Activating Methanol for Chemoselective Transfer Hydrogenation of Chalcones Using an SNS-Ruthenium Complex

Abstract

Methanol is gaining popularity as a transfer-hydrogenating agent in catalytic reduction reactions because of its bulk-scale production and inexpensive nature. Current research interests include the utilization of methanol as a safe and sustainable hydrogen source for chemical synthesis and drug development. In particular, the chemoselective reduction of α,β-unsaturated ketones is of great interest because of their prevalence in many natural products. We investigated the potential application of acridine-derived SNS-Ru pincer complexes in methanol activation for chemoselective reduction of chalcones. Our developed catalytic system showed broad substrate tolerance, including substrates containing reducible functional groups. Control experiments and postsynthetic applications are also highlighted.

The hydrogenation reaction has emerged as a useful technique for the sustainable synthesis of a variety of fine and bulk chemicals which are used in our daily life.[1] [2] This class of reactions permits the conversion of various functional groups, such as acid, ester, or carbonyl groups into the corresponding alcohols; nitriles into imines or amines; and alkyne or alkene systems into alkenes and alkanes, respectively. Despite the progress made, achieving the selective hydrogenation of particular functional groups while keeping other reducible groups intact is extremely challenging, yet holds paramount importance.

For example, the syntheses of many biologically active compounds depends on the successful selective hydrogenations of C=C bonds in α,β-unsaturated carbonyl compounds[3] [4] [5] (see Figure [1]).[6] This transformation is frequently accomplished through the chemoselective hydrogenation of chalcones. Many metal-based single-electron reductants, such as Li in liquid ammonia or Mg in methanol, are employed for this transformation.[7] [8] [9] However, the main issues with using these reductants include safety, tedious purification procedures, and metal leaching. On the other hand, the utilization of metal hydrides results in the generation of stoichiometric amounts of waste. Therefore, chemoselective catalytic hydrogenation is an environmentally friendly way to achieve the required reaction. Although hydrogenation using molecular hydrogen offers a more environmentally friendly and atom-economical approach, it requires specialized equipment and entails safety risks due to the flammability of hydrogen, alongside logistical hurdles in storage and transportation.[10] [11] [12] [13] [14] Consequently, the pursuit of sustainable hydrogenation strategies has become a central objective for the scientific community, with the aim of balancing efficacy with environmental consciousness in chemical synthesis. In this regard, lignocellulosic biomass-derived alcohols could be the most promising candidates; in particular, methanol stands out, as it contains 12.6 wt% of hydrogen, which can overcome the problems associated with the storage and transportation of hydrogen gas.[15] Despite its potential, the activation of methanol in chemical transformations presents challenges, including its high dehydrogenation energy (~84 kcal/mol), unwanted side reactions yielding reactive formaldehyde intermediate,[16] and catalyst poisoning through the formation of metal carbonyls.[17] The development of effective and robust catalysts capable of facilitating methanol dehydrogenation while minimizing side reactions and catalyst deactivation is therefore required to overcome these obstacles.

Table 1 Reaction Optimization for Chemoselective Hydrogenation of Chalcone 4a Using Ru Pincer Complexes

Entry	Cat. (mol%)	Base (equiv)	Time (h)	Temp (°C)	Solvent	Yieldb (%)
						5a	6a
1	Ru-1 (1)	t-BuOK (1)	24	120	toluene	50	10
2	Ru-1 (0.5)	t-BuOK (1)	24	120	toluene	40	5
3	Ru-1 (1)	t-BuONa (1)	24	120	toluene	48	12
4	Ru-1 (1)	KOH (1)	24	120	toluene	50	–
5	Ru-1 (2)	KOH (1)	24	120	toluene	85	10
6	Ru-1 (2)	KOH (1)	18	120	toluene	90	trace
7	Ru-1 (2)	KOH (1)	12	120	toluene	90	–
8	Ru-1 (2)	KOH (1)	6	120	toluene	50	–
9	Ru-1 (2)	KOH (1)	12	100	toluene	66	–
10	Ru-1 (2)	KOH (0.5)	12	120	toluene	60	–
11	Ru-1 (2)	NaOMe (0.5)	12	120	toluene	20	–
12	Ru-1 (2)	NaOH (1)	12	120	toluene	41	–
13	Ru-1 (2)	Na2CO3 (1)	12	120	toluene	25	–
14	Ru-1 (2)	K2CO3 (1)	12	120	toluene	30	–
15	Ru-1 (2)	Cs2CO3 (1)	12	120	toluene	42	–
16	Ru-1 (2)	K2PO3 (1)	12	120	toluene	20	–
17	Ru-1 (2)	KOH (1)	12	120	xylenes	53	–
18	Ru-1 (2)	KOH (1)	12	120	THF	trace	–
19	Ru-1 (2)	KOH (1)	12	120	1,4-dioxane	34	–
20	Ru-1 (2)	KOH (1)	12	120	H2O	–	–
21	Ru-2 (1)	KOH (1)	12	120	toluene	20	–
22	Ru-3 (1)	KOH (1)	12	120	toluene	30	–
23	Ru-1 (1)	–	12	120	toluene	NRc	–
24	–	KOH (1)	12	120	toluene	NR	–

