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ÂŒH Bond Amination Strategies for the Synthesis Of InnovatIve tools In organIc / organometallIc chemIstry CHIMIA 2020, 74, No. 11 895 doi:10.2533/chimia.2020.895 Chimia 74 (2020) 895–903 © J. Zhang, M. H. Pérez-Temprano Intramolecular C(sp3)–H Bond Amination Strategies for the Synthesis of Saturated N-containing Heterocycles Jiayu Zhang and Mónica H. Pérez-Temprano*a Dedicated to the memory of Prof. Kilian Muñiz Abstract: The selective functionalization of C(sp3)–H bonds via intramolecular amination reactions represents a very attractive strategy for the construction of saturated N-containing heterocycles (SNHets). Over the past de- cades, the chemical community has devoted its efforts towards expanding the synthetic toolbox with the aim of facilitating access to these key fragments in a controllable, reproducible and efficient manner. This review covers selected examples of the most recent advances in intramolecular C(sp3)–N bond-forming reactions by three main approaches: (1) the Hofmann-Löffler-Freytag (HLF) reaction; (2) transition-metal-catalyzed nitrene C(sp3)–H inser- tion; and (3) transition-metal-catalyzed ligand-assisted C(sp3)–N bond-forming reactions via a reductive elimina- tion step. We will discuss reactivity, selectivity and the major mechanistic insights into these transformations. Keywords: Amination · C–H functionalization · Catalysis · Cyclization · Heterocycle Jiayu Zhang was born in Shaanxi, China, 1. Introduction in 1993. She received her BSc and MSc Saturated nitrogen-containing heterocycles (SNHets), such as degrees from Shaanxi Normal University azetidine, pyrrolidine or piperidine are among the most prevalent (2016) and Xi’an Jiaotong University molecular fragments in chemical space, especially in medicinal (2019), respectively. During her Master chemistry.[1] Their presence in druglike architectures usually en- studies, Jiayu developed different synthetic hances their biological activities, so these motifs are found in a protocols, involving radical cyclization and wide variety of top-selling pharmaceuticals (Scheme 1a). Despite cyanoalkylations. In October 2019, she their prevalence and importance, the synthesis of these cyclic joined the Muñiz group at the Institute of amines is far from trivial. In this context, intramolecular C(sp3)–N Chemical Research of Catalonia (ICIQ) in bond-forming processes represent a very efficient and practical Tarragona, Spain, to pursue her PhD.After the unexpected passing route for accessing these attractive moieties. These approaches of Prof. Muñiz in March 2020, Jiayu joined the Pérez-Temprano pose different initial challenges such as (i) the thermodynamic and group, also at ICIQ. Currently, her PhD is focused on exploring kinetic stability of the C(sp3)–H bonds or (ii) the site-selectivity new venues for promoting C–heteroatom bond-forming reactions. control of the C–H bond cleavage. Therefore, the synthetic com- munity has devoted tremendous efforts to tackle them, provid- Mónica H. Pérez-Temprano received her ing a versatile toolkit for streamlining the construction of these European PhD degree in 2011 from the molecules.[2] In this work, we cover the principal intramolecular University of Valladolid, under the super- C(sp3)–H amination strategies. We have classified the methods vision of Prof. Espinet and Prof. Casares, into three categories (Scheme 1b): (1) the Hofmann-Löffler- in the field of organometallic chemistry. Freytag (HLF) reaction; (2) transition-metal-catalyzed nitrene In 2012, she joined the research group of C(sp3)–H insertion; and (3) transition-metal-catalyzed ligand-as- Prof. Melanie Sanford at the University of sisted C(sp3)–H activation. We will mainly focus on selected and Michigan as Post-doctoral fellow. In 2015, representative synthetic examples reported over the past 5 years, she began her independent career as Junior since different reviews have revised these reactions previously.[3] Group Leader at the Institute of Chemical We will also gather the most relevant mechanistic information on Research of Catalonia (ICIQ). Her research program is focused on the operative pathways behind each of these reactions. developing mechanism-driven tools for streamlining the develop- ment of sustainable chemical transformations. 2. Hofmann-Löffler-Freytag (HLF) Reaction Intramolecular amination of remote aliphatic C–H bonds via hydrogen atom transfer (HAT) reactions stands out as a very attrac- tive and general procedure for generating mainly pyrrolidine deriv- atives.[4] These transformations are triggered by the thermal or pho- tochemical homolysis of the N–X bond of an N-halogenated amine in acidic media.[5] The protonated aminyl radical thus generated *Correspondence: Dr. M. H. Pérez-Temprano, E-mail: [email protected] Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of participates in an intramolecular 1,5-HAT to give a carbon-centered Science and Technology, Avinguda Països Catalans 16, Tarragona 43007, Spain radical.