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

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.

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.

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

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

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

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

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

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

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)

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

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

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

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

+ Squalene H HO O

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

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”)

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

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

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

Proves that they entered as a unit. The dilute double label experiment enhances the same 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.

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

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

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?

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

Electrophilic Aromatic Substitution

H HH H H H D 2 SO 4 H HH H D H e.g. J&F Sec 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.

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

+ + + 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

+ LUMO X NO 2 + ++ ++ ++ H O2NO2N X + ++ ++ ++ H O2NO2N X ++ ++ ++ H Substituent Effects on Rate (e.g. sec ) X O2NO2N X O2NO2N XX NO (from HONO 2 /H 2 SO 4 ) X Relative Rate (overall) H[1] Cl0.03 NO 2 6  CH 3 25 OH1000 (CH 3 ) 3 N + 1   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  y (or  x ) electrons

Why is -NO 2 e-Withdrawing when -OH is e-Donating? -NO 2 6   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

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

Activate the Electrophile Halogenation (e.g. J&F, Sec. 14.4f) “Cl +” H ++ - 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

Activate the Nucleophile H OH C O O H NH 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.

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

Charles Friedel ( ) James Mason Crafts ( ) AlCl 3 /C-Electrophile: The Friedel-Crafts Reaction

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

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