Photoinduced Mechanisms of C-S Borylation of Methyl(p-Tolyl)sulfane with Bis(pinacolato)diboron: A Dft Investigation

1. Introduction

Over the past few decades, C–S bond cleavage and transformation of aryl sulfides have received considerable attention and have been rapidly developed[1,2]. Generally, the C–S bond activation of aryl sulfides could be achieved by guiding group-assisted or directing group-free transition metal-catalysis[3,4,5,6]. While the formed aromatic C–B bonds are a series of important complexes. Especially, arylboric acid has been widely used as a sensor of sugars in materials science and medical research, an inhibitor of enzymes, and a carrier of nucleosides and sugars in biology[7,8,9,10,11]. Hence, great efforts have been devoted to the formation of aromatic C–B bonds in organic chemistry[12,13,14].

Traditionally, the Miyaura borylation reaction can be used to achieve the various C–heteroatom borylations by the corresponding aromatic electrophiles including aryl sulfides. For example, Miyaura used a palladium-catalyzed coupling reaction between halogenated aromatics and biborate pinacol ester to produce corresponding organic borates[15,16,17]; Yorimitsu[18] and Hosoya[19] finished the C–S borylation of aryl sulfides via Pd- and Rh-catalysis(Scheme 1a and 1b); Yorimitsu[20] and Gao[21] realized the C–S borylation of aryl sulfonium salts via Pd-catalysis(Scheme 1c) and UV irradiation. Unfortunately, most synthetic methods for aryl boronate esters have various disadvantages, including the need for noble metals as catalysts, complex synthetic steps, and low yields[22,23,24].

In recent years, amidst the surge in popularity of green chemistry[25,26,27], considerable attention has been directed towards identifying synthesis methods that are eco-friendly, cost-effective and highly efficient. Under the irradiation of blue LED, Xinqi Li et al. proposed a photocatalytic study that directs C–S bond activation of aryl sulfides via photoinduced aerobic borylation under transition-metal-free conditions in 2022 [28]. This synthetic methodology offers enhanced convenience, cleanliness, and efficacy, thereby bearing significant relevance in chemical research and practical applications. A thorough investigation of its reaction mechanism can undoubtedly facilitate experimenter comprehension and enable the design of similar reactions.

2. Computational Details

All the calculations were performed in Gaussian 09 program package[29]. Every structure was optimized at the M06-2X-D3/ma-def2-SVP[30,31,32] level, and the frequency analysis confirmed that every transition state with only one imaginary frequency and the other structures with no imaginary frequency are correct. The SMD model[33,34]was employed to simulate the solvent effect of 1,4-dioxane, which is defined by the static dielectric constant (eps = 2.21) and the dynamic dielectric constant (epsinf = 1.90). The model has been widely used in the process of investigating the mechanisms of organic chemistry[35,36,37,38,39,40,41,42,43]. Additionally, hole-electron isosurface map, Spin density isosurface maps, frontier molecular orbital (FMO), TD-DFT[44,45] and conceptual density functional theory (CDFT) data were obtained using the Multiwfn program (version 3.3.8)[46]. FMO was drawn using VMD program 1.9.3[47]. All optimized three-dimensional (3D) structures were displayed in CYLview (version 1.0b)[48].

3. Results and Discussion

According to the reference[28], under the irradiation of blue LED, the reactants methyl(p-tolyl)sulfane (R1) and bis(pinacolato)diboron (R2) would go through C-S borylation reaction to yield the final product methyl boronic acid pinacol ester (P) in the solvent of 1,4-dioxane, in which 1,4-benzoquinone (BQ) as the photosensitizer, as depicted in Scheme 2.

