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# Cobalt-Catalyzed Wacker-type Aerobic Oxidation of Olefins into Ketones Enabled by PMHS: Giving Value to a Chemical Waste with Optimal Atom-Economy

Naba Abuhafez, Rafael Gramage-Doria

► To cite this version:

Naba Abuhafez, Rafael Gramage-Doria. Cobalt-Catalyzed Wacker-type Aerobic Oxidation of Olefins into Ketones Enabled by PMHS: Giving Value to a Chemical Waste with Optimal Atom-Economy. ChemCatChem, 2024, 16 (16), pp.e202400333. 10.1002/cctc.202400333. hal-04530330v2

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# Cobalt-Catalyzed Wacker-Type Aerobic Oxidation of Olefins into Ketones Enabled by PMHS: Giving Value to a Chemical Waste with Optimal Atom-Economy

Naba Abuhafez[a] and Rafael Gramage-Doria*[a]

Herein, it is demonstrated that a chemical waste from the silicon industry, namely polymethylhydrosiloxane (PMHS), is a suitable hydrosilane source for the challenging, selective aerobic oxidation of olefins into methyl ketones under cobalt catalysis: a first row, abundant in the Earth's crust and cheap metal with low toxicity. The catalytic system operates under unprecedented, stoichiometric amounts of hydrosilane while the cobalt catalyst loading is kept at a very low 1 mol% with the reactions being finished in less than 1 hour with very high

turnover numbers (1,340) and record-breaking turnover frequency values up to 530 h⁻¹. Mechanistic studies highlight the key role of the porphyrin ligand for stabilizing the active cobalt species that does follow a radical-based reaction pathway under these particular conditions. These results are relevant for replacing the expensive and scarce palladium catalyst, traditionally used for Wacker-type oxidations, by first-row, Earth-abundant transition metals under green conditions including the efficient valorization of a chemical waste such as PMHS.

## Introduction

One of the pillars of circular economy is the possibility to reuse any good or to give a second (or more) live(s) to a given material.[1] In chemical sciences, the term coined circular chemistry has received increasing attention during the last years as it complements well the already popular 12 principles of Green Chemistry developed by Warner and Anastas at the end of 20th century.[2-4] In this respect, searching new reactivities for existing materials considered traditionally as “chemical waste” is appealing and it will provide important advances towards a sustainable economy.[5-8]

Although highly important processes are operating worldwide in chemical industries, still many of them suffer from the formation of side-products due to their poor selectivity.[9-11] On one hand, these side-products could be of interest and their isolation is economically viable and carried out as exemplified in the production of acetone during the oxidation of benzene into phenol, also known as the cumene process.[12] Unfortunately, in other cases, the side-products have little or no utility so far. In particular, this is the case of polymethylhydrosiloxane (PMHS), which is a chemical delivered as side-product during the Rochow-Müller process and related ones from the silicon industry.[13,14] Interestingly, the use of PMHS to replace more expensive, less accessible and more hazardous hydrosilane ones

(i.e. (EtO)₃SiH, PhMeSiH₂, Et₃SiH, PhSiH₃) has appeared as a promising alternative during the last years, in particular in reduction chemistry.[15,16] For instance, the reduction of ketones, esters, imines and amides to the corresponding alcohols and amines, respectively, using PMHS as reducing agent have been reported with or without metal catalysts.[17-21] In the presence of tris(pentafluorophenyl)borane B(C₆F₅)₃, PMHS is a higher reducing agent and it leads to full deoxygenation of ketones to alkanes[22] as well as cascade sequences involving reduction and cyclization for the case of sugars derived from biomass.[23] PMHS has proven particularly useful for the hydroamination of styrenes and hydroxylamines under copper catalysis.[24] Another use of PMHS concerns its utilization in the alcohol production from epoxides *via* a titanocene-catalyzed hydrosilylation.[25] In all these cases the loading of PMHS was found to be in the range from 2 to 5 equivalents with respect to the limiting reagent, typically the substrate starting material being functionalized, thus leading to an overall extremely poor atom-economy.[17-25]

