Improved Conditions for the Visible-Light Driven Hydrocarboxylation by Rh(I) and Photoredox Dual Catalysts Based on the Mechanistic Analyses

The improved catalytic conditions and detailed reaction mechanism of the visible-light driven hydrocarboxylation of alkenes with CO2 by the Rh(I) and photoredox dual catalysts were investigated. The use of the benzimidazoline derivative, BI(OH)H, as a sacrificial electron donor was found to increase the yield of the hydrocarboxylated product by accelerating the reduction process. In addition, the incorporation of the cyclometalated Ir(III) complex as a second photosensitizer with [Ru(bpy)3]2+ photosensitizer also resulted in the promotion of the reduction process, supporting that the catalytic cycle includes two photochemical elementary processes: photoinduced electron and energy transfers.

We recently developed the visible-light driven hydrocarboxylation of alkenes with CO 2 for the first time by means of the photochemical generation of Rh(I) hydride species . 4-Cyanostyrene was transformed to the branched hydrocarboxylated product by using a Rh(I) hydride or chloride complex as a carboxylation catalyst, [Ru(bpy) 3 ] 2+ as a photoredox catalyst, i Pr 2 NEt as a sacrificial electron donor, with visible-light irradiation under CO 2 atmosphere at room temperature (Figure 1). The photoredox catalysis made it possible to take electrons from tertiary amines and drive the reduction process without using a metallic reductant. Since then, several photoredox-catalyzed hydrocarboxylation reactions of unsaturated hydrocarbons with CO 2 have been reported by other groups (Seo et al., 2017b;Hou et al., 2018b;Meng et al., 2018). Concomitantly, difunctionalizations of alkenes such as thiocarboxylation (Ye et al., 2017), carbocarboxylation and silylcarboxylation (Yatham et al., 2017;Hou et al., 2018a) have also been developed by incorporating an appropriate radical precursor with CO 2 . Furthermore, in addition to unsaturated hydrocarbons, various substrates such as aryl and alkyl halides (Meng et al., 2017;Shimomaki et al., 2017), amines (Seo et al., 2017a), imines and enamides (Fan et al., 2018;Ju et al., 2018) have been carboxylated with CO 2 by photoredox catalysis so far (for review, see Yeung, 2019). These examples demonstrated wide applicability of the photoinduced electron transfer to carboxylation reactions.
On the basis of our previous experiments, the reaction mechanism of the hydrocarboxylation by Rh(I) and photoredox dual catalysts was proposed as shown in Figure 2. Initially, the hydrometallation of a styrene derivative by Rh(I) hydride species A gave the Rh(I) benzyl species B (i), and the visible-light promoted nucleophilic addition to CO 2 afforded the Rh(I) carboxylate species C (ii). Then, the reductive quenching cycle of [Ru(bpy) 3 ] 2+ with i Pr 2 NEt mediated 2-electron, 2proton transfers afforded the Rh(III) dihydride carboxylate species D (iii), followed by the base-promoted liberation of the carboxylated product to regenerate the active species A (iv). Although this reaction demonstrated fundamental aspects of the application of photochemical reduction processes to catalytic carboxylation reactions, there still has been room for improvement from the viewpoint of the applicability in organic synthesis: (i) The efficiency of the reaction was not very high. Good yield was obtained with 4-cyanostyrene and moderate yields were obtained with several other substrates. (ii) A large excess amount of a tertiary amine and long reaction time (>24 h) were necessary for completion of the reaction even for the reactive substrates, (iii) A significant amount of the hydrogenated product was produced as a byproduct. In order to resolve these problems, further screenings of the catalytic conditions were desired.
Herein, we explored the improved conditions of the visible-light driven hydrocarboxylation, and the catalytic efficiency was analyzed based on the detailed mechanistic study with a series of stoichiometric reactions of the rhodium intermediates. Through the investigation, the hydrocarboxylation was successfully improved by the alteration of the sacrificial electron donor or the incorporation of the second photosensitizer. The mechanistic study suggested that the promotion of the photochemical reduction process was crucial for the enhancement of the catalytic reaction.

