DFT computational insight into Pd(0)-catalyzed oxidative cross-coupling of 1,2-allenyl ketones and aryl boronic acids: Pd(II)-carbenoid intermediate versus π-allyl-Pd(II) intermediate

Abstract

DFT calculations unveil that the Pd(0)-catalyzed oxidative cross-coupling of 1,2-allenyl ketones with aryl boronic acids may either occur via a Pd(II)-carbenoid intermediate or a π-allyl-Pd(II) intermediate, depending on the substituents on 1,2-allenyl ketones. The substituted substrate prefers to undergo the π-allyl-Pd(II) intermediate mechanism over the Pd(II)-carbenoid intermediate one, while the unsubstituted one reverses the preference. The calculations also show a new mechanism for the transformation of Pd(0) to the Pd(II) complex and rationalize the reason for the non-excellent yield for the unsubstituted 1,2-allenyl ketone.

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1. Introduction

Furans, one of the most important aromatic compounds, are widely existing in a variety of natural products and synthetic medicines.^1–4 Moreover, furan-based functional molecules have potential applications in materials science, especially in the field of furan-based semiconductors for energy conversion and storage.⁵ The past decades have witnessed great advancements in furan syntheses using various methods,^6–10 in which the transition-metal-catalyzed cycloisomerization of 1,2-allenyl ketones has attracted remarkable attention^11–15 due to their availability^16,17 (Scheme 1a). The efficient syntheses of furan derivatives via the intramolecular nucleophilic cyclization of 1,2-allenyl ketones were initially reported by Marshall¹³ and Hashmi et al.¹⁴ (Scheme 1a with G = H, M = Rh^I, Ag^I, or Pd^II) and subsequently developed by Gevorgyan et al.¹⁵ (Scheme 1a with G = alkyl, aryl, halide, silyl, etc., M = Au^I, Ag^I, Cu^I, etc.) using various transition-metal catalysts. It is believed that the cycloisomerization reactions in these pioneering studies occur via a cyclic metal-carbene intermediate mechanism.^18–20

Scheme 1 Transition-metal-catalyzed intramolecular cycloisomerization (a) and intermolecular cross-coupling of 1,2-allenyl ketones with aryl halides (b) or aryl boronic acids (c) to synthesize furans.

Another important methodology for the furan synthesis from 1,2-allenyl ketones using catalysis by transition metals, especially palladium(0) species, is either via their intermolecular cross-coupling reactions with electrophiles, such as aryl halides,²¹ or via their intermolecular oxidative cross-coupling reactions with nucleophiles, for example, aryl boronic acids in the presence of suitable oxidants.^22,23 Ma and co-workers²⁴ reported Pd(0)-catalyzed coupling cyclization reactions of 1,2-allenyl ketones with aryl halides to efficiently and conveniently synthesize poly-substituted furans (Scheme 1b). In 2014, Wang et al.²⁵ developed palladium-catalyzed oxidative cross-coupling of 1,2-allenyl ketones with aryl boronic acids (Scheme 1c), which provides a complementary strategy for highly substituted furans, especially tetra-substituted furans.

To account for these intra- and intermolecular cross-coupling reactions, the previous reports have proposed two different mechanistic pathways, which can be referred to as the metal-carbene intermediate mechanism^18–20 and the π-allyl-metal intermediate mechanism.^26,27 Bao et al.²⁸ have reported an elegant DFT-based computational study on the cross-coupling reaction of 1,2-allenyl ketones with iodobenzene, one of the most commonly used aryl halides. It is found that the viable mechanistic pathway depends on the substituents on either 1,2-allenyl ketones or iodobenzene. Here, we were intrigued by the oxidative cross-coupling reaction of 1,2-allenyl ketones with aryl boronic acids developed by Wang et al.,²⁵ as exemplified by the two reactions in Scheme 1c. According to the experimental results,²⁵ it is not sure whether the reaction proceeds via the metal-carbene intermediate mechanism (pathway I in Scheme 2) or via the π-allyl-metal complex mechanism (pathway II Scheme 2). In other words, exactly how the furan product is formed is uncertain. Moreover, the experimental observations of the different yields using substituted and unsubstituted 1,2-allenyl ketones (excellent vs. good) are still not understood well. These dilemmas prompted us to study the two representative reactions in Scheme 1c by performing DFT calculations. We expect that the calculated results would give a better understanding of the Pd(0)-catalyzed furan synthesis from the oxidative cross-coupling of 1,2-allenyl ketones with aryl boronic acids and provide enlightenment for the design of new catalytic cycles.

