Key role of a π–π complex in diaryl cross-coupling between aryldiazonium salts and arylboronic acids using photosensitizer-free gold/photoredox catalysis

Abstract

DFT and TD-DFT calculations were performed to better understand the photosensitizer-free visible-light-mediated Au-catalyzed cross-coupling between aryldiazonium salts and arylboronic acids. The π–π type complex between the aryldiazonium salt and the arylboronic acid, rather than either the generally accepted gold(I)–aryldiazonium salt complex or the aryldiazonium salt itself, was shown to play the role of a virtual photoinitiator. The oxidation of Au(I) to Au(III), the key aspect of the dual gold/photoredox catalytic aryl–aryl cross-coupling, occurs via the radical addition of an aryl radical rather than an aryldiazo radical on Au(II) species. The transmetalation of the arylboronic acid to Au(III) species was identified as the rate-determining step, and the presence of a tetrafluoroborate anion can assist this process remarkably. The experimentally observed effect of substituents on the aryldiazonium salt and boronic acid on the reactivity was also rationalized.

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1. Introduction

Biaryl compounds are important structural motifs in agrochemicals, pharmaceuticals, materials and organic products.^1,2 Over the last few years, synthetic chemists have made great efforts to develop efficient methods for the construction of biaryl scaffolds via C–C cross-coupling reactions catalyzed by transition metals,^3–6 in particular, Pd-based complexes, the most successful species for the coupling between aryl electrophiles and aryl nucleophiles due to the readily accessible Pd(0)/Pd(II) cycle.^7,8 In contrast, the use of gold catalysts for such reactions has remained highly challenging for a long time because of the high redox potential of the Au^I/Au^III couple (E₀ = +1.41 V),⁹ which means that strong external oxidants,^10–12 suitable ligands¹³ or reactive electrophiles (such as aryldiazonium chlorides)^14–16 are required to access Au^I/Au^III catalytic cycles. In recent years, to avoid the use of strong oxidants in gold catalysis, a new strategy for C–C cross-coupling has been developed by Glorius,^17,18 Toste^19,20 and Fensterbank²¹ that involves merging gold catalysis with photoredox catalysis under mild conditions.²²

The groups of Lee²³ and Fouquet²⁴ reported examples of dual gold/photoredox catalytic aryl–aryl cross-coupling in the presence of a photosensitizer. Furthermore, it was demonstrated by Hashmi^25–27 and Bandini²⁸ that gold-catalyzed C(sp²)–C(sp²) cross-coupling can be even achieved by only using an aryl radical source (aryldiazonium salts) combined with visible light irradiation without an exogenous photosensitizer, and several elegant works have been reported recently by Patil,^29,30 Wong³¹ and Echavarren.³² In particular, the cross-coupling of aryldiazonium salts with electron-rich arylboronic acids, ideal coupling partners for dual catalytic systems,^33–36 has been shown to be a highly efficient approach for the synthesis of biaryl compounds. Scheme 1 shows a prototype of the pioneering works of Hashmi et al.^26,27,37 on the synthesis of diaryl compounds via photosensitizer-free dual gold/photoredox catalysis.

Scheme 1 Photosensitizer-free visible-light-mediated gold-catalyzed diaryl cross-coupling between aryldiazonium salts and arylboronic acids.

It is particularly interesting how the gold/photoredox reaction works without the presence of a photosensitizer. Hashmi et al.^26,27,37 have proposed two potential mechanisms, as shown by catalytic cycles I and II in Scheme 2. In cycle I, the reaction starts from the single electron transfer (SET) between Au^I species A and aryldiazonium salt Ar¹N₂⁺, resulting in Au^II species B and an aryldiazo radical Ar¹N₂˙. Subsequently, B captures Ar¹N₂˙ to form Au^III species C, which then evolves into key aryl Au^III species D by extrusion of N₂ under light irradiation. An alternative pathway (cycle II) for the generation of D could be via direct addition of the aryldiazonium salt to A to form N₂–Au^III–Ar¹ species E, which then carries out elimination of N₂ under light irradiation to afford D. Next, the transmetalation of arylboronic acid 2 to D gives diaryl Ar²–Au^III–Ar¹ species F, from which the aryl–aryl reductive elimination delivers biaryl product Ar¹–Ar²3 and regenerates A. In these two mechanisms, the role of light irradiation is believed to be assisting extrusion of N₂ from C or E (in the absence of light, the formation of a C–N bond to diazonium salts was observed under the same conditions³⁸).

