Rhodium-Catalyzed Carbene Transfer Reactions for Sigmatropic

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Rhodium-Catalyzed Carbene Transfer Reactions for Sigmatropic Rearrangement Reactions of Selenium Ylides Sripati Jana and Rene M. Koenigs* Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany

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ABSTRACT: The rearrangement of selenium ylides is even today almost unexplored, although it would provide access to important organoselenium compounds with broad downstream applications. In this report, the first systematic study of sigmatropic rearrangement reactions of selenium ylides using a simple rhodium catalyst with catalyst loadings as low as 0.01 mol % is described. Selenium oxide pyrolysis of the rearrangement products gives access to important 1,1-disubstituted butadienes.

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Scheme 1. Sigmatropic Rearrangement Reactions of Selenium Ylides and Their Application in the Synthesis of 1,1-Butadienes

rganoselenium compounds are highly valuable reagents or catalysts with widespread application in the synthesis of complex natural products, drugs, or materials and are regarded as an important tool for the efficient introduction of new functional groups.1,2 The reaction of selenium compounds with electrophilic oxygen or nitrogen species is welldocumented and furnishes selenoxides or selenimides that, depending on the substitution pattern of the starting selenide, undergo elimination or rearrangement reactions.3,4 On the contrary, the reaction of organoselenium compounds with electrophilic carbenes has been explored significantly less and no systematic study of their reactivity has previously been reported.5,6 In the presence of an electrophilic carbene, the lighter group VI elements, oxygen and sulfur, react under ylide formation and subsequent rearrangement.7−10 The heavier homologues have been investigated much less, and selenium ylides find very limited applications.5,6 Even today, there are only singular examples that describe sigmatropic rearrangement reactions of selenium ylides via deprotonation reactions of a C−H acidic, preformed selenonium salt under basic reaction conditions.5 In 1995, Uemura reported a single example describing the catalytic rearrangement of a selenium ylide using copper or rhodium catalysts, though the efficiency and yield were poor.6 Against this background, we became interested in studying the catalytic formation of selenium ylides and their application in sigmatropic rearrangements, which would open up pathways toward densely functionalized organoselenium compounds with applications in organic synthesis. More specifically, the development of a relay protocol consisting of [2,3]-sigmatropic rearrangement of an allylic selenide followed by oxidation to the selenium oxide and syn elimination should allow an efficient, stereoselective entry into valuable 1,1-disubstituted butadiene derivatives (Scheme 1). We thus began our investigations by studying the reaction of allylic selenide 1 with phenyl diazoacetate 2 in the presence of different typical carbene transfer catalysts. Rh(II) catalysts © XXXX American Chemical Society

provided the desired rearrangement product 3 in excellent yield (72−99%) in a short reaction time (Table 1, entries 1 and 2). Chiral rhodium catalysts, however, provided the desired reaction product only in diminished yield and with only negligible enantioselectivity (Table 1, entry 4). All other catalysts investigated, based on iron, cobalt, copper, or silver, did not provide the desired reaction product (Table 1, entry 5).11 In all reactions, analysis of the crude reaction mixture revealed only diminutive amounts of byproducts formed and the diazoalkane and selenide remained almost untouched, which might be attributed to catalyst poisoning via complexation of the Lewis basic selenide. In further studies, the influence of solvent was investigated. We observed a marked effect of water and could isolate rearrangement product 3 in almost quantitiative yield in a water/DCM mixture.11 Notably, even with only 0.1 mol % Rh2(OAc)4 catalyst, we could observe a complete conversion of diazoalkane 2 after reaction for 120 min and the product was isolated in excellent yield Received: March 28, 2019

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DOI: 10.1021/acs.orglett.9b01092 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Table 1. Optimization of the Catalytic [2,3]-Sigmatropic Rearrangement of Selenium Ylides

no.a

catalyst

time

1 2 3 4

Rh2(OAc)4 (1 mol %) Rh2(TPA)4 (1 mol %) Rh2(esp)2 (1 mol %) Rh2(S-DOSP)4 (1 mol %)

15 15 30 60

5

Co(salen), FeTPPCl, TBA[Fe], AgOTf, CuOTf1/2C6H6 (1 mol %) Rh2(OAc)4 (1 mol %) Rh2(OAc)4 (1 mol %) Rh2(OAc)4 (0.1 mol %) Rh2(OAc)4 (0.01 mol %)

24 h

6b 7c 8c,d 9c,e

min min min min

45 min 45 min 120 min 24 h

Scheme 2. Substrate Scope of the Catalytic [2,3]Sigmatropic Rearrangement of Selenium Ylides

yield (%) 91 72 76 43 (3% ee) no reaction 84 99 86 67

a

For the reaction, 1 (0.2 mmol) and 2 (1.1 equiv) were dissolved in 1.5 mL of DCM, the catalyst was added, and the mixture was stirred at room temperature. bIn water as the solvent. cIn a 1:4 DCM/water mixture. dOn a 1 mmol scale. eOn a 5 mmol scale.

