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Iron-catalyzed esterification of allylic sp3 C–H bonds with carboxylic acids: Facile access to allylic esters
Tetrahedron Letters 58 (2017) 2490–2494
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Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Iron-catalyzed esterification of allylic sp3 C–H bonds with carboxylic acids: Facile access to allylic esters Bing Lu, Fan Zhu, Dan Wang, Hongmei Sun ⇑, Qi Shen The Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
a r t i c l e
i n f o
Article history: Received 10 April 2017 Revised 10 May 2017 Accepted 12 May 2017 Available online 12 May 2017 Keywords: Iron(III) complex Imidazolinium salt Oxidative cross-coupling C–H bond functionalization Allylic ester
a b s t r a c t The first general and efficient iron-catalyzed esterification of allylic sp3 C–H bonds with carboxylic acids using ionic iron(III) complexes (1–4) as a catalyst and DTBP (DTBP = di-tert-butyl peroxide) as an oxidant is achieved. A variety of allylic esters were synthesized in good to excellent yields using the ionic iron(III) complex 2 as a catalyst in a 5 mol% loading. This reaction is characterized by its high efficiency, broad substrate scope with excellent steric hindrance tolerance and good functional group compatibility. Ó 2017 Published by Elsevier Ltd.
Introduction Allylic esters are of growing interest in organic synthesis, as they are important structural units in bioactive and medical molecules1 as well as fine chemicals.2 They have been traditionally synthesized by the reaction of acids or their derivatives (acyl chlorides and anhydrides) with the corresponding allylic alcohols.3 However, the limited substrate scope and harsh reaction conditions restrict their applications. During the past decade, the direct functionalization of sp3 C–H bonds through oxidative cross-coupling has emerged as one of the most important strategies for the development of new synthetic methodologies in organic synthesis from green and sustainable points of view.4 For example, oxidative esterification of allylic sp3 C–H bonds has shown remarkable potential as an alternative to traditional protocols for the synthesis of allylic esters. In this context, several types of transition metals-based catalytic systems, including palladium,5 copper,6 and copper–aluminum mixed oxide,7 have been developed for the esterification of allylic sp3 C–H bonds. Also, a metal-free esterification of allylic sp3 C–H bonds with carboxylic acids using Bu4NI as a catalyst has been developed.8 Nevertheless, there remains ample opportunity for improving such transformation into a routine tool for synthesis of allylic ester in the laboratory and on an industrial scale.
⇑ Corresponding author. E-mail address: [email protected] (H. Sun). http://dx.doi.org/10.1016/j.tetlet.2017.05.039 0040-4039/Ó 2017 Published by Elsevier Ltd.
Over the past decade, significant efforts have been devoted to the development of iron-catalyzed functionalization of sp3 C–H bonds9 because iron is inexpensive, nontoxic, and environmentally benign.10 In this context, a large number of studies focused on the transformation of sp3 C–H bonds adjacent to a heteroatom11 or benzylic sp3 C–H bonds12 have made significant progress. However, iron-catalyzed functionalization of allylic sp3 C–H bonds is still highly desired but challenging, possibly owing to the difficulty of selective functionalization of allylic sp3 C–H bond without an active group (i.e., a heteroatom and/or an aromatic ring).4f In 2010, the Jiao group reported the first example of an FeCl2-catalyzed cyanation of allylic sp3 C–H bonds via the oxidative cross-coupling of allylarenes with Me3SiN3 using DDQ (DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone) as an oxidant.13 Later, the White group described the first example of intramolecular amination of allylic sp3 C–H bonds to sulfamide using [FePc]Cl (Pc = phthalocyaninato) as a catalyst.14 However, in terms of iron-catalyzed esterification of allylic sp3 C–H bonds to the synthesis of allylic esters, only the Pan group mentioned one example of the esterification of cyclohexene with thiophene2-carboxylic acid in a 45% yield catalyzed by Fe(acac)3 at a 20 mol% loading.11e Therefore, the development of a general methodology for iron-catalyzed oxidative esterification of allylic sp3 C–H bonds to the synthesis of allylic esters is highly desired. We recently reported the synthesis of a series of ionic iron(III) complexes containing an imidazolinium cation (Scheme 1) and their catalytic potential in the esterification of benzylic sp3 C–H bonds with carboxylic acids using di-tert-butyl peroxide (DTBP)
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Scheme 1. Ionic iron(III) complexes containing an imidazolinium cation.
as an oxidant.15h Herein, we continue our studies15 by developing the synthetic route of such kind of ionic iron(III) complexes and their application in the esterification of allylic sp3 C–H bonds with carboxylic acids. Results and discussion In our previous work, complexes 2–4 were synthesized via the reaction of FeBr3 with the corresponding 1,3-dihydrocarbylimidazolinium bromide.15h The present investigation reveals that these iron(III) bromides could be directly synthesized using 1,3dihydrocarbylimidazolinium chlorides as starting materials. As shown in Scheme 2, the reaction of FeBr3 with 1 equiv. of 1,3-dihydrocarbylimidazolinium chloride in the presence of NaBr afforded the desired iron(III) bromides 2–4 as air-stable brown-red crystals in yields ranging from 87% to 90%, which are very similar to the data reported previously.15h This protocol avoids the employment of the corresponding 1,3dihydrocarbylimidazolinium bromides, which were generally synthesized from more expensive alkyl bromides or more corrosive hydrobromide as compared with their chloride derivatives.16 Complexes 2–4 were characterized by elemental analysis, Raman spectroscopy and electrospray ionization mass spectroscopy. The characterization results agreed well with those reported previously.15h The solid-state structures of complexes 3 and 4 were further confirmed by X-ray crystallography. Single crystals suitable for X-ray diffraction studies were grown from cold THF/hexane. The crystallographic data are listed in Table S1 (see the ESIy). Their molecular structures are depicted in Figs. 1–2, with selected bond lengths and angles given in the captions. It is worth noting that the molecular structures of this type of ionic iron(III) complexes have been seldom reported, even if the application of imidazolinium salts as ligands in iron-based catalytic systems has received increasing attention during the past decade.17 To date, only two scientific articles have described structural data for imidazolinium cation-containing ionic iron(III) complexes15h,18 As seen in Figs. 1 and 2, each of the two solid-state structures contained one 1,3-dihydrocarbylimidazolinium cation and one [FeBr4] anion. The structure of the 1,3-dihydrocarbylimidazolinium cation presented either in 3 or 4 changed little after the reaction of the corresponding 1,3-dihydrocarbylimidazolinium salt with FeBr3.19 In both [FeBr4] anions, each iron atom is coordinated by four bromine atoms in a slightly distorted tetrahedral geometry with a mean Br–Fe–Br angle of 109.5° for both of 3 and 4, which is close to the ideal tetrahedral angle of 50°.20 The Fe–Br bond lengths were found to lie in the ranges of 2.301(11)–2.341(12) Å, wherein the mean Fe–Br bond length was 2.333 Å in 3 and 2.308
Scheme 2. Synthesis of ionic iron(III) complexes 2–4.