Screening of Reaction Conditions
On the basis of our previous experiments in terms of the screening of catalytic conditions and the observation of the reaction intermediates under the catalytic conditions, the followings were demonstrated: (i) As a carboxylation catalyst, Rh(I) hydride or chloro complexes with triarylphosphines were applicable. In particular, the µ-chloro bridged Rh(I) dimer [Rh(P(4-CF 3 C 6 H 4 ) 3 ) 2 Cl] 2 (4) was the most effective catalyst. (ii) When 4 was employed under the catalytic conditions, the resting state was the corresponding Rh(I) carboxylate complex, indicating that the rate-determining step was its transformation to the Rh(I) hydride species. This result suggested that the promotion of the reduction process was crucial for the improvement of the catalytic reaction. According to these considerations, the reaction conditions were screened in terms of the photoredox catalyst and sacrificial electron donor, FIGURE 2 | The proposed reaction mechanism of the photocatalytic hydrocarboxylation with the schematic representation of the reductive quenching cycle of a photoredox catalyst.
which would have taken an important part in the reductive quenching cycle.

Photoredox Catalyst
Photoredox catalysts were initially screened by performing the reaction of 4-cyanostyrene (1a) in a mixture of 2.0 mol% of a photosensitizer, 3.5 mol% of 4 and 4.0 equiv. of i Pr 2 NEt under a CO 2 atmosphere at room temperature (Table 1). In the case of [Ru(bpy) 3 ](PF 6 ) 2 (E II * /I 1/2 = +0.77 V, E II/I 1/2 = −1.33 V vs. SCE) (Kalyanasundaram, 1982) as a photoredox catalyst, the hydrocarboxylated (2a) and hydrogenated (3a) products were obtained in 54 and 25% yields, respectively, after visiblelight irradiation for 24 h (Table 1, entry 1). A small amount of polymerized product of 1a was also produced as byproduct. Though no other photosensitizers overcame this activity, the yield of 2a was found to be strongly dependent on the photoredox catalyst. For instance, when [Ru(bpz) 3 ](PF 6 ) 2 (E II * /I 1/2 = +1.45 V, E II/I 1/2 = −0.80 V vs. SCE) (Crutchley and Lever, 1980) or fac-Ir(ppy) 3 (E (Flamigni et al., 2007) (Lowry et al., 2005), moderate yields were obtained (Table 1, entry 4, 5). These results indicated that both sufficient oxidizing ability of the excited state and reducing ability of the one-electron reduced species are at least necessary for the photosensitizer. However, the detailed dependency was not simple, as other factors such as absorption properties, excited-state energies and photochemical stability of the photosensitizer could also affect the catalytic performances. Meanwhile, the screenings of additives with [Ru(bpy) 3 ] 2+ photosensitizer demonstrated that the addition of Cs 2 CO 3 as an inorganic base significantly improved the yield of the hydrocarboxylated product by suppressing the formation of the hydrogenated byproduct: the yield of 2a increased to 67% while the yield of 3a decreased to 1% (Table 1, entry 11).

