Ionic Liquids as Green Solvents - ACS Publications - American

Chapter 24

Downloaded by PENNSYLVANIA STATE UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch024

Polarity Variation of Room Temperature Ionic Liquids and Its Influence on a Diels-Alder Reaction Richard A. Bartsch and Sergei V. Dzyuba Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061

The polarity of 1-X-3-methylimidazolium bis(trifluoromethylsulfonyl)imides, as asssessed by E (30) values, may be substantially altered by incorporation of functional groups into the X substituent. When such RTILs are employed as solvents for the Diels-Alder reaction of cyclopentadiene and methyl acrylate, increased polarity produces an enhanced endo/exo ratio in the reaction products. T

Introduction Air- and moisture-stable room-temperature ionic liquids (RTILs) are emerging as important alternatives to conventional molecular organic solvents (i). Negligible vapor pressure, as well as ease of recovery and reuse, make RTILs a greener alternative to volatile organic solvents. An intriguing feature of RTILs is the ability to tailor certain bulk properties (e.g., melting point, viscosity, hydrophobicity) by varying the nature of the cation and/or anion (2). Hence, RTILs have been deemed 'designer-solvents' (5). Recently, air- and moisture-stable MiV'-dialkylimidazolium salts with PF " and BF " anions have been utilized for a wide range of chemical processes (4) including alkylation, Baylis-Hillman reactions, Diels-Alder reactions, 6

4

© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

289


290 dimerization, enzymatic catalysis (5), Friedel-Crafts reactions, Heck coupling reactions, hydrodimerization, hydrogénation, nucleophilic displacement, polymerization, silica aerogel synthesis (6), Suzuki cross-coupling reactions and Wittig reactions. They have also been employed in several separation methodologies as stationaiy phases for gas chromatography (7), diluents for solvent extraction of neutral molecules and ions (8-13) and liquid phases in supported liquid membranes (14,15). Much less attention has been paid to determining the influence of structural variations within the cationic and/or anionic components of RTILs on their physical properties, such as conductivity, density, phase transitions, polarity, refractive index, surface tension, thermal stability and viscosity (16-32). For example, only a few studies have been conducted to explore their polarity and how the cationic and anionic components influence solvation on the molecular level (16, 19, 20, 22-24, 27, 28, 30, 32).

Probing the Polarity of RTILs The IUPAC recommended definition of solvent polarity (33) is: "Polarity is the sum of all possible, non-specific and specific, intermolecular interactions between the solute ions or molecules and solvent molecules, excluding such interactions leading to definite chemical alterations of the ions or molecules of the solute." A frequently encountered empirical scale of solvent polarity is E (30), which is based the wavelength maximum of the longest intramolecular chargetransfer π-π* absorption of Reichardt's dye (1) (Figure 1). (Historically this is the dye numbered 30 in the initial publication by Dimroth, Reichardt et al.) (34). This zwitterionic dye exhibits one of the largest observed solvatochromic effects of any known organic molecule (35). The charge-transfer absorption wavelength shifts amount to several hunched nanometers in going from a polar solvent (>w β 453 nm in water) to a non-polar solvent ( L x es 925 nm in hexane). The Ε (30) value is calculated from the wavelength of the absorption maximum by the equation: T

Ύ

1

E (30)(kcalmor ) = 2 8 5 9 1 / ^ (nm) T

In a limited number of investigations, RTEL polarities have been probed using solvatochromic (Figure 1) (19, 22-24, 28, 30, 32) andfluorescent(16, 20, 23, 27) dyes. The solvatochromic dyes include Reichardt's dye (1) (22-24, 28, 30, 32\ Nile Red (2) (19, 32) and organometallic complex 3 (22, 24). The measured and calculated £τ(30) values for some 1-alky1-3-methylimidazolium salts (24) are presented in Table I.