Fig. 1. Molecular structure of complex 3 with thermal ellipsoids at the 30% probability level. Hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (°): Fe(1)–Br(1) 2.324(12), Fe(1)–Br(2) 2.337(12), Fe(1)–Br(3) 2.327(12), Fe (1)–Br(4) 2.341(12); Br(1)–Fe(1)–Br(2) 108.78(5), Br(1)–Fe(1)–Br(3) 110.86(5), Br (1)–Fe(1)–Br(4) 108.97(5), Br(2)–Fe(1)–Br(3) 109.56(5), Br(2)–Fe(1)–Br(4) 108.44 (5), Br(3)–Fe(1)–Br(4) 110.18(5).
Fig. 2. Molecular structure of complex 4 with thermal ellipsoids at the 30% probability level. Hydrogens and solvent are omitted for clarity. Selected bond lengths (Å) and angles (°): Fe(1)–Br(1) 2.301(11), Fe(1)–Br(2) 2.311(13), Fe(1)–Br(3) 2.319(13), Fe(1)–Br(4) 2.301(14); Br(1)–Fe(1)–Br(2) 108.47(5), Br(1)–Fe(1)–Br(3) 109.02(5), Br(1)–Fe(1)–Br(4) 108.33(5), Br(2)–Fe(1)–Br(3) 109.09(6), Br(2)–Fe(1)– Br(4) 110.74(6), Br(3)–Fe(1)–Br(4) 111.14(5).
Å in 4. These values were in the expected range as found for other imidazolinium cation-containing ionic iron(III) complexes reported previously15h,18 We began our investigation with the oxidative cross-coupling reaction of 4-methoxybenzoic acid (5a) and cyclohexene (6a) with catalysts 1–4 and related iron(III) salts. The results are summarized in Table 1. Under the optimized conditions, there were obviously different catalytic activities for these iron(III)-based catalysts. For example, complex 2 exhibited the highest activity and gave the desired product 7a in 95% yield at a 5 mol% loading (entry 2). The desired product 7a was still obtained in high yield of 81% even when the loading of 2 was reduced to 3 mol%. In comparison, com-
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Table 1 Screening of optimal conditions.a
Table 2 Esterification of various carboxylic acids with cyclohexene.a
Entry
Catalyst
Oxidant
Yield (%)
1 2 3 4 5 6 7 8 9 10 11
1 2 2 2 2 3 4 [Me4N][FeBr4]f FeBr3 [HSItBu]Brg + FeBr3 [HSItBu]Br
DTBP DTBP DTBP TBHPd DDQe DTBP DTBP DTBP DTBP DTBP DTBP
32 95(88b) 81c 0 0 73 84 28 16 88 0
a Conditions: 4-methoxybenzoic acid (0.5 mmol), cyclohexene (7.5 mmol), catalyst (5 mol%), oxidant (1.0 mol), EtOAc (0.5 mL), 110 °C, 24 h, GC yield using nhexadecane as internal standard, an average of two runs. b Isolated yield. c 2 (3 mol%). d TBHP = tert-butyl hydroperoxide. e DDQ = 2,3-dicyano-5,6-dichlorobenzoquinone. f [Me4N] = tetramethylammonium cation (Ref. 21). g [HSItBu]Br = 1,3-di-tert-butylimidazolinium bromide.
plexes 3 and 4 showed a relatively lower activity, affording the desired product in yields of 73% (entry 6) and 84% (entry 7), respectively. The chloride analogue (1) showed very poor activity, giving 7a in 32% yield (entry 1). Notably, an iron(III) complex of [Me4N] [FeBr4], which containing a tetramethylammonium cation, also exhibited very poor activity (entry 8). As expected, FeBr3 alone gave 7a in merely 16% yield (entry 9). A mixture of FeBr3 and 1,3-di-tert-butylimidazolinium bromide ([HSItBu]Br) in a 1:1 M ratio gave the desired product 7a in 88% yield (entry 10), and no 7a was detected when [HSItBu]Br was employed alone (entry 11). Oxidant screening disclosed that TBHP and DDQ showed no activity (entries 4 and 5). To date, 10 mol% or higher loadings of other transition metalbased catalysts5–7 or metal-free catalyst8 are usually required to achieve satisfactory yields for such kind of transformation. The present results indicates that complex 2 might be among the most efficient catalyst for the oxidative esterification of allylic sp3 C–H bonds to the synthesis of allylic esters, and the presence of a sterically bulky imidazolinium cation with electron-donating N-substituents is of benefit to its high catalytic activity.22 Encouraged by the above results, we explored the substrate scope of this iron-catalyzed esterification of allylic C–H bonds with carboxylic acids to obtain allylic esters. Firstly, a series of carboxylic acids were investigated, as shown in Table 2. Both electron-donating and electron-withdrawing aryl carboxylic acids were successfully coupled with cyclohexene to form corresponding allylic esters in moderate to excellent yields (7b–7n). Interestingly, steric hindrance was well tolerated in this reaction, as the highly hindered 2,4,6-triiso-propylbenzoic acid was converted to corresponding ester in a 91% yield without enhanced reaction conditions (7i). To date, such highly hindered carboxylic acids have rarely been explored as substrates in other sp3 C–H bond esterification reported previously.23 Another notable merit of this protocol is that a series of functional groups can been tolerated in our oxidative conditions, including cyano (7m), nitro (7n), hydroxyl (7o), and ester (7p). Additionally, biphenyl 4-carboxylic acid (7p), biphenyl 2-carboxylic acid (7r), 1-naphthoic acid (7s) and 2-naphthoic acid (7t) gave the desired products in 95%-98% yields. Notably, furan-2-carboxylic acid (7u) and thiophene-2-carboxylic acid
a Conditions: carboxylic acids (0.5 mmol), cyclohexene (7.5 mmol), 2 (5 mol%), EtOAc (0.5 mL), DTBP (1.0 mmol), 110 °C, 24 h, isolated yield.
(7v) were also excellent coupling partners under the optimized conditions. Such kind of heteroaromatic carboxylates have been widely used in organic synthesis,24 however, they were usually less employed as substrates or showed low activities in previously reported studies.8 Several kinds of aliphatic carboxylic acids were also found to be compatible with this esterification protocol, furnishing the corresponding ester products (7w–7y) in 67%–76% yields. We also investigated the substrate scope of various alkenes to further explore the potential of the present protocol. As shown in
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Table 3 Esterification of benzoic acid with various alkenes.a
Scheme 3. Investigation of reaction mechanism.
of an imidazolinium cation should be of benefit to form the diiron(III) intermediate.25 Conclusions a Conditions: benzoic acid (0.5 mmol), alkene (7.5 mmol), 2 (5 mol%), EtOAc (0.5 mL), DTBP (1.0 mmol), 110¯ C, 24 h, isolated yield. b 2 (10 mol%), allylarenes (1.5 mL), no solvent, 16 h. c The reaction was too complicated and no obvious amount of the desired product was obtained.
