Starbon®400-HSO3: A green mesoporous carbonaceous solid acid catalyst for the Ritter reaction

    R Starbon400-HSO 3 : A green mesoporous carbonaceous solid acid catalyst for the Ritter reaction

L´ıgia M.M. Mesquita, Rui M.A. Pinto, Jorge A.R. Salvador, James H. Clark, Vitaliy L. Budarin PII: DOI: Reference:

S1566-7367(15)00238-1 doi: 10.1016/j.catcom.2015.06.010 CATCOM 4360

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

24 April 2015 9 June 2015 10 June 2015

Please cite this article as: L´ıgia M.M. Mesquita, Rui M.A. Pinto, Jorge A.R. Salvador, R James H. Clark, Vitaliy L. Budarin, Starbon400-HSO 3 : A green mesoporous carbonaceous solid acid catalyst for the Ritter reaction, Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.06.010

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ACCEPTED MANUSCRIPT Starbon®400-HSO3: A green mesoporous carbonaceous solid acid catalyst for the Ritter reaction a

b

a,b

c,

Laboratório de Química Farmacêutica, Faculdade de Farmácia, Universidade de Coimbra, 3000-548,

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a

c

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Vitaliy L. Budarin

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Lígia M. M. Mesquita, Rui M. A. Pinto, Jorge A. R. Salvador, * James H. Clark ** and

Coimbra, Portugal b

Centro de Neurociências e Biologia Celular, Universidade de Coimbra, 3004-517 Coimbra, Portugal

c

Green Chemistry Centre of Excellence, Department of Chemistry, The University of York, Heslington,

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YO10 5DD, York, UK

Corresponding authors:

* Tel: +351 239488479; Fax: +351 239 82 126; E-mail: [email protected]

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** E-mail: [email protected]

Abstract ®

technology in the Ritter reaction to afford vic-N-acylamino-

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The application of the Starbon

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hydroxy compounds, in very high yields, is reported. Starbon 400-HSO3 was used as a green and easily recovered mesoporous carbonaceous solid acid catalyst and applied, for the first

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time, to more complex naturally-derived substrates, such as steroids.

Highlights ®



First application of Starbon catalysts to complex naturally-derived substrates.



Significant improvement for the Ritter reaction of epoxysteroids.



2-Oxazolines can be obtained in good yields, under some reaction conditions.



The catalyst is easily recovered, simplifying the reaction work-up.



The green nature of Starbons makes this new process very promising for scale-up.

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Keywords ®

Starbons , Ritter Reaction, Steroids, vic-N-Acylamino-hydroxy compounds, 2-oxazolines. 1

ACCEPTED MANUSCRIPT 1. Introduction The formation of a carbon-nitrogen bond is among the most fundamental processes in organic chemistry. In particular, the synthesis of amides constitute an important reaction in

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pharmaceutical chemistry [1]. Ritter reaction is one of the most widely used strategies to obtain

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N-substituted amide [2, 3]. This reaction consists on the addition of nitriles to a wide variety of compounds capable of forming carbenium ions, such as alkenes, alcohols, carboxylic acids and

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esters, in the presence of concentrated sulphuric acid [2, 3, 4, 5]. Over the years, improved processes have been reported for the synthesis of N-alkyl amides, via acid catalyzed Ritter reaction under homogeneous [6, 7] and heterogeneous conditions [8, 9]. Brønsted acid ionic liquids [10] and organic acids, such as 2,4-dinitrobenzenesulfonic acid (DNBSA) [11], o-

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benzenedisulfonimide [12] and pentafluorophenyl ammonium triflate (PFPAT) [13] have also been recently applied as metal-free catalysts in the Ritter reaction.

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The Ritter reaction has been described using epoxides as substrates to obtain N-substituted amides bearing a vicinal hydroxyl group [14]. Using steroid compounds with epoxides fused to rings A or B, the Ritter reaction proceeds through trans-diaxial ring opening of the epoxide to

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afford vic-N-acylamino-hydroxy steroids. Corrosive acid catalysts, such as BF 3·OEt2 [15, 16,

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17], gaseous BF3 [15], HClO4 [16, 18, 19], SnCl4 [20] and p-toluenosulfonic acid [20] have been reported for this transformation. More recently, a new process using bismuth(III) salts as

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ecofriendly catalysts has been described for the Ritter reaction of epoxysteroids [21, 22].