Scheme 2 Possible mechanisms for photosensitizer-free visible-light-mediated gold-catalyzed diaryl coupling between aryldiazonium salts and arylboronic acids.

Intriguingly, here, by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, we show that both of these cycles are not accessible. Alternatively, we propose a new mechanism in which the reaction requires the assistance of a virtual photoinitiator that is identified as the π–π complex between the aryldiazonium salt and the arylboronic acid, rather than either the generally accepted complex of Au^I with the aryldiazonium salt or the aryldiazonium salt itself. In the newly proposed mechanism, the reaction starts from denitrification of the excited π–π complex to form a triplet aryl cation, which then performs SET with the Au^I complex and the oxidation of Au^I to Au^III, and the reaction proceeds through the radical addition of an aryl radical rather than an aryldiazo radical on Au^II species. We hope that the theoretical results will provide some insights into future reaction protocols.

2. Results and discussion

Our attention first focuses on the catalytic cycles shown in Scheme 2. Calculations were carried out for the reaction shown in Scheme 1 with R = F. Fig. 1 shows the calculated energy profiles for the first steps of cycles I and II. The SET from (4-CF₃C₆H₄)₃PAu^ICl to aryldiazonium salt Ar¹N₂⁺ to give Au^II and aryldiazonium radical Ar¹N₂˙ needs to overcome a barrier as high as 53.7 kcal mol⁻¹ (the blue pathway in Fig. 1). Although such a high energy requirement is within the photon energy that blue LEDs can provide,³⁹ our TD-DFT calculations show that neither (4-CF₃C₆H₄)₃PAu^ICl nor Ar¹N₂BF₄ falls in the blue LED region (Table 1). Moreover, the Au^III–N₂Ar¹ complex C involved in this pathway cannot be successfully located as a minima due to the weak interaction in the metal–azo bond.^40–42 Thus cycle I in Scheme 2 seems not to be viable.

Fig. 1 Calculated Gibbs energy profiles for the first steps of cycles I and II shown in Scheme 2. Bond distances are in Å.

Table 1 TD-DFT predicted lowest vertical excited energies (E_ex, eV), oscillator strengths of transitions (f), maximum absorption wavelengths (λ_max, nm), transition characterizations (H = HOMO, L = LUMO) and the corresponding percentage contributions

Entry	Species	E ex	f	λ max	Characters
1	Aryldiazonium salt (1)	4.25	0.0341	292	H−1 → L (96.2%)
2	Arylboronic acid (2)	4.74	0.0316	261	H → L (84.1%)
3	(4-CH3C6H4)3PAu^ICl (Au^I)	4.86	0.1819	255	H → L (90.5%)
4	MeOH	8.21	0.0008	151	H → L (98.9%)
5	Complex of 1 with Au^I	2.87	0.0147	432	H → L (98.2%)
6	Complex of 1 with MeOH	3.05	0.0074	406	H → L (99.7%)
7	Complex of 1 with 2	3.05	0.0158	406	H → L (89.0%)
8	Complex of 2 with Au^I	4.47	0.0128	277	H → L (58.3%), H → L+1 (20.1%)
9	Complex of 2 with MeOH	4.72	0.0333	263	H → L (84.0%)
10	Complex of MeOH with Au^I	4.76	0.1521	261	H → L (94.0%)

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Alternatively, according to cycle II, the reaction is initiated from the direct oxidative addition of Ar¹N₂⁺ to the Au^I center to form Au^III species E. Two potential pathways (the black and red lines in Fig. 1) have been located, which proceed viaTS1 and TS1′ with and without the assistance of a tetrafluoroborate anion, respectively. The relative Gibbs energies of these two transition states were calculated to be 49.7 and 52.7 kcal mol⁻¹, which are consistent with previous reports in the literature.^42–44 However, these values are too high for a reaction that proceeds at room temperature. Therefore, the mechanism described in cycle II also seems to be problematic.

Our present calculations propose a new energetically viable mechanism, which emphasizes that the reaction proceeds via a photoinitiation mechanism where the π–π complex between the aryldiazonium salt and the arylboronic acid is identified as a virtual photoinitiator. The relevant details are discussed as follows.

