AbstractRhodium(I)-complex [Rh(CO)2I2−] (1) catalyzed two carbonylations of methyl iodide and trimethylamine in NMP (1-methyl-2-pyrolidone) to acetic acid and DMAC (N,N-dimethylacetamide) in the presence of calcium oxide and water. The carbonylation of trimethylamine continued during the carbonylation and consumption of methyl iodide. In total, 183.8 mmol of carbonylated products was produced while consuming 24.1 mmol methyl iodide via acetic acid formation. These results clearly indicated that there were two carbonylation routes of trimethylamine and methyl iodide and the carbonylation rate of trimethylamine was faster than that of methyl iodide. Rhodium(I)-complex [Rh(CO)2I2]− (1) in the presence of trimethylamine was stable enough to be used 25 times with TON (Turnover Number) of 368 for DMAC and TON of 728 for trimethylamine. Inner-sphere reductive elimination in stepwise procedure was suggested for the formation of DMAC instead of acyl iodide intermediate under anhydrous condition. View Full-Text
Keywords: rhodium; carbonylation; trimethylamine; dimethylacetamide; methyl iodide; acetic acid; tetramethylammonium iodide; intramolecular; inner-sphererhodium; carbonylation; trimethylamine; dimethylacetamide; methyl iodide; acetic acid; tetramethylammonium iodide; intramolecular; inner-sphere►▼ Figures
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MDPI and ACS Style
Hong, J.-H. Two Carbonylations of Methyl Iodide and Trimethylamine to Acetic acid and N,N-Dimethylacetamide by Rhodium(I) Complex: Stability of Rhodium(I) Complex under Anhydrous Condition. Catalysts2015, 5, 1969-1982.
Hong J-H. Two Carbonylations of Methyl Iodide and Trimethylamine to Acetic acid and N,N-Dimethylacetamide by Rhodium(I) Complex: Stability of Rhodium(I) Complex under Anhydrous Condition. Catalysts. 2015; 5(4):1969-1982.Chicago/Turabian Style
Hong, Jang-Hwan. 2015. "Two Carbonylations of Methyl Iodide and Trimethylamine to Acetic acid and N,N-Dimethylacetamide by Rhodium(I) Complex: Stability of Rhodium(I) Complex under Anhydrous Condition." Catalysts 5, no. 4: 1969-1982.
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Heck reaction of aryl halides with olefins
In the presence of solvents, secondary phosphine oxide (RR'P(O)H) might undergo tautomerization, which generates a less stable phosphinous acid (RR'POH) species. Subsequently, its coordination to the metal center through the phosphorus atom forms a phosphinous acid–metal complex [54-56]. Thus, the resulting transition-metal complex might function as an active catalyst in various C–C-bond-forming reactions. Ackermann et al. reported the synthesis of stable N-aryl-substituted pyrrole and indole-derived SPO-preligands, which were utilized in Kumada–Corriu cross-coupling reactions . Recently, we reported the synthesis and characterization of imidazole-based secondary phosphine oxide ligand 5 and its application in C–C-bond-forming reactions (Scheme 2) . Furthermore, the application of complex 6 in cross-coupling reactions has been carefully studied. We found that complex 6 is an active catalyst for the Heck reaction of aryl halides with olefins under mild conditions.
To optimize the reaction conditions, a series of reactions under various combinations of bases, solvents and temperatures, employing complex 6 as precatalyst, was pursued. Bromobenzene (1a) and styrene (2a) were chosen as the model substrates in this coupling reaction and the results are presented in Table 1.
aReaction conditions: styrene (1.0 mmol), bromobenzene (1.0 mmol), base, solvent (1 mL), stirred for 12 h. bIsolated yield. cReaction mixture was stirred for 24 h.
Initially, the coupling was carried out by using 1 mol % loading of Pd-complex 6 as a precatalyst, with styrene (2a, 1 mmol), and bromobenzene (1a, 1 mmol) in DMSO (2 mL), and at ambient temperature in the presence of NaOH (1 equiv, Table 1, entry 1). The reaction did not give the coupled product 3a. Moreover, the use of other bases such as NaOAc, Et3N and K2CO3 in the presence of the solvents, DMSO, toluene or acetonitrile were not useful and no coupled product was observed. Interestingly, the reaction showed little progress in the presence of K3PO4 and tetrahydrofuran at 40 °C to obtain 3a in 17% yield (Table 1, entry 6). The yield was slightly improved when the reaction was heated at 60 °C (Table 1, entry 7). When K2CO3 (1.0 equiv) in THF was employed under similar reaction conditions, the yield of trans-stilbene was improved to 46% (Table 1, entry 8). Once K2CO3 had been selected as the most effective base, the next step involved the enhancement of the product yield. The combination of K2CO3 (2 equiv) and DMF (2 mL) resulted in the formation of 84% of 3a at 100 °C (Table 1, entry 10). A further increase in the reaction temperature would lead to decomposition of the palladium complex, which was formed in situ, thus lowered the yield of the product. Therefore, the loading of the precatalyst 6 was increased to 2 mol % and resulted in the formation of trans-stilbene in 92% yield at 80 °C (Table 1, entry 11). Synthetically, it is important to carry out reactions under mild reaction conditions. Nevertheless, low yield (73%) of the product was obtained by reducing the reaction temperature to 50 °C. Thus, a substrate survey was conducted at 60 °C. The optimized reaction conditions were found to be the use of styrene (2a, 1 mmol), bromobenzene (1a, 1 mmol), K2CO3 (2 mmol), and precatalyst 6 (2 mol %) with heating at 60 °C in DMF (1 mL, Table 1, entry 14). It is worthy of noting that the coupling reaction was also performed in the absence of solvent, which gave 82% yield (Table 1, entry 15) of the coupled product.
