Source: https://pubs.rsc.org/en/content/articlehtml/2019/qo/c9qo00092e
Timestamp: 2019-04-25 07:43:25+00:00

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Photoredox/nickel dual catalysis using easily oxidized bis-catecholato hypercoordinated silicon derivatives as radical sources and acyl chlorides as electrophiles allows a new method of formation of dialkyl and alkyl-aryl ketones as well as dibenzyl ketones which are less easily accessed. Flow chemistry can be used.
In addition to being environmentally friendly and very mild, the photoredox conditions used in these reactions are usually compatible with many functionalities.12 They have also been found to be compatible for the delivery of ketones.8i,13 For instance, Doyle and co-workers showed that it is possible to obtain α-amino ketones by using N-aryl amines and electrophiles such as anhydrides or activated esters.14 In the same vein, Molander et al. used this approach to achieve the coupling reaction between acyl chlorides and trifluoroborates as radical precursors to obtain functionalized ketones (Scheme 1).15 Nevertheless, this transformation relies on conditions involving the use of additives such as 2,6-lutidine or fluoride salts and does not provide highly enolizable benzylic ketones which are difficult to access. For these reasons, we turned our attention to the reactivity of highly soluble and easily oxidized silicates toward carboxylic acid derivatives and particularly acyl chlorides in order to obtain benzylic and alkyl ketones (Scheme 1).
Scheme 1 Formation of ketones via dual photoredox/Ni catalysis using acid chloride as the electrophile.
We first tried this reaction with 1.5 equiv. of cyclohexyl silicate 2a (Eox = 0.69 V vs. SCE) as the radical source and 1 equiv. of benzoyl chloride as the electrophile in the presence of Ru(bpy)3(PF6) as a photocatalyst, and NiCl2·dme and dtbbpy (4,4′-di-tert-butyl-2,2′-dipyridyl) as ligands in THF under blue LED irradiation at room temperature. Gratifyingly, the expected ketone 3a was obtained in 64% yield. Following this encouraging result, we searched for the best conditions to achieve this reaction (Table 1). The organic photocatalyst 4CzIPN, known to have a long lived excited state and a high oxidative potential (τ = 5100 ns, E1/2 (Ir(PC*)/Ir(PC˙−) = +1.59 V vs. SCE)16,17 and previously used to oxidize silicates,18 gave ketone 3a in 52% yield (entry 2). To our delight, iridium photocatalyst Ir(dF(CF3)ppy)2(bpy)(PF6) provided the best yield of coupling product 3a (72%). Other solvents (dichloromethane and acetonitrile) and nickel catalysts, for instance Ni(COD)2 as the nickel (0) source, were screened but they did not show any improvement. Therefore, the Ir(dF(CF3)ppy)2(bpy)(PF6)/NiCl2·dme dual catalytic system was selected and it gave the best results.
a NMR yield using 1,3,5 trimethoxybenzene as an internal standard. b [Ru] = Ru(bpy)3(PF6)2. c [Ir] = Ir(dF(CF3)ppy)2(bpy)(PF6). d 16 h of blue LED irradiation (477 nm) instead of 24 h. e No NiCl2·dme was present.
Under these optimized conditions other electrophiles were tested. While no product was observed with the corresponding acyl fluoride derivative, yields of 40%, 31% and 12% were obtained from activated ester 1b, mixed anhydrides 1c and mixed carbonate 1d, respectively (entries 12–14, Table 1).14 Nevertheless, these yields were lower than that previously obtained with benzoyl chloride as a partner. Therefore, we decided to continue this study with acyl chloride partners.
We first varied the silicate partner (Scheme 2) keeping benzoyl chloride 1a as the electrophile. All primary radicals resulted in poor yields of products (below 10%). The main side product in these reactions is the acylation of catechol which suggested that the rate-limiting step is the very slow oxidation of the silicate allowing the catecholate moiety to react on the acyl chloride.19 More activated α-acetoxysilicate resulted only in a slight increase in the yield and adding 6 equiv. of benzoyl chloride for 1 equiv. of silicate did not change it. Not surprisingly, tert-butyl silicate underwent little cross coupling.8a Secondary silicates proved however to be generally more rewarding since cyclopentyl, norbornane and cyclohexyl silicate precursors afforded the cross coupling products in 27% (3h), 49% (3g) and 64% (3a) yields, respectively. Finally, activated silicates such as the newly prepared (see the ESI†) methoxymethylene silicate (Eox = +0.71 V vs. SCE 2i) and benzyl derivatives gave the best results and provided highly enolizable ketones 3i, 3j and 3k in very good yields of 64%, 82% and 80%, respectively.
Scheme 2 Silicate scope in the coupling reaction with benzoyl chloride. aNMR yield using 1,3,5 trimethoxybenzene as an internal standard.
It appeared also judicious to compare the reactivities of the benzyl trifluoroborate and benzyl silicate precursors under these conditions with no additive. The benzyl trifluoroborate 4a afforded the desired product 3j with a yield of 50% as determined by NMR (benzoic acid was also observed in the reaction mixture attesting the incomplete conversion of the reactant) compared to the 82% isolated from benzyl silicate 1j with full conversion of the acyl chloride (Scheme 3) (10% of the monoacylated product was also observed in that case).
