Radical Arylation of Triphenyl Phosphite Catalyzed By
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Subscriber access provided by Universidad de Alicante Article Radical Arylation of Triphenyl Phosphite Catalyzed by Salicylic Acid: Mechanistic Investigations and Synthetic Applications Manel Estruch-Blasco, Diego Felipe-Blanco, Irene Bosque, and José C. González-Gómez J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.0c00795 • Publication Date (Web): 18 May 2020 Downloaded from pubs.acs.org on May 24, 2020 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 37 The Journal of Organic Chemistry 1 2 3 4 5 6 7 8 9 Radical Arylation of Triphenyl Phosphite Catalyzed by Salicylic Acid: Mechanistic 10 11 Investigations and Synthetic Applications 12 13 Manel Estruch-Blasco,† Diego Felipe-Blanco,† Irene Bosque, Jose C. Gonzalez-Gomez* 14 15 16 Instituto de Síntesis Orgánica (ISO) and Departamento de Química Orgánica, Facultad de 17 Ciencias, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain 18 19 Email: [email protected] 20 21 CO H NH2 2 Ar O O 22 + OH <10 mol % Ar + P P OPh 23 P(OPh)3 SA OPh Ar OPh OPh + CH CN [0.20 M] 24 3 29 examples t-BuONO Ar, 20 ºC, 12 h 25 "Persistent radical effect" up to 94% yield 26 Metal free and additive free Availability of starting materials 27 SA as very convenient catalyst Gram-scale and operationally simple 28 Short reaction times at 20 ºC Unique reactivity of diaryl phosphonates 29 30 31 ABSTRACT: A straightforward and scalable methodology to synthesize diphenyl 32 arylphosphonates at 20 ºC within 1-2 h is reported, using inexpensive SA as the catalytic 33 34 promoter of the reaction. Mechanistic investigations suggest that the reaction proceeds via 35 36 radical-radical coupling, consistent with the so-called Persistent Radical Effect. The reaction 37 tolerated a wide range of functional groups and heteroaromatic moieties. The synthetic 38 39 usefulness and the unique reactivity of the obtained phosphonates were demonstrated in 40 41 different one-step transformations. 42 43 44 45 INTRODUCTION 46 47 Aryl phosphonates are privileged scaffolds among pharmaceuticals, agrochemicals, and 48 49 organic materials; as well as being versatile building blocks in organic synthesis or ligands 50 in transition-metal (TM) catalysis.1 Traditionally, these organophosphorus compounds were 51 52 prepared by reaction of electrophilic dialkyl chlorophosphonates with organolithium2 or 53 3 4 54 Grignard reagents. Since the seminal contribution of Hirao and coworkers, the 55 TM-catalyzed cross-coupling of (pseudo)haloarenes with H-phosphonates or trialkyl 56 57 58 59 60 ACS Paragon Plus Environment The Journal of Organic Chemistry Page 2 of 37 1 2 3 phosphites has been one of the most extensively used strategies to prepare aryl 4 5 phosphonates.5-7 Given the recent advancements in visible light photocatalysis,8 the room 6 7 temperature preparation of aryl phosphonates has also been accomplished using this 8 methodology. In this frame, the photoinduced reductive generation of aryl radicals trapped 9 10 by trisubstituted phosphites has been successfully implemented,9 as well as the single- 11 12 electron oxidation of arenes to obtain radical cations that easily add nucleophilic 13 10 14 phosphites. In addition, intensive efforts have been made recently in the development of 15 TM-free synthesis of aryl phosphonates at room temperature. In this context, one successful 16 17 approach relied on the nucleophilic addition of dialkyl phosphites to in situ generated arynes 18 11 19 from 2-(trimethylsilyl)aryl triflates (Kobayashi precursors, Scheme 1a). It has been also 20 recently reported that the combination of diaryliodonium salts with phosphites can form an 21 22 electron donor-acceptor complex, upon irradiation with blue light in the presence of a base, 23 24 leading to the desired aryl phosphonates (Scheme 1b).12 The photoinduced oxidation of 25 26 arylhydrazines to produce aryl radicals that were trapped with trialkyl phosphites was also 27 recently developed,13 using an organic photocatalyst and 1,4-diazabicyclo[2.2.2]octane 28 29 (DABCO) as additive under aerobic conditions (Scheme 1c). The visible light-driven 30 31 generation of aryl radicals from arylazo sulfones in the absence of photocatalyst, which was 32 14 33 trapped with triaryl phosphites, is another elegant approach recently reported (Scheme 1d). 