Metal-free oxidative isocyanides insertion with aromatic aldehydes to aroylated N-heterocycles

A metal-free, mild and efficient protocol for the construction of aroylated N-heterocycles via radical cascade aroylation of phenyl or vinyl isocyanides with aromatic aldehydes has been developed. Both phenanthridine and isoquinoline derivatives are delivered quickly in moderate to good yields with high regioselectivity.

insertion reaction with aromatic aldehydes through PIDA/ TMSN 3 -mediated C-C bond formation, delivering an efficient approach to construction of 6-aroylated phenanthridine and 1aroylated isoquinoline derivatives in one pot (Scheme 1d).
Initially, the reaction was carried out by using 2-isocyanobiphenyl 1a and benzaldehyde 2a as model substrates in benzene with PIFA as the oxidant and TMSN 3 as the additive at room temperature (Table 1, entry 1). Pleasingly, the reaction of 1a with 4 equiv. of 2a in the presence of 2 equiv. of PIFA with TMSN 3 to give the desired 3a in 23% yield. Aer screening various solvents, CH 2 Cl 2 was found to be the most efficient and provided 3a in 27% yield (entries [2][3][4]. Furthermore, replacing the additive TMSN 3 with NaN 3 didn't improve the reaction (entry 5). Subsequently, various oxidants were also evaluated and PIDA gave a better result, providing 3a in 31% yield (entries 6-8). Importantly, a further improvement was achieved upon elevating the temperature to 50 C (entry 9). However, further increasing the temperature resulted in the yield reduction, maybe due to the aldehyde being oxidized to carboxylate under harsh conditions. We also investigated the relative ratio of reactants (entries [10][11][12][13], and found that the addition of the amounts of aldehydes to 6 equivalent amounts along with oxidants and additives to 2.5 equivalent amounts led to the optimum result and gave 3a in 74% yield (entry 12). The blank experiments demonstrated that both oxidants and additives were essential for the isocyanide insertion reactions (entries 14, 15). Consequently, the optimum reaction conditions were determined to be 2-isocyanobiphenyl 1a with aldehyde 2a (6 equiv.) in the presence of PIDA as well as TMSN 3 (2.5 equiv.) at 50 C under air condition. It was noteworthy that the reaction proceeded without any metal catalysts in only 15 minutes.
With optimized conditions in hand, various benzaldehydes 2a-l were rst tested to react with 2-isocyanobiaryl 1a to form the corresponding 6-aroyl phenanthridine products 3a-l. As showed in Table 2, benzaldehydes bearing electron-rich groups (2b-d) such as methyl, ethyl and methoxy at the para-position underwent this oxidative isocyanide insertion process smoothly to afford the desired products in moderate to good yields (63-76%). Weakly electron-withdrawing groups, for example, uoro (2e) and chloro (2f) substituents, gave lower yield. When the benzaldehyde moiety was substituted by strongly electronwithdrawing group such as CF 3 (2g) at the para-position, the yield was sharply decreased and only 13% yield of 3g was obtained (2g was recovered in 82% yield). These results indicated that electronic effects exerted by the substituents on the arene ring of the benzaldehyde component had an important inuence on this reaction. Moreover, lower yields were achieved for the ortho-substituents maybe due to the steric effect (3h-j). In addition, meta-substituted groups also gave the corresponding products 3k and 3l in moderate yield.
The isocyanobiaryl moiety was subsequently evaluated (Table 3). Various substituents on the arene ring without the isocyanide group at the 4-position were tolerant well, affording the desired products in moderate to good yield. For example, electron-donating groups such as t-Bu, OMe, OBn gave the product 3m-o in 47-76% yield. Electron-withdrawing F, CF 3 substituents also formed the product 3p and 3q in 87% and 62% yield, respectively. To explore the regioselectivity, 3-methylisocyanidebiphenyl 1r was employed as substrate to proceed  this oxidative isocyanide insertion process, and gave a mixture of two isomers 3r and 3r 0 , the ratio of which was 1.8 : 1. Orthomethyl derivative 1s gave the targeted product 3s in a slight lower yield maybe owing to the steric effect. In addition, the substituent effects on the aromatic ring attached to isocyanide moiety were also been explored. Not only the electron-rich group such as methyl substituent but also the electronwithdrawing groups such as chloro and triuoromethyl substituents were compatible with this oxidative cyclization process, and formed the products 3u-w in 62-81% yield. Importantly, halogen substituent could be used as a coupling partner for further transformation. Vinyl isocyanides were also explored. Methyl-4-aryl-1-aroylisoquiniline-3-carboxylates 5a-d were prepared in moderate yields starting from the corresponding vinyl isocyanides and 4methoxybenzaldehyde 2. Furthermore, 4-methyl substituent isoquinoline 5e was also formed in a slightly lower yield (41%).
To investigate the mechanism for this oxidative isocyanide insertion reaction, several control experiments were conducted. Benzaldehyde 6 containing ortho terminal alkene was rst reacted with 1a under the standard conditions (Scheme 2a). Phenanthridine derivatives 7, attached by 2,3-dihydro-1Hinden-1-one with a methylene linkage, was formed through acyl radical addition to the intramolecular C-C double bond, followed by the reaction with isocyanide 1a. It provided a novel and efficient approach for the synthesis of multi heterocyclic systems bearing a phenanthridine moiety. Furthermore, when TEMPO, a radical scavenge, was added into the system, the reaction was suppressed and the starting material 1a was recovered in 90% yield (Scheme 2b). These results indicated that the reaction probably involved a radical process (Table 4).
Based on the above results and previous reports, 7 a plausible mechanism pathway for PhI(OAc) 2 /N 3 -mediated synthesis of aroylated nitrogen-containing aromatic heterocycles starting from aryl or vinyl isocyanides with aromatic aldehydes is depicted in Scheme 3. Initial ligand exchange between PhI(OAc) 2 and TMSN 3 provides hypervalent iodide intermediate A, which next undergoes a thermal homolytic cleavage to generate an azide radical and radical B. Then, the azide radical abstracts an H atom from the aldehyde to release the acyl radical C, which attacks the isocyanide 1 to form the imidoyl radical D. Radical D subsequently experiences a homolytic aromatic substituent followed by an oxidative aromatization process and nally afford the targeted aroylated nitrogencontaining aromatic heterocycles 3.
In summary, we have developed a mild and efficient protocol for the construction of aroylated azaheterocyclic arenes via oxidative isocyanide insertion of isocyanides with benzaldehydes under metal-free conditions. Diversied phenanthridine and isoquinoline derivatives are assembled quickly in moderate to good yields with high regioselectivity. Interestingly, benzaldehyde bearing ortho-allyl substituent can also be applied in this reaction, allowing the installation of other heterocycle to Table 3 Variation of the 2-isocyanobiaryl component a a Reaction conditions: 1 (0.2 mmol), 2 (1.2 mmol), PIDA (0.5 mmol), TMSN 3 (0.5 mmol) in CH 2 Cl 2 (2 mL) at 50 C for 15 min.  Table 4 Substrate scope of vinyl isocyanides a a Reaction conditions: 1 (0.2 mmol), 2 (1.2 mmol), PIDA (0.5 mmol), TMSN 3 (0.5 mmol) and K 2 CO 3 (0.4 mmol) in CH 2 Cl 2 (2 mL) at 50 C for 30 min. phenanthridine by a methylene tether. Control experiments reveals that the reaction probably experiences a radical process. Further investigations on the chemistry of isocyanides and biological evaluation of the reductant products are underway in our laboratory.

Conflicts of interest
There are no conicts to declare.