Scheme 3 Comparison of the reactivity between the benzyl trifluoroborate and benzyl silicate. aNMR yield using 1,3,5-trimethoxybenzene as an internal standard.
An initial flow set-up was assembled using a Schlenk flask and a recirculating pump (Scheme 4). The reagents were mixed by magnetic stirring inside the Schlenk flask protected from light, pumped through a PTFE tube (1/16′′ tubing, 134 cm, 0.67 mL at room temperature) under blue LED irradiation (477 nm) which was used as the flow reactor and re-introduced into the Schlenk flask. The coupling of benzyl silicate 2k with benzoyl chloride was used as a model reaction and a yield of 43% for 3j was obtained in 3.8 hours within the flow reactor. Nevertheless the formation of a larger amount of mono- and bis-acetylated catechol products was observed in contrast to the trace amounts from benzyl silicate in batches. This result can be easily explained by the fact that the side reaction does not need light and that it can occur inside the Schlenk reservoir.
Scheme 4 Flow configuration with premixed reactants.
Following this first attempt, a two flux process without recirculation was developed on the same scale as the previous batch reaction (Scheme 5).
Scheme 5 Flow configuration with an in-line micro reactor. aBy NMR using 1,3,5 trimethoxybenzene as an internal standard.
One flux introduced the acyl chloride and the other one the nickel catalyst, the photocatalyst, the ligand and the silicate. The two flux components (F = 1.305 mL h−1) were mixed within a glass milli-mixer (LTF-MX, 0.2 mL at room temperature) under blue LED irradiation (477 nm), and the mixture then flowed through a PTFE tubing (1/16′′ tubing, 134 cm, 0.67 mL at room temperature) under blue LED irradiation (477 nm). The resulting solution was collected in the dark and the desired product was directly isolated by chromatography. A good yield (70%) of the cross-coupling product 3j was obtained in only 20 minutes with total conversion of the acyl chloride with this flow setup (20% of the monoacetylated catechol product and traces of the bisacylated adduct were also observed), while a similar yield (81%) on the same scale can be obtained in a batch reactor after 24 hours. The flow reaction setup enables a shorter reaction time, hence a higher energy efficiency, not to mention the ease of scaling-up with the flow chemistry.
Encouraged by these series of preliminary findings and exploring the synthetic potential of this new transformation, we examined the influence of the acyl chloride partner (Scheme 6) using 2a and 2j as radical sources. The highly electrophilic para-CF3 substituted benzoyl chloride mainly led to the formation of the acylated catechol (mono-acylated and bi-acylated adducts) with poor yields of cross coupling ketones 3l (8%) and 3q (16%). In contrast, electron richer benzoyl chlorides afforded the corresponding ketones in moderate to good yields (51% and 52%, respectively, for the cyclohexyl silicate and 81% and 71%, respectively, for the benzyl silicate). To our delight, primary and secondary alkyl acyl chlorides provided the products in very high yields leading to ketones with various substitution patterns, featuring for example cyclopropyl and cyclobutyl benzylketones (3zb and 3zd). It is noteworthy that double coupling from a diacyl chloride was possible (diketone 3ze). Finally, highly enolizable dibenzyl ketones such as 3w–z could be prepared through this method.
Scheme 6 Scope of this reaction based on the variation of the acyl chloride.
Based on our recent studies and those of other groups on related silicate chemistry,8 the following mechanism can be reasonably proposed (Scheme 7). The excited photocatalyst readily oxidizes the silicate to generate an iridium(II) complex and a radical intermediate. Presumably, the latter adds onto the nickel(II) intermediate resulting from the oxidative addition of the acyl chloride partner onto nickel(0). The resulting nickel(III) complex undergoes reductive elimination releasing both the desired product and a nickel(I) species and is then further reduced by iridium(II) and regenerates the iridium(III) photocatalyst.
In a nutshell, a novel method has been used for obtaining ketones by dual photoredox/Ni dual catalysis using bis-catecholato silicates as radical sources and acyl chlorides as electrophiles. While the reaction proved unsatisfactory with primary silicates or highly electrophilic acid chlorides due to adventitious acylation of the catechol moieties, interesting yields of variously substituted ketones have been generally recorded. Notably, highly enolizable dibenzylic ketones could be prepared. Various applications of flow chemistry to such a photoredox/nickel dual catalysis reaction have been disclosed and a shorter reaction time has been obtained.
To a Schlenk flask was added the appropriate silicate (1.5 equiv.), [Ir] (2 mol%), 4,4′-di-tert-butyl-2,2′-dipyridyl (4 mol%) and NiCl2·dme (4 mol%). The Schlenk flask was sealed with a rubber septum and evacuated/purged with vacuum/argon cycles three times. Degassed THF (0.1 M) was introduced followed by the addition of the electrophile 2 (1 equiv.) and the reaction mixture was irradiated with a blue LED (477 nm) at room temperature for 24 h under an argon atmosphere. The reaction mixture was diluted with diethyl ether, washed with aqueous saturated K2CO3 solution (2 times) and brine (2 times), dried over MgSO4 and evaporated under reduced pressure. The crude residue was purified by flash chromatography.
We thank Sorbonne Université, CNRS and ANR-17-CE07-0018 HyperSilight (PhD grant to EL) for financial support. We also thank Khaoula Jaoudi for the CV of 2i.
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