Document 2JVK78vepZeZozz6vvK6nQ2NN

SCIENCE CHINA Chemistry MINI REVIEWS SPECIAL ISSUE: Dedicated to the 100th Anniversary of Nankai University CrossMark click for updates May 2019 Vol.62 No.5: 525-532 https://doi.org/10.1007/s11426-018-9402-x Recent advances in new trifluoromethoxylation reagents Xiaofei Zhang & Pingping Tang* State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China Received October 31, 2018; accepted December 11, 2018; published online February 22, 2019 The trifluoromethoxy (CF30) group has become a novelmoiety in various fields because ofits unique features. However, despite the promising applications, the synthesis of CF30-containing compounds is still a challenge due to indirect synthetical strategies and volatile reagent which is hard to handle. Until very recently, several innovative reagents were developed to facilitate the trifluoromethoxylation reaction and make CF30-containing compounds more accessible. This review mainly focuses on the recent advances in new trifluoromethoxylation reagents and their usage. trifluoromethoxy group, trifluoromethoxylation reagent, synthetic methods, fluorine Citation: Zhang X, Tang P. Recent advances in new trifluoromethoxylation reagents. Sci China Chem, 2019, 62: 525-532, https://doi.org/10.1007/s11426-0189402-x 1 Introduction Fluorine and fluorine-containing substituents have been exploited extensively in pharmaceuticals, agrochemicals, and organic materials [1-3]. Indeed, about 25% of all the onselling drugs contain at least one fluorine [4-6]. This broad incorporation result from the extreme properties of fluorine which can impart to the features of parent compounds [2,7-10]. Among the fluorinated moieties, trifluoromethoxy group (OCF3) becomes increasingly prominent because of its unique characteristics [11-15]. Introducing OCF3 onto the molecule significantly increases the metabolic stability and lipophilicity (Hansch parameter: ir=1.04) [16,17]. Furthermore, the OCF3 group possesses special electron properties [13,18,19]. The electron-withdrawing ability is greater while the electron-donating by resonance is weaker than chlorine as a result of the low electron density in the non-bonding porbitals ofOCF3 [19,20]. Consequently, there is little electron conjugation with the aromatic ring result in an unusual or- *Corresponding author (email: =@nankai.edu.cn) thogonal orientation of O--CF3 bond with respect to arene plane [21-24]. Despite these novel properties, compounds bearing OCF3 moiety are still relatively rare. Only several OCF3-containing drugs hit the market [13]. The reason might be the absence of reliable methodology to introduce this motif. Although elegant strategies such as electrophilic trifluoromethylation [25-30], oxidative coupling between alcohols and TMSCF3 [31,32], and fluorodecarboxylation [3336] were developed to prepare OCF3-containing compound indirectly, the more efficient direct trifluoromethoxylation still mainly relied on the volatile reagent, trifluoromethyl trifluoromethanesulfonate (TFMT) [19,37], and trifluoromethanolate salts including AgOCF3 [38-43], TASOCF3 [44,45], CsOCF3 [46-48] which obtained or generated in situ from TFMT in the presence of fluoride sources. Trifluoromethoxylation of arenes [37,44], alkyl halides and pseudohalides [39,41,47,49,50], a-diazo esters [38] and intramolecular aminotrifluoromethoxylation [40] can be accomplished with TFMT. However, the harsh condition to prepare TFMT which requires triflic acid along with its volatile property and expensive price limits its application Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 chem.scichina.com link.springer.com 526 Zhang et al. Sci China Chem May (2019) Vol.62 No.5 (Scheme 1) [51,52]. Until very recently, the development of new tri- fluoromethoxylation reagents makes it possible to readily access much more OCF3-substituted molecules than ever. In this review, various trifluoromethoxylation reagents will be discussed with a focus on recently developed ones. Scheme 1 Preparation of TFMT with triflic acid and diphosphorus pentoxide. 