Mechanism and origins of ligand-controlled Pd(II)-catalyzed regiodivergent carbonylation of alkynes

Jian-Biao Liu *, Xin Zhang , Ying-Ying Tian , Xin Wang , Xun-Kun Zhu and De-Zhan Chen *
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Institute of Molecular and Nano Science, Shandong Normal University, Jinan 250014, P. R. China. E-mail: jianbiaoliu.thu@gmail.com; chendzh@sdnu.edu.cn

Received 13th August 2019 , Accepted 3rd September 2019

First published on 4th September 2019


Transition-metal-catalyzed carbonylation provides a useful approach to synthesize carbonyl-containing compounds and their derivatives. Controlling the regio-, chemo-, and stereoselectivity remains a significant challenge and is the key to the success of transformation. In the present study, we explored the mechanism and origins of the ligand-controlled regiodivergent carbonylation of alkynes with competitive nucleophilic amino and hydroxy groups by density functional theory (DFT) calculations. The proposed mechanism involves O(N)-cyclization, CO insertion, N–H(O–H) cleavage, C–N(C–O) reductive elimination and regeneration of the catalyst. The chemoselectivity is determined by cyclization. Instead of the originally proposed switch of competitive coordination sites, a new type of concerted deprotonation/cyclization model was proposed to rationalize the ligand-tuned chemoselectivity. The electron-deficient nitrogen-containing ligand promotes the flow of electrons during cyclization, and so it favors the O-cyclization/N-carbonylation pathway. However, sterically bulky and electron-rich phosphine controls the selectivity by a combination of electronic and steric effects. The improved mechanistic understanding will enable further design of selective transition-metal-catalyzed carbonylation.


Introduction

After over a half-century of developments, the transition-metal-catalyzed carbonylation reaction has achieved significant progress.1 This reaction, especially employing carbon monoxide as the C1 source, has emerged as one of the most efficient protocols for the synthesis of carbonyl-containing compounds and their derivatives.2–4 Nowadays, despite advances in this field, developing regio-, chemo-, and stereoselective carbonylation reactions remains a significant challenge.5 Various ligand-controlled regio- and chemodivergent carbonylation reactions have been recently developed,6–9 and the majority of research concerning chemoselectivity is mainly based on the substrates bearing two or more nucleophilic sites.10,11

By comprehensive utilization of the electronic and steric properties of ligands, several regiodivergent cyclizations have been reported by Jiang's group.12–14 In a very recent study, they reported an intriguing example of the ligand-controlled Pd(II)-catalyzed regiodivergent carbonylation of alkynes for the syntheses of benzofuro[3,2-c]quinolinones and indolo[3,2-c]coumarins, which are important skeletons in many biologically active molecules.15 As shown in Scheme 1a, the nitrogen-containing 1,10-phenanthroline-5,6-dione (L1) facilitates the formation of benzofuro[3,2-c]quinolinone 2, while indolo[3,2-c]coumarin 3 was generated preferentially by using bidentate phosphine L2 (dppm = 1,1-bis(diphenylphosphino)methane). In substrate 1, there exists competitive coordination between the amino and hydroxy groups with catalysts. It was originally proposed that the coordination with electron-deficient L1 causes the palladium center to become more electrophilic and thus coordinate preferentially with the amino group, which promotes O-attack/N-carbonylation cyclization (Scheme 1b). In contrast, electron-rich and sterically bulky L2 favors coordination with the hydroxy group rather than the amino group, thus facilitating N-attack/O-carbonylation cyclization.


image file: c9dt03294k-s1.tif
Scheme 1 (a) Pd-catalyzed carbonylation of alkynes and (b and c) the selectivity-determining mechanism.

Considering that the detailed mechanism of the Pd-catalyzed carbonylation especially the origins of the ligand-controlled chemo- and regioselectivity remain elusive, we herein report a mechanistic study of the title reaction by means of density functional theory (DFT) calculations. Instead of the originally proposed switch of competitive coordination sites, our calculations indicate that the chemoselectivity is controlled by a new type of concerted deprotonation/cyclization. More specifically, the selectivity is mainly controlled by the electronic effect of nitrogen-containing ligand L1, while both the electronic and steric effects control the selectivity with bidentate phosphine L2 (Scheme 1c).