Scheme 2 Pd(II)-Carbenoid intermediate mechanism (pathway I) versus π-allyl-Pd(II) intermediate mechanism (pathway II) for Pd(0)-catalyzed oxidative cross-coupling of 1,2-allenyl ketones with aryl boronic acids to form furans.

2. Computational methods

All of the DFT calculations were carried out with the Gaussian 09 program package.²⁹ The geometry optimizations of all stationary points were performed in the gas phase using the M06 functional³⁰ with the LANL2DZ basis set^31,32 for the palladium atom and the 6-31G(d) basis set³³ for the other atoms. The vibrational frequencies were computed at the same level to verify that the optimized structures correspond to minima (no imaginary frequency) or transition states (one imaginary frequency) and to provide the thermochemical corrections to Gibbs free energies at 298 K. Intrinsic reaction coordinate (IRC)^34,35 calculations were used to identify that each transition state connects to its corresponding minima. Solvent effects were considered using the SMD³⁶ continuum solvation model (in toluene) to improve the accuracy of the calculated relative Gibbs energies. Single-point energies were obtained at the M06 levels with the larger SDD basis set³⁷ for palladium and the 6-311++G(d,p) basis set for the other atoms. The Gibbs free energy of each species is obtained by adding the thermal correction obtained in the gas phase to the single-point energy. It should be noted that for reactions in solvent, in particular for situations involving the change of component numbers, thermal corrections based on the ideal gas phase model remarkably overestimate entropy contributions to free energies due to the neglect of the suppressing effect of the solvent on the rotational and transitional freedoms of species.^38–40 In the present study, to more accurately evaluate the contribution of entropy in solution, we employed the approach proposed by Martin et al.,⁴¹ a correction value of (n–m) × 4.3 kcal mol⁻¹ applies to a transformation from m- to n-components at 298.15 K and 1 atm. The value of 4.3 kcal mol⁻¹ is obtained by considering the actual conditions of a solution reaction medium in the translational entropy. This approximate approach of Martin et al.⁴¹ has been proven by Holm and Rybak-Akimova⁴² and provides reasonable free energies for a process involving multicomponent change.^40,43 All the final energies presented are so-corrected Gibbs energies, which were computed by adding the corrections of entropy in solution to the Gibbs free energy at 298.15 K. The 3D structures were prepared using CYLView.⁴⁴ All the calculated structures and corresponding energies can be found in the ioChem-BD repository⁴⁵ and can be accessed viahttps://doi.org/10.19061/iochem-bd-6-183.