2 2,4-Dinitro(trifluoromethoxy)benzene Langlois and co-workers [53] developed an alternative reagent for TFMT by employing two electron-withdrawing groups onto aryl trifluoromethyl ether. They supposed that CF3O- could be obtained by cleavage of Ar-OCF3 bond through an SNAr mechanism. Indeed, by heating 2,4-dinitro (trifluoromethoxy)benzene (DNTFB) in the presence of tetrabutylammonium triphenyldifluorosilicate (TBAT), CF3O- was released. Thus, trifluoromethoxylation was achieved at moderate to good yields by substitution of bromide at activated position with CF3O- generate in situ (Scheme 2). To improve the kinetics of reaction, microwave irradiation was applied. With microwave irradiation, benzyl bromide could be trifluoromethoxylated in a 70% yield within 20 min at 100 C. Unfortunately, the scope and yield of this method are still required to be improved. 3 Trifluoromethyl aryl sulfonate Although DNTFB can serve as an alternative for TFMT, the reaction is still problematic because of the low reactivity and narrow substrate scope. Exploiting new trifluoromethoxylation reagent could provide a way to overcome the shortcomings. Recently, Umemoto et al. [25] and Togni et al. [54] reported the preparation of trifluoromethyl sulfonates (TFMS), respectively (Scheme 3). Inspired by their work, Tang et al. [54] hypothesized that activated with a suitable nucleophile, TFMS could release trifluoromethoxide anion (CF3O-) in situ and serve as a new trifluoromethoxylation reagent. After screening different nucleophiles, the authors found that TFMS demonstrated a potent synthetic potency in the present of fluoride. With TFMS as a new trifluoromethoxylation reagent, a series of trifluoromethoxylation reactions were developed by Tang and co-workers [55-60]. 3.1 Asymmetric bromotrifluoromethoxylation of alkenes In 2017, Tang and co-workers [59] developed an original strategy for asymmetric bromotrifluoromethoxylation of alkenes with TFMS. With activation of CsF, this new reagent can release CF3O- and form AgOCF3 in the present of silver Scheme 2 Nucleophilic trifluoromethoxylation of halogenated substrates with DNTFB. Scheme 3 Preparation of TFMS with benzenesulfonyl chloride and Togni's reagent. catalyst AgF. By directing and catalysis with chiral ligand (DHQD)2PHAL, a broad scope of olefin substrates including natural products were difunctionalized in an asymmetric way (Scheme 4). They supposed that in situ formed AgOCF3 further coordinates to the quinoline nitrogen of the chiral ligand (DHQD)2PHAL. And styrene substrate is anchored by ,stacking of the quinoline of (DHQD)2PHAL and aromatic ring. While bromine from DBDMH is readily directed toward the double bond, the asymmetric product is yielded (Scheme 5). 3.2 Direct dehydroxytrifluoromethoxylation of alcohols Shortly after, Tang and co-workers [58] found that by generation of alkyl fluoroformate in situ from alcohol, dehydroxytrifluoromethoxylation of alcohols could be achieved with TFMS. The decomposition of trifluoromethoxide anion (CF3O-) is a significant challenge for introducing OCF3 motif to molecule of interest. However, the decomposition product, fluorophosgene, can react with alcohols in situ to yield alkyl fluoroformates. Alkyl fluoroformate is an activated species which readily undergoes the nucleophilic attack of CF3O- generated from TFMS to afford the alky trifluoromethyl Zhang et al. Sci China Chem May (2019) Vol.62 No.5 527 Scheme 6 Strategy for direct dehydroxytrifluoromethoxylation of alcohols. Scheme 4 Asymmetric bromotrifluoromethoxylation of alkenes with TFMS. Scheme 5 Proposed transition-state model. ethers (Scheme 6). With this strategy, various primary and secondary alcohols were transformed into corresponding alkyl trifluoromethyl ethers in moderate to good yields (Scheme 7). Compared with conventional methods [25,32,61-63], this method processes a mild reaction conditions which tolerates a wide scope of functional groups. 