CO2 can be regarded as a nontoxic, economical and abundant carbon source, and therefore has been widely used as a sustainable C1 building block , , ,  through the formation of C–C, C–O or C–N bond to produce various chemicals, e.g. cyclic carbonates , , , oxazolidinones , , carboxylic acid ,  (viz. CO2 functionalization). On the other hand, CO2 reduction to energy related products such as formic acid , , , formaldehyde , , methanol ,  and methane ,  has drawn increasing attention, as these products can serve as important energy carriers. In recent years, reductive functionalization of CO2 combining both reduction of CO2 and formation of C−C, C−N and C−O bonds becomes popular, as this novel approach enlarges the spectrum of CO2–derived products and opens the opportunity for a large synthetic diversity as an entry into the chemical value chain , , , , , , , , , , , , . One promising methodology in this area is the reductive functionalization of CO2 with amine and reductant to produce methylamines, which realizes six-electron reduction of CO2 coupled with the C−N bond construction as shown in Scheme 1 , .
Methylamines are important intermediates which find widespread applications in the preparation of dyes, natural products and fine chemicals, etc. , . Conventionally, methylamines are often prepared from amines by employing formaldehyde, methyl iodide or dimethyl sulfate as methylating agent , . However, toxicity associated with methylating agent and limited substrate scopes are significant drawbacks in those existed procedures. In this regard, a synthetic methodology using CO2 as a C1-building block becomes alternative and promising to methylamines from amines in view of sustainable chemistry , . Among the methods developed, metal-based catalysts including Zn , Ru , , , , Cu , Ni , Cs  have been widely employed. Recently, organocatalytic systems such as proazaphosphatrane superbases , N-heterocyclic carbenes (NHCs) , analogous carbodicarbenes (CDCs) , B(C6F5)3  as well as tetrabutylammonium fluoride (TBAF)  have also been developed as a green alternative. Despite the progress in this area, simple, easy-handling and inexpensive catalysts under mild reaction conditions remain in desirable, yet unmet, demand.
We have recently reported on the efficient reductive functionalization of CO2 to methylamines with hydrosilanes as reactants catalyzed by TBAF . It is proposed that fluoride anion interacts with the silicon atom of hydrosilane to generate the penta- or hex-coordinated silicon intermediate, promoting hydride transfer from the hydrosilane to CO2/formamide. It has been known that amide could coordinate with the hydrosilane to form the active hypervalent organosilicate intermediate via Si atom coordinating with a carbonyl oxygen atom of amide, which promotes hydride transfer to carbonyl/imine group, realizing reduction of carbonyl/imine group. For example, DMF in combination with Cl3SiH has been found to efficiently reduce aldehyde and imine , , . Notably, the DMF-promoted N-formylation of amines using CO2 has been recently reported in which CO2 goes through two-electron reduction to formic acid oxidation level . Herein, we would like to report amide as both promoter and solvent for fixing CO2 with amines using hydrosilane to afford methylamines . Such a protocol allows six-electron reduction of CO2 to methanol oxidation level in combination with C–N bond construction.
Results and discussion
The reductive functionalization of CO2 in the presence of N-methylaniline (1a) with phenylsilane as a reductant (Scheme 2) was investigated as summarized in Table 1. Various solvents were screened initially and toluene, THF and CH3CN were not suitable for this reaction (entries 1–3). When a amide solvent DMF was used, a quantitative conversion of 1a was achieved along with the formation of a mixture of N-methylformanilide 1b and N,N-dimethylaniline 1c (entries 4). We then attempted to improve the product selectivity towards the methylated product 1c. Initially increasing the phenylsilane amount improved the yield of the methylated product 1c. However, a considerable amount of 1b is difficult to be separated in practice (entries 5–6). To our delight, when raising the reaction temperature to 80°C, an almost quantitative yield of 1c could be obtained (entry 7). This similar trend that high temperature favors the formation of the methylated product was also observed in the TBAF and Cs2CO3 catalysis for the same reaction , , hinting that the methylation is more difficult than the formylation. Using N2 balloon instead of CO2, no product could be detected, indicating CO2 is indispensable for this transformation (entry 8). When DEA (N,N-diethylacetamide) instead of DMF acting as promoter and solvent, a 72% yield of 1c could still be attained, which excludes the possibility of the newly formed methyl group of 1c from DMF (entry 9). In the absence of phenylsilane as reductant, this reaction could not occur at all (entry 10). In particular, three control experiments suggest that the newly-formed –CH3 derives from reduction of CO2 (entries 8–10).