2.1 Potential photoinitiators

To verify which species plays a substantial role in the light absorption as a virtual photoinitiator, we carried out TD-DFT calculations for various possible species in the present catalytic system, including four isolated components, aryldiazonium salt 1, arylboronic acid 2, Au^I catalyst and MeOH solvent, as well as the various complexes between them. As shown in Table 1, the maximum absorptions of all four isolated species are beyond the range of visible light (390–780 nm);^45,46 however, the maximum absorptions of the complexes of aryldiazonium salt with 2, the Au^I catalyst and MeOH fall in the visible region. Therefore, only these three complexes are considered as potential photoinitiators under blue light irradiation.^31,43,47 The calculated excitation–decay energy profiles of three complexes are shown in Fig. 2, S1 and S2 in the ESI,† where the complexes can be excited vertically under irradiation with blue LEDs to their respective lowest singlet excited states, which then decay to the corresponding lowest triplet states via intersystem crossing (ISC).⁴⁸ Examination of orbital contributions indicates that the excitation of all three complexes corresponds to a HOMO–LUMO electron transition to form the excited triplet charge transfer complexes (CTCs). Which of these three complexes is the most likely photoinitiator? To answer this question, we studied the subsequent reactions of the excited triplet CTCs, which can be quenched either oxidatively or reductively via SET with the redox-active components in the system,^45,49 resulting in the generation of the aryl radical followed by the oxidation of Au^I species.

Fig. 2 Calculated Gibbs energy profiles of the generation of aryl radical Ar¹˙ (Ar¹ = 4-FPh) and the oxidation of gold(I) using the complex of the aryldiazonium salt with the arylboronic acid as a photoinitiator. Bond distances are in Å.

2.2 Generation of the aryl radical followed by oxidation of Au^I species

In the newly proposed mechanism, the generation of the aryl radical is a key process in the present gold/photoredox catalysis. Based on the three excited triplet complexes of the aryldiazonium salt (Table 1), we studied various possible pathways for the generation of the aryl radical, and Fig. 2 summarizes the energetically most favorable one which originates from the complex of the aryldiazonium salt with the arylboronic acid. Those originating from the other two complexes are given in Fig. S1 and S2† for simplification.

As shown in Fig. 2, from the triplet complex between the aryldiazonium salt and arylboronic acid, ³4, there are three possible pathways that may yield aryl radicals.^43,50 Pathway I starts from the SET of the Au^I catalyst to ³4, giving the reduced state complex 5 and Au^II species with an energy barrier of 12.9 kcal mol⁻¹ calculated according to the Marcus–Hush theory.^51,52 This process is determined to be endothermic by 2.9 kcal mol⁻¹. Subsequently, 5 undergoes ligand exchange with the Au^II species, leading to the anionic aryldiazonium salt radical 6 and the complex of Au^II with the arylboronic acid complex, 7. By carrying out elimination of N₂, 6 affords the aryl radical Ar¹˙viaTS2 with 5.8 kcal mol⁻¹. Finally, the aryl radical preferentially associates the Au^II species 7 with an unpaired electron to form Au^III species 8, and this process is determined to be exothermic by 33.5 kcal mol⁻¹.

Alternatively, pathway II occurs via decomposition of ³4 into singlet state arylboronic acid and triplet aryldiazonium salt, which then performs the elimination of N₂via³TS3 with a barrier of 6.2 kcal mol⁻¹. A third pathway initiates from the direct elimination of N₂ from ³4via³TS4 and this process is identified as a barrierless process with an exothermicity of 16.4 kcal mol⁻¹, generating a triplet aryl cation ³Ar¹⁺, which then performs SET with the Au^I catalyst to obtain Ar¹˙ and Au^II species. This barrier is estimated to be 15.1 kcal mol⁻¹. In comparison with pathways I and II, pathway III is the most energetically favorable for the generation of Ar¹˙.

The generation of Ar¹˙ from the excited complex of the aryldiazonium salt with the Au^I catalyst (Fig. S1†) cannot compete with that from the excited complex of the aryldiazonium salt with the arylboronic acid (Fig. 2) due to the relative higher energy requirement (12.1 kcal mol⁻¹ for N₂ extrusion and 15.1 kcal mol⁻¹ for SET, respectively, Fig. S1†). It should be noted that the calculated barrier for N₂ extrusion, 12.1 kcal mol⁻¹, is comparable with that calculated for the Au-catalyzed 1,2-difunctionalization of alkynes.⁴² In addition, it was found that the complex between the aryldiazonium salt and MeOH solvent could also act as a photoinitiator, as indicated by the calculated low barrier (6.5 kcal mol⁻¹) for the elimination of N₂ from its excited triplet complex (Fig. S2†). From an energy point of view, the π–π complex between the two reactants most likely acts as a photoinitiator, which carries out the π → π* (HOMO–LUMO) transition upon vertical excitation under blue LED irradiation (Fig. S3†) to induce the generation of Ar¹˙ and Au^II. Once formed, Ar¹˙ can immediately perform oxidative addition on the in situ formed Au^II complex, resulting in Au^III complex D.