3.1. Photocatalysis Process

As we all know, the circular conjugated large π-structure is the preferred site for photo-catalytic reaction. The structures of BQ and R1 have the conjugated π-bond, and BQ is usually used as the photosensitizer. Hence, there are two models designed to investigate the photo-catalytic process. When BQ was employed to explore the excitation in first model, the computational results suggest that S₀→S₁ of BQ in Table 1 requires to absorb 2.7539 eV energy from the wavelength of 450.22 nm, which accords with the experimental results (450 nm) within error, and the S₀ →S₁ transition mainly depends on HOMO→LUMO (91.62%). Moreover, the Frontier Molecular Orbital Theory (FMOT) in Figure 1b and 1c shows that this S₀→S₁ excitation focuses on the intramolecular transfer from σ-orbital of benzene ring to π-orbital of benzene ring, which is consistent with ρ_ele and ρ_hole in Figure 2a. While the S₀→S₂ and S₀→S₃ excitations depend on the wavelengths of 405.16 nm and 286.28 nm, respectively, which mainly come from the HOMO-2→LUMO (89.04%) and HOMO-1→LUMO (99.89%) in Table 1; and they cannot be achieved with blue LED (450 nm). In the second model, BQ and R1 form a complex BQ+R1. The TD-DFT calculations suggest that 449.56 nm wavelength can finish the S₀→S₁ excitation of BQ+R1, which accords with the experimental results (450 nm) within error, and the S₀→S₁ transition mainly depends on HOMO→LUMO (57.46 %) and HOMO-3→LUMO (37.46 %). Frontier molecular orbital theory in Figure 1e-g displayed that HOMO→LUMO transition focuses on the intermolecular transfer from R1 to BQ. Meanwhile, the computational results in Table 1 indicate that S₀→S₂ with 436.44 nm could have the possibilities to accord with the experimental results (450 nm) within error. All above description could suggest that the photo-catalytic reaction would successfully achieve the excitation process of BQ.

3.2. R1+BQ → IM1

As highlighted in the reference[28], BQ, as an organic photocatalyst, is easy to become an excited state in the irradiation of blue LED (450 nm). The computed oscillator strengths (f=0) of BQ model in Section 3.1 indicates it has a little possibility to achieve the photoexcitation process. Hence, the second model is prior. The R1+BQ model would absorb the energy of photons to finish the S₀→S₁ excitation. The calculations suggest that this photo-excited process requires 51.52 kcal/mol to become excitation state ¹[R1+BQ]*, which is very activate and has two possible paths, as depicted in Figure 3. In the first path, the singlet excited state ¹[R1+BQ]* would become the ground state R1+BQ by releasing much energy, which results in the triplet oxygen ³O₂ changing into singlet oxygen ¹O₂. Generally, the structure of triplet excited state ³[R1+BQ]* is much more stable than that of singlet excited state ¹[R1+BQ]*. So in the second path, Intersystem Crossing (ISC) would transfer ¹[R1+BQ]* into ³[R1+BQ]* and this process releases 8.63 kcal/mol of energy. Meanwhile, there is a possibility that ³[R1+BQ]* and triplet oxygen ³O₂ could go through energy transfer to become R1+BQ and singlet oxygen ¹O₂ via releasing about 5.00 kcal/mol energy. The obtained ¹O₂, as IM1, is one excited state and very active, which continues to participate the following reaction.

3.3. IM1 → P

As we all know, the formed IM1(¹O₂) is very active, which could participate the following reactions. Hence, ¹O₂ reacts with R1 and R2 in paths a1 and a2, respectively. Moreover, the reaction between ³O₂ and R1 (or R2) has also been investigated in path a1, elaborating in the following section.

3.3.1. Path a1

According to Figure 4, Gibbs free energy surface suggests that it is a SN2 reaction with the energy barrier of 54.87 kcal/mol, which is very high and cannot happen. Considering the large amount of ³O₂, it can also have the possibility to react with R1 or R2 in the system. Figure 4 suggests that R1+³O₂→TS1a1-2→IM2a1-1+IM3a1-1 (blue line) process has an energy barrier of 53.12 kcal/mol and cannot also happen in path a1-2. Finally, the reaction between ³O₂ and R2 has a relative lower energy barrier than those in paths a1-1 and a1-2, 42.32 kcal/mol is still a higher barrier and it cannot continue to occur.

3.3.2. Path a2

Based on the analysis of Fukui function and dual descriptor, B5(R2) atom with the positive value ($f_{\mathrm{B}5\left(\mathrm{R}2\right)}^{\left(2\right)}$=0.0746) in Table 2 is susceptible to nucleophilic attack. In Path a2 of Figure 6, B5 atom of R2 reacts with O1(IM1) atom via transition state TS1a2, which has an energy barrier of 17.22 kcal/mol and the distance of O1(IM1)•••B5(R1) in TS1a2 is 1.78 Å. Additionally, the energy of IM2a2 is 115.70 kcal/mol lower than that of IM1+R2, indicating that this step is an exothermic process in Figure 6. Then the following step would convert intermediate IM2a2 into IM3a2 via an intramolecular homolytic reaction by absorbing energy of 32.68 kcal/mol. The electron spin density isosurface graph of IM3a2 in Figure 5d show that the single electron is distributed on O1 atom, which is the reactive site to participate the following reaction. The condensed dual descriptor (CDD) values of C4 atom of R1 and B5 (or B6) atom of R2 are $f_{\mathrm{C}4\left(\mathrm{R}1\right)}^{\left(2\right)}$=0.0025 and $f_{\mathrm{B}5\left(\mathrm{R}2\right)}^{\left(2\right)}$=0.0746 in Table 2, respectively. Therefore, there are two paths (a2-1 and a2-2) can yield the final product P.

