N-Alkynylpyridinium Salts: Highly Electrophilic Alkyne–Pyridine

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N‑Alkynylpyridinium Salts: Highly Electrophilic Alkyne−Pyridine Conjugates as Precursors of Cationic Nitrogen-Embedded Polycyclic Aromatic Hydrocarbons Naoyuki Toriumi,*,† Norihito Asano,† Kazunori Miyamoto,† Atsuya Muranaka,‡ and Masanobu Uchiyama*,†,‡ †

Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Elements Chemistry Laboratory, RIKEN, Wako-shi, Saitama 351-0198, Japan

‡

S Supporting Information *

Scheme 1. Synthetic Methods for Quinoliziniums

ABSTRACT: We achieved the first synthesis of Nalkynylpyridinium salts, by reacting pyridines with alkynyl-λ3-iodanes. The N-alkynylpyridiniums exhibit highly electron-accepting character with extended πconjugation. The electrophilic alkynyl groups were readily susceptible to Michael addition and 1,3-dipolar cycloaddition to afford various N-alkenylpyridiniums. Ringfused pyridiniums were synthesized through intramolecular cyclization, demonstrating the utility of N-alkynylpyridiniums for the design of various electron-deficient cationic nitrogen-embedded polycyclic aromatic hydrocarbons with unique optical and electrochemical properties.

P

yridine and other sp2 nitrogen-containing heteroaromatics are fundamental structural motifs in organic chemistry, and are widely found in natural products, pharmaceuticals and functional materials.1 The basic and nucleophilic nitrogen atom of pyridine allows formation of diverse cationic pyridinium salts.2 Recently, there has been growing interest in cationic nitrogen-embedded polycyclic aromatic hydrocarbons (cNePAHs), whose structures contain quinolizinium moieties with the pyridinic nitrogen attached to sp2 carbon.3 These materials can be regarded as substructures of nitrogen-doped graphenes, which have many potential applications, for example, in fuel cells, batteries and semiconductors.4 The physicochemical properties of cNe-PAHs are also of fundamental interest. Several synthetic methodologies have been developed for quinolizinium and benzoquinolizinium salts since the first examples were reported in the 1950s.5 Photocyclization of Nstyrylpyridinium6 (Scheme 1a) and ring-closing methathesis (RCM) of N-vinylpyridinium7 (Scheme 1b) give benzo[a]quinolizinium salts in moderate yields, but multistep preparation of the starting materials is required and it is difficult to introduce substituents at the bridging ethylene moiety. Many transition-metal-catalyzed C−H activation annulation reactions have been applied to the construction of substituted quinolizinium salts over the past ten years (Scheme 1c).8 These reactions often offer excellent yields from 2-arylpyridines/N-arylpyridiniums and internal alkynes. However, there are no examples for terminal alkynes and the regioselectivity for © XXXX American Chemical Society

unsymmetrical alkynes is low, limiting potential applications for the synthesis of cNe-PAHs. To overcome this synthetic challenge, we focused on unprecedented N-alkynylpyridinium salts as prospective precursors. Alkynes generally have versatile functionality and can undergo many transformations.8,9 Ammonium salts with an alkynyl group on the nitrogen atom are referred to as “ynammonium” salts. Though their first description dates back to the late 19th century,10 there have been only two reports of successful isolation and characterization of ynammoniums,11 and N-alkynylpyridiniums have never been synthesized. In 1971, pioneering work by Miller revealed that triethylamine reacts with bromoacetylene to form ethynyl(triethyl)ammonium bromide.11a In 1985, Katritzky successfully synthesized N-alkynylacridinium salts from N-alkynylacrydones, which were obtained by the reaction between acridone and bromoacetylenes under basic conditions.11b Both groups attempted N-alkynylation of pyridines using bromoacetylenes, but only obtained pyridine adducts or tars.11b,12 The lack of preparation methods and the unknown chemical properties of ynammonium salts are in sharp contrast to the well-established chemistry of ynamines.13 Herein we report the first synthesis of N-alkynylpyridiniums and other heterocyclic ynammonium Received: January 10, 2018 Published: March 5, 2018 A

