Sodium Iodidecatalyzed Direct Alkoxylation of Ketones With

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DOI: 10.1002/adsc.201500006 Sodium Iodide-Catalyzed Direct a-Alkoxylation of Ketones with Alcohols via Oxidation of a-Iodo Ketone Intermediates

Cuiju Zhu,a Yuanfei Zhang,a Huaiqing Zhao,a Shijun Huang,a Min Zhang,a,* and Weiping Sua,* a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Yangqiao West Road 155, Fuzhou, Fujian 350002, Peoples Republic of China E-mail: [email protected] or [email protected]

Received: January 5, 2015; Published online: February 4, 2015

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500006.

Abstract: The direct a-alkoxylation of ketones with for the a-alkoxylation of carbonyl compounds is alcohols via a sodium iodide-catalyzed oxidative highly desirable. cross-coupling has been developed. This protocol Herein, we described an efficient method for the enables a range of alkyl aryl ketones to cross- construction of a-alkoxy ketones via direct oxidative couple with an array of alcohols in synthetically coupling of ketones with a broad range of alkyl and useful yields. The mechanistic studies provided solid benzyl alcohols as coupling partners using low-cost evidence supporting that an a-iodo ketone was sodium iodide as the catalyst. In principle, two cou- a key reaction intermediate, being converted into pling partners of the direct a-alkoxylation of ketones, an a-alkoxylated ketone via further oxidation to namely, the enol and the alcohol, are both nucleo- a hypervalent iodine species rather than a common philes and therefore electronically mismatched. This nucleophilic substitution, and was generated from kind of cross-coupling between two nucleophiles is the ketone starting material via a radical intermedi- conventionally realized through umpolung of one ate. These new mechanism insights should have an coupling component.[6] In this regard, MacMillan and effect on the design of iodide-catalyzed oxidative co-workers[6a] have recently achieved the Cu-catalyzed cross-coupling reactions between nucleophiles. direct a-amination of carbonyl compounds, and Loh [6b] et al. have discovered the I2-catalyzed a-amination Keywords: alcohols; a-alkoxylation; CÀO coupling; of aldehydes. These two reactions were proposed to hypervalent iodine; ketones proceed via a-halogenation of the carbonyl and sub- sequent nucleophilic substitution of the halide func- tionality by an amine. Metal-catalyzed oxidative cross-coupling has evolved into a powerful tool for

Carbonyl compounds with a-alkoxy substituents have found numerous applications either as building blocks or synthetic intermediates,[1] and are present in many pharmaceuticals and biologically active compounds (Scheme 1).[2] Their prevalence has promoted synthet- ic organic chemists to seek efficient methods for the construction of this kind of molecular framework. Traditionally, the installation of alkoxy substituents at the carbonyl a-position can be accomplished through multistep processes via synthetic intermediates such as silyl enol ethers,[3] enol acetates[4] or a-diazo ke- tones.[5] These methods suffer from the inconvenient Scheme 1. Three examples illustrating the importance of a- multiple oxidation procedures or rigorous control of alkoxy ketones: a) (+)-(aR)-a-methoxy-2’,4’-dihydroxydihy- the reaction conditions. Moreover, the substrate scope drochalcone, a natural product;[2a] b) 2-(benzyloxy)-1,2-di- of these methods is limited as the introduction of phenylethanone, a small molecule that can inhibit apolipo- a long-chain alkoxy group remains underdeveloped. protein E production;[2b] c) 2-methoxy-1,2-diphenyletha- Accordingly, a general and straightforward method none, a photopolymerization initiator.[2c]