Despite the fact that the processes described in the literature for the Ritter reaction in general, as well as for its application to the chemistry of epoxysteroids, in particular, report good reaction yields, there are still a few drawbacks related to the use of metal or organic catalysts under

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homogeneous conditions that demand large consumption of solvents during work-up procedures. Therefore, the development of greener processes, using materials derived from natural sources, preferably catalytic and recyclable, that greatly simplifies the reaction work-up should contribute to increase the chemical efficiency of the Ritter reaction, as applied to the synthesis of compounds of pharmaceutical interest, usually associated with high E-factor values [23, 24]. The development of metal-free synthetic methods avoids the use of toxic and expensive metals and is particularly attractive for the preparation of compounds that do not tolerate metal contamination or solvent residues, such as pharmaceuticals [25]. ®

Starbon materials are a family of mesoporous carbonaceous materials derived from starchy food wastes, with surfaces ranging from hydrophilic to hydrophobic, depending of the degree of ®

carbonization (100-700 ºC) [26, 27]. Since Starbons prepared between the temperatures of 300-600 ºC have aromatic functionality, their sulfonation yields solid acid materials, with great potential in acid-catalyzed processes [28, 29, 30, 31]. The green nature of these mesoporous carbonaceous solid acid catalysts prompted us to investigate their application in the

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ACCEPTED MANUSCRIPT development of new chemical processes applied to the chemistry of natural-derived compounds, such as steroids. In this communication, we report the first use of the Starbon

®

technology for the acid-catalyzed Ritter reaction of epoxysteroids to afford the corresponding vic-N-acylamino-hydroxy compounds, in high yields, after simple filtration of the reaction

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mixture.

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2. Experimental 2.1. General

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Steroid compounds that were used in the preparation of epoxysteroids and nitriles were purchased from Sigma-Aldrich Co. 5β,6β-Epoxysteroids were obtained by β-selective 5

epoxidation of Δ -steroids using a method developed by Salvador et al. [32], whereas 5α,6α5

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epoxysteroids were prepared from the corresponding Δ -steroids by epoxidation with mchloroperbenzoic acid (m-CPBA)]. Solvents were obtained from VWR Portugal and purified according to standard procedure. TLC was done in commercial Kieselgel 60 F 254/Kieselgel 60G

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Merck TLC plates, purchased from VWR Portugal. The revelation was done with EtOH/H 2SO4

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95:5 (v/v), followed by heating at 150 ºC, in an appropriate heating plate. Silica gel 60 (230-400 mesh) was used for flash column chromatography. The crude mixture was immobilized in silica gel, and then transferred to the column, previously filled with stacked silica gel 60 (230-400

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mesh). Analytical grade solvents were used for the preparation of the eluents. MS was performed by direct injection of the sample on a Thermo Finnigan PolarisQ GC/MS Benchtop Ion Trap equipped with a direct insertion probe (Electron impact mass spectroscopy, EI-MS) or on a Thermo Finnigan LCQ Advantage Max Quadrupole Ion Trap (Electrospray ionization mass

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spectroscopy, ESI-MS, with samples dissolved in CH3OH or CH3CN). Melting points were determined on a Buchi Melting point B-540 and are uncorrected. IR spectra were performed in a Jasco FT/IR 420 spectrophotometer. NMR experiments were performed on a Bruker Avance III 400 MHz or a Varian 600 MHz spectrometer. NMR samples were prepared in CDCl3 solution. 1

Calibration of chemical shift scale was made using the CDCl3 signal at 7.26 ppm ( H NMR) or 13

77.0 ppm ( C NMR). Chemical shifts (δ) are given in ppm and coupling constants are presented in Hz.

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2.2. Preparation of Starbon 400-HSO3 ®

Starbon 400-HSO3 solid acid catalyst were prepared as previously reported [28, 29]. Acid group loading on the catalyst was 0.6 mmol/g.

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ACCEPTED MANUSCRIPT 2.3. Typical experimental procedure for the Ritter reaction of epoxysteroids catalyzed by Starbon®400-HSO3 To a solution of 5β,6β-epoxycholestan-3β-yl acetate 1 (26.7 mg, 0.06 mmol) in CH3CN (3.4 mL, ®

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65.2 mmol), Starbon 400-HSO3 (5.0 mg, 5 mol%) was added. After 6 h under magnetic stirring,

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at r.t., the reaction was completed as verified by TLC control. The reaction mixture was then filtered, the catalyst was recovered and the filtrate was concentrated under reduced pressure to

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afford 5α-acetamido-6β-hydroxycholestan-3β-yl acetate 2 [22] as a white solid (28.7 mg, 95% yield). Mp (CH3OH): 179-180 ºC; lit [15] 180-182 ºC; IR (film) 3406, 2935, 1720, 1663, 1512, -1 1

