Letter Cite This: ACS Catal. 2019, 9, 7741−7745
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1,3-Diamine Formation from an Interrupted Hofmann−Löffler Reaction: Iodine Catalyst Turnover through Ritter-Type Amination Thomas Duhamel,†,‡ Mario D. Martínez,† Ioanna K. Sideri,† and Kilian Muñiz*,†,§ †
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Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, 16 Avgda. Països Catalans, 43007 Tarragona, Spain ‡ Universidad de Oviedo, Julian Clavería, s/n, 33006 Oviedo, Spain § ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain S Supporting Information *
ABSTRACT: An iodine-catalyzed Ritter-type amination of nonactivated C−H bonds is presented enabling the formation of 1,3-αtertiary diamines. A sulfamidyl radical serves as the promoter in a guided tertiary C−H iodination through an exclusive 1,6-HAT process. The subsequent Ritter reaction furnishes the C−N bond and establishes an unprecedented concept for catalyst turnover in iodine redox catalysis. The general robustness of the methodology, including broad functional group tolerance, was demonstrated for 24 different 1,3-diamine derivatives, which were synthesized in yields of 42%−99%. KEYWORDS: amination, 1,3-diamines, Hofmann-Löffler reaction, iodine catalysis, Ritter reaction
A
mong the plethora of amino groups, the 1,3-diamine motif is an important structural unit, which is present in several molecules of strategic biological interest, including natural products such as carfentanil, nankakurine A, and the manzacidin family.1 In addition, it has been employed as building blocks in synthetic organic chemistry2 and in ligand engineering for asymmetric transition-metal catalysis.3 In contrast to the related family of 1,2-diamines,4 the corresponding 1,3-diamine scaffold has received significantly less attention in methodology development, and its synthesis has largely remained a domain of transition-metal catalysis.5 We now gave consideration to the concept of the venerable Hofmann−Löffler reaction,6 which mechanistically enables position-selective C−H amination7 through a sulfamidylradical mediated 1,n-hydrogen atom transfer (1,n-HAT). Previously, we have developed C(sp3)−H amination protocols under iodine8 and bromine9 catalysis. While for the class of 1,3-diamines, the direct strategy of C−H functionalization within amidyl radicals would require a challenging 1,4-HAT process and an external nucleophile, we explored alternative amidyl radical precursors based on tracelessly removable functional entities and turned to sulfamides (sulfuric diamides; see Figure 1, conceptual outline). Following the behavior of related sulfamate esters,10 these compounds should enable guided 1,6-HAT processes. We decided to start to investigate the reactivity of sulfamides under the conditions of iodine oxidation catalysis11 for model compound 1a, which contains an activated benzylic carbon position for the rapid cyclization and C−N bond installment (Scheme 1). After optimization of the iodine(III) oxidant,12 the corresponding 1,3-diamine 2a was obtained with an © XXXX American Chemical Society
Figure 1. Structure and synthetic strategy.
Scheme 1. 1,3-Diamine Formation through Hofmann− Löffler Reaction: Initial Substrate Exploration
excellent yield of 90% using molecular iodine as the catalyst source and the iodine(III) reagent PhI(oFBA)2 (oFBA = 2Received: April 17, 2019 Revised: July 22, 2019
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ACS Catalysis fluorobenzoate) as the terminal oxidant. Heterocycle 2a was formed as a single diastereoisomer and the expected stereochemistry of two equatorial phenyl substituents was confirmed by X-ray analysis. However, the observed cyclization to 1,3diamines turned out to be limited to benzylic positions. The subsequent attempt to move from the activated benzylic carbon position in 1a to a nonactivated secondary carbon position in 1b did not provide the desired cyclization product. Instead, the corresponding alkyl iodide 2b was isolated in 40% yield for commercial (diacetoxyiodo)benzene (PIDA) as the oxidant.12 Conversion to the 1,3-diamine 2c was only possible with the strong oxidant PhICl2, thus preventing an overall catalytic transformation. To arrive at a synthetically useful catalytic amination reaction at nonactivated C(sp3)−H positions, we decided to tackle tertiary alkyl positions. Amination reactions of this type13 are highly sought-after, since they provide access to the important class of tert-alkyl amines, which constitute important subgroups in naturally occurring alkaloids.14 Among all the reported synthetic approaches, the Ritter-type amination15 at nonactivated C− H positions stands out as a potent strategy, in which harsh conditions16 or transition-metal catalysis17 have so far dominated the stage. Recently, a new concept was designed to perform multiple selective halogenation reactions within the Hofmann−Löffler manifold.10,18 For halogenations at tertiary C−H bonds, the related sulfamate ester group was of unique effectiveness, because of its low tendency to undergo amination itself.10a,b,d This encouraged us to investigate a guided Ritter-type amination under iodine catalysis to access the formation of 1,3-α-tertiary diamines. Indeed, the underlying strategy (Table 1) was to direct the iodination within the Hofmann−Löffler manifold (1,6-HAT) at a nonactivated tertiary carbon position. Following the manifest stability of 2b toward intramolecular amination, we envisaged that a plausible intermolecular