Sacrificial Electron Donor and Additives
Sacrificial electron donors were then screened in the presence of an excess amount of Cs 2 CO 3 . In order to highlight the reactivity, a less reactive alkene, 3,5-bis(trifluoromethyl)styrene (1b), was used as a substrate. The reactions of 1b were performed in a mixture of 2.0 mol% of [Ru(bpy) 3 ](PF 6 ) 2 , 3.5 mol% of 4, 4.0 equiv. of sacrificial electron donor and 1.2 equiv. of Cs 2 CO 3 under a CO 2 atmosphere at room temperature ( Table 2). When i Pr 2 NEt was employed as a sacrificial electron donor, 32% yield of the hydrocarboxylated product (2b) and a trace amount of the hydrogenated product (3b) were obtained after visiblelight irradiation for 12 h (Table 2, entry 1). Although the use of TEOA (triethanolamine) slightly increased the yield of 2b, the formation of 3b became pronounced probably due to the increase of proton concentration (Table 2, entry 3). On the other hand, the use of BI(OH)H (1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole) successfully accelerated the hydrocarboxylation and increased the yield of 2b considerably with maintaining the low yield of 3b (Table 2, entry 5). Furthermore, the incorporation of BI(OH)H made it possible to reduce the amounts of the photoredox catalyst and the sacrificial electron donor: the use of only 1.0 mol% of [Ru(bpy) 3 ](PF 6 ) 2 and 2.0 equiv. of BI(OH)H gave 70% yield of 2b (Table 2, entry 7). When the reaction was performed under an Ar atmosphere in the presence of Cs 2 CO 3 , no hydrocarboxylated product 2b was obtained. This result confirmed that the carbonate did not work as a source of CO 2 in the present reaction (Table 2, entry 9). BI(OH)H has been known to work as a 2-electron, 2-proton donor with high reducing ability in redox photosensitizing reactions (Hasegawa et al., 2005(Hasegawa et al., , 2006Tamaki et al., 2015). Since a tertiary amine contributed to the reductive quenching cycle of [Ru(bpy) 3 ] 2+ , the increase in the yield of 2b was attributed to the promotion of the reduction process of the Rh(I) carboxylate species, which was the rate-determining step in the hydrocarboxylation. These results indicated that the redox property of a sacrificial electron donor is one of the crucial factors for the efficient promotion of the reaction.

Generality of the Hydrocarboxylation Under the Improved Conditions
Based on the improved conditions using 1b as discussed above, the generality of the hydrocarboxylation was examined using various alkene substrates. 1.0 mol% of [Ru(bpy) 3 ](PF 6 ) 2 , 3.5 mol% of 4, 2.0 equiv. of BI(OH)H and 1.2 equiv. of Cs 2 CO 3 were employed for the hydrocarboxylation ( Table 3).
In the cases of using styrenes with an electron-withdrawing group such as 1a and 4-methoxycarbonyl styrene (1d), the reaction was almost completed after irradiation for 12 h, and the yields of the corresponding hydrocarboxylated products were significantly improved compared with those obtained in the previous conditions where 2.0 mol% of [Ru(bpy) 3 ](PF 6 ) 2 and 4.0 equiv. of i Pr 2 NEt were employed. Moreover, 4-trifluoromethyl styrene (1c) and non-substituted styrene (1e), which exhibited quite low reactivities in the previous conditions, did react to afford significant amounts of the corresponding hydrocarboxylated products though the yields were still not sufficiently high. The yields of the hydrocarboxylated products were also improved in the case of alkyl acrylates (1f and 1g). Consequently, the introduction of BI(OH)H electron donor with Cs 2 CO 3 base successfully resulted in the increase in the yields of the present hydrocarboxylation reaction.

Mechanistic Study
In order to reveal the reaction mechanism of the photocatalytic hydrocarboxylation, the stoichiometric reactions of the possible rhodium intermediates, which corresponded to each elementary step in the proposed catalytic cycle, were examined.

