Regioselective and Chemoselective Reduction of Naphthols Using

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Regioselective and Chemoselective Reduction of Naphthols Using Hydrosilane in Methanol: Synthesis of the 5,6,7,8Tetrahydronaphthol Core Yuan He, Jinghua Tang, Meiming Luo,* and Xiaoming Zeng* Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, China

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S Supporting Information *

ABSTRACT: A regioselective and chemoselective method for catalytic synthesis of biologically interesting 5,6,7,8tetrahydronaphthols by reduction of naphthols was described. The side aromatic hydrocarbons in naphthols were siteselectively reduced, using hydrosilanes in methanol, allowing for retaining functional phenol scaffolds intact. It presents a rare example of using low-cost and air-stable hydrosilane for catalytic reduction of unactivated aromatic hydrocarbons under mild conditions. This reaction is scalable and proceeds in high selectivity without the formation of 1,2,3,4tetrahydronaphthol byproducts with toleration of sensitive functionalities such as bromide, chloride, fluoride, ketone, ester, and amide.

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traditionally involve the hydrogenation of naphthols (see Scheme 1a). However, naphthol hydrogenation usually leads to mixed compounds by partially or fully reducing two aromatic rings.8 Huang and co-workers recently demonstrated that selective hydrogenation of naphthols to 1,2,3,4-tetrahydronaphthols can be achieved using a ruthenium pincer catalyst.9 In contrast, a general, regioselective protocol to form the 5,6,7,8-tetrahydronaphthol core by naphthol reduction has not yet been realized. Recently, reduction using stable organic reductants has attracted broad interest, because of the less-hazardous properties of the reducing reagents, and has been widely used as an operationally simple method in enantioselective reduction of unsaturated olefins, imines, and ketones.10−12 To our knowledge, reduction of aromatics using stable organic reductants has rarely been studied. Buchwald,13 Nagashima,14 and Beller15 described that inexpensive and air-stable hydrosilanes can be used as mild reductants in catalytic reduction of oxygen-containing compounds such as amides, phosphine oxides, and CO2 (see Scheme 1b).16 Herein, we report a regioselective and chemoselective reduction of side aromatics in naphthols that was enabled by using low-cost polymethylhydrogen siloxane (PMHS) as reductant in methanol with rhodium catalysis, providing access to 5,6,7,8-tetrahydronaphthol motifs in an operationally simple manner (see Scheme 1c). We commenced our studies by probing the effect of hydrosilanes on the regioselective reduction of 1-naphthol

elective reduction of unsaturated compounds is a fundamental transformation in organic chemistry, which provides a powerful and useful tool to the synthesis of appealing molecules such as drugs, materials, and common chemicals.1,2 Among numerous unsaturated motifs, aromatic hydrocarbons are difficult substrates for reduction, probably because of the stability caused by aromaticity.3,4 In particular, selectivity and functional group tolerance have long been prominent issues when several aromatics are coexisting in the same substrates. Naphthols contain two different aromatic hydrocarbon rings. Naphthol reduction by partially reducing one of the aromatics is able to access tetrahydronaphthol motifs that are valuable building blocks found in natural products and biologically active molecules.5 For example, compounds A and B contain 5,6,7,8-tetrahydronaphthol subunits, which have shown attractive pharmaceutical utility for the treatment of osteoporosis and malaria (see Figure 1).6,7 Methods that are used to form 5,6,7,8-tetrahydronaphthols

Figure 1. Illustrative examples of biologically active drugs and candidates bearing the 5,6,7,8-tetrahydronaphthol core. © XXXX American Chemical Society

Received: April 21, 2018

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DOI: 10.1021/acs.orglett.8b01273 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