amination could derive from a Ritter-type reaction at the more reactive tertiary carbon position. We used 3a as a model substrate to optimize the reaction conditions. While trying the same system of 20 mol % of molecular iodine and 2 equiv of PhI(mClBA)2 (mClBA = 3chlorobenzoate) as an oxidant,8a we already achieved the good yield of 81% for 4a (Table 1, entry 1). The use of PhI(oFBA)2 slightly improved the yield to 90% (Table 1, entry 2). Moving to PIDA, a yield of 73% was obtained, together with unidentified decomposition products (Table 1, entry 3). Reducing the amount of molecular iodine and PIDA to 10 mol % and 1.2 equiv, respectively, enhanced the selectivity and generated an excellent yield of 95% (Table 1, entry 4). Decreasing the amount of catalyst further, affected the effectiveness of the catalysis (Table 1, entries 5 and 6). Despite an excellent yield of 95% using N-iodosuccinimide NIS,19 more ionic iodine sources did not promote the reaction (Table 1, entries 7 and 8). Finally, exploration of irradiation conditions either under blue or purple LEDs (Table 1, entries 9 and 10) determined optimum conditions for the latter with an isolated yield of 99% for 4a. With optimized conditions in hand, the general applicability of the new methodology for 1,3-α-tertiary diamine formation was explored (Scheme 2). At the outset, it was explored whether a branched substitution pattern was required for the reaction. Gratifyingly, the primary alkyl substitution in 3b led to a comparable yield for 4b (90%), indicating that a Thorpe− Ingold effect is not required. Performing the reaction with 2propionitrile as a solvent in the presence of 20 mol % molecular iodine and 1.5 equiv of PIDA, the corresponding 1,3-diamine 4c was obtained in 83% yield. Comparison with an N-tert-butyl substituent at the sulfamidyl radical provided similar outcome for 4d. To implement the pharmaceutically important class of benzylamines into the 1,3-diamination products, a large number of substrates with different aryl substituents were found compatible, demonstrating the reaction tolerance to either electron-rich or poor arenes (4e−4k, 62%−99%). Next, we investigated the reactivity of a benzyl-protected derivative 3l. The reaction is efficient, providing 4l with a yield of 70%. A successful experiment at 1 mmol scale was also performed, indicating the robustness of the catalysis (4l, 62%). The same holds true for amination at chiral tertiary centers, which form the corresponding chiral tert-alkyl amines 4m and 4n in good yields as equimolar mixtures of diastereomers. For the substrates 3o−3q with different alkyl chains at the internal nitrogen atom containing potentially competing methyl and methylene positions, complete selectivity for the tertiary C−H bond was encountered and 4o−4q were isolated in 60%−82% yield. The C−H amination proceeds equally well at cyclic positions as demonstrated for 4r and 4s. No benzylic functionalization was detected for the former example, and generally, no free radical-derived byproducts have ever been observed under the present protocol. To further demonstrate the exclusive dominance of the guided C−H functionalization, compound 3t was investigated, which exclusively formed 4t, without any reaction at the remote tertiary C−H group. The reaction tolerates functionalized side chains, as demonstrated for the examples of nitrile 4u and acetate 4v. Finally, amination of enantiopure chiral pool derivatives was successfully explored for the proline derivative 4w (70% yield). For the more complex structure of the 5α-androsterone derivative 3x, stereoselective formation of the 5β-aminated product 4x was
Table 1. Ritter-Type Amination through the Hofmann− Löffler Manifold: Reaction Optimization
entry
iodine source (x mol %)
oxidant (y equiv)
yield [%]
1 2 3 4 5 6 7 8 9 10
I2 (20 mol %) I2 (20 mol %) I2 (20 mol %) I2 (10 mol %) I2 (5 mol %) I2 (2.5 mol %) NIS (20 mol %) Bu4NI (20 mol %) I2 (10 mol %) I2 (10 mol %)
PhI(mClBA)2 (2 equiv) PhI(oFBA)2 (2 equiv) PIDA (2 equiv) PIDA (1.2 equiv) PIDA (1.2 equiv) PIDA (1.2 equiv) PIDA (1.2 equiv) PIDA (1.2 equiv) PIDA (1.2 equiv) PIDA (1.2 equiv)
81 90 73 95 48 30 95 0 80a 99b
a Reaction performed under blue LED irradiation. performed under purple LED irradiation.
