Metal-free visible-light-induced borylative/silylative pyridylation of vinylarenes

Abstract

We present a metal-free and mild three-component reaction involving vinylarenes, NHC–BH₃ complexes, hydrosilanes, and 4-cyanopyridine. This versatile reaction encompasses a broad scope, with over 40 examples of vinylarenes, NHC–BH₃ complexes, and hydrosilanes, leading to the synthesis of diverse β-pyridinyl boranes and β-pyridinyl silanes. Furthermore, we demonstrate the potential of this methodology by functionalizing pharmaceutical molecules, showcasing its practical applications. Remarkably, this reaction was performed with simple and inexpensive benzophenone, and notably, it did not require the presence of alkali or any costly transition metals.

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Introduction

Organoboron compounds are effectively employed in a plethora of transformations pertinent to organic synthesis, catalysis, drug invention, materials science, and various other disciplines.¹ As a result, a plethora of borylation reagents and synthetic techniques have been developed until now,² encompassing hydroboration of alkenes and alkynes,³ the Miyaura borylation reaction,⁴ and C–H bond borylation,⁵ all of which have significantly propelled the progress of boron-centred chemistry. Despite these accomplishments, the endeavour to develop practical and widely applicable borylation reactions, which employ conceptually novel mechanisms to synthesize diverse functionalized organoboron compounds, continues to be a prominent aim in organic chemistry.

Recently, the field of boron-centred radical chemistry has gained significant momentum, with a significant proportion of these radicals stemming from N-heterocyclic carbene (NHC)–boranes, along with amine–boranes, which have emerged as key players in driving the advancement of this dynamic research area.⁶ The outstanding research conducted by Curran, Fensterbank, Lacôte, Malacria, and Wang⁷ demonstrates that N-heterocyclic carbene (NHC)–boryl radicals can be formed through the abstraction of hydrogen atoms from NHC–BH₃ complexes, these being potent intermediates that facilitate the synthesis of a diverse array of high-value boron compounds.⁸ For instance, the seminal Giese-type addition of NHC–boryl radicals to unsaturated C–C bonds was independently pioneered by the groups of Curran and Wang,^8,9 marking a significant milestone in the field. More recently, the Wang and Xie groups have independently disclosed a novel addition reaction involving NHC–boryl radicals generated through hydrogen atom transfer to activated alkenes.^10,11 This advancement leverages the cooperative power of homogeneous photocatalysis and thiol catalysis, further expanding the boundaries of boron-centred radical chemistry. NHC–boryl radicals reacted with polyfluoroarenes, gem-difluoroalkenes, trifluoromethyl alkenes, and substituted 1,4-dicyanobenzenes have also been achieved by the Yang, Wu, Wang, and Curran groups.¹²

Despite the considerable advancements achieved in the addition of NHC–boryl radicals to multiple bonds,¹³ the utilization of these radicals in multicomponent reactions has thus far remained relatively scarce. In 2020, Wang and his team discovered a novel three-component reaction catalyzed by Ir(ppy)₃, which involves the oxidation of NHC–BH₃ to generate NHC–boryl radicals, followed by the addition of these NHC–boryl radicals to styrenes (Fig. 1a).^12c In addition, the Quan group reported on the dialkylation of NHC–BH₃ facilitated by a tetrabutylammonium decatungstate (TBADT)-catalyzed hydrogen atom transfer (HAT) process.¹⁴ Nevertheless, the use of transition metals iridium and tungsten in these two processes may introduce limitations regarding scalability and sustainability.

Fig. 1 (a) Iridium metal-catalyzed arylboration of alkenes via oxidation of NHC–BH₃. (b) 4CzIPN-catalyzed silylation pyridylation reaction. (c) Metal-free three-component reactions with NHC–BH₃ complexes via HAT.

Consistent with our continuing dedication to developing a metal-free multicomponent reaction driven by hydrogen atom transfer (HAT) from diverse C–H precursors,¹⁵ we envision a novel approach where the boryl radical is generated through a HAT process with NHC–BH₃ (Fig. 1c). This innovative strategy would enable the synthesis of diverse compounds through three-component reactions involving styrenes and cyano-pyridines, all without the need for any metallic catalysts. Such a development would further broaden the scope of environmentally friendly and cost-effective synthetic methodologies. Here, we showcase that only utilizing a benzophenone catalyst enables the efficient three-component borylative pyridylation of an extensive array of vinylarenes, leading to the synthesis of the targeted β-pyridinyl boranes in satisfactory yields under mild conditions. The key to the success of this reaction lies in the employment of an economical benzophenone catalyst, devoid of both alkaline metals and costly transition metals. Furthermore, this approach presents an innovative three-component pathway to β-pyridinyl silanes, which serve as crucial structural units in diverse biologically active molecules.