Rh Hydride Formation
Initially, the Rh(I) hydride formation step was investigated using Rh(PPh 3 ) 2 (OAc) (5)    . Control experiments demonstrated that [Ru(bpy) 3 ](PF 6 ) 2 , i Pr 2 NEt, and visible-light were all essential for the transformation. Since hydrogen evolution was not evident during the reaction, the contribution of gaseous hydrogen was excluded. Thus, this transformation was considered to proceed via (i) stepwise 2-electron, 2-proton transfers from the tertiary amine by the photoredox catalysis to give the Rh(III) dihydride carboxylate (7), and (ii) the baseassisted elimination of the carboxylic acid to give 6. In terms of step (i), the similar mechanisms have been proposed in photocatalytic hydrogen generation systems by a Rh(I) catalyst (Stoll et al., 2015). The initial single electron transfer to the protonated form of 5 would give the Rh(II) carboxylate monohydride, and the following electron and proton transfers or disproportionation of the two Rh(II) hydride species would give 7 (Figure 4). The presence of the Rh(III) dihydride intermediate was also supported by the fact that Rh(PCy 3 ) 2 (OAc) (5 ′ ) was transformed to Rh(PCy 3 ) 2 (OAc)(H) 2 (7 ′ ) almost quantitatively under the similar conditions although the reaction was relatively slow ( Figure 3B; Supplementary Figure 2). In this case, PCy 3 ligands with strong σ-donation were considered to stabilize the Rh(III) dihydride intermediate to inhibit the following elimination reaction. In order to confirm the carboxylic acid elimination step (ii), the reactivity of 7 was investigated in the presence of base. 7 was alternatively synthesized by the hydrogenation of 5 with H 2 , and was treated with an excess amount of i Pr 2 NEt in the presence of PPh 3 in C 6 D 6 . The reaction readily gave a mixture of 7 and 6 with liberation of [ i Pr 2 NHEt] + [CH 3 COO] − . Furthermore, addition of a small excess amount of [ i Pr 2 NHEt] + [CH 3 COO] − to the C 6 D 6 solution of 6 resulted in the quantitative formation of 7. These results demonstrated that 7 was in equilibrium with 6 in the presence of i Pr 2 NEt and PPh 3 (Figure 5). When the treatment of 7 with i Pr 2 NEt was similarly conducted in DMA, 6 was detected as a sole rhodium species in the reaction mixture, indicating that the equilibrium was almost completely shifted to the product side owing to the solvent effect of DMA.
The possible mechanisms for generation of the hydrogenated product were (i) 2-electron, 2-proton transfers to the Rh(I) benzyl intermediate by the photoredox catalysis to give Rh(III) benzyl dihydride intermediate, which would undergo reductive elimination of the hydrogenated product, and (ii) the alkene insertion to the Rh(III) dihydride intermediate and the successive reductive elimination. Both pathways could be inhibited by lowering proton concentrations, as proton transfers would become inefficient in the former, and the competing carboxylic acid elimination from the dihydride complex would be promoted in the latter. Therefore, the inhibition of the hydrogenated product formation by the addition of Cs 2 CO 3 was attributed to the decrease of the proton concentration in the catalytic system.
The photochemical formation of Rh(I) monohydride species was also feasible by using Rh(I) chloride complex as a Rh(I) source. It was demonstrated by the fact that Wilkinson's type complex Rh(PPh 3 ) 3 Cl was converted to Rh(PPh) 3 H (6) by visible-light irradiation in the presence of a catalytic amount of [Ru(bpy) 3 ](PF 6 ) 2 and an excess amount of i Pr 2 NEt. Therefore, the Rh(I) chloride complex was confirmed to work as a precursor of the Rh(I) hydride active species.

