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Palladium-catalyzed carbonylative synthesis of α,β-unsaturated amides from aryl azides and alkenylaluminum reagent
Journal of Catalysis 383 (2020) 160–163
Contents lists available at ScienceDirect
Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Priority Communication
Palladium-catalyzed carbonylative synthesis of a,b-unsaturated amides from aryl azides and alkenylaluminum reagent Bo Chen, Xiao-Feng Wu ⇑ Department of Chemistry, Zhejiang Sci-Tech University, Xiasha Campus, Hangzhou 310018, People’s Republic of China Leibniz-Institut für Katalyse e. V. an der Universität Rostock, Albert-Einstein-Strabe 29a, 18059 Rostock, Germany
a r t i c l e
i n f o
Article history: Received 31 October 2019 Revised 13 January 2020 Accepted 14 January 2020
Keywords: Palladium catalyst Carbonylation a,b-Unsaturated amides Aryl Azides Alkenylaluminum reagent Carbonylative coupling
a b s t r a c t In this work, an interesting procedure for the synthesis of a,b-unsaturated amides from aryl azides and alkenylaluminum reagent has been developed. With palladium as the catalyst and XPhos as the ligand under carbon monoxide pressure, the desired a,b-unsaturated amides were isolated in good to excellent yields with good functional group tolerance. Remarkably, this procedure also represents an example on addition of organometallic reagent to isocyanates for a,b-unsaturated amides synthesis. Ó 2020 Elsevier Inc. All rights reserved.
1. Introduction
a,b-Unsaturated amide and its derivatives are important structural motifs due to their wide occurrence in natural products and also many other known biological activities [1,2]. Additionally, a, b-unsaturated amides are useful synthetic intermediates for various potent organic transformations as well [3]. Many methodologies for their preparation have been developed in recent years, because of their recognized potent applications in pharmaceuticals and synthetic chemistry [4]. The traditional method for preparing a,b-unsaturated amides was based on reacting of amines with a, b-unsaturated carboxylic acid in the presence of activating reagents [5]. Recently, Martin and co-workers developed a novel protocol for the hydroamidation of alkynes with isocyanates recently. Alkyl bromides have been used as hydride sources here. The method turns parasitic b-hydride elimination into a strategic advantage, rapidly affording a,b-unsaturated amides in good yields [6]. Transition-metal catalyzed carbonylation procedures based on using alkenes and alkynes as the substrates have also been developed for a,b-unsaturated amides preparation [7–9]. Alper’s group described a chemo- and regioselective direct aminocarbonylation of alkynes and aminophenols to form hydroxy-substituted a,bunsaturated amides in good to excellent yields [8]. Our group developed a procedure on palladium-catalyzed selective oxidative ⇑ Corresponding author. E-mail address: [email protected] (X.-F. Wu). https://doi.org/10.1016/j.jcat.2020.01.017 0021-9517/Ó 2020 Elsevier Inc. All rights reserved.