Han and co-workers found that a very remarkable feature of PMHS is the ability to behave as a hydride donor in the Wacker-type oxidation of olefins into ketones by means of iron catalysis under air as oxygen source (Figure 1B).[26] This challenging transformation is highly relevant because it compares well with the classical palladium-catalyzed Wacker reactions (Figure 1A) but using a cheaper and more abundant first row transition metal catalyst. Although Han's reaction conditions are tolerant to a wide spectrum of sensitive functional groups, the iron catalyst loading is relatively high (10-20 mol%) and the PMHS is used in a large and unsustainable excess of 3-5 equivalents with respect to the olefin starting material (Figure 1B). Consequently, there is a vast room for improvement by decreasing the loading of both hydrosilane as well as the metal catalyst. This can be achieved by searching for alternatives to iron within the first row of the transition metal series, with the overall aim

<footnote>[a] Dr. N. Abuhafez, Dr. R. Gramage-Doria
Univ Rennes, CNRS, ISCR-UMR6226, F-35000 Rennes, France
E-mail: rafael.gramage-doria@univ-rennes1.fr</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202400333</footnote>

<footnote>© 2024 The Authors. ChemCatChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.</footnote>
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<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Wacker-type oxidation reaction of olefins under classical palladium catalysis (A), under iron catalysis using PMHS as hydrogen source (state-of-the art, B), under iron- and cobalt-based catalysis with unsustainable silanes (C), and the key advantages of the present work using stoichiometric amounts of PMHS under cobalt catalysis (D). [O] = oxygen source, [H] = hydrogen source</figcaption></figure>

of replacing the costly, scarce and toxic palladium in Wacker-type oxidations. Note that related iron- and cobalt-catalyzed processes work under ambient conditions, but at the expenses of using undesired over-stoichiometric amounts of more expensive and hazardous hydrosilanes such as $PhSiH_3$ or $Et_3SiH$ (Figure 1C).[27-31] In fact, PMHS is the cheapest amongst the many available silane sources.[32] As such, it is urgent to develop highly efficient Wacker-type oxidation reactions based on first-row transition metal catalysts while operating under stoichiometric amounts of hydrosilane.[33] Based on our previous contribution demonstrating that CoTPP (TPP = tetraphenylporphyrinato) is a suitable first-row transition metal-based complex enabling the highly selective catalytic oxidation of aromatic olefins into methyl ketones using $Et_3SiH$ under ambient conditions,[34] we envisioned that it could be possible to upgrade the sustainability of such approach by using only stoichiometric amounts of PMHS as reducing agent under aerobic conditions (Figure 1D). Herein, we demonstrate such strategy in which the catalysis additionally displays a broad functional group tolerance with high activity and very short reaction times with as little as 1 mol% of CoTPP at industrially-relevant temperatures (70-90°C). Extensive control experiments enabled to propose a plausible catalytic cycle involving radical-based cobalt(III) intermediates. It is worthy to note that Matsushita reported similar reaction design but using a highly sophisticated octa-chlorinated cobalt-porphyrin in the presence of expensive and hazardous $Et_3SiH$, being notable the formation of alcohol and hydroperoxide as by-products aside of the methyl ketone product.[35]

## Results and Discussion

Initially, we evaluated the oxidation of 4-tert-butylstyrene starting material (1a) under conditions similar to those reported

by ourselves elsewhere[34] but replacing the expensive and hazardous $Et_3SiH$ hydrosilane reagent for only a single equivalent of PMHS (Table 1). By screening the reaction conditions, we discovered that 1 mol% of CoTPP complex enabled the full oxidation of 1a into the corresponding ketone 2a in a remarkable 88% yield in the presence of one equivalent of PMHS at 70°C under 1 bar of oxygen pressure after 60 minutes in ethanol solvent (Table 1, entry 1).[36] This corresponds to an initial turnover frequency at 10 minutes of $TOF_i = 337 h^{-1}$, which clearly outperforms the previous activity encountered for iron-catalyzed processes.[27-31] The control experiments showed the need of all the reagents for the efficiency of the catalysis (Table 1, entries 2-3). When the reaction was performed under air as an oxidant instead of pure oxygen, 2a was obtained in a low 17% yield (Table 1, entry 4) and no conversion was obtained under argon atmosphere (Table 1, entry 5). These findings strongly suggest that $O_2$ is the source of oxygen in the catalytic reaction. On the other hand, decreasing the temperature led to a significant lowering of the reactivity (Table 1, entries 6-7) whereas increasing the temperature to 90°C gave a higher initial turnover frequency ($TOF_i = 530 h^{-1}$, calculated at 10 minutes) with the same 88% selectivity for ketone 2a formation (Table 1, entry 8). The robustness of the catalytic system was addressed by using as little as 0.01 mol% of CoTPP catalyst, which led to a TON of 1,340 (see Supporting Information). On the other hand, the conversion significantly diminished when solvents such as methanol, dichloromethane, isopropanol and toluene were evaluated (Table 1, entries 9-13). Clearly, the ethanol solvent plays some role on stabilizing the active cobalt species in contrast to other solvents including methanol that has similar pKa. Although hydrogen bonding was evoked in the past as a key factor using $Et_3SiH$ as hydrosilane in place of PMHS at ambient conditions under cobalt catalysis,[34] the precise role of ethanol remains to be addressed for the current case of study without discarding the
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<figure><img src="image_5.png" /></figure>