Hydrometalation and Carboxylation
Since the Rh(I) monohydride species was successfully generated from the Rh(I) carboxylate species by photoredox catalysis, the hydrometallation and subsequent carboxylation processes were then investigated to complete the catalytic cycle. Treatment of Rh(PPh) 3 H (6) with an excess amount of 1a at room temperature readily formed the Rh(I) benzyl species, Rh(PPh 3 ) 2 (η 3 -CHCH 3 (4-CNC 6 H 4 )) (8), almost quantitatively with the liberation of a PPh 3 ligand (Figure 6i). The benzyl ligand in 8 was found to possess η 3 -coordination to the Rh(I) center based on NMR spectroscopic data (Werner et al., 1994). However, the attempt for isolation of 8 was not successful due to the presence of an equilibrium with 6. Therefore, in situ generated 8 was directly used for the carboxylation step.
To investigate the carboxylation process with CO 2 , a DMA solution of a 1:1 mixture of in-situ generated 8 and PPh 3 was exposed to the atmospheric pressure of CO 2 under various conditions. The carboxylation did not proceed under dark even by heating, which was against our expectations based on the general reactivity of organorhodium(I) complexes with CO 2 (Ukai et al., 2006;Mizuno et al., 2011;Suga et al., 2014;Kawashima et al., 2016). Quite interestingly, when the mixture was irradiated by visible-light for 30 min in the presence of 30 mol% of [Ru(bpy) 3 ](PF 6 ) 2 , 8 was successfully converted to    the Rh(I) carboxylate complex, Rh(PPh 3 ) 3 (η 1 -O 2 CCHCH 3 (4-CNC 6 H 4 )) (9), almost quantitatively (Figure 6ii). 31 P{ 1 H} NMR spectroscopy confirmed the clean formation of 9: a pair of the doublet of doublet signals attributed to 8 completely disappeared with the PPh 3 signal, and the doublet of doublet and doublet of triplet signals attributed to 9 appeared in 2: 1 ratio by visible-light irradiation (Figure 7). The control experiments demonstrated that CO 2 , [Ru(bpy) 3 ](PF 6 ) 2 and visible-light were all essential for the carboxylation, suggesting that the nucleophilic addition of 8 to CO 2 was facilitated by the photosensitization of [Ru(bpy) 3 ] 2+ . The luminescence quenching experiment demonstrated that the excited state of [Ru(bpy) 3 ] 2+ was effectively quenched by 8 (Figure 8). The quenching constant was determined to be K q = 2.07 × 10 3 , which was much larger than that by i Pr 2 NEt (K q =  1.56 × 10 2 , Supplementary Figure 3A). This result indicates that either photoinduced electron transfer or triplet-triplet energy transfer to 8 contributed to the quenching (Campagna et al., 2007;Arias-Rotondo and McCusker, 2016;Strieth-Kalthoff et al., 2018). However, the photoinduced electron transfer mechanism was unlikely in this case since (i) the carboxylation of 8 proceeded with a catalytic amount of [Ru(bpy) 3 ](PF 6 ) 2 even in the absence of the sacrificial electron donor, and (ii) the cyclic voltammogram of 8 showed no significant redox peak within the window where the oxidative quenching of [Ru(bpy) 3 ] 2+ was possible. Therefore, the photoinduced triplet-triplet energy transfer from the excited [Ru(bpy) 3 ] 2+ to 8 was considered to be the most likely process in this carboxylation process. Although not very common, several examples on the photocatalytic organic transformations mediated by the triplet-triplet energy transfer were previously reported (Ikezawa et al., 1986;Osawa et al., 2001;Islangulov and Castellano, 2006;Lu and Yoon, 2012;Farney and Yoon, 2014).
In order to investigate the detailed effect of the photoinduced energy transfer, the electronic structure analyses were performed in terms of the ground (S 0 ) and the lowest excited triplet (T 1 ) states of 8 based on DFT/TD-DFT methods. The calculated energy level of the T 1 state of 8 (1.07 eV, based on the comparison between the S 0 and T 1 optimized geometries) was much lower than that of [Ru(bpy) 3 ] 2+ (2.17 eV), indicating that the triplet-triplet energy transfer from the excited [Ru(bpy) 3 ] 2+ to 8 was feasible. In terms of the optimized structures, a notable difference was found on the coordination manner of the benzyl ligand between the S 0 and T 1 geometries. In the S 0 optimized structure, the η 3 -coodination of the benzyl ligand was represented by the similar three Rh-C distances, which coincided with the results of the 1 H NMR observation ( Figure 9A; Table 4). On the other hand, in the T 1 optimized structure, while the Rh-C1 (benzyl carbon) distance remained unchanged, the Rh-C2/C3 distances significantly elongated compared with those of the S 0 structure. These results indicated that the benzyl ligand changed its coordination-mode from η 3 -type (αbenzyl) to η 1 -type (σ-benzyl) in the T 1 state. According to the analysis on the electronic transition characters, the T 1 state was mainly contributed by the transitions of HOMO→LUMO (88%) and HOMO→LUMO+16 (4%) (Supplementary Table 2). The molecular orbital distribution indicated that LUMO and LUMO+16 mainly localized on the Rh (dπ) and benzyl ligand (π * ) while the HOMO localized on the Rh (dσ) center ( Figure 9B). As these LUMOs partially possessed the antibonding character on the Rh-C2/C3 bonds, the photoexcitation induced the dissociation of these Rh-C bonds, which resulted in the isomerization to the σ-benzyl species.
Concerning the acceleration of the carboxylation step, one possibility is the generation of the coordination site by taking σ-benzyl structure in the T 1 state, which would promote the following carboxylation by facilitating coordination of CO 2 to Rh center. Indeed, a similar thermal process has been proposed as a plausible mechanism for the carboxylation of organorhodium(I)  complexes (Darensbourg et al., 1987). Another possibility is the direct nucleophilic addition of the benzyl carbon to CO 2 in the T 1 state. The NBO analysis demonstrated that the natural charge on the C1 atom significantly shifted to the negative side while that on the Rh atom shifted to the positive side in the T 1 state. The increase of the electron density on the C1 atom in the T 1 state would result in the acceleration of the nucleophilic addition to CO 2 . Therefore, the structural and/or electronic factors associated with the transition to the T 1 state are thought to contribute to the carboxylation of 8. The effect of the photoactivation of the Rh(I) π-benzyl complex was also supported by the reactivity of the Rh(I) σ-alkyl complex with CO 2 . When a mixture of 6 and an excess amount of methyl acrylate (1f) was subjected to a CO 2 atmosphere for 3 h even under dark, the quantitative formation of Rh(PPh 3 ) 3 (η 1 -O 2 CCHCH 3 (CO 2 CH 3 )) (10) was indicated by 31 P{ 1 H} NMR spectroscopy. The carboxylation of 1f was confirmed by the fact that the corresponding hydrocarboxylated product (2f) was obtained from the reaction mixture. This result indicated that the photosensitization by [Ru(bpy) 3 ] 2+ was not essential in this case. Thus, the major role of the excitation was thought to be the transformation from π-benzyl to σ-benzyl complexes to generate a coordination site and to make them more nucleophilic.

