Phosphine Oxide-Promoted Rh(I)-Catalyzed C–H Cyclization of Benzimidazoles with Alkenes

Ligands play a critical role in promoting transition-metal-catalyzed C–H activation reactions. However, owing to high sensitivity of the reactivity of C–H activation to metal catalysts, the development of effective ligands has been a formidable challenge in the field. Rh(I)-catalyzed C–H cyclization of benzimidazoles with alkenes has been faced with low reactivity, often requiring very harsh conditions. To address this challenge, a phosphine oxide-enabled Rh(I)–Al bimetallic catalyst was developed for the reaction, significantly promoting the reactivity and allowing the reaction to run at 120 °C with up to 97% yield.


Introduction
Transition-metal-catalyzed C-H functionalization represents one of the most convenient methods for the construction of molecular complexity from readily available nonprefunctionalized substrates, and considerable progress has been achieved during the past several decades [1][2][3][4][5][6]. In the activation of inert C-H bonds either via electrophilic activation or via oxidative addition, transition metal catalysts often play critical roles in affecting reactivity and selectivity. In general, the catalytic ability of metal catalysts mainly relies on intrinsic properties of metals and of accompanying ligands. Therefore, the search for proper ligands to match metal catalysts is critical to most C-H functionalization reactions, and has become a long-term and challenging goal in the field [7][8][9][10]. Rh(I)-catalysis is recognized as a powerful catalytic method for C-H functionalization, and a large number of examples have been reported [11][12][13][14]. However, so far, only three types of ligand are available, namely, monophosphines [15][16][17], bisphosphines [18][19][20][21][22] and diene ligands [23,24] (Scheme 1a). Owing to limited availability of ligands, some Rh(I)-catalyzed C-H functionalizations have to be run under relatively harsh conditions, resulting in difficult selectivity control and low reaction applicability. In 2001, Ellman, Bergman and co-workers reported a Rh(I)-catalyzed C-H cyclization of benzimidazoles with alkenes [25], providing an elegant route to versatile polycyclic imidazoles that widely exist in bioactive molecules (Scheme 1b). However, the use of monophosphine (PCy 3 ) as a ligand was not very effective, in general requiring very harsh conditions (over 160 • C and 20 h). Shortly after, to further optimize the reaction, a monophosphine (PPh 3 )-containing catalyst (RhCl(PPh 3 )) was then investigated; however, there was no obvious increase in reactivity observed, despite using a microwave-assisted technique at a higher temperature (250 • C) [26][27][28][29][30]. Switching back the monophosphine ligand with a Brønsted acid co-catalyst (PCy 3 ·HCl) led to slightly improved reactivity, allowing 5 mol% Rh to be used at a slightly lower temperature (225 • C) (Scheme 1b). Therefore, the search for new types of more effective ligands, instead of traditional monophosphines, is highly desirable for this reaction as well as other Rh(I)-catalyzed C-H activations. Herein, we report the use of phosphine oxide (PO) as a distinctive type of

Results
We recently explored a wide range of inert C-H and C-C bond activation reactions by using phosphine oxide-ligated Ni and Al bimetallic catalysis [34][35][36][37][38][39][40][41][42][43]. However, the replacement of nickel with other transition metals has been faced with big challenges, probably owing to a mismatch between the two metals, or between metal catalysts and substrates. To address this challenge, we turned our attention to the Rh(I)-catalyzed C-H cyclization of benzimidazoles with alkenes, hoping to develop a PO-ligated Rh-Al bimetallic system to improve the reactivity. As shown in Scheme 2, the investigation of transition metals with Ph2P(O)H as a ligand showed that versatile Pd catalysts were completely ineffective (entries 1 and 2), and only Co and Rh catalysts displayed moderate to good reactivity (entries 3-9). Among various Rh catalysts, [Rh(cod)2]BF4 gave the best result, providing the cyclized product in 68% yield (entry 5). Next, a large number of phosphine oxides were then examined (entries [10][11][12], and only bulky Mes-DAPO afforded a better yield: 84% (entry 12). In contrast, traditional phosphines gave very low yields under the same conditions (entries 13 and 14). With Mes-DAPO as the optimal ligand, the survey of Scheme 1. Rh(I)-catalyzed C-H cyclization of benzimidazoles with alkenes.