3.3 Azidotrifluoromethoxylation of styrenes Soon after, Tang and co-workers [57] developed an azidotrifluoromethoxylation reaction by combining photoredox and silver catalyst. With this method, a variety of styrene derivatives containing electron-donation or withdrawing substituents were smoothly converted to the difunctional products. A broad range of function groups were tolerant in this reaction (Scheme 8). After preliminary mechanistic studies, they supposed that Scheme 7 Dehydroxytrifluoromethoxylation of alcohols. under photoredox condition, styrene derivatives undergo azide radical addition. The radical addition product is further oxidized to benzyl carbocation by [Ru(bpy)3]3+. Consequently, the resulting benzyl carbocation is quenched by AgOCF3 generated in situ to afford the product (Scheme 9). This new method is the first trifluoromethoxylation reaction employing organometallic and photoredox catalysis together. 3.4 Oxidative trifluoromethoxylation of alkylsilanes In 2018, an oxidative trifluoromethoxylation of alkylsilanes with TFMS was achieved by Tang and co-workers [56]. Inspired by the previous work of hypervalent iodine mediated fluorination of alkylsilanes [64], Tang et al. envisioned that by changing the nucleophiles, alkylsilanes could be 528 Zhang et al. Sci China Chem May (2019) Vol.62 No.5 Scheme 8 Azidotrifluoromethoxylation of styrenes. Scheme 10 Trifluoromethoxylation of alkylsilanes. et al. [55] developed the benzylic C-H trifluoromethoxylation reaction. With silver catalyst, methylated arenes were transformed to the corresponding trifluoromethoxylated products in moderate to good yields. Common functional groups were tolerated (Scheme 11). The reaction conditions might be different concerned to the electronic properties of the substrates. Further attempt to obtain ditrifluoromethoxylation led to the -fluorobenzyl trifluoromethyl ether products. Methylated arenes bearing electron-donation groups were converted to the corresponding products in moderate yields (Scheme 12). Scheme 9 Proposed mechanism for azidotrifluoromethoxylation. transformed into virous functional products with the same strategy. Indeed, in the presence of AgF and TFMS, the trifluoromethoxylation of alkylsilanes was facilitated by CF3O- generated in situ. Primary alkylsilanes bearing a wide range of substituents were converted into corresponding trifluoromethoxylated products with this method (Scheme 10). 3.5 Oxidative benzylic C-H trifluoromethoxylation Based on the TFMS reagent, the oxidative benzylic C-H trifluoromethoxylation was also achieved by Tang and coworkers [55]. A few works of direct C-H trifluoromethoxylation reactions were reported due to the unsuitability of trifluoromethoxide anion and limited access of trifluoromethoxylation reagent [48,65-68]. Inspired by the previously reported difluoromethylation of arenes [69], Tang 4 Trifluoromethyl benzoate The preparation of TFMS has yet to be limited to trifluoromethylation of sulfonic acids using CF3 oxonium salts [25] or hypervalent iodine reagents [54,63,70]. Recently, Hu and co-workers [71] reported a new trifluoromethoxylation reagent. They envisioned that the reagent should be a stable liquid or solid with suitable activity and could be economically prepared for practical application. Through combining triphosgene, KF and benzoyl bromide, they prepared trifluoromethyl benzoate (TFBz) as the new trifluoromethoxylation reagent (Scheme 13). To test the synthetic utility of this new reagent, Hu et al. employed the arynes trifluoromethoxylation-bromination reaction with TFBz. After screened a range of solvents and fluoride salts, they found