Computational details

Geometry optimizations were performed in acetonitrile using the Gaussian 09 suite of programs16 at the B3LYP17/6-31G(d,p)/ECP28MDF-AVDZ(Pd) level of theory with the corresponding effective core potential (ECP) on Pd.18 Frequency calculations were performed at the same theoretical level to verify the nature of the stationary points and to obtain thermal Gibbs free energy correction at 298.15 K. Intrinsic reaction coordinate (IRC) calculations were conducted to ensure that the transition state connects the corresponding reactant and the product. To refine the electronic energy, single point calculations with B3LYP-D3(BJ)19 and larger 6-311++G(2d,2p) and ECP28MDF-AVTZ(Pd) basis sets were carried out in the solvent. The continuum solvation model SMD20 was utilized to incorporate the bulk solvent effects. The reported free energies at the standard state of 1 M include the electronic energy at B3LYP-D3(BJ)/6-311++G(2d,2p)/ECP28MDF-AVTZ(Pd) in solution (including the solvation free energy correction), the dispersion correction and the thermal correction. Natural population analysis (NPA) charges were calculated by the GenNBO 5.0 program21 using the wavefunction obtained at the B3LYP/6-31G(d,p)/ECP28MDF-AVDZ(Pd) level. Iso-chemical shielding surfaces (ICSSs)22 were generated by Multiwfn.23 The 3D diagrams of molecules were prepared by CYLview.24 The steric contour plots were generated by the web based SambVca 2.0 program.25

Results and discussion

Proposed catalytic cycle

In the absence of coordinating ligands, the most stable form of palladium(II) trifluoroacetate (TFA) is the trimeric complex [Pd(TFA)2]3,26,27 the crystal structure of which has been carefully analyzed.28 [Pd(TFA)2]3 may be dissociated via ligand exchange and the formation of a square planar complex Cat1 has large thermodynamic driving force (eqn (1)). We thus chose stable Cat1 as the active species and also the starting point to investigate the catalytic mechanism. The proposed mechanism of Pd(II)-catalyzed carbonylation of alkynes is shown in Scheme 2 and the divergence occurs at the
 
image file: c9dt03294k-u1.tif(1)
very beginning. Starting from [Pd(L1)(TFA)2], the two possible cyclization steps lead to intermediates with different heterocycles. The sequential CO insertion, N–H/O–H cleavage and reductive elimination generate the product-coordinated Pd(0) complexes. Finally, oxidation of Pd(0) to Pd(II) by benzoquinone (BQ) regenerates the catalyst.

image file: c9dt03294k-s2.tif
Scheme 2 Proposed mechanism of Pd(II)-catalyzed carbonylation of alkynes.

Free energy profiles of the O-cyclization/N-carbonylation pathway

We first investigated the mechanism of Pd-catalyzed carbonylation of 1 with L1 to elucidate the overall catalytic cycle and the rate- and selectivity-determining steps. The free energy profiles of the O-cyclization/N-carbonylation pathway are shown in Fig. 1 and the optimized structures of selected intermediates and transition states are illustrated in Fig. 2. Cat1 first undergoes ligand exchange with substrate 1 to release one TFA, generating complex 4. We also considered the amino N- and hydroxy O-coordination modes, while the corresponding isomers are less stable than 4 by 5.5 and 16.4 kcal mol−1 respectively (see Table S2), indicating that the originally proposed coordination patterns in Scheme 1b are less favorable. Concerted O–H deprotonation/nucleophilic cyclization of 4 occurs readily with an energy barrier of 10.6 kcal mol−1. During this process, the O–H bond is lengthened from 1.04 Å in 4 to 1.28 Å in TS1. The external TFA plays an important role in the cyclization since it facilitates the O–H bond cleavage via hydrogen bonding, thus resulting in enhanced nucleophilicity of oxygen. To the best of our knowledge, the kinds of hydrogen bonding interactions during the cyclization in TS1 have not been explored previously. It should be pointed out that we failed to locate the cyclization transition state without external TFA.
image file: c9dt03294k-f1.tif
Fig. 1 Free energy profiles of [Pd(L1)(TFA)2]-catalyzed carbonylation via the O-cyclization/N-carbonylation pathway.

image file: c9dt03294k-f2.tif
Fig. 2 Optimized structures of selected intermediates and transition states in the O-cyclization/N-carbonylation pathway (bond distances are given in Å).

