A series of strong Brønsted acids has been synthesized in high yields using N-triflylphosphorimidoyl trichloride as reagent. The syntheses proceed efficiently with electron-rich, electron-deficient, and sterically hindered substrates

Kaib, P.

Lee, S.

List, B.

English

A series of strong Brønsted acids has been synthesized in high yields using N-triflylphosphorimidoyl trichloride as reagent. The syntheses proceed efficiently with electron-rich, electron-deficient, and sterically hindered substrates.</jats:p

Benjamin List

Sunggi Lee

Philip Kaib

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N-Triflylphosphorimidoyl Trichloride: A Versatile Reagent for the Synthesis of Strong Chiral Brønsted Acids

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Max-Planck-Institut für Kohlenforschung  Journal Article published in: Synlett 2017, 28, 1478-1480, Thieme Publishing Group https://www.thieme-connect.de/products/ejournals/pdf/10.1055/s-0036-1588782.pdf.  This article may be used for non-commercial purposes, for the copyright please see Thieme Publishing Group (www.thieme-connect.com).  N-Triflylphosphorimidoyl Trichloride: A Versatile Reagent for the Synthesis of Strong Chiral Brønsted Acids  Abstract A series of strong Brønsted acids has been synthesized in high yields using N-triflylphosphorimidoyl trichloride (1) as reagent. The syntheses proceed efficiently with electron rich, electron deficient, and sterically hindered substrates.  Key words N-triflylphosphorimidoyl trichloride, Brønsted acid, N-triflylphosphoramide, N-triflylthiophosphoramide, N,N’-bis(triflyl)phosphoramidimidate  Over the last decade, chiral phosphoric acid catalysts have attracted great attention because of their remarkable reactivity and ease of handling.1 Since Akiyama and Terada had reported successful application of BINOL-derived phosphoric acids or their salts as catalysts in Mannich reactions, numerous catalyst variations have been developed by modifying the 3,3’-substitutents of the BINOL backbone.2 Furthermore, the Yamamoto group demonstrated that the activity of phosphoric acid catalysts can be enhanced by replacing the OH group with an N-triflyl group.3 Due to the higher acidity of the resulting N-triflylphosphoramides, several groups successfully reported asymmetric reactions which could not be accomplished using the original phosphoric acids.4 However, despite of their utility, the synthesis of these catalysts requires a two-step procedure which involves a solvent change and a relatively long reaction time under heating.3a,5 During our studies on the development of even stronger Brønsted acid catalysts, we recently reported a practical method to introduce N-triflyl groups to molecular structures using N-triflylphosphorimidoyl trichloride (1) as a reagent (Scheme 1). We have prepared this substance in a solid state reaction between phosphorous pentachloride (PCl5) and trifluoromethansulfonylamide under reduced pressure.6 When compound 1 was reacted with different BINOLs (2) in the presence of trimethylamine or diisopropylethylamine in THF or toluene, intermediate 3 was formed within 10 min. Adding 0.5 equiv of ammonia or hexamethyldisilazane afforded the corresponding N-triflylphosphoramidimidate 4 in situ. With further heating under reflux, novel imidodiphosphorimidates (IDPi) 5 were obtained successively. On the basis of this observation, we wondered if it was possible to establish a new approach to Yamamoto catalysts, simply by hydrolyzing intermediate 3. Herein we report the fruition of these efforts with a general approach to various N-triflyl substituted chiral Brønsted acids.   