Iron-Catalyzed Radical Acyl-Azidation of Alkenes with Aldehydes

Dec 24, 2018 - Wallentin et al. developed iridium-catalyzed intramolecular radical difunctionalization of alkenes with aromatic carboxylic acids(5) or...
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Letter Cite This: Org. Lett. 2019, 21, 256−260

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Iron-Catalyzed Radical Acyl-Azidation of Alkenes with Aldehydes: Synthesis of Unsymmetrical β‑Azido Ketones Liang Ge,†,‡ Yajun Li,† and Hongli Bao*,†,‡ †

Org. Lett. 2019.21:256-260. Downloaded from pubs.acs.org by UNITED ARAB EMIRATES UNIV on 01/11/19. For personal use only.

Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Center for Excellence in Molecular Synthesis, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou, Fujian 350002, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China S Supporting Information *

ABSTRACT: An iron-catalyzed acyl-azidation of alkenes under mild reaction conditions has been developed. Aromatic aldehydes or aliphatic aldehydes can be used as the acyl radical precursors; TMSN3 is used as the azido source; TBHP is the initiator. The synthesized unsymmetrical β-azido ketones can be easily transformed into valuable functionalized compounds, such as γ-aminol, γ-azido alcohol, β-azido oxime, β-azido ester, and triazoles.

D

Scheme 1. Radical Acyl-Functionalization of Alkenes with Aldehydes

irect vicinal difunctionalization of alkenes which installs two functional groups on a CC bond in a single process has emerged as a powerful tool for the generation of highly useful and complex skeletons.1 Hydroacylation of alkenes, introducing a valuable carbonyl group into the structure, has great advantages in the transformation of alkenes and has been demonstrated to be a very reliable method.2 Approaches that can introduce a carbonyl group and another functional group into a molecule, further broadening the synthetic applications of alkene difunctionalization, are highly desirable but remain challenging. Radical difunctionalization of alkenes is a rapid and convenient method in which to convert alkenes into more complex molecules.3 Initiated by a radical precursor or irradiation, radical hydroacylation of alkenes often employs aldehydes as the source of acyl radicals (Scheme 1a).4 Recently, several radical difunctionalizations of alkenes have been developed. Wallentin et al. developed iridium-catalyzed intramolecular radical difunctionalization of alkenes with aromatic carboxylic acids5 or aromatic carboxylic anhydrides6 as the source of acyl radicals, affording a range of oxindole derivatives under light irradiation. Xu et al. reported that aroyl chlorides could also be used as the precursor to the acyl radicals in such reactions7 and developed a method for the intramolecular synthesis of diverse fused pyran derivatives.8 Intramolecular radical acylation−phosphorylation in the synthesis of phosphonate chroman-4-ones, employing a silver salt as the catalyst and K2S2O8 as an oxidant, has also been reported (Scheme 1b).9 For intermolecular radical difunctionalization of alkenes, Taniguchi et al.10 and Li et al.11 independently disclosed efficient synthetic protocols to obtain β-hydroxy and β-peroxy carbonyl compounds from vinyl arenes using the reaction of carbazates and aldehydes in the presence of O2 or tert-butyl hydroperoxide (TBHP) as the co© 2018 American Chemical Society

oxidant (Scheme 1c). Chen’s group reported that vanadyl species can also trigger the reaction which leads to βhydroxycarbonyl compounds.12 Subsequently, intermolecular radical fluoroacylation of alkenes catalyzed by a silver catalyst has been reported by Duan and co-workers, with αoxocarboxylic acids as the acyl radical precursors and Selectfluor (1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane) bis(tetrafluoroborate) as the fluorine source.13 Although this radical difunctionalization of alkenes has been developed, the scope of the useful functional groups is still narrow and some other disadvantages should be considered. Received: November 19, 2018 Published: December 24, 2018 256

DOI: 10.1021/acs.orglett.8b03688 Org. Lett. 2019, 21, 256−260

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Organic Letters Table 1. Optimization of the Reaction Conditionsa

First, although different acyl radical precursors, such as aldehydes, aromatic carboxylic acids, aromatic carboxylic anhydrides, aroyl chlorides, or even carbazates have been employed, aldehydes provide atom economy and availability and are the best sources. Second, in most cases, the use of aliphatic aldehydes is very rare. Finally, nonfriendly reaction conditions, such as elevated temperatures, precious metal catalysts, or toxic solvents are often involved. Herein we report our development of a ligand-free, ironcatalyzed radical acyl-azidation of alkenes for the synthesis of unsymmetrical β-azido ketones (Scheme 1d). Aliphatic or aromatic aldehydes serve as the acyl radicals, simple alkenes, or activated alkenes as the substrates, and azidotrimethylsilane (TMSN3) serves as the azido source. The β-azido ketones produced in this reaction can be easily transformed into valuable functionalized compounds, such as γ-aminols, γ-azido alcohols, β-azido oximes, β-azido esters, or triazoles. Very recently, Wang et al. reported the only example of acylazidation of alkenes, using (salen)Mn(III) species as the catalyst, iodosobenzene as the oxidant (iodobenzene as the side product), and sodium azide as the azido source.14 The development of more environment-friendly catalytic systems is still highly desired. We initiated our study by evaluating the parameters of the reaction, including the metal catalysts, additives, solvents, and temperatures in the reaction of styrene (1a) with benzaldehyde (2a) and TMSN3 (3) (Table 1). First, metal catalysts were screened with hexane as the solvent at 50 °C. Fe(OTf)3 was found to be the most effective catalyst, with the reaction providing the desired product (4aa) in 40% yield (entries 1− 9). Several other solvents were then tested (entries 10−17), and it was found that MTBE was optimum. The yield of 4aa increased when the reaction time was extended at a reduced temperature (entries 18 and 19). Decreasing or increasing the loading of the catalyst did not improve the yield of 4aa (entries 20 and 21). When TMSN3 was added portionwise during an interval time of 3 h, better performance was observed (entry 22). The best yield of the desired product was obtained when the quantity of TMSN3 was decreased to 2.5 equiv (entry 23). A scale-up reaction (5 mmol) could afford the product 4aa in 63% yield (entry 24). With the optimized reaction conditions in hand, the scope of the alkenes in the reaction was investigated (Scheme 2). Reactions of o-, m-, or p-alkyl-substituted vinylarenes with benzaldehyde (2a) afforded the corresponding products (4ba−4ea) in 58−83% yield. Vinylarenes containing halogens reacted to give 4fa−4ia in moderate yields (47−65%). Vinylarenes bearing an ester or a free carboxyl group also participated in this reaction, affording products 4ja and 4ka respectively, in moderate yields. 1,1-Disubstituted vinylarenes, such as 4la, are a suitable substrate for this reaction, and a good yield (64%) can be obtained. Heterocyclic compounds can be used to carry out the reaction, affording the desired product (4ma) with a 42% yield. α,β-Unsaturated carbonyl compounds were also found to be compatible with the reaction conditions, giving the acylazidation products (4oa−4sa) in moderate yields (36−74%). Conjugated dienes proved to be suitable substrates for the reaction, delivering the product (4ta) with a terminal CC bond in 56% yield, but unactivated alkenes showed a lower efficiency affording the corresponding product (4ua) in only 25% yield. Subsequently, we studied the substrate scope of aromatic aldehydes (Scheme 3). In general, the reactions of a variety of

entry

catalyst

solvent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18c 19d 20e 21f 22g 23h 24j

