Chem 125 Lecture 2911/12/08

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may not be copied or distributed further.

It is not readily understood without reference to notes or the wiki from the lecture.

Pasteur's"Bargain-Basement"Moldy Racemic Acidfrom Thann Pharmacy

(probably apocryphal anecdote via L. F. Fieser ~1960)

Penicillium glaucum had "eaten" (R,R)

Purified l-(S,S)-Tartaric Acid ???

(Remember the smell of carvones!)

React with One Enantiomer

Diastereomeric ReactionsHave Different Rates

React Racemate withResolved Chiral Reagentor Catalyst (e.g. Enzyme)

Nature's WayPrepare only one Enantiomer

Use resolved starting material

Or resolved reagent/catalyst

Eisai - 7389Purchase 5 stereocenters.Make the rest.1 by chiral Simulated Bed Chromatography.2 by allyl silane additions.2 by asymmetric dihydroxylations.2 by oxy-michael reactions.3 by asymmetric Ni/Cr reactions.1 by Jacobsen epoxidation.1 by conjugate reduction.1 by enolate alkylation1 by ketal formation

Best regards,

Frank

Only One Chiral Separation

Specific or general selective reactions that preferentially

form one isomer.

In starting materials

19 Stereocenters

d-(+) l-(-)

How does Optical Activity work?

COOH

COOH

H

OH

HO

H

COOH

COOH

OH

H

H

HO

COOH

COOH

H

OH

HO

H

COOH

COOH

OH

H

H

HO ?

Molecular Chirality and Optical Rotation

Laurence Barron

Chirality pervades much of modern science: from the physics of elementary particles, through organic stereochemistry, to the structure and behaviour of the molecules of life, with much else besides (nonlinear optics; nanotechnology; materials, electrical engineering; pharmaceuticals; astrobiology; origin of life; etc.).

Chirality Means Right- or Left-Handedness

‘Rassi-maid’ by Hans Erni

Lord Kelvin (1824-1907)

Originally William Thomson. First introduced the word chirality into science. Best known for inventing the absolute (Kelvin) temperature scale.

River Kelvin

I call any geometrical figure or group of points chiral, and say that it has chirality if its image in a plane mirror, ideally realized, cannot be brought into coincidence with itself. (Lord Kelvin, Baltimore Lectures, 1904)

Lord Kelvin’s Definition of Chirality

Mirror-image chiral molecules (enantiomers) show optical rotation of equal magnitude but opposite sign.(Louis Pasteur, 1848)

Natural Optical Rotation

Isotropic collections of chiral molecules (e.g. a sugar solution) show natural optical activity phenomena such as optical rotation.

Mirror-image enantiomers generate equal and opposite optical rotation angles.

Not to be confused with magnetic optical rotation (the Faraday effect).

Magnetic Optical Rotation

A static magnetic field B parallel to the incident light beam induces magnetic optical rotation in collections of achiral molecules (the Faraday effect, 1846). Reversing the magnetic field direction reverses the sense of optical rotation.

Michael Faraday (1791-1867)

B

Symmetry and Optical Rotation

“The magnetic rotation has neither right-handed nor left-handed quality (that is to say, no chirality). This was perfectly understood by Faraday and made clear in his writings, yet even to the present day we frequently find the chiral rotation and the magnetic rotation of the plane of polarised light classed together in a manner against which Faraday’s original description of his discovery contains ample warning.” Lord Kelvin (Baltimore Lectures, 1904)

Chiral phenomena such as natural optical rotation are characterized by time-even pseudoscalar observables. The quantum states of the system must have mixed parity and definite reversality like (L) and (R).

Magnetic optical rotation is not a chiral phenomenon (the observable is a time-odd axial vector). The quantum states must have definite parity and mixed reversality like (J,M) and (J,-M).

Chirality and Circularly Polarized Light

In order to detect molecular chirality, some sort of chiral probe must be used.

Right- and left-circularly polarized light beams are mirror-image chiral systems and so can act as chiral probes:

Chiral molecules respond slightly differently to right- and left-circularly polarized light. A difference in absorption corresponds to circular dichroism; a difference in refractive index leads to optical rotation.

The instantaneous electric field vectors of right- and left-circularly polarized light beams propagating along z.

right

left

z

z

Changing Time

at Fixed Position

counter-clockwise

Fixed Time

clockwise

Circular Differential Refraction

Linearly polarized light can be described as a coherent superposition of right- and left-circularly polarized waves of equal amplitude.

A difference in refractive index for the right- and left-circularly polarized beams means a difference in velocity. So the phase relation between the two contrarotating electric vectors will change, resulting in a rotation of the plane of polarization.

