Synthesis of aromatic aldehydes via silver(I)-catalyzed formylation of aryl bromides in DMF promoted by samarium metal in air

Abstract Under ambient temperature conditions, a novel method has been developed for the conversion of aryl bromides to aromatic aldehydes, utilizing DMF (N,N-dimethylformamide) as both the starting material and solvent, in the presence of samarium metal and a trace of silver salts. This research has successfully explored an efficient approach for Barbier-type addition reactions, leading to the establishment of a novel C-C bond. A variety of bromides have been investigated as substrates, and aldehydes are readily obtained in moderate to high yields under mild conditions. The presence of 1 mol% of silver nitrate and potassium iodide is sufficient to catalyze the reaction, with the primary role of potassium iodide being to generate soluble silver iodide in the organic solvent, thereby catalyzing the reaction. The mechanism of the silver(I)-catalyzed process is discussed. GRAPHICAL ABSTRACT


Introduction
In organic chemistry, aldehyde compounds play a crucial role and find widespread applications in various industries such as pharmaceuticals and fragrances.1d ] Despite their effectiveness, these methods often suffer from drawbacks such as the need for toxic high-pressure gas (CO), low yields, issues with chemical selectivity and regioselectivity, and reliance on expensive catalysts.DMF (N,N-dimethylformamide) is commonly used as a versatile solvent in various chemical reactions and serves as a widely applicable multifunctional reagent.In recent years, researchers have frequently utilized DMF as a reaction precursor, leading to significant advancements in transition metal-catalyzed aminoformylation, cyanation reactions, and C-H activation processes. [2]iven these advancements, exploring a mild and efficient method for generating aromatic aldehydes is highly appealing.
Samarium reagents have garnered widespread attention in organic synthesis due to their unique properties. [3]Not only do they exhibit significant reduction coupling capabilities, but they also offer the advantages of low toxicity and cost-effectiveness, making them highly valuable and promising in the field of organic synthesis. [4]However, owing to the relatively weak reactivity of samarium metal, it is rarely employed directly.Typically, additives are introduced to activate samarium metal. [5]It has been observed that silver(I) demonstrates excellent efficacy in building C-C bonds. [6]Most studies on silver(I) catalysis primarily focus on the use of simple silver(I) salts or in situ generated silver(I) complexes. [7]The use of silver(I) as an additive dates back to the last century, [8] and compared to other transition metals such as rhodium, [9] palladium, [10] and nickel, [11] relatively fewer studies have investigated the catalytic activity of silver(I) compounds.
In our recent research, we have developed a synergistic catalytic system involving transition metal and samarium reagents to form a samarium-mediated cooperative catalysis.
Here we have discovered that the cooperative catalysis between silver(I) and samarium metal exhibits excellent performance in the catalysis field.Building upon the exceptional catalytic performance of the synergistic catalytic system mediated by silver(I) and samarium metal, we embarked on the reaction of aryl bromides with DMF in the Sm/AgNO 3 /KI system, leading to the formation of corresponding aldehyde compounds.