Dissociation of HTFA from 5 and the following coordination with CO forms a stable complex 7, which makes the concerted deprotonation/cyclization irreversible. In 7, the L1 ligand becomes monodentate. The subsequent CO insertion viaTS2 and isomerization generate a more stable intermediate 9, in which the amino group coordinates with the metal center to get prepared for the following cyclization. Intermediate 9 then undergoes N–H activation via carboxylate-assisted concerted metalation–deprotonation (CMD) transition state TS3, and the subsequent C–N reductive elimination viaTS4 eventually produces product-coordinated complex 12. The last step of the catalytic cycle is catalyst regeneration via oxidation of Pd(0) to Pd(II) by BQ.29 This specific process occurs by two steps of protonation and the corresponding Gibbs free energy changes are given in Fig. S1. On the basis of the calculated free energy changes of the whole catalytic cycle, the rate-limiting step of the O-cyclization/N-carbonylation pathway is the C–N reductive elimination with an overall barrier of 21.7 kcal mol−1 (9 to TS4). This overall barrier is consistent with the experimental conditions (room temperature and 3 h). Chemoselectivity is determined by the first step, namely the concerted deprotonation/cyclization.

The protonation of 5 may occur as a competitive side reaction, delivering the benzofuran product. As shown in Fig. 3, 5 first isomerizes to intermediate 14, in which HTFA comes close to the metal center. The subsequent protonation viaTS5 is facile and reversible, which produces benzofuran and regenerates Cat1. However, CO insertion also occurs readily to generate a more stable intermediate 9. Therefore, the protonation of the O-cyclization intermediate is suppressed thermodynamically and CO insertion occurs and eventually affords benzofuro[3,2-c]quinolinone 2, which is consistent with experimental observations.15


image file: c9dt03294k-f3.tif
Fig. 3 Free energy profiles of the protonation of intermediate 5.

Free energy profiles of the N-cyclization/O-carbonylation pathway

The free energy changes of the other competing pathways are compared in Fig. 4 and the optimized structures are given in Fig. 5. This reaction pathway has similar elementary steps to those mentioned above. First, Cat1 undergoes ligand exchange with substrate 1 to generate 4′, which differs from 4 in the orientation of phenol and aniline groups. The concerted N–H deprotonation/cyclization is irreversible and requires 13.4 kcal mol−1 from Cat1. The irreversible deprotonation/cyclization again suggests that the transition state of this step determines the chemoselectivity. Comparing the two competing pathways, the N–H deprotonation/cyclization transition state TS1′ is 2.8 kcal mol−1 less favorable than the O–H deprotonation/cyclization transition state TS1. This result agrees well with experimental observations that only product 2 was obtained when using ligand L1. The subsequent CO insertion and O–H cleavage are both facile with small energy barriers, which are similar to the results in Fig. 1. C–O reductive elimination of 10′viaTS4′ eventually produces product-coordinated complex 12′. C–O reductive elimination requires a higher barrier (29.1 kcal mol−1, 10′ to TS4′) as compared with C–N reductive elimination (21.7 kcal mol−1, 9 to TS4).
image file: c9dt03294k-f4.tif
Fig. 4 Free energy profiles of [Pd(L1)(TFA)2]-catalyzed carbonylation via the N-cyclization/O-carbonylation pathway.

image file: c9dt03294k-f5.tif
Fig. 5 Optimized structures of selected intermediates and transition states in the N-cyclization/O-carbonylation pathway (bond distances are given in Å).

