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Synthesis of Trifluoromethylated Cycloheptatrienes from N-Tosyl- Hydrazones: Transition-Metal-Free Büchner Ring Expansion

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Synthesis of Trifluoromethylated Cycloheptatrienes from N-Tosyl- Hydrazones: Transition-Metal-Free Büchner Ring Expansion SYNLETT0936-52141437-2096 © Georg Thieme Verlag Stuttgart · New York 2015, 26, 59–62 59 cluster Syn lett Z. Zhang et al. Cluster Synthesis of Trifluoromethylated Cycloheptatrienes from N-Tosyl- hydrazones: Transition-Metal-Free Büchner Ring Expansion Zhikun Zhanga Cs2CO3 (3.0 equiv) a Jiajie Feng NNHTs 4 Å MS a Yan Xu + a Ar CF3 neat, 120 °C, 3 h Songnan Zhang Ar CF3 Yuxuan Yea Tianjiao Lia Xi Wang*a Jun Chenb Yan Zhanga Jianbo Wang*a a Beijing National Laboratory of Molecular Sciences (BNLMS) and Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, P. R. of China+861(06)2751708 [email protected] b Beijing Institute of Microchemistry, No.15 Xinjiangongmen Road, Haidian District, Beijing, 100091, P. R. of China Received: 30.09.2014 N2 Ar CF Accepted after revision: 21.10.2014 cat. Rh(II) or Ir(III) 3 Published online: 18.11.2014 + (a) DOI: 10.1055/s-0034-1378937; Art ID: st-2014-u0823-c Ar CF3 ref. 4, 5 Abstract A transition-metal-free Büchner reaction using trifluoro- methylated N-tosylhydrazones as substrates is reported. A series of tri- Ar NNHTs cat. Pd(0) fluoromethylated cycloheptatriene derivatives can be synthesized by R + ArBr R CF this straightforward method. CF3 ref. 6, 7 3 Key words diazo compounds, carbene, trifluoromethylation, Büchner N2 reaction, cycloheptatriene (b) 1 Ar CF3 Ar2 cat. Pd(0) or Trifluoromethyl-bearing organic molecules have found + Ar2 Br NNHTs ref. 6 1 wide applications in pharmaceuticals, agrochemicals, and Ar CF3 1 functional materials. However, trifluoromethyl-bearing or- 1 Ar CF3 ganic molecules do not exist in the nature. Consequently, it is highly important to develop general and practical meth- ods to synthesize trifluoromethyl-bearing compounds. R NNHTs Great efforts have devoted to this area and significant prog- transition-metal-free + R (c) 2 ress has been made in recent years. Ar CF3 this study On the other hand, diazo compounds possess diverse re- Ar CF3 activities and are highly useful in organic synthesis. In par- Scheme 1 Reactions with trifluoromethyl-bearing diazo compounds or ticular, diazo compounds have found extensive applications N-tosylhydrazones This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. as metal carbene precursors in transition-metal-catalyzed reactions.3 Thus, the diazo compounds bearing the trifluo- romethyl group have become attractive for the synthesis of In connection to our interest in both carbene chemistry trifluoromethyl-containing organic compounds. For exam- and trifluoromethylations, herein we report a transition- ple, transition-metal-catalyzed cyclopropenation and metal-free Büchner reaction using trifluoromethylated N- cyclopropanation with 1-aryl-2,2,2-trifluorodiazoethanes tosylhydrazones as the substrates (Scheme 1, c). The rhodi- have been reported by Davis4 and Katsuki,5 respectively um(II)-catalyzed intramolecular Büchner reaction as an ef- (Scheme 1, a). Recently, we6 and Valdés7 have independent- fective approach toward seven-membered carbocycles have ly reported the palladium-catalyzed cross-coupling reac- been extensively studied by the groups of McKervey, Doyle, tion with 1-aryl-2,2,2-trifluoro ketone and 1-alkyl-2,2,2- Moody, and others.9 The intermolecular Büchner reaction trifluoro ketone N-tosylhydrazones. The diazo compounds without transition-metal catalysis usually suffers low selec- could be generated in situ in these cases.8 These reactions tivity due to other competing reaction pathways.3b The provide unique methods for the synthesis of trifluorometh- Büchner reaction with 1-aryl-2,2,2-trifluorodiazoethanes ylated alkenes and dienes (Scheme 1, b). reported herein is not interfered by the aromatic sp2 C–H © Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 59–62 60 Syn lett Z. Zhang et al. Cluster insertion side reactions, thus providing a convenient meth- With the optimized reaction conditions (Table 1, entry od for the synthesis of trifluoromethylated cyclohepta- 7), the substrate scope of the reaction was investigated by trienes. using a series of N-tosylhydrazones.11 As illustrated in Table The initial study was carried out with N-tosylhydrazone 2, a variety of N-tosylhydrazones 1a–l bearing different 1a (0.2 mmol) and benzene (2a, 2.0 mL) as the substrates, substituents on the aromatic rings were reacted smoothly with Cs2CO3 (3.0 equiv) as the base at 80 °C. After