that although arynes were very reactive, a stabilized CF3O- species was still required to promote this bifunctional reaction. And a combination of KF/cis-dicyclohexano-18-crown-6 and EtOAc solvent was found to be the optimum conditions. With this reaction conditions, a range of trifluoromethoxylation-bromination products were obtained (Scheme 14). Hu et al. [71] applied different trifluoromethoxylation protocols to further evaluate the synthetic utility of TFBz. As Zhang et al. Sci China Chem May (2019) Vol.62 No.5 529 zole N-OCF3 (Scheme 16). They hypothesized that with appropriate N-OCF3 reagent, N-OCF3 bond could undergo photo-induced homolytic cleavage to generate OCF3 radical. By screening different N-OCF3 compounds, they developed a benzimidazole analog which could efficiently produce OCF3 radical under irradiation of blue light-emitting diode (LED). The OCF3 radical generated by photoexcitation of the re- Scheme 11 Oxidative benzylic C-H trifluoromethoxylation. Scheme 14 Trifluoromethoxylation-halogenation of arynes. Scheme 12 Benzylic C-H trifluoromethoxylation/fluorination. Scheme 13 Preparation of trifluoromethyl benzoate. shown in Scheme 15, various methods for the synthesis of trifluoromethyl ethers can be facilitated by TFBz. Scheme 15 Trifluoromethoxylation with TFBz. 5 Benzimidazole N-OCF3 trifluoromethoxylation reagent In 2018, Ngai and co-workers [68] reported a radical trifluoromethoxylation reagent with a scaffold of benzimida- Scheme 16 Synthesis of benzimidazole N-OCF3 trifluoromethoxylation reagent. 530 Zhang et al. Sci China Chem May (2019) Vol.62 No.5 agent was subsequently trapped by arenes in an intermolecular manner. With Ru(bpy)3(PF6)2 as a photoredox catalyst, both arenes and heteroarenes containing different functional groups were converted to trifluoromethoxylated products in moderate to good yields (Scheme 17). However, the regioselectivity of the reaction is still needed to be improved. 6 Pyridine N-OCF3 trifluoromethoxylation reagent by excess amount of arenes. After oxidation and deprotonation of the cyclohexadienyl radical, the trifluorome- Scheme 18 Synthesis of pyridine N-OCF3 trifluoromethoxylation reagent. Togni and co-workers [66] developed a pyridine trifluoromethoxylation reagent bearing a N-OCF3 motif which was synthesised by one step reaction form commercially available chemicals (Scheme 18). After screening a range of substituents, pyridinium with cyan on the para-position was found to be the most preferred. The reagent could react with a variety of arenes with common functional groups in the present of photoredox catalyst [Ru(bpy)3](PF6)2. Several bioactive molecules were also functionalized in moderate yields to demonstrate the synthetic utility (Scheme 19). A series of experiments were carried out to study the mechanism. With supportive results, a preliminary mechanistic proposal was made (Scheme 20). Under blue light irradiation, the photocatalyst Ru(bpy)32+ is excited, followed by oxidative quenching through single electron transfer (SET) between pyridinium N-OCF3 reagent and the photocatalyst. The resulting pyridinium radical undergoes homolytic cleavage to afford an N or O-centered radical. Then the thermodynamic favored trifluoromethoxyl radical is trapped Scheme 19 Trifluoromethoxylation of arenes with pyridine N-OCF3 reagent. Scheme 17 C-H trifluoromethoxylation of arenes and heteroarenes. Scheme 20 Proposed mechanism of trifluoromethoxylaton of arenes. Zhang et al. Sci China Chem May (2019) Vol.62 No.5 531 thoxylated product is afforded. 