Assemblies of 1,4-Bis(diarylamino)naphthalenes and Aromatic Amphiphiles: Highly Reducing Photoredox Catalysis in Water

Abstract

Host–guest assemblies of a designed 1,4-bis(diarylamino)naphthalene and V-shaped aromatic amphiphiles consisting of two pentamethylbenzene moieties bridged by an m-phenylene unit bearing two hydrophilic side chains emerged as highly reducing photoredox catalysis systems in water. An efficient demethoxylative hydrogen transfer of Weinreb amides has been developed. The present supramolecular strategy permits facile tuning of visible-light photoredox catalysis in water.

Visible-light photoredox catalysis has become a useful strategy in organic synthesis because it provides a smart redox protocol under operationally easy conditions.[1] Recently, extensions to photoredox reactions in water have been actively studied due to their various merits such as environmentally benign green chemistry and the applications of hydrated electrons.[2] [3] In general, innately water-soluble metal complexes such as [Ru(bpy)₃]Cl₂, and organic dyes such as Eosins have been used as photocatalysts (Scheme [1a]).[3a] [c] [h] On the other hand, to eliminate the redox limitation, the use of designed photocatalysts with appropriate redox properties is essential. Recently, several hydrophobic photocatalysts solubilized in water by covalent linking of hydrophilic units have been developed (Scheme [1b]).[3e] [f] However, the synthesis of complicated structures requires laborious multistep processes and sometimes causes unexpected degradation of photophysical properties. In addition to the choice of an appropriate photocatalyst, micellar systems have been usually adopted in the previous systems because of efficient mixing of hydrophobic substrates in water.

In 2020, König’s group reported that assemblies composed of Ir photocatalysts, surfactants, and electron donors in water can serve as highly reducing catalytic systems for reductive activation of C–Cl bonds of alkyl chlorides in a process known as assembly-promoted single-electron transfer (APSET).[3d] At about the same time, our group developed a capsule-like supramolecular photoredox catalyst 3_POX , assembled from hydrophobic phenoxazine photocatalyst guest POX [4] and molecular capsule host 2 _n [5] through multiple interactions, including hydrophobic effect and CH–π/π–π interactions (Scheme [1c]).[6] Catalyst 3_POX is readily prepared by simple protocols including grinding, addition of water, sonication, and filtration under air. The system is noble-metal free and shows good photocatalytic activity in water under air at room temperature, especially for pinacol coupling. These results prompted us to develop a new capsule-like water-soluble photoredox catalyst by changing the inner organic photoredox catalyst (OPC) guest.[7] In 2019, we found that N,N,N′,N′-tetraphenylnaphthalene-1,4-diamine (BDN; 1a) can serve as a highly reducing OPC. Since then, various derivatives have been developed as reducing photocatalysts in organic solvents.[8] Here, we present the design of a new series of organic supramolecular photocatalysts 3, in which BDN derivatives 1 are incorporated as the guest OPCs. In addition, we report that Weinreb amides, which are generally difficult to reduce, readily undergo photocatalytic reduction with efficient promotion by a supramolecular photoredox catalyst in water.

We previously revealed that hydrophobic OPCs are easily encapsulated by capsule-like host 2 _n consisting of V-shaped aromatic amphiphiles.[6] We therefore prepared the BDN derivatives N,N,N′,N′-tetrakis(4-tert-butylphenyl)naphthalene-1,4-diamine (1b) and N,N,N′,N′-tetrabiphenyl-4-ylnaphthalene-1,4-diamine (1c).[9] Selected photo- and electrochemical properties of 1a–c are summarized in Table [1]. Note that the tert-butyl and phenyl substituents significantly enhance visible-light absorption (ε = 3.7 × 10² (1a), 2.1 × 10³ (1b), 3.6 × 10³ (1c) M^–1 cm^–1 at 425 nm), indicating that 1b and 1c can utilize visible-light energy more efficiently than 1a. The reducing power in the excited state, as estimated from the fluorescent spectra (λ_max) and oxidation potentials (E _ox,dpv) suggested that 1a–c might serve as highly reducing photocatalysts (E*_ox) in an organic solvent under visible-light irradiation.