Table 3, cyclopentene reacted well with benzoic acid (8a), while cyclooctene gave the desired product in a lower yield under the same reaction conditions (8b). Notably, 2,3-dimethylbut-2-ene exhibited very high selectivity, affording merely the mono-esterification product 8c in a yield of 90%. Interestingly, a very high regioselectivity is also observed in our protocol when allylbenzene and allylnaphthalene were employed, wherein linear allylic ester was isolated in yields of 86% (8d) and 82% (8e), respectively. This phenomenon is quite different from that observed with palladium-based catalysts, wherein p-allyl-Pd(II) intermediates could lead to a L/B (L = linear product, B = branched product) regioselectivity. To date, a control regioselectivity to the linear isomer is still highly desired in the field of palladium-catalyzed esterification of allylic sp3 C–H bond.5 Somewhat to our surprise, methylenecyclohexene failed to participate in this esterification reaction (8f). Finally, a simple conjugated diene was tested under optimized conditions, however, the reaction was too complicated and no obvious amount of desired product 8g could be detected, similar to that reported previously.13 Although an exact mechanism of the present reaction requires further investigation, our current experimental studies suggest that this transformation may involve radical species because the reaction was completely stopped by the addition of TEMPO as a radical inhibitor (Scheme 3, eqn. 1). Cyclohex-2-enol was not detected during the reaction (Scheme 3, eqn. 2), which suggests that the present esterification reaction possibly occurred without an alcohol intermediate. When tert-butyl perester was used as a reactant, the allylic ester 7b was obtained only in 31% yield (Scheme 3, eqn. 3), indicating that this reaction may not proceed through a tert-butyl perester intermediate. Recently, Pan11e group proposed that a l-oxo, l-carboxylate diiron(III) intermediate might be formed in iron-catalyzed esterification of sp3 C–H bonds adjacent an oxygen atom. We suggest that the present esterification reaction possibly also proceeded via a l-oxo, l-carboxylate diiron(III) intermediate (see Supplementary material S5). The role
An efficient iron-catalyzed oxidative esterification of allylic sp3 C–H bonds with carboxylic acids is realized using well-defined ionic iron(III) complex as a catalyst and DTBP as an oxidant. The present reaction is suitable for a wide range of carboxylic acids and derivatives of cyclohexene with outstanding tolerance to sterically hindered substrates and good compatibility to various functional groups. As these ionic iron(III) complexes containing an 1,3dihydrocarbylimidazolinium cation are easily prepared as well as stable in air, this work provides a new and general synthetic tool for the synthesis of allylic esters with easy-to-hand or on large scale. Further studies on the detailed reaction mechanism and other direct functionalization of sp3 C–H bonds are currently underway in our laboratory. Acknowledgements This project was supported by the National Natural Science Foundation of China (Grant No. 21172164 and 21472134), the Key Laboratory of Organic Chemistry of Jiangsu Province, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2017.05. 039. References 1. (a) Quang DN, Hashimoto T, Stadler M, Asakawa Y. J Nat Prod. 2004;67:1152; (b) Ankisetty S, ElSohly HN, Li X-C, et al. J Nat Prod. 2006;69:692; (c) Covell DJ, Vermeulen NA, Labenz NA, White MC. Angew Chem Int Ed. 2006;45:8217; (d) Saito T, Fuwa H, Sasaki M. Org Lett. 2009;11:5274; (e) Jogalekar AS, Kriel FH, Shi Q, Cornett B, Cicero D, Snyder JP. J Med Chem. 2010;53:155; (f) Chang C-L, Zhang L-J, Chen RY, et al. J Nat Prod. 2010;73:1482. 2. (a) Nicolaou KC, Winssinger N, Hughes R, Smethurst C, Cho SY. Angew Chem Int Ed. 2000;39:1084; (b) Stanley LM, Bai C, Ueda M, Hartwig JF. J Am Chem Soc. 2010;132:8918. 3. (a) Xiang J, Orita A, Otera J. Angew Chem Int Ed. 2002;41:4117; (b) Mohan KVVK, Narender N, Kulkarni SJ. Green Chem. 2006;8:368; (c) Meyer ME, Ferreira EM, Stoltz BM. Chem Commun. 2006;1316; (d) Zhang L, Luo Y, Fan R, Wu J. Green Chem. 2007;9:1022;
2494
4.
5.
6.
7. 8. 9. 10.
11.
B. Lu et al. / Tetrahedron Letters 58 (2017) 2490–2494
(e) Otera J. Esterification: methods, reactions, and applications. Weinheim: WileyVCH; 2010. (a) Giri R, Shi B-F, Engle KM, Maugel N, Yu J-Q. Chem Soc Rev. 2009;38:3242; (b) Li H, Li B-J, Shi Z-J. Catal Sci Technol. 