Adv. Synth. Catal. 2015, 357, 331 – 338 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 331 Cuiju Zhu et al. COMMUNICATIONS forging new chemical bonds linking two nucleophilic Table 1. Optimization studies for the catalytic a-alkoxylation components.[7] For example, the amine-assisted Cu- of ketones.[a] catalyzed oxidative cross-coupling of aldehydes with alkenylboronic acids has been established for the enantioselective a-alkenylation of aldehydes.[8] On the other hand, hypervalent iodine compounds have proved to be versatile reagents for diverse oxi- dative transformations[9] including a-functionaliza- [10] Entry [I] Oxidant Additive Yield tions of carbonyl compounds. On the basis of the [b] advances in using hypervalent iodine reagents for or- (mol%) (equiv.) [%] ganic syntheses, aryl iodide- or iodide salt-catalyzed 1Bu4NI (20) TBHP TsOH (0.4) 0 oxidative transformations have recently been devel- 2Bu4NI (20) K2S2O8 TsOH (0.4) 0 oped, in which hypervalent iodine intermediates are 3Bu4NI (20) H2O2 TsOH (0.4) 0 [11] in-situ generated in catalytic processes. For in- 4Bu4NI (20) m-CPBA TsOH (0.4) 0 stance, Ochiai and Ishihara independently reported 5Bu4NI (20) m-CPBA BF3·Et2O (0.4) 0 aryl iodide- and Bu NI-catalyzed a-oxyacetylations of 6Bu4NI (20) m-CPBA NsOH (0.4) 36 4 7BuNI (20) m-CPBA NsOH (1.0) 70 ketones, respectively.[11c,f] Although a number of 4 8BuNI (20) m-CPBA NsOH (1.5) 85 methods have been established for the a-hydroxyl- 4 [12] 9 NaI (20) m-CPBA NsOH (1.5) 90 (86) ation of carbonyl compounds, the catalytic direct a- 10 KI (20) m-CPBA NsOH (1.5) 83 alkoxylation of carbonyl compounds with alcohols re- 11 LiI (20) m-CPBA NsOH (1.5) 70 mains a great challenge because of the undesired 12 PhI (20) m-CPBA NsOH (1.5) 60 Baeyer–Villiger oxidation of ketones to esters,[13] the 13 – m-CPBA NsOH (1.5) 0 oxidative consumption of alcohols,[14] the weak nucle- 14 NaI (100) – NsOH (1.5) 0 ophilicity of alcohols and the 1, 2-addition of alcohols [a] Reaction conditions: 1a (0.2 mmol), 2a (0.5 mL), iodide to the carbonyl C=O bond. An illustrative example catalyst, oxidant (2.5 equiv.), additive, CH3CN (2.0 mL), reported by Cheng and co-workers is the Bu4NI-cata- 808C, 24 h. TBHP =tert-butyl hydroperoxide, NsOH= lyzed reaction of ketones with benzylic alcohols using para-nitrobenzenesulfonic acid. TBHP as the oxidant that produced a-acyloxy ke- [b] GC yield using dodecane as an internal standard (values tones rather than a-alkoxy ketones.[15] Very recently, in parentheses refers to the isolated yield). Jiao and co-workers described the Cu-catalyzed aero- bic oxidative esterification of ketones with alcohols, further highlighting the complications of oxidative cross-coupling between ketones and alcohols.[16] To of 1a and activate 1a towards oxidation at the a-posi- date, the only example closely related to the direct a- tion with varyious oxidants such as TBHP, K2S2O8, alkoxylation of carbonyl compounds was reported by H2O2, m-CPBA, but the reaction did not give any de- Ishihara who achieved a quaternary ammonium sired product (entries 1–5). To our delight, employing iodide-catalyzed intramolecular a-phenolation of ke- 2.5 equiv. of m-CPBA as the oxidant, in combination tones.[17] To the best of our knowledge, our findings with 40 mol% para-nitrobenzenesulfonic acid represent the first example of a catalytic direct a-al- (NsOH) as the additive, led to formation of the de- koxylation of carbonyl compounds. Importantly, the sired product 3a in 36% yield (entry 6). We speculat- mechanistic studies provide evidence supporting ed that increasing the amount of acid would push for- a new reaction pathway for iodide-catalyzed oxidative ward the enolization of the ketones, thus favoring ac- cross-couplings between nucleophiles that involves an tivation of the ketones. As expected, an 85% yield a-iodo ketone as intermediate. The coupling of an a- was obtained after adding 1.5 equiv. of NsOH to the iodo ketone with a weak nucleophile such as alcohol reaction system (entry 8). Inorganic iodide salts such proceeds via further oxidation to a hypervalent iodine as NaI, KI and LiI also proved to be effective cata- intermediate rather than the usual nucleophilic substi- lysts and led to comparable conversions, among which tution. This mechanistic insight should be helpful for NaI gave the best result (entries 9–11). Notably, the design of iodide-catalyzed oxidative cross-cou- Baeyer–Villiger oxidation of ketones was not ob- pling reactions. served under the optimized conditions, probably be- Initially, we screened a variety of reaction parame- cause the a-position of the enol from the acid-pro- ters using the reaction of propiophenone 1a with an moted ketone tautomerization is much more reactive excess amount of methanol 2a as a model system than other positions. However, accompanying product (Table 1). To the model reaction carried out in 3a, a small amount of a-iodo ketone (e.g., 2-iodo-1-

