Pyridinium Salt Forming Rh(III)-Catalyzed Annulation Reaction of

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Pyridinium Salt Forming Rh(III)-Catalyzed Annulation Reaction of Secondary Allylamines with Internal Alkynes and Its Application to Surface Modification of a Mesoporous Material Ye Ri Han,†,§ Su-Hyang Shim,†,§ Dong-Su Kim,‡ and Chul-Ho Jun*,† †

Department of Chemistry, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea New Drug Development Center, Daegu-Gyeongbuk Medical Innovation Foundation, Daegu 41061, Republic of Korea

‡

S Supporting Information *

ABSTRACT: A Rh(III)-catalyzed C−H activation reaction has been developed for the preparation of pyridinium salts from secondary allylamines and internal alkynes. The pyridinium salts formed by this N-annulation reaction have interesting fluorescence properties. This protocol has been applied to the surface modification of mesoporous silica materials to generate functionalized silica that can be used for the detection of nitrobenzene.

equiv), [Cp*RhCl2]2 (5 mol %), and Cu(OAc)2·H2O (2 equiv) leads to generation of pyridinium salt 3a in an 84% isolated yield (eq 1).

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ationic N-heterocycles have received significant interest as pharmaceutical and fluorescent materials.1 Many protocols are available to prepare these substances. Recently, we developed a novel N-annulation reaction, involving transition metal catalyzed C−H activation of benzyl amine, which can be employed to prepare cationic N-heterocycles.1i This transition metal-catalyzed C−H activation reaction is an atom economical one-pot process.2 In an earlier effort, we also developed a new method for the synthesis of benzoquinolizinium salts by a Rh(III)-catalyzed double N-annulation reaction of primary allylamines and diarylacetylenes.1b In the investigation described below, we extended this strategy to the design of a new efficient procedure for the preparation of pyridinium salts using secondary allylamines instead of primary amines. Moreover, in this effort we have demonstrated that the pyridinium salts generated in this manner fluoresce over a wide wavelength range depending on the nature of the pyridine ring substituents. Furthermore, we showed that the protocol can be applied to the synthesis of pyridinium salt-immobilized mesoporous silica by carrying out the Rh(III)-catalyzed N-annulation reaction on the surface of secondary allylamine-impregnated mesoporous silica SBA-15, which is produced by a process we previously uncovered involving co-condensation of sec-allylamine-linked methallylsilane and TEOS (tetraethyl orthosilicate).3 Finally, we demonstrated that the resulting pyridinium salt-immobilized SBA-15 serves as a recyclable detector of nitrobenzene. In the first phase of this effort, we observed that reaction of a methanol solution of N-phenethyl-N-methallylamine (1a) and diphenylacetylene (2a) in the presence of HBF4 (1.5 © XXXX American Chemical Society

A plausible mechanism for the reaction forming 3a is given in Scheme 1. The first step in the route involves rhodium(III)-catalyzed, chelation-assisted cleavage of the allylic C−H bond of 1a to form the five-membered rhodacyclic complex 4a. Next, the internal alkyne 2a inserts into 4a to produce the seven-membered rhodacyclic complex 5a, which undergoes reductive elimination with the assistance of HBF4 to produce pyridinium salt 3a and Rh(I) complex 6a. Finally, Rh(III) complex 7a is regenerated by Cu(OAc)2·H2O promoted oxidation of 6a. Reactions of a variety of secondary allylamines 1 with internal alkynes 2 were carried out using the conditions described above to explore the scope of the process. In all cases, reactions produced the corresponding pyridinium salts 3 in high to moderate isolated yields (Table 1, entries 1−14). Interestingly, the generated pyridinium salts fluoresce at wavelength maxima that depend on the nature of the pyridine Received: November 24, 2017

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DOI: 10.1021/acs.orglett.7b03654 Org. Lett. XXXX, XXX, XXX−XXX

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Figure 1. Fluorescence spectra of selected pyridinium salts 3h, 3a, 3l, and, 3m.