three with benzene, affording the desired products in moderate hours, the product 3a was isolated in 23% yield (Table 1, en- yields (3a–l). N-Tosylhydrazones bearing halogen substitu- try 1). To our delight, the direct C–H insertion product was ents, such as fluoro, chloro, bromo, and iodo, all give the not detected.10 Next, the effect of temperature was exam- corresponding products in decent yields. In particular, ac- ined. The yield could be slightly improved at high tempera- tive iodide 1e is compatible under the reaction conditions ture (Table 1, entry 2). It was found that adding 4 Å (Table 2, entry 5). The substrates bearing electron-with- molecular sieves (4 Å MS) as additive could further improve drawing substituents, such as nitro and trifluoromethyl, the yield (Table 1, entries 4 and 5). Besides, the concentra- gave the corresponding products in moderate yields (Table tion has marginal effect on the reaction (Table 1, entries 4– 2, entries 7 and 8). 9). Finally, the effect of base was studied, and Cs2CO3 was However, the substrate with strong electron-donating found to afford the best result (Table 1, entries 10–14). 4-methoxy substituent only gave the product in low yield (Table 2, entry 11). Notably, the reaction also worked well Table 1 Optimization of Reaction Conditionsa with the substrate bearing the CF2CF2CF3 group instead of the trifluoromethyl group (Table 2, entry 12). The reaction NNHTs seems to be significantly affected by the steric effect of the substituent, as the substrate bearing the 2-methyl group CF3 + CF3 gave only trace amount of the expected product (Table 2, entry 13). 1a 2a 3a Table 2 Scope of N-Tosylhydrazones Entry Base Additive Temp (°C) 2a (mL) Yield (%)b (equiv) Cs2CO3 (3.0 equiv) NNHTs 1Cs2CO3 none 80 2 23 4 Å MS (3) + neat, 120 °C c Ar Rf 2Cs2CO3 none 110 2 33 Ar Rf (3) 1a–l 2 3a–l c 3Cs2CO3 4 Å MS 80 2 33 a (3) Entry Substrate 1a–l Ar Rf Yield (%) 4CsCO 4 Å MS 120 2 42 2 3 1 1a Ph CF 3a 61 (3) 3 c 2 1b 4-FC6H4 CF3 3b 57 5Cs2CO3 4 Å MS 80 1 20 (3) 3 1c 4-ClC6H4 CF3 3c 67 6CsCO 4 Å MS 120 3 51 2 3 4 1d 4-BrC6H4 CF3 3d 68 (3) 5 1e 4-IC6H4 CF3 3e 62 7Cs2CO3 4 Å MS 120 4 61 (3) 6 1f 2-BrC6H4 CF3 3f 64 8Cs2CO3 4 Å MS 120 5 51 7 1g 4-F3CC6H4 CF3 3g 69 (3) This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. 8 1h 3-O2NC6H4 CF3 3h 56 9CsCO 4 Å MS 120 10 57 2 3 9 1i 4-MeC H CF 3i 52 (3) 6 4 3 c 10 1j 2-naphthyl CF3 3j 51 10 K2CO3 (3) 4 Å MS 2 2 25 11 1k 4-MeOC H CF 3k 27 11 LiOt-Bu 4 Å MS 4 4 30c 6 4 3 (3) 12 1l 4-FC6H4 C3F7 3l 52 12 NaOt-Bu 4 Å MS 4 4 55c 13 1m 2-MeC6H4 CF3 3m trace (3) a Isolated yield. 13 KOt-Bu 4 Å MS 4 4 49 (3) Furthermore, we proceeded to expand the substance 14 K3PO4 (3) 4 Å MS 4 4 53 scope of arenes and examined the reactions in several aro- a The reaction was carried in 0.2 mmol scale with 4 Å MS (50 mg). Reaction time is 3 h and the substances were reacted at a sealed tube. matic solvents. In these cases, the reaction will meet the b Unless otherwise noted, it refers to isolated yield. c Yield determined by GC–MS. © Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 59–62 61 Syn lett Z. Zhang et al. Cluster problem of regioselectivity. Three aromatic solvents have Supporting Information been examined: anisole (4a), 1,3-dimethoxybenzene (4b), and mesitylene (4c). Supporting information for this article is available online at http://dx.doi.org/10.1055/s-0034-1378937. SupponritIgof rmoaitnSupponritIgof rmoaitn In the case of anisole (4a), two regiosomeric products 5aa and 5ab were isolated in approximately equal amount (Scheme 2, a). For 1,3-dimethoxybenzene (4b), attributed References and Notes to the steric effects, the formal C–C bond insertion only oc- curs at the sterically less hindered sites, giving 5b as the (1) (a) Kirsch, P. Modern Fluoroorganic Chemistry: Synthesis, Reac- tivity, Applications; Wiley-VCH: Weinheim, 2004. (b) Gladysz, J. sole product in moderate yield (Scheme 2, b). For the sym- A.; Curran, D. P.; Horváth, I. T. Handbook of Fluorous Chemistry; metric substrate mesitylene (4c), the reaction afforded the Wiley-VCH: Weinheim, 2005. (c) Horváth, I. T. Fluorous Chemis- expected product 5c, but in 27% yield due to steric hin- try; Springer: Berlin/Heidelberg, 2011. (d) Müller, K.; Faeh, C.; drance (Scheme 2, c). Diederich, F. Science 2007, 317, 1881. (e) Furuya, T.; Kamlet, A. S.; Ritter, T. Nature (London, U.K.) 2011, 473, 470. (f) Wang, J.; OMe Sánchez-Roselló, M.; Aceña, J. L.; del Pozo, C.; Sorochinsky, A. E.; OMe Fustero, S.; Soloshonok, V. A.; Liu, H. Chem. Rev. 2014, 114, 2432. Cs2CO3 (3.0 equiv) OMe 4 Å MS (2) For selected reviews, see: (a) Xu, X.-H.; Matsuzaki, K.; Shibata, + (a) N.
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