2011;1:191; (c) Baudoin O. Chem Soc Rev. 2011;40:4902; Girard SA, Knauber T, Li C-J. Angew Chem Int Ed. 2014;53:74; (e) Noisier AFM, Brimbl MA. Chem Rev. 2014;114:8775; (f) Li C-J. From C-H to C-C bonds: cross-dehydrogenative-coupling. Cambridge: The Royal Society of Chemistry; 2014; (g) Liu C, Liu D, Lei A. Acc Chem Res. 2014;47:3459. (a) Chen MS, White MC. J Am Chem Soc. 2004;126:1346; (b) Chen MS, Prabagaran N, Labenz NA, White MC. J Am Chem Soc. 2005;127:6970; (c) Mitsudome T, Umetani T, Nosaka N, et al. Angew Chem Int Ed. 2006;45:481; (d) Covell DJ, White MC. Angew Chem Int Ed. 2008;47:6448; (e) Campbell AN, White PB, Guzei IA, Stahl SS. J Am Chem Soc. 2010;132:15116; (f) Vermeulen NA, Delcamp JH, White MC. J Am Chem Soc. 2010;132:11323; (g) Thiery E, Aouf C, Belloy J, Harakat D, Bras JL, Muzart J. J Org Chem. 2010;75:1771; (h) Henderson WH, Check CT, Proust N, Stambuli JP. Org Lett. 2010;12:824; (i) Gormisky PE, White MC. J Am Chem Soc. 2011;133:12584; (j) Stang EM, White MC. Angew Chem Int Ed. 2011;50:2094; (k) Takenaka K, Akita M, Tanigaki Y, Takizawa S, Sasai H. Org Lett. 2011;13:3506; (l) Kondo H, Yu F, Yamaguchi J, Liu G, Itami K. Org Lett. 2014;16:4212. (a) Kharasch MS, Sosnovsky G, Yang NC. J Am Chem Soc. 1959;81:5819; (b) Eames J, Watkinson M. Angew Chem Int Ed. 2001;40:3567; (c) Andrus MB, Lashley JC. Tetrahedron. 2002;58:845; (d) Marín-Barrios R, Guerra FM, García-Cabeza AL, Moreno-Dorado FJ, Massanet GM. Tetrahedron. 2012;68:1105. García-Cabeza AL, Marín-Barrios R, Moreno-Dorado FJ, Ortega MJ, Massanet FM, Guerra FM. Org Lett. 2014;16:1598. Shi E, Shao Y, Chen S, et al. Org Lett. 2012;14:3384. (a) Sun C-L, Li B-J, Shi Z-J. Chem Rev. 2011;111:1293; (b) Jia F, Li Z. Org. Chem. Front.. 2014;1:194. (a) Bolm C, Legros J, Paih JL, Zani L. Chem Rev. 2004;104:6217; (b) Sherry BD, Fürstner A. Acc Chem Res. 2008;41:1500; (c) Czaplik WM, Mayer M, Cvengroš J, Wangelin AJV. ChemSusChem. 2009;2:396; (d) Bauer I, Knölker H-J. Chem Rev. 2015;115:3170. (a) For selected examples of iron-catalyzed direct functionalization of sp3 C-H bonds adjacent to an oxygen atom: Li Z, Yu R, Li H. Angew Chem Int Ed. 2008;47:7497; (b) Guo X, Pan S, Liu J, Li Z. J Org Chem. 2009;74:8848; (c) Pan S, Liu J, Li H, Wang Z, Guo X, Li Z. Org Lett. 2010;12:1932; (d) Wei W-T, Zhou M-B, Fan J-H, et al. Angew Chem Int Ed. 2013;52:3638; (e) Zhao J, Fang H, Zhou W, Han J, Pan Y. J Org Chem. 2014;79:3847; (f) . For selected examples of iron-catalyzed direct functionalization of sp3 C-H bonds adjacent to a nitrogen atom:Volla CMR, Vogel P. Org Lett. 2009;11:1701; (g) Han W, Ofial AR. Chem Commun. 2009;6023; (h) Liu P, Zhou C-Y, Xiang S, Che C-M. Chem Commun. 2010;46:2739; (i) Ratnikov MO, Xu X, Doyle MP. J Am Chem Soc. 2013;135:9475.
12. (a) Li Z, Cao L, Li C-J. Angew Chem Int Ed. 2007;46:6505; (b) Wang Z, Zhang Y, Fu H, Jiang Y, Zhao Y. Org Lett. 2008;10:1863; (c) Li Y-Z, Li B-J, Lu X-Y, Lin S, Shi Z-J. Angew Chem Int Ed. 2009;48:3817; (d) Song C-X, Cai G-X, Farrell TR, et al. Chem Commun. 2009;6002; (e) Xia Q, Chen W, Qiu H. J Org Chem. 2011;76:7577; (f) Pan S, Liu J, Li Y, Li Z. Chin Sci Bull. 2012;57:2382; (g) Liu X, Chen Y, Li K, Wang D, Chen B, Chin J. Chem.. 2012;30:2285; (h) Cheng Y, Dong W, Wang L, Parthasarathy K, Bolm C. Org Lett. 2014;16:2000; (i) He Y, Mao J, Rong G, Yan H, Zhang G. Adv Synth Catal. 2015;357:2125; (j) Guo S, Li Y, Wang Y, Guo X, Meng X, Chen B. Adv Synth Catal. 2015;357:950; (k) Guo L-N, Wang S, Duan X-H, Zhou S-L. Chem Commun. 2015;51:4803; (l) Yang K, Song Q. Org Lett. 2015;17:548; (m) Shi J-L, Zhang J-C, Wang B-Q, Hu P, Zhao K-Q, Shi Z-J. Org Lett. 2016;18:1238. 13. Qin C, Jiao N. J Am Chem Soc. 2010;132:15893. 14. Paradine SM, White MC. J Am Chem Soc. 2012;134:2036. 15. (a) Yan CH, Wang LL, Gao HH, Sun HM, Shen Q. Chin Sci Bull. 2012;57:1953; (b) Deng H-N, Xing Y-L, Xia C-L, Sun H-M, Shen Q, Zhang Y. Dalton Trans. 2012;41:11597; (c) Xia Y, Yan CH, Li Z, et al. Chin Sci Bull. 2013;58:493; (d) Wang LL, Lu B, Zhu AD, Sun HM, Shen Q. Chin Sci Bull. 2013;58:3624; (e) Xia C-L, Xie C-F, Wu Y-F, Sun H-M, Shen Q, Zhang Y. Org Biomol Chem. 2013;11:8135; (f) Zhang Q, Li Z, Sun H, Shen Q. Chin Sci Bull. 2014;59:4728; (g) Zhu F, Lu B, Sun H, Shen Q. Tetrahedron Lett. 2016;57:4152; (h) Lu B, Zhu F, Sun H-M, Shen Q. Org Lett. 2017;19:1132. 16. (a) Naumann S, Schmidt FG, Schowner R, Frey W, Buchmeiser MR. Polym Chem. 2013;4:2731; (b) Iglesias M, Beetstra DJ, Knight JC, et al. Organometallics. 2008;27:3279. 17. (a) Hatakeyama T, Hashimoto S, Ishizuka K, Nakamura M. J Am Chem Soc. 2009;131:11949; (b) Rosa JN, Reddy RS, Candeias NR, Cal PMSD, Gois PMP. Org Lett. 2010;12:2686; (c) Reddy RS, Rosa JN, Veiros LF, Caddick S, Gois PMP. Org Biomol Chem. 2011;9:3126; (d) Chua Y-Y, Duong HA. Chem Commun. 2014;50:8424; (e) Chua Y-Y, Duong HA. Chem Commun. 2016;52:1466. _ C R Chim. 18. Demir S, Gköçe Y, Roisnel T, Sortais J-B, Darcel C, Özdemir I. 2014;17:541. 19. Arduengo III AJ, Goerlich JR, Marshall WJJ. Am Chem Soc. 1995;117:11027. 20. Bäcker T, Breunig O, Valldor M, Merz K, Vasylyeva V, Mudring A-V. Cryst Growth Des. 2011;11:2564. 21. Wyrzykowski D, Kruszyn´ski R, Kłak J, Mrozin´ski J, Warnke Z. Inorg Chim Acta. 2008;361:262. 22. Ye S, Yang W, Coon T, et al. Chem Eur J. 2016;22:4748. 23. Majji G, Rout SK, Rajamanickam S, Guin S, Patel BK. Org Biomol Chem. 2016;14:8178. 24. (a) Waetzig SR, Tunge JA. J Am Chem Soc. 2007;129:4138; (b) Chen L, Bruneau C, Dixneuf PH, Doucet H. Green Chem. 2012;14:1111. 25. (a) Neuse EW, Meirim MG. Transition Met Chem. 1984;9:205; (b) Marlin DS, Olmstead MM, Mascharak PK. Inorg Chem. 2003;42:1681.