3. Results and discussion

3.1 Formation of the Pd(II)-aryl complex

According to Scheme 2, the reaction starts from the oxidation of the Pd(0) complex by 1,4-benzoquinone (BQ) to form Pd(II) species A, which is then converted into Pd(II)-aryl complex Bvia transmetalation with aryl boronic acid 2. We first preformed calculations for the so-called Pd(II) species A where Pd bonds to two adjacent C atoms of BQ. However, such a Pd(II) species cannot be located, and upon optimization, it always evolves into structure A′, a coordination complex between Pd(0) and BQ, as indicated by calculated structural parameters (Fig. 1). In this case, the formation of Pd(II)-aryl complex B needs to undergo an oxidative transmetalation process^46,47 instead of the separated oxidation and transmetalation processes proposed in the experiment (Scheme 2). The calculated results for the formation of Bvia a direct oxidative transmetalation process (i.e. the aryl group directly translates to the Pd center) are shown in Fig. 1, where Pd(PPh₃)₂ is used as the active species as proposed in previous research.^48,49 The formation of the initial coordination complex A′ is calculated to be exergonic by 15.8 kcal mol⁻¹. As indicated in Fig. 1, the present calculations consider two potential oxidative transmetalation pathways, which proceed viaTS1 and TS2, respectively. A′ undergoes a direct oxidative transmetalation with aryl boronic acid 2viaTS1, leading to IM1, which then isomerizes into the Pd(II) species B. This process requires crossing a barrier as high as 64.4 kcal mol⁻¹, suggesting that it is not responsible for the formation of B. Another oxidative transmetalation pathway occurs viaTS2 that is similar to TS1 except for the release of a PPh₃ ligand from IM2 upon coordination of aryl boronic acid 2 to give intermediate IM3. The barrier is reduced to 42.9 kcal mol⁻¹, which is still too high to be overcome under the experiment conditions (70 °C).

Fig. 1 Calculated Gibbs energy profile for the formation of Pd(II)-aryl complex Bvia the direct oxidative transmetalation processes. Selected bond distances are in Å.

In addition, we have also performed calculations for two potential pathways assisted by iPr₂NEt. However, as shown in Fig. S1,† these pathways are less favorable than those shown in Fig. 1. Thus, we consider that iPr₂NEt does not play a substantial role in the oxidative transmetalation process.

Alternatively, the energetically viable pathway for the formation of Pd(II)-aryl complex B is shown in Fig. 2. In this pathway, from IM2, the initial oxidative transmetalation occurs viaTS3 with the aryl borate translating instead of the aryl group translating to the Pd center discussed above, leading to Pd(II)-borate species IM4 possessing a protonated η³-quinone ligand. Subsequently, a second boronic acid molecule adds to IM4 through O → B coordination interaction to generate IM5, from which the transmetalation of the aryl group from boron to palladium⁵⁰ takes place viaTS4 to form Pd(II)-aryl species IM6. Then via isomerization of the η³-quinone ligand to p-hydroxyphenoxide and the addition of PPh₃, IM6 evolves into a more stable Pd(II)-aryl species IM7, which then undergoes a σ-bond metathesis of the B–O/O–H bonds viaTS5 to give the Pd(II)-aryl complex B. Other energetically less favorable pathways to form Pd(II)-aryl complex B are shown in Fig. S2 and S3.†

Fig. 2 Calculated energetically viable pathway for the formation of Pd(II)-aryl complex B. Selected bond distances are in Å.

From calculated data shown in Fig. 2, it is observed that A′ is the rate-determining intermediate, and TS4 is the rate-determining transition state. Thus, the overall barrier involved in the formation of the Pd(II)-aryl complex B is estimated to be 32.2 kcal mol⁻¹, which is much lower than those shown in Fig. 1, 42.9 and 64.4 kcal mol⁻¹, respectively.

In addition, we also considered the possibility of the 1,2-allenyl ketone for performing the intramolecular nucleophilic cyclization reaction under the catalysis of the reactive Pd(0) species, Pd(PPh₃)₂ (Fig. S4†). It is found that the barrier of such a process is much higher than that shown in Fig. 2 (39.0 vs. 32.2 kcal mol⁻¹).

3.2 Reaction of substituted 1,2-allenyl ketone 1_a

Our attention first focuses on the reaction of the substituted 1,2-allenyl ketone 1_a (R¹ = Ph, R² = Me), which led to an excellent furan yield (93%) in the experiment of Wang et al.²⁵ The calculated results are summarized in Fig. 3, where the Pd(II)-carbenoid intermediate pathway (red line) and the π-allyl-Pd(II) intermediate pathway (black line) are compared.

