Photo-induced terminal alkyne insertion into arene ring to synthesize boron-doped polycycles

Abstract

The classic Büchner reaction involves the insertion of highly reactive carbenoids into benzene rings to form cycloheptatrienes. Achieving a Büchner-type ring expansion using stable terminal alkynes as carbon sources, however, has remained a formidable challenge. Herein, we report that under light irradiation, ethynyl- and BMes₂ (Mes = 2,4,6-trimethylphenyl)-substituted heteroarenes undergo an intramolecular Büchner-type ring expansion. This clean reaction generates heteroatom-doped polycyclic compounds through concerted inert aromatic C–C bond cleavage, carbon atom insertion, and hydrogen atom transfer (HAT). To our knowledge, this is the first example of a Büchner-type ring expansion employing a stable terminal alkyne to form a C7 ring, offering a new strategy for constructing boron-containing polycyclic architectures. Theoretical studies reveal that the photochemical transformation is enabled by intramolecular allene- and boron-radical species along with an intramolecular HAT process.

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Introduction

Arene compounds are fundamental in organic chemistry and widely utilized in pharmaceuticals, petrochemicals, and materials science.^1,2 Over decades, the functionalization and transformation of arenes via C–H activation^3–5 or dearomatization^6,7 have been well-established. In contrast, the activation of the arene ring's C=C bonds for skeletal modification remains challenging under mild conditions, largely due to the inherent aromaticity, low polarity, and competitive C–H functionalization pathways of arenes.^8–17 Nevertheless, the selective cleavage of aromatic C=C bonds offers a promising strategy for the efficient modification of arene rings and their conversion into high-value compounds.^1,18–23

The Büchner ring expansion represents a classic example of carbon-atom insertion into the inert C=C bond of a benzene ring, typically leading to the formation of a seven-membered ring. Traditional versions of this reaction rely on highly reactive carbene species as the carbon source, which often afford mixtures of cycloheptatriene isomers with low regioselectivity (Fig. 1a).^24–26 The introduction of precious-metal catalysts has significantly advanced this field, substantially improving regioselectivity in Büchner-type reactions (Fig. 1b).^27–34

Fig. 1 Conceptual overview of previously reported examples of Büchner-type ring expansion reactions (a–d) and summary of this work (e).

Beyond transition metals, highly reactive low-valent main-group species are also known to participate in aromatic ring-expansion processes.^35–41 For example, in 2002, Kira and co-workers reported that a silylene can undergo Büchner-type ring-expansion reactions with benzene and toluene via photochemical insertion of the Si atom into aromatic C=C bonds (Fig. 1a).⁴² Okazaki and co-workers later observed the [1 + 2] cycloaddition and ring expansion reactions of silylenes and aromatic compounds under thermal conditions.⁴³ More recently, Inoue, Rieger, and co-workers demonstrated that acyclic iminosilylene compounds can undergo either intramolecular or intermolecular Büchner-type ring expansion by inserting the Si atom into aromatic C=C bonds, with the reaction outcome depending on the substituents on silicon.^26,44 Notably, Goicoechea, Aldridge, and co-workers described a rare example of an isolable low-valent aluminum compound [K(2.2.2-crypt)][(NON)Al] (NON = 4,5-bis(2,6-diisopropyl-anilido)-2,7-di-tert-butyl-9,9-dimethylxanthene)-involved oxidative activation of the C=C bond in benzene to afford a seven-membered AlC₆H₆ metallacycle at room temperature (Fig. 1a).⁴⁵

