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Palladium-Catalyzed Synthesis of Aldehydes from Aryl Iodides and Formic acid with Propylphosphonic Anhydride as the Activator

An interesting palladium-catalyzed carbonylative procedure for the synthesis of aromatic aldehydes from aryl iodides has been developed. By using propylphosphonic anhydride as the activator for formic acid, moderate to good yields of the corresponding aldehydes were produced with formic acid as the carbonyl and hydrogen donors. Interestingly, neither additional phosphine ligand nor inert gas protection is needed here.


Results
Initially, we chose iodobenzene as the model substrate to establish this idea (Table 1). Using the combination of Pd(OAc) 2 and PPh 3 as the catalytic system, with formic acid as the source of formyl group and NEt 3 as the base in DMF at 100 °C for 5 hours, 10% of benzaldehyde was formed with the total conversion of iodobenzene (Table 1, entry 1). Interestingly, the reaction was totally inhibited when pyridine was used as the base (Table 1, entry  2). Then the reaction was tested without phosphine ligand, and even better yield of benzaldehyde was formed  (Table 1, entry 3). Subsequently, the amounts of formic acid and propylphosphonic anhydride were tested, and 80% of benzaldehyde can be produced with higher loading of propylphosphonic anhydride (Table 1, entry 5). The conversion of iodobenzene decreased when the reaction was carried out at lower temperature (Table 1, entry 6). To our surprise, the same arrange yield of benzaldehyde can be formed with 2.5 mmol of NEt 3 (Table 1, entry 7). This phenomenon implies that propylphosphonic acid as the produced by-product not necessarily to be neutralized. However, no reaction occurred in the absence of base (Table 1, entry 8). Then several other solvents were tested, but no improved yield can be obtained (Table 1, entries 9-13).
After established the optimum catalytic system, we started the scope and limitation testing. As shown in Fig. 1, moderate to good yields of the corresponding aldehydes can be produced in general. Both electron-donating and electron-withdrawing substituents on the aromatic iodides can be well tolerated. 2-Iodonaphthalene and 1-iodonaphthalene are suitable substrates as well, good yields of the desired aldehydes were produced (Fig. 1, entries [16][17]. Heterocyclic substrates can be applied and smoothly transformed as well, moderate to good yields of the corresponding products can be detected (Fig. 1, entries 18-21).

Discussion
A plausible reaction mechanism is proposed and shown in Fig. 2 as well. Initially, oxidative addition of aryl iodide to Pd(0) species generates an arylpalladium complex. Then, carbon monoxide, prepared in-situ from formic acid, can be inserted into the arylpalladium complex to give an aroylpalladium species. Finally, ligand exchange of the aroylpalladium complex with another molecular of formic acid leads to an acylpalladium formic acid complex, which undergoes decarboxylation and reductive elimination to give the expected aldehyde product and regenerate the Pd(0) species.
In summary, an attractive palladium-catalyzed carbonylative procedure for transforming aryl iodides into the corresponding aldehydes has been developed. By using propylphosphonic anhydride as the activator for formic  acid, moderate to good yields of the corresponding aldehydes can be formed with formic acid as the carbonyl and hydrogen donors. Interestingly, neither additional phosphine ligand nor inert gas protection is needed here.

Methods
General Procedure. Under air, Pd(OAc) 2 (0.03 mmol, 1.5 mol%) was added to an oven-dried tube. Then aryl iodide (1 mmol), DMF (2 mL), HCO 2 H (4.5 mmol), NEt 3 (2.5 mmol), and propylphosphonic anhydride (0.8 mmol; 50% in DMF) were added to the reaction tube via syringe. Subsequently, the tube was sealed and stirred at 100 °C for 5 h. Then the tube was cooling down to room temperature and 100 mg of hexadecane was added into the tube as internal standard. After properly mixed, a part of the mixture was subjected to GC analysis for determination of the yield and conversion.