Results and discussion
The investigation of reaction conditions was initially conducted, and the summarized results are presented in Table 1.As shown in Table 1, in the Sm/AgNO 3 /KI system, we conducted a study on the reaction between bromobenzene and DMF.It was observed that changing the temperature had a limited impact on the reaction, while the quantity of the catalyst significantly influenced the reaction yield.Based on the research results, it is evident that at room temperature, when the catalyst was maintained at 1 mol%, the reaction was sufficiently catalyzed, yielding an 89% yield (Table 1, Entry 2).When the quantity of catalyst was insufficient, both the reaction time prolonged and the yield decreased (Table 1, Entry 1).Conversely, when excess silver nitrate was used (Table 1, Entries 3 and 4), although the reaction time was shorter, there was hardly any significant improvement in product yield.Additionally, the reaction carried out in moist air exhibited a slightly reduced yield, suggesting that the reaction was inhibited by the presence of water (Table 1, Entry 5).Secondly, when zinc or magnesium was employed individually as substitutes for samarium, or when samarium was omitted entirely (Table 1, Entries 6 and 7), no discernible products were generated.When silver nitrate and potassium iodide were separately added (Table 1, Entries 8 and 9), no products were observed either.We also investigated the replacement of silver nitrate with sodium sulfide, potassium fluoride, sodium chloride, and sodium bromide (Table 1, Entries 10,  11, 12 and 13).The results revealed that the reaction occurred only when sodium bromide was used as a substitute, albeit with a relatively lower yield, possibly due to the lower solubility of sodium bromide compared to potassium iodide.However, when silver nitrate was replaced with silver acetate and potassium iodide was not added (Table 1, Entries 14), the change in reaction yield was minimal.This indicates that the presence of potassium iodide has little influence on the reaction, emphasizing the predominant role of silver(I)-catalyzed reaction in this process.Due to the relatively high cost of silver acetate and economic considerations, we have opted to conduct the reaction using silver nitrate and potassium iodide instead of choosing silver acetate.In previous studies, some additives were found to have specific effects in conjunction with samarium.Through our investigation, it was found that the addition of TMSCl, HMPA, or I 2 (Table 1, Entries 15, 16 and 17) did not significantly improve the yield and, in some cases, even resulted in a decrease.Therefore, this reaction does not require the addition of additional additives to promote the reaction.In conclusion, considering all factors, the optimal reaction conditions were determined to be at room temperature under dry conditions with a catalyst concentration of 1 mol%.
Under the optimal reaction conditions, the reactivity of various bromides with DMF was explored, and the scope of the reaction was expanded, as shown in Table 2.We extended the applicability range of the reaction using bromides.
As is well known, transition metals find extensive applications in the construction of C-C bonds. [12]In this regard, Barbier and colleagues discovered the intermolecular SmI 2 -mediated Barbier reaction in 1977, which proceeds through the formation of organosamarium intermediates via two consecutive single-electron reductions. [13]It is noteworthy that silver is widely employed as a catalyst in the field of organic synthesis. [14]In our research, the addition of silver nitrate and potassium iodide serves the purpose of generating silver iodide.This is particularly advantageous as silver iodide is soluble in organic solvents, thereby facilitating the progress of the reaction.
Recent research results from Flowers II suggest that the addition of organic iodine reagents to ketones can generate organosamarium via Sm/Ni exchange. [15]Based on these studies, it is highly likely that organosamarium species rather than free radical intermediates are involved in the reaction process.In Muzart's review, the formylation of organolithium compounds by DMF is another widely used method for introducing formyl groups, where corresponding aldehydes are typically formed after the addition of organolithium compounds to DMF, followed by hydrolysis of the alkoxide. [16]onsequently, a plausible mechanism involves an oxidative addition reaction between Ar-Br and AgI, leading to an intermediate, which is then transformed into organosamarium via a transmetalation process.Finally, organosamarium undergoes a formylation reaction with the carbonyl group in DMF, followed by hydrolysis of the alkoxide to produce aromatic aldehydes.

Conclusion
This mechanistic proposal represents an initial deduction, and further experimental research is required to explore and confirm the generation of organosamarium intermediates.In summary, this reaction, characterized by its mild conditions, ligand-free nature, one-pot synthesis, simplicity of operation, high yields, and excellent chemical selectivity, provides an innovative approach for the construction of new C-C bonds in organic synthesis, opening up novel avenues.Furthermore, it offers another unique pathway for the samarium/silver combination, demonstrating extensive prospects in the field of organic transformations.

General information
All NMR spectra were measured in CDCl 3 and recorded on Bruker Avance-500 spectrometer ( 1 H NMR 500 MHz, and 13 C NMR 125 MHz) and Avance-Neo-400 spectrometer ( 1 H NMR 400 MHz, and 13 C NMR 101 MHz) with TMS or the residual signals of the solvent (δ 7.26 for 1 H NMR and δ 77.16 for 13 C NMR) as the internal standard.Chemical shifts are expressed in δ values (ppm) and coupling constants are given in J values (Hz).IR spectra were recorded on a Bruker Tensor-27 spectrometer.Melting points were measured on RY-1 melting point apparatus, and the values are uncorrected.
All chemical reagents and solvents were purchased from commercial sources and used without further purification unless otherwise specified.Before use, distill N,N-dimethylformamide and then dry it with 3 A molecular sieves.All reaction mixtures were stirred magnetically and were monitored by thin-layer chromatography using silica gel pre-coated glass plates, which were visualized with an ultraviolet analyzer.

Table 1 .
optimization of the reaction conditions.