In transition-metal-free systems, photochemical strategies have been proven effective for cleaving aromatic C=C bonds and transforming arene skeletons, often by generating highly reactive radical intermediates, particularly with lighter main group elements such as N, C, and B.^46–51 Early work by Sundberg, DeGraff, Chapman, and co-workers in the 1970s showed that photolysis of phenyl azide with amines afforded N-insertion products via nitrene intermediates.^9,52,53 More recently, Ruffoni, Leonori, and co-workers demonstrated that blue-light irradiation of nitroarenes generates a singlet nitrene capable of Büchner-type expansion to form azepanes.²³ Impressively, Hudnall and co-workers found that photolysis at 380 nm switches a diamidocarbene from singlet to triplet spin state, enabling reversible Büchner-type reactions with arenes.²⁴ In the related boron chemistry, Wang, Li et al. reported that peri-(2-pyridyl)- and BMes₂-substituted naphthalene/acenaphthene derivatives undergo B-atom insertion into the arene ring with mesityl migration, yielding benzoborepins (Fig. 1c).⁵⁴ Takaya and co-workers later showed that ortho-phosphino-substituted triarylboranes undergo reversible boron insertion upon irradiation, producing borabicyclo[3.2.0]heptadiene derivatives (Fig. 1d).⁵⁵ Despite these advances in main-group-atom insertion into arenes, especially via carbene pathways,⁵⁶ the corresponding process employing a stable terminal alkyne as a C1 source remains unknown. While transition metals have been shown to promote the insertion of internal alkynes into arenes to form expanded C7 rings,^57,58 the cleavage of an aromatic C=C bond and insertion of a Csp atom from a terminal alkyne without any metal has not been reported.

The electron-deficient character of boron endows boron-incorporated conjugated molecules with distinct electronic properties,⁵⁹ rendering them valuable in organic synthesis^60–62 and optoelectronic materials.^63,64 Boron also plays a key role in photoinduced transformations, enabling the conversion of simple boron-containing frameworks into complex architectures through photoisomerization.^65–70 Moreover, photo-promoted ring opening and boron-insertion reactions offer a direct route to novel boron-doped organic molecules (Fig. 1c and d), which have potential applications in organic functional materials.^54,55

Inspired by the above photo-promoted Büchner-type ring expansion reactions, particularly those involving boron-engaged photo-transformation, we designed and synthesized a series of air-stable BMes₂-substituted aromatic heterocycles bearing an adjacent ethynyl group to exploit potential synergy between the boron center and the alkyne under UV light irradiation (Fig. 1e). We found that the carbon atom of the terminal ethynyl group inserts into a robust C=C bond of a mesityl ring, assisted by the boron atom, without the need for harsh conditions or highly reactive substrates (Fig. 1e and 2a). This work establishes a new strategy for arene C=C bond cleavage/insertion and provides access to novel, highly conjugated polycyclic boron compounds fused with various aromatic rings, which are challenging to prepare by conventional methods.

Fig. 2 (a) Photo-promoted transformation of 1 to 2; ¹H NMR (b) and ¹¹B NMR (c) tracking experiments show the conversion of 1a to 2a in C₆D₆ under 313 nm light irradiation at room temperature. ^a10 days. ^bisolated yields, ^c174 hours. ^dNMR yield.

Results and discussion

Synthesis and characterization

The 2-dimesitylboryl-3-ethynyl-heteroarene compounds 1a–1e were prepared by modified two-step reactions (See SI). First, 2-bromo-3-ethynylheteroarene or 3-ethynylheteroarene derivatives with a trimethylsilane protecting group at the terminal carbon atom of the alkynyl group were lithiated, followed by quenching with FBMes₂ to afford the dimesitylboryl-substituted intermediates pre-1a to pre-1d in moderate to high yields (33–84%). Subsequent deprotection with TBAF to remove the trimethylsilyl group gave the target compounds 1a–1d in high yields (80–95%). The deuterated compound 1e was prepared by lithiation of 1b and followed by quenching with D₂O. They were characterized by multinuclear NMR spectra (¹H, ¹³C{¹H}, ¹¹B{¹H}) and HRMS analysis. The crystal structures of 1a–1d were also determined by single-crystal X-ray diffraction analysis (Fig. 4a and See SI Fig. S25–S28), and the DFT-optimized structures agree well with the crystallography data.

These compounds all have strong and broad absorption bands at λ_abs = 275 to 400 nm (ε = 7200 to 21 100 mol⁻¹ cm⁻¹), and exhibit weak emission with λ_em = 422 to 452 nm, Φ_FL ≈ 0.2–5.6% and with the apparent Stokes shift (6809 cm⁻¹, 5826 cm⁻¹, 5646 cm⁻¹, 5654 cm⁻¹ for 1a–1d respectively) owing to a charge transfer from the mesityl group (π, HOMO) to the boron center (p_π, LUMO) (See SI Fig. S22, S23 and Table S1).