Fig. 3 Calculated Gibbs energy profile for the formation of furan 3_a from the cross-coupling of substituted 1,2-allenyl ketone 1_a with the Pd(II)-aryl complex B either via the Pd(II)-carbenoid intermediate pathway (red line) or via the π-allyl-Pd(II) intermediate pathway (black line). Selected bond distances are in Å.

From the Pd(II)-aryl complex B, the substituted 1,2-allenyl ketone 1_a undergoes a ligand exchange with PPh₃ to form species C_a, the common precursor for the Pd(II)-carbenoid intermediate pathway and the π-allyl-Pd(II) intermediate pathway. Along the Pd(II)-carbenoid pathway, C_a undergoes intramolecular cyclization viaTS6_a by the nucleophilic attack of the carbonyl oxygen atom on the terminal carbon atom of the 1,2-allenyl ketone, leading to the cyclic Pd(II)-carbenoid intermediate D_a, from which the aryl group on the Pd center migrates to the carbene carbon atom through TS7_a to give species E_a. The relative energies of TS6_a and TS7_a are 5.7 and 6.1 kcal mol⁻¹, respectively, and thus the predicted barrier for the formation of E_a is 27.9 kcal mol⁻¹, the energy difference between TS7_a and IM7 in Fig. 2.

On the other hand, C_a can evolve into a π-allyl-Pd(II) intermediate G_avia the migratory insertion of the aryl group into the internal carbon C2 of the allenyl moiety viaTS8_a, which is more stable by 0.9 kcal mol⁻¹ than TS6_a (4.8 vs. 5.7 kcal mol⁻¹). Other three migratory insertion transition states of the aryl group into 1,2-allenyl ketone, which are structurally similar to TS8_a but energetically less favorable, are shown in Fig. S5.† Subsequently, G_a can be converted to E_a by performing an intramolecular nucleophilic cyclization process viaTS9_a with an energy barrier of 27.7 kcal mol⁻¹. From an energy point of view, the barrier of the reaction via the π-allyl-Pd(II) intermediate pathway (27.7 kcal mol⁻¹ from G_a to TS9_a) is comparable with that via the Pd(II)-carbenoid intermediate pathway (27.9 kcal mol⁻¹ from IM7 to TS7_a). However, it is noted that in the π-allyl-Pd(II) intermediate pathway, the transformation from C_a to G_a is a highly exothermic (irreversible) process, which is in contrast with the almost thermally neutral transformation (reversible) from C_a to D_a. In this case, it seems that E_a may be mainly obtained via the π-allyl-Pd(II) intermediate pathway but cannot exclude the Pd(II)-carbenoid pathway.

Once formed, E_a can carry out the β-H elimination viaTS10_a to generate the ultimate tetra-substituted furan derivative 3_a and IM8 with the addition of the PPh₃ ligand. After then, IM8 isomerizes into F, from which the O–H reductive elimination viaTS11 leads to the regeneration of the reactive Pd(PPh₃)₂ species. It should be noted that during the final reductive elimination process, the appearance of iPr₂NEt is substantial, which assists the regeneration of the reactive Pd(PPh₃)₂ species by forming an acid–base adduct with the generated phenic acid. The calculated barriers of the β-H elimination and the O–H reductive elimination are 18.6 and 21.1 kcal mol⁻¹, respectively.

The above results indicate that for the substituted 1,2-allenyl ketone, the π-allyl-Pd(II) intermediate mechanism with an energy barrier of 27.7 kcal mol⁻¹ may mainly contribute to the formation of the product and the Pd(II)-carbenoid intermediate mechanism also makes partial contributions. This situation is different from the reactions of the substituted 1,2-allenyl ketones with aryl halides reported recently by Bao et al.,²⁸ who showed that the Pd(II)-carbenoid intermediate mechanism is beneficial when the aryl reagent is aryl iodide rather than the present aryl boronic acid.

In addition, from D_a, the 1,2-H shift process could directly lead to the tri-substituted side product 2,5-biphenyl-3-methylfuran viaTS15 as shown in Fig. S6;† however, this process requires crossing an energy barrier of 36.1 kcal mol⁻¹ (from IM7 to TS15) and is difficult to occur under the experimental conditions (70 °C), explaining no yield of the side product 2,5-biphenyl-3-methylfuran in the experiment.