Addition of the Second Photosensitizer
The above mechanistic study revealed that a photosensitizer played two key roles in the hydrocarboxylation cycle: one is a "photoredox catalyst" to reduce the Rh(I) carboxylate species, and the other is a "triplet photosensitizer" to promote carboxylation of the Rh(I) benzyl species. With a single photosensitizer, the excited state of the photosensitizer was quenched by either a tertiary amine for the electron transfer or a Rh(I) benzyl species for the energy transfer, and these two processes competed during the reaction. Since the former was related to the rate-determining step when using i Pr 2 NEt as a sacrificial electron donor, the incorporation of the second photosensitizer possessing suitable redox properties for the reductive quenching cycle was expected to facilitate the catalytic reaction.
On the basis of the idea, 2.0 mol% of a cyclometalated Ir(III) complex was added as a second photosensitizer to a mixture of 1b, 3.5 mol% of 4, 2.0 mol% of [Ru(bpy) 3 ](PF 6 ) 2 and 4.0 equiv. of i Pr 2 NEt, and the solution was irradiated under CO 2 atmosphere at room temperature ( Table 5). To excite both photosensitizers, a wide range of UV-visible-light (380-750 nm) was applied to the reactions. As expected, the addition of the second photosensitizer was found to be effective. For instance, when [Ir(ppy) 2 (dtbbpy)](PF 6 ) was added, the reaction was completed after irradiation for only 6 h, and the yield of 2b was increased more than five-fold compared to that of the reaction without the second photosensitizer (Table 5, entry 2). According to the redox properties of [Ir(ppy) 2 (dtbbpy)](PF 6 ), the acceleration of the reaction was thought to be attributed mainly to the high reducing ability of the one-electron reduced species to promote the reduction process. The yield of 2b further increased when incorporating [Ir(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 ) as a second photosensitizer ( Table 5, entry 3), and its concentration could be reduced to 1.0 mol% without lowering the yield ( Table 5, entry 4). This result was assumed to be due to the high oxidizing ability of the excited state in addition to the sufficient reducing ability of the one-electron reduced species. The excited state of [Ir(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 ) was found to be able to work as an energy transfer agent of 8 based on the luminescence quenching experiment (K q = 2.76 × 10 4 , Supplementary Figure 3B). However, it is considered to contribute to the reaction mainly as an electron transfer agent under the catalytic conditions owing to the efficient quenching by the sacrificial electron donor. On the other hand, the addition of fac-Ir(ppy) 3 resulted in only a small acceleration, which was probably attributable to the inferior oxidizing ability in the excited state ( Table 5, entry 6). These results demonstrate that photosensitizers possessing both high oxidizing ability of the excited state and high reducing ability of the one-electron reduced species are advantageous as a second photosensitizer. The positive result on the addition of the two appropriate photosensitizers reflected the fact that the catalytic cycle was composed of the multiple photochemical processes, and the acceleration of the reduction process led to the enhancement of the catalytic activity when using i Pr 2 NEt as a sacrificial electron donor.