CH3CN at 808C in the presence of 20 mol% Bu4NI as phenylpropan-1-one 5) was always formed in the a catalyst, a variety of Lewis acids[18] such as TsOH, model reaction. When PhI was used as the catalyst in

BF3·Et2O were introduced to promote the enolization place of NaI, the yield of 3a dropped to 60%

332 asc.wiley-vch.de 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Synth. Catal. 2015, 357, 331 – 338 COMMUNICATIONS Sodium Iodide-Catalyzed Direct a-Alkoxylation of Ketones with Alcohols

Scheme 2. Scope of the ketone coupling partner. Reaction conditons: 1 (0.2 mmol), 2a (0.5 mL), NaI (20 mol%), NsOH

(1.5 equiv.), m-CPBA (2.5 equiv.), CH3CN (2 mL); isolated yields.

(entry 12). Control experiments confirmed that both cohols and benzyl alcohols could be directly synthe- iodide salt and oxidant were necessary for the reac- sized by this method (4c–4k), which are not readily tion to occur (entries 13 and 14). accessible using conventional methods because of the With the optimized conditions in hand, we first ex- complicated reactivity of the long-chain alcohols. amined the substrate scope of this reaction with re- Thus, this new CÀO coupling process provided a syn- spect to ketones. As shown in Scheme 2, propiophe- thetically useful and operationally simple approach to none derivatives with both electron-donating and the a-alkoxylation of ketones. electron-withdrawing substituents on the benzene ring To gain insights into the reaction mechanism, were efficiently coupled with methanol to produce a series of experiments was performed. In the optimi- the desired products in good yields (3a–3j). The toler- zation studies, the model reactions conducted at dif- ance of various substituents such as fluoro, chloro, ferent temperatures always generated 2-iodo-1-phe- bromo groups provides opportunities for further syn- nylpropan-1-one 5 accompanying the formation of a- thetic elaboration. Unfortunately, other electron-with- methoxylation product 3a, which led us to consider drawing substituents such as the ester, nitro, and whether 5 is an intermediate of this reaction process. cyano group were deleterious to the reactivity of pro- The reaction of 5 with methanol in the absence of piophenones, affording the desired products in the di- NaI under otherwise identical conditions also afford- minished yields (not being included in Table 1). Nota- ed 3a in 75% yield(Scheme 4a). In addition, the inter- bly, as exemplified by 1-(2-thiophenyl)-1-propanone, mediate 5 (20 mol%) could serve as the catalyst in the phenyl moiety of propiophenone could be re- place of NaI to catalyze the a-methoxylation of pro- placed with a heterocyclic group (3l). Lengthening piophenone in 70% yield (Scheme 4b). These results the alkyl chain had little impact on the reaction out- provided evidence supporting that 5 is a possible in- come, as shown in the case of butyrophenone (3m). termediate lying in the catalytic cycle of this a-me- Differing from propiophenone, however, acetophe- thoxylation reaction. The next question to be an- none gave rise to disubstituted products (3n). This swered is how an intermediate 5 is converted into a- method was applicable for a facile synthesis of the methoxylation product 3a. A control experiment re- practical photopolymerization initiator[2c] 3k (60% vealed that after removing the oxidant m-CPBA, the yield). reaction of 5 with methanol did not happen regardless Next, we evaluated the scope of alcohols by using of the presence or absence of NsOH (Scheme 4c), propiophenone as the coupling partner (Scheme 3). which ruled out the possibility that the conversion of This reaction was applicable to a variety of primary 5 into 3a proceeds via a nucleophilic substitution of alkyl and benzyl alcohols, and gave the corresponding iodide by methanol. The need for m-CPBA in the products in synthetically useful yields (4a–4k). In con- conversion of 5 into 3a implied that this transforma- trast, secondary alcohols such as isopropyl alcohol af- tion may involve the oxidation of 5 to a very reactive forded the product in modest yield (4l) due in part to iodine(III) species such as a-iodoso ketone that un- steric factors. What deserves to be mentioned is the dergoes adduct formation with methanol and subse- fact that a-alkoxy products from long-chain alkyl al- quent reductive elimination to generate 3a.[9f]