A wide range of olefins with different and diversely substituted aryl bromides were subjected to cross-coupling to produce the corresponding 1,2-disubstituted olefins. The results are summarized in Table 2.
aReaction conditions: olefin (1.0 mmol), aryl halide (1.0 mmol), Pd-complex 6 (2.0 mol %), K2CO3 (2.0 mmol), DMF (1 mL), 60 °C, 12 h. bIsolated yield. cProducts were characterized by 1H, 13C NMR and GC–MS. dThe yield corresponds to employing 4-chloro anisole as the aryl halide source.
Both aryl bromide and aryl iodide performed well (Table 2, entries 1 and 2) under these conditions. However, the aryl chloride was found to be less reactive giving the corresponding product 3a in 62% yield (Table 2, entry 3). The oxidative coupling was found to be selective in the case of 4-bromostyrene (2b), which gives 90% yield of 4-bromo trans-stilbene (3b) without the observation of any side product (Table 2, entry 4). The presence of either an electron-withdrawing or electron-donating group on the aromatic ring of olefin did not affect the reactivity and yield of product. The reactions led to the formation of excellent yields of the corresponding products 3e and 3f in 90% and 95% yields, respectively (Table 2, entries 7 and 8). As known, aromatic rings having substituents such as, -CH2OH, -CHO, -COCH3 -CN and -CF3 are rather useful in organic synthesis. However, in earlier reported oxidative coupling conditions these functional groups were not compatible and gave low yields of products. Therefore, these highly modifiable groups were screened under these catalytic conditions. Thus, 4-vinylbenzyl alcohol (2g), 4-vinyl benzaldehyde (2h), 4-vinylacetophenone (2i), 4-cyanostyrene (2j) and 4-trifluoromethylstyrene (2k) were smoothly converted to their corresponding coupled products 3g–3k in excellent yields (Table 2, entries 10–14). The selectivities and yields of the coupled products were excellent regardless of ortho-, meta-, or para-substitution patterns on either styrenes or aryl halides under these catalytic conditions. For example, the coupling of substituted methylstyrenes (Table 2, entry 15) or alkyl-substituted aryl halides (Table 2, entry 16) gave 88–90% isolated yields of 3l and 3m. To investigate whether the reaction was compatible with a heteroaryl olefin, 2-vinylpyridine (2m) was subjected to this reaction. It produced the corresponding coupled product 3n in 78% yield (Table 2, entry 17). Furthermore, using these optimized conditions, bromobenzene (1a) was examined with different vinyl esters to determine the scope of this procedure. The results are given in Table 2, entries 18–20. Notably, the performances were in agreement with the previous expectations and yields are excellent in the preparation of α,β–unsaturated esters. The corresponding α,β-unsaturated esters 3o–3q were obtained in 90–95% yields, respectively.
Heck reaction of arylboronic acids with olefins
The phosphine- and base-free coupling of arylboronic acids with olefins under mild reaction conditions were studied as well to broaden the scope of cross-coupling reactions. To search for the optimized reaction conditions, phenylboronic acid (4a) and styrene (2a) were chosen as the model substrates and Pd(OAc)2 was employed as the catalyst. Various reaction conditions were tested and the results are presented in Table 3. Initially, a Pd(OAc)2 catalyzed Heck reaction was performed employing polar sovents, dimethylacetamide (DMAc) and DMF, at 25 °C in the presence of 0.5 equiv of N-bromosuccinimide (NBS). This resulted in the formation of trans-stilbene (3a) in 52% and 40% yield, respectively (Table 3, entries 1 and 2). However, the same reaction under the control conditions (i.e., in the absence of NBS) resulted in production of a trace amount of the coupled product 3a (Table 3, entry 3). When the coupling reaction was carried out at 90 °C in DMAc solvent, the yield of 3a decreased, due to the formation of side product, such as bromobenzene, from the corresponding phenylboronic acid (Table 3, entry 4). Therefore, it is believed that NBS plays an important role in this catalytic reaction. Furthermore, we focused our attention to other solvents such as MeOH, CH2Cl2, CH3CN, Me2O, t-Bu2O, THF, DMSO and 1,4-dioxane, which resulted in low yields of arylated product. Subsequently, the reaction was subjected to the apolar solvent toluene. The expected product trans-stilbene (3a) was obtained in 68% yield at 25 °C for 18 h (Table 3, entry 5). The yield of the desired product did not improve even when the reaction was stirred for 24 h (Table 3, entry 6). On the other hand, lowering the additive (NBS) to 10 mol % did not show any improvement to the formation of trans-stilbene (3a) (Table 3, entries 7 and 8). A sharp decline in the formation of trans-stilbene (3a) (Table 3, entry 9) was observed on increasing the quantity of NBS to a stoichiometric amount (1.0 equiv). This was probably due to the formation of other competitive side product(s). Interestingly, the coupled product was obtained with improved yield of 76% by using 30 mol % NBS (Table 3, entry 10). Next, we turned our attention to the improvement of the yields of trans-stilbene by adjusting other reaction parameters. Thus, the addition of K2CO3 as base along with NBS under similarly performed reaction conditions led to no formation of the targeted product. The addition of molecular sieves was not a good choice either . The other additives such as LiBr and CuBr were also examined. Still, no coupled product was obtained in the presence of LiBr (30 mol %, Table 3, entry 13). On the other hand, the employment of CuBr (30 mol %) with the presence of Pd(OAc)2 resulted in a 42% yield of trans-stilbene (3a) (Table 3, entry 14). Thus, the optimized reaction conditions for the Heck reaction here is the use of arylboronic acid (1 mmol), olefin (1 mmol), Pd(OAc)2 (5 mol %), NBS (30 mol %), toluene (1 mL) at 25 °C under stirring for 12 h.