Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes

Transcription

1 Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes Markus Schwarz and Bernd Speiser Institut für rganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, D Tübingen bernd.speiser@uni-tuebingen.de Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.1/19

2 Introduction: Electrosynthesis of Compound Libraries? combinatorial synthesis of compound libraries multiple reactions of similar starting compounds small amount of individual product(s) screening: quickly assess some properties of library elements redox behavior ( redox screening ) synthesis products ( synthesis screening ) mechanisms ( mechanistic screening ) reaction channels ( reactivity screening ) standard method in drug design and catalyst optimization electrosynthesis? Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.2/19

3 Introduction: Approaches and Examples earlier approaches: Yudin et al.: spatially adressable electrolysis platform galvanostatic T. Siu, W. Li, and A.K. Yudin, J. Comb. Chem. 2, (2000) Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.3/19

4 Introduction: Approaches and Examples earlier approaches: Yudin et al.: spatially adressable electrolysis platform galvanostatic Breinbauer et al.: mediated electrolysis with polymer bead support S. Nad and R. Breinbauer, Angew. Chem. 116, (2004); Angew. Chem. Intl. Ed. 43, (2004) Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.3/19

5 Introduction: Approaches and Examples earlier approaches: Yudin et al.: spatially adressable electrolysis platform galvanostatic Breinbauer et al.: mediated electrolysis with polymer bead support Moeller et al.: micro-electrolysis on a chip E. Tesfu, K. Maurer, and K.D. Moeller, J. Am. Chem. Soc. 128, (2006) Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.3/19

6 Introduction: Approaches and Examples earlier approaches: Yudin et al.: spatially adressable electrolysis platform galvanostatic Breinbauer et al.: mediated electrolysis with polymer bead support Moeller et al.: micro-electrolysis on a chip our system: combi-secm instrument C C-bond forming reaction: reductive coupling of α,β-unsaturated esters and allylbromides electrolyte: room temperature ionic liquid Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.3/19

7 The Experiment: Instrument T. Erichsen, S. Reiter, V. Ryabova, E.M. Bonsen, W. Schuhmann, W. Märkle, C. Tittel, G. Jung, and B. Speiser, Rev. Sci. Instrum. 76, (2005) Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.4/19

8 The Experiment: Instrument T. Erichsen, S. Reiter, V. Ryabova, E.M. Bonsen, W. Schuhmann, W. Märkle, C. Tittel, G. Jung, and B. Speiser, Rev. Sci. Instrum. 76, (2005) Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.4/19

9 The Experiment: Instrument % $ $! $ W. Märkle, B. Speiser, C. Tittel, and M. Vollmer, Electrochim. Acta 50, (2005) Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.4/19

10 The Experiment: Instrument Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.4/19

11 The Experiment: Reaction cathodic reduction, allylbromide (1) + α,β-unsaturated ester (2) + R 4 R 6 CR 7 R 5 R 1 R 3 R 4 R 5 CR R 1 R 3 R 4 R 5 CR 7 R 1 R e - - Br - R 3 Br 1 R 1 R 2 R 3 C R 1 R 2 R 2 R 3 C + R 6 R 4 R 6 CR 7 R 5 2 C combinatorial diversity (R 1 R 7 ) R 3 R 2 R 4 R 5 CR 7 R 1 R 2 R 6 3 R R 3 R 4 R 5 CR 7 2 C R 1 R 2 3 R 6 R 4 CR 7 R 5 R e - R 4 R 6 + CR 7 R 5 R 1 R 2 R 3 Br R 7 C R 6 R 3 - Br - R 4 R R 7 C R 6 R 3 + e - R 4 R 1 R 5 R 2 R 5 R 2 4 Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.5/19

12 The Experiment: Reaction cathodic reduction, allylbromide (1) + α,β-unsaturated ester (2) R 1 R 2 R 4 CR 7 R e - - Br - R 3 Br R R 1 R 2 + R 4 R 6 CR 7 R 5 R 3 C R 1 R 2 R 1 R 2 R 3 R 4 R 5 CR 7 R 3 C + R 6 R 4 R 6 CR 7 R C combinatorial diversity (R 1 R 7 ) R 3 R 1 R 2 R 4 R 5 CR 7 R 1 R 2 R 3 R 4 R 5 CR 7 two reaction pathways (primary reduction of 1 or 2) R 6 3 R R 3 R 4 R 5 CR 7 2 C R 1 R 2 3 R 6 + e - R 4 R 6 + CR 7 R 5 R 1 R 2 R 3 Br R 7 C R 6 R 3 - Br - R 4 R R 7 C R 6 R 3 + e - R 4 R 1 R 5 R 2 R 5 R 2 4 Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.5/19

