 21, 5552−5555

Letter

Organic Letters electrophilic character and are alkynophilic.19 Regarding the catalytic activation of allenes by Au nanoparticles toward nucleophilic attack, there is only one known example,20 in which an allenyl carboxylic acid undergoes intramolecular lactonization, using dendrimer supported Au nanoparticles stabilized with N-heterocyclic carbene ligands. In this communication we report the ligandless Au nanoparticlecatalyzed cycloisomerization of allenyl ketones into furans in almost quantitative yields and short reaction times. Recently, we presented our studies regarding the regioselective hydrosilylation,21 dehydrogenative disilylation,22 and diboration or silaboration23 of allenes catalyzed by Au nanoparticles supported on titania (Au/TiO2). Herein we report that allenes bearing a ketone functionality in conjugation (allenones) do not undergo any addition, but instead, they readily and cleanly cycloisomerize into the corresponding furans. Allenone 1 was examined as a key substrate, and under the optimum conditions presented in Table 1, quantitative isomerization into furan 1a can be Table 1. Optimization of Conditions Regarding the Cycloisomerization of Allenone 1

a

A side product from the conjugate addition of methanol to 1 was also formed in 34% relative yield. bA side product from the conjugate addition of CF3COOH to 1 was also formed in 60% relative yield.

achieved after 1 h at room temperature in the presence of 1 mol % Au/TiO2 (in CH2Cl2). External additives have peculiar results. Trifluoracetic acid (1 equiv), which is an essential reagent in the analogous Au(III)-catalyzed transformation,13 indeed accelerates the reaction, but the formation of a side product from its conjugate addition to the allenone was seen in ∼60% relative yield. On the other hand, by addition of 1 equiv of acetic acid, the reaction was accelerated and went to completion within 10 min without forming any side product. Searching for other suitable additives (Table 1), no further optimization was achieved. Notably, polar aprotic additives such as DMF or DMSO retard the reaction rate. The possible role of acetic acid will receive further comment in the following mechanistic discussion. Having optimized the reaction conditions (1 mol % Au/ TiO2, DCM as solvent, 1 equiv of acetic acid, 25 °C), we examined the scope and limitations of the cycloisomerization in a series of conjugated allenyl ketones,24 and the results are presented in Figure 1. Monosubstituted and 1,3-disubstituted

Figure 1. Cyclization of conjugated allenones into furans catalyzed by Au/TiO2. aThe reactions were performed on approximately 0.2 mmol scale. The cyclization of 1 was also performed in 1 mmol scale, and product 1a was isolated in 92% yield. b1,2-Dichloroethane was used as solvent, 70 °C. cThe precursor allenone 23 was used as a mixture with its isomeric alkynone 23′ in a relative ratio 23/23′ = 62/38 (see the Supporting Information). Within 30 min of reaction, 23 had been completely consumed and the relative ratio in the reaction mixture was 23a/23′ = 72/28. After 5 additional hours, 23′ had been also isomerized into 23a, which possibly implies the isomerization of alkynone 23′ into allenone 23 under the reaction conditions. 5553

DOI: 10.1021/acs.orglett.9b01869 Org. Lett. 2019, 21, 5552−5555

Letter

Organic Letters allenones isomerize smoothly at ambient conditions, yielding furans in very high yields. Notably, no chromatographic purification of the products is necessary. 1,1-Disubstituted and 1,2,3-trisubstituted allenes require heating to 70 °C (1,2dichloroethane, DCE) to cyclize, and given the amount of moisture in the solvent, partial hydration of the allene functionality may be observed leading to 1,3-diketones as minor side products. On the other hand, 3,3-disubstituted allenones (substrates 26 and 27) are completely unreactive. Cyclopropyl-substituted furan 16a is quantatively formed from the cyclization of the corresponding allenone 16, which possibly implies the lack of development of a typical zwitterionic intermediate in the process (see also Scheme 3 below). Our process is purely heterogeneous, and the catalytic system is recyclable and reusable. Through a simple filtration after the first run, the catalyst can be recovered, washed with DCM, dried in an oven at 90 °C for 2 h, and then reused without showing any deterioration of its activity. This recycling process was repeated successfully in 4 additional runs. Additionally, ICP-MS measurements on the supernatant solution of the reaction (DCM as solvent with added 1 equiv of acetic acid) revealed that the Au content was very low (2 ppm), indicative of the purely heterogeneous nature of our transformation. To investigate the effect of acetic acid on the rate acceleration of cyclization, we used CD3COOD as an additive. In the analogous Au(III)-catalyzed cyclization,13 where the addition of a protic acid is essential, it was reported that in the presence of CF3COOD/D2O, incorporation of a D atom on the C-3 position of the furan ring takes place (Scheme 2). This

Scheme 3. Proposed Mechanism in the Au/TiO2-Catalyzed Cycloisomerization of Allenones and the Rationalization of Product Formation in the Cyclization of Allenone 3-D

somewhat reminiscent of the cationic metal-catalyzed cyclization of allenones. The experiments presented in Scheme 2 regarding the use of deuterated acids as additives, however, indicate that the protodeauration in the chemisorbed intermediate II occurs intramolecularly via proton transfer from the adjacent carbon atom. In fact, the cyclization outcome of labeled allenone 3-D is in agreement with this proposal. Intramolecular H transfer in intermediate III leads to 3a-D, and D transfer to 3a′-D. The relative ratio 3a-D/3a′-D essentially represents a product primary isotope effect of kH/kD = 3.5 ± 0.4. Therefore, the role of acetic acid in accelerating the reaction rate is unclear. In conclusion, we report herein the superb catalytic ability of ligandless supported Au(0) nanoparticles on TiO2 in catalyzing the cycloisomerization of allenyl ketones into furans. In contrast to the typical cationic metal-catalyzed protocols, including catalysis by Au(I) and Au(III), the protodemetalation step on the chemisorbed Au nanoparticle cyclized intermediate occurs intramolecularly. The reaction rate is significantly accelerated by stoichiometric amounts of acetic acid, but the acid does not participate in the protodeauration step. Our catalytic system is simple, recyclable, and reusable, and in most of cases there is no need for chromatographic purification of the products.