nium salt-immobilized SBA-15 could be fabricated by Rhpromoted reaction of secondary allyl amine-impregnated SBA15 10a (Figure 2). It has been reported that methallylsilane derivatives can be utilized to construct functional mesoporous silica or for surface modification of silica by using the grafting method.5 In order to apply this process, the secondary allylamine-linked methallylsilane 1h was prepared by reaction of 11-chloroundecyldimethallylsilane (8a) with methallylamine (9a). Then, 1h was utilized as the organosilane precursor in the formation of secondary allylamine-impregnated SBA-15 10a (Figure 2a). Specifically, co-condensation of 1h with TEOS (1h/TEOS = 1:99) in the presence of P123 (block copolymer of PEG−PPG−PEG) and HCl generated the 10a (0.17 mmol/g loading rate).3 Finally, N-annulation of 10a with diphenylacetylene (2a) in the presence of HBF4 (1.5 equiv), [Cp*RhCl2]2 (5 mol %), and Cu(OAc)2·H2O (2 equiv) yields the pyridinium salt-immobilized SBA-15 11a.

ring substituents (Table 1 and Figure 1).4 For example, salts containing only alkyl R1 and R2 substituents have emission maxima between 419 and 423 nm (entries 1−4), while those containing phenyl as the R1 or R2 group emit around 442− 459 nm (entries 5−6). Moreover, the nature of the R3 group has a profound effect on fluorescence emission wavelength (entries 7−14). Pyridinium salt 3h containing R3 = n-propyl emits at a much lower wavelength than analogs having R3 = aryl. Finally, pyridinium salts containing para-electron-withdrawing substituted phenyl groups at R3 emit at lower wavelengths (entries 9−10) compared to their electron donating substituted counterparts (entries 11−13). In order to explore the N-annulation process further, we assessed its application to the preparation of fluorescent materials. Specifically, we proposed that fluorescent pyridi-

Table 1. Scope of Secondary Amines (1) and Internal Alkynes (2) in the Pyridinium Salt (3) Synthesisa

entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14

allylamine (1) R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1

= = = = = = = = = = = = = =

CH3, R2 = CH3 (1b) n-(C4H9), R2 = CH3 (1c) (CH2)3Ph, R2 = CH3 (1d) CyHex, R2 = CH3 (1e) Ph, R2 = CH3 (1f) Ph, R2 = Ph (1g) (CH2)2Ph, R2 = CH3 (1a) (CH2)2Ph, R2 = CH3 (1a) (CH2)2Ph, R2 = CH3 (1a) (CH2)2Ph, R2 = CH3 (1a) (CH2)2Ph, R2 = CH3 (1a) (CH2)2Ph, R2 = CH3 (1a) (CH2)2Ph, R2 = CH3 (1a) (CH2)2Ph, R2 = CH3 (1a)

alkyne (2) R3 R3 R3 R3 R3 R3 R3 R3 R3 R3 R3 R3 R3 R3

= = = = = = = = = = = = = =

Ph (2a) Ph (2a) Ph (2a) Ph (2a) Ph (2a) Ph (2a) n-C3H7 (2b) Ph (2a) p-F-C6H4 (2c) p-CI-C6H4 (2d) p-CH3−C6H4 (2e) p-OMe-C6H4 (2f) p-N(CH3)2-C6H4 (2g) thiophene (2h)

yield (%)b 65 56 89 72 72 82 70 84 78 60 61 80 94 63

(3b) (3c) (3d) (3e) (3f) (3g) (3h) (3a) (3i) (3j) (3k) (3l) (3m) (3n)

wavelengthc (nm) 419 421 424 423 459 442 388 436 428 427 450 486 570 497

Reaction of secondary allylamines 1 (0.2 mmol), internal alkynes 2 (0.4 mmol) in 0.3 mL of MeOH at 130 °C containing [Cp*RhCl2]2 (5 mol %), Cu(OAc)2·H2O (0.4 mmol), and HBF4 (0.3 mmol). bAll yields are for isolated material. cWavelength maxima of fluorescence emission spectra.

a

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DOI: 10.1021/acs.orglett.7b03654 Org. Lett. XXXX, XXX, XXX−XXX

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Figure 2. (a) Synthesis of pyridinium salt-impregnated SBA-15 11a by surface reaction of 10a with diphenylacetylene (2a) in the presence of HBF4 and Rh(III)/Cu(II) complex at 100 °C. (b) TEM image of 11a. (c) Fluorescence spectra of 11a, prepared with different reaction time.

