Received: December 22, 2013 | Accepted: January 8, 2014 | Web Released: January 16, 2014
CL-131192
Sodium Hypochlorite-promoted Novel Synthesis of Organic Ammonium Tribromides and Application of Phenanthroline Hydrotribromide in Chemoselective Oxidation of Organic Sulfides by Hydrogen Peroxide Madhudeepa Dey, Rupa R. Dey, and Siddhartha S. Dhar* Department of Chemistry, National Institute of Technology Silchar, Assam 788010, India (E-mail:
[email protected])
A novel method of synthesis of organic ammonium tribromides (OATBs) is developed by using an inexpensive and eco-friendly sodium hypochlorite as oxidant for conversion of Br¹ to Br3¹. The OATBs thus prepared include both quaternary ammonium tribromides and N-heterocyclic tribromides. A new addition to the family of OATBs is made in the form of phenanthroline hydrotribromide. The efficacy of this new tribromide as catalyst is ascertained in the oxidative transformation of organic sulfides to their corresponding sulfoxides and sulfones by hydrogen peroxide.
The synthesis and application of organic ammonium tribromides (OATBs) have garnered overwhelming interest in the past two decades.1 Quite a few quaternary ammonium tribromides such as tetrabutylammonium tribromide (TBATB), tetraethylammonium tribromide (TEATB), and cetyltrimethylammonium tribromide (CTMATB) have been reported to be highly useful reagents for a variety of organic functional group transformations. Moreover, OATBs such as pyridine hydrotribromide (PyHTB), quinolinium tribromide (QHTB), N-methylpyrrolidin-2-one hydrotribromide (MPHT), phenyltrimethylammonium tribromide (PTATB), and bipyridine hydrotribromide (BHTB) have been well documented in the literature as alternative sources of bromine that offer the advantages of safety, selectivity, mild reaction conditions, and stability.2 It is evident from the available literature that OATBs find increasing applications as reagents for a variety of organic reactions including functional group transformations like bromination, oxidation, epoxidation, acylation, etc.3,1c For example they act as vital reagents for preparation of bromo organics, for bromination of alcohols, enones, alkenes, and activated aromatics4 and also as bifunctional catalyst in oxidative bromination of aromatic compounds. Thus, looking into the spectrum of uses of OATBs in synthetic organic chemistry it was considered pragmatic to design newer synthetic protocols for the synthesis of OATBs. Traditional methods of synthesis of OATBs utilize liquid bromine, Br2, for their synthesis which severely handicaps their applicability to large-scale industrial processes. Therefore as a part of our continued research endeavor toward development of newer improved and greener methods for synthesis of OATBs and other organic substrates,1c,1d,5 we report herein for the first time a new transition-metal-free, simple, and efficient method for the synthesis of OATBs employing an inexpensive and useful environmentally benign oxidant, sodium hypochlorite, NaOCl. The use of NaOCl as potent oxidant in synthetic organic chemistry has been well documented in literature.6 However, its application in the synthesis of OATBs is hitherto unknown. Therefore it has been chosen here as the oxidizing agent for the Chem. Lett. 2014, 43, 631–633 | doi:10.1246/cl.131192
