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And Monofluoromethyl-Containing Amides Via Ritter Reaction Tetrahedron Letters 47 (2006) 6753–6756 Facile preparation of difluoromethyl- and monofluoromethyl-containing amides via Ritter reaction Jun Liu, Chuanfa Ni, Ya Li, Laijun Zhang, Guanyu Wang and Jinbo Hu* Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Rd., Shanghai 200032, China Received 1 June 2006; revised 16 July 2006; accepted 18 July 2006 Available online 4 August 2006 Abstract—Both secondary and tertiary difluoromethylated carbinols were found to readily react with acetonitrile under the catalysis of concentrated sulfuric acid to give the corresponding difluoromethylated acetamides in good yields, which is remarkably more efficient than the previously reported Ritter reactions with corresponding trifluoromethylated carbinols. Similarly, monofluoro- methylated and (benzenesulfonyl)difluoromethylated carbinols have shown good reactivity in the Ritter reactions. Since the acet- amides can be mildly deacetylated to give amines, the present methodology provides a convenient way for the synthesis of both difluoromethyl- and monofluoromethyl-containing amines starting from simple carbonyl compounds. Ó 2006 Elsevier Ltd. All rights reserved. During the past two decades, fluorine has been probably methylated amides 3 are scarce.5a,6 Previously, we devel- the most highlighted halogen element (so-called ‘a small oped the nucleophilic difluoromethylation methods for atom with a big ego’). Today, around 20% of pharma- the efficient synthesis of difluoromethyl alcohols and ceuticals on the market and up to a quarter of those in amines from aldehydes, ketones, and aldimines.5a,b the development pipeline contain fluorine.1 The incorpo- Herein, we wish to report our continuing work in the ration of fluorine atom(s) into organic molecules can preparation of difluoromethyl amides 3 via Ritter reac- often impart profound and unexpected chemical, tion with difluoromethylated carbinols 2 that can be biological, and physical properties. Such highly intrigu- obtained through nucleophilic difluoromethylation of ing fluorine effects (also called ‘fluorine magic’) have carbonyl compounds 1 (Scheme 1). spurred organic chemists to develop efficient and conve- nient methods for the synthesis of fluorine-containing Ritter reaction has long been known as a useful 2 organic molecules. Recently, difluoromethyl (CF2H) approach for the synthesis of amides and amines (after functionality has been realized to be an important group hydrolysis of amides) from alcohols or alkenes.7 How- to modulate the biological properties within bioactive ever, although the Ritter reaction has been frequently molecule, given the fact that the CF2H group has similar used in the nonfluorinated systems including the high lipophilicity as trifluoromethyl (CF3) group, and Merck’s industrial-scale synthesis of anti-HIV drug Cri- 8 more importantly, the CF2H group often behaves as a xivan (indinavir), its application for the preparation of hydrogen donor through hydrogen bonding and is thus fluorinated amides or amines was not well explored. The highly useful in applications where a more lipophilic Ritter reactions between tertiary trifluoromethylcarbi- 1 2 hydrogen bond donor other than hydroxyl (OH) group nols 4 (Scheme 2,RF =CF3;R,R = aryl, alkyl, etc.) is required.2,3 In some cases, the difluoromethylated and acetonitrile under the catalysis of concentrated compounds exhibit increased bioactivity over their tri- fluoromethylated counterparts.4 Although several meth- F ods for the synthesis of difluoromethylated compounds H 5 F O O are available, the reports on the synthesis of difluoro- HO CF2 H R1 N R3 R1 R2 R1 R2 R2 H Keywords: Fluorine; Difluoromethyl; Amide; Ritter reaction. 321 * Corresponding author. Tel.: +86 21 54925174; fax: +86 21 64166128; e-mail: [email protected] Scheme 1. 0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2006.07.079 6754 J. Liu et al. / Tetrahedron Letters 47 (2006) 6753–6756 11,13 + Table 1. Preparation of difluoromethylated amides 1 RF OH H RF CH3CN R1 R2 1 2 CH CN, H SO - H2O R R HO CF2H 3 2 4 HF2C NHAc R1 R2 1 2 4 5 reflux, 70~80 oC R R 2 3 H2O RF a RF N C CH3 NHCOCH3 Entry Carbinol 2 Product 3 Yield (%) - H+ 1 2 R1 R2 R R CF2H 6 7 1 2a 97 NHAc (RF= CF3, CF2H, CFH2, CF2SO2Ph, etc.) MeO (3a) Scheme 2. CF2H 2 2b 47 sulfuric acid were reported by both Kaluszyner’s6c and NHAc Prakash’s groups.9 However, a similar reaction could (3b) not work effectively with secondary trifluoromethylcar- 1 2 9,10 NHAc binols (4, when RF =CF3,R or R = H). Obvi- ously, here the strong electron-withdrawing CF group Ph CF H 3 2 75 significantly destabilizes the carbocation species 5, (3c) 1 2 1 2 3 2c and R ,R groups (R ,R= aryl, alkyl, and other CF2H electron-donating groups, but not H) are essentially Ph NHAc Trace important to make the reaction proceed (Scheme 2). (3c') We envisioned that since