2020-02-14 16:49:20
Yuan Cheng,[a] Xiongyu Ou,[a] Jimei Ma,[a] Linhao Sun,[a] and Zhong-Hua Ma*
Introduction
Using water as a reaction media offers advantages for cost, safety, and environment, and is therefore considered green and ideal for laboratory and industry processes. Hence many efforts have been devoted to the development of aqueous catalytic reactions.[1] In aqueous suspension, some reactions involving organic substrates are accelerated, which has been defined as “on water” reactions by Sharpless.[1b] Several approaches have been aimed to strengthen the interaction of organic substrates under “on water” conditions, including the use of surfactants as amphiphilic additives, the use of phase-transfer catalysts,[2] and the specifically functionalized design of the catalysts structure. Water also brings some benefits for product isolation. Various types of reactions forming C-C and C-heteroatom bond have been carried out in water with enhanced reactivity and selectivity, such as the nucleophilic attack of a carbanion species in conjugate addition reaction, known as Michael-type reactions.[1a,1c] For example, a Michael addition of ß,ß-disubsti- tuted nitroalkenes with malonate derivatives was reported by Song, in which the otherwise unreactive substrate systems were demonstrated as on-water catalytic reaction.[1a]
An effective promotion for nucleophilic addition is via the use of Brønsted or Lewis acids to activate electrophiles.[3] How- ever, the activating ability of Brønsted acids is commonly weak- ened in presence of water. A few studies,[4] initiated by Kobaya- shi's pioneering work,[3e] were developed on Brønsted acid-surfactant-combined catalysts and their applications in aqueous systems. The surfactant segments assemble into micelles as microreactor, whose hydrophobic cores trap the labile sub- strates and exclude water molecules,[3b,5] while the micelle exte- rior remains hydrated, and allows the permeation of protons for substrate activation.[6] This surfactant-combined strategy has also been extended to solid Brønsted acids.
The hydrophobic assembly is probably caused by fluorocar- bon chain except hydrocarbon chain. Perfluoroalkyl chains have a stronger hydrophobic character than hydrocarbon chains due to their greater molecular cross-sectional area.[7] A classic exam- ple is -C8F17, which is applied in fluorous biphasic media for fluorous catalyst recycling.[8] Even relatively short -C4F9 chains exhibit significant hydrophobicity. Our previous research dem- onstrated that -C4F9 hindered access of water molecules to ad- jacent SO2NHSO2 acid sites,[9] and hydrogen was impeded to bind with the negative conjugate base SO2N–SO2.[10] Similarly, in Wang's reports, pyrrolidine perfluorobutyl sulfonamide was used in aqueous catalysis.[11] On the other hand, perfluoroalkyl sulfonylimides, (RFSO2)2NH, known as nitrogen acids, is widely applied in catalytic field instead of SO3H analogues to practice challenging tasks because of their strong acidity.[3c,3d,9] Gener- ally, compounds with high fluorine content (> 60 %) strongly prefer to go into fluorous phase in fluorocarbon–hydrocarbon systems.[12] These distinct properties could provide water- shielded microenvironments for aqueous reactions. However, too long fluorocarbon chain will inevitably lead to low miscibility of organic and fluorous substrates, weaken the interaction of organic substrates, and increas the cost.
Herein, in the structure of designed sulfonimide catalyst 1a/ 1b, fluorine content and acid density are well balanced by split- ting long perfluoroalkyl chain into two relatively short perfluoroalkyl chains, attached to respective imide Brønsted acid sites (Scheme 1). This method adjusts fluorine content at relative low level with elevated acid density per molecule. The acidity is examined by using the Hammett acidity function, the 31P NMR shift of Et3P=O probe, and conductance titration. The binary acid catalyst has been applied in water to promote the nucleo- philic addition (the Friedel–Crafts reactions). The substrates used herein were indoles, a widely distributed core structure in nature, and there is a large number of applications of synthetic indoles as pharmaceuticals and agrochemicals.[3a,13] With the use of 1a, Friedel–Crafts alkylation of indoles in water was easily performed, via 1,4-addition with ß-monosubstituted vinyl ket- ones and condensation with aldehydes, respectively. The prod- ucts are isolated without column chromatography or recrystalli- zation. The aqueous solution containing 1a is recyclable at least three runs. Furthermore, typically poor-reactive ß,ß-disubsti- tuted vinyl ketones were also activated by the strong acidity of the catalyst, giving medium to good product yields.