Rate-Determining Step in the Hydrocarboxylation With BI(OH)H
The previous experiments demonstrated that the ratedetermining step of the catalytic cycle was the reduction process of the Rh(I) carboxylate species 9 when employing i Pr 2 NEt as a sacrificial electron donor. In order to investigate the contribution of BI(OH)H to the catalytic cycle, the similar examination was carried out using BI(OH)H as a sacrificial electron donor instead of i Pr 2 NEt. Interestingly, the resting-state was found to be Rh(I) π-benzyl intermediate 8 when a mixture of 1a, catalytic amounts of 6 and [Ru(bpy) 3 ](PF 6 ) 2 , and 1.2 equiv. of BI(OH)H was irradiated by visible-light under CO 2 atmosphere (Supplementary Figure 4). In that case, 9 was not detectable even after prolonged irradiation, indicating that the rate-determining step obviously altered from the reduction process to the carboxylation process by changing the sacrificial electron donor. This result was also supported by the fact that the acceleration of the reaction by the addition of a second photosensitizer was not observed in the case of the reaction using BI(OH)H as a sacrificial electron donor (

CONCLUSION
In this study, the improved catalytic conditions of the visiblelight driven hydrocarboxylation by Rh(I) and [Ru(bpy) 3 ] 2+ catalysts were explored, and the detailed reaction mechanism was investigated. On the basis of the stoichiometric reactions of the possible rhodium intermediates, the proposed catalytic cycle was confirmed to be composed of (i) the hydrometallation of alkenes by Rh(I) monohydride species, (ii) the photochemical carboxylation of the Rh(I) benzyl species with CO 2 , (iii) the photoinduced 2-electron, 2-proton transfers to the Rh(I) carboxylate species, and (iv) the base-assisted carboxylic acid elimination. One strategy for the enhancement of the catalytic reaction was to employ BI(OH)H possessing superior reducing ability as a sacrificial electron donor instead of i Pr 2 NEt. It successfully improved the efficiency of the reaction, which had been major challenges in the previous catalytic conditions. The alteration of the resting-state by changing the sacrificial electron donor indicated that the addition of BI(OH)H significantly promoted the reduction process of the Rh(I) carboxylate species through the enhancement of the reductive quenching efficiency of [Ru(bpy) 3 ] 2+ . Another strategy for the enhancement of the efficiency was to add the second photosensitizer in charge of the reductive quenching cycle. The acceleration of the catalytic reaction by the addition of the appropriate cyclometalated Ir(III) complex together with [Ru(bpy) 3 ] 2+ supported this hypothesis. These two effective strategies suggested that the promotion of the reduction processes was a key to enhance the catalytic activity in the present system. In addition to expand the versatility of the present hydrocarboxylation, this study would provide fundamental insights into the catalytic organic transformations by transitionmetal/photoredox dual catalysis.