first time for conversion of bromide to tribromide leading to the synthesis of a variety of OATBs including nitrogen-containing heterocyclic tribromides such as PyHTB, MPHT, BHTB, and phenanthroline hydrotribromide (PhenHTB). It is also revealed from exhaustive literature survey that PhenHTB has not yet been reported to date and thus is added as a new member to the family of OATBs. The new reagent is expected to be stable, long-lived, and easy to handle. Furthermore mention may be made of the chemistry involving selective oxidation of organic sulfur compounds that have attracted attention of researchers for many decades. Such transformations are of immense significance in synthetic and bioorganic chemistry.7 We wish to report herein a very efficient, cost-effective, and mild synthetic protocol for a wide variety of organic ammonium tribromides, and exploration of a new reagent, 1,10-phenanthroline hydrotribromide as a catalyst for oxidative transformation of organic sulfides to sulfoxides and sulfones by hydrogen peroxide. A variety of quaternary ammonium bromides, QABs and Nheterocyclic compounds were converted to the corresponding tribromides by using NaOCl as a reagent for oxidizing Br¹ to Br3¹ (Schemes 1 and 2). Two equivalents of potassium bromide, KBr, were used for QABs to transform them into their respective QATBs while three equivalents of KBr were needed for transformation of N-heterocycles such as phenanthroline to their corresponding tribromides, PhenHTB (Scheme 3). It may be reiterated that NaOCl has never been used for the synthesis of OATBs and proved to be an excellent and efficient oxidant for such conversion. A small amount of dilute H2SO4 is also necessary to obtain the respective tribromides. It may be further claimed that PhenHTB is the first reported tribromide. All the synthesized tribromides were characterized by UVvisible spectroscopic analysis. The tribromides exhibit a strong absorpNaOCl + 3 Br - + 2 H+
Br3- + NaCl + H2O
Scheme 1. Oxidation of Br¹ to Br3¹ by NaOCl. R4NBr + 2 KBr
NaOCl, H+ R4NBr3 Grinding R 4 N+ 1. = Me4N+ 2. = Et 4N+ 3. = n -Bu4N + 4. = Benzyl-Me3N+ 5. = Cetyl-Me3N+
Scheme 2. Formation of ammonium tribromides from their corresponding quaternary ammonium bromides.
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O
H+, NaOCl + 3 KBr N
N
PhenHTB (10 mol %) H2O2 (1 equiv)
r.t. N
N HBr3
S
R1
PhenHTB (20-30 mol %)
Scheme 3. A representative example of formation of a tribromide from N-heterocyclic compound.
tion band at ca. 270 nm,8 characteristic of Br3¹ anion. PhenHTB was additionally analyzed for its CHN and the total bromine content was estimated by volumetric method. All the results obtained are consistent with theoretical values. The analytical results also suggest that only one nitrogen atom of phenanthroline gets protonated thus forming a monotribromide rather than a ditribromide. A survey of the available literature also points to the fact that a monotribromide is formed as the product.3b,2c,9 As an example, two recent reports on ditribromides such as 1,2-bis(N-pyridinium)ethane ditribromide1b and ethylenebis(Nmethylimidazolium) ditribromide2d indicate that stable ditribromides of N-heterocyclic compounds have CH2CH2 groups between two N-heterocycles. Tribromides obtained from an Nheterocyclic compound always lead to the formation of monotribromide. The analytical results suggest that tribromide obtained from bipyridine and phenanthroline in the present methodology are also monotribromide consistent with literature. In a typical synthesis of OATBs, a