Results and discussion

We initiated our research by employing 20 mol% benzophenone as the catalyst, styrene as the alkene component, and NHC–BH₃ as the source of boryl radicals, under irradiation with 395 nm light for 12 hours, resulting in the isolation of the desired product in 53% yield (Table S1,† entry 1). When the reaction time was prolonged to 36 hours, the desired product was isolated in 78% yield (Table S1,† entry 3), demonstrating a significant improvement in production efficiency. After an extensive screening of various catalysts (Table S1,† entries 6–9), benzophenone emerged as the most suitable choice for the reaction. To enhance product yield, an attempt was made to investigate the NHC–BH₃ loading; however, this adjustment did not result in a corresponding increase in the yield of the desired product (Table S1,† entries 10–13). We further delved into the effect of varying both the concentration and the solvent (Table S1,† entries 14–19). Surprisingly, increasing the concentration led to a slight decrease in product yield. In addition, switching to alternative solvents also resulted in a decrease in the yield (DMSO) or totally suppressed the reaction (DMF and DCM), indicating that the initial conditions were optimal for maximizing the yield. When the reaction was conducted in the dark or in the absence of a catalyst, no product was observed. However, when the reaction was performed under air, the yield was significantly reduced to 46%. These results indicated that light irradiation, benzophenone, and the exclusion of oxygen were crucial to the success for this reaction (Table S1,† entries 20–22).

After identifying the optimal conditions for the reaction, we initially explored the versatility of vinylarenes as substrates in the multicomponent reaction. As shown in Scheme 1, the borylative pyridylation reaction occurred smoothly with a variety of substituted vinylarenes. Diverse styrenes proved to be efficient linchpins in the formation of borylative pyridylation products, facilitated by the reaction with NHC–BH₃ and cyanopyridine. These reactions afforded borylative pyridylation products in good yields (ranging from 30% to 78%) with styrenes that possess electron-withdrawing and electron-donating functional groups. The electronic properties of styrenes had a pronounced impact on their reactivity, leading to notable variations. As an illustration, the target product was obtained in a relatively low yield of just 30% when using electron-rich styrenes, which is attributed to the destabilizing effect of the methoxy group on the benzyl radical (4h). In addition, the desired products were efficiently obtained in good yields through the successful transformation of α-methyl substituted styrene (4k and 4l). To our delight, 2-vinylthiophene as a vinylheterocycle has also been transformed to the corresponding product in 71% yield (4m). Moreover, we found that only the Giese addition product of the boryl radical was isolated with 2-vinylpyridine and 4-vinylpyridine as the substrates (4n and 4o). To demonstrate its potential pharmaceutical applications, we applied our boryl radical-involved multicomponent reaction in late-stage functionalization reactions. In particular, we treated substrates derived from ibuprofen, phytol, geraniol, oxaprozin, epiandrosterone, and estrone with NHC–BH₃ under our reaction conditions and obtained the borylative pyridylation products in 31–81% yields (4p–4u).

Scheme 1 Scope of vinylarenes. Reaction conditions: compound 1 (0.4 mmol), compound 2 (0.2 mmol), compound 3a (0.6 mmol), and benzophenone (20 mol%) in MeCN (4 mL), room temperature, 395 nm blue LEDs, and 36 h, with an isolated yield.

We subsequently tested the substrate scope of NHC–BH₃ in this multicomponent reaction and found that NHC–boranes were efficiently converted to the corresponding products in 66–72% yields (Scheme 2, 4v–4x).

Scheme 2 Scope of NHC–BH₃ complexes. Reaction conditions: compound 1 (0.4 mmol), compound 2 (0.2 mmol), compound 3 (0.6 mmol), and benzophenone (20 mol%) in MeCN (4 mL), room temperature, 395 nm blue LEDs, and 36 h, with an isolated yield.