DOI: 10.1021/jacs.8b00356 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society Table 1. Screening of Alkynyl-λ3-iodanes*

entry

R

product

solvent

temperature

yields [%]c

1a 2a 3a 4b 5b 6b

TMS (2a) TIPS (2b) TBDPS (2c) H (2d) Ph (2e) t Bu (2f)

3aa 3ab 3ac 3ad 3ae 3af

ClCH2CH2Cl ClCH2CH2Cl ClCH2CH2Cl CH2Cl2 CH2Cl2 CH2Cl2

30 °C 85 °C 85 °C −78 °C to rt −78 °C to rt −78 °C to rt

92 (88) quant. (94) 80 (53) 69 (57) n.d. trace

*

Reaction conditions: a1a (1.1 equiv), 2a−c (1 equiv), 16 h. b1a (1 equiv), 2d−f (1 equiv), 12 h. cNMR yields determined using mesitylene as an internal standard. Isolated yields shown in parentheses. n.d. = not detected.

Table 2. Scope of the N-Alkynylation Reaction

salts (Scheme 1d). The key strategy for the present achievement is the use of alkynyl-λ3-iodanes as potent alkynyl transfer reagents.14 The highly electrophilic nature of the alkyne moiety of N-alkynylpyridiniums enables their facile conversion into cNe-PAHs via Michael addition and successive intramolecular cyclization. To initiate our study, we investigated the reactions of 4phenylpyridine (1a) with various alkynyl(phenyl)(triflato)-λ3iodanes 2a−f (Table 1), which were readily prepared from silylor stannylacetylenes using the reported PhI(OAc)2/Tf2O system.14c The functional groups at the β-alkynyl termini of the iodanes greatly influence the reactivities because alkynyl transfer reactions are generally initiated by a nucleophilic attack on the β-acetylenic carbon.14e Trimethylsilylethynyl-λ3-iodane (2a) reacted smoothly with 1a at 30 °C to give 3aa in 88% yield. This transformation could be operated on a gram-scale, providing 3aa in 89% yield (1.07 g). Iodanes with bulkier TIPS and TBDPS groups also afforded the corresponding Nalkynylpyridiniums 3ab and 3ac at elevated temperature. The discrepancy between the isolated yield and NMR yield of 3ac is due to strong adsorption of the product on silica gel during column chromatography, whereas 3aa and 3ab were purified by recrystallization from the crude mixtures. We found that low temperature is appropriate for sterically less hindered ethynylλ3-iodane (2d). However, the reactions gave complex mixtures, and little or no formation of the desired pyridiniums was detected when iodanes 2e and 2f were employed, although complete consumption of the starting materials was confirmed by 1H NMR analysis. ESI-MS measurements suggested addition of the counter triflate anion to the N-alkynylpyridiniums in these cases. Isolated N-alkynylpyridiniums 3aa−ad are stable for at least 6 months under ambient conditions, and were characterized by ESI-MS and 1H, 13C, and 19F NMR, IR and UV−vis spectroscopies. The introduction of an alkynyl group at the sp2-nitrogen was unambiguously confirmed by X-ray singlecrystal diffraction analysis of 3ad (Figure S4-1). This is the first example of X-ray structural elucidation of an ynammonium salt. We next examined the substrate scope of this N-alkynylation, focusing on 2a and 2d as appropriate alkynyl-λ3-iodanes (Table 2). Pyridines with phenyl substituents 1b and 1c were converted to 3ba and 3ca with 2a, respectively, in excellent yields, though sterically hindered 1b required a high temperature. We found that 1b and the even more crowded 2,4,6triphenylpyridine (1i) readily underwent the alknylation using 2d. Electron-rich and simple pyridines 1d−f afforded the

*

NMR yields determined using mesitylene as an internal standard. Reaction conditions: a1 (1.1 equiv), 2a (1 equiv), ClCH2CH2Cl, 30 °C, 16 h. b1 (1 equiv), 2d (1 equiv), CH2Cl2, −78 °C to rt, 12 h. cRun at 85 °C. dRun at 85 °C for 60 h. eRun in MeCN at −40 °C to rt.