Ni(OTf)2 CuOTf AgOTf Zn(OTf)2 Sc(OTf)2 Fe(OTf)3 FeCl2 FeCl3 FeBr3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3 Fe(OTf)3

hexane hexane hexane hexane hexane hexane hexane hexane hexane benzene THF cyclohexane DCE 1,4-dioxane Ether MTBE DME MTBE MTBE MTBE MTBE MTBE MTBE MTBE

temp 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 40 25 25 25 25 25 25

°C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C

yieldb − 12% − − − 40% 7% 7% 3% 41% 16% 37% 11% 39% 36% 49% 34% 60% 64% 46% 58% 71% 76%(67%)i 63%

a

1a (0.5 mmol), 2a (2.5 mmol), 3 (1.5 mmol), TBHP (70% in water, 1.25 mmol), cat. (2 mol %), solvent (1 mL) at given temperature for 12 h. b1H NMR yield. c40 °C, 24 h. d25 °C, 24 h. eFe(OTf)3 (1 mol %), 25 °C, 24 h. fFe(OTf)3 (5 mol %), 25 °C, 24 h. g3 (1.5 mmol) was added portion-wise, Fe(OTf)3 (2 mol %), 25 °C, 24 h. h3 (1.25 mmol) was added portionwise, Fe(OTf)3 (2 mol %), 25 °C, 24 h. i Isolated product in parentheses. j1a (5 mmol), 2a (25 mmol), 3 (15 mmol), TBHP (70% in water, 12.5 mmol), Fe(OTf)3 (2 mol %), MTBE (12 mL) 25 °C, 24 h.

aryl aldehydes bearing either electron-donating or electronwithdrawing groups proceeded with moderate to good yields (4jb−4jf). It was noted that electron-donating groups are more efficient than electron-withdrawing groups. Multiple substituted aryl aldehydes were found to afford the corresponding products (4jg and 4jh) in moderate yields. Aryl aldehyde containing hydroxyl groups also underwent this reaction, giving the product (4ji) in 36% yield. The reaction of naphthaldehydes occurred to produce 4jj and 4jk with moderate yields. Functional groups, such as (trimethylsilyl)ethynyl and morpholino, were compatible with the reaction conditions and gave 4jm and 4jn, respectively. Heteroarenes, prevalent in many biologically relevant molecules, are compatible with the reaction conditions, and aldehydes containing thiophene, furan, and benzo[b]thiophene motifs when subjected to the reaction provided the desired products (4jo−4jq) with moderate yields. Encouraged by the success with aromatic aldehydes, aliphatic aldehydes and α,β-unsaturated aldehydes were then investigated giving the results summarized in Scheme 4. The reactions of a variety of aliphatic aldehydes with 1j proceeded smoothly under the standard conditions to give the corresponding products (4jr−4jy) in good yields. Not only saturated aliphatic aldehydes (4jr−4jt) but also unsaturated 257

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Organic Letters Scheme 2. Substrate Scope for Acyl-Azidation of Alkenesa,b

aliphatic aldehydes (4jv−4jy) are suitable substrates. The reaction was further supported by the compatibility of α,βunsaturated aldehydes. Myrtenal and β-methyl acrolein could be successfully converted into the corresponding acyl-azidation products (4jx and 4jy) in good yields. Further transformations of the acyl-azidation products were explored (Scheme 5). It is noteworthy that carbonyl azides Scheme 5. Transformations of Acyl-Azidation Productsa

a

(i) Cu(OAc)2, 2-aminophenol, DCM/H2O, rt. (ii) LAH (2.0 equiv), ether, rt, 12 h. (iii) NaBH4, MeOH, rt. (iv) NH2OH−HCl, NaOH, H2O, EtOH, 60 °C, 10 h. (v) Sc(OTf)3 (5 mol %), m-CPBA (5.0 equiv), 40 °C, 48 h.

could be used successfully in the efficient synthesis of a range of molecules which could be useful building blocks. The click reaction of 4aa with butyn-2-one gave β-carbonyl triazole (5). γ-Aminol (6) was prepared from 4aa after a simple reduction reaction. With the reduction conditions carefully controlled, the γ-azido alcohol (7) could be obtained with the azido group untouched. The condensation reaction of an acyl-azidation product with hydroxylamine hydrochloride gave the oxime (8) which may be applied to the synthesis of biological or polymeric materials. A β-amino acid derivative (9) could be produced through the Baeyer−Villiger oxidation reaction. Furthermore, the acyl-azidation products could also be converted into triazoles (10−13) with internal alkynes (Scheme 6).

a

1 (0.5 mmol), 2a (2.5 mmol), 3 (1.25 mmol), TBHP (1.25 mmol), MTBE (1 mL), 24 h, 3 was added portionwise. bIsolated product. c Fe(OTf)3 (10 mol %), MTBE (0.5 mL), PhH (0.5 mL). dFe(OTf)3 (10 mol %). eDetermined by GC-MS.