aminocarbonylation of styrenes with nitroarenes for the synthesis of a,b-unsaturated amides [9]. Additionally, palladium-catalyzed aminocarbonylation of enol triflates or iodoalkenes has been proven as an potent procedure for a,b-unsaturated amides synthesis as well [10]. Although significant progresses have been achieved in a,b-unsaturated amides synthesis, new procedure is always attractive as it can give the possibility to start from different substrates. Herein, we report a new method for the synthesis of a,b-unsaturated amides which is achieved by palladiumcatalyzed carbonylation of aryl azides and alkenylaluminum reagent. The initial investigation was carried out with azidobenzene and (E)-diisobutyl(oct-1-en-1-yl)aluminum which was prepare from oct-1-yne with diisobutylaluminum hydride (DIBAL-H) as the substrates. To our delight, the target (E)-N-phenylnon-2-enamide can be successfully obtained in 73% yield with Pd/C as the catalyst and XPhos as the ligand in THF at 80 °C for 16 h under CO pressure (Table 1, entry 1). The yield of the target product can be further improved by using Pd(OAc)2 as the catalyst (Table 1, entry 2). Subsequently, with Pd(OAc)2 as the catalyst, different phosphine ligands were tested (Table 1, entries 3–8). XPhos was found still to be the best ligand for this reaction, which may due to its promising properties on electron donation and sterically hindrance. Then a set of solvents were tested, the reaction efficiency decreased significantly in these cases and THF is the optimal solvent (Table 1, entries 9–11). The reaction temperature was varied as well;
161
B. Chen, X.-F. Wu / Journal of Catalysis 383 (2020) 160–163 Table 1 Selected results of the optimization of the reaction conditions.a
a b c d e
Entry
Catalyst
Ligand
Solvent
Yield %b
1 2 3 4 5 6 7 8 9 10 11 12d 13e
Pd/C Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2
XPhos XPhos PPh3 DPPP DPEphos DPPM DPPE Xantphos XPhos XPhos XPhos XPhos XPhos
THF THF THF THF THF THF THF THF Toluene 1,4-dioxane DCE THF THF
73 82(76c) trace trace 65 23 42 77 21 53 7 65 75
Reaction conditions: 1a (0.5 mmol), 2a (0.7 mmol in hexane), catalyst (2 mol %), ligand (2 mol %), solvent (1.0 mL), 10 bar CO, 80 °C, 16 h. NMR yield. Isolated yield. 60 °C. 100 °C.
inferior results were observed at 60 °C or 100 °C (Table 1, entries 12–13). Here, it is also important to mention that 65% yield of the target product can be obtained even under 1 bar of CO. However, due to the low boiling point of THF, we decided continual our studies with 10 bar of CO in order to avoid the leaching of solvent. With the optimal reaction conditions in hand, we explored the substrates scope of this carbonylative reaction. At first, we tested the generality of substituted phenyl azide substrates. As shown in Scheme 1, azidobenzene with electron-donating or -withdrawing substituents can be well tolerated, and delivering the desired products in good to excellent yields (3aa-3ia, 3la3na). Then, we tried substrates with functional groups including alkenyl, cyano, which may be incompatible with aluminum reagents. Fortunately, for these compounds, the reaction could also give the expected products in 64% and 84% yields, respectively (3ja, 3 ka). Notably, phenyl ether-, phenyl sulfide-, phenyl- substituted azidobenzenes were also well tolerated in this transformation and delivered the corresponding a,b-unsaturated amides in 74%, 84% and 76% yields, respectively (3oa-3qa). Additionally, the reaction with 1-azidonaphthalene also proceeded smoothly and gave the target products in 75% yield (3ra). Subsequently, different types of alkenylaluminum reagents for this reaction were tested (Scheme 2). Several aliphatic alkenylaluminums were prepared from the corresponding aliphatic alkynes with DIBAL-H proceeded well under our conditions to afford the desired products in good to excellent yield (3ab-3ah). Interestingly, diene aluminum compound can be successfully transformed in this reaction and gave the target product in 76% yield (3ai). Aromatic alkenylaluminums can also work well in this reaction, good yields of the corresponding products can be isolated (3aj-3am). In the case of using indole-substituted alkenylaluminum as the substrate, 54% of the desired product was isolated (3an). Concerning the reaction pathway, on the basis of our results and the literatures,[11] a possible reaction mechanism is proposed and shown in Scheme 3. First, from azidobenzene as the substrate, the palladium-nitrene species A is formed simultaneously with the release of N2. Subsequent insertion of CO into A occurred to give the intermediate B. After reductive elimination, the isocyanate will be afforded and the released palladium species which is ready for
Scheme 1. Synthesis of a,b-Unsaturated Amides from Aryl Azides. Reaction conditions: aryl azides (0.5 mmol), alkenylaluminum reagent 2a (0.7 mmol in hexane), Pd(OAc)2 (2 mol %), XPhos (2 mol %), THF (1.0 mL), 10 bar CO, 80 °C, 16 h, isolated yield.