Photochemical reactivity studies

Compounds 1a–1d could be isolated by column chromatography on silica gel, and do not decompose after heating to 70 °C (Fig. S11). Therefore, they are air- and thermally-stable, but they are all sensitive to UV light irradiation, typically with a notable color change from colorless to dark purple (2a, 2b, 2d) or green (2c). First, we conducted a photochemical reactivity study using 2-dimesitylboryl-3-ethynylthiophene (1a) as the starting material. A solution of 1a (5.0 mg in 0.5 mL toluene-d₈ or benzene-d₆) was irradiated with a UV light source (313 nm, 160 W) at room temperature under N₂ atmosphere. The colorless solution gradually turned into a dark purple solution after several hours of irradiation (Fig. 2b), and the UV-vis absorption spectrum of 2a exhibits a new broad band at λ ≈ 550 nm (Fig. 4c). Monitoring the photoreaction by ¹H and ¹¹B{¹H} NMR spectra revealed that this reaction was clean, with high regioselectivity without any detectable intermediates, and produced only one product, 2a, with some starting material, 1a, remaining after more than 100 hours of irradiation (Fig. 2b and c). Transient absorption spectroscopy revealed that the conversion from 1 to 2 occurs over ∼7 ns, making the detection and capture of the intermediates difficult (Fig. S15). EPR and radical trapping experiments were used to investigate the involvement of the radical intermediates in this photo-transformation process (Fig. S16, S19–S21). The ¹¹B{¹H} NMR spectrum showed two resonance signals at δ = 67.50 and 47.82 ppm, and the former signal belongs to the starting material 1a, while the other upfield-shifted resonance signal corresponds to the product 2a, which is similar to other trisubstituted boron-containing compounds featuring B=N⁷¹ or B=C^72,73 bond. After around 150 hours of irradiation, the ¹H and ¹¹B{¹H} NMR spectra indicated that around 76% of 1a was consumed and converted into the product 2a based on the ¹H NMR spectra. Importantly, compound 2a is air-stable and can be purified by silica gel column chromatography. The photoreaction can be performed on a large synthetic scale to prepare 2a in 42% isolated yield. The ¹H NMR spectrum (in THF-d₈) of isolated 2a was consistent with the spectrum of the product generated in the photoreaction of the NMR tracking experiments, where singlet alkenyl protons were observed at δ = 7.42, 6.57, 6.47 ppm with only one mesityl aromatic Mes-H signal at δ = 6.82 ppm (See SI Fig. S73). Compound 2a was fully characterized by NMR spectroscopic analysis, HRMS, and X-ray crystallography (See the SI and Fig. 4b). The control experiments confirmed that the presence of air (Fig. S12) or lowering the temperature (Fig. S13) is unfavorable for this photo-transformation.

To investigate the steric and electronic influence of the substrate on this unusual photoreaction, compounds that bear different substituents, heteroarenes, or extended π-conjugated heteroarenes (1b–1d) were prepared and well characterized (Fig. 2a). Compounds 1b–1d undergo a similar structural photo-transformation to that of 1a. The photochemical reaction rate and the isolation yield (50%) of 1b are similar to those of 1a. However, for 1c and 1d, prolonged irradiation (30 h) was required to produce 2c and 2d in ∼70% yields, as suggested by in situ ¹H NMR spectroscopic studies (See SI Fig. S3 and S4). It is therefore evident that the conversion rates of the compounds containing an additional conjugated benzene ring (1c and 1d) are slightly lower than those of 1a and 1b (Fig. S8 and S9). The products 2b–2d were also characterized by NMR spectroscopy and HRMS. The structures of 2b–2d were disclosed by the single-crystal X-ray diffraction analysis and were found to be similar to 2a (Fig. 4b and See SI S29–S32). Compound pre-1a was taken as an example to investigate the photoreactivity of the internal alkyne substrates. The ¹H NMR tracking experiments showed that pre-1a does not undergo any structural changes upon irradiation with UV light (Fig. S10).