Adv. Synth. Catal. 2015, 357, 331 – 338 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim asc.wiley-vch.de 333 Cuiju Zhu et al. COMMUNICATIONS

Scheme 3. Scope of the alcohol coupling partner. Reaction conditions: 1a (0.2 mmol), 2 (0.5 mL), NaI (20 mol%), NsOH

(1.5 equiv.), m-CPBA (2.5 equiv.), CH3CN (2 mL); isolated yields.

Scheme 4. Experiments investigating the participation of 2-iodo-1-phenylpropan-1-one.

Furthermore, the role of NsOH in the conversion in 60% overall yield theoretically (Scheme 5a). This of 5 into 3a was investigated. The reaction of 2-iodo- observation seems to indicate that NsOH not only 1-phenylpropan-1-one 5 with NsOH in the absence of served as an acid to promoted enolization of ketones methanol gave a-sulfonyloxy ketone 6 in 90% yields, and oxidation of 5 by m-CPBA, but also a reactant to and the reaction 6 with methanol formed 3a in 67% participate in the formation active intermediates. yield. Thus, this two-step process generated 3a from 5 However, in the absence of NsOH, the reaction of 5

334 asc.wiley-vch.de 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Synth. Catal. 2015, 357, 331 – 338 COMMUNICATIONS Sodium Iodide-Catalyzed Direct a-Alkoxylation of Ketones with Alcohols

Scheme 5. Experiments investigating the participation of a-sulfonyloxy ketone.

Scheme 6. Experiments investigating the involvement of a C-centered radical. with methanol still gave 3a in 27% yield.(Scheme 5b). (TEMPO) was observed to have no effect on the oxi- As such, methanol is able to directly take part in the dative conversion of 5 into 3a (Scheme 6a). In con- oxidative coupling with 5 without recourse to forma- trast, one equiv. of TEMPO did impede the a-me- tion of an a-sulfonyloxy ketone intermediate, in thoxylation of propiophenone and the 5-catalyzed which an acid plays a role in facilitating the conver- (20 mol%) ketone alkoxylation under the standard sion of the a-iodo ketone to the a-methoxylation conditions (Scheme 6b and c). In the light of this, we product. speculated that the a-iodination of propiophenone for Finally, the generation of intermediate 5 was ex- the formation of 5 might involve a radical pro- plored. 2,2,6,6-Tetramethylpiperidine 1-oxyl cess,[11f,19] which is supported by the fact that exposure

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Scheme 7. Proposed mechanism. of diphenylcyclopropyl ketone 7 to the standard con- In conclusion, we have established the direct a-al- ditions resulted in the rearranged conjugated diene 8 koxylation of ketones with alcohols via the NaI/m- (Scheme 6d). CPBA-catalyzed oxidative cross-coupling for the first Based on the above results, a plausible mechanism time. This protocol allows for cross-coupling of a vari- for this reaction is proposed in Scheme 7. Firstly, the ety of alkyl aryl ketones with an array of alcohols, propiophenone 1a is oxidized to form a radical inter- thus providing a straightforward approach to a-alkoxy mediate that abstracts an iodine atom from the in-situ ketones. The reaction mechanism has been studied in generated I2 to deliver the key intermediate 2-iodo-1- detail, which offered the solid evidence for identifica- phenylpropan-1-one 5. Then, 5 undergoes oxidation tion, generation and conversion of reaction intermedi- to form a hypervalent iodine(III) species. At this ates. The a-iodo ketone is identified as the key inter- point, there are two possible accesses to the final mediate, its conversion into a-alkoxylated ketone pro- product: (i) adduct formation of NsOH with the hy- ceeds via oxidation to hypervalent iodine species and pervalent iodine(III) species followed by reductive its generation involves a radical process. This mecha- elimination to furnish 6 that releases the final product nistic insight should influence the design of new via SN2 nucleophilic substitution by MeOH (Path A); iodide-catalyzed cross-coupling reactions between nu- (ii) adduct formation of MeOH with hypervalent io- cleophiles. Further studies to expand the substrate dine(III)ACHTUNGRE species (Path B) followed by reductive elim- scope of this transformation to other compounds such ination to release the final product 3a along with hy- as esters, aldehydes or the phenols, and even studies poiodous acid. To close the catalytic cycle, hypoio- towards an asymmetric version of this new transfor- dous acid either serves as active iodine species to mation are ongoing in our laboratory. effect the direct iodination of 1a to intermediate 5,or undergoes disproportionation to I2 and HIO3 species followed by I trapping the ketone radical intermedi- 2 Experimental Section ate to form 5 and reduction of HIO3 to the catalytical- ly active iodine species (NaIO3 could catalyze ketone alkoxylation in place of NaI, see the Supporting Infor- General Procedure mation for details). A 25-mL Schlenk tube equipped with a stir bar was charged with NaI (6.0 mg, 0.04 mmol, 20 mol%), NsOH (61.0 mg,