Results and Discussion
Catalyst Design
Compounds 1a/1b comprise two N-H groups, respectively at- tached to perfluoroalkyl groups (Scheme 1). Perfluoroalkyl groups bring some advantages of stabilizing the anionic charge of the conjugate bases due to its electron-withdrawing effect. Furthermore, there is an extensive delocalization system over N, O and phenyl ring, which conspire to increase the propensity of dissociation of N–H bonds, largely improving the intrinsic acidity and decreases the dependence on external solvation.[14] An example is the analogue (CF3SO2)2NH, whose pKa values were 2.8, 2.7 and 2.1, respectively in H2O, MeOH, and DMSO. The values demonstrate the effect of the two factors on high acidity.[15] Compounds 1a/1b is thus reasoned to serve as strong acid.
1b was given by acidification of 1b·2Na, but decomposed when handled at ca. 55–65 °C to remove the remained solvent; whereas 1a was obtained. The result suggests that a certain linkage in 1b is more susceptible to the intrinsic acidity at higher temperature than that in 1a. The most likely linkage is -CONH-, which is possibly decomposed by the strong acidity, but stabilized by -C4F9 in 1a. It is inferred that the nonpolar segments, including -C4F9 groups and phenyl ring, aggregate to protect -CONH- group. Especially in the presence of water, due to the hydrophobic hydration effect, the aggregation splits each other's H-bond association of the water molecules near hydrophobic segments, and the bulk water thus hardly en- croaches -CONH- group.[6a,16] The similar aggregation much less efficiently happens to the shorter -CF3 of 1b. 1a is thus a stable amphiphile.
A simple Tyndall effect trial was conducted to confirm the formation of the hydrophobic aggregation (Figure 1a). The Tyn- dall effect is often used as a measurement of a colloid, and the intensity of the effect is proportional to the mean volume of the particles. In our trial, obvious Tyndall phenomenon hap- pened to 1a solution, showing the nano-level aggregation ex- isted. As a comparison, no similar phenomenon was observed for CF3SO3H solution. Furthermore, TEM images of 1a aqueous solution is shown in Figure 1(b). A large amount of nano-size aggregates was observed, with a size of 15–20 nm. Their rela- tively small diameters implied a stacked hydrophobic segment among molecules.
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1-acetamidocycloheptane-1-carboxylic acidCatalog No.:AA019KUC CAS No.:1097102-48-1 MDL No.:MFCD13633884 MF:C10H17NO3 MW:199.2469 |
3-cyclobutanecarbonyl-5-nitro-1H-indoleCatalog No.:AA01AC52 CAS No.:1097117-03-7 MDL No.:MFCD11545710 MF:C13H12N2O3 MW:244.2460 |
2-[(2,4-difluorophenyl)sulfanyl]-2-phenylacetic acidCatalog No.:AA01AB3D CAS No.:1097125-21-7 MDL No.:MFCD12569683 MF:C14H10F2O2S MW:280.2898 |
2-[(2,5-difluorophenyl)sulfanyl]-2-phenylacetic acidCatalog No.:AA01AB60 CAS No.:1097125-22-8 MDL No.:MFCD12569684 MF:C14H10F2O2S MW:280.2898 |
2-[(3,4-difluorophenyl)sulfanyl]-2-phenylacetic acidCatalog No.:AA01AB4C CAS No.:1097125-23-9 MDL No.:MFCD12654648 MF:C14H10F2O2S MW:280.2898 |
3-[(2-Oxo-2,3-dihydro-1h-indol-1-yl)methyl]furan-2-carboxylic acidCatalog No.:AA01AAKW CAS No.:1097140-95-8 MDL No.:MFCD12652091 MF:C14H11NO4 MW:257.2414 |
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2-(5-amino-1H-indol-1-yl)-1-(morpholin-4-yl)ethan-1-oneCatalog No.:AA01A7W3 CAS No.:1097161-86-8 MDL No.:MFCD12654343 MF:C14H17N3O2 MW:259.3037 |