<table><caption>**Table 1.** Optimization of reaction conditions for the cobalt-catalyzed aerobic Wacker-type Markovnikov-selective oxidation of **1a** using stoichiometric amounts of PMHS and control experimentation.[a]</caption><thead><tr><th>Entry</th><th>Deviation from above conditions</th><th>Conv.(%)<sup>b)</sup></th><th>2a:3a<sup>b)</sup></th></tr></thead><tbody><tr><td>1</td><td>none</td><td>&gt; 99</td><td>88:12</td></tr><tr><td>2</td><td>w/o CoTPP</td><td>0</td><td>n.d.</td></tr><tr><td>3</td><td>w/o PMHS</td><td>0</td><td>n.d.</td></tr><tr><td>4</td><td>air instead of O<sub>2</sub></td><td>17</td><td>17:0</td></tr><tr><td>5</td><td>argon instead of O<sub>2</sub></td><td>0</td><td>n.d.</td></tr><tr><td>6</td><td>25 °C instead of 70 °C</td><td>4</td><td>n.d.</td></tr><tr><td>7</td><td>60 °C instead of 70 °C</td><td>74</td><td>62:12</td></tr><tr><td>8</td><td>90 °C instead of 70 °C</td><td>&gt; 99</td><td>88:12</td></tr><tr><td>9</td><td>EtOH with 4% water content</td><td>41</td><td>34:7</td></tr><tr><td>10</td><td>MeOH instead of EtOH</td><td>4</td><td>n.d.</td></tr><tr><td>11</td><td>DCM instead of EtOH</td><td>0</td><td>n.d.</td></tr><tr><td>12</td><td>isopropanol instead of EtOH</td><td>39</td><td>32:7</td></tr><tr><td>13</td><td>toluene instead of EtOH</td><td>0</td><td>n.d.</td></tr><tr><td>14</td><td>H<sub>2</sub>TPP instead of CoTPP</td><td>0</td><td>n.d.</td></tr><tr><td>15</td><td>Co(OAc)<sub>2</sub>·4H<sub>2</sub>O instead of CoTPP</td><td>0</td><td>n.d.</td></tr></tbody></table>

[a] Reaction conditions: **1a** (0.325 mmol), PMHS (74 µL, 0.325 mmol), CoTPP (0.00325 mmol), O₂ (1 bar, balloon), EtOH (1 mL), dodecane (0.081 mmol). [b] Conversion of **1a**, and yields of **2a** and **3a** determined by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) using dodecane as the internal standard, H₂TPP = tetraphenylporphyrin, n.d. = not determined.

eventual increase of O₂ solubility in ethanol under this precise reaction conditions.[37] For the sake of comparison, the use of free-base tetraphenylporphyrin (H₂TPP) and Co(OAc)₂·4H₂O, respectively, as eventual catalysts led to no reactivity and full recovery of the starting material **1a** (Table 1, entries 14–15), thereby emphasizing the relevance of the porphyrin ligand around cobalt. Moreover, a kinetic study indicated that the reaction was completed in 20 minutes with the formation of **2a** being directly correlated to the consumption of the substrate **1a** (Figure 2).