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Iron-catalyzed esterification of allylic sp3 C–H bonds with carboxylic acids: Facile access to allylic esters Bing Lu, Fan Zhu, Dan Wang, Hongmei Sun ⇑, Qi Shen The Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
a r t i c l e
i n f o
Article history: Received 10 April 2017 Revised 10 May 2017 Accepted 12 May 2017 Available online 12 May 2017 Keywords: Iron(III) complex Imidazolinium salt Oxidative cross-coupling C–H bond functionalization Allylic ester
a b s t r a c t The first general and efficient iron-catalyzed esterification of allylic sp3 C–H bonds with carboxylic acids using ionic iron(III) complexes (1–4) as a catalyst and DTBP (DTBP = di-tert-butyl peroxide) as an oxidant is achieved. A variety of allylic esters were synthesized in good to excellent yields using the ionic iron(III) complex 2 as a catalyst in a 5 mol% loading. This reaction is characterized by its high efficiency, broad substrate scope with excellent steric hindrance tolerance and good functional group compatibility. Ó 2017 Published by Elsevier Ltd.
Introduction Allylic esters are of growing interest in organic synthesis, as they are important structural units in bioactive and medical molecules1 as well as fine chemicals.2 They have been traditionally synthesized by the reaction of acids or their derivatives (acyl chlorides and anhydrides) with the corresponding allylic alcohols.3 However, the limited substrate scope and harsh reaction conditions restrict their applications. During the past decade, the direct functionalization of sp3 C–H bonds through oxidative cross-coupling has emerged as one of the most important strategies for the development of new synthetic methodologies in organic synthesis from green and sustainable points of view.4 For example, oxidative esterification of allylic sp3 C–H bonds has shown remarkable potential as an alternative to traditional protocols for the synthesis of allylic esters. In this context, several types of transition metals-based catalytic systems, including palladium,5 copper,6 and copper–aluminum mixed oxide,7 have been developed for the esterification of allylic sp3 C–H bonds. Also, a metal-free esterification of allylic sp3 C–H bonds with carboxylic acids using Bu4NI as a catalyst has been developed.8 Nevertheless, there remains ample opportunity for improving such transformation into a routine tool for synthesis of allylic ester in the laboratory and on an industrial scale.
⇑ Corresponding author. E-mail address: [email protected] (H. Sun). http://dx.doi.org/10.1016/j.tetlet.2017.05.039 0040-4039/Ó 2017 Published by Elsevier Ltd.
Over the past decade, significant efforts have been devoted to the development of iron-catalyzed functionalization of sp3 C–H bonds9 because iron is inexpensive, nontoxic, and environmentally benign.10 In this context, a large number of studies focused on the transformation of sp3 C–H bonds adjacent to a heteroatom11 or benzylic sp3 C–H bonds12 have made significant progress. However, iron-catalyzed functionalization of allylic sp3 C–H bonds is still highly desired but challenging, possibly owing to the difficulty of selective functionalization of allylic sp3 C–H bond without an active group (i.e., a heteroatom and/or an aromatic ring).4f In 2010, the Jiao group reported the first example of an FeCl2-catalyzed cyanation of allylic sp3 C–H bonds via the oxidative cross-coupling of allylarenes with Me3SiN3 using DDQ (DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone) as an oxidant.13 Later, the White group described the first example of intramolecular amination of allylic sp3 C–H bonds to sulfamide using [FePc]Cl (Pc = phthalocyaninato) as a catalyst.14 However, in terms of iron-catalyzed esterification of allylic sp3 C–H bonds to the synthesis of allylic esters, only the Pan group mentioned one example of the esterification of cyclohexene with thiophene2-carboxylic acid in a 45% yield catalyzed by Fe(acac)3 at a 20 mol% loading.11e Therefore, the development of a general methodology for iron-catalyzed oxidative esterification of allylic sp3 C–H bonds to the synthesis of allylic esters is highly desired. We recently reported the synthesis of a series of ionic iron(III) complexes containing an imidazolinium cation (Scheme 1) and their catalytic potential in the esterification of benzylic sp3 C–H bonds with carboxylic acids using di-tert-butyl peroxide (DTBP)
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Scheme 1. Ionic iron(III) complexes containing an imidazolinium cation.
as an oxidant.15h Herein, we continue our studies15 by developing the synthetic route of such kind of ionic iron(III) complexes and their application in the esterification of allylic sp3 C–H bonds with carboxylic acids. Results and discussion In our previous work, complexes 2–4 were synthesized via the reaction of FeBr3 with the corresponding 1,3-dihydrocarbylimidazolinium bromide.15h The present investigation reveals that these iron(III) bromides could be directly synthesized using 1,3dihydrocarbylimidazolinium chlorides as starting materials. As shown in Scheme 2, the reaction of FeBr3 with 1 equiv. of 1,3-dihydrocarbylimidazolinium chloride in the presence of NaBr afforded the desired iron(III) bromides 2–4 as air-stable brown-red crystals in yields ranging from 87% to 90%, which are very similar to the data reported previously.15h This protocol avoids the employment of the corresponding 1,3dihydrocarbylimidazolinium bromides, which were generally synthesized from more expensive alkyl bromides or more corrosive hydrobromide as compared with their chloride derivatives.16 Complexes 2–4 were characterized by elemental analysis, Raman spectroscopy and electrospray ionization mass spectroscopy. The characterization results agreed well with those reported previously.15h The solid-state structures of complexes 3 and 4 were further confirmed by X-ray crystallography. Single crystals suitable for X-ray diffraction studies were grown from cold THF/hexane. The crystallographic data are listed in Table S1 (see the ESIy). Their molecular structures are depicted in Figs. 1–2, with selected bond lengths and angles given in the captions. It is worth noting that the molecular structures of this type of ionic iron(III) complexes have been seldom reported, even if the application of imidazolinium salts as ligands in iron-based catalytic systems has received increasing attention during the past decade.17 To date, only two scientific articles have described structural data for imidazolinium cation-containing ionic iron(III) complexes15h,18 As seen in Figs. 1 and 2, each of the two solid-state structures contained one 1,3-dihydrocarbylimidazolinium cation and one [FeBr4] anion. The structure of the 1,3-dihydrocarbylimidazolinium cation presented either in 3 or 4 changed little after the reaction of the corresponding 1,3-dihydrocarbylimidazolinium salt with FeBr3.19 In both [FeBr4] anions, each iron atom is coordinated by four bromine atoms in a slightly distorted tetrahedral geometry with a mean Br–Fe–Br angle of 109.5° for both of 3 and 4, which is close to the ideal tetrahedral angle of 50°.20 The Fe–Br bond lengths were found to lie in the ranges of 2.301(11)–2.341(12) Å, wherein the mean Fe–Br bond length was 2.333 Å in 3 and 2.308
Scheme 2. Synthesis of ionic iron(III) complexes 2–4.