Table 1 Selected Photophysical and Electrochemical Properties of 1a–c in Organic Solvents

Photocatalyst	λabs (nm)	ε (M–1 cm–1) at 425 nm	λem a (nm)	Ε0,0 (eV)	ΦF	E ox,dpv b (V vs. Cp2Fe)	E ox* (V vs. Cp2Fe)	Ref.
1a	380	3.7 × 102	457	2.71	97	+0.30/+0.52	–2.41	[8a]
1b c	390	2.1 × 103	473	2.62	>0.99	+0.17/+0.42	–2.45	[8b]
1c	396	3.6 × 103	467	2.65	45	+0.29/+0.49	–2.36	–

^a In DCE ([1] = 1.0 × 10^–4 M) under air at rt (excitation at 370 nm).

^b In DCE ([1] = 1.0 mM, [NBu₄](PF₆) = 0.10 M) with a Pt disk working electrode, a wire counter electrode, and a Ag/AgNO₃ reference electrode under N₂ at rt; scan rate: 100 mV/s. Ferrocene was used as a reference.

^c Data for 1b were obtained in acetone.

Assemblies 3a–c were prepared by a similar protocol to that used to prepare 3_POX .[6] UV/vis spectra recorded in water (Figure [1]) confirmed that 1a, 1b, and 1c are solubilized by 2 _n ([1a] = 1.3 mM, [1b] = 1.2 mM, [1c] = 1.3 mM), suggesting that in water, the corresponding assemblies 3a–c are formed almost quantitatively (based on 2). Figure [1] reveals that host 2 _n provides no absorption band in the visible region. Thus, the host does not hamper visible-light absorption by the guest OPC.

With the water-soluble organic supramolecular photocatalysts 3a–c in hand, we studied their photocatalytic reactions in water. In particular, to evaluate their performance as a one-electron reducing system, we examined the reductive cleavage of the N–O bond in Weinreb amides. A noncatalytic method based on the action of an organic super-electron donor was previously reported by Murphy and co-workers.[11]

Table 2 Photocatalytic Demethoxylative Hydrogen Transfer of Amide 4a in Water^a

Entry	Photocatalyst and conditions	Yieldb (%)
1	3a	28
2	3b	57
3	3c	99
4	3POX	66
5	1c without 2	13
6	–	–
7	3c without Et3N	5
8	3c in dark	0

^a A 4 mL sample bottle was charged with 4a (0.12–0.13 mmol), Et₃N (0.24–0.26 mmol), and an aqueous solution of 3 (~1.2 –1.3mM, 2 mL), and then irradiated 1 cm away from blue LED lamps (λ = 425 nm). After the reaction, the mixture was extracted with CH₂Cl₂ and volatiles were removed in vacuo.

^b Determined by ¹H NMR with 1,3,5-trimethoxybenzene as an internal standard.

A 4 mL sample bottle was charged with an aqueous solution (2 mL) of photocatalyst 3 (2.4–2.6 μmol) and N-methoxy-N,4-dimethylbenzamide (4a) (0.12–0.13 mmol) (E _red = –3.06 V vs. Cp₂Fe in MeCN) and Et₃N (2 equiv) were added under air. The mixture was then irradiated by blue LEDs (425 nm) for five hours. Demethoxylative hydrogen transfer proceeded to give N,4-dimethylbenzamide (5a) in NMR yields of 28–99% (Table [2], entries 1–3). Interestingly, catalyst 3_POX , previously reported by us,[6] gave a lower yield of 66% (entry 4), suggesting that the choice of the encapsulated photocatalyst is crucial for the reaction. Catalyst 3c emerged as the best catalyst under the conditions used. These results suggest the catalytic activity of catalysts 3 can be readily tuned by appropriate choice of the combination of OPC and amphiphile. In addition, if the amphiphile, the photocatalyst, or Et₃N is absent, the reaction is significantly retarded (entries 5–7). Furthermore, visible-light irradiation was shown to be essential for the present reaction (entry 8).