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


291

3 Figure L Solvatochromic dyes used to probe RTIL polarity.

Table I. Absorption Maxima and E (30) Values for Some î-R-3-methy limidazolium Salts T

1

R Anion ^ (nm) ErtëQ) (kcal mol') Bu PF " 546.5 52.3 Bu BF * 545.0 52.5 Bu Tf N~ 555.5 51.5 Bu TfO 547.0 52.3 Oct PF " 558.0 51.2 Oct ΉΝ 559LO 51.1 SOURCE: Data from Reference 24, Copyright 2001 Royal Society of Chemistry 6

4

2

6

2

For l-butyl-3-methylimidazolium salts with PF , B F , bis(trifluoromethylsulfonyl)imide (Tf N~) and trifluoromethylsulfonate (TfO) anions, the £ (30) values vaiy only slightly in the range of 51.5-52.5. Changing R from butyl to octyl with PF and Tf IsT anions produces single unit decreases in the £τ(30) values. These observations reveal that neither elongation of the alkyl group nor variation of the anion produces a pronounced change in the Ε (30) value of the RTIL. The £t(30) values for several common molecular organic solvents are given in Table II. From comparison of the data in Tables I and II, it is concluded that the 1 -R-3 -methylimidazolium salts are similar in polarity to ethanol. However, it has been found that the precise positioning of the RTIL polarity on a scale of molecular organic solvent polarities may vary somewhat with the identity of the solvatochromic or fluorescent dye (25). 6

4

2

6

T

2

τ


292 Table Π. Ε (30) Values of Molecular Organic Solvents


τ

Erf30) (hcalmof) Solvent Water 63.1 55.5 MeOH 51.9 EtOH 48.4 2-PrOH 43.2 DMF 42.2 Acetone 1.2-Dichloroethane 41.3 SOURCE: Data from Reference 35, Copyright 1994 Amerrican Chemical Society

Expanding the Polarity Range for RTILs We surmised that incorporation of functional groups into one nitrogen substituent in l-X-3-methylimidazolium salts might produce a significant alteration in the polarity. (Although others have reported a very limited number of imidazolium salts with functional groups in their substituents, their polarities were not determined (7, 16, 36-38). For testing of this proposal, preparation of the series of compounds shown in Figure 2 was undertaken. For this RTIL series, the Tf N" anion was chosen over PFe ' due to the lower melting points and viscosities of 1,3-disubstituted bistrifyhmides (29). -

2

Me^ C3H7 C10H21

CH Ph 2

CH CHpMe 2

CH CHiOH 2

CH CO^t 2

CH CO^I 2

Figure 2. Structures for the proposed l-X-3-methylimidazolium bistrifylimide series. Synthesis of members of this series was accomplished in two steps, first by alkylation of 1-methylimidazole under the conditions shown in Figure 3. Then anion metathesis with LiNTf in water (29) transformed the l-X-3-methyl2


293 imidazolium bromide or chloride into the corresponding bistrifylimide salt. (Although the synthesis of l-(2-methoxyethyl)-3-methylimidazolium bistrifyl imide was reported earlier by others {16), this new synthetic route is much simpler.) Most members of the series were free-flowing liquids. However, with X = CH C02Et and CH2CO2H, the RTILs were found to be very viscous. 2

l^JK

|(g,N-X

140"C,20min

X = C3H* C H i , CH CH OMe Downloaded by PENNSYLVANIA STATE UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch024

1 0

M

6

N

N-^V

2

BnO

2

2

w

Μ

[ j N Ci-CH2CH OH 2

|C+)N-CH œ Et 2

2

cr

Bf

BrCH CQ H 2

2

MeCKreflux^h

[@N-CH C0 H (25%) 2

2

^

Figure 3. Alkylation of 1-methylimidazole to form l-X-3-methylimidazolium halides. Absoiption maxima for Reichardt's Dye (1) in the remaining five RTDLs were measured and the J5r(30) values were calculated. The £ (30) values and water contents for this series of 1 -X-3 -methylimidazolium bistrifylimides are recorded in Table ΙΠ. T


294

Table ΠΙ. 2?(30) Values for l-X-3-methyIimidazolium bistrifylimides T

i

Water content (oom) Er(S0)(kcalmoT ) 52.0 3810 51.9 160 52.1 C10H21 1450 52.5 CH2PI1 2180 54.1 3050 CH CH OMe 61.4 CH2CH2OH 3380 60.8 CH,CH,OH 390 SOURCE: Data from Reference 32, Copyright 2002 Elsevier Science X C3H7 CsHy