Furthermore, the reaction using N,N-dimethylacetamide (DMAc) as promoter and solvent could give 65% yield of 1c (entry 11). Nevertheless, N,N-dimethylethylamine (DMEA) and 3-methyl-2-butanone (MBO), without carbonyl group and nitrogen atom in their molecules compared with DMA, could not deliver 1c at all (entries 12, 13). These results indicate that amide group plays a crucial role in this transformation (entries 11–13). This is in accordance with the reduction of aldehydes, ketones and imines catalyzed by amide in which amide interacts with the silicon reagent to form the active hypervalent silicon center and promotes the hydride transfer , , .
Halving the amount of DMF did not influence the yield of the methylated product 1c (entry 14). Further lowering DMF to 20 μL (1 equiv. relative to 1a), 11% yield of 1c was obtained (entry 15). In addition, 4 h is enough to complete the reaction (entry 16).
Subsequently, various hydrosilanes were investigated to optimize this reductive functionalization of CO2 with 1a (entries 17–21). Diphenylsilane showed good reactivity with 65% yield of 1c, while other hydrosilanes including dimethylphenylsilane, triethoxysilane, triethylsilane and polymethylhydrosiloxane (PMHS) were inactive for this reaction (entries 18–21).
After having established the amide-promoted reductive functionalization of CO2 with N-methylaniline, we further examined the generality of this methylation protocol as listed in Scheme 3. Para-substituted N-methylanilines with electron-donating or -withdrawing groups performed smoothly to give the corresponding products in good to excellent yields (1c–8c). Comparatively, p-Cl-substituted N-methylaniline exhibited the highest reactivity (4c). Notably, electron-deficient aniline (p-fluoro) was converted to 5c in good yield. In addition, o-substituted N-methylaniline was also viable substrate (9c). N-ethyl, N-allyl and N-isopropyl anilines were successfully methylated with CO2 (10c–12c), but the amine with bulky group showed low reactivity (12c), being ascribed to the steric hindrance. It is remarkable that substrates with alkenyl (11c), alkynyl (6c), carbonyl (7c) and ester group (8c) were transformed into the corresponding methylated products with the sensitive group intact, showing good functional group compatibility. Notably, this protocol was also applicable to aliphatic amines such as piperidine and morpholine (13c–14c). In addition, primary amines performed in a similar fashion to that of secondary amines, resulting in the production of dimethylated products dominantly by increasing the amount of hydrosilane and doubling the reaction time (15c–20c). Furthermore, imine substrate e.g. benzylidene phenylamine was subject to this protocol and the corresponding N-methylated amine 21c was also obtained in 73% yield (Scheme 4).
To gain insight into the reaction mechanism, several control experiments were performed as depicted in Scheme 5. Formamide 1b was firstly tested under the optimized reaction conditions, yet no expected product 1c was detected, revealing that the formamide 1b is by-product rather than the intermediate (eq. 1). N,N-diphenyl-N,N-dimethylurea (1d) was another possible intermediate in this reductive methylation, which could be quantitatively reduced to give N,N-dimethylaniline 1c in the presence of PhSiH3 catalyzed by [RuCl2(dmso)4] . However, 1c could not be obtained in this case (eq. 2), ruling out the formation of the urea as intermediate.
It has very recently been reported that DMF as both promoter and solvent could promote reduction of CO2 to silyl acetals coupled with silyl formates and silyl methoxides in the presence of diphenylsilane . On the other hand, N,N-dimethyl-N,N-diphenylmethanediamine 1e which is assumed to derive from condensation between the silyl acetal and N-methylaniline 1a , was detected in the reaction mixture by GC-MS. The reaction using N,N-dimethyl-N,N-diphenylmethanediamine 1e as substrate under the optimized conditions afforded quantitative yield of 1c, indicating that 1e may be the real intermediate in this system.
