Chemistry 125: Lecture 62 March 28, 2011 NMR Spectroscopy Decoupling 13 C NMR and Double Labeling...

Chemistry 125: Lecture 62March 28, 2011

NMR SpectroscopyDecoupling

13C NMR and Double LabelingCorrelation and 2D NMR

Electrophilic Aromatic Substitution This

For copyright notice see final page of this

file

Proton Decoupling

13C H

100 MHz25 MHz

(in frame rotating at 100 MHz)

C13 NMR spectrum

while irradiating H

(100 MHz)

H average

H up H down

Decoupling Power determines the rate of this precession.

Pulse (25MHz) to observe C13

or

If it is fast enough, the 13C doublet

collapses.

40 db (inverse log measure of rf power)

CH2

CHCH3

C

CH2

CHCDCl3

Observe 13C while decoupling 1H at various powers.


CH2

CHCH3

C

CH2

CH

CDCl3


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

H1

F19

P31

C13

O17

1%

99.98%

6%

Proton-Decoupled 13C NMR Assignments forthe Artificial Sweetner Neotame Monohydrate

Prof. Eric Munson, Kansas Univ.

One peak per carbon, pretty

well spread out

Why no 13C-13C splitting?

Only 1-4% of 13Cs have an adjacent 13C

in the same molecule.

C=O CaromC-X

e-neg

Calkane

7

2-D NMR

Dilute 13C Double Labeling

Power of Correlation:

Double Labeling

Introduction:Lanosterol Biogenesis

Cf. Frames 6-13 of Lecture 52and e.g. J&F, Sec. 12.13 pp. 554-562

+

Squalene

H +

+

+

+

+

+

HOO

Squalene

+

+

+

+

+

HO

HH

H

CH3

HH

CH3

H+

CH3H3C

H3C CH3

CH3

CH3

CH3H3C

Lanosterol(source of cholesterol

& steroid hormones)

Squalene

+

+

+

+

+

HO

HH

H

CH3

HH

CH3

H+

CH3H3C

H3C CH3

CH3

CH3

CH3H3C

Lanosterol(source of cholesterol

& steroid hormones)

3°

3°

3°

3°3°

Cute StoryIs it True?

(“Wait for NMR”)

HO

HH

CH3H3C

H3C CH3

CH3

CH3

CH3H3C

Squalene

Lanosterol

13C Label

Single Label Enrichment

Enriched Peaks

(100x stronger than natural-abundance peaks)

HO

HH

CH3H3C

H3C CH3

CH3

CH3

CH3H3C

Squalene

Lanosterol

13C Label

Single Label Enrichment

Enriched Peaks

HO

HH

CH3H3C

H3C CH3

CH3

CH3

CH3H3C

Squalene

Lanosterol

13C Double Label

Proves that they entered as a unit.

The dilute double label experiment enhances the same 12 13C peaks as the single label experiments, but only 8 of them show spin-spin splitting (because their C-C bond stays intact). HO

HH

CH3H3C

H3C CH3

CH3

CH3

CH3H3C

13C Double Label

Squalene

DILUTE !

Double-Doublet (proton decoupled)13C-13C splitting (neighboring 13Cs)

Power of Correlation

both labeled, but notin the same molecule

Few single precursor molecules have any C13 label, but thosethat are labeled have two C13s.

These are both labeled, in the same molecule

Strongly confirms the rearrangement scheme.

Dilute 13C Double Labeling

Power of Correlation:

2-D NMR

1H to 1HCorrelation by NOE

(through-spacemagnetic interaction)

protonsin proteinpolymer

proximity(< 6Å) of

With Molecular Mechanics Constraints gives 3-D Structure (without crystal!)

NH at d7.25

is within 6Å of NHs at d8.9, 8.3, 8.25, 7.7

Narrow range; mostly HN-C=O protons

Identify NH withamino acid by

coupling throughCH to R

H

HO

NHR O

NHR

Less-congested, off-diagonal peaks appear when “tickling” one signal on the diagonal enhances another.

diago

nal s

hows n

ormal

spec

trum

heav

ily co

nges

ed by

overl

appin

g sign

als

1H vs. 1H Correlation

in TIME

0.3 sec 40°C

d(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 d.

(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

H3C CH3

H3C

CH3

CH3

H3C CH3

H3C

CH3

CH3

CH3

+

A B

C D

A D

or 1-Anywhere?

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

Electrophilic Aromatic

Substitution

Electrophilic Aromatic Substitution

H

HH

H

H

H

D2SO4

H

HH

H

D

H

e.g. J&F Sec. 14.4

H

HH

H

H

H

D

A/D via intermediate +

D2SO4

C6D6etc.