13 The Experiment: Reaction cathodic reduction, allylbromide (1) + α,β-unsaturated ester (2) R 1 R 2 R 4 CR 7 R e - - Br - R 3 Br R R 1 R 2 + R 4 R 6 CR 7 R 5 R 3 C R 1 R 2 R 1 R 2 R 3 R 4 R 5 CR 7 R 3 C + R 6 R 4 R 6 CR 7 R C combinatorial diversity (R 1 R 7 ) R 3 R 1 R 2 R 4 R 5 CR 7 R 1 R 2 R 3 R 4 R 5 CR 7 two reaction pathways (primary reduction of 1 or 2) R 6 R isomeric products depending on symmetry of educts 3 R 3 R 4 R 5 CR 7 2 C R 1 R 2 3 R 6 + e - R 4 R 6 + CR 7 R 5 R 1 R 2 R 3 Br R 7 C R 6 R 3 - Br - R 4 R R 7 C R 6 R 3 + e - R 4 R 1 R 5 R 2 R 5 R 2 4 Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.5/19

14 The Experiment: Design of the Experiment galvanostatic reduction of allylbromides and α,β-unsaturated esters: S. Satoh et al., Bull. Chem. Soc. Jpn. 54, (1981) changes: galvanostatic potentiostatic (in DMF) DMF ionic liquid electrolyte ([BMIM]BF 4 ) macro cell miniaturization (200 µl) single reaction combinatorial (6 8 = 48 elements) 3 C N N C 4 9 BF 4 Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.6/19

15 The Experiment: Components 6 allylbromides 3 C 3 C C 3 3 C 3 C Br Br 3 C Br Br Br 1a 1b 1c 1d 1e 1f Br 8 α,β-unsaturated esters C 3 C 3 3 C 3 C C3 C3 C3 2a 2b 2c 2d C 3 N 3 C 3 C C 3 C3 3 C C3 N 2 3 C C 3 C 3 2e 2f 2g 2h Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.7/19

16 Potentiostatic Reduction in DMF: Allylbromide + Diethyl Fumarate Br 3 C C3 1a; c = 1.2 mm; v = 0.2 V s 1 2a; c = 1.2 mm; v = 0.2 V s 1 Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.8/19

17 Potentiostatic Reduction in DMF: Allylbromide + Diethyl Fumarate Br 3 C C3 1a; c = 1.2 mm; v = 0.2 V s 1 2a; c = 1.2 mm; v = 0.2 V s 1 co-electrolysis of equimolar amounts (0.4 mmol) in DMF/0.1 M NBu 4 PF 6 at V 20 h, 139 C, 1.8 F extraction with ether GC-MS: formation of ethyl-3-(ethoxycarbonyl)-5-hexanoate 4aa Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.8/19

18 Potentiostatic Reduction in DMF: Allylbromide + Diethyl Fumarate Br 3 C C3 1a; c = 1.2 mm; v = 0.2 V s 1 2a; c = 1.2 mm; v = 0.2 V s 1 co-electrolysis of equimolar amounts (0.4 mmol) in DMF/0.1 M NBu 4 PF 6 at V 20 h, 139 C, 1.8 F extraction with ether GC-MS: formation of ethyl-3-(ethoxycarbonyl)-5-hexanoate 4aa transfer galvanostatic potentiostatic ok Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.8/19

19 Miniaturized Potentiostatic Reduction in [BMIM]BF 4 : Allylbromide + Diethyl Fumarate cyclic voltammetric monitoring before (full) and after (dotted) electrolysis (v = 0.3 V s 1 ) E p (2a) = 1.45 V, E p (1a) 2 V decrease of negative extension of potential window decrease of i p (2a) Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.9/19

20 Miniaturized Potentiostatic Reduction in [BMIM]BF 4 : Allylbromide + Diethyl Fumarate 3 C C3 C 2 5 C C 2 5 C 4aa 3 C 3 C C 3 C 3 c(1a) = c(2a) = 0.2 mm, reaction volume = 1 ml potentiostatic electrolysis at V 13.5 h, 2.31 C, 1.2 F extraction with hexane, GC-MS Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.10/19