the difluoromethyl (CF2H) and CF H monofluoromethyl (CFH2) groups have relatively weak- 2 er electron-withdrawing property, the Ritter reaction 4 2d NHAc 93 with secondary difluoromethylcarbinols and mono- fluoromethylcarbinols may proceed smoothly. Based Me2N (3d) on these considerations, we firstly prepared a variety of structurally diverse difluoromethyl-containing carbi- OMe nols 2a–i in satisfactory to excellent overall yields from 5 2e NHAc 92 corresponding aldehydes and ketones by using the (3e) previously reported procedure (Scheme 3).5c The Ritter CF2H reactions between 2 and acetonitrile were typically CF2H performed in the presence of concentrated sulfuric acid as a catalyst at 70–80 °C(Table 1).11 As we expected, 6 2f NHAc 79 a variety of secondary difluoromethylcarbinols 2a–g (3f) could smoothly react with acetonitrile to give the corre- sponding difluoromethylated acetamides 3a–g (Table 1, entries 1–7). In most cases, the product yields were good HF2C NHAc to excellent, which is remarkably more efficient than 7 2g H 55 the previously reported similar reactions with trifluo- (3g) romethylcarbinols.6c,9 Amidation of difluorocarbinol 2c occurred via intramolecular rearrangement to give HF C NHAc 3c as the major product (entry 3). The reactions with 2 8 2h CH3 82 the tertiary difluorocarbinols 2h and 2i gave the amides (3h) HF2C NHAc 1) PhSO2CF2H, LHMDS, O o THF-HMPA, - 78 C HO CF2H 9 2i 82 R1 R2 R1 R2 2) Na(Hg), CH3OH, Na2HPO4 (3i) o 1 -20~0 C 2 a Isolated yield. 2a: R1 = H, R2 = 4-methoxyphenyl (93%) 2b: R1 = H, R2 = 2-naphthyl (78%) 2c: R1 = H, R2 = (E)-PhCH=CH- (69%) 2d: R1 = H, R2 = 4-(dimethylamino)phenyl (79%) 3h and 3i in good yields (entries 8 and 9). Monofluorom- 2e: R1 = H, R2 = 2-methoxyphenyl (86%) ethylcarbinol 9 was also prepared and its reaction with 2f: R1 = H, R2 = 4-(tert-butyl)phenyl (84%) acetonitrile under similar Ritter reaction conditions 2g: R1 = H, R2 = Ph (66%) 1 2 gave the monofluoromethylated amide 10 in 94% yield 2h: R = CH3, R = Ph (64%) 2i: R1 = Ph, R2 = Ph (62%) (Scheme 4). It is noteworthy to mention that in our reactions (as shown in Table 1 and Scheme 4) we did Scheme 3. not observe the formation of any dehydrated product J. Liu et al. / Tetrahedron Letters 47 (2006) 6753–6756 6755 OH In summary, difluoromethylated, monofluoromethyl- CHO PhSO2CFH2, n-BuLi SO2Ph ated and (benzenesulfonyl)difluoromethylated amides were successfully prepared with a simple Ritter reaction THF, -78 oC, 74% F Cl Cl procedure. Our results indicate that difluoromethyl, 8 monofluoromethyl, and (benzenesulfonyl)difluoro- methyl groups are all relatively weaker electron-with- Na(Hg), Na HPO drawing functionalities compared to the trifluoromethyl 2 4 65% o o group, which enabled us to efficiently prepare both CH3OH, -20 C~0 C secondary and tertiary difluoromethylated (or monoflu- oromethylated) amides in good yields. Since the difluo- F OH romethylated and monofluoromethylated amides can CH3CN,H2SO4 F NHAc be deacetylated under mild conditions, the present meth- o reflux, 70 C Cl odology provides a useful approach for the preparation Cl 10 9 of difluoromethyl- and monofluoromethyl-containing 94% amines. Scheme 4. Acknowledgements OH We gratefully thank the National Natural Science CHO SO2Ph PhSO2CF2H, LHMDS Foundation of China (20502029), Shanghai Rising-Star F THF-HMPA F Program (06QA14063), and the Chinese Academy of Me2N Me2N -78 oC, 90% Sciences (Hundreds-Talent Program) for financial 11 support. CH3CN, H2SO4 87% reflux, 70 oC Supplementary data NHAc Supplementary data associated with this article can be SO2Ph found, in the online version, at doi:10.1016/j.tetlet. F F 2006.07.079. Me2N 12 Scheme 5. References and notes 1. (a) Thayer, A. M. Chem. Eng. News 2006, 84, 15–24, 27– 1,1-difluoroalkene or 1-fluoroalkene. Kaluszyner et al. 32; (b) Yarnell, A. Chem. Eng. News 2006, 84, 12–18. found that during the Ritter reactions with dichloro- 2. (a) Uneyama, K. Organofluorine Chemistry; Blackwell: methylcarbinols and methylcarbinols, alkenes were ob- New Delhi, 2006; (b) Chambers, R. D. Fluorine in Organic tained as the major products presumably through an Chemistry; Blackwell and CRC: Oxford, 2004; (c) Kirsch, acid-catalyzed dehydration mechanism.6c P. Modern Fluoroorganic Chemistry: Synthesis, Reactivity, Applications; Wiley-VCH: Weinheim, 2004; (d) Organo- fluorine Compounds: Chemistry and Applications; Hiyama, We also tried the Ritter amidation with (benzenesulfon- T., Ed.; Springer: New York, 2000. yl)difluoromethylated carbinol 11, and the amide prod- 3. Erickson, J. A.; McLoughlin, J. I. J. Org. Chem. 1995, 60, uct 12 was successfully obtained in 87% yield (Scheme 1626–1631. 5). 4. Graneto, M. J.; Phillips, W. J. US patent 5,093,347, 1992. 5. For recent examples, see: (a) Li, Y.; Hu, J. Angew. Chem., In order to demonstrate the possibility of further appli- Int. Ed. 2005, 44, 5882–5886; (b) Ni, C.; Hu, J.
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