Letter Cite This: ACS Catal. 2018, 8, 3030−3034
pubs.acs.org/acscatalysis
CO2‑Catalyzed Oxidation of Benzylic and Allylic Alcohols with DMSO Daniel Riemer, Bhavdip Mandaviya, Waldemar Schilling, Anne Charlotte Götz, Torben Kühl, Markus Finger, and Shoubhik Das* Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany S Supporting Information *
ABSTRACT: CO2-catalyzed transition-metal-free oxidation of alcohols has been achieved. Earlier, several methodologies have been explored for alcohol oxidations based on transition-metal catalysts. However, owing to the cheaper price, easy separation and nontoxicity, transition-metal-free systems are in high demand to the pharmaceutical industries. For this reason, various primary and secondary alcohols have been selectively oxidized to the corresponding carbonyl compounds using CO2 as a catalyst in the presence of different functional groups such as nitrile, nitro, aldehyde, ester, halogen, ether, and so on. At the end, transition-metal-free syntheses of pharmaceuticals have also been achieved. Finally, the role of CO2 has been investigated in detail, and the mechanism is proposed on the basis of experiments and DFT calculations. KEYWORDS: CO2, alcohols, carbonyl compounds, oxidations, pharmaceuticals
O
Scheme 1. Transition-Metal-Free Oxidation Reactions
xidation of primary and secondary alcohols to the corresponding carbonyl compounds is of fundamental importance in organic synthesis.1 Several novel strategies have been achieved using transition-metal-based catalysts.2 However, transition-metal-catalyzed oxidations hold several limitations such as the need for mild reaction conditions, low catalyst loadings, expensive metal catalysts and ligands, or avoidance of costly and toxic additives. In addition, removal of trace amounts of transition-metal residues from desired products is quite costly and challenging especially in the pharmaceutical industry.3 Compared with transition-metal-catalyzed oxidation systems, transition-metal-free systems are highly appealing owing to the fact that they are cheap, nontoxic, and easy to separate from the reaction mixtures.4 A recent publication by Pfizer’s medicinal chemists showed that the three most popular oxidation methods for the oxidation of alcohols to the corresponding carbonyls used at Pfizer are the Dess−Martin periodinane or its precursor IBX as the oxidant, the Swern oxidation and the TPAP/NMO system.5 Similar to Swern oxidations, few other oxidation methods are well-known in organic syntheses, such as the Pfitzner−Moffatt oxidation, Parikh−Doering oxidation, Albright−Goldman oxidation, and so on (Scheme 1).6,7 Although they are highly effective in the pharmaceutical industry, they have poor atom efficiencies such as the use of a huge excess of promoters and bases, involvement of toxic reagents (e.g., oxalyl chloride), and significant scale-up issues. Use of clean reagents and minimizing the quantity and toxicity of released waste is the main target of current green and sustainable chemistry.8 For this purpose, the cheap, abundant, and nontoxic nature of CO2 has brought immense interest for its use as a promoter in oxidation reactions such as oxidative coupling of CH4, oxidative dehydrogenation of alkanes, alkyl aromatics, C−C bond formation between aldehydes, and so © XXXX American Chemical Society
on.9 However, kinetic inertness and high thermodynamic stability of the CO2 molecule hinder wide applications in organic syntheses.10 Nonetheless, several useful chemicals such as urea, formic acid, methanol, cyclic carbonates, lactone, Received: December 20, 2017 Revised: February 22, 2018 Published: February 22, 2018 3030
DOI: 10.1021/acscatal.7b04390 ACS Catal. 2018, 8, 3030−3034
Letter
ACS Catalysis carboxylic acids, methylated compounds, carbamates, salicylic acid, among others, have been synthesized and some of them even in industrial scales as well.11 On the basis of our previous experience with CO2-promoted C−C bond formation reactions, we rationalized that CO2 could be a promoter for the oxidation of alcohols to the corresponding carbonyls as well.9c Indeed, we found that CO 2 promoted the oxidation of cinnamyl alcohol to cinnamaldehyde in the presence of a base. A variety of inorganic and organic bases was applied to optimize the reaction conditions for the synthesis of cinnamaldehyde (Table S1, Supporting Information), and to our delight, use of K3PO4 gave an optimized yield of 90%. Efforts to lower the temperature did not attain better activity. Other bases such as KOtBu, KOH, DBU, DMAP, K2CO3, and NaHCO3 showed less activity under the same reaction conditions. Additionally, a decrease of the base loading from 20 mol % to 10 mol % gave significantly lower yield. Efforts to scale up the reaction to 5 mmol scale under optimized reaction conditions gave 89% of isolated yield. The reaction also worked in the presence of 20 mol % of CO2; however, a slight decrease of the yield was observed (see Supporting Information, Table S2). Not surprisingly, no formation of product was observed in the absence of base or CO2 atmosphere or even in the presence of O2. With these optimized conditions in hand, a number of primary and secondary alcohols was converted to the corresponding aldehydes and ketones (Scheme 2). Different aromatic, heteroaromatic, aliphatic, and alicyclic alcohols have shown good to excellent activity under the optimized reaction conditions. In addition, different cyclic secondary alcohols have been converted to the corresponding valuable ketones (22a− 26a). ortho-Substitution in the aromatic ring did not hamper the product formation and different symmetrically and nonsymmetrically substituted secondary alcohols reacted excellently under the optimized reaction conditions as well (16a−21a). It should be noted that in none of the cases did we observe any overoxidized carboxylic acid products. Our reaction conditions also tolerated halogens; ether and benzyloxy groups attached to the alcohol substrates (3a, 5a, 9a, 13a). Gratifyingly, veratraldehyde (9b), which is a key intermediate for many pharmaceutical drug syntheses such as verazide, amiquinsin, toborinone, hoqquizil, and so on, was obtained in 90% yield in 5 mmol scale. Furthermore, we grafted oxidizable functional groups such as thiomethyl and aldehyde onto the aromatic ring of the corresponding alcohol substrates, and to our delight, these two functional groups were well tolerated under the optimized reaction conditions (33a and 38a).12 In none of the cases have we observed any oxidized or carboxylated byproducts. With our oxidation protocol in hand, we were interested in obtaining transition-metal-free syntheses of pharmaceutical drugs. For example, combretastatin A4 is a potent microtubule-targeting and vascular damaging agent, which has been used for cancer chemotherapy.13 In addition, DMU-212 is also a pharmaceutical drug for breast cancer treatment.14 To our delight, successful syntheses of these two drugs were achieved in 5 mmol scale starting from 3,4,5-trimethoxy benzyl alcohol (Scheme 2; 6a). Initially, 3,4,5-trimethoxy benzyl alcohol was oxidized to the corresponding aldehyde, and finally drug compounds were synthesized in an excellent yield via Wittig reaction (Scheme 3).15
Scheme 2. General Substrates Scope for CO2-Promoted Oxidations of Primary and Secondary Alcoholsa
a Reaction conditions: Substrates (0.25 mmol), K3PO4 (20 mol %), DMSO (2.5 mL), CO2 (balloon), 90 °C, 48 h. All are isolated yields. 1 equiv of K3PO4 was used in case of entries 3a, 28a, 30a, and 37a.
Finally, we became interested in transition-metal-free homologation of α,β-unsaturated aldehydes as the corresponding homologated products have wide structural similarities with different natural products such as navenone B, 11-cis-retinal, and others.16 This homologation reaction started from cinnamaldehyde via the addition of vinyl magnesium bromide followed by hot water rearrangement and finally CO2-promoted oxidation, which led to the corresponding homologated α,βunsaturated aldehyde in 81% yield (Scheme 4).17 We believe that this can be a future strategy for the synthesis of polyenes starting from commercially available starting materials and reagents in a transition-metal-free pathway. 3031
DOI: 10.1021/acscatal.7b04390 ACS Catal. 2018, 8, 3030−3034
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ACS Catalysis Scheme 3. Transition-Metal-Free Syntheses of Combretastatin A4 and DMU-212
Scheme 5. Experimental Evidences for the Crucial Role of CO2 in the Oxidation Step and Plausible Mechanism
Scheme 4. Transition-Metal-Free Homologation of α,βUnsaturated Aldehydes
Supporting Information). That supported the role of DMSO as actual oxidant, and indeed DMS was found in the reaction mixture but quantification was impossible due to its low boiling point (31 °C).19 However, replacement of DMSO by butyl sulfoxide generated dibutyl sulfide in equimolar ratio to the oxidized product of cinnamyl alcohol (Scheme 5; eq 5). Finally, replacement of DMSO by DMS18O, incorporated 99% 18O in the final cinnamaldehyde (Scheme 5; eq 6). To further evaluate the nature of the rate-determining step, the reaction process was monitored under the optimized reaction conditions over the whole reaction time (Figure 1A). The reaction rate showed a first-order rate dependence with
After the substrate-scope exploration and application of this CO2-promoted oxidation reactions, we finally became interested to examine the actual role of CO2 in this reaction. In our previous work, on the basis of control experiments, we proposed a reaction sequence which consisted of catalyzed benzoin condensation and an oxidation step whose key features were the formation of an intermediate carbonate ester followed by nucleophilic substitution by DMSO and finally deprotonation and proton shift which led to the product formation.9c Regarding the different nature of the substrates, we reinvestigated this mechanism. The pivotal role of CO2 was demonstrated under N2 atmosphere, where no oxidation was observed (Scheme 5; eq 1). Unfortunately, we were not able to observe the formation of an intermediate O-carboxylated product from cinnamyl alcohol under 13CO2 atmosphere due to its high instability. However, addition of ethyl iodide trapped the corresponding 13C-labeled cinnamyl ethyl carbonate product (Scheme 5; eq 4).18 To address the question of the oxidant, the reaction was conducted under O2 atmosphere in the absence of CO2 but resulted in a low yield of 5% (Scheme 5; eq 2). Furthermore, only CO2 was observed after the reaction but no other reduced products of CO2 such as CO, HCOOH or HCOO− were found using neither in situ gas GC nor NMR measurements (see