Table 3 Scope of the Present Reductive Cleavage of the N–O Bond in Weinreb Amides[10]

Entry	Substrate	R	Time (h)	Product	Yielda (%)
1	4a	4-MeC6H4	3	5a	88 (99)
2	4b	Ph	5	5b	73 (86)
3	4c	4-PhC6H4	5	5c	69 (91)
4	4d	4-MeOC6H4	24	5d	72 (72)
5	4e	4-FC6H4	5	5e	62 (81)
6	4f	4-ClC6H4	5	5f	72 (82)
7	4g	4-BrC6H4	3	5g	69 (93)
8	4h	2-pyridyl	5	5h	62 (65)
9	4i	2-furyl	5	5i	43 (54)
10	4j	2-MeC6H4	24	5j	86 (87)
11	4k	2-i-PrC6H4	48	5k	84 (96)
12	4l	2-PhC6H4	24	5l	77 (98)
13	4m	Bn	24	5m	– (5)
14	4n	(E)-PhCH=CH	5	5n	– (–)

^a Isolated yield (NMR yield in parentheses).

Next, we examined the scope of the reductive transformation of Weinreb amides 4 with 3c as a photocatalyst (Table [3]). In addition to the synthesis of 5a in 88% isolated yield (Table [3], entry 1), the electronically unbiased benz­amides 5b and 5c were obtained in good yields of 73 and 69%, respectively (entries 2 and 3). On the other hand, the reactions of amide substrates bearing an electron-donating methoxy group (4d) or substituents at the ortho-positions (4j–l) were so sluggish that they required longer reaction times of 24 or 48 hours (entries 4 and 10–12). Notably, the reaction was compatible with halo groups (5e–g; yield: 62–72%) (entries 5–7) and heteroaromatics (4h: 62%; 4i: 43%) (entries 8 and 9). In addition, aliphatic amide 4m afforded a small amount of product 5m (5% NMR yield, 24 h) (entry 13), whereas the alkenyl substrate 4n gave none of the corresponding amide (entry 14). Note that although substrates 4c and 4l are solid chemicals that are insoluble in water, the reactions proceeded smoothly, especially in the case of 4c. This is one of the excellent features of the present capsule-like supramolecular photocatalyst. The hydrophobic cavity formed by the catalyst and amphiphiles can act as a unique reaction site. In fact, the reaction of 4a using assembly 3c in water was more efficient than those using 1c in organic solvents (see the Supporting Information).

To gain structural information for catalyst 3c, NMR and dynamic light-scattering (DLS) analyses were conducted (Figure [2]). The diffusion-ordered spectroscopy (DOSY) NMR spectrum of 3c (Figure [2a]) displayed a single band with log D = –9.54, which is smaller than that of the assembly consisting only of 2, suggesting that a single complex is formed. The DLS analysis supported the formation of small particles with an average core diameter of 1.7 nm (Figure [2b]). The observed value agreed with the core diameter of a structure composed of one photocatalyst molecule and three amphiphile molecules, as optimized by molecular mechanics on Material Studio ver. 5.5. (Figure [2c]). In addition, no virtual elution of 1c from 3c into the aqueous phase after two weeks was noted on monitoring the UV/vis spectrum of 3c (see the Supporting Information), indicating that the host–guest structure is stable and easy to handle as a catalyst.

