1. Chemistry 125: Lecture 62 March 28, 2011 NMR Spectroscopy Decoupling 13 C NMR and Double Labeling Correlation and 2D NMR Electrophilic Aromatic Substitution This For copyright notice see final page of this file

2. Proton Decoupling 13 C H 100 MHz25 MHz (in frame rotating at 100 MHz) C 13 NMR spectrum while irradiating H (100 MHz) H average H upH down Decoupling Power determines the rate of this precession. Pulse (25MHz) to observe C 13 or If it is fast enough, the 13 C doublet collapses.

3. 40 db (inverse log measure of rf power) CH 2 CH CH 3 C CH 2 CH CDCl 3 Observe 13 C while decoupling 1 H at various powers.

4. 40 db (inverse log measure of rf power) Observe 13 C while decoupling 1 H at various powers.

5. 20 db (inverse log measure of rf power) Observe 13 C while decoupling 1 H at various powers.

6. 15 db (inverse log measure of rf power) Observe 13 C while decoupling 1 H at various powers.

7. 10 db (inverse log measure of rf power) Observe 13 C while decoupling 1 H at various powers.

8. 5 db (inverse log measure of rf power) Observe 13 C while decoupling 1 H at various powers.

9. 2 db (inverse log measure of rf power) Observe 13 C while decoupling 1 H at various powers.

10. 1 db (inverse log measure of rf power) CH 2 CH CH 3 C CH 2 CH CDCl 3 Observe 13 C while decoupling 1 H at various powers. NOE (Nuclear Overhauser Enhancement) RF excitation of a nucleus strengthens the signal from nearby nuclei. Bad for integration Good for determining structure (see below)

11. Precession Frequencies in Magnetic Field of ~23.5 kGauss MHz H1H1 F 19 P 31 C 13 O 17 1% 99.98% 6%

12. Proton-Decoupled 13 C NMR Assignments for the Artificial Sweetner Neotame Monohydrate Prof. Eric Munson, Kansas Univ. One peak per carbon, pretty well spread out Why no 13 C- 13 C splitting? Only 1-4% of 13 Cs have an adjacent 13 C in the same molecule. C=OC arom C-X e-neg C alkane 7

13. 2-D NMR Dilute 13 C Double Labeling Power of Correlation:

14. Double Labeling Introduction: Lanosterol Biogenesis Cf. Frames 6-13 of Lecture 52 and e.g. J&F, Sec. 12.13 pp. 554-562

15. + Squalene H + + + + + + HO O

16. Squalene + + + + + HO H H H CH 3 H H H + H3CH3C H3CH3C H3CH3C Lanosterol (source of cholesterol & steroid hormones)

17. Squalene + + + + + HO H H H CH 3 H H H + H3CH3C H3CH3C H3CH3C Lanosterol (source of cholesterol & steroid hormones) 3° Cute Story Is it True? (“Wait for NMR”)

18. HO H H CH 3 H3CH3C H3CH3C H3CH3C Squalene Lanosterol 13 C Label Single Label Enrichment Enriched Peaks (100x stronger than natural-abundance peaks)

19. HO H H CH 3 H3CH3C H3CH3C H3CH3C Squalene Lanosterol 13 C Label Single Label Enrichment Enriched Peaks

20. HO H H CH 3 H3CH3C H3CH3C H3CH3C Squalene Lanosterol 13 C Double Label

21. Proves that they entered as a unit. The dilute double label experiment enhances the same 12 13 C peaks as the single label experiments, but only 8 of them show spin-spin splitting (because their C-C bond stays intact). HO H H CH 3 H3CH3C H3CH3C H3CH3C 13 C Double Label Squalene DILUTE ! Double-Doublet (proton decoupled) 13 C- 13 C splitting (neighboring 13 Cs) Power of Correlation both labeled, but not in the same molecule Few single precursor molecules have any C 13 label, but those that are labeled have two C 13 s. These are both labeled, in the same molecule Strongly confirms the rearrangement scheme.

22. Dilute 13 C Double Labeling Power of Correlation: 2-D NMR

23. 1 H to 1 H Correlation by NOE (through-space magnetic interaction) protons in protein polymer proximity (< 6Å) of With Molecular Mechanics Constraints gives 3-D Structure ( without crystal!) NH at  7.25 is within 6Å of NHs at  8.9, 8.3, 8.25, 7.7 Narrow range; mostly HN-C=O protons Identify NH with amino acid by coupling through CH to R H H O N HR O N HR Less-congested, off-diagonal peaks appear when “tickling” one signal on the diagonal enhances another. diagonal shows normal spectrum heavily congesed by overlapping signals

24. 1 H vs. 1 H Correlation in TIME 0.3 sec 40°C  (ppm) Note: ppm scale is slanted and "wackbards". The protons in methyls C and D are near + charge (see resonance structures), thus deshielded from lack of electron density, and appear furthest to right, i.e. at highest . (Range of peaks is 150 Hz in 60 MHz spectrometer = 2.5 ppm.) + + ++ ++ ++ ++ ++ ++ C A D B B C Methide Shift: 1- 2 (as shown) “2-Dimensional” NMR H3CH3C CH 3 H3CH3C H3CH3C H3CH3C + A B C D A D or 1-Anywhere?