Results
We recently explored a wide range of inert C-H and C-C bond activation reactions by using phosphine oxide-ligated Ni and Al bimetallic catalysis [34][35][36][37][38][39][40][41][42][43]. However, the replacement of nickel with other transition metals has been faced with big challenges, probably owing to a mismatch between the two metals, or between metal catalysts and substrates. To address this challenge, we turned our attention to the Rh(I)-catalyzed C-H cyclization of benzimidazoles with alkenes, hoping to develop a PO-ligated Rh-Al bimetallic system to improve the reactivity. As shown in Scheme 2, the investigation of transition metals with Ph 2 P(O)H as a ligand showed that versatile Pd catalysts were completely ineffective (entries 1 and 2), and only Co and Rh catalysts displayed moderate to good reactivity (entries 3-9). Among various Rh catalysts, [Rh(cod) 2 ]BF 4 gave the best result, providing the cyclized product in 68% yield (entry 5). Next, a large number of phosphine oxides were then examined (entries [10][11][12], and only bulky Mes-DAPO afforded a better yield: 84% (entry 12). In contrast, traditional phosphines gave very low yields under the same conditions (entries 13 and 14). With Mes-DAPO as the optimal ligand, the survey of Al-Lewis acids (entries [15][16][17][18][19] revealed that Me 2 AlCl was the best one, providing the product in 97% yield (entry 19). Moreover, with this catalytic system, the reaction temperature can be decreased to 120 • C without observing significant loss of yield (entries Al-Lewis acids (entries [15][16][17][18][19] revealed that Me2AlCl was the best one, providing the product in 97% yield (entry 19). Moreover, with this catalytic system, the reaction temperature can be decreased to 120 °C without observing significant loss of yield (entries [20][21][22][23]. Control experiments showed that the absence of either a Rh catalyst or Al-Lewis acid led to no reaction (entries 24 and 25), and the removal of phosphine oxide gave a very low yield (12%) (entry 26), suggesting that any component of Rh, Al or PO ligand would be essential to the reactivity. With the optimal conditions in hand, we explored the scope of benzimidazoles (Scheme 3). Electron-deficient substituents such as carboester (2b), CF3 (2c), F (2d) and Cl (2e, 2f and 2g) on the phenyl ring of benzimidazoles can be highly compatible with the reaction, providing the corresponding products in 93-97% yield. In addition, electron-deficient heterocycle-bearing imidazole (2h) and fused aromatic ring-bearing imidazole (2i) were also suitable substrates, affording the corresponding products in 95% yield and 96% yield, respectively. In contrast, electron-donating substituents on the phenyl ring led to a Scheme 2. Reaction optimization. Reaction conditions: 1a (0.2 mmol), catalyst (10 mol%), Lewis acid (20 mol%) and ligand (10 mol%) in toluene under N 2 ; yield for isolated product. MAD = methylaluminum bis (2,6-di-tert-butyl 4-methylphenoxide).
With the optimal conditions in hand, we explored the scope of benzimidazoles (Scheme 3). Electron-deficient substituents such as carboester (2b), CF 3 (2c), F (2d) and Cl (2e, 2f and 2g) on the phenyl ring of benzimidazoles can be highly compatible with the reaction, providing the corresponding products in 93-97% yield. In addition, electrondeficient heterocycle-bearing imidazole (2h) and fused aromatic ring-bearing imidazole (2i) were also suitable substrates, affording the corresponding products in 95% yield and 96% yield, respectively. In contrast, electron-donating substituents on the phenyl ring led to a relatively lower yield. For example, C6-and C7-monomethylated benzimidazoles gave the corresponding products in 61% yield and 52% yield, respectively (2j and 2k). In addition to terminal alkenes, an internal alkene was also tolerated, providing the corresponding product in 53% yield with the increased loading of catalysts.
Molecules 2023, 28, x FOR PEER REVIEW 4 of 10 relatively lower yield. For example, C6-and C7-monomethylated benzimidazoles gave the corresponding products in 61% yield and 52% yield, respectively (2j and 2k). In addition to terminal alkenes, an internal alkene was also tolerated, providing the corresponding product in 53% yield with the increased loading of catalysts. After the completion of the investigation of substrate scope, we turned our attention to enantioselective control with chiral phosphine oxides (Scheme 4). Ellman et al. used a chiral bisphosphine-Rh(I) system to obtain a β-chiral stereocenter, while the construction of a more sterically hindered α-chiral stereocenter via Rh(I) catalysis still remains an elusive challenge. We examined a wide range of chiral phosphine oxides and found that only bulky phosphine oxide (L1) bearing a flexible chain gave 27% ee, suggesting that chiral After the completion of the investigation of substrate scope, we turned our attention to enantioselective control with chiral phosphine oxides (Scheme 4). Ellman et al. used a chiral bisphosphine-Rh(I) system to obtain a β-chiral stereocenter, while the construction of a more sterically hindered α-chiral stereocenter via Rh(I) catalysis still remains an elusive challenge. We examined a wide range of chiral phosphine oxides and found that only bulky phosphine oxide (L 1 ) bearing a flexible chain gave 27% ee, suggesting that chiral phosphine oxide-Rh-Al would be a feasible enantioselective catalytic system. Surprisingly, a cyclohexane-1,2-diamine-derived phosphine oxide-Co-Al system provided better yield and ee (65% yield and 45% ee) in the same reaction, suggesting that a phosphine oxide ligand could also be a potential chiral ligand for Co(I) catalyzed C-H activation reactions.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 10 phosphine oxide-Rh-Al would be a feasible enantioselective catalytic system. Surprisingly, a cyclohexane-1,2-diamine-derived phosphine oxide-Co-Al system provided better yield and ee (65% yield and 45% ee) in the same reaction, suggesting that a phosphine oxide ligand could also be a potential chiral ligand for Co(I) catalyzed C-H activation reactions.