Photocatalytic Reactions
For screening conditions with alkenes (1a-1g), a DMA solution (0.6 mL) of an alkene (0.060 mmol), [Rh(P(4-CF 3 C 6 H 4 ) 3 ) 2 Cl] 2 (4, 4.5 mg, 0.0021 mmol), photoredox catalyst(s), sacrificial electron donor and inorganic base (defined amounts) was prepared in a glass tube (ϕ 2.0 cm, 18 cm) under an argon atmosphere. Then the headspace gas was replaced by an atmospheric pressure of CO 2 , and the reaction vessel was put in a water bath placed at a distance of 10 mm from light sources. The mixture was irradiated with visible-light from blue LED lamp (λ irr. = 425 nm, two sockets) or UV-visible-light from Xe lamp (λ irr. = 380-800 nm) for defined time in the closed system. The product mixture was analyzed by 1 H NMR and GC to determine the NMR yield of the hydrocarboxylated product (2a-2g) and the GC yield of the hydrogenated product (3a, 3b), respectively (internal standard: 1,1,2,2-tetrachloroethane).
For isolation of the methyl esters of the hydrocarboxylated products (2a, 2b, 2d), a DMA solution (1.2 mL) of a styrene (0.12 mmol), 4 (9.0 mg, 0.0042 mmol), [Ru(bpy) 3 ](PF 6 ) 2 (1.0 mg, 0.0012 mmol), BI(OH)H (58 mg, 0.24 mmol), and Cs 2 CO 3 (47 mg, 0.14 mmol) was prepared in a glass tube (ϕ 2.0 cm, 18 cm), and irradiated with visible-light from blue LED lamp (λ irr. = 425 nm, three sockets) for defined time after replacement of the headspace gas by an atmospheric pressure of CO 2 . After irradiation, the reaction mixture was diluted with diethyl ether and extracted with H 2 O three times. The combined aqueous layer was acidified by 1N HCl aq., and then extracted with diethyl ether three times. The combined organic layer was dried over MgSO 4 , filtered and evaporated under reduced pressure to give the hydrocarboxylated product. Then, the product was dissolved in Et 2 O-MeOH, and TMSCHN 2 (excess) was added at 0 • C. The mixture was stirred at 0 • C for 30 min and the solvent was removed under reduced pressure. The crude product was purified by preparative TLC (AcOEt/n-hexane = 1/5) to give the corresponding methyl-esterified product.

Theoretical Study
Theoretical calculations were performed at the DFT level with the Gaussian 09 package. The geometry optimizations were performed using the mPW1PW91 functional (Adamo and Barone, 1998). The LanL2DZ basis set was used for all atoms and extended by a polarization function (except for H) (Dunning and Hay, 1976;Wadt and Hay, 1985a,b). To address solvation effects, the conductor-like polarizable continuum model (CPCM, N,N-Dimethylacetamide) (Tomasi et al., 2005) was used for the ground and excited states. For validation, vibrational frequencies were calculated for the ground and excited states. The orbital plots as well as the graphical representations were performed using Molekel (Varetto, 2009). Natural bond orbital (NBO) analysis was used to predict and interpret the computational results (Glendening et al., 2001). Total ZPE energies and cartesian coordinates of computed structures are given in Supplementary Table 3.

DATA AVAILABILITY
All datasets generated for this study are included in the manuscript and/or the Supplementary Files.

AUTHOR CONTRIBUTIONS
NN and KM performed the experiments. KS instructed the experiments of NN. KM performed the theoretical calculations. NI and JT supervised the project. KM and NI wrote the paper.

FUNDING
The authors are grateful for the financial supports by an ACT-C program JPMJCR12Y3 from JST and JSPS KAKENHI Grant no. 15H05800, 17H06143, and 24245019.