2.3 Transmetalation and reductive elimination

Fig. 3 shows the calculated results for the subsequent reaction of the Au^III–Ar¹ species with arylboronic acid, which delivers the final diaryl cross-coupling product via two elementary steps: transmetalation and reductive elimination.

Fig. 3 Calculated Gibbs energy profiles for the transmetalation and the subsequent reductive elimination to form the final product. Bond distances are in Å.

The left Gibbs energy profile in Fig. 3 describes a direct transmetalation pathway of the arylboronic acid to the cationic π-coordinated Au^III complex 8. This pathway involves the rupture of the Au–Cl bond viaTS5, a four-centered transition state, and involves a barrier of 34.3 kcal mol⁻¹, resulting in cationic diaryl Au^III species 9 with the release of B(OH)₂Cl. The barrier is too high for a reaction at room temperature. The high energy requirement is attributed to the high energy requirement for breaking the strong Au–Cl bond.⁵³

Following the experimental observation that the presence of the BF₄⁻ counterion remarkably improves the yield, we calculated the BF₄⁻-assisted transmetalation process, as shown by the right black energy profile in Fig. 3. The transition state involved is denoted as TS6, where as indicated by the calculated bond distances, a three-center–two-electron B–F–B bond is formed via the interaction between the negatively charged fluorine atom in BF₄⁻ and the positively charged boron atom in the arylboronic acid. In the presence of the BF₄⁻ counterion, the transmetalation process was determined to be exothermic by 11.5 kcal mol⁻¹ and the barrier of the reaction was reduced to 24.1 kcal mol⁻¹ in comparison with the reaction without the assistance of the BF₄⁻ counterion. These calculated results are consistent with the previous studies by Toste et al.,^54,55 who substantiated the fluorine-assisted transfer of aryl groups from boron to gold.

The BF₄⁻-assisted transmetalation results in the neutral diaryl Au^III species F, which then undergoes reductive elimination viaTS7 to obtain the desired C(sp²)–C(sp²) cross-coupling products 3 and regenerate the Au^I catalyst. The barrier of this process is estimated to be only 8.8 kcal mol⁻¹, indicating that the reductive elimination is facile.

Based on the above results, we summarize the mechanism in Scheme 3. In this mechanism, the π–π complex ¹G between aryldiazonium salt 1 and arylboronic acid 2 plays the role of photoinitiator. Under blue LED irradiation, the complex ¹G is excited to its triplet excited state ³G, which then undergoes elimination of N₂ and SET with gold(I) catalyst A to give the cationic gold(II) species B and aryl radical Ar¹˙. Subsequently, the addition of Ar¹˙ to B results in the key Au^III–Ar¹ intermediate D, from which the biaryl Ar¹–Au^III–Ar² complex F can be obtained via transmelalation with the assistance of a BF₄⁻ anion. Finally, F undergoes reductive elimination to release the diaryl cross-coupling product 3 and regenerate the Au^I species A. From the present calculations, it is clear that the transmetalation process with an energy barrier of 24.1 kcal mol⁻¹ can be identified as the rate-determining step of the reaction and the overall reaction is estimated to be exothermic by 68.6 kcal mol⁻¹.

Scheme 3 Newly proposed mechanism.

