Читать книгу Methodologies in Amine Synthesis - Группа авторов - Страница 33

In the successful examples directly employing N—H bonds as the radical precursors, the N—H bonds of most aminating reagents possess relatively low pK_a values to facilitate their effective oxidation. Besides, electron‐deficient functional groups such as acyl groups and sulfonyl groups (–Ts) adjacent to the N‐atom can also significantly stabilize the generated N‐radical species. Thus, amidyl or sulfonamidyl radicals often appear as the key intermediates in the N‐radical‐based C—N bond formation.

During the past few years, proton‐coupled electron transfer (PCET) strategy has been adopted in synthetic chemistry by Knowles' group, particularly for the homolytic activation of strong N—H bonds to form N‐radical species that could further engage in various C—N bonds constructing transformations. Different from the straightforward hydrogen atom transfer (HAT) process, in a typical PCET‐enabled amination, the most crucial step is the formation of a discrete hydrogen bond complex between the N—H bond of the substrate and the catalytic Brønsted base to modulate the redox potential, which enables the following concerted exchange step of an electron and a proton to access the N‐radical species.

In 2015, employing alkene 1 with a remote amidyl substituent as the substrate, Knowles and coworkers sequentially developed PCET‐based intramolecular carbo‐ and hydroamination using a combination of an iridium complex photocatalyst and a catalytic Brønsted base (Scheme 3.2) [9]. As is proposed by the authors, the N—H bond of 1 first interacts with the phosphate base, forming a hydrogen bond to lower the requirement of redox potential for the following electron transfer process. The concerted process of a one‐electron oxidation by photoexcited Ir^III* species and a proton abstraction by the base provides the key amidyl radical 2, which then cyclizes onto the attached C=C bonds to form a new C—N bond with an adjacent C‐centered radical (3). Subsequently, the nucleophilic radical 3 can further add onto an electrophilic olefin acceptor to deliver the carboamination product 4 after a tandem electron/proton transfer process. On the other hand, in the presence of catalytic thiophenol, radical 3 can also undergo a HAT process with the hydrogen donor to furnish the hydroamination product 5, while the resulting PhS^· is then engaged in a SET process with the Ir^II species to afford PhS⁻ and regenerate ground‐state photocatalyst Ir^III. The following protonation of PhS⁻ by the conjugate acid of the phosphate base indicates the closure of both catalytic cycles.

Later in 2016, Knowles' group employed the same strategy in the selective alkylation of remote C(sp³)—H bonds, which engages a 1,5‐HAT process with the aid of the amidyl radicals produced via PCET [10]. Simultaneously, a similar work was reported by Rovis' group [11]. Therefore, PCET activation has been proven as a powerful reaction mode for direct N‐radical formation from strong N—H bonds. Furthermore, it is also a promising platform to broadly explore this strategy in the construction of diversified C—N bonds.

Scheme 3.2 PCET‐mediated intramolecular carbo/hyrdroamination of alkenes.

Source: Modified from Gentry and Knowles [9].

More recently, Knowles and coworkers reported a sulfonamidyl‐based hydroamination strategy, again via PCET activation (Scheme 3.3) [12]. Hydroamination products 7 and 10 could be smoothly furnished from intramolecular cyclization of 6 and intermolecular reaction between 8 and 9, respectively, employing 2,4,6‐triisopropyl‐thiophenol (TRIP thiol) as the HAT catalyst via the same pathway as depicted in Scheme 3.2. Apart from the broad substrate scope demonstrated, a series of tandem amination/C–H alkylation sequences were performed to highlight the synthetic versatility (Scheme 3.3b). The terminal alkene 11 was first subjected to the intermolecular anti‐Markovnikov hydroamination with p‐methoxyphenyl (PMP) sulfonamide 12 to afford the alkylated sulfonamide 13, and the newly installed secondary sulfonamide was then activated in a second oxidative PCET event using the same Ir/phosphate pair, leading to the site‐selective abstraction of the δ‐C—H bond to afford a carbon‐centered radical that can be further trapped by an electron‐deficient olefin 14 to afford the final product 15.

The reviving synthetic organic electrochemistry has been providing environmentally benign alternatives to the traditional synthetic methods because of its generally high atom economy and good functional group compatibility. In the electrochemical C—N bond formation from unfunctionalized N–H/C–H precursors, a series of advances have been achieved by Xu's group in the recent years. In 2016, Xu's group disclosed an electrocatalytic hydroamination of alkenes, in which an inexpensive organometallic reagent ferrocene was employed as a redox catalyst to enable the direct generation of amidyl radicals from N‐aryl amides 16 (Scheme 3.4) [13]. Based on the optimization of the reaction conditions, a mixed solvent tetrahydrofuran (THF)/MeOH in 5 : 1 ratio is demonstrated as the optimal choice, while no reaction takes place in the single solvent MeOH, which is also supported by the cyclic voltammetry (CV) observations.

Scheme 3.3 PCET‐mediated intra/intermolecular amination of alkenes. (a) Hydroamination of alkenes with primary or secondary sulfonamides. (b) Tandem amination/C–H alkylation.