b
Reaction 7742
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ACS Catalysis Scheme 2. Guided Ritter-Type C(sp3)−H Amination: Scope
strated for 4l. It can be deprotected in an orthogonal manner to the two amides 5a and 5b, respectively, and consecutively to monobenzylated 1,3-diamine 5c, maintaining an amino differentiation. Regarding the mechanistic basis for the present transformation, acylhypoiodite (IOAc) is generated at the outset from comproportionation between molecular iodine and PIDA (Figure 2).20 This electrophilic iodine(I) catalyst state
Figure 2. Mechanism of the guided Ritter-type amination within an interrupted iodine-catalyzed Hofmann−Löffler reaction.
iodinates the NH bond of the substrate 3a to form the required intermediate N−I bond A. Unlike other amide sources,8a sulfamides provide effective N-iodination with IOAc. In the presence of light, homolysis to the N-centered radical species B occurs.21 A subsequent 1,6-HAT process to C occurs, followed by an iodination through a radical chain mechanism22 with a quantum yield of 120, to selectively provide the tert-alkyl iodide D. This is the first example of an iodine-catalyzed Hofmann−Löffler process comprising a 1,6HAT. As in the related cases involving a 1,5-HAT,8,9 the experimental value kH/kD = 5.5 from a competition reaction points to the H-abstraction as the most probable slow step. At this stage, a Ritter reaction can proceed to the product and to the regeneration of the iodine catalyst through oxidation. As documented previously,8a,c oxidation to an alkyl iodine(III) prior to the Ritter reaction should provide a more efficient iodine catalysis cycle. The successful application of alkyl iodine(III) intermediates in nonguided Ritter-type aminations has previously been explored by Minakata.23 In this work, an alkyl iodine(III) was required for the Ritter-type amination to occur. Under our conditions, PIDA can act as terminal oxidant to generate the tert-alkyl iodine(III) intermediate E. Despite the application of the iodine high oxidation state as a driving force for nucleophilic substitution, 8a,c no cyclization by the sulfamide is observed interrupting the Hofmann−Löffler pathway.24 Instead, acetonitrile solvent undergoes rapid and irreversible Ritter-type amination at the tertiary carbon position, releasing the original acylhypoiodite(I) catalyst. Water traces in acetonitrile provide the corresponding acetamide 4a. Based on the observed acid
a
Reaction performed with 20 mol % of I2 and 1.5 equiv of PIDA. Isopropionitrile was used as a solvent. bReaction performed over 40 h. c Reaction performed with 15 mol % of I2 and 1.3 equiv of PIDA. d Reaction performed with 1 mmol of 3l. eA 1:1-mixture of diastereoisomers was obtained. fYield based on recovering starting material. gReaction performed over 24 h. hReaction performed with 2 equiv of AcOH as additive. iStarting from the 5α-androsterone isomer. jNaOH, ethylene glycol, 200 °C. k1,3-Propanediamine, 150 °C.
observed (yield of 65%) and confirmed by single-crystal X-ray diffraction. This stereochemical outcome can be rationalized from shielding of the α-face at the stage of acetonitrile attack. Deprotection of the sulfamide directing group was demon7743
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ACS Catalysis
nucleophile engages in the final carbon−heteroatom bond formation.
effect, we cannot exclude that the corresponding N-protonated species may be participating throughout the catalytic cycle.25 Regarding the overall transformation, so far, there has been no precedence for a metal-free catalytic Ritter-type amination of nonactivated tertiary C−H bonds. Our work demonstrates that such a process is feasible. It also introduces the intermolecular Ritter reaction as a useful concept for ensuring catalyst turnover for the given example of sulfamides, which do not generate a nucleophilic nitrogen group for intramolecular C−N bond formation. As a result, the present protocol is the first to use the sulfamide group as a source for sulfamidyl radical formation under catalytic conditions. Previous work on related sulfamate esters in Hofmann-Löffler chemistry has remained stoichiometric with regard to halogen10a,b,d or pseudo-halogen26 reagents. Importantly, when turning to the corresponding sulfamate ester 6, the iodine-catalyzed amination did not proceed (Scheme 3). In order to generate an iodine-catalyzed entry
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01566. Details on experimental procedures for the catalytic reactions, spectroscopic data for the products (PDF) Crystallographic data for 2a (CIF) Crystallographic data for 4m (CIF) Crystallographic data for 4w (CIF) Crystallographic data for 2c (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Scheme 3. Guided Oxygenation Reactions
Kilian Muñiz: 0000-0002-8109-1762 Author Contributions
Experiments were performed by T.D., M.D.M., and I.S., and the manuscript was written by T.D. and K.M. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. E. C. Escudero-Adán and Dr. M. Martı ́nez for the X-ray structure determinations, Dr. G. Goti for support with the quantum yield determination, and the Spanish Ministry for Economy and Competitiveness and FEDER (Grant No. CTQ2014-56474R to K.M.), and the region of Catalonia for financial support.
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REFERENCES
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