Rh(I)-Catalyzed regioselective arylcarboxylation of acrylamides with arylboronic acids and CO₂

Abstract

The first Rh(I)-catalyzed regioselective arylcarboxylation of electron-deficient acrylamides with arylboronic acids under atmospheric pressure of CO₂ has been developed. A range of acrylamides and arylboronic acids were compatible with this reaction under redox-neutral conditions, leading to a series of malonate derivatives that are versatile building blocks in organic syntheses.

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In recent years, numerous transformations for fine chemical synthesis have been achieved through the use of carbon dioxide (CO₂), which is nontoxic, low cost, abundant, and sustainable, and is often considered an ideal one-carbon (C1) building block.¹ In particular, notable achievements have been made in the functionalization of alkenes with CO₂via hydrocarboxylation with transition-metal catalysis and/or photocatalysis,² including a few examples using Rh(I)-catalysis^2d,g,l,r,t (Scheme 1a). Difunctionalization of alkenes with CO₂, in contrast, has been less explored,^3–5 although several elegant transformations of this kind have been achieved by Xi's,^5a,g Popp's,^5b,h Martin's,^5c Yu's^5d,f and Wu's^5e groups. To date, however, only a single method of arylcarboxylation of alkenes is available through visible-light-driven reductive arylcarboxylation with CO₂ and aryl halides, and has been demonstrated by our group recently [Scheme 1b(i)],⁶ besides two isolated examples of titanocene-catalyzed arylcarboxylation of dienes reported by Xi's group [Scheme 1b(ii)].^5g However, the substrates of this photocatalytic method were limited to styrenes and not applicable to electron-deficient α,β-unsaturated substrates such as acrylamides. Therefore, a complementary method that could overcome this limitation is highly desirable.

Scheme 1 Carboxylation of alkenes with CO₂.

As part of our continuing interest in developing Rh-catalyzed carboxylation using CO₂,⁷ we considered whether it is possible to achieve a Rh-catalyzed difunctionalization of α,β-unsaturated compounds with CO₂.⁸ Compared with the Rh-catalyzed 1,4-addition of α,β-unsaturated compounds with organometallic reagents, Rh-catalyzed difunctionalization of these compounds is much less documented and the corresponding arylcarboxylation has not been achieved to date with Rh catalysis.⁹ Herein, we report the first Rh(I)-catalyzed arylcarboxylation of acrylamides with arylboronic acids and CO₂.

After extensive investigation of reaction conditions (see also the ESI†), a set of promising reaction conditions were revealed by using N-methyl-N-phenylacrylamide 1a and phenylboronic acid 2a as the model substrates with [Rh(cod)Cl]₂ as the catalyst in the presence of Cs₂CO₃ under CO₂ (1 atm) in DMA. Pleasingly, 40% of the desired phenylcarboxylation product 3a was detected, while 25% of Heck-type product 4 through a β-H elimination process and 7% of 1,4-addition product 5 were also identified as the major side products (Table 1, entry 1). It should be noted that traces of hydrocarboxylation and alkene reduction products could also be detected. Moreover, for the ease of reaction result analysis, the initial carboxylic acid products were converted into their ester counterparts after the phenylcarboxylation reaction ceased. In order to suppress the formation of side products 4 and 5 and increase the desired product, ligands such as dppe and IPr·HCl were added to tune the reactivity of the Rh catalyst (entries 2 and 3, see also the ESI† for more ligands used). Unfortunately, no significant improvement was observed. To our delight, however, the yield of the desired product was slightly increased when an organic base TMEDA (B1) or PMDETA (B2) was introduced into the reaction (entries 4 and 5). Subsequently, different silver salts, which are often used to increase the reactivity of Rh catalysts, were extensively investigated as additives for the reaction (entries 6–9; see also the ESI†), and it was found that AgOTf was the best choice to produce the best yield of 3a, leading to the least yield of 4 and 5 (entry 9). Comparing the results of entry 5 and entries 6–8, it appears that the silver salt mainly inhibited the formation of side product 4. However, the exact role of the silver salt is not clear at present.¹⁰ Notably, the amine additive (PMDETA) might also be a potential tridentate ligand to form a silver complex in the solution to increase the solubility of the silver salts such as AgCl.^11,10c Interestingly, CuCl could also slightly improve the reaction (entry 10, see also the ESI†). Other bases or solvents were also evaluated, but no increase in the yield of the desired product was observed (entries 11–14). Other Rh catalysts were then tested (entries 15–18), and only [Rh(cod)OH]₂ could lead to rather high yield of the product (entry 17). Importantly, the efficiency of the reaction was reduced evidently with a lower loading of the [Rh(cod)Cl]₂ catalyst (entry 19) or without B2 (entry 20). Moreover, no product was received without the Rh catalyst (entry 21). Finally, when the reaction was run under argon instead of CO₂, no desired product was found, indicating that CO₂ was not generated in situ from caesium carbonate (entry 22).