reduction occurred in high regioselectivity without the formation of 1,2,3,4-tetrahydronaphthol regioisomer. Studying the effect of hydrosilanes on the reaction suggested that PMHS largely improved the transformation of naphthol (>99% conversion), giving 2 in 70% yield, combined with a small amount of over-reduced compound of decahydronaphthalen-1ol (9% yield) (Table 1, entry 5). Interestingly, the reaction media largely influenced the conversion of 1-naphthol. The use of alcohol as a medium allowed for the reaction proceeding effectively, whereas it was completely inhibited in a nonprotonic solvent such as THF and dichloromethane (Table 1, entries 6−10). Meanwhile, putting phosphine ligands into the reaction led to low conversion. It was noteworthy that basically Rh/C catalyst did not afford the reduced products under present conditions (Table 1, entry 11).17 Meanwhile, no reduced product was formed when performing the reaction using PMHS in methanol without rhodium complex (Table 1, entry 12). With the optimal reaction conditions in hand, we next examined the substrate scope for the synthesis of diverse substituted 5,6,7,8-tetrahydronaphthol compounds. As shown in Scheme 2, the incorporation of unsaturated groups of carboxylic acid and ester at the ortho position of 1-naphthols had no impact on the regioselective reduction of aromatic hydrocarbons, allowing for these functionalities to be kept intact to give 5,6,7,8-tetrahydronaphthol compounds 5−7 in 72%−92% yields. It was found that phenyl group on the aryoxycarbonyl scaffold of naphthol can be tolerated by the reaction system (8). Notably, sensitive functionalities of bromide, chloride, fluoride, trifluoromethyl, and ketone groups that are usually not compatible with catalytic hydrogenation or reduction using metal hydrides, were well-retained in the resulting motifs (11, 12, and 16−19). The installation of unsaturated phenyl substituents at the ortho position of 2naphthols did not affect the selectivity and efficiency of the reduction, giving the desired products 13−20 in good to excellent yields. Furthermore, naphthalene-2,3-diol was amenable to the regioselective reduction in the formation of 5,6,7,8-tetrahydronaphthalene-2,3-diol (22) in 77% yield. Interestingly, amide scaffolds that were reduced to primary amine group when using tetramethyldisiloxane and triethoxysilane with platinum and zinc catalysis were tolerated by the reaction system, leading to amide-substituted 5,6,7,8-tetrahydronaphthol derivatives (26−29). By the treatment of 1,1′binaphthalene-2,2′-diol in the reaction, we found that two side aromatics were simultaneously reduced to form 5,5′,6,6′,7,7′,8,8′-octahydro-[1,1′-binaphthalene]-2,2′-diol (31) in 95% yield. In addition, substituted side-aromatic hydrocarbon in 2-naphthol can also be reduced, albeit giving relatively low conversion (32). Inspired by these results, the feasibility of using PMHS for catalytic reduction of heteroarenes was examined next. Nheterocycle rings in quinolinols can be regioselectively reduced with this protocol at elevated temperature, resulting in the formation of 1,2,3,4-tetrahydroquinolinols 33 and 34 in good yields (see Figure 2).18 Besides naphthol substrates, naphthalenyl 4-methylbenzenesulfonate was also suitable for the conversion by reducing the corresponding side aromatic (35). Moreover, the application of this method in the preparation of 5,6,7,8-tetrahydronaphthalenamine was successful (36). It is noted that performing the reduction on a 1-g scale did not affect the efficiency of the selectivity (see Scheme 3). The latestage functionalization of 5,6,7,8-tetrahydronaphthalen-2-ol by

Scheme 1. Naphthol Reduction and Examples of Using Hydrosilanes in Catalytic Reduction

using a rhodium complex. In the presence of 1.5 mol % of [RhCl(cod)]2, we were pleased to find that PhSiH3 could be used to reduce the side aromatic of 1-naphthol in methanol, and the related compound of 5,6,7,8-tetrahydronaphthol (2) was detected by gas chromatography/mass spectroscopy (GC/ MS) analysis. After removal of the volatiles under vacuum, the crude product was purified by column chromatography on silica gel to give 2 in 35% yield (Table 1, entry 1). Notably, the Table 1. Regioselective Reduction of 1-Naphthol Using Hydrosilanes with Rhodium Catalysisa

entry

hydrosilane

solvent

Rh complex

yield of 2 (%)

1 2 3 4 5 6 7 8 9 10 11 12

PhSiH3 Ph2SiH2 (MeO)3SiH (EtO)3SiH PMHS PMHS PMHS PMHS PMHS PMHS PMHS PMHS

MeOH MeOH MeOH MeOH MeOH EtOH n-PrOH n-BuOH THF DCM MeOH MeOH

[Rh(cod)Cl]2 [Rh(cod)Cl]2 [Rh(cod)Cl]2 [Rh(cod)Cl]2 [Rh(cod)Cl]2 [Rh(cod)Cl]2 [Rh(cod)Cl]2 [Rh(cod)Cl]2 [Rh(cod)Cl]2 [Rh(cod)Cl]2 Rh/C