[6] Subsequently, this species is trapped by recombination 896 CHIMIA 2020, 74, No. 11 InnovatIve tools In organIc / organometallIc chemIstry (a) Drugs containing saturated N-heterocycles (SNHets) scaffolds such as starting materials containing acid-sensitive groups or to the synthesis of key natural product intermediates.[3b] OH O O N HO N SH N N NH Hofmann, 1885 Löffler &Freytag, 1909 Captopril Varenicline Nicotine addiction H SO ,140 oC H SO ,140 oC Procyclidine Hypertension 2 4 N 2 4 N N Br Anti-Parkinson drug then KOH N then NaOH H Br N N N Nicotine O Suárez's modification O I (1 equiv) N 2 N R1 PhI(OAc)2 (3 equiv) R1 1,5-HAT R1 −HI N R1 O NH h or ∆ N I N I N R2 O R2 R2=NO ,CN, PO(OEt) R2 H 2 Azelastine 2 2 in situ R F Paroxetine Antihistamine Heterocyclic structures synthesized by using neutral HFL conditions Antidepressant drug Cl O O gelsemicine (b) Intramolecular C(sp3)−H amination strategies for synthesizing SNHets OMe R TM-catalyzed O N O N N HLF reaction R nitrene insertion FG NH NH gelsedine Et OMe Et X N NX OMe H H n n n Scheme 3. Seminal reports on δ C(sp3)–H amination via N-centered radi- X=halogen X=H2,N2,IAr TM-catalyzed FG =electron- cals via N–X homolysis. ligand-assisted withdrawing group C(sp3)−N bond formation Inspired by these literature precedents, over the past years, dif- DG NH ferent research groups have contributed to develop more robust and straightforward cyclization events. For example, major accomplish- H n ments have been made in developing iodine-catalyzed methodolo- Scheme 1. (a) Bioactive molecules containing saturated N-containing gies. The presence of stoichiometric amounts of I tends to provide 2 heterocycles (SNHets). (b) Current synthetic strategies for accessing undesired oxidation reactions of weaker α-aminyl C–H bonds, SNHets via intramolecular C(sp3)–H amination reactions. which compete with 1,5-HAT processes, due to theAcOI homolysis- derived radicals.[11] In this context, Muñiz and co-workers designed with a caged X•, or participates in halogen atom abstraction from very elegant strategies to suppress these side-reactions. In 2015, the protonated starting material, usually to generate a δ-halogenated they reported the first example of an HLF reaction using iodine as a amine. The final cyclization proceeds via an S 2 reaction, usually catalyst (Scheme 4a). In this work, the authors used 10 mol% of I in N 2 upon treatment with a base (Scheme 2). The formation of six-mem- the presence of a hypervalent iodine oxidant, PhI(mCBA) (mCBA 2 bered rings is rare and is typically observed in systems where the = 3-chlorobenzoate).[12a] This method is applicable to the amina- formation of pyrrolidine derivatives is not viable. tion of a wide variety of primary, secondary, and tertiary C(sp3)–H The first example of the conversion of an N-halogenated amine bonds in excellent yields. From a mechanistic perspective, this reac- into a SNHet was reported in the early 1880s by Hofmann. In this tion is proposed to proceed by two interconnected catalytic cycles seminal work, the treatment of N-bromo-2-propylpiperidine with encompassing a radical chain reaction induced by visible light. In hot sulfuric acid, and subsequent basification, afforded the forma- 2017, the same group reported a dual light-activated cooperative tion of a bicyclic tertiary amine.[7] In 1909, Löffler and Freytag iodine and photoredox catalysis protocol for the synthesis of 2-aryl- extended the scope to the use of secondary amines, providing a pyrrolidines (Scheme 4b).[12b] In this work, the iodine catalyst ac- versatile method for the synthesis of pyrrolidines.[8] In the 1960s, tivates a remote C(sp3)–H bond (1,5-HAT process) under visible Corey and Wawzonek provided the first experimental evidence sup- light irradiation, while the organic photoredox catalyst (TPT) al- porting the radical reaction pathway, such as the observation of no lows the regeneration of I , after the product release. In order to 2 cyclized product in the absence of radical initiators.[9] Despite these overcome the unfavorable kinetics of 1,6-H abstraction from ami- remarkable synthetic advances, the early reports on HLF reactions nyl radical,[6a] the Muñiz group fine-tuned the reaction conditions were limited by the pre-formation of the haloamine and/or the harsh of their previous iodine-catalyzed methodologies to access six- and acidic reaction conditions. In this context, Suárez and co-workers seven-membered rings in a selective manner (Scheme 4c).[12c] In surmounted these limitations by two major breakthroughs (Scheme this report, the authors envisioned the synergistic cooperation of 3).First, the authors were capable of generating the N–halogen bond two catalytic cycles. In the first cycle, an N-centered succinimidoyl in situ from the corresponding amine in the presence of iodine and radical, generated from NBS, abstracts a hydrogen atom from the hypervalent iodine reagents. Second, the use of electron-withdraw- benzylic position of the corresponding starting material, which is ing groups (–CN, –NO , etc.) on the nitrogen atom makes the result- the weakest C–H bond.
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