Fig. 1. Molecular structure of complex 3 with thermal ellipsoids at the 30% probability level. Hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (°): Fe(1)–Br(1) 2.324(12), Fe(1)–Br(2) 2.337(12), Fe(1)–Br(3) 2.327(12), Fe (1)–Br(4) 2.341(12); Br(1)–Fe(1)–Br(2) 108.78(5), Br(1)–Fe(1)–Br(3) 110.86(5), Br (1)–Fe(1)–Br(4) 108.97(5), Br(2)–Fe(1)–Br(3) 109.56(5), Br(2)–Fe(1)–Br(4) 108.44 (5), Br(3)–Fe(1)–Br(4) 110.18(5).
Fig. 2. Molecular structure of complex 4 with thermal ellipsoids at the 30% probability level. Hydrogens and solvent are omitted for clarity. Selected bond lengths (Å) and angles (°): Fe(1)–Br(1) 2.301(11), Fe(1)–Br(2) 2.311(13), Fe(1)–Br(3) 2.319(13), Fe(1)–Br(4) 2.301(14); Br(1)–Fe(1)–Br(2) 108.47(5), Br(1)–Fe(1)–Br(3) 109.02(5), Br(1)–Fe(1)–Br(4) 108.33(5), Br(2)–Fe(1)–Br(3) 109.09(6), Br(2)–Fe(1)– Br(4) 110.74(6), Br(3)–Fe(1)–Br(4) 111.14(5).
Å in 4. These values were in the expected range as found for other imidazolinium cation-containing ionic iron(III) complexes reported previously15h,18 We began our investigation with the oxidative cross-coupling reaction of 4-methoxybenzoic acid (5a) and cyclohexene (6a) with catalysts 1–4 and related iron(III) salts. The results are summarized in Table 1. Under the optimized conditions, there were obviously different catalytic activities for these iron(III)-based catalysts. For example, complex 2 exhibited the highest activity and gave the desired product 7a in 95% yield at a 5 mol% loading (entry 2). The desired product 7a was still obtained in high yield of 81% even when the loading of 2 was reduced to 3 mol%. In comparison, com-
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Table 1 Screening of optimal conditions.a
Table 2 Esterification of various carboxylic acids with cyclohexene.a
Entry
Catalyst
Oxidant
Yield (%)
1 2 3 4 5 6 7 8 9 10 11
1 2 2 2 2 3 4 [Me4N][FeBr4]f FeBr3 [HSItBu]Brg + FeBr3 [HSItBu]Br
DTBP DTBP DTBP TBHPd DDQe DTBP DTBP DTBP DTBP DTBP DTBP
32 95(88b) 81c 0 0 73 84 28 16 88 0
a Conditions: 4-methoxybenzoic acid (0.5 mmol), cyclohexene (7.5 mmol), catalyst (5 mol%), oxidant (1.0 mol), EtOAc (0.5 mL), 110 °C, 24 h, GC yield using nhexadecane as internal standard, an average of two runs. b Isolated yield. c 2 (3 mol%). d TBHP = tert-butyl hydroperoxide. e DDQ = 2,3-dicyano-5,6-dichlorobenzoquinone. f [Me4N] = tetramethylammonium cation (Ref. 21). g [HSItBu]Br = 1,3-di-tert-butylimidazolinium bromide.
plexes 3 and 4 showed a relatively lower activity, affording the desired product in yields of 73% (entry 6) and 84% (entry 7), respectively. The chloride analogue (1) showed very poor activity, giving 7a in 32% yield (entry 1). Notably, an iron(III) complex of [Me4N] [FeBr4], which containing a tetramethylammonium cation, also exhibited very poor activity (entry 8). As expected, FeBr3 alone gave 7a in merely 16% yield (entry 9). A mixture of FeBr3 and 1,3-di-tert-butylimidazolinium bromide ([HSItBu]Br) in a 1:1 M ratio gave the desired product 7a in 88% yield (entry 10), and no 7a was detected when [HSItBu]Br was employed alone (entry 11). Oxidant screening disclosed that TBHP and DDQ showed no activity (entries 4 and 5). To date, 10 mol% or higher loadings of other transition metalbased catalysts5–7 or metal-free catalyst8 are usually required to achieve satisfactory yields for such kind of transformation. The present results indicates that complex 2 might be among the most efficient catalyst for the oxidative esterification of allylic sp3 C–H bonds to the synthesis of allylic esters, and the presence of a sterically bulky imidazolinium cation with electron-donating N-substituents is of benefit to its high catalytic activity.22 Encouraged by the above results, we explored the substrate scope of this iron-catalyzed esterification of allylic C–H bonds with carboxylic acids to obtain allylic esters. Firstly, a series of carboxylic acids were investigated, as shown in Table 2. Both electron-donating and electron-withdrawing aryl carboxylic acids were successfully coupled with cyclohexene to form corresponding allylic esters in moderate to excellent yields (7b–7n). Interestingly, steric hindrance was well tolerated in this reaction, as the highly hindered 2,4,6-triiso-propylbenzoic acid was converted to corresponding ester in a 91% yield without enhanced reaction conditions (7i). To date, such highly hindered carboxylic acids have rarely been explored as substrates in other sp3 C–H bond esterification reported previously.23 Another notable merit of this protocol is that a series of functional groups can been tolerated in our oxidative conditions, including cyano (7m), nitro (7n), hydroxyl (7o), and ester (7p). Additionally, biphenyl 4-carboxylic acid (7p), biphenyl 2-carboxylic acid (7r), 1-naphthoic acid (7s) and 2-naphthoic acid (7t) gave the desired products in 95%-98% yields. Notably, furan-2-carboxylic acid (7u) and thiophene-2-carboxylic acid
a Conditions: carboxylic acids (0.5 mmol), cyclohexene (7.5 mmol), 2 (5 mol%), EtOAc (0.5 mL), DTBP (1.0 mmol), 110 °C, 24 h, isolated yield.
(7v) were also excellent coupling partners under the optimized conditions. Such kind of heteroaromatic carboxylates have been widely used in organic synthesis,24 however, they were usually less employed as substrates or showed low activities in previously reported studies.8 Several kinds of aliphatic carboxylic acids were also found to be compatible with this esterification protocol, furnishing the corresponding ester products (7w–7y) in 67%–76% yields. We also investigated the substrate scope of various alkenes to further explore the potential of the present protocol. As shown in
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Table 3 Esterification of benzoic acid with various alkenes.a
Scheme 3. Investigation of reaction mechanism.
of an imidazolinium cation should be of benefit to form the diiron(III) intermediate.25 Conclusions a Conditions: benzoic acid (0.5 mmol), alkene (7.5 mmol), 2 (5 mol%), EtOAc (0.5 mL), DTBP (1.0 mmol), 110¯ C, 24 h, isolated yield. b 2 (10 mol%), allylarenes (1.5 mL), no solvent, 16 h. c The reaction was too complicated and no obvious amount of the desired product was obtained.