Figure 3. Fluorescence spectra of 11a upon addition of nitrobenzene (NB) (0.2−2.8 mM).

Inspection of a transmission electron microscope (TEM) image of 11a shows that the material contains a uniform distribution of the mesopores, which is maintained well after the surface modification reaction of 10a (Figure 2b, see SI). To optimize the surface modification process, reaction of 10a was carried out for different times while monitoring product formation using the intensity of fluorescence at 420 nm (excitation at 300 nm) in Figure 2c. We observed that, as the progress of the N-annulation reaction of 10a increases, the fluorescence intensity of the generated pyridinium salt on mesoporous SBA-15 11a also increases. Second, the fluorescence intensity was observed to reach a plateau after 2 h after initiation of the reaction.6 The ability to use fluorescent pyridinium salt-modified SBA15 11a to detect potential explosives was probed. It is known that a common family of explosives are nitro-organic compounds and that these substances are efficient quenchers of fluorescent compounds probably due to the electron transfer quenching effect.7 Indeed, when nitrobenzene (NB) is added to a suspension of 11a (1.5 mg, 1 × 10−4 mmol) in CH2Cl2, the intensity of fluorescence decreases (Figure 3). Also as the amount of nitrobenzene increases in the range of 0.2−2.6 mM, the fluorescence intensity gradually decreases and becomes nearly fully quenched at 2.8 mM of nitrobenzene with high quenching efficiency (see SI). The silica-based pyridinium salt can be recycled and used to detect nitrobenzene repeatedly (Figure 4). After quenching the fluorescence of a suspension of 11a, nitrobenzene was thoroughly removed by washing with CH2Cl2/acetone. This led to recovered 11a, which was used again to detect nitrobenzene. Finally, the process can be repeated three times without loss of fluorescence intensity of 11a or quenching by nitrobenzene.

Figure 4. Recycling experiment of 11a by addition and washing of nitrobenzene (NB).

In the effort described above, we developed a simple catalytic method to prepare a wide range of fluorescence pyridinium salts from secondary allylamines and internal alkynes. Interestingly, the synthetic protocol was applied to surface modification of SBA-15 to generate fluorescent pyridinium salt-embedded SBA-15. Finally, this functionalized SBA-15 was used to detect nitrobenzene in a recyclable manner.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03654. Compound characterization data, 1H and 13C NMR spectra (PDF) C

DOI: 10.1021/acs.orglett.7b03654 Org. Lett. XXXX, XXX, XXX−XXX

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Omega 2017, 2, 3572. (b) Shanmugaraju, S.; Mukherjee, P. S. Chem. Commun. 2015, 51, 16014. (c) Kaleeswaran, D.; Vishnoi, P.; Murugavel, R. J. Mater. Chem. C 2015, 3, 7159. (d) Das, G.; Biswal, B. P.; Kandambeth, S.; Venkatesh, V.; Kaur, G.; Addicoat, M.; Heine, T.; Verma, S.; Banerjee, R. Chem. Sci. 2015, 6, 3931. (e) Zhang, S.; Han, L.; Li, L.; Cheng, J.; Yuan, D.; Luo, J. Cryst. Growth Des. 2013, 13, 5466. (f) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Chem. Rev. 2012, 112, 1105. (g) Pramanik, S.; Zheng, C.; Zhang, X.; Emge, T. J.; Li, J. J. Am. Chem. Soc. 2011, 133, 4153.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+82)-2-3147-2644. ORCID

Dong-Su Kim: 0000-0003-0388-4633 Chul-Ho Jun: 0000-0002-5578-2228 Author Contributions §

These authors equally contributed to this work. The manuscript was written through the contributions of all the authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the grant from LG Chem. (Grant 2015-11-0067) and the National Research Foundation of Korea (NRF) (Grant 2017-R-1-A2b4009460).

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REFERENCES

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DOI: 10.1021/acs.orglett.7b03654 Org. Lett. XXXX, XXX, XXX−XXX