3.3 Reaction of unsubstituted 1,2-allenyl ketone 1_b

With the clear mechanism in hand, our attention now turns to the reaction of unsubstituted 1,2-allenyl ketone to rationalize the experimental observation that when the R¹ and R² groups of the 1,2-allenyl ketone substrate are replaced with H atoms, denoted as 1,2-allenyl ketone 1_b, the furan yield is reduced to 63% from 93%. The calculated results are shown in Fig. 4.

Fig. 4 Calculated Gibbs energy profile for the formation of furan 3_b from the cross-coupling of unsubstituted 1,2-allenyl ketone 1_b with the Pd(II)-aryl complex Bvia the Pd(II)-carbenoid intermediate pathway (red line) or the π-allyl-Pd(II) intermediate pathway (black line). Selected bond distances are in Å.

Intriguingly, in comparison with Fig. 3, there are three essential differences. First, TS6_b is now slightly more favorable than TS8_b (4.6 vs. 5.0 kcal mol⁻¹), implying that C_b may prefer to form the Pd(II)-carbenoid intermediate D_b. Second, TS7_b is more stable than TS6_b by 5.1 kcal mol⁻¹, and thus once formed, D_b would evolve immediately into E_bviaTS7_b rather than reversibly return to C_b. Third, the barrier from G_b to TS9_b is 41.3 kcal mol⁻¹, which is much higher than that in Fig. 3, 27.7 kcal mol⁻¹, and in this case, the further conversion of G_b would be obstructed. These results indicate that for the unsubstituted 1,2-allenyl ketone 1_b, the furan product may be only obtained via the Pd(II)-carbenoid intermediate mechanism with a barrier of 26.4 kcal mol⁻¹ (the energy difference between TS6_b and IM7), and the remarkable reduction of the furan yield can be attributed to the high stability of the π-allyl-Pd(II) intermediate G_b, whose further conversion involves a too high barrier (41.3 kcal mol⁻¹) to be overcome under the thermal reaction conditions.

3.4 Effects of substituents on 1,2-allenyl ketones

According to the above calculated results, it is very clear that the R¹ and R² groups of the 1,2-allenyl ketone play a substantial role in determining whether the reaction proceeds according to the Pd(II)-carbenoid intermediate mechanism or the π-allyl-Pd(II) intermediate mechanism. When the R¹ and R² groups are H atoms (the unsubstituted 1,2-allenyl ketone), the π-allyl-Pd(II) intermediate mechanism seems impossible due to the high energy barrier (41.3 kcal mol⁻¹) of the nucleophilic cyclization process from G_b to TS9_b, and thus the product may be formed via the Pd(II)-carbenoid intermediate mechanism with a barrier of 26.4 kcal mol⁻¹. In contrast, when R¹ is a phenyl group and R² is a methyl group (the substituted 1,2-allenyl ketone), the π-allyl-Pd(II) intermediate mechanism is relatively easier and thermodynamically much favorable, therefore dominantly contributing to the formation of the product with a barrier of 27.7 kcal mol⁻¹ (the energy difference between G_a and TS9_a).

It should be noted that the calculated rate-determining barriers of the unsubstituted allenyl ketone and the substituted one, 26.4 vs. 27.7 kcal mol⁻¹, seem to be inverse to the corresponding yields observed experimentally, 63% vs. 93%. However, the lower yield of 3_b can be attributed to the chemo-divergence in the reaction of the unsubstituted allenyl ketone. As shown in Fig. 4, the pathway viaTS8_b competes with that viaTS6_b, and from TS8_b the reaction delivers the stable intermediate G_b, which cannot be transformed into the desired product due to its high stability. In contrast, for the reaction of the substituted allenyl ketone (Fig. 3), both pathways viaTS6_a and TS8_a contribute to the product formation, although the pathway viaTS8_a is believed to be the decisive pathway. From these calculated thermodynamic and kinetic data, we can qualitatively understand the experimentally observed different product yields from the unsubstituted and substituted allenyl ketones.