7 Benzotriazole N-OCF3 trifluoromethoxylation reagent Soon after the work of their first radical trifluoromethoxylation reagent, Ngai and co-workers [67] reported a more efficient benzotriazole scaffold trifluoromethoxylation reagent (Scheme 21). This new reagent could generate OCF3 radical executively through a redox mechanism. Thus, the Narylated by-products were terminated. With this new reagent, not only mono-, di-, and tri-substituted (hetero)arenes were smoothly converted to trifluoromethoxylated products smoothly, electron-rich arenes could also be functionalized (Scheme 22). Based on KIE, on/off experiments and DFT calculation, a photo induced redox mechanism was proposed. And instead of directly homolytic cleavage of N-OCF3 bond, the reagent is firstly reduced through single electron transfer (SET) from *Ru(bpy)32+. The resulting radical releases the OCF3 radical to form a neutral molecular. OCF3 radical is trapped by arenes which is further oxidized by Ru(bpy)33+ to give carbocations. Finally, deprotonation of carbocations affords the desired products. 8 Conclusions Over last few years, a variety of trifluoromethoxylation reagents have been developed. These reagents upon activated can generate trifluoromethoxide anion (CF3O-) or trifluoromethoxyl radical (CF3O). Even though, by employing these in situ generated trifluoromethoxide intermediates the library of CF3O-containing compounds has been expanded, the access to these compounds is still limited. There is still a requirement for exploiting new trifluoromethoxylation reagents to incorporate the CF3O onto molecules. Especially the one involved with breakthrough strategy such as trifluoromethoxonium ion (CF3O+) is highly expected in this field. Acknowledgements This work was supported by the National Key Research and Development Program of China (2016YFA0602900), the National Natural Science Foundation of China (21522205, 21672110), and the Fundamental Research Funds for the Central Universities. Conflict of interest The authors declare that they have no conflict of interest. Scheme 21 Synthesis of benzotriazole N-OCF3 trifluoromethoxylation reagent. Scheme 22 Trifluoromethoxylation of arenes with benzotriazole N-OCF3 reagent. 1 Hiyama T, Yamamoto H. Fluorine-containing materials. In: Tamejiro H, Ed. Organofluorine Compounds: Chemistry and Applications. Berlin, Heidelberg: Springer Berlin Heidelberg, 2000. 183-233 2 Gillis EP, Eastman KJ, Hill MD, Donnelly DJ, Meanwell NA. J Med Chem, 2015, 58: 8315-8359 3 O'Hagan D. J Fluorine Chem, 2010, 131: 1071-1081 4 Wang J, Snchez-Rosell M, Acea JL, del Pozo C, Sorochinsky AE, Fustero S, Soloshonok VA, Liu H. Chem Rev, 2014, 114: 2432-2506 5 Bgu JP, Bonnet-Delpon D. J Fluorine Chem, 2006, 127: 992-1012 6 Ilardi EA, Vitaku E, Njardarson JT. J Med Chem, 2014, 57: 2832- 2842 7 Mller K, Faeh C, Diederich F. Science, 2007, 317: 1881-1886 8 Hagmann WK. J Med Chem, 2008, 51: 4359-4369 9 Purser S, Moore PR, Swallow S, Gouverneur V. Chem Soc Rev, 2008, 37: 320-330 10 Smart BE. J Fluorine Chem, 2001, 109: 3-11 11 Manteau B, Pazenok S, Vors JP, Leroux FR. J Fluorine Chem, 2010, 131: 140-158 12 Mamada M, Shima H, Yoneda Y, Shimano T, Yamada N, Kakita K, Machida T, Tanaka Y, Aotsuka S, Kumaki D, Tokito S. Chem Mater, 2015, 27: 141-147 13 Landelle G, Panossian A, Leroux F. Curr Top Med Chem, 2014, 14: 941-951 14 Kirsch P, Bremer M. Angew Chem Int Ed, 2000, 39: 4216-4235 15 Jeschke P, Baston E, Leroux F. Mini Rev Med Chem, 2007, 7: 1027- 1034 16 Hansch C, Leo A, Unger SH, Kim KH, Nikaitani D, Lien EJ. J Med Chem, 1973, 16: 1207-1216 17 Park BK, Kitteringham NR, O'Neill PM. Annu Rev Pharmacol Toxicol, 2001, 41: 443-470 18 Castagnetti E, Schlosser M. Chem Eur J, 2002, 8: 799-804 19 Marrec O, Billard T, Vors JP, Pazenok S, Langlois BR. J Fluorine Chem, 2010, 131: 200-207 532 Zhang et al. Sci China Chem May (2019) Vol.62 No.5 20 Hansch C, Leo A, Taft RW. Chem Rev, 1991, 91: 165-195 21 Serfaty IW, Hodgins T, McBee ET. J Org Chem, 1972, 37: 2651-2655 22 Herkes FE. J Fluorine Chem, 1977, 9: 113-126 23 Manteau B, Genix P, Brelot L, Vors JP, Pazenok S, Giornal F, Leuenberger C, Leroux FR. Eur J Org Chem, 2010, 2010(31): 6043- 6066 24 Shishkov IF, Geise HJ, Van Alsenoy C, Khristenko