Eosin Y-catalyzed visible-light-mediated aerobic oxidative cyclization of N,N-dimethylanilines with maleimides

Summary A novel and simple strategy for the efficient synthesis of the corresponding tetrahydroquinolines from N,N-dimethylanilines and maleimides using visible light in an air atmosphere in the presence of Eosin Y as a photocatalyst has been developed. The metal-free protocol involves aerobic oxidative cyclization via sp3 C–H bond functionalization process to afford good yields in a one-pot procedure under mild conditions.


Eosin Y-catalyzed visible-light-mediated aerobic oxidative cyclization of N,N-dimethylanilines with maleimides
Introduction Over the past several years, visible light photoredox catalysis has become a powerful and promising tool and has been productively used to drive chemical transformations in the field of organic synthesis [1][2][3][4][5][6]. The approach takes full advantage of visible light, which is clean, abundant, and renewable. The pioneering work in this research area, reported by the groups of MacMillan [7][8][9], Yoon [10,11], Stephenson [12,13] and others [14][15][16][17][18], has demonstrated that ruthenium and iridium complexes as visible light photoredox catalysts are capable of catalyzing a broad range of useful reactions. A variety of new methods have been developed to accomplish known and new chemical transformations by means of these transition metalbased photocatalysts so far. However, the ruthenium and iridium catalysts usually are highcost, potentially toxic and not sustainable. Similar to the redox properties of these organometallic complexes, some metal-free organic dyes such as Eosin Y, Rose Bengal, Fluorescein, and Methylene Blue, have shown superiority of their applications as photocatalysts, which are easy to handle, environmentally friendly, inexpensive, and have great potential for applications in visible-light-mediated photoredox reactions [19][20][21][22][23][24][25][26][27].
More recently, visible-light-induced sp 3 C-H bond functionalization adjacent to nitrogen atoms has been extensively studied and has become a fundamental organic transformation [28][29][30][31][32][33][34][35][36][37][38]. Tertiary amine A generally generates a nucleophilic α-amino-alkyl radical B or an electrophilic iminium ion C via visiblelight photoredox catalysis. Unfortunately the research of the α-aminoalkyl radical is limited in photochemical synthesis because it tends to form the iminium ion by one electron oxidation (Scheme 1) [39][40][41]. In the context of this research background, we investigated the α-aminoalkyl radical route to achieve the aerobic oxidative cyclization of N,N-dimethylanilines with maleimides to form the corresponding tetrahydroquinoline derivatives under organic dye Eosin Y catalysis. Swan and Roy reported the reaction using benzoyl peroxide as catalyst at low temperature as early as 1968 [42]. In 2011, Miura and co-workers achieved this transformation using a copper catalyst and air as the terminal oxidant [43]. Bian and co-workers presented the same reaction using [Ru(bpy) 3 ] 3+ as photoredox catalyst under irradiation with visible light next year [44]. Herein, we show an environmentally friendly aerobic oxidative cyclization methodology that avoids the use of metal catalysts and makes full use of air as oxidant.

Results and Discussion
Our investigations for the envisaged protocol commenced with the reaction of N,N-dimethylaniline (1a) (0.5 mmol) with N-phenylmaleimide (2a) (0.25 mmol) in MeCN (3 mL) in the presence of 3 mol % Eosin Y under an air atmosphere (with no air bubbling). The reaction mixture was irradiated with visible light (two 9 W blue LEDs) at room temperature. Gratifyingly, the desired product tetrahydroquinoline 3a was obtained in 82% yield after 18 h (  tetrahydroquinoline product 3a was decreased to 77% (Table 1, entry 5). When the molar proportion of 1a and 2a was adjusted to 1:1 and 1:2, the yield of 3a decreased (Table 1, entries 6 and  7). Then, a series of control experiments were carried out, which indicated that Eosin Y, visible light and air are all essential for the reaction ( Table 1, entries 8-10).
Next, we optimized the reaction conditions with respect to solvent and catalyst dosage. MeCN was found to be the best solvent ( Table 2, entry 1) among DMF, DCE, DCM, DMSO, acetone, dioxane, and MeNO 2 . When the amount of Eosin Y was decreased from 3 mol % to 2 mol % or increased from 3 mol % to 4 mol %, the yield of the tetrahydroquinoline product 3a was slightly reduced ( Table 2, entries 9 and 10). Thus, the optimum catalyst dosage of Eosin Y was found to be 3 mol %.
With the optimized conditions in hand, the substrate scope of this reaction was examined (Scheme 2). The reaction is mild and tolerates many functional groups. N,N-dimethylaniline and substituted N,N-dimethylanilines incorporating methyl and bromo on the phenyl ring reacted with 2 to afford the corresponding tetrahydroquinolines 3 in good yields. N-arylmaleimides with electron-donating groups such as methyl, methoxy Scheme 3: A proposed reaction mechanism. and electron-withdrawing groups such as chloro, bromo, and N-methylmaleimide all underwent the aerobic oxidative cyclization to give the corresponding products in good yields. When using 4,4'-methylenebis(N,N-dimethylaniline) as the substrate, the reaction occurred only on one side and the yield of the product 3p is 52%. The reaction of N,N,3-trimethylaniline and N-phenylmaleimide resulted in the formation of a mixture of regioisomers 3q1 and 3q2 with 81% combined yield. The major product was the sterically more hindered 3q1 [43][44][45].
On the basis of our observations and literature reported [19,36,38,43,44], a proposed mechanism for the formation of the corresponding tetrahydroquinolines 3 form N,N-dimethylanilines 1 and maleimides 2 is depicted in Scheme 3. On absorption of visible light, the ground state of Eosin Y (EY) is induced to its single excited state ( 1 EY * ), which moves to its more stable triplet excited state ( 3 EY * ) through inter system crossing (ISC) [46,47]. 3 EY * may undergo an oxidative or reductive quenching cycle [48][49][50]. In this mechanism, a single electron transfer (SET) from 1 to 3 EY* generates the amine radical cation 4, and at the same time, 3

Conclusion
In conclusion, we report an efficient metal-free method for the synthesis of corresponding tetrahydroquinolines from N,Ndimethylanilines and maleimides using molecular oxygen as oxidant and Eosin Y as catalyst under the irradiation of visible light. The protocol is significantly green because it utilizes visible light and atmospheric oxygen as the greenest reagents, and metal-free, cheap Eosin Y with a relatively low loading as the photocatalyst to deliver the product at room temperature in a simple one-pot procedure. This methodology expands the range of substrates in the area of visible light photoredox reactions.

Supporting Information
Supporting Information File 1 Experimental section and characterization of the synthesized compounds.