^a Reaction conditions: 4a (0.5 mmol), MeOH (0.2 mL), KOH (0.5 mmol), solvent (2 mL), 30 mL sealed tube, under argon

^b Isolated yield.

^c NR = no reaction.

In recent years, significant progress has been made in designing noble- and nonnoble-metal-based catalysts tailored for methanol dehydrogenation and subsequent applications in hydrogenations of alkynes, nitriles, imines, aldehydes, ketones, etc., or in methylations of ketones, amines, amides alcohols, chalcones, etc.[13] However, the highly reactive nature of in situ-formed formaldehyde can directly lead to the formation of the corresponding methylated products. It is challenging to stop this reaction at the stage of hydrogenated intermediates while the in situ generated electrophilic synthon formaldehyde is present.[16] In this regard, only a handful of reports describe chemoselective hydrogenations of α,β-unsaturated ketones. In 2019, Xiao and co-workers developed a Rh-catalyzed protocol for the selective hydrogenation α,β-unsaturated ketones.[18] In 2022, the Sundararaju group developed an elegant Ir-catalyzed elegant protocol for the selective hydrogenation of chalcones at ambient temperatures.[19] Very recently, Kundu and co-workers developed another Ir complex-catalyzed protocol that proceeds through a proton-responsive mechanism.[6] Hence, the development of a more sustainable and more cost-effective protocol is required. In this respect, Patil and Pratihar reported a chemoselective hydrogenation of chalcones using a Ru catalyst and excess MeOH.[20] Therefore, the development of an efficient and relatively inexpensive catalyst for the activation of MeOH for chemoselective transfer hydrogenation processes is highly desirable. As a part of our ongoing focus on developing Ru-catalyzed sustainable protocols[21] [22] [23] [24] [25] for various catalytic transformations using methanol, we envisioned that a chemoselective hydrogenation of α,β-unsaturated ketones would be advantageous.

Initially, we started our optimization by using our developed SNS-Ru catalyst Ru-1 with (2E)-1,3-bis(4-methoxyphenyl)prop-2-en-1-one (4a) as a model substrate. When the chalcone 4a and methanol were heated at 120 °C in the presence of 1 mol% of catalyst Ru-1 and one equivalent of t-BuOK under an argon atmosphere for 24 hours, a mixture of the hydrogenated product 5a (50%) and the α-methylated product 6a (10%) was isolated by column chromatography (Table [1], entry 1). In an attempt to obtain the C=C-hydrogenated product selectively, we reduced the loading of catalyst Ru-1 from 1 to 0.5 mol%; however, this reduced the yield of both products (entry 2). t-BuONa delivered similar yields of the two products (entry 3). The selectively C=C-hydrogenated product 5a was isolated in 50% yield by employing KOH as a base, keeping other parameters unaltered (entry 4). On increasing the Ru-1 loading to 2 mol%, the yield of 5a was increased to 85% with the formation of 10% of the α-methylated product 6, which was suppressed by reducing the reaction time to 12 hours (entries 5–7). Further reduction of the time, temperature, or the base concentration had a detrimental effect (entries 8–10). Weaker bases such as NaOH, Na₂CO₃, K₂CO₃, Cs₂CO₃, and K₃PO₄ produced inferior results (entries 12–16). Screening of various solvents (xylenes, THF, 1,4-dioxane, or water) showed that these were unsuitable for the chemoselective hydrogenation of chalcone (entries 17–20). Two other catalysts, Ru-2 and Ru-3, delivered lower yields of 5a (entries 21 and 22). Control experiments indicated that the presence of both the catalyst and a suitable amount of base was important to obtain our desired product (entries 23 and 24). To check the chemoselectivity of the model reaction, the crude product from the optimized reaction conditions (entry 7) was analyzed by NMR spectroscopy.The results (see the Supporting Information) indicated a clean conversion of the chalcone 4a into the α-branched ketone product 5a under our optimized reaction conditions.