Scheme 1 Preparation of N-triflylphosphorimidoyl trichloride 1 and its application to synthesis of imidodiphosphorimidates 5 Sunggi Leea, Philip S. J. Kaiba, Benjamin List*a aMax-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1 45470 Mülheim an der Ruhr, Germany  list@mpi-muelheim.mpg.de     Indeed, most BINOLs (2a-f), upon reaction with reagent 1 in dichloromethane and DIPEA, gave the corresponding intermediate 3 within 10 min. With sterically hindered BINOL 2g, the reaction took 1 h until completion. Further reaction with water required only 10 min with chlorides 3a-f and 1 h with compound 3g to furnish the corresponding acids. Products 5a-g were obtained in >80% yield regardless of the electronic or steric properties of the BINOL staring material (Table 1).  Table 1 Substrate scope of the Yamamoto-type Brønsted acid synthesisa  Entry Product Configuration R Yield (%) 1 6a S Ph 98 2 6b R 4-Ph-C6H4 97 3 6c S 1-Naph 90 4 6d R 2-Naph 96 5 6e S 9-Phenanthryl 89 6 6f S 3,5-(CF3)2-C6H3 97 7 6g S 2,4,6-i-Pr3-C6H2b 82 aReactions were performed with 1.0 equiv of 2, 1.1 equiv of 1, and 5.0 equiv of DIPEA in 0.25 mL of CH2Cl2 for 10 min, and then 20 µL of H2O was added to hydrolyse the intermediates 3. bIn this case, substitution and hydrolysis reactions each took 1 h.  Next, we applied our method to synthesize other strong Brønsted acids (Scheme 2). In 2008, the Yamamoto group exchanged the oxo-group of their catalysts to a thio-group. The resulting more acidic N-triflylthiophosphoramides successfully enabled catalytic enantioselective protonation reactions.7 Later, our group exchanged the oxo-group with an N-triflyl imino-group expecting an even further increase in acidity.8 In order to also obtain these two stronger acid motifs, intermediate 3 was reacted with H2S or with triflamide respectively. The target acids 7 and 8 were readily obtained within 20 min or 1 day, depending on the substrates.   Scheme 2 Synthesis of N-Triflylthiophosphoramides and N,N’-bis(Triflyl)phosphoramidimidates In summary, we have established a simple and practical route to synthesize strong chiral Brønsted acids. The method is effective for the preparation of N-triflylphosphoramides with electron deficient, electron rich, and  sterically demanding substrates.9 Furthermore, both of N-triflylthiophosphoramides and N,N’-bis(triflyl)phosphoramidimidates were prepared in high yields within one day. Further use of reagent 1 in catalyst development is currently underway in our laboratory.  Acknowledgment We thank the technicians of our group and the analytical departments of the institute for support of this work. Funding from the European Research Council (Advanced Grant CHAOS) is gratefully acknowledged.  Supporting Information Supporting information for this article is available online at https://www.thieme-connect.de/DOI/DOI?10.1055/s-0036-1588782. References and Notes (1) (a) Akiyama, T. Chem. Rev. 2007, 107, 5744. (b) Terada, M. Synthesis 2010, 1929. (c) Kampen, D.; Reisinger, C. M.; List, B. Top Curr. Chem. 2010, 291, 395. (d) Asymmetric Organocatalysis Workbench Edition; List, B.; Maruoka, K., Eds.; Thieme: Stuttgart, 2012. (e) Parmar, D.; Sugiono, E.; Raja, S.; Rueping, M. Chem. Rev. 2014, 114, 9047. (f) Akiyama, T.; Mori, K. Chem. Rev. 2015, 115, 9277. (2) (a) Akiyama, T.; Itoh, J.; Yokota, K.; Fuchibe, K. Angew. Chem. Int. Ed. 2004, 43, 1566. (b) Uraguchi, D.; Terada, M. J. Am. Chem. Soc. 2004, 126, 5356. See also: (c) Hatano, M.; Moriyama, K.; Maki, T.; Ishihara, K. Angew. Chem. Int. Ed. 2010, 49, 3823. (d) Klussmann, M.; Ratjen, L.; Hoffmann, S.; Wakchaure, V.; Goddard, R.; List, B. Synlett 2010, 2189. (e) Terada, M.; Kanomata, K. Synlett 2011, 1255. Also see: Mao, Z.; Mo, F.; Lin, X. Synlett 2016, 546. (g) Tay, J.-H.; Nagorny, P. Synlett 2016, 551. (h) Lai, Z.; Sun, J. Synlett 2016, 555. (i) Lebée, C.; Blanchard, F.; Masson, G. Synlett 2016, 559. (j) Qin, L.; Wang, P.; Zhang, Y.; Ren, Z.; Zhang, X.; Da, C.