Observable!

Or other electrophiles in place of D+

e.g. NO2+, Br+, HOSO2 , R+, R-C=O R-C=O+HOSO2

+

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.

1H vs. 1Hcorrelation in time

0.3 sec 40°C

CA

DB

Remember

+

+

+

SHMo2 (Simple Hückel Molecular Orbital Program)

Benzene Pentadienyl

Cation addition converts ring to chain (destroys aromaticity).

H

HH

H

H

H

D+

H

HH

H

H

H

D

+

Locus ofodd electron in

radical,

+ charge(LUMO)in cation.

- charge (HOMO)in anion,

SOMO(nonbonding)

Cf.

e.g. J&F, Sec. 14.4b

+

LUMO

X

NO2

++

+ +

H

O2N

X

+++

+

H

O2N

X

+

+ +

H

Substituent Effects on Rate (e.g. sec. 14.9-14.10)

X

O2N

X

O2N

X X

NO2

+ +NO2

+

(from HONO2/H2SO4)

X Relative Rate (overall)

H [1] Cl 0.03 NO2 6 10-8

CH3 25 OH 1000

(CH3)3N+ 1 10-8

donation / withdrawal

p (or ) e-donation eases formation of cation intermediates

p (or ) e-withdrawal retards formation of cation intermediates

HO H+

Note: +NO2 is O=C=O with an extra proton in the center

productsintermediates

Cf. J&F, Table 14.2

4 py (or px) electrons

Why is -NO2 e-Withdrawing when -OH is e-Donating?

-NO2 6 10-8p e-withdrawal retards formation of cation intermediates

-OH 1000 p e-donation eases

N

O O

O

H

pO-1

0

High HOMO; Good Overlap with Phenyl

(able to donate, but not willing)

Low OMO;Good Overlapwith Phenyl

HOMO as high as pO;

OH is a Donor

NO2 is a Acceptor

but No Overlap with Phenyl

NO2 isallylic

(willing, but not able)

(willing and able to accept)

Low LUMO;Good Overlapwith Phenyl pC

C=C

Substituent Effects on Orientation (e.g. J&F, Sec. 14.9-14.10)

X

NO2

+

(from HONO2/H2SO4)

X- Relative Rate (per replaceable H)

H- [1] [1] [1] Cl- 0.0008 0.03 0.13

O2N- 9 10-80.6 10-8 0.03 10-8

H3C- 1 39 46 (CH3)3C- 4 6 72

DirectingActivating

Deactivating

Act

Deact

Deact

s p

(CH3)3N- 3 10-8 0.6 10-8 + _

ortho meta para

EtOC=O 0.006 0.0006

- 0.003

o/p

m

o/p

“e-donating”

“e-withdrawing”

(steric hindrance)?

X

NO2

++

+ +

H

O2N

X

+++

+

H

O2N

X

+

+ +

H intermediates

Activate the ElectrophileHalogenation (e.g. J&F, Sec. 14.4f)

“Cl+”

H

d+

d-

+29 +38

AlCl3 “Lewis acid” catalyst

AlCl5Cl2 vs.

LUMOsSurface Potentials

(Al, like B, has an empty valence AO)

AlCl4 leaving group

Cf. [Br2]2

for rate of addition to

alkene

Activate the Nucleophile

H

OH

C

O

O

OH

H

NH2

100 atm125°C

Self-Igniting Rocket Fuel(1944)

N

O

O

+

Salicylic AcidKolbe (1860)

OH

C

O

OH

H+

Aspirin

loweredHOMO

raisedHOMO

C

O

H3C

Acetylation of anilinemakes its nitration

controllable.

Taming Aniline Nitration

ortho19%

meta2%

para79%

HNO3

H2SO4

(pyridine)

OH

(e.g. J&F, Sec. 14.4f)

Charles Friedel(1832-1890)

James Mason Crafts(1839-1917)

AlCl3/C-Electrophile: The Friedel-Crafts Reaction

Par

is (W

urtz

- M

édic

ine)

Cor

nell

MIT

MITP

aris

(Fri

edel

- M

ines

)

Pres

iden

t

AlCl3/C-Electrophile: The Friedel-Crafts Reaction

1877

End of Lecture 62March 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). 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

http://creativecommons.org/licenses/by-nc-sa/3.0/us

Chemistry 125: Lecture 62 March 28, 2011 NMR Spectroscopy Decoupling 13 C NMR and Double Labeling...

Documents

Transcript of Chemistry 125: Lecture 62 March 28, 2011 NMR Spectroscopy Decoupling 13 C NMR and Double Labeling...