In accord with previous reports,[6] [11] a possible reaction mechanism is shown in Scheme [2].[12] The capsule-like photocatalyst 3c efficiently incorporates amide 4 into its cavity. The photoexcited 1c* in the cavity then transfers an electron to 4, giving the anionic radical 4′ together with the oxidized radical cation 1c⁺ . Reduction by Et₃N then regenerates the ground-state photocatalyst 3c (1c). These electron-transfer events proceed efficiently, possibly through the above-mentioned APSET processes. The anionic radical 4′ is sufficiently hydrophilic to be extruded to the aqueous outer space of the capsule and then undergoes a demethoxylative 1,2-spin-center shift to give intermediate 5′. Finally, 5′ abstracts a hydrogen atom from the oxidized amine to give the product 5. Additionally, the reaction did not proceed well when the spin-trapping agent N-tert-butyl-α-phenylnitrone was added to the reaction mixture (see the Supporting Information); radical species are therefore involved in the present reaction.

In conclusion, we have developed new assemblies of a BDN catalyst encapsulated in a capsule-like host consisting of V-shaped aromatic amphiphiles. The present supramolecular photocatalyst serves as a highly reducing OPC system in water. The system is effective for demethoxylative hydrogen transfer of Weinreb amides. In addition, the strategy involving a combination of an OPC and amphiphiles is useful for facile tuning of photoredox catalysis in water. Further development of supramolecular photoredox catalysts composed of an OPC and amphiphiles is ongoing in our laboratory.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

The present work is dedicated to Professor Shunichi Fukuzumi on the occasion of his 70th birthday. All doctors, nurses, and researchers who are fighting against the COVID-19 pandemic are gratefully acknowledged.

• References and Notes


For selected recent references on photoredox catalysis, see:
• 1a Visible Light Photocatalysis in Organic Chemistry . Stephenson CR. J, Yoon TP, MacMillan DW. C. Wiley-VCH; Weinheim: 2018
  Search in Google Scholar
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• 1e McAtee RC, McClain EJ, Stephenson CR. J. Trends Chem. 2019; 1: 111
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For selected examples of photoredox catalysis in water, see:
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• 8a Noto N, Koike T, Akita M. ACS Catal. 2019; 9: 4382
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• 9 CCDC 2097141 contains the supplementary crystallographic data for compound 1c. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures
• 10 Photocatalytic Demethoxylation of Weinreb Amides: General Procedure A 4 mL sample bottle was charged with amide 4 (0.13 mmol), Et₃N (0.26 mmol), and 1.3 mM aq 3c (2 mL, 2 mol%), and then placed 1 cm away from blue LED lamps (λ = 425 nm). The mixture was irradiated, with stirring and cooling by a fan for 3–48 h. H₂O (5 mL) was then added and the resulting mixture was extracted with CH₂Cl₂ (3 × 15 mL). The organic layer was dried (Na₂SO₄) and concentrated, and the crude product was purified by gel-permeation chromatography. N,4-Dimethylbenzamide (5a) White solid; yield: 16.3 mg (88%, 0.109 mmol). ¹H NMR (400 MHz, CDCl₃): δ = 7.65 (d, J = 8.0 Hz, 2 H, Ar), 7.22 (d, J = 8.0 Hz, 2 H, Ar), 6.10 (br s, 1 H, NH), 3.00 (d, J = 4.7 Hz, 3 H, CH ₃), 2.39 (s, 3 H, CH ₃). N-Methylbiphenyl-4-carboxamide (5c) White solid; yield: 20.0 mg (69%, 0.0904 mmol). ¹H NMR (400 MHz, CDCl₃): δ = 7.84 (d, J = 8.3 Hz, 2 H, Ar), 7.65–7.59 (m, 4 H, Ar), 7.46 (dd, J = 7.1, 7.7 Hz, 2 H, Ar), 7.38 (t, J = 7.4 Hz, 1 H, Ar), 6.28 (br s, 1 H, NH), 3.04 (d, J = 4.8 Hz, 3 H, CH ₃).
• 11 Cutulic S, Murphy J, Farwaha H, Zhou S.-Z, Chrystal E. Synlett 2008; 2132
• 12 Quenching experiments for 3c with quenchers 4a and Et₃N in water showed a significant increase in fluorescence intensity. In addition, 4a affected the lifetime of the fluorescent excited species. These results also suggest that unique assemblies are formed in the present system (see the Supporting Information).