1256, 1030 cm ; H NMR (CDCl3, 400 MHz) δ 0.70 (s, 3H, 18-CH3), 0.87 and 0.87 (2d, J = 6.6 Hz, each 3H, 26-CH3 and 27-CH3), 0.92 (d, J = 6.5 Hz, 3H, 21-CH3), 1.32 (s, 3H, 19-CH3), 2.02

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(s, 6H, 3β-OCOCH3 and 6β-NHCOCH3), 4.67 (t, J = 2.9 Hz, 1H, 6α-H), 4.84 (ddd, J1 = 16.6 Hz, J2 = 11.1 Hz and J3 = 5.2 Hz, 1H, 3α-H), 5.21 (s, 1H, NH);

13

C NMR (CDCl3, 100 MHz) δ 62.8

(C5), 68.5 (C6), 70.9 (C3), 169.5 (6β-NHCOCH3), 170.8 (3β-OCOCH3); EI-MS m/z (%): 503(2) +

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M , 443 (39), 428 (100), 384 (52), 351 (46), 253 (46), 143 (52), 91 (18).

Experimental details and selected analytical data for products 4, 6, 8, 9 and 10 are available as

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Electronic Supplementary Information (ESI), in Appendix A. Additionally; it is also provided relevant discussion about the structural elucidation of the new steroid compounds 6, 9 and 10 13

C NMR spectra

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1

as well as their H and

3. Results and discussion

Initially, standardization of the protocol was carried out with 5β,6β-epoxysteroid 1 and CH3CN, ®

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using Starbon 400-HSO3, as catalyst (Table 1). In the absence of acid catalyst, compound 1 was recovered unchanged, after 24 h of reaction, in CH 3CN, at r.t. (Table 1, entry 1). Using a ®

catalytic amount of Starbon 400-HSO3 (5 mol %), the Ritter reaction product 5α-acetamido-6βhydroxycholestan-3β-yl acetate 2 was obtained, in very high yields (Table 1, entries 2 and 3), however a shorter reaction time was obtained under lower concentration of 1 (Table 1, entry 2). The ring opening of 5β,6β-epoxysteroid 1 was stereoselective. The trans-diaxial nucleophilic attack occurred by the α-face, allowing the introduction of the acetamide group at the bridgehead C5. The product 2 was easily isolated as a white powder, after simple filtration and CH3CN removal, under reduced pressure. The recovered catalyst was subsequent applied on further reactions (Table 1, entries 4 and 5). Although catalytic activity was retained, the reactions become slower by using the recovered catalyst (Table 1, entries 4 and 5). The nature of this catalyst activity loss was not investigated in this work; however similar findings have been reported with other solid acid catalysts [33].

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ACCEPTED MANUSCRIPT Table 1. ® a Ritter reaction of 5β,6β-epoxycholestan-3β-yl acetate 1, catalyzed by Starbon 400-HSO3. C8H17 H

H AcO

Starbon®400-HSO3

H

AcO

CH3CN, r.t.

OH 2

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®

Starbon 400-HSO3 (mol %) --5 5 c 5 c 10 5

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1 (mmol) 0.06 0.06 0.11 0.06 0.06 0.06

NH CH3

1

Entry 1 2 3 4 5 6

O

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O

T

H

a

Time (h) 24 6 11 24 15 48

Yield of 2 (%) --95 98 93 95 d, e ---

b

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Reaction conditions: CH3CN was used both as solvent and reactant [3.4 mL (65.2 mmol)], in an open b c ® reaction vessel, at r.t.; Isolated yield; Reaction performed using Starbon 400-HSO3 recovered from d e previous reactions; Reaction performed under nitrogenous atmosphere; Very low conversion of 1 into 2, as observed by analysis of the TLC plate.

When the reaction was performed under nitrogen atmosphere, very low conversion of 1 was

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obtained (Table 1, entry 6). Thus, the presence of H2O, provided when the reactions are carried out in an open vessel, under non-inert atmosphere, is absolutely necessary for full substrate

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conversion. From this data it is plausible to suggest the following mechanistic approach: the ®

Starbon 400-HSO3 solid acid, that were found to have almost an equal proportion of Brønsted

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and Lewis acid sites [31] catalyzes the generation of a carbenium ion, which adds to the nitrile to afford a nitrilium ion. This cationic species is subsequently trapped by H 2O affording the corresponding N-substituted amides (Figure 1), which is in agreement with the known reaction

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mechanism for Ritter reaction [2].

O

HO

H+



AcO



N

CCH3

AcO

OH

H

AcO

O

OH

H AcO

- H+

N CCH3

N C

CH3 OH

OH AcO

H

N

C

CH3

O

Figure1. Reaction mechanism for the acid-catalyzed Ritter reaction of 5β,6β-epoxysteroids.