On the basis of the above experimental results and previous reports on the amide-catalyzed carbonyl and imine reduction , , , a possible mechanism for the present reductive functionalization of CO2 with amine and phenylsilane as a reductant was proposed as depicted in Scheme 6. At first, in the presence of phenylsilane and amide, CO2 is reduced to the silyl formate I, which undergoes further reduction to obtain the silyl acetal species II. Then, the nitrogen atom of the amine substrate nucleophilically attacks the carbon atom of the intermediate II affording the aminal IV . The aminal IV is reduced to the desired methylated product V with amide as promoter and phenylsilane as reductant. On the other hand, the silyl formate I may be attacked by the amine to give the formamide III , , which could account for the formation of formamide as a by-product.
In conclusion, an amide-promoted reductive functionalization of CO2 with secondary amines and an imine in the presence of phenylsilane as a reductant affording corresponding methylamines has been developed under atmospheric pressure of CO2 and 80°C. Notably, a tentative mechanism that the amine attacks the bis(silyl)acetal species deriving from CO2 reduction to deliver the aminal which is further reduced to the methylated product was proposed. Such a protocol provides the six-electron reduction of CO2 coupled with construction of carbon-nitrogen bond and further studies to expand the utility of this system are in progress in our laboratory.
The starting materials were commercially available and were used without further purification except solvents. All compounds were characterized by 1H NMR, 13C NMR and mass spectroscopy, which are consistent with those reported in the literature. NMR spectra were determined on Bruker 400 in CDCl3. GC-MS data were performed on Finnigan HP G1800 A. GC analyses were performed on a Shimadzu GC-2014 equipped with a capillary column (RTX-17 30 m×0.25 μm) using a flame ionization detector.
Under inert atmosphere (Ar), a 10 mL Schlenk flask was charged successively with amine (0.25 mmol), hydrosilane, and DMF (2 mL). The reaction mixture was stirred at a typical temperature for the desired time under an atmosphere of CO2 (99.999%, balloon). After the reaction, 1,3,5-trimethyoxybenzene (42 mg) was added as an internal standard and then a sample was taken to be injected into the GC to determine the conversion and yield. All catalytic reactions were performed at least twice to ensure reproducibility. To identify the structure of the methylated product, the reaction mixture was concentrated and purified by silica gel column chromatography (petroleum ether-EtOAc) to afford the corresponding methylamine. Spectral characterizations of the products (1c–21c) are as follows and corresponding NMR charts are appended in the Supplementary Material.
N,N-dimethylaniline 1c(15c) (eluent ratio of petroleum ether-EtOAc=50:1)
Yellow oil. 1H NMR (400 MHz, CDCl3) δ 7.24 (dd, J=8.8, 7.3 Hz, 2H), 6.85–6.62 (m, 3H), 2.94 (s, 6H). 13C NMR (101 MHz, CDCl3) δ 150.58, 129.03, 116.60, 112.62, 40.61. GC-MS (EI, 70 eV) m/z (%) 121.15 (81.29), 120.15 (100.00), 77.05 (29.56).
White solid. 1H NMR (400 MHz, CDCl3) δ 6.88–6.81 (m, 2H), 6.79–6.72 (m, 2H), 3.76 (s, 3H), 2.86 (s, 6H). 13C NMR (101 MHz, CDCl3) δ 152.12, 145.52, 115.04, 114.60, 55.72, 41.92. GC-MS (EI, 70 eV) m/z (%) 151.25 (59.49), 136.20 (100.00).
Yellow oil. 1H NMR (400 MHz, CDCl3) δ 7.05 (d, J=8.1 Hz, 2H), 6.69 (d, J=8.2 Hz, 2H), 2.89 (s, 6H), 2.25 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 148.77, 129.54, 126.09, 113.19, 41.05, 20.22. GC-MS (EI, 70 eV) m/z (%) 35.20 (77.14), 134.15 (100.00), 91.10 (22.96).