Having identified the optimal reaction conditions, we examined the applicability of our developed catalytic protocol to a wide range of substrates. Initially, chalcones containing electron-donating substituents on both the aromatic nuclei delivered excellent yields (71–80%) of the desired α-branched ketone products 5a–c (Scheme [1]). When any one of the aromatic rings was substituted with an electron-rich substituent, our protocol also produced excellent isolated yields (73–80%) of the desired compounds 5d–h. A chalcone containing a polyaromatic nucleus was also found to be suitable for chemoselective hydrogenation by our protocol and delivered a 77% isolated yield of the corresponding ketone 5i. Next, the chalcones containing halogens were examined under the optimized reaction conditions. Interestingly, α,β-unsaturated ketones containing a chloro or iodo group on the aromatic nucleus furnished good yields (68–74%) of the corresponding α-branched ketones, whereas a bromo-substituted chalcone derivative afforded a 50% yield of the corresponding α-branched product 5l along with a 30% yield of the debrominated product. A substrate with a highly electron-withdrawing fluoro group delivered a moderate yield of the target product 5o. Chalcones containing a heteroaromatic 2-thienyl or 2-furyl group were then tested under optimized conditions. Notably, they gave 75-80% isolated yields of the corresponding α-branched ketones 5p–s. Furthermore, an α,β-unsaturated ketone containing a decyl chain underwent smooth chemoselective hydrogenation by our protocol to give a 72% yield of the corresponding α-branched ketone 5t. Tetralone-substituted chalcones gave the desired products 5u–w in yields of 65–56%. Pleasingly, chalcones with reducible nitrile, alkenyl, or alkynyl functional groups, were well tolerated under our established protocol and chemoselectively provided the corresponding α-functionalized ketones 5x–z in yields of 50–67%. We also investigated the transfer hydrogenation of cyclohex-2-en-1-one, which delivered a 71% yield of cyclohexanone (5za).

Table 3 Calculated Green Chemistry Metrics

Metric	Equation	Calculation	Value
E Factor	mass of total waste/mass of product [total waste (kg)/total product (kg)]	[(1.072 + 1.267 + 0.224 + 5.201) – (0.7 + 4.33)]/0.7	3.90 kg waste/kg of product
Atom economy	(molecular weight of product/sum of molecular weights of all starting materials) × 100	[270.3280/(268 + 32.04)] × 100%	90%
Atom efficiency	(% yield) × atom economy	70 × 90%	63%
Reaction mass efficiency	(mass of product/total mass of reactants) × 100	0.7/(1.072 + 0.12) × 100%	59%
Carbon efficiency	(amount of carbon in product/amount of total carbon present in reactants) × 100	(17/18) × 100%	94.4%

The synthetic applicability of our current Ru-catalyzed protocol for chemoselective hydrogenation of α,β-unsaturated ketones was established by a preparative-scale synthesis of 5a (70%) (Scheme [2]; top). Product 5a was then used to synthesize the pharmaceutically important triazole 7 [26] [27] in 77% yield by a click-type reaction (Scheme [2]; bottom). The α-functionalized ketones are therefore useful as key intermediate for the synthesis of azafluorene derivatives with complex molecular scaffolds.[28]

To gain insights into the mechanism, some control experiments were performed. The transfer hydrogenation of chalcone 4a in the presence of methanol-d ₄ gave the corresponding reduced product 8a-D2 with deuterium incorporation, confirming that methanol acted as the hydrogen source (Scheme [3]). The isolated product was also characterized by ²H NMR, which showed two distinct peaks at δ = 2.93 and 3.13 ppm. The reaction proceeded smoothly in the presence of the radical scavengers TEMPO and BHT, suggesting that a radical route is not involved. The addition of 2.1 equivalents of mercury to the reaction did not harm the yield of the desired product, confirming the homogeneous nature of the catalytic system. The yield of the transfer-hydrogenated product was significantly reduced by the addition of 18-crown-6, which suggests that the K⁺ counterion has an important role in the process. Based on reports in the literature,[29] a plausible catalytic cycle is presented in the Supporting Information (SI; S8).

Several green chemistry metrics were calculated for the hydrogenation of the unsaturated carbonyl compound 4a using methanol as a hydrogen source (Table [3]). Table [4] lists the masses and molecular weights of the reaction components used in the calculations. Green chemistry metrics for two other products, 5g and 5h, were also evaluated and are presented in the SI.