desired products 3da−fa. This N-alkynylation was also extended to other heteroaromatics such as imidazole, pyrazole, quinoline, isoquinoline and tetraazaporphyrin15 to give novel N-alkynylated species (3ga, 3ha and 3kd−md) in moderate to high yields. However, the reactions got messy and did not yield the desired products when pyridines with electron-withdrawing substituents such as CF3, halogen and ester were used. To probe the effects of the alkynyl group on the physicochemical properties, we compared the absorption and fluorescence spectra of N-methyl-4-phenylpyridinium triflate (3a′) with those of 3aa−ad in MeCN (Table 3, Figure S3-1). Red-shifted absorption and fluorescence bands were observed at 331−338 and 401−416 nm, respectively, for 3aa−ad, as compared with those of 3a′. Fluorescence quantum yields of B

DOI: 10.1021/jacs.8b00356 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

Table 4. One-Pot Synthesis of Quinoliziniumsa

Table 3. Optical and Electrochemical Properties of Pyridinium Derivatives* compound

λabs [nm]

3a′ 3aa 3ab 3ac 3ad

293 335 337 338 331

ε [M−1 cm−1]

λflu [nm]

ΦF

Epc, red [V vs Fc/Fc+]

× × × × ×

373 406 403 401 416

0.718 0.663 0.002 0.011 0.373

−1.55 −1.02 −1.04 −0.97 −1.03

1.99 2.74 3.33 4.24 2.60

104 104 104 104 104

*

Measured in MeCN.

the N-alkynylpyridiniums cover a wide range (3aa: ΦF = 0.663; 3ab: ΦF = 0.002; 3ac: ΦF = 0.011; 3ad: ΦF = 0.373), probably due to the charge-transfer character of the S1→S0 transition in 3ab and 3ac (Figure S3-2). Extension of π-conjugation by the alkyne moiety was further demonstrated in 3md, which has longer absorption and fluorescence maxima (667; 685 nm) than those of 1m (593; 607 nm) in CHCl3 (Figure S3-3). These results indicate that N-alkynylation of heteroaromatic dyes can be a useful method for extending their absorption and emission wavelengths. The properties of the N-alkynylpyridiniums were further studied by cyclic voltammetry (CV) (Table 3, Figure S3-4). The first reduction potentials (Epc, red) of 3aa−ad ranged from −0.97 to −1.04 V (vs Fc/Fc+), being considerably higher than that of 3a′ (−1.55 V). Thus, Nalkynylpyridiniums with extended π-conjugation have high electron-accepting abilities. To showcase the synthetic utility of the electrophilic Nalkynylpyridiniums, we then examined reactivities at the alkyne moiety. Michael addition of nucleophilic hydrogen halides and thiol to the triple bond of 3ad gave trans-adducts 4a−d in high yields at room temperature (Scheme 2). The structure of 4a

a

All the reactions were run in MeCN.

The reported syntheses of quinolizinium salts also suffer from low regioselectivity as to asymmetrical substituents (Scheme 1c). To address this problem, functionalization of quinoliziniums was demonstrated using 3ba (Scheme 3). Scheme 3. Synthesis of Functionalized Quinolizinium

Stepwise dibromination and photocyclization readily yielded quinolizinium 7 and its bromo and silyl substituents can be useful scaffolds for various transformations to introduce different peripheral substituents. We supposed that N-alkynylpyridiniums would undergo intramolecular cyclization in the presence of neighboring nucleophilic nitrogen atoms, yielding novel cNe-PAHs with multiple nitrogens. Reactions of 2-pyrazolyl- and imidazolylpyridines with 2d gave cyclized products 8a and 8b, respectively, without detectable formation of the corresponding Nalkynylated intermediates (Table 5). These azaquinoliziniums have donor−acceptor structures with electron-rich azole and electron-deficient pyridinium moieties. cNe-PAHs 8c−e with more extended π-conjugations were also obtained in 51−58% yields. Conventional cNe-PAHs 5a−e exhibited absorption maxima at 345−389 nm and UV−violet fluorescence at 357−416 nm