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0.3 mmol, 1.5 equiv.), m-CPBA (103.5 mg, 85%, 0.5 mmol, 130, 12901–12903; d) C. He, S. Guo, J. Ke, J. Hao, H. 2.5 equiv.). The tube was fitted with a rubber septum after Xu, H. Chen, A. Lei, J. Am. Chem. Soc. 2012, 134, using a Schlenk line to evacuate the air and fill the tube 5766–5769; e) Y. Ma, S. Zhang, S. Yang, F. Song, J. with nitrogen. CH3CN (2.0 mL), methanol (0.5 mL), and You, Angew. Chem. 2014, 126, 8004–8008; Angew. propiophenone (0.2 mmol) were added in turn to the Chem. Int. Ed. 2014, 53, 7870–7874; f) B. M. Trost, Schlenk tube through the rubber septum using syringes, and D. A. Thaisrivongs, E. J. Donckele, Angew. Chem. 2013, then the septum was replaced with a Teflon screwcap under 125, 1563–1566; Angew. Chem. Int. Ed. 2013, 52, 1523– a nitrogen flow. The reaction mixture was stirred at 808C 1526; g) P. Hu, Y. Shang, W. Su, Angew. Chem. 2012, for 24 h. After cooling down, the reaction mixture was dilut- 124, 6047–6051; Angew. Chem. Int. Ed. 2012, 51, 5945– ed with 10 mL of diethyl ether, filtered through a pad of 5949; h) Y. Wei, W. Su, J. Am. Chem. Soc. 2010, 132, silica gel, followed by washing the pad of the silica gel with 16377–16379. the same solvent (40 mL). The filtrate was extracted three [8] J. M. Stevens, D. W. C. MacMillan, J. Am. Chem. Soc. times with 30 mL of water. The organic phase was dried 2013, 135, 11756–11759. over Na2SO4, filtered, concentrated under reduced pressure [9] a) V. V. Zhdankin, P. J. Stang, Chem. Rev. 2008, 108, (some alcohols with high boiling point should be subjected 5299–5358; b) N. R. Deprez, M. S. Sanford, Inorg. to high vacuum). The residue was then purified by flash Chem. 2007, 46, 1924–1935; c) R. J. Phipps, N. P. Grim- chromatography on silica gel to provide the corresponding ster, M. J. Gaunt, J. Am. Chem. Soc. 2008, 130, 8172– product. 8174; d) N. R. Deprez, D. Kalyani, A. Krause, M. S. Sanford, J. Am. Chem. Soc. 2006, 128, 4972–4973; e) A. R. Dick, J. W. Kampf, M. S. Sanford, Organome- tallics 2005, 24, 482–485; f) G. Shan, X. Yang, Y. Zong, Acknowledgements Y. Rao, Angew. Chem. 2013, 125, 13851–13855; Angew. Chem. Int. Ed. 2013, 52, 13606–13610. Financial support from the 973 Program (2011CB932404, [10] a) E. Skucas, D. W. C. MacMillan, J. Am. Chem. Soc. 2011CBA00501) and NSFC (21332001, 21431008 and 2012, 134, 9090–9093; b) A. E. Allen, D. W. C. MacMil- 21301173) is greatly appreciated. lan, J. Am. Chem. Soc. 2011, 133, 4260–4263; c) A. E. Allen, D. W. C. MacMillan, J. Am. Chem. Soc. 2010, 132, 4986–4987; d) J. Yu, J. Tian, C. Zhang, Adv. Synth. References Catal. 2010, 352, 531–546; e) R. M. Moriarty, B. A. Ber- glund, R. Penmasta, Tetrahedron Lett. 1992, 33, 6065– [1] a) J. A. Murphy, M. Mahesh, G. McPheators, R. V. 6068; f) G. F. Koser, Aldrichimica Acta 2001, 34,89; Anand, T. M. McGuire, R. Carling, A. R. Kennedy, g) J. C. Lee, T. Hong, Synth. Commun. 1997, 27, 4085– Org. Lett. 2007, 9, 3233–3236; b) J. Aziz, E. Brachet, A. 4090; h) J. C. Lee, S. Kim, W. C. Shin, Synth. Commun. Hamze, J.-F. Peyrat, G. Bernadat, E. Morvan, J. 2000, 30, 4271–4275; i) J. C. Lee, Y. S. Jin, Synth. Bignon, J. Wdzieczak-Bakala, D. Desravines, J. Dubois, Commun. 1999, 29, 2769–2774; j) E. Merritt, B. Olofs- M. Tueni, A. Yassine, J.-D. Brion, M. Alami, Org. son, Synthesis 2011, 517–538. Biomol. Chem. 2013, 11, 430. [11] a) M. Ochiai, K. Miyamoto, Eur. J. Org. Chem. 2008, [2] a) D. C. Rueda, M. De Mieri, S. Hering, M. Hambur- 4229–4239; b) T. Dohi, Y. Kita, Chem. Commun. 2009, ger, J. Nat. Prod. 2014, 77, 640–649; b) M. Singh, J. T. 2073–2085; c) M. Ochiai, Y. Takeuchi, T. Katayama, T. Schott, M. A. Leon, R. T. Granata, H. K. Dhah, J. A. Sueda, K. Miyamoto, J. Am. Chem. Soc. 2005, 127, Welles, M. A. Boyce, H. S. Oseni-Olalemi, C. E. Mor- 12244–12245; d) T. Dohi, A. Maruyama, M. Yoshimura, daunt, A. J. Vargas, Bioorg. Med. Chem. Lett. 2012, 22, K. Morimoto, H. Tohma, Y. Kita, Angew. Chem. 2005, 6252–6255; c) T. Corporation, European Patent 117, 6349–6352; Angew. Chem. Int. Ed. 2005, 44, 6193– 2055698A1, 2009. 6196; e) M. Uyanik, T. Yasui, K. Ishihara, Bioorg. Med. [3] R. M. Moriarty, O. Prakash, M. P. Duncan, R. K. Vaid, Chem. Lett. 2009, 19, 3848–3851; f) M. Uyanik, D. H. A. Musallam, J. Org. Chem. 1987, 52, 150–153. Suzuki, T. Yasui, K. Ishihara, Angew. Chem. 2011, 123, [4] S. Rozen, E. Mishani, M. Kol, J. Am. Chem. Soc. 1992, 5443–5446; Angew. Chem. Int. Ed. 2011, 50, 5331–5334; 114, 7643–7645. g) A. Tanaka, H. Togo, Synlett 2009, 3360–3364; h) A. [5] a) M. S. Newman, P. F. Beal, J. Am. Chem. Soc. 1950, Tanaka, K. Moriyama, H. Togo, Synlett 2011, 1853– 72, 5161–5163; b) E. J. Corey, S. Knapp, Tetrahedron 1858; i) Y. Yamamoto, H. Togo, Synlett 2006, 798–800; Lett. 1976, 17, 4687–4690. j) H. Kikui, K. Moriyama, H. Togo, Synthesis 2013, 45, [6] a) R. W. Evans, J. R. Zbieg, S. Zhu, W. Li, D. W. C. 791–797; k) J. Sheng, X. Li, M. Tang, B. Gao, G. MacMillan, J. Am. Chem. Soc. 2013, 135, 16074–16077; Huang, Synthesis 2007, 1165–1168; l) Q. Jiang, B. Xu, b) J. S. Tian, K. W. Ng, J. R. Wong, T. P. Loh, Angew. A. Zhao, J. Jia, T. Liu, C. Guo, J. Org. Chem. 2014, 79, Chem. 2012, 124, 9239–9243; Angew. Chem. Int. Ed. 8750–8756. 2012, 51, 9105–9109; c) N. Halland, A. Braunton, S. [12] a) Y. F. Liang, N. Jiao, Angew. Chem. 2014, 126, 558– Bachmann, M. Marigo, K. A. Jørgensen, J. Am. Chem. 562; Angew. Chem. Int. Ed. 2014, 53, 548–552; b) T. Soc. 2004, 126, 4790–4791. Arai, T. Sato, H. Noguchi, H. Kanoh, K. Kaneko, A. [7] a) C. Liu, H. Zhang, W. Shi, A. Lei, Chem. Rev. 2011, Yanagisawa, Chem. Lett. 2006, 35, 1094–1095; c) G. J. 111, 1780–1824; b) C. S. Yeung, V. M. Dong, Chem. Chuang, W. Wang, E. Lee, T. Ritter, J. Am. Chem. Soc. Rev. 2011, 111, 1215–1292; c) S. Lin, C.-X. Song, G.-X. 2011, 133, 1760–1762; d) C. E. Frank, Chem. Rev. 1950, Cai, W.-H. Wang, Z.-J. Shi, J. Am. Chem. Soc. 2008, 46, 155–169; e) I. Hermans, J. Peeters, P. Jacobs, Top.

