DFT study on the mechanism of palladium(0)-catalyzed reaction of o-iodoanilines, CO2, and CO
Abstract
The reaction mechanism of palladium(0)-catalyzed reaction of o-iodoanilines, CO2, and CO has been studied theoretically. The calculations suggest that the reaction proceeds via C(sp2)
I bond oxidative addition, CO insertion, CsI·OAc cluster anion formation, NHH bond activation, ligand exchange of one PPh3 ligand with CsI·HOAc cluster, CO2 insertion, and C(sp2)O bond reductive elimination steps. The CO2 insertion involves the rate-determining transition state with a free energy barrier of 18.4 kcal/mol, consistent with the experimental reaction condition (60 °C). The influence of solvent on the product yield was analyzed. It is found that the CO2 insertion occurs with the coordination of PPh3 ligand in THF while it needs the help of base CsOAc in toluene. The role of bases KOAc and NaOAc has been calculated and compared with CsOAc. The stability of the rate-determining intermediate increases in the order of Cs < K < Na, resulting in the increase of the reaction energy barrier.
Graphical abstract
Introduction
Utilization of carbon dioxide (CO2) [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]] or carbon monoxide (CO) [[15], [16], [17], [18], [19], [20]] as a sole C1 resource in chemical synthesis has attracted extensive research interest, because CO2 or CO is abundantly available, inexpensive, and renewable. In this regard, considerable efforts have been devoted toward the development of various methodologies for converting CO2 or CO into value-added chemical products [4,[21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]]. On the one hand, transition-metal catalyzed carboxylation using CO2 is among the most straightforward approaches for CO2 conversion. In the past decades, lots of protocols have been established to achieve carboxylation of various organic substances using CO2 as carboxylative reagent [[22], [23], [24], [25], [26]]. On the other hand, in recent years, many reactions couple CO with some compounds of transition metals to prepare ketone compounds [[17], [18], [19]].
Isatoic anhydrides [[39], [40], [41], [42], [43], [44], [45], [46], [47], [48]] as important structural motifs and fundamental building blocks, are broadly employed in various fields such as medicinal chemistry, and RNA structure probing chemistry [[49], [50], [51]]. Recently, Lu and co-workers have developed a mild and efficient Pd(0)-catalyzed easily available o-iodoanilines, CO2, and CO to prepare a variety of isatoic anhydrides [52]. The reaction proceeds at 60 °C in tetrahydrofuran (THF) with an equivalent of base CsOAc. For the reaction of o-iodoaniline S1 with CO S2 and CO2 S3 (Eq. 1), the yield of product P decreases from 99 % to 64 %, when the solvent THF was replaced by toluene. In addition, it is found that the relatively high yield (99 % and 94 %) of product P was obtained for base CsOAc and KOAc, respectively, while NaOAc gives the relatively low yield (46 %) of product P. The possible mechanism was proposed by authors that the reaction proceeds via oxidative addition, CO insertion, NH activation, the CO2 insertion, and subsequent reductive elimination to give the isatoic anhydrides. In addition, the reaction mechanism involving the six-membered palladium carbamate may be possible.
Although the possible reaction mechanism was proposed in experiments [52], some basic issues have not been well understood. In this work, the detailed reaction mechanism for reaction of o-iodoaniline S1, with CO S2, and CO2 S3 (Eq. 1) to generate the product isatoic anhydrides P will be calculated with the help of density functional theory methods. We embark on elucidating the questions which catalytic mechanism is feasible by means of comparing potential energy surfaces, and which step is the rate-determining one. The reaction mechanisms will be closely elaborated in main text step by step. The further theoretical investigation will reveal the origin of the difference of yields. Through the detailed comparison, we hope to provide a mechanistic understanding of how the isatoic anhydrides reaction is achieved in this system and help to design more efficient catalysts.
Section snippets
Computational details
Molecular geometries of all complexes were optimized at the B3LYP level [[53], [54], [55], [56]] of density functional theory. The effective core potential (ECP) of Hay and Wadt with double-ζ LANL2DZ [57,58] was chosen to describe Pd, P, I, and Cs atoms. The polarization functions [59] were added: Pd (ζf = 1.472), P (ζd = 0.387), I (ζd = 0.289), Cs (ζd = 0.306). For other main group atoms C, O, N, H, Na, and K, the 6−31 g(d,p) basis set was used. Frequency calculations at the same level of
Results and discussion
The catalytic cycle proposed by the experimental authors was shown in Scheme 1 including two pathways. One pathway involved oxidative addition, CO insertion, CsOAc-assisted N–H activation, CO2 insertion, and reductive elimination steps. Another pathway involved the formation of six-membered palladium carbamate G through oxidative addition, CsOAc-assisted N–H activation, and CO2 insertion, followed by CO insertion and reductive elimination steps. Besides these two pathways, we speculate another
Conclusions
The reaction mechanism of palladium-catalyzed o-iodoaniline, CO2, and CO to form isatoic anhydride has been theoretically investigated by using density functional theory calculations. Four reaction mechanisms have been calculated. The difference between these pathways is the order of CsOAc combination and CO insertion. It is found that the experimental reaction mechanism is preferred involving the following steps: C(sp2)I bond oxidative addition of o-iodoaniline, CO insertion into Pd-C(sp2)
CRediT authorship contribution statement
Bing-wen Li: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Mei-yan Wang: Resources, Writing - review & editing, Supervision, Data curation. Jing-yao Liu: Writing - review & editing.
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.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grants 21773083 and 21203073). The authors are grateful to Computing Center of Jilin Province for essential support.
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