the next catalytic cycle. Finally, the reaction of isocyanate with alkenylaluminum reagent produces the desired final product 3aa. In conclusion, we have developed an interesting procedure for the synthesis of a,b-unsaturated amides started from aryl azides
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S. Buchholz, and S. Schareina (all in LIKAT) is gratefully acknowledged. General procedure Under an Argon atmosphere, a 4 mL screwcap vial was charged with Pd(OAc)2 (2 mol %), XPhos (2 mol %), azidobenzene (0.5 mmol), THF (1 mL) and an oven-dried stirring bar, then injected alkenylaluminum solution which are prepared from the corresponding alkynes with DIBAL-H. The vial was closed by a Teflon septum and a phenolic cap and connected to the atmosphere through a needle. Then the vial was fixed in an alloy plate and put into Parr 4560 series autoclave (300 mL). At room temperature, the autoclave is flushed with carbon monoxide for three times and 10 bar of carbon monoxide was charged. The autoclave was placed on a heating plate equipped with magnetic stirring and an aluminum block. The reaction was heated at 80 oC for 16 hours. Afterwards, the autoclave was cooled to room temperature and the pressure carefully released. After removal of solvent under reduced pressure, pure product was obtained by column chromatography on silica gel (eluent: heptane/ethyl acetate 5:1). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2020.01.017. Scheme 2. Synthesis of a,b-Unsaturated Amides from Alkenylaluminum reagents. Reaction conditions: phenyl azides (0.5 mmol), alkenylaluminum reagent 2 (0.7 mmol in hexane), Pd(OAc)2 (2 mol %), XPhos (2 mol %), THF (1.0 mL), 10 bar CO, 80 °C, 16 h, isolated yield.
Scheme 3. Proposed reaction mechanism.
and alkenylaluminum reagents. In the presence of palladium catalyst under carbon monoxide pressure, the desired a,b-unsaturated amides were isolated in good to excellent yields with good functional group tolerance. Remarkably, this work also represents an example on the addition of organometallic reagent to isocyanates. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment C.B. thanks the Chinese Scholarship Council (CSC) for financial support. The analytical support of Dr W. Baumann, Dr C. Fisher,
References [1] (a) R.J. Andersen, J.E. Coleman, E. Piers, D.J. Wallace, Tetrahedron Lett. 38 (1997) 317–320; (b) C. Marrano, P. de Macédo, J.W. Keillor, Bioorg. Med. Chem. 9 (2001) 1923– 1928; (c) R. Hitayezu, M.M. Baakdah, J. Kinnin, K. Henderson, A.J. Tsopmo, J. Cereal Sci. 63 (2015) 35–40; (d) S. Son, B.A. Lewis, J. Agric. Food Chem. 50 (2002) 468–472; (e) L. Dai, C. Zang, S. Tian, W. Liu, S. Tan, Z. Cai, T. Ni, M. An, R. Li, Y. Gao, D. Zhang, Y. Jiang, Med. Chem. Lett. 25 (2015) 34–37. [2] (a) K.S. Putt, V. Nesterenko, R.S. Dothager, P.J. Hergenrother, ChemBioChem 7 (2006) 1916–1922; (b) T.W. Schultz, J.W. Yarbrough, S.K. Koss, Cell Biol. Toxicol. 22 (2006) 339– 349. [3] (a) M.J. Caulfield, G.G. Qiao, D.H. Solomon, Chem. Rev. 102 (2002) 3067–3083; (b) X. Mu, T. Wu, H.-Y. Wang, Y.-L. Guo, G.S. Liu, J. Am. Chem. Soc. 134 (2012) 878–881; (c) J.-H. Fan, W.-T. Wei, M.-B. Zhou, R.