21 Miniaturized Potentiostatic Reduction in [BMIM]BF 4 : Allylbromide + Diethyl Fumarate 3 C C3 C 2 5 C C 2 5 C 4aa 3 C 3 C C 3 C 3 transfer DMF [BMIM]BF 4 ok miniaturization ok Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.10/19

22 Miniaturized Potentiostatic Reduction in [BMIM]BF 4 : A Small Collection 1a/2a 1c/2a 1a/2c 1c/2c Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.11/19

23 Miniaturized Potentiostatic Reduction in [BMIM]BF 4 : A Small Collection 3C C3 C25C C25C 4aa 3C 3C C3 C3 1a/2a 1c/2a 1a/2c 1c/2c ester more easy to reduce coupling product identified by molecular ion and main fragments identity of 4aa assigned based on ester radical anion attack on allylbromide MS does not allow to differentiate between isomers easily Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.11/19

24 Miniaturized Potentiostatic Reduction in [BMIM]BF 4 : A Small Collection 3C C3 C25C C25C 4aa 1a/2a 3C 3C C3 C3 3C C3 C25C CC25 2C 3C C3 3ca C25C C25C 4ca C3 unidentified C3 3C 3C C3 C3 1c/2a 1a/2c 1c/2c ester more easy to reduce coupling product identified by molecular ion and main fragments 4ca is main product isomeric side product, tentatively assigned as 3ca Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.11/19

25 Miniaturized Potentiostatic Reduction in [BMIM]BF 4 : A Small Collection 3C C3 C25C C25C 4aa 1a/2a 3C 3C C3 C3 3C C3 C25C CC25 2C 3C C3 3ca C25C C25C 4ca C3 unidentified C3 3C 3C C3 C3 1c/2a 3C C3 3C CC3 2C 3ac 1a/2c 1c/2c allylbromide more easy to reduce coupling product identified by M + and main fragments (very low yield) identity of 3ac assigned based on attack of allyl anion on ester Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.11/19

26 Miniaturized Potentiostatic Reduction in [BMIM]BF 4 : A Small Collection 3C C3 C25C C25C 4aa 1a/2a 3C 3C C3 C3 3C C3 C25C CC25 2C 3C C3 3ca C25C C25C 4ca C3 unidentified C3 3C 3C C3 C3 1c/2a 3C C3 3C CC3 2C 3ac 1a/2c 1c/2c allylbromide more easy to reduce no products identified Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.11/19

27 Miniaturized Potentiostatic Reduction in [BMIM]BF 4 : A Small Collection 3C C3 C25C C25C 4aa 1a/2a 3C 3C C3 C3 3C C3 C25C CC25 2C 3C C3 3ca C25C C25C 4ca C3 unidentified C3 3C 3C C3 C3 1c/2a 3C C3 3C CC3 2C 3ac 1a/2c 1c/2c electrolysis with subsequent GC-MS provides screening technique Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.11/19

28 Interpretation of Mass Spectra: The Example of 4aa M + small molecular ion peak M + Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.12/19

29 Interpretation of Mass Spectra: The Example of 4aa α-cleavage F1 M + small molecular ion peak M + α-cleavage of ester group F1 Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.12/19

30 Interpretation of Mass Spectra: The Example of 4aa α-cleavage F1 F2 = F1 - M + small molecular ion peak M + α-cleavage of ester group F1 hydrogen loss F2 Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.12/19

31 Interpretation of Mass Spectra: The Example of 4aa C F4 F1 α-cleavage C F3 F2 = F1 - M + small molecular ion peak M + α-cleavage of ester group F1 hydrogen loss F2 cleavage of C F3 or F4 Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.12/19

32 The Combinatorial Experiment: A Collection of 48 Electrolyses Microtiter Plate Loading A B C D E F G 1 WW 1a/2a 1b/2a 1c/2a 1d/2a 1e/2a 1f/2a WW 2 WW 1a/2b 1b/2b 1c/2b 1d/2b 1e/2b 1f/2b WW 3 WW 1a/2c 1b/2c 1c/2c 1d/2c 1e/2c 1f/2c WW 4 WW 1a/2d 1b/2d 1c/2d 1d/2d 1e/2d 1f/2d WW 5 WW 1a/2e 1b/2e 1c/2e 1d/2e 1e/2e 1f/2e WW 6 WW 1a/2f 1b/2f 1c/2f 1d/2f 1e/2f 1f/2f WW 7 WW 1a/2g 1b/2g 1c/2g 1d/2g 1e/2g 1f/2g WW 8 WW 1a/2h 1b/2h 1c/2h 1d/2h 1e/2h 1f/2h WW WW = wash well 48 combinations of allylbromides and esters sequential electrolysis (30 min) in wells (volume: 200 µl) mixing washing Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.13/19