solution of 20 mmol of KBr in 10 mL of 2 M H2SO4 was added to 10 mmol of quaternary ammonium bromide. Sodium hypochlorite (10 mL of 4%) was then added to the resulting solution and stirred for ca. six to ten minutes to obtain bright orange-yellow crystalline tribromide compound. The synthesized tribromide compound thus formed was isolated by suction filtration and washed with water (10 mL) three to four times and dried under vacuum over anhydrous CaCl2 to get orange crystals of quaternary ammonium tribromides in very good to excellent yields. N-Heterocyclic tribromides were prepared by adding an amount of 30 mmol of KBr in 10 mL of 2 M H2SO4 to 10 mmol of N-heterocyclic compound. To the resulting solution 10 mL of 4% sodium hypochlorite was added and stirred for ca. four to five minutes. The bright orange-yellow crystalline compound of tribromide thus formed was separated by suction filtration and washed with water (5 mL) three to four times and dried under vacuum over anhydrous CaCl2 to obtain orange crystals of the tribromide in excellent yields. This new tribromide PhenHTB is characterized by CHN analysis, UVvis, FT-IR, and 1H NMR spectroscopy. UVvis (max): 269 nm; FT-IR (KBr): 3390, 3048, 1535, 1184, 713 cm¹1; 1H NMR (400 MHz, CDCl3): ¤ 9.33 (s, 1H), 8.8 (d, J = 8.4 Hz, 2H), 8.1 (d, J = 11.2 Hz, 2H), 8.0 (d, J = 1.6 Hz, 2H), 7.2 (t, J = 13.4 Hz, 2H); Anal Calcd for C12H9N2Br3: C, 34.24; H, 2.16; N, 6.65; Br, 56.95%. Found: C, 33.5; H, 2.0; N, 6.4; Br, 57.02%. After the successful synthesis of PhenHTB, it was considered worthwhile to investigate its efficacy as catalyst for selective oxidative transformation of organic sulfides to corresponding sulfoxides and sulfones. The oxidation of sulfides was carried out in acetonitrilewater solvent system and 30% H2O2 as the oxidant. For preliminary studies, methyl phenyl sulfide was chosen as a representative substrate. The oxidation was then
632 | Chem. Lett. 2014, 43, 631–633 | doi:10.1246/cl.131192
H2O2 (2-3 equiv)
Room Temperature, 70-94%
S
R1
O S O
R1
Scheme 4. Oxidation of alkyl aryl sulfides to their corresponding sulfoxides and sulfones. Table 1. Oxidation of sulfides to sulfoxides using 30% H2O2 catalyzed by PhenHTB in CH3CNH2Oa Entry 1 2 3 4 5 6 7 8 9 10
Substrate PhSCH3 4-MeOPhSCH3 4-MePhSCH3 4-ClPhSCH3 PhSPh PhCH2SCH2Ph PhSC2H5 4-NO2PhSCH2Ph t-C4H9SCH3 PhSCH2Ph
Time Isolated /min yield/% PhSOCH3 30 92 4-MeOPhSOCH3 30 94 4-MePhSOCH3 35 91 4-ClPhSOCH3 25 87 PhSOPh 25 87 PhCH2SOCH2Ph 25 80 PhSOC2H5 26 89 4-NO2PhSOCH2Ph 60 84 t-C4H9SOCH3 32 70 PhSOCH2Ph 30 88 Sulfoxideb
a
All reactions were carried out at room temperature using oxidant-to-substrate molar ratio of 1:1 and 10 mol % PhenHTB used as catalyst in CH3CNH2O as the solvent. bAll products were identified by their IR and NMR spectral data and comparison of their mp with published data.10
carried out by taking catalytic amount of PhenHTB (10 mol %) and 30% H2O2 (1 equiv) as oxidant under acetonitrilewater as solvent (Scheme 4). A controlled reaction carried out without the catalyst either shows no product formation or in some cases formation of products in extremely poor yields. Also it was observed that in absence of peroxide the reaction did not take place at all using 10 mol % of PhenHTB indicating that the tribromide does not act as oxidant at this concentration. Therefore, the main oxidant in the reported protocol is PhenHTB acts as catalyst only. It may be further