2

2

As expected, the change from X = propyl to decyl did not produce a significant variation in the £ (30) value. Similarly when X = benzyl, the 2?t(30) value was essentially the same as with X = alkyl. On the other hand, when X = 2-methoxyethyl, a significantly larger £ (30) value shows that the polarity is enhanced. With X = CH CH OH, a much larger value of £ (30) was obtained. This reveals a markedly enhanced polarity of the RTIL to OIK intermediate between water and methanol (Table II). Since the water content of 1 -alkyl-3-methyhmidazolium hexafluorophosphates has been shown to influence their £ (30) values (30), two members of the 1 -X-3 -methylimidazolium bistrifylimide series were subjected to special drying. The £ (30) values for the dried RTILs with X = C*H and CftCH OH were found to be in good agreement with those obtained with the "wet" RTILs. Thus incorporation of a functional group into the X substituent of l-X-3methylimidazolium bistrifylimides is shown to markedly expand the range of RTIL polarities. Further investigation of this phenomena with other functional groups and substituents is underway. T

T

2

2

T

T

T

7

2

Influence of RTIL Polarity on A Diels-Alder Reaction Among the most useful synthetic processes for carbon-carbon bond formation is the Diels-Alder reaction (39). RTILs are suitable media for DielsAlderreactionsand have been employed as both sovents and catalysts (40-47). For molecular organic solvents, the endolexo ratio of Diels-Alder reaction products is related to the polarity of the solvent in which the reaction is performed (48). Increasing the solvent polarity enhances the endolexo ratio. For comparison, a study of a Diels-Alder reaction (Figure 4) with the five members of the RTIL series was undertaken to probe the influence of ionic liquid solvent polarity on the endolexo ratio.


295

+

CH =CHC0 Me 2

2

R

m

^

+ CO^Me (endo)

^ ^ C 0

2

M e

Η (exo)


Figure 4. The Diels-Alder reaction. The Diels-Alder reaction of cyclopentadiene and methyl acrylate wass reported to be heterogeneous in 1-alky 1-3-methylimidazolium RTILs (44) with the endolexo ratio depending on both the concentration of reactants and the reaction time. However, we found the reactions to be homogeneous. The influence of concentration and reaction time was evaluated with 1-methy 1-3propylimid-azolium bistrifylimide as solvent. No change in the endolexo ratio was observed over arangeof concentrations (0.3-1 M) andreactiontimes (2-24 hours). Reactions of equimolar amounts of cyclopentadiene and methyl acrylate in the RTIL proceeded in high yield (>95%) in 2 hours at room temperature. The endolexoratiosobtained for reactions conducted in the RTIL series and Ετ(30) values for the solvents are compared in Table IV. Although the variation in the endolexoratiois modest, a correlation with the RTIL polarity is clearly evident. Greater polarity produces a higher endolexo ratio. Table IV. Comparison of the l-X-3-methylimidazolium bistrifylimide polarity and stereoselectivity of the Diels-Alder reaction of cyclopentadiene with methyl acrylate. Et(30) (kcalmoï ) endo/exo 52.0 4.3 52.1 4.3 C10H21 52.5 CH Ph 4.9 54.1 5.7 CH CH OMe 61.4 CH,CH,OH 6.1 SOURCE: Data from Reference 32, Copyright 2002 Elsevier Science Ltd. X CH 3

1

7

2

2

2

Experimental Section RTIL Synthesis The 1 -methy 1-3-propylimidazolium, 1 -decyl-3-methylimidazolium and 1benzy 1-3 -methyhmidazolium bistrifylimides were prepared by the procedures reported in Reference 29.