Source: Modified from Zhu et al. [12].

Scheme 3.4 Electrocatalytic intramolecular hydroamination of alkenes.

Source: Modified from Zhu et al. [13].

As depicted in their mechanistic proposal (Scheme 3.4a), this reaction is supposed to begin with the anodic oxidation of ferrocene ([Cp₂Fe]) and the simultaneous cathodic reduction of cosolvent methanol. Subsequently, the electrochemically generated base MeO⁻ deprotonates the amide group of substrate 16 to afford anionic intermediate 18, which can be easily oxidized by [Cp₂Fe]⁺ to provide the key amidyl radical 19, along with the regenerated mediator [Cp₂Fe]. Radical 19 then cyclizes onto its tethered alkene to furnish intermediate 20, which further acquires a hydrogen atom from the H‐atom donor 1,4‐cyclohexadiene (1,4‐CHD) to yield the desired product 17. The scope exploration reveals that various carbamates, ureas, and amides can serve as viable substrates, providing the corresponding products in good to high yields under the standard conditions (Scheme 3.4b, 17a–17d). Notably, in the reactions of diene substrates 16a and 16b, tandem cyclization processes occurred to furnish polycyclic products 17e and 17f with high efficiency. Moreover, under standard conditions but in the absence of 1,4‐CHD, substrate 16c finally turns into indoline 21a after an oxidative termination step.

Again, Xu and coworkers employed the same electrochemical N‐radical formation strategy in the synthesis of highly functionalized (aza)indoles 23 through intramolecular N‐radical species addition of 22 to their remotely attached alkynyl moieties (Scheme 3.5) [14]. In this transformation, ferrocene is also selected as the redox mediator upon anodic oxidation, and the simultaneous cathodic reduction converts the cosolvent methanol into MeO⁻ base and H₂ gas. Subsequently, a SET process between the oxidized [Cp₂Fe]⁺ and anion 24 from deprotonation of 22 produces an electron‐deficient, N‐centered radical 25 and meanwhile regenerates [Cp₂Fe]. Radical 25 would then preferentially undergo a 6‐exo‐dig cyclization to vinyl radical 26, followed by a second cyclization to give the delocalized radical 27, as also supported by the density functional theory (DFT) calculations. Finally, rearomatization of 27 after an oxidation/deprotonation sequence delivers the final product 23. A broad substrate scope is demonstrated under the standard conditions, exhibiting high functional group tolerance (Scheme 3.5b). Notably, the late‐stage modification of ethinyl estradiol proceeds smoothly to deliver the indole‐functionalized estradiol 23a. The acid/base‐sensitive chiral amino esters (23b, 23c) as well as a free alcohol (23d) are also well tolerated under the electrolysis.

Scheme 3.5 Electrochemical synthesis of multifunctionalized (aza)indoles. RVC, reticulated vitreous carbon.

Source: Modified from Hou et al. [14].

In 2017, Xu and coworkers adopted substrate 28 with a polysubstituted alkene moiety to achieve an electrochemical aza‐Wacker‐type cyclization, wherein the key amidyl radical 29 is generated by direct anodic oxidation of the amidyl N—H bond without the aid of a base (Scheme 3.6) [15]. The following intramolecular cycloaddition of radical 29 to its pendant alkenyl group furnishes C‐centered radical 30, which is more prone to suffer further oxidation rather than H‐atom abstraction, to deliver the corresponding cation 31. The desired product 32 is finally generated from cation 31 after deprotonation. This electrochemical protocol achieves the challenging intramolecular oxidative amination of sterically demanding alkenes in the absence of a metal catalyst and is proven compatible with a wide range of carbamates (32a–32d), amides (32e, 32f), and ureas (32g, 32h). Moreover, product 32i with a steroid‐based core is smoothly afforded in 70% yield from a tetrasubstituted alkene under electrolysis, demonstrating the extra synthetic potential of this protocol.

Scheme 3.6 Electrochemical intramolecular oxidative amination of tri‐ and tetra‐substituted alkenes.

Source: Modified from Xiong et al. [15].

A regiospecific electrochemical [3+2] annulation for the preparation of various imidazo‐fused N‐heteroaromatic compounds was more recently described by Xu's group (Scheme 3.7) [16]. Employing a novel tetra‐arylhydrazine (34) as the catalyst in a mixed solvent of acetonitrile and water, this electrosynthesis smoothly converts substrate 33 into 35 through a cyclization/C—N bond cleavage sequence and the release of one molecule of CO₂. Conversely, another product 36, wherein the carbonyl survived, could be obtained from certain substrates under another set of reaction conditions using THF/methanol as the solvent. Further attempts subjecting urea‐linked 37 under the second conditions also resulted in the corresponding imidazopyridines 38 with methoxycarbonyl substituents installed. The mechanism of this transformation is quite similar to the aforementioned ones, and the DFT studies reveal that the C—N bond‐forming pathway of 41a is both thermodynamically and kinetically favored over the alternative C=C bond‐forming pathway.

Scheme 3.7 Electrochemical synthesis of imidazo‐fused N‐heteroaromatic compounds.

Source: Modified from Hou et al. [16].