In order to investigate the origin of the proton in the central boron-containing six-membered ring (e.g., H3 in Fig. 4b), we carried out the following deuterium labelling reactions. The deuterium labelled compound 1e was prepared and irradiated under the same conditions as 1a–1d, which underwent a similar photo-transformation process based on the NMR spectroscopic analysis (Fig. 3). Compared with the ¹H NMR spectrum of undeuterated 2b, the typical singlet peak at δ = 7.28 ppm is diagnosed as the proton atom in the boron-containing six-membered ring, which is absent in the ¹H NMR of 2e (Fig. 3, top one). In addition, the ²H NMR spectrum of 2e also detected the D resonance signal at 7.37 ppm (See SI Fig. S98) with a similar chemical shift as in the ¹H NMR spectra of 2b (Fig. 3, bottom one). Thus, the deuterium labelling experiments unambiguously confirmed that the H-atom transferred to the Csp² atom of the boron-containing six-membered ring originates from the Csp-H of the alkyne group, which is in line with theoretical findings on the mechanism (vide infra). These results also highlight the synergistic effect of boron, and the terminal alkyne group plays a key role in these unprecedented photo-promoted Büchner-type ring expansion reactions.

Fig. 3 Comparison of the ¹H NMR spectra of a mixture of 1b, 2b (bottom, 5 mg of 1b in C₆D₆ irradiated at 313 nm for 48 hours) and a mixture of deuterium-labelling 1e, 2e (top, 10 mg of 1e in C₆D₆ irradiated at 313 nm for 48 hours) with the assignment of the aromatic peaks. The solvent peaks are labelled with *.

Solid-state structures

The molecular structures of 2a–2d are shown in Fig. 4b and the SI Fig. S29–S32, which confirm the presence of a boron-embedded polycyclic framework. Given the structural similarity among 2a–2d, the discussion focuses on the solid-state structure of 2a, in which the fused five-membered ring is derived from the original thiophene ring. One of the mesityl groups on the boron center of the original 1a was inserted by the terminal Csp atom of the alkynyl group, undergoing Büchner-type ring expansion to form a new seven-membered ring in 2a. The C≡C bond of 1a was transformed into a C=C bond in 2a, which is part of the boron-doped six-membered ring. The newly formed seven-membered ring unit is similar to cycloheptatriene, featuring alternating C=C (∼1.32 Å) and C–C bonds (∼1.53 Å), which is consistent with the chemical shift of olefinic protons (δ = 6.19 to 6.14 ppm) on the seven-membered ring. However, the difference between 2a and cycloheptatriene is that the seven carbon atoms of the former are all sp²-hybridized, whereas in the latter, six C atoms are sp²-hybridized, and one C atom is sp³-hybridized. The B(1)–C(1) bond length is 1.518(2) Å, which is between the B–C single bond length (B1–C16 = 1.5808(18) Å of 2a) and reported B=C double bond distances (∼1.37–1.44 Å),^74,75 thus suggesting partially double-bond character for the former. In the 2a molecule, the C3–C4 bond and all seven C–C bonds in the heptagonal ring also exhibit partial double-bond character, as indicated by their bond lengths (Fig. 4b). Thus, the structural data suggest that the polycyclic compounds 2a–2d represent a class of highly conjugated molecules consistent with the intense absorptions in their electronic absorption spectra (Fig. 4c). Compounds 2a–2d exhibit very similar absorption bands, including a broad band around 550 nm, which is responsible for the dark purple or green color (Fig. 4c). TD-DFT theoretical analysis (B3LYP/6-31g(d)) suggests that these absorption bands of 2a–2d originate from charge transfer (CT) transitions involving mainly the HOMOs to LUMOs (Fig. 4c and See SI Tables S19–S26) of the conjugated central boron-containing six-membered ring and the newly formed seven-membered ring with highly delocalized π-electronic character.

