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Letter Cite This: Org. Lett. 2018, 20, 7856−7859

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Atom- and Step-Economical Ruthenium-Catalyzed Synthesis of Esters from Aldehydes or Ketones and Carboxylic Acids Sofiya A. Runikhina,† Dmitry L. Usanov,‡ Alexander O. Chizhov,§ and Denis Chusov*,† †

Org. Lett. 2018.20:7856-7859. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 12/21/18. For personal use only.

Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences, Vavilova St. 28, Moscow 119991, Russian Federation ‡ Broad Institute of MIT and Harvard, 415 Main Street, Cambridge, Massachusetts 02142, United States § N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect, 47, Moscow 119991, Russian Federation S Supporting Information *

ABSTRACT: We developed a ruthenium-catalyzed reductive ester synthesis from aldehydes or ketones and carboxylic acids using carbon monoxide as a deoxygenative agent. Multiple factors influencing the outcome of the reaction were investigated. Best results were obtained for commercially available and inexpensive benzene ruthenium chloride; as low as 0.5 mol % of the catalyst is sufficient for efficient reaction. Competitive studies demonstrated that the presence of even 1000 equiv of alcohol in the reaction mixture does not lead to the corresponding ester, which clearly indicates that the process is not a simple reductive esterification but a novel type of Ru-catalyzed redox process.

E

sters represent an irreplaceable class of chemicals, which are widely used in natural products, pharmaceuticals, agrochemicals, fragrances, and food additives.1 Traditional synthesis of esters involves the reaction of alcohols with carboxylic acids or, more frequently, with activated carboxylic acid derivatives. The latter approach often suffers from stoichiometric and superstoichiometric amounts of waste, which rapidly become highly problematic with the increase of the reaction scale. In light of the wide accessibility and synthetic importance of aldehydes, in many cases the alternative synthetic approach to esters from carboxylic acids and aldehydes can be more convenient as well as step- and atom-economical.2 Furthermore, from a sustainability point of view, it is highly desirable to develop new methodologies that are capable of utilizing industrial byproducts as components of chemical synthesis. In this regard, we have been interested in the synthetic use of the deoxygenative potential of carbon monoxide,3 a multimillion-ton byproduct of several major industrial processes (e.g., steel making).4 Herein, we report expansion of this paradigm to a new type of chemistry, namely, atom- and step-economical synthesis of esters from aldehydes or ketones and carboxylic acids. We studied several potential catalysts for the model reaction of reductive ester synthesis from naphthaldehyde with acetic acid in the presence of carbon monoxide (Figure 1 and Table 1). Ruthenium(III) complex 1 with six strong ligands led only to trace amounts of the product (Table 1, entry 1), and the same was observed for ruthenium(II) cyclohexadienyl complex 2 (Table 1, entry 2). However, cyclopentadienyl ruthenium(II) complex 3 with a labile arene ligand did lead to the desired ester with 7% yield (Table 1, entry 3). Surprisingly, one of the © 2018 American Chemical Society

Figure 1. Ruthenium precatalysts.

best catalysts in hydrogen-transfer processes (Shvo catalyst,5 4) showed similar activity (Table 1, entry 4). Complexes 1−4 were outperformed by simple ruthenium chloride, which furnished the product with almost 3-fold higher yield (Table 1, entry 5). The best activity was observed for ruthenium(II) complexes with chlorides and labile ligands such as cyclooctadiene, benzene, and p-cymene (Table 1, entries 6−8). The optimum temperature was found to be in the range of 140−160 °C (see the Supporting Information); however, the reaction can be run under much milder conditions with Received: October 22, 2018 Published: December 7, 2018 7856

DOI: 10.1021/acs.orglett.8b03375 Org. Lett. 2018, 20, 7856−7859

Letter

Organic Letters Table 1. Catalyst Effect on the Reductive Ester Synthesis of Naphthaldehyde with Acetic Acid Using Carbon Monoxide as a Reducing Agent

entrya

catalyst

temperature (°C)

pressure (bar)

yieldb (%)

1 2 3 4 5 6 7 8 9c 10c 11c 12c 13c 14c 15c 16c

1 2 3 4 RuCl3 5 6 7 6 6 6 6 6 6 6 6

150 150 150 150 150 150 150 150 160 150 140 130 120 150 150 150

30 30 30 30 30 30 30 30 30 30 30 30 30 10 5 2

trace trace 7 10 27 41 44 47 64 72 67 39 21 70 70 58

a

0.3 mmol scale; 10 equiv of acetic acid and 5 equiv of water were used. bYields were determined by NMR. cTHF was used as a solvent.