Next, the substrate scope of the aerobic CoTPP-catalyzed oxidation using stoichiometric amounts of PMHS was next investigated using the optimized reaction conditions from Table 1 (entry 1). A variety of functionalized styrene derivatives were fully oxidized in 40 minutes with high selectivity towards the corresponding methyl ketones **2** over the alcohol side-products **3** (Scheme 1). For instance, alkyl-containing styrenes **1a** and **1b** were oxidized into their corresponding methyl ketones **2a** and **2b** in 82% and 70% isolated yields, respectively. Unfunctionalized styrene **1c** delivered acetophenone **2c** in 81% isolated yield. Styrene derivatives containing electron-withdrawing group such as trifluoromethyl, chloride, bromide and as well as benzyl chloride were readily oxidized in good to excellent yields providing the corresponding ketones

<figure><img src="image_6.png" /><figcaption>**Figure 2.** Kinetic profile for the aerobic CoTPP-catalyzed oxidation of 4-tert-butylstyrene **1a** using PMHS.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Scheme 1.** Evaluation of the substrate scope for olefin substrates in the aerobic CoTPP-catalyzed oxidation using stoichiometric amounts of PMHS as the hydrogen source. [a] 15% of hydrogenated olefin was formed too. [b] 16 hours instead of 40 minutes.</figcaption></figure>
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<figure><img src="image_8.png" /></figure>

**2d–2g.** In these cases, the alcohol side-products formed in a very low range of 3–10% compared to the major ketone product that formed in 90–97% selectivity. Ether and ester-containing ketones **2h** and **2i** were obtained in 79% and 93% isolated yield, respectively. Under these conditions, the carboxylic acid product **2j** was isolated in 90% isolated yield with no detectable formation of the alcohol side-product. This observation contrasts with the absence of reactivity encountered for this particular substrate **1j** in the reaction conditions developed previously by us under cobalt catalysis using the most expensive and costly Et₃SiH as the hydrosilane.[34] Polyaromatic substrates were also compatible as shown in the synthesis of 2-acetonaphthone (**2k**) and 4-acetylbiphenyl (**2l**) in 84% and 94% yields respectively. Furthermore, no reactivity was observed for the heteroaromatic 2-vinylpyridine and 4-vinylpyridine as well as for highly inert aliphatic olefins (Scheme 1). Other challenging olefins were evaluated. For instance, internal olefins, such as 1,2-di-substituted trans-β-methylstyrene (**1m**) furnished propiophenone (**2m**) in a remarkable 65% selectivity and 58% of isolated yield (Scheme 1). For this particular substrate, the cobalt-catalyzed oxidation afforded up to 20% of alcohol **3m** and 15% olefin hydrogenation as side-product. Beyond the reactivity of styrene derivatives, we also evaluated the reactivity of allyl derivatives. The allyl derivative containing electron-withdrawing CF₃ group **1n** furnished the corresponding ketone product **2n** in a remarkable 87% selectivity and it was isolated in 80% yield (Scheme 1). On the other hand, the reaction using 2-methyl allyl–benzene **1o** afforded only 40% of the corresponding ketone product **2o** after 16 hours (Scheme 1), which outperforms previous observations for this substrate applying our previously-developed reaction conditions.[34] Following this electronic effect trend, the reaction performed with unfunctionalized allyl–benzene led to no reactivity (Scheme 1). It is relevant to consider that allyl-containing derivatives can follow undesired reaction pathways (isomerization/oxidation or oxidative C=C bond cleavage) in the presence of metal catalysts under similar reaction conditions,[38,39] which is not the case for the current cobalt-based catalytic system. Overall, this substrate scope compares well with previous reports using iron catalysts at high catalyst loadings (10–20 mol%) and over-stoichiometric amounts of PMHS (3 equivalents)[26] as well as those reports involving iron- or cobalt-catalyzed Wacker-type oxidations of olefins to ketones using unsustainable PhSiH₃ or Et₃SiH as hydrosilane reagents.[27–31,34]

In order to gain insights into the reaction mechanism operating for this aerobic CoTPP-catalyzed oxidation of olefins using stoichiometric amounts of PMHS as the hydrogen source, we performed additional experiments (Scheme 2). For instance, there was a complete absence of reactivity when the catalysis was attempted in the presence of TEMPO [TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxyl] (Scheme 2A), thus strongly supporting a radical-based mechanism. We also ruled out the possibility of epoxides or alcohols as transient intermediates towards the formation of ketone considering their lack of reactivity under the catalytic conditions (Scheme 2B and 2C). We also noted that ketone is not the intermediate towards the

formation of alcohol considering the absence of reactivity of the former under the herein developed catalysis using stoichiometric amounts of PMHS (Scheme 2D). Consequently, a common intermediate is responsible for both the formation of the ketone resulting from Markovnikov selectivity and the formation of the alcohol side-product. In addition, the reaction mechanism does not follow an epoxyde/isomerization pathway as reported elsewhere for the anti-Markovnikov oxidation of olefins into aldehydes with ruthenium- or iron-porphyrin catalysts.[40–43] Note that the same activity and selectivity was observed when the reaction was conducted in the dark, thereby ruling out any excited state cobalt intermediates.