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ACCEPTED MANUSCRIPT This new process was then extended to other substrates (Table 2). Thus, the 5β,6βepoxysteroids 3 and 5 were efficiently converted into the corresponding 5α-acetamido-6βhydroxy compounds 4 and 6, in 95% and 98% yield, respectively (Table 2, entries 1 and 2). ®

These results showed the excellent catalytic activity of Starbon 400-HSO3 in the Ritter reaction

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of 5β,6β-epoxysteroids, which are prone to side reactions, under acidic conditions, such as

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rearrangements involving C18-methyl migration [22].

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The use of other nitriles has also been investigated (Table 2, entries 4 and 5). When methylthioacetonitrile was used as both solvent and reactant, product 9 was efficiently obtained in 90% yield, after 5 h (Table 2, entry 4). In order to maintain the straightforward work-up, a lower amount of this liquid nitrile was used. Under these reaction conditions, 10 mol% of

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catalyst were found necessary for full substrate conversion.

The Ritter reaction of the 5β,6β-epoxysteroid 1 has also been accomplished with

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methylsulfonylthioacetonitrile, a solid nitrile (Table 2, entry 5). The reaction was performed in 1,4-dioxane and a aqueous work-up was needed to eliminate the excess of unreacted nitrile. Under these reaction conditions, a different reaction outcome was observed, since no vic-N-

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acylamino-hydroxy product was obtained. Instead, the 5α-N-6β-O-fused 2-oxazoline compound 10 was isolated, in 80% yield, after purification by flash column chromatography on SiO 2. The

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preparation of 2-oxazolines has also been previously reported under Ritter reaction conditions

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[34, 35].

4. Conclusions

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In conclusion, the application of the Starbon technology to more complex naturally-derived substrates, such as steroids, has been described for the first time. A new procedure for the ®

Ritter reaction of epoxides using Starbon 400-HSO3 as catalyst was developed. The use of the mesoporous carbonaceous solid acid really improves this process due to the following reasons: absence of metal catalyst, green nature of the catalyst and easiness of its recovery, very simple work-up consisting of just filtration followed from nitrile removal under reduced pressure, high yields, relatively short reaction times at r.t. and great chemo- and stereoselectivity. Other ®

applications of Starbons to the chemistry of natural products are currently being investigated.

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ACCEPTED MANUSCRIPT Table 2. ® a Ritter reaction of the epoxysteroids 1, 3, 5 and 7, catalyzed by Starbon 400-HSO3. Solvent/ Time Entry Substrate Product Reagent (h)

Yield b (%)

O O

T

H H

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H

AcO AcO

H

O

3

1

CH3CN

c

H

H

O

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AcO

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H

5

2

CH3CN

C8H17 H

H

4 H H H

AcO

O

NH

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O

OH

98 C8H17 H

H AcO

c

d

8

45

60 C8H17 H H

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H AcO

H

O

O

H3 C

1

4

CH3 O

CH3CN

H

H

H

OH HN

C8H17 H

AcO

H

6

20

D H

95 O

H

7

3

c

OH

CH3

CH3

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H

AcO

NH

6

O

H

O

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H

H

CH3SCH2CN

e

NH

H

OH

S

9

5

90 C8H17 H H

C8H17 H

H

H

N

H AcO

5 a

H

AcO

H

O

H3C

O

1

1,4-Dioxane/ f CH3SO2CH2CN ®

O

34

S O d

10

80 b

c

Reaction conditions: 5 mol % of Starbon 400-HSO3, in an open reaction vessel, at r.t.; Isolated yield; d CH3CN was used both as solvent and reactant (2.5 mL/0.05 mmol of substrate); Purification by flash e column chromatography on SiO2, using petroleum ether 40-60 ºC/ethyl acetate 2:1 (v/v) as eluent; ® f Reaction performed with 0.06 mmol of 1, 6.0 mmol of nitrile and 10 mol % of Starbon 400-HSO3; Reaction performed with 0.06 mmol of 1, 2.0 mL of 1,4-dioxane and 4.2 mmol of nitrile.

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ACCEPTED MANUSCRIPT

Acknowledgment J. A. R. Salvador thanks Universidade de Coimbra for financial support. The authors also

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acknowledge the Nuclear Magnetic Resonance Laboratory of the Coimbra Chemistry Centre

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(www.nmrccc.uc.pt), Universidade de Coimbra, for obtaining part of the NMR data.

Appendix A. Supplementary data

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Supplementary data to this article can be found online.

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