Methodologies in Amine Synthesis. Группа авторов

addition products. Nonetheless, the direct nucleophilic amination of alkenes depends on the prior oxidation of the C=C bonds to form the crucial C‐centered radical cation intermediates, which is followed by subsequent nucleophilic addition of N–H partners.

During the past decade, Nicewicz's group has developed a variety of visible‐light‐induced anti‐Markovnikov hydrofunctionalization protocols for alkenes via the addition of nucleophiles to the in situ generated C‐centered radical cations, employing the strongly oxidizing acridinium salts as photocatalysts. Based on their previous work on the intramolecular hydroamination of alkenes 174 using a combination of catalytic Mes‐Acr⁺BF₄⁻ and thiophenol as the photocatalyst and H‐atom donor, respectively (Scheme 3.31a) [44], they later reported an intermolecular anti‐Markovnikov hydroamination of alkenes by replacing the thiophenol cocatalyst with a combination of phenyl disulfide and 2,6‐lutidine (Scheme 3.31b) [45]. Therein, trifluoromethanesulfonamide (TfNH₂) and azoles (177) are employed as the N–H nucleophiles to approach a variety of phenethylamine derivatives 178. In line with their previous work on nucleophilic alkene functionalization, the authors suggest that the key step is the SET process between alkene 176 and the photoexcited acridinium (Mes‐Acr⁺*), which results in the generation of key radical cation intermediate 179. Subsequently, nucleophiles TfNH₂ or 177 undergo reversible addition to the least‐substituted side of 179 to form the stabilized C‐radical 180, which may further undergo a HAT process with a hydrogen atom donor, presumably thiophenol, to deliver the desired product 178. On the other hand, the putative HAT reagent thiophenol is supposed to come from phenyl disulfide after a homolysis/single‐electron reduction/protonation sequence.

In 2017, Lei and coworkers developed an external oxidant‐free photocatalytic CDC amination and etherification of alkenes with azoles and alcohols using a dual catalytic system combining photocatalyst Mes‐Acr⁺ClO₄⁻ and a Co‐based cocatalyst (Scheme 3.32) [46]. The single‐electron oxidation of alkene 181 by the photoexcited Mes‐Acr⁺* is considered as the key initial step, providing Mes‐Acr^· and radical cation species 186. Subsequently, the employed nucleophile 182 (or 183) reacts with 186 to form the more stable radical intermediate 187, which is further oxidized into the corresponding cationic species 188 by either Mes‐Acr⁺* or Co^III. Then, the final anti‐Markovnikov product 184 (or 185) is delivered after deprotonation of 188. As for the Co catalytic cycle, the reduced Co^II undergoes further reduction by Mes‐Acr^· to yield Co^I species, which is protonated twice and then releases one molecule H₂ to regenerate the Co^III species.

Scheme 3.31 Anti‐Markovnikov hydroaminations of alkenes by two component organic photoredox systems.

      Source: Modified from Nguyen and Nicewicz [44].

Scheme 3.32 Photocatalytic dehydrogenative cross‐coupling of alkenes with azoles and alcohols via a dual catalytic system.

      Source: Modiifed from Yi et al. [46].

In 2018, again in Lei's laboratory, a photoinduced oxidative [4+2] annulation of imines and alkenes was developed by utilizing a dual photoredox/cobaloxime catalytic system in a mixed solvent of 1,2‐DCE and hexafluoroisopropanol (HFIP) (Scheme 3.33) [47]. This method not only obviates the application of stoichiometric oxidants but also exhibits excellent atom economy by generating hydrogen gas as the only by‐product, achieving high regioselectivity and trans‐diastereoselectivity even if alkenes are employed in Z/E mixtures. On the basis of Stern–Volmer studies and CV experiments, a plausible mechanism is proposed. Firstly, single‐electron oxidation of 190 by the excited‐state photosensitizer leads to the generation of the alkene radical cation intermediate 192 and to the reduced Mes‐Acr^·. Subsequently, the nucleophilic attack of 189 furnishes the radical 193 after deprotonation, in which the C=C single bond prefers a more stable trans‐configuration upon rotation as proposed by the authors. Next, the radical cyclization of 193 gives the intermediate 194, which is further oxidized to furnish 195. After elimination of a proton, the desired 3,4‐dihydroisoquinoline product 191 is afforded. On the other hand, the reduced photosensitizer is oxidized by the Co^III species to complete the photoredox catalytic cycle. As for the cobalt side, the Co^II species is reduced to form a Co^I intermediate, which can be protonated to produce Co^III‐hydride. The Co^III‐hydride species would then release H₂ upon the interaction with protons.

Scheme 3.33 Photoinduced oxidative [4+2] imine/alkene annulation with H₂ liberation.

      Source: Modified from Hu et al. [47].

      3.3.3 Activated C(sp³)—H Bond Amination

In the early years, C(sp³)–H amination reactions were mainly achieved via transition metal catalysis or with hypervalent iodine reagents. Recently, quite a few methods for CDC amination of activated C(sp³)—H bonds, such as benzylic C(sp³)–H and C(sp³)–H in α-position to a nitrogen atom, have been realized via the formation of the corresponding cationic species and subsequent addition of nitrogen nucleophiles by means of photo/electrochemistry.

      3.3.3.1 Benzylic C(sp³)—H Bond Amination

As an example for the direct amination of benzylic C(sp³)—H bonds, Pandey's group disclosed a photocatalytic CDC amination of alkylbenzenes 196 with N‐methoxyamides 197 under metal‐free and external oxidant‐free conditions (Scheme 3.34) [48]. The 410 nm wavelength visible light source is obtained by using a combination of Pyrex and a CuSO₄:NH₃ solution filter from a 450 W Hanovia medium pressure lamp. Compound 9,10‐dicyanoanthracene (DCA) is employed as a photosensitizer, which facilitates an array of C(sp³)–H amination reactions between 196 and 197 with good functional group tolerance (198a–198f). Notably, this protocol is also applicable to intramolecular C—N bond construction to provide five‐membered rings (198g). The mechanistic proposal is depicted in Scheme 3.34b. First, the single‐electron oxidation of 197 by the photoexcited DCA* yields the N‐methoxyamide radical cation 199, which then converts into radical intermediate 200 after