-S. Synlett 2016, 571. (k) Jiang, F.; Zhang, Y.-C.; Yang, X.; Zhu, Q.-N.; Shi, F. Synlett 2016, 575. (l) Kanomata, K.; Terada, M. Synlett 2016, 581. (m) Zhou, Y.; Liu, X.-W.; Gu, Q.; You, S.-L. Synlett 2016, 586. (n) Monaco, M. R.; Properzi, R.; List, B. Synlett 2016, 591. (3) (m) Monaco, M. R.; Pupo, G.; List, B. Synlett 2016, 1027. (4) Nakashima, D.; Yamamoto, H. J. Am. Chem. Soc. 2006, 128, 9626.  (5) (a) Enders, D.; Narine, A. A.; Toulgoat, F.; Bisschops, T. Angew. Chem. Int. Ed. 2008, 47, 5661; (b) Rueping, M.; Nachtsheim, B. J.; Moreth, S. A.; Bolte, M. Angew. Chem. Int. Ed. 2008, 47, 593; (c) Rueping, M.; Theissmann, T.; Kuenkel, A.; Koenigs, R. M. Angew. Chem. Int. Ed. 2008, 47, 6798; (d) Zeng, M.; Kang, Q.; He, Q.-L.; You, S.-L. Adv. Synth. Catal. 2008, 350, 2169; (e) Lee, S.; Kim, S. Tetrahedron Lett. 2009, 50, 3345; (f) Enders, D.; Seppelt, M.; Beck, T. Adv. Synth. Catal. 2010, 352, 1413; (g) Guan, H.; Wang, H.; Huang, D.; Shi, Y. Tetrahedron 2012, 68, 2728; (h) Guo, B.; Schwarzwalder, G.; Njardarson, J. T. Angew. Chem. Int. Ed. 2012, 51, 5675; (i) Han, Z.-Y.; Chen, D.-F.; Wang, Y.-Y.; Guo, R.; Wang, P.-S.; Wang, C.; Gong, L.-Z. J. Am. Chem. Soc. 2012, 134, 6532; (j) Borovika, A.; Nagorny, P. Tetrahedron 2013, 69, 5719; (k) Cui, Y.; Villafane, L. A.; Clausen, D. J.; Floreancig, P. E. Tetrahedron 2013, 69, 7618; (l) Enders, D.; Rembiak, A.; Seppelt, M. Tetrahedron Lett. 2013, 54, 470; (m) Wang, P.-S.; Li, K.-N.; Zhou, X.-L.; Wu, X.; Han, Z.-Y.; Guo, R.; Gong, L.-Z. Chem. Eur. J. 2013, 19, 6234; (n) Hong, X.; Küçük, H. B.; Maji, M. S.; Yang, Y.-F.; Rueping, M.; Houk, K. N. J. Am. Chem. Soc. 2014, 136, 13769; (o) Kong, L.; Han, X.; Jiao, P. Chem. Commun. 2014, 50, 14113; (p) Li, N.; Chen, D.-F.; Wang, P.-S.; Han, Z.-Y.; Gong, L.-Z. Synthesis 2014, 46, 1355; (q) Wu, X.; Li, M.-L.; Wang, P.-s. J. Org. Chem. 2014, 79, 419; (r) Lin, J.-S.; Yu, P.; Huang, L.; Zhang, P.; Tan, B.; Liu, X.-Y. Angew. Chem. Int. Ed. 2015, 54, 7847; (s) Liu, J.; Zhou, L.; Wang, C.; Liang, D.; Li, Z.; Zou, Y.; Wang, Q.; Goeke, A. Chem. Eur. J. 2016, 22, 6258. (t) Also see: Hatano, M; Ishihara, H.; Goto, Y.; Ishihara, K. Synlett 2016, 564. (6) Rueping, M.; Nachtsheim, B. J.; Koenigs, R. M.; Ieawsuwan, W. Chem. Eur. J. 2010, 16, 13116. (7) (a) Kaib, P. S. J.; Schreyer, L.; Lee, S.; Properzi, R.; List, B. Angew. Chem. Int. Ed. 2016, 55, 13200. (b) Xie, Y.; Cheng, G. J.; Lee, S.; Kaib, P. S. J.; Thiel, W.; List, B. J. Am. Chem. Soc. 2016, 138, 14538. (8) (a) Cheon, C. H.; Yamamoto, H. J. Am. Chem. Soc. 2008, 130, 9246. (b) Cheon, C. H.; Yamamoto, H. Org. Lett. 2010, 12, 2476. (c) Yokosaka, T.; Kanehira, T.; Nakayama, H.; Nemoto, T.; Hamada, Y. Tetrahedron 2014, 70, 2151. (d) Sai, M.; Yamamoto, H. J. Am. Chem. Soc. 2015, 137, 7091. (9) Kaib, P. S. J.; List, B. Synlett 2016, 27, 156.  (10) General Procedure:  In a flame-dried vial under Ar, the corresponding (S)- or (R)-BINOL (1.0 equiv) was dissolved in anhydrous CH2Cl2 (0.20 M). TfNPCl3 (1.1 equiv) and DIPEA (5.0 equiv) were added and the mixture was stirred for 10 min at ambient temperature. After the full consumption of the starting material (as indicated by TLC), the second nucleophile was added (20 µL for H2O, 2.0 equiv for H2S and TfNH2). After an additional 10 min of stirring, the reaction mixture was dried over Na2SO4, filtered, concentrated, and purified by column chromatography on silica gel to afford the desired product as a salt. Acidification in CH2Cl2 with HCl (3.0 M) followed by drying under reduced pressure afforded the desired product as a free acid.                                       

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N-Triflylphosphorimidoyl Trichloride: A Versatile Reagent for the Synthesis of Strong Chiral Brønsted Acids

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