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Photoredox Α‑Vinylation of Α‑Amino Acids and N‑Aryl Amines Adam Noble and David W

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Photoredox Α‑Vinylation of Α‑Amino Acids and N‑Aryl Amines Adam Noble and David W Communication pubs.acs.org/JACS Open Access on 07/14/2015 Photoredox α‑Vinylation of α‑Amino Acids and N‑Aryl Amines Adam Noble and David W. C. MacMillan* Merck Center for Catalysis at Princeton University, Princeton, New Jersey 08544, United States *S Supporting Information radicals to the direct synthesis of allylic amines from N-aryl ABSTRACT: A new coupling protocol has been amines or α-amino acids. Specifically, we hypothesized that allyl developed that allows the union of vinyl sulfones with amines should be accessible via exposure of α-amino radicals to photoredox-generated α-amino radicals to provide allylic olefins that can generically participate in radical-based vinylation amines of broad diversity. Direct C−H vinylations of N- mechanisms (i.e., incorporate leaving groups that are susceptible aryl tertiary amines, as well as decarboxylative vinylations to single-electron β-elimination pathways). As an overarching of N-Boc α-amino acids, proceed in high yield and with goal of this work we hoped to demonstrate that photoredox- excellent olefin geometry control. The utility of this new generated α-amino radicals provide a complementary approach allyl amine forming reaction has been demonstrated via the to allyl amine production in comparison to stoichiometric metal- syntheses of several natural products and a number of based technologies. Herein, we describe the successful execution established pharmacophores. of these ideals and present the first direct C−H vinylation of N- aryl tertiary amines and the first decarboxylative olefination of N- Boc α-amino acids using vinyl sulfones in the presence of isible light photoredox catalysis has recently emerged as a iridium-based MLCT catalysts. V powerful activation strategy for the discovery and invention Critical to our proposal of generating allylic amines from α- of new chemical transformations.1 The capacity of metal-to- amino radicals was the identification of an appropriate alkene ligand charge transfer (MLCT) catalysts to simultaneously act as SOMO-phile, an olefinic coupling partner that would be strong reductants and oxidants under the influence of low-energy susceptible to radical addition yet could further participate in photon sources has enabled the design of many novel reaction single-electron β-elimination to deliver the required vinylation mechanisms that can be chemoselectively triggered using visible adduct. In this context, Nozaki et al., among others, have light. One emerging application of photoredox catalysis is its use ingeniously demonstrated that unsaturated sulfones can in the direct functionalization of unactivated sp3 C−H bonds,2 an generically react with alkyl radicals to produce a wide variety of area of research that has become a fundamental goal of modern olefin containing products.14,15 For our purposes, we presumed organic chemistry.3 In this context, our laboratory has become that vinyl sulfones should be useful substrates for the proposed interested in the activation of C−H bonds to generate α-amino photoredox vinylation given that (i) they are electron-deficient radicals, which are versatile intermediates that can participate in olefins which should readily couple with nucleophilic α-amino radical−radical couplings4 to afford benzylic amines or be radicals and (ii) the radical C−C bond forming step should be trapped with radical acceptors to forge α-N-alkylation adducts.5,6 rapidly followed by β-sulfone elimination to produce a sulfinyl Generation of α-amino radicals under photoredox conditions radical, a species that should rapidly undergo single-electron typically proceeds via single-electron oxidation of an amine and reduction via the reduced state of the photocatalyst. subsequent deprotonation of the resulting radical cation.4,5 The specific mechanistic details of our proposed photoredox Recently, our group has also reported an alternative strategy for vinylation reaction are outlined in Scheme 1. It has been α-amino radical formation via the decarboxylation of N-tert- established that irradiation of Ir[dF(CF3)ppy]2(dtbbpy)PF6 α 7,8 fl fl butoxycarbonyl (N-Boc) -amino acids, aCO2-extrusion [dF(CF3)ppy = 2-(2,4-di uorophenyl)-5-tri uoromethylpyri- mechanism that has implications for the use of biomass dine, dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine] (1, depicted as III feedstocks in conjugate additions and organometallic couplings. Ir in Scheme 1) with visible light will generate the excited-state III * III/II Ir species 2, which is a strong oxidant (E1/2 [ Ir ] = +1.21 V Allylic amines have long been attractive targets for new 16 reaction development due to their importance as (i) function- vs SCE in MeCN). This photoexcited complex should readily alized building blocks9 and (ii) versatile intermediates in the undergo single-electron transfer (SET) with a tertiary amine 3 10 red 17 production of medicinal agents. Indeed, over the last three (e.g., N-phenylpyrrolidine (R = Ph), E1/2 = +0.70 V vs SCE) decades, numerous methods have been developed for the to form a radical cation that upon deprotonation will deliver the α III preparation of allyl amines, typically proceeding through the -amino radical 5 (for R = Ph). Similarly, the excited-state Ir addition of vinyl metal reagents to CN bonds11 or the direct species 2 could also undergo SET with the carboxylate formed by 12 α red deprotonation of -amino acid 4 (e.g., Boc-Pro-OCs, E1/2 = amination of allylic substrates. Moreover, deprotonation-based 7a methods for the direct α-vinylation of nitrogen centers have been +0.95 V vs SCE) to generate a carboxyl radical, which upon loss α reported via the α-lithiation, transmetalation of tertiary of CO2 would deliver -amino radical 5 (for R = Boc). Reaction β carbamates, followed by metal-catalyzed cross-couplings with of 5 with a vinyl sulfone would then generate the -sulfonyl vinyl halides.3d,13 Taking inspiration from our previously developed photoredox Received: June 17, 2014 arylation protocols,4,7 we sought to extend the utility of α-amino Published: July 14, 2014 © 2014 American Chemical Society 11602 dx.doi.org/10.1021/ja506094d | J. Am. Chem. Soc. 2014, 136, 11602−11605 Journal of the American Chemical Society Communication Scheme 1. Proposed Mechanism for the Vinylation Reaction (ppy)2(dtbbpy)PF6 (11)andIr[dF(CF3)ppy]2(dtbbpy)PF6 (1), provided enhanced E-selectivities (entries 3 and 4, ≥95:5 E:Z).19,20 The choice of base was also found to have a dramatic effect on the yield while maintaining the high E-selectivity, with CsOAc proving to be optimal (entry 5, 81% yield). Finally, the use of 1,2-dichloroethane (DCE) as the reaction medium was found to provide slightly higher yields than toluene, delivering allyl amine 10 in 91% yield and with a 98:2 E:Z ratio (entry 7). The required participation of both the photocatalyst and light were confirmed via control experiments (entries 8 and 9). Having identified optimal reaction conditions, we next examined the scope of the C−H vinylation reaction with respect to the amine substrate. As shown in Table 2, this protocol is a Table 2. Direct C−H Vinylation: Amine Scope radical 6, which after elimination of a sulfinyl radical would provide the allylic amine product. Completion of the photo- catalytic cycle would then be accomplished via reduction of the fi • red sul nyl radical (For PhSO2 /PhSO2Na, E1/2 = +0.50 V vs 18 II III/II − 16 SCE) with Ir 7 (E1/2 [Ir ]= 1.37 V vs SCE) to give a sulfinate anion while reconstituting the photocatalyst 1. With this mechanistic hypothesis in mind, we set out to explore the feasibility of our proposed vinylation reaction (Table 1). Our initial investigations began by exposure of N-phenyl- Table 1. Initial Studies Towards the C−H Vinylation Reaction entry photocatalyst base solvent yielda E:Z 1 Ir(ppy)3 NaOAc DMA 28% 21:79 a 2 Ir(ppy)3 NaOAc toluene 57% 91:9 All reactions performed with 0.50 mmol vinyl sulfone, 2.5 equiv 3 11 NaOAc toluene 33% >98:2 amine, 3.0 equiv CsOAc, and 1.0 mol % photocatalyst 1 in DCE (5.0 4 1 NaOAc toluene 40% 95:5 mL). Yields are of isolated products. E:Z ratio determined by 1H NMR 5 1 CsOAc toluene 81% 96:4 analysis. 6b 1 CsOAc toluene 87% 97:3 b 7 1 CsOAc DCE 91% 98:2 successful with a range of tertiary N-aryl amines (products 10 and b − 8 none CsOAc DCE <5% 12−24,64−98% yield, 94:6 to >98:2 E:Z). A variety of cyclic b,c − 9 1 CsOAc DCE 0% amines are well tolerated, with five-, six-, and seven-membered aDetermined by 1H NMR analysis using an internal standard. rings giving their respective products in excellent yields b c Performed with 3.0 equiv base and at 0.1 M in solvent. Performed (products 10, 12, and 13,84−98% yield, ≥97:3 E:Z). This in the absence of light. vinylation protocol is relatively unaffected by the nature of the N- aryl group, with substrates possessing electron donating or pyrrolidine (8)to(E)-(2-(phenylsulfonyl)vinyl)benzene (9), in withdrawing functionalities demonstrating good efficiency − the presence of Ir(ppy)3, NaOAc, and a 26 W household (products 14 18). Heteroatom-containing cyclic amines were fluorescent light bulb. To our delight, the desired allylic amine also found to be suitable (products 19 and 20) along with acyclic product 10 was observed under these new photocatalytic substrates (products 21 and 22). Vinylation of a number of conditions, albeit in modest yield and stereoselectivity (entry pharmacologically important fused nitrogen heterocycles such as 1, 28%, 21:79 E:Z). The efficiency of this vinylation protocol was N-benzyl-protected indolines and tetrahydroquinolines was improved by changing to less polar solvents, with toluene demonstrated to be possible, with excellent regioselectivity for providing higher yield and selectivity compared to N,N- the α-methylene ring position over the benzylic α-amino site dimethylacetamide (DMA) (entry 2, 57% yield, 91:9 E:Z).
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