Corresponding Authors

Publication History

Received: 31 July 2021

Accepted after revision: 23 September 2021

Accepted Manuscript online:
23 September 2021

Article published online:
13 October 2021

© 2021. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

• References and Notes


For selected recent references on photoredox catalysis, see:
• 1a Visible Light Photocatalysis in Organic Chemistry . Stephenson CR. J, Yoon TP, MacMillan DW. C. Wiley-VCH; Weinheim: 2018
  Search in Google Scholar
• 1b Marzo L, Pagire SK, Reiser O, König B. Angew. Chem. Int. Ed. 2018; 57: 10034
  CrossrefPubMedSearch in Google Scholar
• 1c Buzzetti L, Crisenza GE. M, Melchiorre P. Angew. Chem. Int. Ed. 2019; 58: 3730
  CrossrefPubMedSearch in Google Scholar
• 1d Kancherla RM. K, Sagadevan A, Rueping M. Trends Chem. 2019; 1: 510
  CrossrefSearch in Google Scholar
• 1e McAtee RC, McClain EJ, Stephenson CR. J. Trends Chem. 2019; 1: 111
  CrossrefPubMedSearch in Google Scholar
• 2 Sun K, Lv Q.-Y, Chen X.-L, Qu L.-B, Yu B. Green Chem. 2021; 23: 232
  CrossrefPubMedSearch in Google Scholar