Go to Google Images for animations (Google ‘circularly polarized light’ and open the www.enzim.hu site).

From P.W. Atkins, Physical Chemistry (OUP)

)( RL nnl

−=λπθΔ

θΔ

A Scattering Picture of Optical Rotation

A circularly polarized light wave ‘bouncing’ from one group to the other as it scatters from a simple two-group chiral molecular structure will sample the chirality. The scattered intensity of right- and left-circularly polarized waves will be slightly different for a given handedness of the chiral structure.

The Rotational Strength

The optical rotation angle is given by

Using quantum-mechanical perturbation theory this becomes

The molecular quantity responsible for optical rotation (and circular dichroism) is the rotational strength:

and m are electric and magnetic dipole moment operators, respectively, so the optical activity ultimately originates in interference between electric and magnetic dipole transitions during the light scattering process!€

R j ← n( ) = Im n μ j • j m n( ) .

)( RL nnl

−=λπθΔ

€

Δθ =2μolN

3h

ω2

ω jn2 −ω2

Im n μ j • j m n( )j≠n

∑ πc/λ

The Carbonyl Chromophore

The carbonyl chromophore is an important source of optical activity in many organic compounds. The carbonyl group itself has a plane of symmetry so is not chiral/optically active. Optical activity is induced in its electronic transitions via perturbations from the chiral environment provided by the rest of the molecule.

n←∗π transition at ~ 290 nm

linear displacement of charge(electric)

helical motionof charge

(both)

n

π*

n

π* n transition is magnetic dipole-allowed, electric dipole-forbidden. Electric dipole character is induced by mixing of the oxygen dYZ orbital into

the π orbital:

€

R π ∗+ λ dYZ( ) ← n( ) = Im n μZ π ∗+ λ dYZ( ) π ∗+ λ dYZ( ) mZ n( )

This generates a ZmZ component (from .m) of the rotational strength:

C O

C O

C O

C O

C OC O

rotation of charge(magnetic)

π* n dXZ n π + dXZ) n

x

z

y

Quantum-Chemical Calculations

€

Δθ =2μolN

3h

ω2

ω jn2 −ω2

Im n μ j • j m n( )j≠n

∑

The optical rotation can be calculated using ab initio quantum-chemical programs (Gaussian, Dalton). Often sufficiently good to determine absolute configuration from the sign and magnitude.

(S)-()-thalidomide (R)-)-thalidomide(teratogenic) (sedative)

Tetelman Lecture

From Cosmic Chiralityto Life on Mars:

Lord Kelvin’s LegacyDavies Auditorium

Becton Center5:00 PM Today

Who Cares?Living Things

Food & Drug Administration

Drug Companiestheir Lawyers & USPTO (Patent Office)

"Chiral Switch"

Pain Reliever

COOH

Isobutyl

PropionicAcidPhenyl

Ibuprofen (Advil, Motrin)

(S) Active Pain Reliever

(R) Inactive

Sold as racemate

Sedative

Thalidomide

(S) Sedative

(R) Teratogen

Sold as racemate (1957-62)

N

N

OOO

O

H

(S) (R) 0.12 / hr (R) (S) 0.17 / hr

(S) eliminated 0.24 / hr (R) eliminated 0.08 / hr

in vivo racemization (human)

10,000 birth defects

?

O••

5-Methoxy-2-{[(4-methoxy-3,5-dimethyl-2-pyridinyl)methyl]sulfinyl}1H-benzimidazole

Gastric Proton Pump Inhibitor (Acid Reflux)

World's largest selling drug in 2000 ($6.2B)

"Omeprazole""Prilosec"

omeprazole1

2

34

5

6 12

34

5

67

8

9

BenzimidazolePyridine

OTC ?

O••

5-Methoxy-2-{[(4-methoxy-3,5-dimethyl-2-pyridinyl)methyl]sulfinyl}1H-benzimidazole

"Nexium"

O•• ••

Gastric Proton Pump Inhibitor (Acid Reflux)

World's largest selling drug in 2000 ($6.2B)

"Omeprazole""Prilosec"

"esomeprazole"

(racemate)

(S)

N

N

O

O

O

S T E R E OB I N O C U L A R S T E R E O

N

N

O

O

O

Stereoviewing

from X-rayof Ohishi et al. 1989 fromX-ray of Ohishi et al. 1989

(S)-Omeprazole - Stereopair View

left eye right eye

(S)-Omeprazole - Stereopair View

from X-ray ofOhishi et al. 1989

right-eye view left-eye view

Central frame perceived in stereo

How it should look when you

stare dreamily into the distance

“through” the stereo-pair above

until the blue lines “swim” together and superimpose.

End of Lecture 29Nov. 12, 2008