In path a2-1, IM3a2 would react with R2 at the sites O1(IM3a2) atom and C4(R2) atom via transition state TS2a2-1. The computational results displayed that it is a SN2 reaction and the distances of O1(IM3a2)•••C4(R2) and C4(R2)•••S3(R2) are 2.12 Å and 1.97 Å in TS2a2-1, respectively. Moreover, this IM3a2+R1→IM4a2-1+ IM5a2-1 process is an exothermic procedure and 26.59 kcal/mol would be released. The formed IM5a2-1 is a radical and the single electron is distributed in S3 atom in Figure 5c. The next process is an intramolecular ring-closure reaction happened between B5-B6(R2) bond and S3 atom via a three-membered(B5, B6 and S3) transition state TS3a2-1, which needs 7.55 kcal/mol energy and the distances of B5•••S3, B6•••S3 and B5•••B6 is 2.61 Å, 2.60 Å and 1.74 Å, respectively. The generated IM6a2-1 is one three-membered intermediate, and it would go through ring-opening reaction to generate byproduct BP and IM7a2-1 radical via transition state TS4a2-1. It is a fast step and 5.78 kcal/mol is the energy barrier in Figure 6. Furthermore, Figure 5 e, f and j reveal it is one single electron transfer process and the single electron is distributed in B5 atom of IM7a2-1 in Figure 5f. Finally, a SN2 reaction happened between C4(R1) and B5(IM7a2-1) atoms via TS5a2-1, which needs 13.33 kcal/mol energy. The distance of C4(R1)•••B5(IM7a2-1) decreased to 1.56 Å in P from 2.27 Å in TS5a2-1, indicating that the IM7a2-1 intermediate could successfully move to the C4(R1) atom of R1. The energy of obtained IM5a2-1+P was 34.94 kcal/mol lower than that of IM7a2-1+R1, showing that this process is an exothermic reaction in Figure 6. Meanwhile, IM5a2-1 can continue to also attack R2 reaction to realize recycling.

The B5 or B6 atom of R2 is an electron-deficient structure. So, in Path a2-2, IM3a2 with one single electron can also attack the B5 or B6 atom of R2 to complete the SN2 reaction. The calculations show that this IM3a2+R2→IM5a2-2 process with the energy barrier of 8.96 kcal/mol via TS2a2-2 is an exothermic procedure (45.42 kcal/mol) in Figure 6. Meanwhile, the B5–B6 bond of R2 is broken and an intermediate IM4a2-2 is formed. The structure of IM5a2-2 is as the same as IM7a2-1 in path a2-1, so the final reaction of IM5a2-2+R1→P+IM6a2-2 process is as the same as IM7a2-1+R1→P+IM5a2-1 procedure in path a2-1.

4. Conclusions

In summary, this paper systematically investigated the detailed formation mechanisms of methyl boronic acid pinacol ester (P) through the photoinduced aerobic borylation reaction between methyl(p-tolyl)sulfane (R1) and bis(pinacolato)diboron (R2). The computations were performed at the M06-2X-D3/ma-def2SVP level, with SMD model to simulate the solvent effect of 1,4-dioxane. Additionally, the photo-catalytic mechanisms of BQ+R1 model were explored using TDDFT method at the M06-2X-D3/ma-def2-SVP level. The computational results revealed that R1+BQ is preferentially excited into excited state ¹[R1+BQ]*, which has the possibility to change into ³[R1+BQ]* via intersystem crossing (ISC). Hence, there are two possibilities existed can change ³O₂ into ¹O₂ by energy transfer process from ¹[R1+BQ]* and ³[R1+BQ]*, respectively. Subsequently, two paths (a2-1 and a2-2) were proposed to convert R2 into product P via a series of reactions. The Gibbs free energy surfaces path a2-2 was identified as the optimal route with lower energy barriers, and the electron spin density isosurface graphs can reveal that it is a free radical transfer process. The clear mechanisms offer valuable insights for the design of similar reactions and showcase the potential of computational methods in advancing catalysis research.