Table 3, cyclopentene reacted well with benzoic acid (8a), while cyclooctene gave the desired product in a lower yield under the same reaction conditions (8b). Notably, 2,3-dimethylbut-2-ene exhibited very high selectivity, affording merely the mono-esterification product 8c in a yield of 90%. Interestingly, a very high regioselectivity is also observed in our protocol when allylbenzene and allylnaphthalene were employed, wherein linear allylic ester was isolated in yields of 86% (8d) and 82% (8e), respectively. This phenomenon is quite different from that observed with palladium-based catalysts, wherein p-allyl-Pd(II) intermediates could lead to a L/B (L = linear product, B = branched product) regioselectivity. To date, a control regioselectivity to the linear isomer is still highly desired in the field of palladium-catalyzed esterification of allylic sp3 C–H bond.5 Somewhat to our surprise, methylenecyclohexene failed to participate in this esterification reaction (8f). Finally, a simple conjugated diene was tested under optimized conditions, however, the reaction was too complicated and no obvious amount of desired product 8g could be detected, similar to that reported previously.13 Although an exact mechanism of the present reaction requires further investigation, our current experimental studies suggest that this transformation may involve radical species because the reaction was completely stopped by the addition of TEMPO as a radical inhibitor (Scheme 3, eqn. 1). Cyclohex-2-enol was not detected during the reaction (Scheme 3, eqn. 2), which suggests that the present esterification reaction possibly occurred without an alcohol intermediate. When tert-butyl perester was used as a reactant, the allylic ester 7b was obtained only in 31% yield (Scheme 3, eqn. 3), indicating that this reaction may not proceed through a tert-butyl perester intermediate. Recently, Pan11e group proposed that a l-oxo, l-carboxylate diiron(III) intermediate might be formed in iron-catalyzed esterification of sp3 C–H bonds adjacent an oxygen atom. We suggest that the present esterification reaction possibly also proceeded via a l-oxo, l-carboxylate diiron(III) intermediate (see Supplementary material S5). The role
An efficient iron-catalyzed oxidative esterification of allylic sp3 C–H bonds with carboxylic acids is realized using well-defined ionic iron(III) complex as a catalyst and DTBP as an oxidant. The present reaction is suitable for a wide range of carboxylic acids and derivatives of cyclohexene with outstanding tolerance to sterically hindered substrates and good compatibility to various functional groups. As these ionic iron(III) complexes containing an 1,3dihydrocarbylimidazolinium cation are easily prepared as well as stable in air, this work provides a new and general synthetic tool for the synthesis of allylic esters with easy-to-hand or on large scale. Further studies on the detailed reaction mechanism and other direct functionalization of sp3 C–H bonds are currently underway in our laboratory. Acknowledgements This project was supported by the National Natural Science Foundation of China (Grant No. 21172164 and 21472134), the Key Laboratory of Organic Chemistry of Jiangsu Province, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2017.05. 039. References 1. (a) Quang DN, Hashimoto T, Stadler M, Asakawa Y. J Nat Prod. 2004;67:1152; (b) Ankisetty S, ElSohly HN, Li X-C, et al. J Nat Prod. 2006;69:692; (c) Covell DJ, Vermeulen NA, Labenz NA, White MC. Angew Chem Int Ed. 2006;45:8217; (d) Saito T, Fuwa H, Sasaki M. Org Lett. 2009;11:5274; (e) Jogalekar AS, Kriel FH, Shi Q, Cornett B, Cicero D, Snyder JP. J Med Chem. 2010;53:155; (f) Chang C-L, Zhang L-J, Chen RY, et al. J Nat Prod. 2010;73:1482. 2. (a) Nicolaou KC, Winssinger N, Hughes R, Smethurst C, Cho SY. Angew Chem Int Ed. 2000;39:1084; (b) Stanley LM, Bai C, Ueda M, Hartwig JF. J Am Chem Soc. 2010;132:8918. 3. (a) Xiang J, Orita A, Otera J. Angew Chem Int Ed. 2002;41:4117; (b) Mohan KVVK, Narender N, Kulkarni SJ. Green Chem. 2006;8:368; (c) Meyer ME, Ferreira EM, Stoltz BM. Chem Commun. 2006;1316; (d) Zhang L, Luo Y, Fan R, Wu J. Green Chem. 2007;9:1022;
2494
4.
5.
6.
7. 8. 9. 10.
11.
B. Lu et al. / Tetrahedron Letters 58 (2017) 2490–2494
(e) Otera J. Esterification: methods, reactions, and applications. Weinheim: WileyVCH; 2010. (a) Giri R, Shi B-F, Engle KM, Maugel N, Yu J-Q. Chem Soc Rev. 2009;38:3242; (b) Li H, Li B-J, Shi Z-J. Catal Sci Technol. 