For selected examples of photoredox catalysis in water, see:
• 3a Kerzig C, Goez M. Chem. Sci. 2016; 7: 3862
  CrossrefPubMedSearch in Google Scholar
• 3b Bu M.-j, Cai C, Gallou F, Lipshutz BH. Green Chem. 2018; 20: 1233
  CrossrefSearch in Google Scholar
• 3c Bu M.-j, Lu G.-p, Jiang J, Cai C. Catal. Sci. Technol. 2018; 8: 3728
  CrossrefSearch in Google Scholar
• 3d Giedyk M, Narobe R, Weiß S, Touraud D, Kunz W, König B. Nat. Catal. 2019; 3: 40
  CrossrefSearch in Google Scholar
• 3e Kerzig C, Wenger OS. Chem. Sci. 2019; 10: 11023
  CrossrefPubMedSearch in Google Scholar
• 3f Eisenreich F, Meijer EW, Palmans AR. A. Chem. Eur. J. 2020; 26: 10355
  CrossrefPubMedSearch in Google Scholar
• 3g Santos MS, Cybularczyk-Cecotka M, König B, Giedyk M. Chem. Eur. J. 2020; 26: 15323
  CrossrefPubMedSearch in Google Scholar
• 3h Liu J, Yao H, Li X, Wu H, Lin A, Yao H, Xu J, Xu S. Org. Chem. Front. 2020; 7: 1314
  CrossrefSearch in Google Scholar
• 4 Pearson RM, Lim CH, McCarthy BG, Musgrave CB, Miyake GM. J. Am. Chem. Soc. 2016; 138: 11399
  CrossrefPubMedSearch in Google Scholar
• 5a Kondo K, Suzuki A, Akita M, Yoshizawa M. Angew. Chem. Int. Ed. 2013; 52: 2308
  CrossrefPubMedSearch in Google Scholar
• 5b Okazawa Y, Kondo K, Akita M, Yoshizawa M. Chem. Sci. 2015; 6: 5059
  CrossrefPubMedSearch in Google Scholar
• 5c Okazawa Y, Kondo K, Akita M, Yoshizawa M. J. Am. Chem. Soc. 2015; 137: 98
  CrossrefPubMedSearch in Google Scholar
• 5d Kondo K, Akita M, Yoshizawa M. Chem. Eur. J. 2016; 22: 1937
  CrossrefPubMedSearch in Google Scholar
• 6 Noto N, Hyodo Y, Yoshizawa M, Koike T, Akita M. ACS Catal. 2020; 10: 14283
  CrossrefSearch in Google Scholar
• 7a Romero NA, Nicewicz DA. Chem. Rev. 2016; 116: 10075
  CrossrefPubMedSearch in Google Scholar
• 7b Silvi M, Melchiorre P. Nature 2018; 554: 41
  CrossrefPubMedSearch in Google Scholar
• 7c Lee Y, Kwon MS. Eur. J. Org. Chem. 2020; 6028
  Crossref
• 7d Koike T, Akita M. Trends Chem. 2021; 3: 416
  CrossrefSearch in Google Scholar
• 7e Vega-Peñaloza A, Mateos J, Companyó X, Escudero-Casao M, Dell’Amico L. Angew. Chem. Int. Ed. 2021; 60: 1082
  CrossrefPubMedSearch in Google Scholar
• 8a Noto N, Koike T, Akita M. ACS Catal. 2019; 9: 4382
  CrossrefSearch in Google Scholar
• 8b Noto N, Takahashi K, Goryo S, Takakado A, Iwata K, Koike T, Akita M. J. Org. Chem. 2020; 85: 13220
  CrossrefPubMedSearch in Google Scholar
• 8c Taniguchi R, Noto N, Tanaka S, Takahashi K, Sarkar SK, Oyama R, Abe M, Koike T, Akita M. Chem. Commun. 2021; 57: 2609
  CrossrefPubMedSearch in Google Scholar
• 9 CCDC 2097141 contains the supplementary crystallographic data for compound 1c. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures
• 10 Photocatalytic Demethoxylation of Weinreb Amides: General Procedure A 4 mL sample bottle was charged with amide 4 (0.13 mmol), Et₃N (0.26 mmol), and 1.3 mM aq 3c (2 mL, 2 mol%), and then placed 1 cm away from blue LED lamps (λ = 425 nm). The mixture was irradiated, with stirring and cooling by a fan for 3–48 h. H₂O (5 mL) was then added and the resulting mixture was extracted with CH₂Cl₂ (3 × 15 mL). The organic layer was dried (Na₂SO₄) and concentrated, and the crude product was purified by gel-permeation chromatography. N,4-Dimethylbenzamide (5a) White solid; yield: 16.3 mg (88%, 0.109 mmol). ¹H NMR (400 MHz, CDCl₃): δ = 7.65 (d, J = 8.0 Hz, 2 H, Ar), 7.22 (d, J = 8.0 Hz, 2 H, Ar), 6.10 (br s, 1 H, NH), 3.00 (d, J = 4.7 Hz, 3 H, CH ₃), 2.39 (s, 3 H, CH ₃). N-Methylbiphenyl-4-carboxamide (5c) White solid; yield: 20.0 mg (69%, 0.0904 mmol). ¹H NMR (400 MHz, CDCl₃): δ = 7.84 (d, J = 8.3 Hz, 2 H, Ar), 7.65–7.59 (m, 4 H, Ar), 7.46 (dd, J = 7.1, 7.7 Hz, 2 H, Ar), 7.38 (t, J = 7.4 Hz, 1 H, Ar), 6.28 (br s, 1 H, NH), 3.04 (d, J = 4.8 Hz, 3 H, CH ₃).
• 11 Cutulic S, Murphy J, Farwaha H, Zhou S.-Z, Chrystal E. Synlett 2008; 2132
• 12 Quenching experiments for 3c with quenchers 4a and Et₃N in water showed a significant increase in fluorescence intensity. In addition, 4a affected the lifetime of the fluorescent excited species. These results also suggest that unique assemblies are formed in the present system (see the Supporting Information).

• Supplementary Material