Fig. 4 Crystal structures of 1a (a) and 2a (b), thermal ellipsoids set at 30% probability, hydrogen atoms are omitted for clarity, selected bond lengths [Å] and angles [°] for 1a: B1–C1: 1.546 (3), B1–C7: 1.578 (3), B1–C16: 1.581 (3), C1–C2: 1.391(3), C2–C5: 1.404(3), C5–C6: 1.215(3); for 2a: B1–C1: 1.518(2), B1–C5: 1.5733(18), B1–C16: 1.5808(18), C1–C2: 1.3931(19), C2–C3: 1.4345 (18), C3–C4: 1.3622(19), C4–C5: 1.4877(18), C5–C6: 1.3906(18), C6–C7: 1.4492(18), C7–C8: 1.357(2), C8–C9: 1.435(2), C9–C10: 1.3469(19), C4–C10: 1.4851(17), C10–C13 1.511(2), C11–C12 1.363(2); C1–B1–C5 113.18(11), C5–B1–C16 129.60(12), C1–B1–C16 117.10(11), C2–C1–B1 123.37(11), C1–C2–C3 119.53(12), C2–C3–C4 123.18(12), C3–C4–C5: 121.26(11), C4–C5–B1: 117.22(11). (c) Absorption spectra of 2a–2d in dichloromethane (10⁻⁴ M) and the calculated HOMO, LUMO diagrams, and oscillator strength (f) involved in the S₀ → S₁ vertical excitation (B3LYP/6-31g(d)) for 2a. (d) Calculated NICS(0), NICS(1) values (ppm) of 2a.

Density functional theory calculations and mechanistic studies

To gain insight into the electronic structure of 2a–2d, density functional theory (DFT) calculations were performed at the B3LYP/6-31g(d) level of theory. The nucleus-independent chemical shift (NICS) values of 2a–2d were calculated to illustrate the aromaticity of these novel polycyclic compounds, and the NICS values are listed in the SI Tables S27–S30. Both of the NICS(0) and NICS(1) values of the central BC5 six-membered rings are negative (NICS(0) = −4.12, −4.47, −4.57, −5.32 ppm and NICS(1) = −6.78, −6.95, −7.73, −8.09 ppm for 2a–2d respectively), indicating π-aromatic character of the central BC5 six-membered rings in 2a–2d. However, the aromaticity of the central BC5 six-membered rings in 2a–2d is weaker compared with that of the SC4 ring in 2a with more negative NICS(0) (−12.18 ppm) and NICS(1) (−9.92 ppm) values, or the phenyl ring in 2c (NICS(0) = −9.86 ppm and NICS(1) = −11.12 ppm). The C7 seven-membered rings in 2a–2d also show weak aromaticity based on the NICS values (NICS(1) = −3.39, −3.04, −3.24, −3.97 ppm for 2a–2d, respectively).