4-Chloro-N,N-dimethylaniline 4c(18c) (eluent ratio of petroleum ether-EtOAc=50:1)
Yellow oil. 1H NMR (400 MHz, CDCl3) δ 7.17 (d, J=8.9 Hz, 2H), 6.64 (d, J=8.7 Hz, 2H), 2.92 (s, 6H). 13C NMR (101 MHz, CDCl3) δ 149.10, 128.75, 121.36, 113.59, 40.64. GC-MS (EI, 70 eV) m/z (%) 157.10 (25.27), 156.10 (39.00), 155.10 (82.99), 154.10 (100.00).
Dark green solid. 1H NMR (400 MHz, CDCl3) δ 6.93 (t, J=8.8 Hz, 1H), 6.77–6.42 (m, 1H), 2.88 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 156.73, 154.39, 147.47, 147.46, 115.42, 115.21, 113.93, 113.85, 41.30.
GC-MS (EI, 70 eV) m/z (%) 139.25 (89.70), 138.20 (100.00), 123.15 (26.03), 122.15 (26.55), 95.10 (25.48).
4-Dimethylaminophenyl acetylene 6c
Yellow solid. 1H NMR (400 MHz, CDCl3) δ 7.36 (d, J=8.9 Hz, 2H), 6.61 (d, J=8.8 Hz, 2H), 2.96 (s, 7H). 13C NMR (101 MHz, CDCl3) δ 150.43, 133.21, 111.72, 108.85, 84.87, 74.76, 40.16.
Colorless liquid. 1H NMR (400 MHz, CDCl3) δ 7.92 (d, J=9.0 Hz, 2H), 6.66 (d, J=9.0 Hz, 2H), 4.32 (q, J=7.1 Hz, 2H), 3.04 (s, 6H), 1.36 (t, J=7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 166.97, 164.97, 153.17, 131.20, 117.69, 110.85, 60.10, 40.13, 14.46.
Ethyl 4-(dimethylamino)benzoate 8c
Colorless liquid. 1H NMR (400 MHz, CDCl3) δ 7.88 (d, J=8.9 Hz, 2H), 6.70 (d, J=8.9 Hz, 2H), 3.07 (s, 6H), 2.51 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 196.25, 153.38, 130.46, 125.43, 110.59, 39.96, 25.89.
Colorless liquid. 1H NMR (400 MHz, CDCl3) δ 7.15 (t, J=6.6 Hz, 2H), 7.02 (d, J=8.2 Hz, 1H), 6.94 (t, J=7.3 Hz, 1H), 2.69 (s, 6H), 2.32 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 152.66, 132.04, 131.10, 126.38, 122.50, 118.28, 44.18, 18.33.
GC-MS (EI, 70 eV) m/z (%)135.25 (100.00), 134.25 (76.46), 120.20 (67.51), 119.20 (15.40), 118.20 (35.79), 104.15 (29.86), 91.10 (45.56), 65.10 (26.91).
Yellow liquid. 1H NMR (400 MHz, CDCl3) δ 7.22 (t, J=7.7 Hz, 2H), 6.84–6.52 (m, 3H), 3.39 (q, J=7.0 Hz, 2H), 2.89 (s, 3H), 1.10 (t, J=7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 149.05, 129.11, 115.97, 112.33, 46.75, 37.38, 11.13. GC-MS (EI, 70 eV) m/z (%) 135.15 (31.98), 120.15 (100.00), 77.05 (33.12).
N-allyl-N-methylaniline 11c (eluent ratio of petroleum ether-EtOAc=100:1)
Colorless liquid. 1H NMR (400 MHz, CDCl3) δ 7.31–7.24 (m, 2H), 6.76 (d, J=8.4 Hz, 2H), 6.72 (t, J=7.3 Hz, 1H), 5.88 (m, 1H), 5.17–5.24 (m, 2H), 3.95 (d, J= 6.4Hz, 2H), 2.98 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 153.4, 131.2, 126.5, 113.3, 112.5, 109.3, 56.2, 35.0.