The higher barrier from G_b to TS9_b than that from G_a to TS9_a can be understood by performing the distortion/interaction analysis for these two processes. According to the distortion/interaction model,⁵¹ also known as the activation-strain model,⁵² the activation energy (ΔE^‡) is decomposed into the distortion or strain energy (ΔE_dist), which is associated with the structural deformation of the reactants/fragments along the reaction coordination, and the interaction energy (ΔE_int) between these deformed reactants/fragments. For the present two processes, we used the fragmentation scheme shown in Fig. 5, where by cutting the Pd–C bond each structure is decomposed into two fragments, the palladium catalyst moiety and the allyl substrate moiety. These two fragments are then saturated with hydrogen atoms, whose positions were optimized while all other atoms were kept fixed.⁵³

Fig. 5 Fragmentation scheme used in distortion/interaction analysis.

Table 1 lists the calculated results of the distortion/interaction analysis for TS9_a and TS9_b. It is found that the distortion energy of the palladium catalyst fragment in TS9_b (28.6 kcal mol⁻¹) is larger than that in TS9_a (22.9 kcal mol⁻¹), while the allyl fragment in TS9_b (32.7 kcal mol⁻¹) is smaller than that in TS9_a (39.9 kcal mol⁻¹). Thus, the total distortion energies are quite close in TS9_a (61.8 kcal mol⁻¹) and TS9_b (61.3 kcal mol⁻¹). On the other hand, the interaction energy between the two deformed fragments in TS9_a (−32.6 kcal mol⁻¹) is remarkably larger in comparison with that in TS9_b (−21.8 kcal mol⁻¹), which means the weaker electronic interaction of the two fragments in TS9_b and thus does not favor the stabilization of TS9_b. Therefore, the weaker interaction between the two fragments contributes to the higher barrier from G_b to TS9_b.

Table 1 Distortion/interaction analysis of TS9_a and TS9_b

Transition state	ΔE^‡	ΔEdist-1	ΔEdist-2	ΔEdist	ΔEint
TS9 a	29.2	22.9	39.9	61.8	−32.6
TS9 b	39.5	28.6	32.7	61.3	−21.8

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4. Conclusion

In conclusion, by performing DFT calculations we have shown the mechanism details of the synthesis of furan derivatives via Pd(0)-catalyzed oxidative cross-coupling of 1,2-allenyl ketones with aryl boronic acids. For the substituted 1,2-allenyl ketone, the reaction proceeds mainly via the π-allyl-Pd(II) intermediate mechanism and partly via the Pd(II)-carbenoid intermediate mechanism. In contrast, for the unsubstituted 1,2-allenyl ketone, the reaction occurs only via the Pd(II)-carbenoid intermediate mechanism. The present calculations also revealed a new mechanism for the formation of the Pd(II)-aryl complex, which involves the key oxidative transmetalation process instead of the prior oxidation of Pd(0) by BQ followed by the transmetalation. The non-excellent yield from the unsubstituted 1,2-allenyl ketone originates from the excessive stability of the π-allyl-Pd(II) intermediate. Expectedly, these mechanistic insights could provide a guide for developing new strategies synthesizing furan derivatives from coupling between 1,2-allenyl ketones and aryl boronic acids.

Author contributions

Jihong Xu performed the calculations and wrote the original draft, Yanhong Liu analyzed the results, Chengbu Liu supervised this work, and Dongju Zhang wrote and reviewed this paper.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21833004 and 22273051). We acknowledge the Shandong University Supercomputing Center for providing the computational resources.

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Footnote

† Electronic supplementary information (ESI) available: Additional computational results and Cartesian coordinates of all optimized structures located in this work (PDF). See DOI: https://doi.org/10.1039/d2cy01506d

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