Scheme 4.
Investigation of enantioselective control.

Discussion
To gain more insights into the reaction, some mechanistic experiments were conducted (Scheme 5a). A deuterium-labeling experiment showed that C2-D was distributed in several positions in the product, suggesting that the migratory insertion of an alkene into the Rh-H species could be a reversible step.

Discussion
To gain more insights into the reaction, some mechanistic experiments were conducted (Scheme 5a). A deuterium-labeling experiment showed that C2-D was distributed in several positions in the product, suggesting that the migratory insertion of an alkene into the Rh-H species could be a reversible step.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 10 phosphine oxide-Rh-Al would be a feasible enantioselective catalytic system. Surprisingly, a cyclohexane-1,2-diamine-derived phosphine oxide-Co-Al system provided better yield and ee (65% yield and 45% ee) in the same reaction, suggesting that a phosphine oxide ligand could also be a potential chiral ligand for Co(I) catalyzed C-H activation reactions.

Scheme 4.
Investigation of enantioselective control.

Discussion
To gain more insights into the reaction, some mechanistic experiments were conducted (Scheme 5a). A deuterium-labeling experiment showed that C2-D was distributed in several positions in the product, suggesting that the migratory insertion of an alkene into the Rh-H species could be a reversible step. In addition, parallel experiments revealed a low kinetic isotope effect (k H /k D = 1.44), implying that the C-H activation step may not be involved in the rate-determining step.
On the basis of these results and previous studies, a plausible mechanism is proposed in Scheme 5b: the combination of the PO ligand, Rh and Al-Lewis acid in situ formed a bimetallic catalyst, which then coordinated to form benzimidazole at the Al terminus. Then, the Rh was directed to activate C2-H via oxidative addition, followed by migratory insertion of an alkene and reductive elimination to generate the desired product. Although the current result is not significantly exciting, it demonstrates for the first time that phosphine oxides can be used as new and effective ligands for Rh(I)-or Co(I)-catalyzed C-H activation reactions, in which proper ligands are still quite scarce. Moreover, phosphine oxides allow co-catalysis of Lewis acid metals to assist Rh(I) or Co(I) catalysis, providing more options for catalyst design.

Procedure for Mechanistic Experiments
The deuterium-labeling experiment and parallel experiments were set up following the general procedure by using 1a or d-1a as substrates, respectively. Aliquots were taken at proper intervals. The yield was determined using 1 H NMR with CH 2 Br 2 as an internal standard. Data points represent the average of two runs.

Conclusions
A phosphine oxide-ligated Rh-Al bimetal-catalyzed selective C2-H cyclization of benzimidazoles with alkenes was developed, providing a series of polycyclic imidazoles in up to 97% yield under relatively mild conditions (120 • C and 3 h). This work demonstrated that the phosphine oxide ligand is a distinctive type of ligand for Rh(I) catalysis compared with traditional monophosphines, bisphosphines and diene ligands, and a wide range of applications may be expected in the future.