In conclusion, we have developed a highly selective protocol for the hydrogenation of chalcones using an SNS-Ru catalyst with methanol as a transfer-hydrogenating agent.[29] A deuteration study confirmed that methanol acts as a transfer-hydrogenating agent. This protocol demonstrates a broad substrate scope with excellent chemoselectivity. The utility of the method was illustrated by scaling up the process and by a postsynthetic modification to give a medicinally important triazole.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

We acknowledge the Department of Chemistry (COE-FAST: 5-5/ 2014-TS VII and FIST: SR/FST/CS-II/2017/23C) and CIF, IIT Guwahati, for instrumental facilities. A.M. and K.M. thank IITG for their fellowships. H.J.P. acknowledges UGC for a fellowship.

• References and Notes

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• 30 Transfer Hydrogenation of α,β-Unsaturated Carbonyl Compounds; General Procedure A 30 mL oven-dried pressure tube containing a magnetic stirrer bar was charged with the appropriate chalcone (0.5 mmol), KOH (0.5 mmol), and catalyst Ru-1 (2 mol %). MeOH (0.2 mL) and toluene (2 mL) were then added and the tube was sealed under argon and placed in a preheated oil bath at 120 °C for 12 h. On completion of the reaction, the tube was cooled to r.t. and the crude mixture was filtered through a small plug of Celite. The solvent was evaporated and the product was purified by column chromatography (silica gel, EtOAc–hexane). 1,3-Bis(4-methoxyphenyl)propan-1-one (5a) Purified by column chromatography [silica gel (100–200 mesh), EtOAc–hexane (15:85)] to give a white solid; yield: 108 mg (80%). ¹H NMR (400 MHz, CDCl₃): δ = 7.86 (d, J = 9.0 Hz, 2 H), 7.09 (d, J = 8.8 Hz, 2 H), 6.85 (d, J = 8.9 Hz, 2 H), 6.76 (d, J = 8.6 Hz, 2 H), 3.79 (s, 3 H), 3.71 (s, 3 H), 3.14 (t, J = 7.7 Hz, 2 H), 2.92 (t, J = 8.0 Hz, 2 H). ¹³C NMR (150 MHz, CDCl₃): δ = 198.2, 163.6, 158.2, 133.7, 130.5, 130.2, 129.6, 114.1, 113.9, 55.7, 55.5, 40.6, 29.7.

Corresponding Author

Publication History

Received: 27 June 2024

Accepted after revision: 12 August 2024

Accepted Manuscript online:
12 August 2024

Article published online:
09 September 2024

© 2024. Thieme. All rights reserved

Georg Thieme Verlag KG
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• References and Notes