mentioned that no significant overoxidation of sulfide to sulfone was observed even on prolonging the reaction time indicating that sulfoxide is the main product under the given reaction conditions. The same protocol was adopted to transform sulfides to sulfones by increasing the concentration of catalyst to 2030 mol % and oxidant (30% H2O2) to 20 mmol, respectively. In the oxidation of organic sulfides to sulfoxides, a solution of organic sulfide (10 mmol) and PhenHTB (10 mol %) in acetonitrilewater (10 mL, 1:1) was stirred and 30% hydrogen peroxide was added to the resulting solution drop wise (10 mmol) at room temperature for an appropriate time (30 70 min) (Table 1). After completion of the reaction as monitored by TLC, the reaction mixture was poured in water and the excess hydrogen peroxide was destroyed by the addition of hydrogensulfite (aq) followed by the filtration through a small Buckner funnel. After filtration the organic products were extracted with ether. The ether layer was washed with water (2 mL) and dried over Na2SO4. The organic solvent was
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Table 2. Oxidation of sulfides to sulfones using 30% H2O2 catalyzed by PhenHTB in CH3CNH2Oa Time /min PhSO2CH3 95 PhSCH3 4-MeOPhSO2CH3 37 4-MeOPhSCH3 4-MePhSO2CH3 62 4-MePhSCH3 4-ClPhSO2CH3 25 4-ClPhSCH3 25 PhSPh PhSO2Ph PhCH2SO2CH2Ph 30 PhCH2SCH2Ph PhSO2C2H5 32 PhSC2H5 4-NO2PhSCH2Ph 4-NO2PhSO2CH2Ph 72 t-C4H9SO2CH3 30 t-C4H9SCH3 PhSO2CH2Ph 35 PhSCH2Ph Sulfoneb
Entry Substrate 1 2 3 4 5 6 7 8 9 10
Isolated yield/% 88 86 88 88 88 82 77 70 85 81
a
All reactions were carried out at room temperature using oxidant-to-substrate molar ratio of 2:1 and 2030 mol % PhenHTB used as catalyst in CH3CNH2O as the solvent. bAll products were identified by their IR and NMR spectral data and comparison of their mp with published data.10
+ Br2 N
N HBr3
N HBr
N
Br 1
R
S
2
R
1
+ Br2
2
R S R
. Br
1 H2O2, H2O - 2 HBr O 1
R
S O
2
R
- H2O
H2O
OH 1
R S R
O 2
1
2
R S R
+
2 HBr
OOH 2HBr + H2O2
Br2 + 2H2O
Scheme 5. Proposed mechanism for oxidation of sufides to sulfoxides and sulfones by H2O2 catalysed by PhenHTB.
removed under reduced pressure to give the corresponding sulfoxide. The products were purified by column chromatography on silica gel using ethyl acetatehexane (1:4) as eluent. Evaporation of the solvent yielded the corresponding sulfoxides. Similar procedure was adopted for the formation of sulfones from sulfides (Table 2). But in this case the catalyst amount was increased to 2030 mol % and also the amount of 30% H2O2 was increased to 20 mmol. From a mechanistic point of view, the oxidation of sulfides to sulfoxides and sulfones is presumed to be achieved by the reaction of water and Br2 formed in situ from PhenHTB via the intermediate 1 (Scheme 5). Intermediate 1 produces sulfoxide by reacting with water and sulfone by reaction with H2O2 and water. In both these reactions, HBr is produced which gets converted into bromine (1 mol of Br2 is equivalent to 1 mol of PhenHTB) by H2O2. Thus, the real oxidant is H2O2 and Br2 released by PhenHTB is only the catalyst since it is generated at the end of the reactions.
Chem. Lett. 2014, 43, 631–633 | doi:10.1246/cl.131192