Table 1 Optimization of reaction conditions^a

Entry	Catalyst	Co-Base	Additive	Yield^b (%) (3a/4/5/1a)
a Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), CO2 (1 atm, closed), catalyst (2.5 mol%), ligand (5 mol%), Cs2CO3 (0.2 mmol), additive (5 mol%), Co-base (50 mol%), and DMA (1 mL), 60 °C, 24 h. b Yield was determined by ^1H NMR with CH2Br2 as the internal standard. c dppe (5 mol% was added as a ligand, dppe: 1,2-bis(diphenylphosphino)ethane). d IPr·HCl (5 mol% was added, IPr·HCl: 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride. e CsF (0.2 mmol) as a base. f KO^tBu (0.2 mmol) as a base. g DMF as solvent. h NMP as solvent. i [Rh(cod)Cl]2 (1 mol%) as a catalyst. j Ar (1 atm, closed) was used instead of CO2. B1: N,N,N′,N′-tetramethylethylenediamine (TMEDA). B2: N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA).
1	[Rh(cod)Cl]2	—	—	40/25/7/—
2^c	[Rh(cod)Cl]2	—	—	43/18/16/—
3^d	[Rh(cod)Cl]2	—	—	36/23/5/—
4	[Rh(cod)Cl]2	B1	—	47/27/6/—
5	[Rh(cod)Cl]2	B2	—	49/27/3/—
6	[Rh(cod)Cl]2	B2	AgF	73/5/14/—
7	[Rh(cod)Cl]2	B2	Ag2O	75/6/13/—
8	[Rh(cod)Cl]2	B2	AgCl	77/6/13/—
9	[Rh(cod)Cl] 2	B2	AgOTf	83/6/5/—
10	[Rh(cod)Cl]2	B2	CuCl	58/18/7/—
11^e	[Rh(cod)Cl]2	B2	AgOTf	13/14/57/2
12^f	[Rh(cod)Cl]2	B2	AgOTf	43/14/12/—
13^g	[Rh(cod)Cl]2	B2	AgOTf	19/9/52/—
14^h	[Rh(cod)Cl]2	B2	AgOTf	15/9/73/—
15	Rh(cod)2OTf	B2	AgOTf	59/16/8/—
16	[Rh(CO)2Cl]2	B2	AgOTf	—/10/—/81
17	[Rh(cod)OH]2	B2	AgOTf	80/9/11/—
18	[Rh(coe)2Cl]2	B2	AgOTf	—/—/—/84
19^i	[Rh(cod)Cl]2	B2	AgOTf	46/14/3/18
20	[Rh(cod)Cl]2	—	AgOTf	46/17/13/—
21	—	B2	AgOTf	—/—/—/53
22^j	[Rh(cod)Cl]2	B2	AgOTf	—/8/70/—

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With the optimized reaction conditions in hand, the method was then tested with a range of arylboronic acids (Table 2). First, good isolated yields were obtained with model phenylboronic acid through esterification with different aryl halides (3a′ and 3a′′). To our delight, arylboronic acids with electron-rich or electron-deficient substituents at the ortho- or meta-position of the phenyl group were tolerated, providing moderate to good yields of malonates (3b–3f). It was also found that several types of substituents at the para-position were suitable for the reaction (3g–3k). It is worth noting that multisubstituted phenylboronic acids could also react with 1a to produce the desired products in acceptable yields (3l–3o). It should also be mentioned that the formation of the side products from Heck-type coupling or 1,4-addition could not be suppressed completely for the above examples (see the ESI† for details).