Yellow liquid. 1H NMR (400 MHz, CDCl3) δ 7.22 (t, J=7.9 Hz, 2H), 6.79 (d, J=8.2 Hz, 2H), 6.69 (t, J=7.2 Hz, 1H), 4.18–3.96 (m, 1H), 2.72 (s, 3H), 1.15 (d, J=6.6 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ 150.10, 129.07, 116.38, 113.28, 48.87, 29.75, 19.26. GC-MS (EI, 70 eV) m/z (%) 149.20 (19.30), 134.15 (100.00), 77.05 (20.49).
Colorless oil. 1H NMR (400 MHz, CDCl3) δ 2.33 (s, 4H), 2.24 (s, 3H), 1.68–1.51 (m, 4H), 1.41 (s, 2H). 13C NMR (101 MHz, CDCl3) δ 56.46, 46.86, 25.95, 23.70. GC-MS (EI, 70 eV) m/z (%) 99.15 (42.36), 98.15 (100.00), 71.10 (22.54).
Colorless oil. 1H NMR (400 MHz, CDCl3) δ 3.80–3.61 (m, 4H), 2.41 (s, 4H), 2.29 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 66.67, 55.20, 46.23. GC-MS (EI, 70 eV) m/z (%) 101.15 (100.00), 100.15 (36.36), 71.10 (60.10).
White solid. 1H NMR (400 MHz, CDCl3) δ 7.30 (d, J=9.1 Hz, 2H), 6.59 (d, J=8.8 Hz, 2H), 2.92 (s, 6H). 56.46, 46.86, 25.95, 23.70. 13C NMR (101 MHz, CDCl3) δ 149.46, 131.64, 114.06, 108.45, 40.53. GC-MS (EI, 70 eV) m/z (%) 201.00 (93.41), 200.00 (100.00), 199.00 (99.32), 198.00 (97.76), 118.15 (45.22), 77.10 (20.98).
Colorless oil. 1H NMR (400 MHz, CDCl3) δ 3.01 (m, 1H), 2.71 (s, 6H), 2.23 (m, 2H), 1.93 (m, 2H), 1.72 (m, 1H), 1.24–1.47 (m, 5H). 13C NMR (101 MHz, CDCl3) δ 65.54, 39.53, 26.96, 25.23, 25.17.
Colorless liquid. 1H NMR (400 MHz, CDCl3) δ 7.24–7.32 (m, 2H), 7.15–7.20 (m, 5H), 6.64–6.73 (m, 3H), 4.47 (s, 2H), 2.98 (s, 3H); 13C NMR (101 MHz, CDCl3) δ 137.9, 134.5, 131.7, 129.6, 128.7, 127.6, 126.7, 126.5, 126.1, 125.4, 63.2, 51.1.
Colorless liquid. 1H NMR (400 MHz, CDCl3) δ 7.25 (dd, J=8.6, 7.4 Hz, 4H), 6.84 (d, J=8.1 Hz, 4H), 6.78 (t, J=7.3 Hz, 2H), 4.76 (s, 2H), 2.87 (s, 6H). 13C NMR (101 MHz, CDCl3) δ 149.31, 129.27, 117.86, 113.76, 70.44, 36.31. GC-MS (EI, 70 eV) m/z (%) 226.20 (2.83), 121.15 (16.02), 120.15 (100.00), 107.15 (61.88), 106.15 (71.81), 79.10 (21.17), 77.10 (39.67).
This work was financially supported by National Key Research and Development Program (2016YFA0602900), National Natural Science Foundation of China (21472103, 21421001, 21672119), Funder ID: 10.13039/501100001809, the Natural Science Foundation of Tianjin Municipality (16JCZDJC39900).
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In the preparation of this paper, a catalyst-free protocol was published for methylation of amines using CO2 as C1 source with DMF as solvent, see: H. Niu, L. Lu, R. Shi, C.-W. Chiang, A. Lei. Chem. Commun. 53, 1148, (2016). Google Scholar
The online version of this article offers supplementary material (https://doi.org/10.1515/pac-2017-0304).
About the article
Published Online: 2018-01-30
Published in Print: 2018-07-26
Citation Information: Pure and Applied Chemistry, Volume 90, Issue 7, Pages 1099–1107, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: https://doi.org/10.1515/pac-2017-0304.http://creativecommons.org/licenses/by-nc-nd/4.0/.