296 l-(2-Hydroxyethyl)-3-methylimidazolium chloride A solution of 1-methylimidazole (10.0 mL, 125 mmol) and 2-chloroethanol (8.4 mL, 125 mmol) was refluxed for 4 hours to give a 90% yield of the product as a tan, oily solid. H NMR (300 MHz, DMS(W ): ô 3.68 (t, 2H), 3.83-3.90 (m, 3H), 4.23 (t, 2H), 5.09 (br s, 1H), 7.69 (s, 1H), 7.71 (s, 1H). Analysis Calculated (Found) for Q»HnClN 0: C, 44.32 (44.30); H, 6.82 (6.96); N, 17.23 (17.25). !

6


2

l-(2-Methoxyethyl)-3-methylimidazolium bromide Reaction of 1-methylimidazole (5.00 mL, 53 mmol) and 2-bromoethyl methyl ether (4.24 mL, 53 mmol) at 140° C for 20 minutes according to the procedure for the preparation of 1 -alky 1-3 -methylimidazolium bromides in Reference 49 gave a 99% yield of an oil. H NMR (300 MHz, DMSO-d ): δ 3.27 (s, 3H), 3.61-3.74 (m, 3H), 4.35 (s, 3H), 7.68 (s, 1H), 7.71 (s, 1H), 9.06 (s, 1H). Analysis Calculated (Found) C Hi3BrN 0: C, 38.03 (37.68); H, 5.93 (6.02); N, 12.67 (12.72). !

6

7

2

l-(Hydroxycarboxymethyl)-3-methylimidazolium bromide When 1-methylimidazole (5.00mL, 63 mmol) was added to a solution of bromoacetic acid (8.71 g, 63 mmol) in acetonitrile (25 mL), refluxing commenced. After refluxing for 8 hours and then stirring at room temperature for 8 hours, the white precipitate was filtered, washed with diethyl ether, and dried in vacuo to give a 25% yield of white solid with mp = 165-172°C (dec). *H NMR (300 MHz, DMSO-^): δ 3.96 (s, 3H), 5.27 (s, 2H), 7.81-7.86 (m, 2H), 9.30 (s, 1H). Analysis Calculated (Found) for CeHpBrN^: C, 32.60 (32.50); H, 4.10 (4.04), N, 12.67(12.54). U(Ethoxycarboxymethyl)-3-methylimidazolium bromide To 1-methylimidazole (5.00 mL, 63 mmol) cooled in an ice-bath, ethyl bromoacetate (7.00 mL, 63 mmol) was added dropwise over a 15-minute period forming a gel. The ice bath was removed and the solution was heated at ~100°C for 15 minutes to provide a quantitative yield of a golden oil. H NMR (300 MHz, DMSO-e/ ): δ 1.25 (s, 3H); 3.84 (s, 3H); 4.21 (q, 2H), 5.31 (s, 2H), 7.79 (s, 2H), 9.19 (s, 1H). Analysis Calculated (Found) for CeHuBdfcQz: C, 38.57 (38.67); H, 5.26 (5.39); N, 11.25 (11.11). !

6

l-X-3-methylimidazolium bistrifylimides Using the procedure given in Reference 29 for conversion of 1-alky 1-3methylimidazolium bromides into the corresponding 1 -alkly-3 -methylimid azolium bistrifylimides, the following bistrifylimide salts were prepared in high yields: X = CH CH OH as a liquid with T = -79°C. H NMR (300 MHz, acetone-^,): δ 3.97 (q, 2H), 3.99 (s, 3H), 4.45 (t, 2H), 7.70-7.76 (m, 2H), 9.00 (s, 1H). Analysis Calculated (Fourni) for CgHi^NaOsS^ C, 23.59 (23.48); H, 2.72 (2.84); N , 10.32 (10.21). X = CH CH OMe as a liquid with % = -81°C. H !

2

2

g

l

2

2


297 NMR (300 MHz, acetone-*): δ 2.06 (t, 3H), 3.35 (s, 3H), 3.82 (t, 2H), 4.10 (s, 3H), 4.55 (t, 2H), 7.72-7.76 (m, 2H), 9.04 (s, 1H). Analysis Calculated (Found) for C Hi3F N305S : C, 25.66 (25.34); H, 3.11 (3.09); N, 9.97 (9.87). X = CH C0 H as a hygroscopic, viscous oil *H NMR (300 MHz, acetone-*): δ 4.15 (s, 2H), 5.38 (s, 2H), 7.77-7.80 (m, 2H), 9.10 (s, 1H). Analysis Calculated (Found) for CgHpFgNsOeSa: C, 22.81 (23.10); H, 2.15 (2.37); N, 9.97 (9.80). X = CH C0 Et as a viscous liquid with % = -56°C and T = 16°C. *H NMR (300 MHz, acetone-*): δ 1.27 (t, 3H), 4.13 (s, 3H), 4.18-4.30 (m, 2H), 5.34 (s, 2H), 7.70-7.82 (m, 2H), 9.07 (s, 1H). Analysis Calculated (Found) for C10H13F6N3O6S2: C, 26.73 (26.89); H, 2.92 (3.01); N , 9.35 (9.27) Downloaded by PENNSYLVANIA STATE UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch024

9

6

2

2

2

2

2

m

Procedure for Diels-Alder Reactions A weighed amount of freshly distilled cyclopentadiene was added to 2.0 mL of the RTIL in a vial at room temperature. With magnetic stirring, an equivalent amount of methyl aciylate was added via a syringe and the vial was capped The contents were stirred for 2 hours at room temperature and then extracted with hexanes and analyzed by gas chromatography on a Caibowax column or in the case of X - C10H21 by H NMR spectroscopy, since that RTIL was slightly soluble in hexane. Yields in all cases were >95% as determined by H NMR spectroscopy. l

!

Acknowledgement This research was supported by the Texas Higher Education Coordinating Board Advanced Research Program.

References 1. 2. 3. 4.

For a recent review see: Sheldon, R. A. Chem. Commun. 2001, 2399. Earle, M . J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391. Freemantle, M . Chem. Eng. News 1998, 76 (March 30), 32. For reviews see: Welton, T. Chem. Rev. 1999, 2071; Olivier-Bouibigou, H.; Magna, L. J. Mol. Catal. A; Chemical 2002, 182-183, 419. 5. For a recent review see: Sheldon, R. Α.; Lau, R. M . ; Sorgedrager, M . J.; van Rantwijk, F.; Seddon, K . R. Green Chem. 2002, 4, 147. 6. Dai, S.; Ju, H.; Gao, J., Lin, J. S.; Pennycook, S. J.; Barnes, C. E. Chem. Commun. 2000, 243. 7. Armstrong, D. L.; He, L.; Liu, Y . S. Anal. Chem. 1999, 71, 3873 and references cited therein. 8. Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998. 1765.



