http://www.diva-portal.org This is the published version of a paper published in ChemistryOpen. Citation for the original published paper (version of record): Trillo, P., Slagbrand, T., Tinnis, F., Adolfsson, H. (2017) Mild Reductive Functionalization of Amides into N-Sulfonylformamidines. ChemistryOpen, 6(4): 484-487 https://doi.org/10.1002/open.201700087 Access to the published version may require subscription. N.B. When citing this work, cite the original published paper. Permanent link to this version: http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-138585 DOI:10.1002/open.201700087 Mild Reductive Functionalization of Amides into N- Sulfonylformamidines Paz Trillo,[a] Tove Slagbrand,[a] Fredrik Tinnis,*[a] and Hans Adolfsson*[a, b] The development of aprotocolfor the reductivefunctionaliza- tion of amides into N-sulfonylformamidines is reported. The one-pot procedure is based on amild catalytic reduction of tertiaryamides into the corresponding enamines by the use of Mo(CO)6 (molybdenum hexacarbonyl) and TMDS (1,1,3,3-tetra- methyldisiloxane). The formed enamines were allowed to react with sulfonyl azidestogive the target compounds in moderate to good yields. The amidine functional group is frequently found in biological- ly activecompounds possessing anti-inflammatory,antibacteri- al, antiviral, antibiotic, and anestheticproperties.[1] They are also employed as intermediates and precursors in organic syn- thesis of importantheterocyclic compounds such as imida- zoles, quinazolines,isoquinolines, and pyrimidines.[2] Further- more, amidines are employed as ligandsinmetal complexes and as protecting groups for primary amines.[3] Scheme1.Preparation of amidines through a–c) electrophilic amide activa- The stability of amides makes this functional group valuable tion and d) reductive functionalization of amides. to include in anarray of different compounds such as pharma- ceuticals, agrochemicals, and other organic materials.[4] On the other hand, the inherent stability is contributing to the reluc- tance of employing amides as synthetic intermediates. The amide activation and subsequentamidine synthesis were also concept of activation and functionalization of amides is well reported by Velavan et al. andPhakhodee et al. (Scheme 1b [13, 14] known and the discoveryoftriflic anhydride (Tf2O) as an amide and 1c). The direct condensation of sulfonamides with activating agent constituted amajor advance within this N,N-dimethylformamide dimethyl acetal (DMF-DMA) was fur- [5] [15] field. In recent years, research based on Tf2Oasanamide ac- thermore reported by Sharma and co-workers, and N-sulfo- tivator has progressed immensely and abroad variety of mild nylformamidines can also be prepared from cyanamides.[16] transformations have been reported.[6] Besides classical amide The development of the chemoselectivereduction of [7] [8] [9] [10] activation reagents such as POCl3, SOCl2, PCl3, PCl5, and amides has been very successful and, today,functional groups [11] [17] (COCl)2, triflic anhydride was also employed by Charette and such as ketones, aldehydes,and imines can be tolerated. Grenon for the transformation of amides into amidines Consequently,the reductivefunctionalization of amides is now [12] (Scheme 1a). Later,protocols based on AlMe3 and Ph3P/I2 for an emerging fieldwithin organic chemistry. This area of re- search can be divided in two divisions, reduction of amides for [a] Dr.P.Trillo, T. Slagbrand, Dr.F.Tinnis, Prof. H. Adolfsson the formation of electrophilicspecies (iminium ion) or nucleo- Department of Organic Chemistry,Stockholm University philic species(enamine) with the subsequenttrapping/func- 106 91 Stockholm (Sweden) [18] E-mail:[email protected] tionalization thereof. Herein, we demonstrate amild proto- [email protected] col for the reductivefunctionalization of amides (via enamines) [b] Prof. H. Adolfsson for the formation of N-sulfonylformamidines (Scheme 1d). Department of Chemistry,Umeå University We have previously reported on ahighly chemoselective 901 87 Umeå (Sweden) protocolfor amide reduction into either amines or alde- E-mail:[email protected] hydes.[19] The Mo(CO) -catalyzed system was further investigat- Supporting Information and the ORCID identification number(s) for the 6 author(s) of this article can be found underhttps://doi.org/10.1002/ ed and it was discovered that enamines could also be ac- open.201700087. cessed.[20] Recently,wedemonstrated the reductivefunctionali- 2017 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA. zation of amides into 4,5-dihydroisoxazoles and triazolines.