• 1 Johnstone RA. W, Wilby AH, Entwistle ID. Chem. Rev. 1985; 85: 129
  CrossrefSearch in Google Scholar
• 2 Magano J, Dunetz JR. Org. Process Res. Dev. 2012; 16: 1156
  CrossrefSearch in Google Scholar
• 3 Wang D, Astruc D. Chem. Rev. 2015; 115: 6621
  CrossrefPubMedSearch in Google Scholar
• 4 Filonenko GA, van Puten R, Hensen EJ. M, Pidko EA. Chem. Soc. Rev. 2018; 47: 1459
  CrossrefPubMedSearch in Google Scholar
• 5 Prasanna R, Guha S, Sekar G. Org. Lett. 2019; 21: 2650
  CrossrefPubMedSearch in Google Scholar
• 6 Sau A, Mahapatra D, Dey S, Panja D, Saha S, Kundu S. Org. Chem. Front. 2023; 10: 2274
  CrossrefSearch in Google Scholar
• 7 Taschner MJ, Shahirpour A. J. Am. Chem. Soc. 1985; 107: 5570
  CrossrefSearch in Google Scholar
• 8 Hudlicky T, Sinai-Zingde G, Natchus MG. Tetrahedron Lett. 1987; 28: 5287
  CrossrefSearch in Google Scholar
• 9 Davies SG, Rodríguez-Solla H, Tamayo JA, Garner AC, Smith AD. Chem. Commun. 2004; 2502
  CrossrefPubMed
• 10 Gu Y, Norton JR, Salahi F, Lisnyak VG, Zhou Z, Snyder SA. J. Am. Chem. Soc. 2021; 143: 9657
  CrossrefPubMedSearch in Google Scholar
• 11 Kisets I, Zabelinskaya S, Gelman D. Organometallics 2022; 41: 76
  CrossrefSearch in Google Scholar
• 12 Li W, Wang Y, Chen P, Zeng M, Jiang M, Jin Z. Catal. Sci. Technol. 2016; 6: 7386
  CrossrefSearch in Google Scholar
• 13 Bhor MD, Panda MD, Jagtap MD, Bhanage BM. Catal. Lett. 2008; 124: 157
  CrossrefSearch in Google Scholar
• 14 Shabade AB, Sharma DM, Bajpai P, Gonnade RG, Vanka K, Punji B. Chem. Sci. 2022; 13: 13764
  CrossrefPubMedSearch in Google Scholar
• 15 Sivakumar G, Kumar R, Yadav V, Gupta V, Balaraman E. ACS Catal. 2023; 13: 15013
  CrossrefSearch in Google Scholar
• 16 Ganguli K, Mandal A, Kundu S. ACS Catal. 2022; 12: 12444
  CrossrefSearch in Google Scholar
• 17 Shinoda S, Itagaki H, Saito Y. J. Chem. Soc., Chem. Commun. 1985; 860
  Crossref
• 18 Aboo AH, Begum R, Zhao L, Faaroqi ZH, Xiao J. Chin. J. Catal. 2019; 40: 1795
  CrossrefSearch in Google Scholar
• 19 Garg N, Somasundharam HP, Dahiya P, Sundararaju B. Chem. Commun. 2022; 58: 9930
  CrossrefPubMedSearch in Google Scholar
• 20 Patil RD, Pratihar S. J. Org. Chem. 2024; 89: 1361
  CrossrefPubMedSearch in Google Scholar
• 21 Biswas N, Srimani D. J. Org. Chem. 2021; 86: 9733
  CrossrefPubMedSearch in Google Scholar
• 22 Biswas N, Srimani D. J. Org. Chem. 2021; 86: 10544
  CrossrefPubMedSearch in Google Scholar
• 23 Biswas N, Sharma R, Sardar B, Srimani D. Synlett 2023; 34: 622
  Thieme ConnectSearch in Google Scholar
• 24 Sardar B, Biswas N, Srimani D. Organometallics 2023; 42: 55
  CrossrefSearch in Google Scholar
• 25 Sardar B, Pal D, Sarmah R, Srimani D. Chem. Comm. 2023; 59: 9267
  CrossrefPubMedSearch in Google Scholar
• 26 Lauria A, Delisi R, Mingoia F, Terenzi A, Martorana A, Barone G, Almerico AM. Eur. J. Org. Chem. 2014; 3289
  Crossref
• 27 Moses EJ, Moorhouse AD. Chem. Soc. Rev. 2007; 36: 1249
  CrossrefPubMedSearch in Google Scholar
• 28 Mondal A, Pal D, Phukan HJ, Roy M, Kumar S, Purakayastha Purakayastha, Guha AK, Srimani D. ChemSusChem. 2024; 17: e202301138
  CrossrefPubMedSearch in Google Scholar
• 29 Ye X, Plessow PN, Brinks MK, Schelwies M, Schaub T, Rominger F, Paciello R, Limbach M, Hofmann P. J. Am. Chem. Soc. 2024; 136: 5929
  Search in Google Scholar
• 30 Transfer Hydrogenation of α,β-Unsaturated Carbonyl Compounds; General Procedure A 30 mL oven-dried pressure tube containing a magnetic stirrer bar was charged with the appropriate chalcone (0.5 mmol), KOH (0.5 mmol), and catalyst Ru-1 (2 mol %). MeOH (0.2 mL) and toluene (2 mL) were then added and the tube was sealed under argon and placed in a preheated oil bath at 120 °C for 12 h. On completion of the reaction, the tube was cooled to r.t. and the crude mixture was filtered through a small plug of Celite. The solvent was evaporated and the product was purified by column chromatography (silica gel, EtOAc–hexane). 1,3-Bis(4-methoxyphenyl)propan-1-one (5a) Purified by column chromatography [silica gel (100–200 mesh), EtOAc–hexane (15:85)] to give a white solid; yield: 108 mg (80%). ¹H NMR (400 MHz, CDCl₃): δ = 7.86 (d, J = 9.0 Hz, 2 H), 7.09 (d, J = 8.8 Hz, 2 H), 6.85 (d, J = 8.9 Hz, 2 H), 6.76 (d, J = 8.6 Hz, 2 H), 3.79 (s, 3 H), 3.71 (s, 3 H), 3.14 (t, J = 7.7 Hz, 2 H), 2.92 (t, J = 8.0 Hz, 2 H). ¹³C NMR (150 MHz, CDCl₃): δ = 198.2, 163.6, 158.2, 133.7, 130.5, 130.2, 129.6, 114.1, 113.9, 55.7, 55.5, 40.6, 29.7.

• Supplementary Material