Taking into account all the above-described findings together with mechanistic studies from previous contributions,[26–31,34] we propose the plausible reaction mechanism displayed in Scheme 3 regarding this cobalt-catalyzed Wacker-type oxidation of olefins using stoichiometric amounts of PMHS under aerobic conditions. We assume that initial reaction between CoTPP and PMHS under oxygen atmosphere could result in the formation of cobalt(III) hydride species Co–H. Addition of the olefin to Co–H would result in the formation of the cobalt(III) alkyl intermediate A' or its radical version A''. This is the step that controls the chemoselectivity of the reaction because the hydrogen atom is added to the terminal carbon of the olefin double bond. Further reaction with O₂ may lead to the cobalt–peroxo species B' or B'', and further rearrangement may lead mainly to the ketone product **2** or to the minor alcohol side-product **3**, the latter via a Mukaiyama-type hydration pathway.[44–47] This results in the formation of the cobalt(III)–hydroxide species C that could react with PMHS leading to the catalytically active Co–H species back again, thus enabling the catalytic cycle to continue. Note that the formation of cobalt(III) species throughout the catalytic cycle is addition-

<figure><img src="image_9.png" /><figcaption>**Scheme 2.** Experiments relevant for the mechanistic understanding of the aerobic CoTPP-catalyzed oxidation of olefins using stoichiometric amounts of PMHS as the hydrogen source.</figcaption></figure>
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<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Scheme 3.** Postulated reaction mechanism for the aerobic CoTPP-catalyzed oxidation of olefins to ketones using PMHS as the hydrogen source (the tetraphenylporphyrin ligand around the cobalt center is not depicted for the sake of clarity).</figcaption></figure>

ally supported by the lack of reactivity found for pyridine-
containing olefins (Scheme 1), which are known to bind in a
kinetically inert way to cobalt(III)-porphyrin complexes.[48–57]

Conclusions

In summary, we have developed a highly environmentally
friendly procedure for the aerobic oxidation of olefins into
ketones with high selectivity as well as a broad functional group
tolerance using stoichiometric amounts of PMHS, a chemical
waste from the silicon industry that is readily affordable and
less hazardous than related hydrosilanes such as Et₃SiH or
PhSiH₃. This industrially-relevant reaction operates under a mild
1 bar of O₂ pressure at benign temperatures (70–90 °C). These
reaction conditions are compatible with practical implementa-
tion and, more importantly, they provide a second useful life to
a chemical waste such as PMHS under highly efficient
conditions. In addition, the catalysis features a high reactivity as
shown by full conversion of the olefins in less than an hour
using only 1 mol% of the CoTPP catalyst, which is an available
first-row metal complex. Experimental studies suggest a similar
reaction mechanism as reported elsewhere,[26–31,34] but here it
remarkably involves the readily available and cheap PMHS as
the source of hydride for the formation of the catalytically
active cobalt(III)-hydride species Co–H. Although as a proof-of-
concept we managed to recycle and reuse the CoTPP catalyst
up to two times without erosion of both activity and selectivity
(see Supporting Information), further studies will be devoted to
a comprehensive study of the stability of the CoTPP catalyst. In
view to further improve the sustainable impact of this trans-

formation, we anticipate that implementing such reaction
design into metal or covalent organic frameworks, or other
heterogeneous supports may lead to interesting approaches in
the industrially-relevant Wacker-type oxidation reaction.[58,59]

Acknowledgements

This work was financially supported by the Centre National de la Recherche Scientifique (CNRS), Université de Rennes, the Agence Nationale de la Recherche ANR-JCJC (ANR-19-CE07-0039), Région Bretagne (ARED 2020 No. 1715) and Collège de France (PAUSE, Ph.D. grant to NA).

Conflict of Interests

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available in
the supplementary material of this article.

Keywords: ketones • olefins • silanes • first row metals • waste valorisation

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Manuscript received: February 17, 2024

Revised manuscript received: March 12, 2024

Accepted manuscript online: April 2, 2024

Version of record online: May 13, 2024