LV, Vilkov LV, Senyavian VM, van der Veken B, Herrebout W, Lokshin BV, Garkusha OG. J Mol Struct, 2001, 567-568: 339-360 25 Umemoto T, Adachi K, Ishihara S. J Org Chem, 2007, 72: 6905-6917 26 Liang A, Han S, Liu Z, Wang L, Li J, Zou D, Wu Y, Wu Y. Chem Eur J, 2016, 22: 5102-5106 27 Koller R, Stanek K, Stolz D, Aardoom R, Niedermann K, Togni A. Angew Chem Int Ed, 2009, 48: 4332-4336 28 Santschi N, Geissbhler P, Togni A. J Fluorine Chem, 2012, 135: 83- 86 29 Matousek V, Pietrasiak E, Sigrist L, Czarniecki B, Togni A. Eur J Org Chem, 2014, 2014(15): 3087-3092 30 Hojczyk KN, Feng P, Zhan C, Ngai MY. Angew Chem Int Ed, 2014, 53: 14559-14563 31 Liu JB, Chen C, Chu L, Chen ZH, Xu XH, Qing FL. Angew Chem Int Ed, 2015, 54: 11839-11842 32 Liu JB, Xu XH, Qing FL. Org Lett, 2015, 17: 5048-5051 33 Zhang QW, Brusoe AT, Mascitti V, Hesp KD, Blakemore DC, Kohrt JT, Hartwig JF. Angew Chem Int Ed, 2016, 55: 9758-9762 34 Zhou M, Ni C, He Z, Hu J. Org Lett, 2016, 18: 3754-3757 35 Chatalova-Sazepin C, Binayeva M, Epifanov M, Zhang W, Foth P, Amador C, Jagdeo M, Boswell BR, Sammis GM. Org Lett, 2016, 18: 4570-4573 36 Krishanmoorthy S, Schnell SD, Dang H, Fu F, Prakash GKS. J Fluorine Chem, 2017, 203: 130-135 37 Zhang QW, Hartwig JF. Chem Commun, 2018, 54: 10124-10127 38 Zha GF, Han JB, Hu XQ, Qin HL, Fang WY, Zhang CP. Chem Commun, 2016, 52: 7458-7461 39 Barbion J, Pazenok S, Vors JP, Langlois BR, Billard T. Org Process Res Dev, 2014, 18: 1037-1040 40 Chen C, Chen P, Liu G. J Am Chem Soc, 2015, 137: 15648-15651 41 Chen S, Huang Y, Fang X, Li H, Zhang Z, Hor TSA, Weng Z. Dalton Trans, 2015, 44: 19682-19686 42 Chen C, Luo Y, Fu L, Chen P, Lan Y, Liu G. J Am Chem Soc, 2018, 140: 1207-1210 43 Zhang W, Chen J, Lin JH, Xiao JC, Gu YC. iScience, 2018, 5: 110- 117 44 Huang C, Liang T, Harada S, Lee E, Ritter T. J Am Chem Soc, 2011, 133: 13308-13310 45 Farnham WB, Smart BE, Middleton WJ, Calabrese JC, Dixon DA. J Am Chem Soc, 1985, 107: 4565-4567 46 Arlt J, Jansen M. Chem Ber, 1991, 124: 321-327 47 Sokolenko TM, Davydova YA, Yagupolskii YL. J Fluorine Chem, 2012, 136: 20-25 48 Qi X, Chen P, Liu G. Angew Chem Int Ed, 2017, 56: 9517-9521 49 Kolomeitsev AA, Vorobyev M, Gillandt H. Tetrahedron Lett, 2008, 49: 449-454 50 Katsuhara Y, DesMarteau DD. J Am Chem Soc, 1980, 102: 2681- 2686 51 Noftle RE, Cady GH. Inorg Chem, 1965, 4: 1010-1012 52 Taylor SL, Martin JC. J Org Chem, 1987, 52: 4147-4156 53 Marrec O, Billard T, Vors JP, Pazenok S, Langlois BR. Adv Synth Catal, 2010, 352: 2831-2837 54 Koller R, Huchet Q, Battaglia P, Welch JM, Togni A. Chem Commun, 2009, 84: 5993 55 Yang H, Wang F, Jiang X, Zhou Y, Xu X, Tang P. Angew Chem Int Ed, 2018, 57: 13266-13270 56 Wang F, Xu P, Cong F, Tang P. Chem Sci, 2018, 9: 8836-8841 57 Cong F, Wei Y, Tang P. Chem Commun, 2018, 54: 4473-4476 58 Jiang X, Deng Z, Tang P. Angew Chem Int Ed, 2018, 57: 292-295 59 Guo S, Cong F, Guo R, Wang L, Tang P. Nat Chem, 2017, 9: 546-551 60 Liu J, Wei Y, Tang P. J Am Chem Soc, 2018, 140: 15194-15199 61 Umemoto T. Chem Rev, 1996, 96: 1757-1778 62 Koller R, Stanek K, Stolz D, Aardoom R, Niedermann K, Togni A. Angew Chem, 2009, 121: 4396-4400 63 Brantley JN, Samant AV, Toste FD. ACS Cent Sci, 2016, 2: 341-350 64 Xu P, Wang F, Fan G, Xu X, Tang P. Angew Chem Int Ed, 2017, 56: 1101-1104 65 Venturini F, Navarrini W, Famulari A, Sansotera M, Dardani P, Tor- telli V. J Fluorine Chem, 2012, 140: 43-48 66 Jelier BJ, Tripet PF, Pietrasiak E, Franzoni I, Jeschke G, Togni A. Angew Chem Int Ed, 2018, 57: 13784-13789 67 Zheng W, Lee JW, Morales-Rivera CA, Liu P, Ngai MY. Angew Chem Int Ed, 2018, 57: 13795-13799 68 Zheng W, Morales-Rivera CA, Lee JW, Liu P, Ngai MY. Angew Chem Int Ed, 2018, 57: 9645-9649 69 Xu P, Guo S, Wang L, Tang P. Angew Chem Int Ed, 2014, 53: 5955- 5958 70 Fiederling N, Haller J, Schramm H. Org Process Res Dev, 2013, 17: 318-319 71 Zhou M, Ni C, Zeng Y, Hu J. J Am Chem Soc, 2018, 140: 6801- 6805