Scheme 2. Mechanistic Studies in the Au/TiO2-Catalyzed Cyclization of Allenones

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is reasonable, because the role of the proton is to deaurate the cyclized intermediate. In our case, even when adding up to 20 equiv of acetic acid-d4, no D incorporation was seen on any position of the furan ring (Scheme 2). Additionally, in the cyclization of the deuterium-labeled allenone 3-D, the two isotopoisomeric products 3a-D and 3a′-D were formed in a relative ratio 3a-D/3a′-D = 3.5/1.25 A suggested mechanism is shown in Scheme 3. We propose that after interaction of the allene on Au(0) nanoparticle Aun (intermediate I), cyclization occurs to form intermediate II in which Aun bears a partial negative charge in accordance to previous proposals on the nature of Au−C bonds on nanoclusters.20−22,26 The mechanism up to this point is

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01869. Copies of 1H, 13C NMR of all products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] 5554

DOI: 10.1021/acs.orglett.9b01869 Org. Lett. 2019, 21, 5552−5555

Letter

Organic Letters ORCID

Eycken, E. V. Green Chem. 2015, 17, 3314. (g) Stratakis, M.; Garcia, H. Chem. Rev. 2012, 112, 4469. (18) (a) Liang, S.; Jasinski, J.; Hammond, G. B.; Xu, B. Org. Lett. 2015, 17, 162. (b) Zeng, X.; Chen, B.; Lu, Z.; Hammond, G. B.; Xu, B. Org. Lett. 2019, 21, 2772. (c) Oliver-Meseguer, J.; DomenechCarbo, A.; Boronat, M.; Leyva-Perez, A.; Corma, A. Angew. Chem., Int. Ed. 2017, 56, 6435. (19) (a) Tang, D.; Chen, Z.; Tang, Y.; Zhang, J.; Xu, Z.; Zhang, J. J. Phys. Chem. C 2014, 118, 18510. (b) Bistoni, G.; Belanzoni, P.; Belpassi, L.; Tarantelli, F. J. Phys. Chem. A 2016, 120, 5239. (c) Ye, R.; Zhukhovitskiy, A. V.; Deraedt, C. V.; Toste, F. D.; Somorjai, G. A. Acc. Chem. Res. 2017, 50, 1894. (d) Stenlid, J. H.; Brinck, T. J. Am. Chem. Soc. 2017, 139, 11012. (e) Stratakis, M.; Lykakis, I. N. Synthesis 2019, 51, 2435. (20) Ye, R.; Zhukhovitskiy, A. V.; Kazantsev, R. V.; Fakra, S. C.; Wickemeyer, B. B.; Toste, F. D.; Somorjai, G. A. J. Am. Chem. Soc. 2018, 140, 4144. (21) Kidonakis, M.; Stratakis, M. Org. Lett. 2015, 17, 4538. (22) Kidonakis, M.; Kotzabasaki, V.; Vasilikogiannaki, E.; Stratakis, M. Chem. - Eur. J. 2019, DOI: 10.1002/chem.201901408. (23) Kidonakis, M.; Stratakis, M. ACS Catal. 2018, 8, 1227. (24) Regarding the methodologies for the synthesis of the allenones used herein, see: (a) Liu, J.; Ma, S. Tetrahedron 2013, 69, 10161. (b) Kuang, J.; Ma, S. J. Org. Chem. 2009, 74, 1763. (c) Li, J.; Kong, W.; Yu, Y.; Fu, C.; Ma, S. J. Org. Chem. 2009, 74, 8733. (d) ref 10. (25) The cyclization of allenone 3-D (96% D) produced, in addition to 3a-D and 3a′-D in a relative ratio 3.5/1, approximately 15% of unlabeled product 3a (see the Supporting Information), which means that 3-D undergoes partial deuterium depletion under the reaction conditions, possibly through tautomerization into the corresponding alkynone (see also the footnote of Figure 1). Partial depletion of deuterium at the C-1 of terminal alkynes has been observed in the past in Au nanoparticle-catalyzed reactions. For a typical example, see reference 17c. (26) (a) Oliver-Meseguer, J.; Boronat, M.; Vidal-Moya, A.; Concepcion, P.; Rivero-Crespo, M. A.; Leyva-Perez, A.; Corma, A. J. Am. Chem. Soc. 2018, 140, 3215. (b) Corma, A.; Juarez, R.; Boronat, M.; Sanchez, F.; Iglesias, M.; Garcia, H. Chem. Commun. 2011, 47, 1446.

Manolis Stratakis: 0000-0002-0962-8297 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.K. thanks the ΕΛΙΔΕΚ foundation for financially supporting his doctoral studies. ProFI (FORTH, Heraklion, Greece) is acknowledged for obtaining the HRMS spectra of the unknown compounds, and professor S. Pergantis is acknowledged for assistance in the ICP-MS measurements.

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DOI: 10.1021/acs.orglett.9b01869 Org. Lett. 2019, 21, 5552−5555