[21] This is an openaccessarticleunder the termsofthe Creative Commons In the case of the latter compounds, the generated enamines Attribution-NonCommercial-NoDerivs License, which permits useand were trapped with organic azides and, during the evaluation distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are of the substrate scope, it wasobserved that N-sulfonylforma- made. midinewas formed upon the use of sulfonyl azide (Scheme 2). ChemistryOpen 2017, 6,484 –487 484 2017 The Authors. PublishedbyWiley-VCH Verlag GmbH &Co. KGaA, Weinheim Scheme2.One-pottransformation of amides into N-sulfonylformamidines. As the sulfonyl group is an important class of pharmacophore and N-sulfonylamidines, in particular, exhibit variousimportant biological and pharmaceuticalactivities, we decided to opti- mize the conditions for the synthesis of this class of com- [22] pounds. The Mo(CO)6-catalyzedreduction of tertiaryamide 1a gives full conversion into the corresponding enamine 1a’, as determinedby1HNMR spectroscopy using an internal standard. The screening of the reaction conditions toward the forma- Scheme4.Evaluation of the amide scope. tion of N-sulfonylformamidine showedthat two equivalents of the sulfonyl azide were needed and the reaction then went to completion within 30 min. Awide varietyofbenzenesulfonyl was in favor of enamine in the case of longeraliphatic chains; azides were then evaluated,which gave the corresponding N- however,only amine wasessentially formed whenemploying sulfonylformamidines in yields between 47 and 79% acetamides. Thus, it was necessary to use phenylacetamides (Scheme3, 3a–3o). Several functional groups such as halide, throughout the scope. The reduction of the different amides ketone, secondary amide, ester,amine, cyano, and nitro was straightforwardand produced the correspondingenam- groups were tolerated. Benzyl- and aliphatic-substituted sulfo- ines in excellent yields (>95%, according to 1HNMR spectros- nyl azides could also be employed and compounds 3p and 3q copy). The subsequentinsitu reactionwith sulfonyl azide were isolated in 83 and 58%yields, respectively. yielded avariety of formamidines substituted with N,N-dimeth- yl (3r, 3s), piperidine(3t, 3u), N,N-dibutyl (3v), N,N-diisopropyl (3w), indoline (3x), morpholine (3y), 2,6-dimethylpiperidine (3z), and N-methyl-N-phenyl (3aa). The overall substrate evaluation shows that this protocol allowsfor N-sulfonylamidines to be synthesized with versatility in both the sulfonyl and the amine part. The products were obtained in moderatetogood yields and most of the com- poundsare novel and have not been previously reported. The developedprotocol for the transformation of carboxa- mides into N-sulfonylformamidines was further evaluated on apreparative scale (Scheme 5). Although we never experi- enced anyissues, one should alwaysbeaware of the explosion risks associatedwith organic azides.[23] The reaction with amide 1a (10 mmol) was performed by using atwo-necked round- bottomed flask under an inert atmosphere and amidine 3a was isolated in 70%yield (1.6 g). The decomposition of triazolines can lead to avariety of dif- ferent compounds such as triazoles, aziridines,and amidines.[24] In arecent study,wedemonstrated the preparationand isola- Scheme3.Evaluation of sulfonylazidesinthe transformation of amides into amidines. Next, we performed an evaluation of different amides in the preparation of N-sulfonylformamidines (Scheme4). During our previousinvestigation of amide transformation into triazolines, it was found that aliphatic amides constituted achallenging Scheme5.One-pottransformation of amide 1a into sulfonylformamidine class of substrates for enamine formation.[21b] The product ratio 3a on apreparative scale. ChemistryOpen 2017, 6,484 –487 www.chemistryopen.org 485 2017 The Authors. PublishedbyWiley-VCH Verlag GmbH &Co. KGaA, Weinheim tion of awide variety of different triazolines.[21b] It was also the trapping reagent for phenyldiazomethane. The p-methoxy- shown that, in some cases,depending on the electronic nature substituted amide 1g was reduced to the corresponding en- of the amide or the azide, triazoles would form spontaneously amine (1g’)and then treated with both sulfonyl azide 2a and by elimination of the amine moiety.The instability of sulfonyl- benzoicacid in situ. The side-product 1-(diazomethyl)-4-me- substituted triazolines is known,but their disintegration does thoxybenzene (5)derived from triazoline 4g was successfully not necessarily form amidines. For instance, Bakulev and co- intercepted (Scheme6c). 4-Methoxybenzyl benzoate (6)could workers reported on the synthesis of 1H-1,2,3-triazolesvia sul- be confirmed by the crude 1HNMR spectrum and also by sub- fonyl-substituted triazolines derived from enamines andsulfo- sequent isolation/characterization(see the Supporting Informa- nyl azides.[24c] Furthermore, the collapse