-J. Song, J.-H. Li, Angew. Chem., Int. Ed. 53 (2014) 6650–6654; (d) K. Liu, L.-C. Sui, Q. Jin, D.-Y. Li, P.-N. Liu, Org. Chem. Front. 4 (2017) 1606– 1610; (e) Z.-Y. He, J.-Y. Guo, S.-K. Tian, Adv. Synth. Catal. 360 (2018) 1544–1548. [4] (a) J.M. Concellón, J.A. Pérez-Andrés, H. Rodríguez-Solla, Angew. Chem., Int. Ed. 39 (2000) 2773–12275; (b) X.-R. Song, B. Song, Y.-F. Qiu, Y.-P. Han, Z.-H. Qiu, X.-H. Hao, X.-Y. Liu, Y.-M. Liang, J. Org. Chem. 79 (2014) 7616–7625; (c) Z. Liu, F. Huang, P. Wu, Q. Wang, Z. Yu, J. Org. Chem. 83 (2018) 5731–5750; (d) X.-R. Song, B. Song, Y.-F. Qiu, Y.-P. Han, Z.-H. Qiu, X.-H. Hao, X.-Y. Liu, Y.-M. Liang, J. Org. Chem. 79 (2014) 7616–7625. [5] (a) C.A.G.N. Montalbetti, V. Falque, Tetrahedron 61 (2005) 10827–10852; (b) E. Valeur, M. Bradley, Chem. Soc. Rev. 38 (2009) 606–631; (c) V.J. Pattabiraman, J.W. Bode, Nature 480 (2011) 471–479; (d) C.L. Allen, J.M. Williams, Chem. Soc. Rev. 40 (2011) 3405–3415; (e) J.W. Bode, Top. Organomet. Chem. 44 (2012) 13–34; (f) H. Lundberg, F. Tinnis, N. Selander, H. Adolfsson, Chem. Soc. Rev. 43 (2014) 2714–2742; (g) R.M. de Figueiredo, J.-S. Suppo, J.-M. Campagne, Chem. Rev. 116 (2016) 12029–12122; (h) A.O. Porras, D. Gamba-Sánchez, J. Org. Chem. 81 (2016) 11548–11555. [6] X.-Q. Wang, M. Nakajima, E. Serrano, R. Martin, J. Am. Chem. Soc. 138 (2016) 15531–15534. [7] (a) R. Shi, H. Zhang, L. Lu, P. Gan, Y. Sha, H. Zhang, Q. Liu, M. Beller, A. Lei, Chem. Commun. 51 (2015) 3247–3250; (b) J.H. Park, S.Y. Kim, S.M. Kim, Y.K. Chung, Org. Lett. 9 (2007) 2465–2468; (c) T. Fujihara, Y. Katafuchi, T. Iwai, J. Terao, Y. Tsuji, J. Am. Chem. Soc. 132 (2010) 2094–2098; (d) B. El Ali, A.M. El-Ghanam, M. Fettouhi, J. Tijani, Tetrahedron Lett. 41 (2000) 5761–5764; (e) U. Matteoli, A. Scrivanti, V. Beghetto, J. Mol. Catal. A: Chem. 213 (2004)
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Contents lists available at ScienceDirect
Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Priority Communication
Palladium-catalyzed carbonylative synthesis of a,b-unsaturated amides from aryl azides and alkenylaluminum reagent Bo Chen, Xiao-Feng Wu ⇑ Department of Chemistry, Zhejiang Sci-Tech University, Xiasha Campus, Hangzhou 310018, People’s Republic of China Leibniz-Institut für Katalyse e. V. an der Universität Rostock, Albert-Einstein-Strabe 29a, 18059 Rostock, Germany
a r t i c l e
i n f o
Article history: Received 31 October 2019 Revised 13 January 2020 Accepted 14 January 2020
Keywords: Palladium catalyst Carbonylation a,b-Unsaturated amides Aryl Azides Alkenylaluminum reagent Carbonylative coupling
a b s t r a c t In this work, an interesting procedure for the synthesis of a,b-unsaturated amides from aryl azides and alkenylaluminum reagent has been developed. With palladium as the catalyst and XPhos as the ligand under carbon monoxide pressure, the desired a,b-unsaturated amides were isolated in good to excellent yields with good functional group tolerance. Remarkably, this procedure also represents an example on addition of organometallic reagent to isocyanates for a,b-unsaturated amides synthesis. Ó 2020 Elsevier Inc. All rights reserved.