2011;1:191; (c) Baudoin O. Chem Soc Rev. 2011;40:4902; Girard SA, Knauber T, Li C-J. Angew Chem Int Ed. 2014;53:74; (e) Noisier AFM, Brimbl MA. Chem Rev. 2014;114:8775; (f) Li C-J. From C-H to C-C bonds: cross-dehydrogenative-coupling. Cambridge: The Royal Society of Chemistry; 2014; (g) Liu C, Liu D, Lei A. Acc Chem Res. 2014;47:3459. (a) Chen MS, White MC. J Am Chem Soc. 2004;126:1346; (b) Chen MS, Prabagaran N, Labenz NA, White MC. J Am Chem Soc. 2005;127:6970; (c) Mitsudome T, Umetani T, Nosaka N, et al. Angew Chem Int Ed. 2006;45:481; (d) Covell DJ, White MC. Angew Chem Int Ed. 2008;47:6448; (e) Campbell AN, White PB, Guzei IA, Stahl SS. J Am Chem Soc. 2010;132:15116; (f) Vermeulen NA, Delcamp JH, White MC. J Am Chem Soc. 2010;132:11323; (g) Thiery E, Aouf C, Belloy J, Harakat D, Bras JL, Muzart J. J Org Chem. 2010;75:1771; (h) Henderson WH, Check CT, Proust N, Stambuli JP. Org Lett. 2010;12:824; (i) Gormisky PE, White MC. J Am Chem Soc. 2011;133:12584; (j) Stang EM, White MC. Angew Chem Int Ed. 2011;50:2094; (k) Takenaka K, Akita M, Tanigaki Y, Takizawa S, Sasai H. Org Lett. 2011;13:3506; (l) Kondo H, Yu F, Yamaguchi J, Liu G, Itami K. Org Lett. 2014;16:4212. (a) Kharasch MS, Sosnovsky G, Yang NC. J Am Chem Soc. 1959;81:5819; (b) Eames J, Watkinson M. Angew Chem Int Ed. 2001;40:3567; (c) Andrus MB, Lashley JC. Tetrahedron. 2002;58:845; (d) Marín-Barrios R, Guerra FM, García-Cabeza AL, Moreno-Dorado FJ, Massanet GM. Tetrahedron. 2012;68:1105. García-Cabeza AL, Marín-Barrios R, Moreno-Dorado FJ, Ortega MJ, Massanet FM, Guerra FM. Org Lett. 2014;16:1598. Shi E, Shao Y, Chen S, et al. Org Lett. 2012;14:3384. (a) Sun C-L, Li B-J, Shi Z-J. Chem Rev. 2011;111:1293; (b) Jia F, Li Z. Org. Chem. Front.. 2014;1:194. (a) Bolm C, Legros J, Paih JL, Zani L. Chem Rev. 2004;104:6217; (b) Sherry BD, Fürstner A. Acc Chem Res. 2008;41:1500; (c) Czaplik WM, Mayer M, Cvengroš J, Wangelin AJV. ChemSusChem. 2009;2:396; (d) Bauer I, Knölker H-J. Chem Rev. 2015;115:3170. (a) For selected examples of iron-catalyzed direct functionalization of sp3 C-H bonds adjacent to an oxygen atom: Li Z, Yu R, Li H. Angew Chem Int Ed. 2008;47:7497; (b) Guo X, Pan S, Liu J, Li Z. J Org Chem. 2009;74:8848; (c) Pan S, Liu J, Li H, Wang Z, Guo X, Li Z. Org Lett. 2010;12:1932; (d) Wei W-T, Zhou M-B, Fan J-H, et al. Angew Chem Int Ed. 2013;52:3638; (e) Zhao J, Fang H, Zhou W, Han J, Pan Y. J Org Chem. 2014;79:3847; (f) . For selected examples of iron-catalyzed direct functionalization of sp3 C-H bonds adjacent to a nitrogen atom:Volla CMR, Vogel P. Org Lett. 2009;11:1701; (g) Han W, Ofial AR. Chem Commun. 2009;6023; (h) Liu P, Zhou C-Y, Xiang S, Che C-M. Chem Commun. 2010;46:2739; (i) Ratnikov MO, Xu X, Doyle MP. J Am Chem Soc. 2013;135:9475.
12. (a) Li Z, Cao L, Li C-J. Angew Chem Int Ed. 2007;46:6505; (b) Wang Z, Zhang Y, Fu H, Jiang Y, Zhao Y. Org Lett. 2008;10:1863; (c) Li Y-Z, Li B-J, Lu X-Y, Lin S, Shi Z-J. Angew Chem Int Ed. 2009;48:3817; (d) Song C-X, Cai G-X, Farrell TR, et al. Chem Commun. 2009;6002; (e) Xia Q, Chen W, Qiu H. J Org Chem. 2011;76:7577; (f) Pan S, Liu J, Li Y, Li Z. Chin Sci Bull. 2012;57:2382; (g) Liu X, Chen Y, Li K, Wang D, Chen B, Chin J. Chem.. 2012;30:2285; (h) Cheng Y, Dong W, Wang L, Parthasarathy K, Bolm C. Org Lett. 2014;16:2000; (i) He Y, Mao J, Rong G, Yan H, Zhang G. Adv Synth Catal. 2015;357:2125; (j) Guo S, Li Y, Wang Y, Guo X, Meng X, Chen B. Adv Synth Catal. 2015;357:950; (k) Guo L-N, Wang S, Duan X-H, Zhou S-L. Chem Commun. 2015;51:4803; (l) Yang K, Song Q. Org Lett. 2015;17:548; (m) Shi J-L, Zhang J-C, Wang B-Q, Hu P, Zhao K-Q, Shi Z-J. Org Lett. 2016;18:1238. 13. Qin C, Jiao N. J Am Chem Soc. 2010;132:15893. 14. Paradine SM, White MC. J Am Chem Soc. 2012;134:2036. 15. (a) Yan CH, Wang LL, Gao HH, Sun HM, Shen Q. Chin Sci Bull. 2012;57:1953; (b) Deng H-N, Xing Y-L, Xia C-L, Sun H-M, Shen Q, Zhang Y. Dalton Trans. 2012;41:11597; (c) Xia Y, Yan CH, Li Z, et al. Chin Sci Bull. 2013;58:493; (d) Wang LL, Lu B, Zhu AD, Sun HM, Shen Q. Chin Sci Bull. 2013;58:3624; (e) Xia C-L, Xie C-F, Wu Y-F, Sun H-M, Shen Q, Zhang Y. Org Biomol Chem. 2013;11:8135; (f) Zhang Q, Li Z, Sun H, Shen Q. Chin Sci Bull. 2014;59:4728; (g) Zhu F, Lu B, Sun H, Shen Q. Tetrahedron Lett. 2016;57:4152; (h) Lu B, Zhu F, Sun H-M, Shen Q. Org Lett. 2017;19:1132. 16. (a) Naumann S, Schmidt FG, Schowner R, Frey W, Buchmeiser MR. Polym Chem. 2013;4:2731; (b) Iglesias M, Beetstra DJ, Knight JC, et al. Organometallics. 2008;27:3279. 17. (a) Hatakeyama T, Hashimoto S, Ishizuka K, Nakamura M. J Am Chem Soc. 2009;131:11949; (b) Rosa JN, Reddy RS, Candeias NR, Cal PMSD, Gois PMP. Org Lett. 2010;12:2686; (c) Reddy RS, Rosa JN, Veiros LF, Caddick S, Gois PMP. Org Biomol Chem. 2011;9:3126; (d) Chua Y-Y, Duong HA. Chem Commun. 2014;50:8424; (e) Chua Y-Y, Duong HA. Chem Commun. 2016;52:1466. _ C R Chim. 18. Demir S, Gköçe Y, Roisnel T, Sortais J-B, Darcel C, Özdemir I. 2014;17:541. 19. Arduengo III AJ, Goerlich JR, Marshall WJJ. Am Chem Soc. 1995;117:11027. 20. Bäcker T, Breunig O, Valldor M, Merz K, Vasylyeva V, Mudring A-V. Cryst Growth Des. 2011;11:2564. 21. Wyrzykowski D, Kruszyn´ski R, Kłak J, Mrozin´ski J, Warnke Z. Inorg Chim Acta. 2008;361:262. 22. Ye S, Yang W, Coon T, et al. Chem Eur J. 2016;22:4748. 23. Majji G, Rout SK, Rajamanickam S, Guin S, Patel BK. Org Biomol Chem. 2016;14:8178. 24. (a) Waetzig SR, Tunge JA. J Am Chem Soc. 2007;129:4138; (b) Chen L, Bruneau C, Dixneuf PH, Doucet H. Green Chem. 2012;14:1111. 25. (a) Neuse EW, Meirim MG. Transition Met Chem. 1984;9:205; (b) Marlin DS, Olmstead MM, Mascharak PK. Inorg Chem. 2003;42:1681.