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1-(cyclopropylmethoxy)isoquinoline-3-carboxylic acidCatalog No.:AA01AIAX CAS No.:1097166-34-1 MDL No.:MFCD11546080 MF:C14H13NO3 MW:243.2579 |
Methyl 2-(2,6-dichlorophenyl)sulfanylacetateCatalog No.:AA01FAGV CAS No.:1097167-77-5 MDL No.:MFCD11641638 MF:C9H8Cl2O2S MW:251.1296 |
2-(3-Chloro-4-fluorophenoxy)-2-phenylacetic acidCatalog No.:AA01AGIN CAS No.:1097168-10-9 MDL No.:MFCD12542597 MF:C14H10ClFO3 MW:280.6788 |
N-(2-methoxyethyl)thieno[3,2-d]pyrimidin-4-amineCatalog No.:AA01C45C CAS No.:1097168-21-2 MDL No.:MFCD11641715 MF:C9H11N3OS MW:209.2681 |
1-[(2-chloroethyl)sulfanyl]-2,4-difluorobenzeneCatalog No.:AA01DQ8Y CAS No.:1097168-39-2 MDL No.:MFCD11641763 MF:C8H7ClF2S MW:208.6560 |
Fmoc-l-delta-azidoornithineCatalog No.:AA003CK1 CAS No.:1097192-04-5 MDL No.:MFCD11052921 MF:C20H20N4O4 MW:380.3972 |
1-Boc-2-(2-methoxy-2-oxoethylcarbamoyl)pyrrolidineCatalog No.:AA009LNQ CAS No.:1097194-13-2 MDL No.:MFCD01046139 MF:C13H22N2O5 MW:286.3242 |
Methyl 5-amino-5,6,7,8-tetrahydronaphthalene-2-carboxylate hydrochlorideCatalog No.:AA00HBI4 CAS No.:1097196-62-7 MDL No.:MFCD22571285 MF:C12H16ClNO2 MW:241.7139 |
(R)-4-(1-Amino-ethyl)-benzoic acid methyl ester hydrochlorideCatalog No.:AA00928U CAS No.:1097196-96-7 MDL No.:MFCD12910780 MF:C10H14ClNO2 MW:215.6767 |
5-(1-((tert-Butoxycarbonyl)amino)ethyl)isoxazole-3-carboxylic acidCatalog No.:AA0099AC CAS No.:1097257-19-6 MDL No.:MFCD24532820 MF:C11H16N2O5 MW:256.2551 |
(1-Methyl-1h-pyrrolo[2,3-b]pyridin-3-yl)methanolCatalog No.:AA00HBID CAS No.:1097323-08-4 MDL No.:MFCD21769727 MF:C9H10N2O MW:162.1885 |
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2-(Methyl(pyridin-4-yl)amino)ethanolCatalog No.:AA007SEJ CAS No.:109736-44-9 MDL No.:MFCD14588720 MF:C8H12N2O MW:152.1937 |
Methyl 5-acetamidopyridine-3-carboxylateCatalog No.:AA009TR6 CAS No.:109737-31-7 MDL No.:MFCD22392129 MF:C9H10N2O3 MW:194.1873 |
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3,4-Diaminobutanoic acid dihydrochlorideCatalog No.:AA01AHYB CAS No.:109754-82-7 MDL No.:MFCD09836079 MF:C4H12Cl2N2O2 MW:191.0563 |
D,L-ERYTHRO-1-PHENYL-2-DECANOYLAMINO-3-MORPHOLINO-1-PROPANOL HCLCatalog No.:AA008RA3 CAS No.:109760-77-2 MDL No.:MFCD08703011 MF:C23H39ClN2O3 MW:427.0204 |
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tert-butyl N-(4-methylthiophen-3-yl)carbamateCatalog No.:AA01ACAF CAS No.:1097629-78-1 MDL No.:MFCD24471163 MF:C10H15NO2S MW:213.2966 |
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tert-Butyl ((3-bromo-4,5-dihydroisoxazol-5-yl)methyl)carbamateCatalog No.:AA003UFZ CAS No.:109770-82-3 MDL No.:MFCD08741480 MF:C9H15BrN2O3 MW:279.1310 |
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tert-Butyl 2-ethynylbenzylcarbamateCatalog No.:AA0093XX CAS No.:1097731-47-9 MDL No.:MFCD24471043 MF:C14H17NO2 MW:231.2903 |
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2-(2-Bromophenyl)morpholineCatalog No.:AA00HBIM CAS No.:1097796-83-2 MDL No.:MFCD11646196 MF:C10H12BrNO MW:242.1124 |
2-(NAPHTHALEN-1-YL)MORPHOLINECatalog No.:AA01EHRM CAS No.:1097797-03-9 MDL No.:MFCD11646204 MF:C14H15NO MW:213.2750 |
2-(2,4-difluorophenyl)morpholineCatalog No.:AA01AFQH CAS No.:1097797-34-6 MDL No.:MFCD11646218 MF:C10H11F2NO MW:199.1972 |
2-(3,4-difluorophenyl)morpholineCatalog No.:AA0095D2 CAS No.:1097797-38-0 MDL No.:MFCD11646219 MF:C10H11F2NO MW:199.1972 |
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1,4-Dioxane-2-carbohydrazideCatalog No.:AA019T5H CAS No.:1097812-58-2 MDL No.:MFCD11645181 MF:C5H10N2O3 MW:146.1445 |