1. Introduction
a,b-Unsaturated amide and its derivatives are important structural motifs due to their wide occurrence in natural products and also many other known biological activities [1,2]. Additionally, a, b-unsaturated amides are useful synthetic intermediates for various potent organic transformations as well [3]. Many methodologies for their preparation have been developed in recent years, because of their recognized potent applications in pharmaceuticals and synthetic chemistry [4]. The traditional method for preparing a,b-unsaturated amides was based on reacting of amines with a, b-unsaturated carboxylic acid in the presence of activating reagents [5]. Recently, Martin and co-workers developed a novel protocol for the hydroamidation of alkynes with isocyanates recently. Alkyl bromides have been used as hydride sources here. The method turns parasitic b-hydride elimination into a strategic advantage, rapidly affording a,b-unsaturated amides in good yields [6]. Transition-metal catalyzed carbonylation procedures based on using alkenes and alkynes as the substrates have also been developed for a,b-unsaturated amides preparation [7–9]. Alper’s group described a chemo- and regioselective direct aminocarbonylation of alkynes and aminophenols to form hydroxy-substituted a,bunsaturated amides in good to excellent yields [8]. Our group developed a procedure on palladium-catalyzed selective oxidative ⇑ Corresponding author. E-mail address: [email protected] (X.-F. Wu). https://doi.org/10.1016/j.jcat.2020.01.017 0021-9517/Ó 2020 Elsevier Inc. All rights reserved.
aminocarbonylation of styrenes with nitroarenes for the synthesis of a,b-unsaturated amides [9]. Additionally, palladium-catalyzed aminocarbonylation of enol triflates or iodoalkenes has been proven as an potent procedure for a,b-unsaturated amides synthesis as well [10]. Although significant progresses have been achieved in a,b-unsaturated amides synthesis, new procedure is always attractive as it can give the possibility to start from different substrates. Herein, we report a new method for the synthesis of a,b-unsaturated amides which is achieved by palladiumcatalyzed carbonylation of aryl azides and alkenylaluminum reagent. The initial investigation was carried out with azidobenzene and (E)-diisobutyl(oct-1-en-1-yl)aluminum which was prepare from oct-1-yne with diisobutylaluminum hydride (DIBAL-H) as the substrates. To our delight, the target (E)-N-phenylnon-2-enamide can be successfully obtained in 73% yield with Pd/C as the catalyst and XPhos as the ligand in THF at 80 °C for 16 h under CO pressure (Table 1, entry 1). The yield of the target product can be further improved by using Pd(OAc)2 as the catalyst (Table 1, entry 2). Subsequently, with Pd(OAc)2 as the catalyst, different phosphine ligands were tested (Table 1, entries 3–8). XPhos was found still to be the best ligand for this reaction, which may due to its promising properties on electron donation and sterically hindrance. Then a set of solvents were tested, the reaction efficiency decreased significantly in these cases and THF is the optimal solvent (Table 1, entries 9–11). The reaction temperature was varied as well;
161
B. Chen, X.-F. Wu / Journal of Catalysis 383 (2020) 160–163 Table 1 Selected results of the optimization of the reaction conditions.a
a b c d e
Entry
Catalyst
Ligand
Solvent
Yield %b
1 2 3 4 5 6 7 8 9 10 11 12d 13e
Pd/C Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2
XPhos XPhos PPh3 DPPP DPEphos DPPM DPPE Xantphos XPhos XPhos XPhos XPhos XPhos
THF THF THF THF THF THF THF THF Toluene 1,4-dioxane DCE THF THF
73 82(76c) trace trace 65 23 42 77 21 53 7 65 75
Reaction conditions: 1a (0.5 mmol), 2a (0.7 mmol in hexane), catalyst (2 mol %), ligand (2 mol %), solvent (1.0 mL), 10 bar CO, 80 °C, 16 h. NMR yield. Isolated yield. 60 °C. 100 °C.