(86%). In additional experiments, we further reduced the catalyst loading to parts per million levels of Rh2(OAc)4 (0.01 mol %, 100 ppm) and the rearrangement product was obtained on a 5 mmol scale in good yield (1.2 g, 67%), which corresponds to a turnover number of 6700 and demonstrates the applicability of this reaction on a gram scale (Table 1, entry 9). Encouraged by these observations, we next investigated the applicability and generality of this rearrangement reaction. Different diazoesters, including styryl diazoacetate, smoothly reacted with allyl selenide 1 to the desired homoallylic selenides in excellent yields (Scheme 2). The substitution pattern of the aryl ring of diazoester 2 had little influence on the reaction yield, and different electron-donating groups and halogens in all positions of the aromatic ring were compatible with the reaction. Notably, ethyl diazoacetate, as a model acceptor-only diazoalkane, also smoothly underwent the rearrangement reaction yet with a slightly diminished yield of reaction product 3q. We subsequently investigated different allylic selenides and were delighted to observe that the selenoDoyle−Kirmse reaction smoothly furnishes homoallylic selenides in good to high yields and a broad variety of functional groups, such as nitro, nitrile, and trifluoromethyl, are well-tolerated (Scheme 2). Similarly, an aliphatic selenide reacted in this transformation to provide reaction product 3ae in good isolated yield. Very intriguingly, thiophene and pyridine heterocycles are compatible under these reaction conditions, and no poisoning of the rhodium catalyst occurred. The corresponding heterocyclic substituted homoallylic selenides were obtained in very good yields (Scheme 2, entries 3ac and 3ad). Upon investigation of the cinnamyl-substituted selenide, the rearrangement product was obtained in good yield, with only little diastereoselectivity (Scheme 2, entry 3af). It should be noted that the observed diastereoselectivity is in the same range as the diastereoselectivity for classic Doyle− Kirmse reactions.12 We next studied the reaction of propargyl selenides 4 with α-aryldiazoacetates, which provides efficient access to allenyl selenides. To our delight, the desired allenyl selenides could be

isolated under the reaction conditions presented here in excellent isolated yields (Scheme 3). Scheme 3. Substrate Scope of the Reaction of Propargylic Selenide with Different Diazoalkanes

To further investigate the generality of selenium-based rearrangement reactions, we next turned our attention to selenium compounds that should promote either [1,2]sigmatropic rearrangement reactions or [2,3]-sigmatropic rearrangements. If ethyl 2-(phenylselanyl)acetate 6a is used, a Sommelet−Hauser or [2,3]-sigmatropic rearrangement takes place and allows access to α-seleno-substituted esters 10a. The reaction product corresponds to a formal o-C−H functionalization of the aromatic substituent of 2 (Scheme 4). The robustness of the Sommelet−Hauser reaction was also investigated using 100 ppm Rh2(OAc)4, and ester 10a was obtained in 34% yield, which corresponds to a turnover B

DOI: 10.1021/acs.orglett.9b01092 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

homoallylic selenide by addition of hydrogen peroxide (Scheme 6a). Indeed, this approach proved to be highly

Scheme 4. Substrate Scope of the Sommelet−Hauser Rearrangement

Scheme 6. Applications in Olefin Synthesis

number of 3400. This transformation also readily takes place in the presence of cyano-substituted selenide 6b as well as different donor−acceptor substituted diazoalkanes and now gives access to o-C−H functionalization products in good to high isolated yields. Notably, meta-substituted diazoalkanes reacted in a highly regioselective fashion to yield the corresponding triply substituted phenyl acetic acid ester 10i in high isolated yield. In the case of benzylic selenide 11, the desired [1,2]sigmatropic or Stevens rearrangement readily took place, though the reaction product was obtained only as a complex mixture. When the solvent was changed to only water, the rearrangement product was obtained in good isolated yield. Similarly, a variety of different donor−acceptor diazoesters smoothly reacted to selectively provide the Stevens rearrangement product (Scheme 5). In an experiment with 100 ppm Rh2(OAc)4 as a catalyst, Stevens product 12a was obtained in 29% yield on a 5 mmol scale reaction. Organoselenium compounds play an important role in the construction of new functional groups, and the selenium oxide pyrolysis reaction is one of the textbook examples of applications of selenium compounds.1 We thus tested a onepot protocol consisting of an initial rhodium-catalyzed rearrangement reaction and a subsequent oxidation of the