Scheme 1. Reductive Ester Synthesis versus Classical Esterification

Table 2. Influence of Water on the Reaction Efficiency Figure 2. Substrate scope. The reactions were conducted under 30 atm CO at 150 °C for 22 h. Yields were determined by NMR. Isolated yields are shown in parentheses. a180 °C. b1 mol % of 6 was used. entrya

R

pressure (bar)

water (equiv)

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1-naphthyl 1-naphthyl o-chloro o-chloro o-chloro o-chloro o-chloro o-chloro o-chloro p-chloro p-chloro 2,5-dimethyl 2,5-dimethyl o-fluoro o-fluoro

2.6 2.6 2.6 2.6 2.6 2.6 2.6 30 30 30 30 30 30 30 30

7.13 0.13 10.13 7.13 5.13 3.13 2.13 7.13 0.13 7.13 0.13 7.13 0.13 7.13 0.13

58 57 76 76 62 43 38 71 73 74 85 75 16 67 27

Scheme 2. Experiment with an Isotopically Labeled Substrate

increased catalyst loading or reaction time. Surprisingly, the influence of the pressure was not critical: the reaction can proceed even in Schlenk tubes under 2 bar pressure (see the Supporting Information). Interestingly, the reaction in alcoholic media did not lead to the esterification of acetic acid with the solvent despite the 1000-fold excess thereof (Scheme 1), which clearly implies that the process involves more advanced ruthenium catalysis than simple water gas shift reduction6 of the aldehyde with subsequent esterification. This was corroborated by the fact

a

0.3 mmol scale; 10 equiv of acetic acid was used. bYields were determined by NMR.

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DOI: 10.1021/acs.orglett.8b03375 Org. Lett. 2018, 20, 7856−7859

Letter

Organic Letters

In summary, we have found a new type of reaction, namely, reductive ester synthesis from carboxylic acids and aldehydes or ketones. The methodology takes advantage of the unique deoxygenative potential of carbon monoxide, which renders the methodology more atom-economical and less wasteful in comparison to the existing synthetic alternatives. A crossesterification product was not observed even in the presence of 1000 equiv of competing alcohol, which indicates that the process involves more advanced ruthenium catalysis than simple water gas shift reduction6 of the aldehyde with subsequent esterification. The catalytic mechanism apparently depends not only on the catalyst and reaction conditions but also on the substrate nature.

Scheme 3. Putative Mechanistic Scheme for the Developed Catalytic Process

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03375. Experimental section and spectra of obtained compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected].

that the presence of water did not have an effect on the model reactions of acetic acid with 1-naphthaldehyde or ochlorobenzaldehyde respectively (see Table 2 and the Supporting Information). It is noteworthy, however, that at lower pressure the reaction with o-chlorobenzaldehyde was accelerated in the presence of water; this phenomenon was not observed for 1-naphthaldehyde. For ortho-fluoro and 2,5dimethyl benzaldehydes, water increases the reaction rate even at 30 bar pressure. We believe that the reaction mechanism might therefore be substrate-dependent. With the optimized conditions in hand, we proceeded to identification of the substrate scope (Figure 2). Good yields were observed for all the tested substrates with various substitution patterns (1a−1p). Product 1k, a derivative of levulinic acid, is particularly notable in light of the intense ongoing research on the production of platform chemicals from biomass via levulinic acid.7 We successfully used this substrate not only in a cross-reaction but also in an intramolecular transformation leading to γ-valerolactone 1o, which is a highly valuable chemical intermediate8 and a “green fuel” candidate with a demand for environmentally friendly, step-economical, and cost-effective preparation methods. In an interesting dichotomy for the ketone moiety, it can stay untouched or it can react with an acid to give the product by simply changing the reaction temperature (1o versus 1k). We synthesized 18O-p-chlorobenzaldehyde and tried it in the reaction with acetic acid (Scheme 2). No 18O isotope was found in the reaction product (see the Supporting Information). On the basis of this observation together with the fact that 1000-fold excess of the butanol did not compete with the main reaction (Scheme 1), we propose the putative catalytic cycle shown in Scheme 3. The catalytic species might insert into an activated C−OH bond of the hemiacetal or hemiketal intermediate to provide hydroxo-complex A. Intramolecular hydroxylation of the Ru-bound CO then leads to intermediate B. Its decarboxylation gives Ru-hydride species C, which, upon reductive elimination, leads to the ester product and regenerated catalyst.

ORCID

Denis Chusov: 0000-0001-6770-5484 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the Russian Science Foundation (Grant No. 16-13-10393). We thank Dr. Dmitry Pripadchev for his help with the isotope experiment. The contribution of the Center for Molecule Composition Studies of INEOS RAS is gratefully acknowledged.

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DOI: 10.1021/acs.orglett.8b03375 Org. Lett. 2018, 20, 7856−7859