Scheme 3. Substrate Scope for Acyl-Azidation of Aromatic Aldehydesa,b

Scheme 6. Further Transformations of Acyl-Azidation Products

a

1j (0.5 mmol), 2 (2.5 mmol), 3 (1.25 mmol), TBHP (1.25 mmol), MTBE (1 mL), 24 h, 3 was added portionwise. bIsolated product.

Scheme 4. Substrate Scope for Acyl-Azidation of Aliphatic Aldehydes and α,β-Unsaturated Aldehydesa,b

We performed preliminary experiments to probe the mechanism of the reaction (Scheme 7). Radical trapping experiments were conducted with 1.5 equiv of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) and BHT (dibutylhydroxytoluene). These reactions did not afford any desired product, and instead, compounds 14, 15, and 16 were detected by GC-MS analysis (Scheme 7a). This is consistent with the radical nature of the reaction, in which the otherwise propagating radical cycle was effectively interrupted by TEMPO and BHT. The formation of compound 14 indicates that an acyl radical was produced in the reaction, while the formation of compound 16 suggests that an azido radical was produced in the reaction. To further support this hypothesis, compound 17 bearing a cyclopropylmethyl moiety (a radical clock) was synthesized (Scheme 7b). As expected, the reaction

a

1j (0.5 mmol), 2 (2.5 mmol), 3 (1.25 mmol), TBHP (1.25 mmol), MTBE (1 mL), 24 h, 3 was added portionwise. bIsolated product.

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Organic Letters

In conclusion, a ligand-free iron-catalyzed acyl-azidation of alkenes under mild reaction conditions has been developed with aromatic aldehydes or aliphatic aldehydes as the acyl radical precursors, TMSN3 as the azido source, and TBHP as the initiator. A range of alkenes has been employed for the synthesis of unsymmetrical β-azido ketones, which can be easily transformed into valuable functionalized compounds, such as γ-aminol, γ-azido alcohol, β-azido oxime, β-azido ester, and triazoles. Mechanistic studies suggest the involvement of an acyl radical, and a radical pathway has been proposed for the mechanism.

Scheme 7. Mechanistic Studies

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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/acs.orglett.8b03688.

of 17 with 2a and TMSN3 afforded the ring-opened product 18 in 58% yield, further supporting the involvement of radical species in the reaction. Based on these mechanistic studies, a catalytic mechanism for the acyl-azidation reaction is proposed (Scheme 8).

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Scheme 8. Proposed Catalytic Cycle for Acyl-Azidation Reaction

Experimental procedures, characterization of new compounds, synthetic application, mechanistic studies, NMR spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yajun Li: 0000-0001-6690-2662 Hongli Bao: 0000-0003-1030-5089 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Key R&D Program of China (2017YFA0700103), the NSFC (Grant Nos. 21502191, 21672213, and 21871258), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), and the Haixi Institute of CAS (Grant No. CXZX-2017-P01) for financial support.

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Initially, an iron(II) species (A) undergoes a single-electron transfer with TBHP to afford the iron(III) species (B) and a tert-butyloxyl radical, which can decompose to a methyl radical or release an azido radical from TMSN3. Then, the tertbutyloxyl radical (or the methyl radical or the azido radical) abstracts an H-atom from the aldehyde to afford an acyl radical.11b,15 The acyl radical then is trapped by an alkene to produce a more stable carbon-centered radical. Upon ligand exchange with TMSN3, the iron(III) species (B) is converted into an iron(III) species (C), which then undergoes azido group transfer with the stabilized carbon-centered radical to afford the acyl-azidation product, regenerating the iron catalyst. 259

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