In conclusion, the present work demonstrates a highly efficient yet mild and environmentally acceptable protocol for synthesis of organic ammonium tribromides and also to examine the potential of PhenHTB as a catalyst for selective transformation of organic sufides. Despite of availability of quite a few of methods for such transformation, the present methodology for synthesis of tribromides can certainly be claimed as one of the most efficient and eco-friendly. As a part of our further objective to assess the potential application of the new tribromide reagent, viz., PhenHTB, we have used this reagent as catalyst for oxidation of organic sulfides to sulfoxides and sulfones. The results obtained are quite satisfactory indicating that methodology can be used as a new and useful alternative to the existing procedures for similar transformation.11 This paper is dedicated to Professor Teruaki Mukaiyama in celebration of the 40th anniversary of the Mukaiyama aldol reaction. References and Notes 1 a) J. K. Joseph, S. L. Jain, B. Sain, Eur. J. Org. Chem. 2006, 590. b) V. Kavala, S. Naik, B. K. Patel, J. Org. Chem. 2005, 70, 4267. c) M. Dey, S. S. Dhar, M. Kalita, Synth. Commun. 2013, 43, 1734. d) M. Dey, S. S. Dhar, Green Chem. Lett. Rev. 2012, 5, 639. 2 a) A. Bekaert, O. Provot, O. Rasolojaona, M. Alami, J.-D. Brion, Tetrahedron Lett. 2005, 46, 4187. b) M. H. Ali, S. Stricklin, Synth. Commun. 2006, 36, 1779. c) A. Ghorbani-Choghamarani, M. Nikoorazm, H. Goudarziafshar, M. Abbasi, Bull. Korean Chem. Soc. 2011, 32, 693. d) R. Hosseinzadeh, M. Tajbakhsh, M. Mohadjerani, Z. Lasemi, Monatsh. Chem. 2009, 140, 57. 3 a) S. Singhal, S. L. Jain, B. Sain, Synth. Commun. 2011, 41, 1829. b) M. K. Chaudhuri, A. T. Khan, B. K. Patel, Tetrahedron Lett. 1998, 39, 8163. 4 a) U. Bora, G. Bose, M. K. Chaudhuri, S. S. Dhar, R. Gopinath, A. T. Khan, B. K. Patel, Org. Lett. 2000, 2, 247. b) U. Bora, M. K. Chaudhuri, D. Dey, S. S. Dhar, Pure Appl. Chem. 2001, 73, 93. c) S. Adimurthy, S. Ghosh, P. U. Patoliya, G. Ramachandraiah, M. Agrawal, M. R. Gandhi, S. C. Upadhyay, P. K. Ghosh, B. C. Ranu, Green Chem. 2008, 10, 232. d) L.-Q. Wu, C.-C. Yang, Y.-F. Wu, L.-M. Yang, J. Chin. Chem. Soc. 2009, 56, 606. 5 a) M. Dey, K. Deb, S. S. Dhar, Chin. Chem. Lett. 2011, 22, 296. b) R. R. Dey, S. S. Dhar, Chin. Chem. Lett. 2013, 24, 866. 6 a) K. R. Seddon, Nat. Mater. 2003, 2, 363. b) N. Fukuda, T. Ikemoto, J. Org. Chem. 2010, 75, 4629. c) R. A. Sheldon, I. W. C. E. Arends, G.-J. ten Brink, A. Dijksman, Acc. Chem. Res. 2002, 35, 774. d) J. H. Ramsden, R. S. Drago, R. Riley, J. Am. Chem. Soc. 1989, 111, 3958. 7 a) H. Zhang, C. Chen, R. Liu, Q. Xu, W. Zhao, Molecules 2010, 15, 83. b) P. Kowalski, K. Mitka, K. Ossowska, Z. Kolarska, Tetrahedron 2005, 61, 1933. c) D. Azarifar, K. Khosravi, Eur. J. Chem. 2010, 1, 15. d) F. Gregori, I. Nobili, F. Bigi, R. Maggi, G. Predieri, G. Sartori, J. Mol. Catal. A: Chem. 2008, 286, 124. e) M. Bakavoli, A. M. Kakhky, A. Shiri, M. Ghabdian, A. Davoodnia, H. Eshghi, M. Khatami, Chin. Chem. Lett. 2010, 21, 651. 8 M. K. Chauduri, U. Bora, S. K. Dehury, D. Dey, S. S. Dhar, W. Kharmawphlang, B. M. Chaudhary, L. K. Mannepalli, U.S. Patent, No. 7,005,548, 2006. 9 M. Joshaghani, A. R. Khosropour, H. Jafary, I. MohammadpoorBaltork, Phosphorus, Sulfur Silicon Relat. Elem. 2005, 180, 117. 10 a) S. Hussain, S. K. Bharadwaj, R. Pandey, M. K. Chaudhuri, Eur. J. Org. Chem. 2009, 3319. b) S. P. Das, J. J. Boruah, H. Chetry, N. S. Islam, Tetrahedron Lett. 2012, 53, 1163. 11 Supporting Information is available electronically on the CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
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