To enhance the synthetic versatility of this metal-free multicomponent reaction, we embarked on an exploration of reactions employing alternative heteroatom hydrogen (X–H) precursors. Organosilicon compounds are ubiquitous in numerous facets of contemporary chemistry, notably in chemical synthesis where organosilanes play a pivotal role in classic reactions, including the Fleming–Tamao, Hosomi–Sakurai, and Hiyama coupling reactions.¹⁶ In addition, the integration of silicon into pharmaceutical compounds has been shown to enhance their pharmacokinetic properties.¹⁷ Hence, the pursuit of novel methodological advancements aimed at the selective synthesis of intricate organosilicon molecules is of immense importance in organic synthesis.¹⁸

As depicted in Scheme 3, our excitement stemmed from the discovery that a wide array of hydrosilanes served as efficient substrates under the optimized conditions for borylative pyridylation reactions, affording silapyridylation products in yields ranging from moderate to outstanding. In stark contrast to prior investigations, this methodology transcends the limitation of solely utilizing sterically congested silane, as it demonstrates compatibility with a diverse range of sterically less hindered hydrosilanes as well. Trialkylsilanes, including those with varying alkyl group sizes such as triethylsilane 5a, tripropylsilane 5b, triisopropylsilane 5c, and tributylsilane 5d, all yielded the corresponding silapyridylation products in excellent yields. Furthermore, the silapyridylation reaction proceeded smoothly with triarylsilanes (5j–5l), yielding the corresponding products in good to excellent yields. Remarkably, this procedure exhibits the potential to be extended to primary and secondary silanes (5f and 5g), affording products that possess two Si–H bonds and one Si–H bond, respectively.

Scheme 3 Substrate scope of hydrosilanes and styrenes. Reaction conditions: compound 1 (0.4 mmol), compound 2 (0.2 mmol), compound 3 (4 mmol), and benzophenone (20 mol%) in MeCN (4 mL), room temperature, 395 nm blue LEDs, and 36 h, with an isolated yield.

The scope of styrenes was explored subsequently. As shown in Scheme 3, styrenes with an electron-withdrawing and electron-donating substituent at the para-position, such as halogen (5n and 5p), trifluoromethyl (5o), ester (5r), methyl (5m), and methoxy (5y), delivered the expected products in good yields. Upon further investigation into this transformation with vinyl heterocycles, it was discovered that both 2-vinylpyridine (5t) and 2-vinylthiophene (5s) exhibited satisfactory performance, yielding the respective heterocycle β-pyridinyl silane products in 74% and 48% yields. Furthermore, 1,1-disubstituted styrenes can be effectively utilized to yield products (5x and 5y) featuring a quaternary carbon center in high yields. This mild and efficient reaction has been successfully employed in the late-stage functionalization of ibuprofen (5aa).

To demonstrate the potential application of our metal-free photoredox catalytic borylation methods, we successfully conducted a reaction by treating 4a with pinacol and N-chlorosuccinimide (NCS) in toluene at room temperature, resulting in the formation of β-pyridine boronate 11 in satisfactory yield (Fig. 2a). To gain insight into the underlying mechanism of this transformation, we designed and performed several meticulous control experiments. Among them, a radical clock experiment utilizing vinylcyclopropane as the olefin substrate was carried out, yielding the ring-open product 8 in 55% yield as a mixture of Z/E isomers in a 1 : 1 ratio (Fig. 2b).

Fig. 2 (a) Synthetic application; (b) radical quenching experiment with TEMPO; (c) radical clock experiment; (d) KIE experiment; and (e) light on/off experiment.

Notably, when 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was introduced as a radical scavenger into the reaction mixture, the yield of the desired product was markedly diminished (Fig. 2c). The measurement of the kinetic isotope effect (k_H/k_D) for borane BH/BD was determined to be 1.0 (Fig. 2d), suggesting that the breaking of the B–H bond may not be the rate-determining step. In addition, we performed light on/off experiments, whose results pointed to the necessity of light irradiation for the reaction to proceed (Fig. 2e). Collectively, these findings strongly suggest that this protocol likely involves boron-centred radical intermediates as key players in the catalytic cycle (Fig. S7†).

Conclusions

In summary, we have disclosed a versatile strategy for the borylative and silylative pyridylation of vinylarenes, employing NHC–BH₃ complexes and hydrosilanes. This approach enables the late-stage functionalization of complex, pharmaceutically significant molecules with good to excellent yields. Furthermore, it demonstrates broad tolerance for various functional groups, offering a unique method for accessing boron- and silicon-containing complex compounds. Our laboratory is currently engaged in intensified efforts to explore additional benzophenone-catalyzed multicomponent reactions.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Jiangxi Provincial Natural Science Foundation (20224BAB203013) and the start-up Fund of Nanchang University (9167-28170030).

References

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo01702a

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