25. "3-D" Version of contour plot on previous slide

26. Electrophilic Aromatic Substitution

27. H HH H H H D 2 SO 4 H HH H D H e.g. J&F Sec. 14.4 H HH H H H D A/D via intermediate + D 2 SO 4 C6D6C6D6 etc. Observable! Or other electrophiles in place of D + e.g. NO 2 +, Br +, HOSO 2, R +, R-C=O R-C=O + HOSO 2 + In electrophilic addition to alkenes, a nucleophile would add in the next step, but here it is easier to lose H + and recover aromatic stabilization.

28. 1 H vs. 1 H correlation in time 0.3 sec 40°C CACA DBDB Remember

29. + + + SHMo2 (Simple Hückel Molecular Orbital Program) BenzenePentadienyl Cation addition converts  ring to  chain (destroys aromaticity). H HH H H H D+D+ H HH H H H D + Locus of odd electron in radical, + charge (LUMO) in cation. - charge (HOMO) in anion, SOMO (nonbonding) Cf. e.g. J&F, Sec. 14.4b

30. + LUMO X NO 2 + ++ ++ ++ H O2NO2N X + ++ ++ ++ H O2NO2N X ++ ++ ++ H Substituent Effects on Rate (e.g. sec. 14.9-14.10) X O2NO2N X O2NO2N XX NO 2 + + + (from HONO 2 /H 2 SO 4 ) X Relative Rate (overall) H[1] Cl0.03 NO 2 6  10 -8 CH 3 25 OH1000 (CH 3 ) 3 N + 1  10 -8  donation /  withdrawal  (or  ) e-donation eases formation of cation intermediates  (or  ) e-withdrawal retards formation of cation intermediates HOH + Note: + NO 2 is O=C=O with an extra proton in the center products intermediates Cf. J&F, Table 14.2 4  y (or  x ) electrons

31. Why is -NO 2 e-Withdrawing when -OH is e-Donating? -NO 2 6  10 -8  e-withdrawal retards formation of cation intermediates -OH1000  e-donation eases N OO O H  p O 0 High HOMO; Good Overlap with Phenyl (able to donate, but not willing) Low OMO; Good Overlap with Phenyl HOMO as high as pO; OH is a  Donor NO 2 is a  Acceptor but No Overlap with Phenyl NO 2 is allylic (willing, but not able) (willing and able to accept) Low LUMO; Good Overlap with Phenyl pCpC  C=C

32. Substituent Effects on Orientation (e.g. J&F, Sec. 14.9-14.10) X NO 2 + (from HONO 2 /H 2 SO 4 ) X- Relative Rate (per replaceable H) H-[1] Cl-0.00080.030.13 O 2 N- 9  10 -8 0.6  10 -8 0.03  10 -8 H 3 C-13946 (CH 3 ) 3 C-4672 Directing Activating Deactivating Act Deact  (CH 3 ) 3 N- 3  10 -8 0.6  10 -8 + _ orthometapara EtOC=O 0.0060.0006 - 0.003 o/po/p m o/po/p “e-donating” “e-withdrawing” (steric hindrance) ? X NO 2 + ++ ++ ++ H O2NO2N X + ++ ++ ++ H O2NO2N X ++ ++ ++ H intermediates

33. Activate the Electrophile Halogenation (e.g. J&F, Sec. 14.4f) “Cl +” H ++ -- +29 +38 AlCl 3 “Lewis acid” catalyst AlCl 5 Cl 2 vs. LUMOsSurface Potentials (Al, like B, has an empty valence AO) AlCl 4 leaving group Cf. [Br 2 ] 2 for rate of addition to alkene

34. Activate the Nucleophile H OH C O O H NH 2 100 atm 125°C Self-Igniting Rocket Fuel (1944) N O O + Salicylic Acid Kolbe (1860) OH C O H+H+ Aspirin lowered HOMO raised HOMO C O H3CH3C Acetylation of aniline makes its nitration controllable.

35. Taming Aniline Nitration ortho 19% meta 2% para 79% HNO 3 H 2 SO 4 ( pyridine ) OH (e.g. J&F, Sec. 14.4f)

36. Charles Friedel (1832-1890) James Mason Crafts (1839-1917) AlCl 3 /C-Electrophile: The Friedel-Crafts Reaction

37. Paris (Wurtz - Médicine) Cornell MIT Paris (Friedel - Mines) President AlCl 3 /C-Electrophile: The Friedel-Crafts Reaction 1877

38. End of Lecture 62 March 28, 2011 Copyright © J. M. McBride 2011. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0) Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol. Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0