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Catal. 2008, 50, 124–132; f) K. U. Ingold, Acc. Chem. [16] X. Huang, X. Li, M. Zou, S. Song, C. Tang, Y. Yuan, N. Res. 1969, 2, 1–9; g) Y. Monguchi, T. Takahashi, Y. Iida, Jiao, J. Am. Chem. Soc. 2014, 136, 14858–14865. Y. Fujiwara, Y. Inagaki, T. Maegawa, H. Sajiki, Synlett [17] M. Uyanik, H. Okamoto, T. Yasui, K. Ishihara, Science 2008, 2008, 2291–2294; h) H. Sakurai, I. Kamiya, H. Ki- 2010, 328, 1376–1379. tahara, H. Tsunoyama, T. Tsukuda, Synlett 2009, 245– [18] a) Y. Nishimoto, Y. Onishi, M. Yasuda, A. Baba, 248. Angew. Chem. 2009, 121, 9295–9298; Angew. Chem. [13] G. J. ten Brink, I. W. C. E. Arends, R. A. Sheldon, Int. Ed. 2009, 48, 9131–9134; b) M. Yasuda, T. Somyo, Chem. Rev. 2004, 104, 4105–4124. A. Baba, Angew. Chem. 2006, 118, 807–810; Angew. [14] A. De Mico, R. Margarita, L. Parlanti, A. Vescovi, G. Chem. Int. Ed. 2006, 45, 793–796. Piancatelli, J. Org. Chem. 1997, 62, 6974–6977. [19] a) D. Griller, K. U. Ingold, Acc. Chem. Res. 1980, 13, [15] S. Guo, J. T. Yu, Q. Dai, H. Yang, J. Cheng, Chem. 317–323; b) M. Newcomb, C. C. Johnson, M. B. Manek, Commun. 2014, 50, 6240–6242. T. R. Varick, J. Am. Chem. Soc. 1992, 114, 10915– 10921.

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