34 Furthermore, a Sandmeyer-type reaction of in situ formed diazonium salts with triaryl 35 36 phosphites promoted by stoichiometric amounts of p-toluene sulfonic acid (TsOH), has also 37 15 38 allowed the efficient preparation of aryl phosphonates (Scheme 1e). Despite these 39 remarkable contributions, a TM-free synthesis of aryl phosphonates at room temperature, 40 41 from readily available starting materials, without stoichiometric additives or excess of 42 43 reagents, is still desired. 44 45 Given the high reactivity and low selectivity of aryl radicals, a sustained catalytic aryl 46 47 radical generation is a more convenient approach for a successful synthetic 48 transformation.16,17 We have recently demonstrated that aryl radicals can be catalytically 49 50 generated from readily available anilines that are in situ transformed into diazonium salts 51 18 52 with tert-butyl nitrite (TBN), by using salicylic acid (SA) as a catalyst at room 53 temperature.19 We do think that SA is a very convenient catalyst because is inexpensive, non- 54 55 toxic, and a renewable feedstock derived from natural salicin. As part of our ongoing 56 57 58 59 60 ACS Paragon Plus Environment Page 3 of 37 The Journal of Organic Chemistry 1 2 3 program, we decided to explore these mild and environmentally benign conditions to obtain 4 5 diphenyl arylphosphonates. 6 7 Scheme 1. Context of the TM-free synthesis of diaryl phosphonates 8 9 (a) Chen, Q., et al.: Reference 11 10 OTf CsF (5 equiv) Cs2CO3 (1 equiv) 11 Ar + P(OR)3 12 TMS CH3CN, 25 ºC, 24 h (2 equiv) (1 equiv) 13 14 (b) Lecroq, W., et al.: Reference 12 K CO (1 equiv) 15 2 3 Ar2IOTf + P(OR)3 Blue LEDs, 16 h 16 (1.5 equiv) (1 equiv) CH3CN, 35 ºC 17 (c) Li, R., et al.: Reference 13 18 Eosin B (5 mol%) O 19 + P(OR) DABCO (50 mol%) ArNHNH2 3 P 20 (1 equiv) (2 equiv) white LEDS, 45 ºC Ar OR 21 CH3CN, open air, 6 h OR 22 (d) Qiu, D., et al.: Reference 14 Blue LEDs, 24 h 23 ArN2SO2Me + P(OR)3 24 (1 equiv) (3.5 equiv) CH3CN, 35 ºC 25 (e) Wang, S., et al.: Reference 15 26 TsOH×H2O (1.2 equiv) 27 P(OPh) (3 equiv) ArNH t-BuONO (1.5 equiv) 3 28 2 (1 equiv) CH3CN, 0 ºC, 15 min 0 ºC- 25 ºC, 8 h 29 30 31 RESULTS AND DISCUSSION 32 33 Since at 60 ºC the diazonium salts can react with CH3CN to form acetamides via N- 34 20 35 arenenitrilium salts (Ritter reaction), we carefully controlled the reaction temperature at 20 36 ºC. We were pleased to observe how the addition of 10 mol % of salicylic acid (SA) 37 38 significantly accelerated the formation of phosphonate 2a in CH3CN (Table 1, entries 1-2). 39 40 Importantly, the reaction was complete within 1 h without preforming of the diazonium salt, 41 42 and without requiring an excess of triphenyl phosphite (TPP) or any other additive. We have 43 seen that the presence of minor amounts of H O is not critical for the success of the reaction, 44 2 45 but its addition to the reaction media was not beneficial neither (entry 3). The reaction 46 47 performance was worst under air atmosphere, which is in accordance with the intermediacy 48 49 of radicals, as also suggested by its complete inhibition in the presence of 2,2,6,6- 50 Tetramethylpiperidine 1-oxyl (TEMPO, entries 4-5). Given the precedents in the 51 52 photogeneration of aryl radicals from aryl diazonium salts,21 we carefully checked that the 53 54 ambient light was not promoting the reaction (entry 6). Importantly, no reaction was 55 observed using P(OEt) (entry 7), and the addition of TPP after the formation of the 56 3 57 58 59 60 ACS Paragon Plus Environment The Journal of Organic Chemistry Page 4 of 37 1 2 3 diazonium salt was detrimental to the reaction yield (entry 8).