2.4 Substituent effect

According to Hashmi et al.,^26,27 the substituent on the aryldiazonium salt significantly affects the reactivity: electron-poor aryldiazonium salts afford the biaryl products in high yields (77–84%), while electron-rich aryldiazonium salts result in the product in low yields (∼30%). To elucidate the effect of the different substituent groups on the reactivity, we carried out calculations for the reactions of two other representative aryldiazonium salts that bear the electron-withdrawing CF₃ group and the electron-donating OMe group, respectively. The calculated results of two key steps, generation of an aryl radical and transmetalation, are shown in Fig. 4, and the results of the reaction with the F substituent discussed above are also given for comparison. The calculated barriers of these two steps are 8.5 and 19.7 kcal mol⁻¹ for the CF₃-substituted aryldiazonium salt (1_CF3), 15.1 and 24.1 kcal mol⁻¹ for the F-substituted aryldiazonium salt (1_F), and 26.2 and 27.4 kcal mol⁻¹ for the OMe-substituted aryldiazonium salt (1_OMe). Clearly, the electron-poor aryldiazonium salt is significantly more favorable for both the elementary steps than the electron-rich aryldiazonium salt, which is in good agreement with experimental observation. The much lower energy requirement of the SET process in the CF₃-substitutent-involving reaction is due to the strong electron-withdrawing property of the CF₃ group, which makes the C atom in the para-position of CF₃ substituent more positively charged in comparison with F and OMe substitutents and hence more favorable for the oxidation of the Au^I species (Fig. 4a). On the other hand, the lower barrier of the transmetalation in the CF₃-substitutent-involving reaction (Fig. 4b) can be attributed to the stronger electrostatic interaction between the Au center and the C² atom, making the corresponding transition state (TS8) more stable than those in the reactions involving F and OMe substituents (TS6 and TS9). Additionally, we also performed calculations for the rate-determining step (transmetalation) of the systems involving boronic acids with electron-donating substituents (OMe and NMe₂) and the results are shown in Fig. S6.† For these two systems, the calculated barriers of the transmetalation are 32.0 and 31.8 kcal mol⁻¹, which are clearly much higher than those for the systems involving boronic acids with electron-withdrawing substituents, such as COOMe in Fig. 4. This can be attributed to the weaker electrostatic interaction between the Au center and the C² atom induced by the electron-donating substituents, making the transmetalation process less favorable. These calculated results are consistent with the experimental observation that boronic acids bearing electron-withdrawing substituents appear to react more efficiently than those bearing electron-donating substituents.

Fig. 4 Generation of an aryl radical (a) and transmetalation transition states (b) involved in the reactions of aryldiazonium salts bearing –CF₃, –F, and –OMe groups, respectively. Gibbs free energy barriers (red), bond distances (black) and NPA charges (blue) are in kcal mol⁻¹, Å and e, respectively.

3. Conclusions

In summary, by performing systematic DFT and TD-DFT calculations we proposed a new mechanism for the photosensitizer-free visible-light-mediated gold-catalyzed cross-coupling between aryldiazonium salts and arylboronic acids. In this new mechanism, the π–π complex between the aryldiazonium salt and the arylboronic acid is indicated to act as a photoinitiator to generate the key aryl radical and the Au^III intermediate. The rate-determining step of the reaction is the transmetalation of the arylboronic acid with the assistance of a tetrafluoroborate anion to the Au^III center. The calculated substituent effect of aryldiazonium salts and arylboronic acids on the reactivity is consistent with the experimental observation. The theoretical results provide new understanding for the reaction under study.

4. Computational details

The reaction shown in Scheme 1 was chosen as the prototype reaction of the photosensitizer-free gold/photoredox-catalyzed diaryl cross-coupling between arylboronic acids and aryldiazonium salts reported by Hashmi et al.^26,27,37 All DFT and TD-DFT calculations were carried out using the Gaussian 16 program.⁵⁶ Geometry optimizations of stationary points were performed using the M06 suite⁵⁷ of meta-GGA functionals with the spin-restricted method for closed-shell species and the spin-unrestricted method for open-shell species. The basis set used was the LANL2DZ effective core potential (ECP) basis set⁵⁸ augmented by an f-type polarization function (ζ_f = 1.050)⁵⁹ for gold atoms and 6-31G(d,p)⁶⁰ for other atoms. The solvent effect of methanol was taken into account using the polarizable continuum model (PCM)⁶¹ in the geometry optimization calculations. Frequency analyses were performed at the same level of theory to confirm the optimized structures to be local minima (with zero imaginary frequency) or first-order saddle points (with one imaginary frequency). The intrinsic reaction coordinate (IRC)⁶² calculations were also performed to ensure that the transition states connected the reactant or product properly. All reported energies of intermediates and transition states were solvated Gibbs energies. The excitation energies and absorption wavelengths of several species were obtained by performing TD-DFT calculations^63,64 at the M06/LANL2DZ(f)/6-31G(d,p) level with the PCM solvation effect of methanol solvent. Natural population analyses (NPA)⁶⁵ were carried out using the NBO 3.1 program as implemented in the Gaussian 16 package.

Author contributions

Yanhong Liu performed the calculations and wrote the original draft; Rongxiu Zhu 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 (Grant Nos. 21773139 and 21833004).

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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: 10.1039/d1qo01464a

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