298 9. Dai, S.; Ju, Y. H.; Bames, C. E. J. Chem. Soc, Dalton Trans. 1999, 1201. 10. Visser, A. E.; Swatloski, R. P.; Reichert, W. M . ; Griffin, S. T.; Rogers, R. D. Ind. Eng. Chem. Res. 2000, 39, 3596. 11. Chun, S.; Dzyuba, S. V.; Bartsch, R. A. Anal. Chem. 2001, 73, 3737. 12. Deitz, M . L.; Dzielawa, J. A. Chem. Commun. 2001, 2124. 13. Bartsch, R. Α.; Chun, S.; Dzyuba, S. V. Ionic Liquids. Industrial Applications to Ionic Liquids, ACS Symposium Series 818; Rogers, R. D.; Seddon, K. R., Eds.; American Chemical Society, Washington, DC, 2002; Chapt 5, pp 58-68. 14. Branco, L. C.; Crespo, J. G.; Afonso, C. A. M . Angew. Chem. Int. Ed. 2002, 41, 2771. 15. Scovazzo, P.; Visser, A. E.; Davis, J. H., Jr.; Rogers, R. D.; Koval, Carl Α.; DuBois, D. L.; Noble, R. D. Ionic Liquids. Industrial Applications to Ionic Liquids, ACS Symposium Series 818; Rogers, R. D.; Seddon, K. R., Eds.; American Chemical Society, Washington, DC, 2002; Chapt 6, pp 69-87. 16. Bonhôte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M . Inorg. Chem. 1996, 35, 1168. 17. Holbrey, J. D.; Seddon, K. R. J. Chem.Soc.,Dalton Trans. 1999, 2133. 18. Seddon, K. R.; Stark, Α.; Torres, M.-J. Pure Appl. Chem. 2000, 72, 2275. 19. Carmichael, A. J.; Seddon, K. R. J. Phys. Org. Chem. 2000, 13, 591. 20. Aki, S. Ν. V. K. ; Brennecke, J. F.; Samanta, A. Chem. Commun. 2001, 413. 21. Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. Α.; Rogers, R. D. Green Chem. 2001, 3, 156. 22. Wasserscheid, P.; Dunkin, I. R Chem. Commun. 2001, 1186. 23. Fletcher, Κ. Α.; Storey, I. Α.; Hendricks, A . E.; Pandey, S.; Pandey, S. Green Chem. 2001, 3, 210. 24. Muldoon, M . J.; Gordon, C. M.; Dunkin, I. R. J. Chem.Soc.,Perkin Trans. 2 2001, 433. 25. Noda, Α.; Hayamizu, K.; Watanabe, M . J. Phys. Chem. Β 2001, 105, 4603. 26. Lau, G.; Watson, P. R. Langmuir 2001, 17, 6138. 27. Baker, S. N.; Baker, G. Α.; Kane, Μ. Α.; Bright, F. V. J. Phys. Chem. Β 2001, 105, 9663. 28. Park, S.; Kazlauskas, R. J. J. Org. Chem. 2001,66, 8395. 29. Dzyuba, S. V.; Bartsch, R. A. ChemPhysChem 2002, 3, 161. 30. Baker, S. N.; Baker, G. Α.; Bright, F. V. Green Chem. 2002, 4, 165. 31. Quinn, Β. M . ; Ding, Z., Moulton, R.; Bard, A. J. Langmuir, 2002, 18, 1734. 32. Dzyuba, S. V.; Bartsch, R. A. Tetrahedron Lett. 2002, 43, 4657. 33. Muller, P. PureAppl.Chem. 1994, 66, 1077. 34. Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. Liebigs Ann. Chem. 1963, 662, 1. 35. Reichardt, C. Chem. Rev. 1994, 94, 2319. 36. Visser, A. E.; Swatloski, R. P.; Reichert, W. M . ; Mayton, R.; Sheff, S.; Wierzbicki, Α.; Davis, J. H., Jr.; Rogers, R. D. Chem. Commun. 2001, 135. 37. Kimizuki, N.; Nakashima, T. Langmuir 2001, 17, 6759.



299 38. Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H., Jr. J. Am. Chem. Soc. 2002, 124, 926. 39. For a recent review, see: Kumar, A. Chem. Rev. 2001, 101, 1. 40. Jaeger, D. Α.; Tucker, C. E. TetrahedronLett.1989, 30, 1785. 41. Howarth, J.; Hanlon, K.; Fayne, D.; McCormae, P. Tetrahedron Lett. 1997, 38, 3097. 42. Lee, C. W. Tetrahedtron Lett. 1999, 40, 2461. 43. Earle, M . J.; McCormac, P. B.; Seddon, K. R. Green Chem. 1999, 1, 23. 44. Fischer, T.; Sethi, Α.; Welton, T.; Woolf, J. Tetrahedron Lett. 1999, 40, 793. 45. Zulfiqar, F.; Kitazume, T. Green Chem. 2000, 2, 137. 46. Song, C. E.; Shim, W. H.; Roh, E. J.; Lee, S.-g.; Choi, J. H . Chem. Commun. 2001, 1122. 47. Berson, J. Α.; Hamlet, Z.; Mueller, W. A. J. Am. Chem. Soc. 1962, 84, 297. 48. Sethi, A. R.; Wleton, T. Ionic Liquids. Industrial Applications to Ionic Liquids, ACS Symposium Series 818; Rogers, R. D.; Seddon, K. R., Eds.; American Chemical Society, Washington, DC, 2002; Chapt 19, pp 241-246. 49. Dzyuba, S. V.; Bartsch, R. A. J. Heterocyclic Chem. 2001, 38, 265.


Ionic Liquids as Green Solvents - ACS Publications - American

Recommend Documents