During the first cyclization step, the resulting N-cyclization intermediate 5′ is 10.9 kcal mol−1 more stable than the O-cyclization intermediate 5. The energy difference can be understood by the different aromaticity within the heterocycle. For simplicity, we chose benzofuran and indole as model complexes and used ICSSs to present an intuitive picture of aromaticity (Fig. 6). The blue isosurfaces covering the benzo-fused five-membered heterocycles imply the strong aromaticity of both benzofuran and indole. However, the aromaticity of the N-heterocycle in indole is stronger than that of the O-heterocycle in benzofuran, as indicated by the area of the corresponding blue isosurfaces. Therefore, N-cyclization is thermodynamically more favorable due to the relatively strong aromaticity of the forming heterocycle.


image file: c9dt03294k-f6.tif
Fig. 6 ICSSs for benzofuran and indole (isovalue = 10 ppm).

Comparing the net C–N reductive elimination (11TS412) and C–O reductive elimination (11′TS4′12′), the former requires a barrier of 5.9 kcal mol−1 and is exergonic by 17.8 kcal mol−1, while the latter requires a barrier of 21.6 kcal mol−1 and is exergonic by only 1.8 kcal mol−1, indicating that the C–N reductive elimination is both kinetically and thermodynamically more favorable. We herein used the schematic correlation diagram for the frontier molecular orbitals to explain the barrier difference. As indicated in Fig. 7a, the activation energy of C–N/C–O reductive elimination is mainly caused by the destabilization of the antisymmetric bonding σ-orbital. As reductive elimination occurs, the σ-orbital will transfer from the ligand-based orbital to the metal-based orbital and the electronic configuration of palladium changes concomitantly from d8 to d10. For 11TS4 and 11′TS4′, the energies of the σ-orbital increase by 13.2 kcal mol−1 and 21.7 kcal mol−1 respectively, which agree well with the corresponding activation free energies. The result that C–N reductive elimination is significantly more exergonic than C–O reductive elimination can be attributed to the stronger C–N bond compared with the C–O bond. The bonding difference between the forming C–N and C–O bonds in the products is well revealed by the calculated Wiberg bond index (WBI; 1.07 for C–N in 2 and 0.92 for C–O in 3).


image file: c9dt03294k-f7.tif
Fig. 7 (a) Correlation diagram for the frontier molecular orbitals involved in reductive elimination and (b) the energy changes of the σ-orbital for 11TS4 and 11′TS4′.

Origins of ligand-controlled regiodivergent carbonylation

Based on the aforementioned mechanistic understanding, the chemoselectivity is accurately tuned by the initial cyclization step. We next examined the two competing cyclizations with L2 to illustrate the ligand effect. Compared with the results of L1, a remarkable difference is that the formation of substrate-coordinated complex 4-L2 becomes exergonic (Fig. 8a), while the formation of 4′-L2 is still endergonic. A careful examination of the optimized structures of 4-L2 and 4′-L2 reveals that the main difference lies in the position of the outer-sphere OTf, which is directed by the orientation of O–H/N–H⋯OTf hydrogen bonding (Fig. 9). In 4-L2, the OH group points outward so there are less steric interactions between the bulky dppm ligand and OTf. However, the orientation of the N–H bond in 4′-L2 makes the formation of hydrogen bonds with OTf difficult because of the steric repulsion between dppm and OTf. This structural difference accounts for the energy difference during the generation of the two substrate-coordinated complexes.
image file: c9dt03294k-f8.tif
Fig. 8 (a) Free energy profiles of competing cyclization with L1 and L2; (b) NPA charges in 1 and the four transition states; (c) electrostatic potential (ESP) surfaces of the four transition states (units in a.u.); and (d) steric contour maps of TS1-L2 and TS1′-L2.

image file: c9dt03294k-f9.tif
Fig. 9 Optimized structures of 4-L2 and 4′-L2. Irrelevant hydrogen atoms are omitted for clarity.