To gain a deeper understanding of the mechanism in these unprecedented photo-promoted Büchner-type ring expansion reactions, theoretical calculations by using the density functional theory methods have been employed at the B3LYP-D3/Def2-TZVP/SMD(toluene)//B3LYP/6-31G(d,p)/SMD(toluene) level (Fig. 5). First, the ground-state reactant 1a absorbs UV light to generate the excited singlet state with an absorption wavelength of 321.51 nm. After the thermal relaxation and internal conversion, the S1 state of 1 has been located with the free energy of 4.7 kcal mol⁻¹, where the free energy of the triplet state species ³2 is set to relative zero. Subsequently, the excited triplet species ³2 is formed via intersystem crossing. The two corresponding singly occupied molecular orbitals on the alkynyl and boranyl moieties were obtained from DFT calculations of the resulting excited triplet state. This indicates an effective electron transfer from the alkynyl group to the boron atom in the excitation process. Then, the terminal allenyl radical could undergo a radical addition to the phenyl moiety. Considering the possible sites, the following discusses two pathways. In the first pathway, the radical addition to the carbon atom bonded to the methyl group would proceed readily via the transition state 3-ts with a free energy barrier of only 14.1 kcal mol⁻¹, whereas the radical addition to the carbon atom bonded to the boron center via 14-ts would be kinetically unfavorable with a very high energy barrier of 42.5 kcal mol⁻¹, and is therefore not discussed in further. After the addition, the seven-membered ring intermediate 4 with the allenyl group forms with an exergonicity of 3.1 kcal mol⁻¹. Subsequently, the radical undergoes another intramolecular radical addition to the allenyl moiety, affording the fused ring intermediate 6 via the three-membered ring transition state 5-ts with a free energy barrier of 10.6 kcal mol⁻¹. It should be noted that the fused ring intermediate could open the ring to generate expanded ring intermediate ³8 via the reverse electrocyclic transition state 7-ts with a small free energy barrier of 4.4 kcal mol⁻¹, accompanied by the release of ring strain. At last, the triplet alkenyl radical could form the final product 2a via a formal 1,2-hydrogen-atom transfer (HAT) process. The stepwise intermolecular HAT process proceeds via transition states 9-ts and 12-ts, with free energy barriers of 5.7 and 14.5 kcal mol⁻¹, respectively. Notably, a minimum energy crossing point (MECP) between the triplet and singlet states of intermediate 8 has also been located, with an energy barrier of 5.4 kcal mol⁻¹. Subsequently, a 1,2-proton shift occurs via transition state 17-ts with a free energy barrier of 5.6 kcal mol⁻¹. Considering the concentration of triplet ³8, the intramolecular proton shift after MECP would be a favorable pathway with less entropy loss, which is also consistent with the deuterium-labelling experimental results (Fig. S14). Furthermore, the trace water, benzene, toluene, and terminal alkyne have also been considered in the 1,2-HAT process, which are unfavorable for these reactions (See the SI for full details).

Fig. 5 A plausible reaction mechanism and the free energy profile for the light-promoted expansion ring process.

Conclusions

To conclude, photolyzing a series of air-stable ethynyl and boron functional groups substituted heteroarenes 1a–1d has enabled us to discover an unprecedented Büchner-type ring expansion reaction to produce boron-doped polycyclic compounds 2a–2d. It should be emphasized that these reactions are the first examples of Büchner-type ring-expansion reactions using a stable terminal alkyne as the carbon-atom-transfer source to form a C7-ring in any synthetic system, thereby developing a new synthetic strategy for boron-containing polycyclic compounds. DFT study on the electronic structures of 2a–2d revealed the π-electron delocalization through the newly formed central boron-containing six-membered ring and seven-membered ring, and the central boron-containing six-membered ring exhibits substantial aromaticity, reflected by calculated NICS values, which is in line with their electronic absorption spectra. Theoretical studies on the reaction mechanism revealed that the formation of biradical species, the intramolecular HAT process, and the synergistic effect of boron and the terminal alkyne group are important for this complex photo-transformation, which involves many chemical bond cleavages and formations. This work develops a new strategy for modifying the arene skeleton under mild reaction conditions and provides a promising approach to the construction of novel conjugated polycyclic aromatic boron-containing compounds with potential functional applications.

Author contributions

H. J. L. conceived the original idea and wrote the manuscript. H. J. L., B. Y., and J. D. supervised the work, analyzed all the data, and proofread the manuscript. B. X., X. L., M. F., and H. J. L synthesized and characterized all the compounds. H. X. J., S. J. L., and Y. L. conducted the computational studies. H. J., R. F., and B. H. analyzed the crystallographic characterization data. R. F. analyzed the EPR spectra. W. Z. and H. J. L. conducted and analyzed the 2D NMR data.

Conflicts of interest

There are no conflicts to declare.

Data availability

CCDC 2456755–2456761 and 2483356 contain the supplementary crystallographic data for this paper.^76a–h

Supplementary information (SI): synthetic methods, characterization data, and computational details. See DOI: https://doi.org/10.1039/d6sc02310j.

Acknowledgements

We gratefully acknowledge funding and support from the National Natural Science Foundation of China (No. 22101007, 22471242, 22509189), the Natural Science Foundation of Henan Province (No. 242300421344), and Zhengzhou University. The authors thank Prof. Xiangkun Jia for assistance with the transient absorption spectroscopy testing and analysis.

Notes and references

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