35 18 44 23 70 42 63 50 ndb ndb ndb ndb

a

Reaction conditions: 1-naphthol (0.3 mmol), hydrosilane (0.9 mmol), Rh complex (0.0045 mmol), solvent (2 mL), rt, 12 h. Isolated yields are given. bNot detected. B

DOI: 10.1021/acs.orglett.8b01273 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 2. Regioselective and Chemoselective Reduction of Naphthols Using PMHS in Methanol*

Scheme 3. Gram-Scale Reduction and Transformations of 5,6,7,8-tetrahydronaphthalen-2-ol (9)

hour (see the Supporting Information for details). After that, the reduction proceeded swiftly to afford the compound 5,6,7,8-tetrahydronaphthol in 75% yield within the next 3 h. We found that alkenyl group can be reduced under present conditions; the formation of heterogeneous rhodium can be considered by the reaction of [RhCl(cod)]2 with PMHS in methanol, which may be responsible for naphthol reduction. A mercury poisoning experiment suggested that the reaction was completely inhibited by the addition of a large amount of Hg(0) (eq 1 in Scheme 3). By performing the reduction of naphthol in CD3OD, it was shown that deuterium was introduced into the product and mainly distributed at the C5, C6, C7, and C8 positions of 5,6,7,8-tetrahydronaphthol (eq 2 in Scheme 3). Together with the result that no reaction occurred in a nonprotonic solvent, alcohol may play an important role in assisting the rhodium catalyst to achieve the arene reduction with hydrosilane. In summary, we have developed a practical reduction protocol to naphthols and related derivatives by using low-cost polymethylhydrogen siloxane with Rh catalysis in methanol. The reaction occurred in high regioselectivity and chemoselectivity by reducing the side aromatic hydrocarbons, allowed for scalable formation of biologically interesting 5,6,7,8tetrahydronaphthol core structure. Because of mild reaction conditions, a wide range of sensitive functional groups can be tolerated by the reduction system. It presents a rare example of

*

Reaction conditions: naphthol (0.3 mmol), PMHS (0.9 mmol), [RhCl(cod)]2 (1−5 mol %), MeOH (2 mL), room temperature (rt), 12 h. aThe reaction was performed at 40 °C. bThe reaction was conducted at 100 °C. cPHMS (1.8 mmol) was used. d24 h.

Figure 2. Regioselective reduction of quinolinols, naphthalenyl 4methylbenzenesulfonate, and 1-naphthylamine using PMHS in methanol with Rh catalysis.

oxidative reactions with DDQ, PhI(OAc)2, and Selectfluor provided access to 6-hydroxy-3,4-dihydronaphthalen-1(2H)one (37), methoxy- and fluoro-substituted tetrahydronaphthalen-2-one derivatives 38 and 39, respectively. The reaction profile was studied by choosing the reduction of 2-naphthol as a model. It was shown that only a trace amount of 5,6,7,8-tetrahydronaphthol was formed in the initial C

DOI: 10.1021/acs.orglett.8b01273 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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using hydrosilane as reductant for catalytic reduction of unactivated aromatic hydrocarbons in alcohol media. Further studies on the mechanism and enantioselective reducing aromatics using hydrosilanes are undergoing.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01273.

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Experimental procedures, characterization data for all products related, and spectrum (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M. Luo). *E-mail: [email protected] (X. Zeng). ORCID

Meiming Luo: 0000-0003-1766-9010 Xiaoming Zeng: 0000-0003-0509-1848 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Nos. 21202128 and 21572175 to X.Z.), SCU and BNLMS for financial support of this research.

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REFERENCES

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Organic Letters (18) For selected examples, see: (a) Ren, D.; He, L.; Yu, L.; Ding, R.-S.; Liu, Y.-M.; Cao, Y.; He, H.-Y.; Fan, K.-N. J. Am. Chem. Soc. 2012, 134, 17592−17598. (b) Wei, Z.; Chen, Y.; Wang, J.; Su, D.; Tang, M.; Mao, S.; Wang, Y. ACS Catal. 2016, 6, 5816−5822. (c) Wu, J.; Barnard, J. H.; Zhang, Y.; Talwar, D.; Robertson, C. M.; Xiao, J. Chem. Commun. 2013, 49, 7052−7054.

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DOI: 10.1021/acs.orglett.8b01273 Org. Lett. XXXX, XXX, XXX−XXX