Table 2 Substrate scope of arylboronic acids

Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol, 1.5 equiv.), CO₂ (1 atm, closed), [Rh(cod)Cl]₂ (2.5 mol%), Cs₂CO₃ (0.2 mmol), AgOTf (5 mol%), PMDETA (50 mol%), and DMA (1 mL), 60 °C, 24 h; then MeI, or BuBr, or BnBr (1 mmol), and K₂CO₃ (0.4 mmol). Isolated yields; the yield of 3a in parentheses is that of 1 mmol (1a) scale reaction.a Ag₂O (5 mol%) as an additive.b KO^tBu (0.6 mmol) as a base.c Cs₂CO₃ (0.6 mmol) as a base.d 2 (0.4 mmol) was used.e AgCl (5 mol%) as an additive.f AgF (5 mol%) as an additive.g [Rh(cod)Cl]₂ (5 mol%) as a catalyst.

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The scope of acrylamides was then examined. As shown in Table 3, acrylamides with electron-donating and electron-withdrawing groups at the meta- or para-position of the aryl group of acrylamides were well tolerated (3ab–3ak), affording the desired products smoothly. Unfortunately, acrylamides bearing substituents at the ortho-position of the acrylamide's aryl group exhibited low reactivity, which is possibly due to the increased steric hindrance. Moreover, a di-substituted acrylamide was also a viable substrate for this reaction (3al). When replacing the N-protecting group Me with Ph and Bn (3am and 3an), the corresponding products were still generated. Notably, indoline- and tetrahydroquinoline-derived acrylamides were compatible with the reaction, yielding the desired products in good yields (3ao and 3ap). In addition, it is noteworthy that the side products from Heck-type coupling or 1,4-addition were also present and were hard to suppress for these examples (see the ESI† for details).

Table 3 Substrate scope of acrylamides

Reaction conditions: 1 (0.2 mmol), 2 (1.5 equiv.), CO₂ (1 atm, closed), [Rh(cod)Cl]₂ (2.5 mol%), Cs₂CO₃ (0.6 mmol), AgOTf (5 mol%), PMDETA (50 mol%), and DMA (1 mL), 60 °C, 24 h. Isolated yields.a Cs₂CO₃ (0.2 mmol) as a base.b [Rh(cod)Cl]₂ (5 mol%) as a catalyst.c AgF (5 mol%).d AgCl (5 mol%).

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To gain insight into this novel arylcarboxylation method, some control experiments were performed. As shown in Scheme 2, subjecting side product 5 to standard conditions could not lead to the desired product 3a (Scheme 2a). No 3a could be formed without adding the Rh catalyst under standard conditions and with AgCl as the additive which could be formed from [Rh(cod)Cl]₂ and AgOTf in the reaction (Scheme 2b). These two experiments indicated that the conjugate addition and the nucleophilic addition of CO₂ by the rhodium enolate should occur sequentially without the hydrolysis of the rhodium enolate,^9d and that the weakly basic conditions of the reaction were not sufficient to induce carboxylation of the α-position of the amide 5. Further experiments also indicated the presence of a rhodium enolate (Scheme 2c) and its importance for the second functionalization after conjugate addition (Scheme 2d). Finally, the product could be elaborated to generate oxindole-type compounds which are versatile building blocks for organic synthesis and are often found as the core structure of natural products and molecules of medicinal importance (Scheme 2e).¹²

Scheme 2 Control experiments and product elaboration.

Based on the above results, a plausible mechanism is proposed (Scheme 3). First, ligand exchange converts the [Rh(cod)Cl]₂ catalyst to an active rhodium species A, which undergoes transmetalation with arylboronic acids 2, leading to a [Rh]–Ar species B. Coordination of B onto 1a, followed by a regioselective addition, produces a rhodium enolate D. This key intermediate D will attack CO₂ to afford the carboxylate intermediate E, which ultimately leads to the desired product 3a. On the other hand, rhodium enolate D can also undergo either β-H elimination to produce side product 4 or direct protonation to produce side product 5, respectively.

Scheme 3 Proposed catalytic cycle.

Conclusions

In summary, we have developed the first Rh(I)-catalyzed regioselective arylcarboxylation of electron-deficient acrylamides with CO₂ under redox-neutral conditions. A range of acrylamides and arylboronic acids were suitable for this reaction, leading to a series of malonate derivatives that could be converted into other important structures such as oxindoles. Preliminary mechanistic studies indicated the importance of the rhodium enolate. Further investigation of the Rh-catalyzed carboxylation strategy with CO₂ is underway in our laboratory.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully thank the financial support from NSFC (Grant No. 21871257 and 21801240) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and data for new compounds. See DOI: 10.1039/d0gc02667k

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