For L2, the concerted O–H deprotonation/cyclization requires a barrier of 15.8 kcal mol−1 (4-L2 to TS1-L2), while the competing concerted N–H deprotonation/cyclization requires a barrier of only 8.1 kcal mol−1. Therefore, the chemoselectivity is reversed by switching ligand L1 to L2. We adopted the “arrow-pushing” schemes (Fig. 8b) to illustrate how the electronic variations of ligands affect the two competing cyclization processes.30 Electron-deficient L1 apparently facilitates the flow of electrons during cyclization. In this case, the O-cyclization viaTS1 is more favorable since the interaction between the more negative oxygen and carbon is stronger, which is well revealed by the calculated NPA charges. For L2, the ligand becomes electron-rich (see the ESP surfaces in Fig. 8c), which decreases remarkably the charge of the palladium center. Compared with the results of L1, a notable decrease in the NPA charge of O in TS1-L2 was observed, while the charge of N in TS1′-L2 remains unchanged. As a consequence, the two cyclization transition states with L2 should be comparable from the viewpoint of electronic effects. The fact that TS1′-L2 is more favorable as compared to TS1-L2 indicates that the steric interactions between the ligand and the substrate also play important roles in affecting the chemoselectivity.

To provide a clearer illustration of the ligand–substrate interactions, we plotted the steric contour maps31–34 of the bisphosphine ligands for the cyclization transition states with L2. The steric contour maps are derived from the van der Waals surface of the ligand in the transition state. The palladium atom is placed at the origin of the coordinate system and the z-axis is defined by the palladium atom and the midpoint of the two P atoms of L2. Then the van der Waals surface of the ligand is built up and the contour line of zero is drawn through all points on the van der Waals surface having the same z coordinate as the Pd atom. The positive contour lines (coloured in yellow and red) indicate regions on the ligand van der Waals surface having a positive z coordinate, i.e., closer to the half-space containing the substrate. Thus, more significant ligand–substrate repulsion is expected if the substrate is located in the yellow or red region. As indicated in Fig. 8d, the substrate in TS1′-L2 is placed away from the ligand while the substrate in TS1-L2 is closer to the ligand. Therefore, less steric interactions between the substrate and L2 are expected in TS1′-L2.

Conclusions

In summary, the mechanism and origins of the ligand-controlled Pd-catalyzed regiodivergent carbonylation of alkynes have been elucidated by DFT calculations. The competitive nucleophilic amino and hydroxy groups existing in the substrate lead to two possible reaction pathways. The proposed catalytic cycle involves O(N)-cyclization, CO insertion, N–H(O–H) cleavage, C–N(C–O) reductive elimination and oxidation of Pd(0) by BQ. The reductive elimination is the rate-limiting step of the overall reaction and the irreversible cyclization determines the chemoselectivity. Interestingly, the initial nucleophilic cyclization occurs concertedly with deprotonation by base, which is unexplored previously. The C–N reductive elimination is both kinetically and thermodynamically more favorable than the C–O reductive elimination, which correlates well with the destabilization of the antisymmetric bonding orbital and the bonding difference between the forming C–N and C–O bonds.

Our calculations well reproduced the experimentally observed ligand-controlled selectivity. For both nitrogen-containing L1 and phosphine L2, N-cyclization is significantly more exergonic due to the relatively strong aromaticity of the forming heterocycle. However, the chemoselectivity is kinetically tuned by the initial cyclization step with different ligands. Since an electron-deficient ligand promotes the flow of electrons during cyclization, ligand L1 thus favors O-cyclization/N-carbonylation leading to benzofuro[3,2-c]-quinolinones. In contrast, the sterically bulky and electron-rich ligand L2 disfavors this pathway due to the electronic effects and the steric interactions between the substrate and the ligand. These mechanistic insights will be helpful for the design of new ligands for controlling the chemo- and regioselectivity reactions of substrates with competitive groups.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC No. 21601110) and the Natural Science Foundation of Shandong Province (ZR2019YQ11).

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Footnote

Electronic supplementary information (ESI) available: Additional computational results and Cartesian coordinates of all optimized structures. See DOI: 10.1039/C9DT03294K

This journal is © The Royal Society of Chemistry 2019