inferior results were observed at 60 °C or 100 °C (Table 1, entries 12–13). Here, it is also important to mention that 65% yield of the target product can be obtained even under 1 bar of CO. However, due to the low boiling point of THF, we decided continual our studies with 10 bar of CO in order to avoid the leaching of solvent. With the optimal reaction conditions in hand, we explored the substrates scope of this carbonylative reaction. At first, we tested the generality of substituted phenyl azide substrates. As shown in Scheme 1, azidobenzene with electron-donating or -withdrawing substituents can be well tolerated, and delivering the desired products in good to excellent yields (3aa-3ia, 3la3na). Then, we tried substrates with functional groups including alkenyl, cyano, which may be incompatible with aluminum reagents. Fortunately, for these compounds, the reaction could also give the expected products in 64% and 84% yields, respectively (3ja, 3 ka). Notably, phenyl ether-, phenyl sulfide-, phenyl- substituted azidobenzenes were also well tolerated in this transformation and delivered the corresponding a,b-unsaturated amides in 74%, 84% and 76% yields, respectively (3oa-3qa). Additionally, the reaction with 1-azidonaphthalene also proceeded smoothly and gave the target products in 75% yield (3ra). Subsequently, different types of alkenylaluminum reagents for this reaction were tested (Scheme 2). Several aliphatic alkenylaluminums were prepared from the corresponding aliphatic alkynes with DIBAL-H proceeded well under our conditions to afford the desired products in good to excellent yield (3ab-3ah). Interestingly, diene aluminum compound can be successfully transformed in this reaction and gave the target product in 76% yield (3ai). Aromatic alkenylaluminums can also work well in this reaction, good yields of the corresponding products can be isolated (3aj-3am). In the case of using indole-substituted alkenylaluminum as the substrate, 54% of the desired product was isolated (3an). Concerning the reaction pathway, on the basis of our results and the literatures,[11] a possible reaction mechanism is proposed and shown in Scheme 3. First, from azidobenzene as the substrate, the palladium-nitrene species A is formed simultaneously with the release of N2. Subsequent insertion of CO into A occurred to give the intermediate B. After reductive elimination, the isocyanate will be afforded and the released palladium species which is ready for
Scheme 1. Synthesis of a,b-Unsaturated Amides from Aryl Azides. Reaction conditions: aryl azides (0.5 mmol), alkenylaluminum reagent 2a (0.7 mmol in hexane), Pd(OAc)2 (2 mol %), XPhos (2 mol %), THF (1.0 mL), 10 bar CO, 80 °C, 16 h, isolated yield.
the next catalytic cycle. Finally, the reaction of isocyanate with alkenylaluminum reagent produces the desired final product 3aa. In conclusion, we have developed an interesting procedure for the synthesis of a,b-unsaturated amides started from aryl azides
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S. Buchholz, and S. Schareina (all in LIKAT) is gratefully acknowledged. General procedure Under an Argon atmosphere, a 4 mL screwcap vial was charged with Pd(OAc)2 (2 mol %), XPhos (2 mol %), azidobenzene (0.5 mmol), THF (1 mL) and an oven-dried stirring bar, then injected alkenylaluminum solution which are prepared from the corresponding alkynes with DIBAL-H. The vial was closed by a Teflon septum and a phenolic cap and connected to the atmosphere through a needle. Then the vial was fixed in an alloy plate and put into Parr 4560 series autoclave (300 mL). At room temperature, the autoclave is flushed with carbon monoxide for three times and 10 bar of carbon monoxide was charged. The autoclave was placed on a heating plate equipped with magnetic stirring and an aluminum block. The reaction was heated at 80 oC for 16 hours. Afterwards, the autoclave was cooled to room temperature and the pressure carefully released. After removal of solvent under reduced pressure, pure product was obtained by column chromatography on silica gel (eluent: heptane/ethyl acetate 5:1). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2020.01.017. Scheme 2. Synthesis of a,b-Unsaturated Amides from Alkenylaluminum reagents. Reaction conditions: phenyl azides (0.5 mmol), alkenylaluminum reagent 2 (0.7 mmol in hexane), Pd(OAc)2 (2 mol %), XPhos (2 mol %), THF (1.0 mL), 10 bar CO, 80 °C, 16 h, isolated yield.
Scheme 3. Proposed reaction mechanism.
and alkenylaluminum reagents. In the presence of palladium catalyst under carbon monoxide pressure, the desired a,b-unsaturated amides were isolated in good to excellent yields with good functional group tolerance. Remarkably, this work also represents an example on the addition of organometallic reagent to isocyanates. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment C.B. thanks the Chinese Scholarship Council (CSC) for financial support. The analytical support of Dr W. Baumann, Dr C. Fisher,
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