versatile in the stereoselective synthesis of the Z-configured, 1,1-disubstituted butadiene 13 in 56% yield over two steps; a consecutive reaction protocol with purification of the intermediate homoallylic selenide produced 13 in similar yield (60%).13 The stereochemical outcome of this syn elimination can be rationalized by the transition state in Scheme 6c, in which the vinyl group and the phenyl ring are arranged, for steric reasons, preferentially in a trans conformation that results in the Z configuration of the butadiene product. Most importantly, the stereochemistry is complementary to protocols relying on the reaction of diazoalkanes with electrophilic palladium−allyl complexes.14 A similar protocol could also be applied to the product of the Stevens rearrangement to provide trisubstituted olefine 14, yet only as a 1:1 mixture of diastereoisomers (Scheme 6b), which may be a result of the similar steric hindrance of both transition states (Scheme 6c). Finally, we decided to study the mechanism of this rearrangement reaction. More specifically, it would be highly important to understand if rearrangement reactions of selenium ylides undergo rearrangement via metal-bound or free ylide intermediates. We therefore conducted a set of control experiments using different Rh(II) catalysts and aryldiazoacetates in the diastereoselective rearrangement reaction of selenide 15, yet the catalyst had an only minor effect on the selectivity of the rearrangement reaction, which is indicative of a rearrangement process without participation of the catalyst and, thus, a free ylide reaction mechanism (Scheme 7a). This is also in line with the observation mentioned above when using chiral Rh(II) catalysts, which gave an only negligible enanantiomeric excess in the rearrangement reaction of 1 (Table 1, entry 4). We also probed our recently developed photochemical protocol in this transformation;15 however, only the dimerization reaction of the diazoalkane was observed. In further control experiments, we investigated the influence of the aryldiazoacetate and studied different esters. Although a

Scheme 5. Substrate Scope of the Stevens Rearrangement

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DOI: 10.1021/acs.orglett.9b01092 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 7. (a) Control Experiment with Different Rhodium Catalysts, (b) Control Experiments with Different Aryldiazoacetates, and (c) Proposed Reaction Mechanism of the Rearrangement Reaction of Selenium Ylides

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Detailed experimental procedures and spectral data for all compounds, including copies of 1H, 13C, and 19F NMR spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rene M. Koenigs: 0000-0003-0247-4384 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.M.K. gratefully acknowledges the Dean’s Seed Fund of RWTH Aachen University. Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), via Project 408033718.

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ABBREVIATIONS TPP, meso-tetraphenylporphyrin; TBA[Fe], [Bu 4N][Fe(CO)3(NO)]

similar product yield was observed when using sterically demanding esters, no improvement in the diastereoselectivity was observed (Scheme 7b). On the basis of the literature of rearrangement reactions of sulfur ylides and the data presented here, we propose the following mechanism. Treatment of diazoalkane 2 with the Rh(II) catalyst furnishes a Rh(II)−carbene complex, which undergoes an addition reaction with selenium compound 1 providing a metal-bound selenium ylide 16. Upon dissociation of the metal complex, a free ylide 17 is formed, which undergoes the sigmatropic rearrangement (Scheme 7c). In summary, we herein report on the first systematic study of catalytic rearrangement reactions of organoselenium compounds with diazoalkanes. Using a cheap rhodium catalyst, donor−acceptor diazoalkanes readily undergo a carbene transfer reaction and formation of an intermediate selenium ylide that, depending on the substitution pattern of the selenide, undergoes either [2,3]- or [1,2]-sigmatropic rearrangement reactions (64 examples in total, ≤99% yield) via a free ylide mechanism in excellent yield and catalyst loadings as low as 100 ppm. Following this methodology, an efficient onepot protocol for the stereoselective synthesis of (Z)-1,1disubstituted butadienes was realized.

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REFERENCES

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DOI: 10.1021/acs.orglett.9b01092 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b01092 Org. Lett. XXXX, XXX, XXX−XXX