33 The Combinatorial Experiment: A Collection of 48 Electrolyses Screening for Fragments in MS after 30 min: A B C D E F G 1 WW F1/F2/F3/F4 M + /F1/F2/F3/F4 M + /F1/F3/F4 F1/F2/F3/F4 WW 2 WW F1/F2/F4 F1/F2/F3/F4 M + /F1/F2/F4 F1/F2/F4 WW 3 WW WW 4 WW M + /F1/F4 F1/F3 F1/F4 WW 5 WW WW 6 WW WW 7 WW WW 8 WW F1/F3/F4 F1/F4 F4 F1/F4 F1 WW Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.14/19

34 The Combinatorial Experiment: A Collection of 48 Electrolyses Screening for Fragments in MS after 30 min: A B C D E F G 1 WW F1/F2/F3/F4 M + /F1/F2/F3/F4 M + /F1/F3/F4 F1/F2/F3/F4 WW 2 WW F1/F2/F4 F1/F2/F3/F4 M + /F1/F2/F4 F1/F2/F4 WW 3 WW WW 4 WW M + /F1/F4 F1/F3 F1/F4 WW 5 WW WW 6 WW WW 7 WW WW 8 WW F1/F3/F4 F1/F4 F4 F1/F4 F1 WW esters 2c, 2e, 2f, and 2g do not react at all to expected products (rows 3, 5, 6, 7) allylbromides 1e and 1f do only react with 2h to expected products (columns F, G) Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.14/19

35 The Combinatorial Experiment: A Collection of 48 Electrolyses Screening for Fragments in MS after 30 min: A B C D E F G 1 WW F1/F2/F3/F4 M + /F1/F2/F3/F4 M + /F1/F3/F4 F1/F2/F3/F4 WW 2 WW F1/F2/F4 F1/F2/F3/F4 M + /F1/F2/F4 F1/F2/F4 WW 3 WW WW 4 WW M + /F1/F4 F1/F3 F1/F4 WW 5 WW WW 6 WW WW 7 WW WW 8 WW F1/F3/F4 F1/F4 F4 F1/F4 F1 WW esters 2c, 2e, 2f, and 2g do not react at all to expected products (rows 3, 5, 6, 7) allylbromides 1e and 1f do only react with 2h to expected products (columns F, G) how can this pattern be explained? Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.14/19

36 The Mechanistic Pattern: Esters with Electron-Donating Substituents are Not Reactive R 4 R 5 R 6 R 7 R 4 R 5 R6 R 7 C 3 3 C 3 C C3 C3 3 C N 2 2c 2e 2f very negative reduction potential allylbromide is reduced allyl anion attacks electron-donating substituents R 4 or R 5 decrease positive charge density esters become less reactive C3 Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.15/19

37 The Mechanistic Pattern: Nitrophenyl Substituted Ester is Not Reactive N C 3 2g two reduction waves characteristic for N 2 reduction radical anion of 2g stabilized and unreactive Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.16/19

38 The Mechanistic Pattern: Allylbromides with Electron-Withdrawing Substituents are Not Reactive R 1 R 3 R 1 R 3 R 1 R 3 3 C C 3 C C 3 C 3 C 3 C 3 C Br Br 3 C 1e 1f 2h C C 3 C 3 relatively positive reduction potential allylbromides are reduced electron-withdrawing substituents decrease charge density in allyl anions allyl anions become less reactive exception: reaction with ester 2h even less negative potential and ester reduction mechanism prevails Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.17/19

39 Conclusions C C-bond forming reaction transfer from classical galvanostatic electrolysis conditions in DMF to miniaturized potentiostatic reaction in an ionic liquid combi-secm approach with GC-MS allows synthesis screening: which combinations do react as intended? explanations through mechanism differentiation between reaction channels (isomers) difficult: instead of MS use NMR (coupling with PLC, GC)? Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.18/19

40 Acknowledgements Deutsche Forschungsgemeinschaft for funding Wolfgang Schuhmann, Bochum, for cooperation Graeme Nicholson, Tübingen, for technical assistance with GC-MS experiments Combinatorial Electrosynthesis in Microtiter Plate Wells with Ionic Liquid Electrolytes p.19/19