Figure 1. Reaction monitoring under optimized reaction conditions (A), plots for determination of reaction order in respect to cinnamyl alcohol (B) and K3PO4 (C), and Arrhenius−Eyring plot (D). 3032
DOI: 10.1021/acscatal.7b04390 ACS Catal. 2018, 8, 3030−3034
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ACS Catalysis
The combined experimental and computational evidence point to a mechanism where CO2 played the pivotal role to transform the hydroxyl to a good leaving group. Based on our inability to observe the intermediate carbonate ester without a trapping agent, the first step should not be rate-determining but a pre-equilibrium. Its unfavorable thermodynamics explain why we were not able to observe the ester or the negative activation entropy. Nucleophilic attack by DMSO was demonstrated by 18 O-labeling experiments showing that the oxygen of the alcohol is quantitatively replaced by the one of DMSO. The following actual oxidation step by deprotonation of the dimethylsulfide group and proton shift is well established for DMSO-based oxidation reactions. However, activation of the alcohol occurs very mildly by CO2 in contrast to these known procedures. In conclusion, we have demonstrated the first CO2-catalyzed oxidation of alcohols to the corresponding carbonyl compounds. Notably, this methodology has been achieved without using transition-metal catalysts and tolerated various functional groups such as halide, ether, nitro, nitrile, aldehyde, ester, and so on. We have also shown transition-metal-free application of our catalytic system for the synthesis of valuable pharmaceuticals. Finally, detailed mechanistic studies clearly demonstrated the role of CO2 in the reaction. We believe this methodology could find interest in the synthesis of pharmaceuticals and in the synthesis of natural products.
respect to the starting material (Figure 1B) but a zeroth-order rate dependence on K3PO4, which suggested that the base was not involved in the rate-determining step (Figure 1C). Monitoring the reaction process at different temperatures between 30 and 110 °C, the activation enthalpy was determined at ΔH‡ = 21.1 kcal mol−1 (88.4 kJ mol−1) and the activation entropy at ΔS‡ = −30.9 cal mol−1 K−1 (− 129.7 J mol−1 K−1) (Figure 1D). The negative entropy value suggested an associative process involved in the rate-limiting step. The mechanism of the oxidation reaction was further investigated by means of DFT calculations (Scheme 6). Scheme 6. Calculated Pathway of CO2 Promoted Oxidation of Cinnamyl Alcohola
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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/acscatal.7b04390. Experimental details, characterization data, and spectra for the compounds of the synthesized compounds (PDF)
a
D3(BJ)-B3LYP/def2-TZVPP, COSMO (DMSO) Corrected SinglePoint Energies Relative to Starting Compound A in kcal mol−1.
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Previously, we proposed that the first step of the mechanism was the formation of a carbonate ester (B) by a pre-equilibrium which is computed 11.3 kcal mol−1 uphill. This step is required to transform the hydroxyl group into a better leaving group. However, the transition state TSB/D of direct substitution by a DMSO molecule is connected with a rather high barrier of 42.7 kcal mol−1 relative to the starting compound reflecting the low nucleophilicity of DMSO. Instead, dissociation of the HCO3− anion to form an allyl cation C is computed at 31.5 kcal mol−1 and thus much lower than TSB/D. No transition states of the dissociation of HCO3− and attack of DMSO at C could be located due to the flat energy profiles of these steps. Formation of intermediate C readily explains the importance of πconjugation to stabilize the intermediate cation but also raises the question of selectivity. However, both the allylic isomer C′ and the internal isomer D′ (Scheme 6) are at least 5.3 kcal mol−1 less stable than C or D, respectively, which would correspond to a 1:1000 ratio well below the experimental detection limit. Once D is formed, deprotonation to E and internal proton shift (TSE/F) result in irreversible product formation. The barrier of 5.2 kcal mol−1 in respect to E for this step suggests rapid reaction once DMSO has attacked the allylic cation. However, the nature of the rate-determining step stays ambiguous as C and TE/F have both comparable energies in relation to the starting compound (see also Figure S8). Tentatively, we assign the formation of the allyl cation C that would be surrounded by DMSO in solution until deprotonation triggers product formation.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shoubhik Das: 0000-0001-9102-3707 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Fonds der Chemischen Industrie (FCI, LiebigFellowship to S.D.) for the financial support. We are highly thankful to Prof. Dr. Lutz Ackermann for his kind support behind our work. Special support from Prof. Inke Siewert for GC measurements is also acknowledged.
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