Scheme 2. Michael Addition to N-Alkynylpyridinium

was confirmed by single-crystal X-ray diffraction analysis (Figure S4-2). We were also pleased to find that 3ad was converted to corresponding N-alkenylpyridiniums under mild conditions by diiodination with LiI/I2 and 1,3-dipolar cycloaddition with N-tert-butyl-α-phenylnitrone and benzyl azide (Scheme S3-1). The smooth regioselective hydrohalogenations to the triple bond prompted us to investigate photochemical cyclization to construct quinolizinium salts.16 We performed a one-pot cyclization procedure starting from pyridine derivatives (Table 4). Sequential alkynylation, hydrochlorination and photocyclization−dehydrochlorination using 2-phenylpyridine afforded 5a in 57% yield; this was verified by a stepwise procedure (Scheme S3-2). Other ring-fused quinolizinium derivatives 5b−e, which are basic components of cNe-PAHs, were obtained in moderate yields. It should be noted that such quinolizinium salts without peripheral substituents require multistep procedures in previous methods (Scheme 1a,b)5−7 and have been impossible to access via the C−H activation annulation approach (Scheme 1c).8 The absence of bulky substituents promotes intermolecular π−π interactions, which are important for the fabrication of organized structures in materials science.3,17

Table 5. Synthesis of Donor−Acceptor cNe-PAHs

C

DOI: 10.1021/jacs.8b00356 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society (Figure S3-5). On the other hand, the optical properties of the donor−acceptor cNe-PAHs 8a−e are considerably different from those of 5a−e (Figure 1). Pyrazole- and imidazole-fused

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X-ray data for compound 3ad (CIF) X-ray data for compound 4a (CIF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Naoyuki Toriumi: 0000-0001-5963-4735 Kazunori Miyamoto: 0000-0003-1423-6287 Atsuya Muranaka: 0000-0002-3246-6003 Masanobu Uchiyama: 0000-0001-6385-5944 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by JSPS KAKENHI (S) (No. 17H06173) (to M.U.) and JSPS Research Fellowships for Young Scientists (to N.T.).

Figure 1. Electronic absorption (solid) and fluorescence (broken) spectra of 8a−e in MeCN. Photographs of 8a−d in MeCN under UV light are shown on the right.

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compounds 8a and 8b exhibited blue fluorescence at 463 and 434 nm with large Stokes shifts of 8800 and 5100 cm−1, respectively. These fluorescence wavelengths are considerably longer than those of the isoelectronic quinoliziniums 5a and 5d (5a: 362 nm; 5d: 357 nm), though the positions of the absorption maxima were not much changed (5a: 353 nm; 5d: 345 nm; 8a: 329 nm; 8b: 355 nm). Benzimidazole-fused azaquinolizinium 8c exhibited green fluorescence at 514 nm. Introducing electron-donating methoxy groups into the benzene moiety led to a further red-shift of fluorescence to 577 nm for 8d, with a decreased quantum yield. Weak and broad absorption bands at 400−500 nm with no detectable fluorescence were observed for the ring-expanded compound 8e. These optical properties clearly indicate that the pyridinium unit in azaquinoliziniums indeed acts as an acceptor unit. The large Stokes shifts of 8a−d can suppress fluorescence quenching in concentrated solution and in the solid state. The first reduction potentials (Epc, red) of 5a−e and 8a−e ranged from −1.31 to −1.68 V (vs Fc/Fc+) (Table S3-1), suggesting that modification of the π-systems of cNe-PAHs would enable fine-tuning of the electron-accepting abilities. In summary, we have prepared the first N-alkynylpyridinium salts and revealed their highly electrophilic nature by means of spectroscopic and electrochemical measurements. We demonstrated their facile conversion to various N-alkenylpyridiniums via Michael addition and 1,3-dipolar cycloaddition. Further, cNe-PAHs including quinolizinium and azaquinolizinium units were synthesized through intramolecular cyclization reactions, which showed a variety of optical and electrochemical properties. These studies indicate that N-alkynylation of pyridines can be a promising strategy for bottom-up synthesis of novel cationic functional molecules. Further application to construction of diverse cNe-PAHs with extended π-conjugations is in progress.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b00356. Experimental/computational details, characterization data and physicochemical properties (PDF) D

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DOI: 10.1021/jacs.8b00356 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX