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Diastereoselectivity in Organic Chemistry: Felkin-Ahn Model and Carbonyl Conformations, Exercises of Organic Chemistry

London Business School (LBS)Organic Chemistry

The concepts of diastereoselectivity, the Felkin-Ahn model for carbonyl conformations, and the effect of electronegative atoms on carbonyl conformation. It also covers carbonyl chelation and stereoselectivity, the aldol reaction's chair-like transition state, and selective production of cis and trans enolates of ketones. essential for students studying organic chemistry, particularly those focusing on stereochemistry and reaction mechanisms.

What you will learn

How does the effect of electronegative atoms on carbonyl conformation impact diastereoselectivity?
How does the aldol reaction form syn and anti isomers?
What factors influence the selective production of cis and trans enolates of ketones?

What is carbonyl chelation and how does it affect stereoselectivity?
What is the Felkin-Ahn model for carbonyl conformations?

Typology: Exercises

2021/2022

Uploaded on 09/27/2022

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Chapter 34 — Diastereoselectivity

-  The Felkin-Ahn model for carbonyl conformations and

diastereoselective nucleophilic attack

-  The effect of electronegative atoms on carbonyl conformation

-  Carbonyl chelation and stereoselectivity

---------------------------------------------------------------------

-  The aldol reaction’s chair-like transition state and

stereoselective formation of syn and anti isomers

-  Selective production of cis and trans enolates of ketones

-  Stereospecificity vs. stereoselectivity (and a pre-midterm

review of reactions)

Partial preview of the text

Download Diastereoselectivity in Organic Chemistry: Felkin-Ahn Model and Carbonyl Conformations and more Exercises Organic Chemistry in PDF only on Docsity!

Chapter 34 — Diastereoselectivity

The Felkin-Ahn model for carbonyl conformations and

diastereoselective nucleophilic attack

The effect of electronegative atoms on carbonyl conformation
Carbonyl chelation and stereoselectivity

The aldol reaction’s chair-like transition state and

stereoselective formation of syn and anti isomers

Selective production of cis and trans enolates of ketones
Stereospecificity vs. stereoselectivity (and a pre-midterm

review of reactions)

The conformations of acyclic carbonyls

The Felkin-Ahn model for carbonyl conformations

Ph•••O

eclipsed

H•••O

eclipsed

nothing eclipsed,

largest substituent

perpendicular

Alpha-substituted carbonyls assume conformations that:

1) avoid all eclipsed interactions

2) have the largest substituent perpendicular to the plane of

the carbonyl

Ph

H

O

Me

O

H

Ph

Me H

H

O

Ph Me

O

H

Ph Me

O

H

Ph

Me

H

O

Ph

Me

H

O

H

Ph

H

Me

H

O

Ph

H

Me

O

R

L

M

S

L

R

O

M S

O

R

L

S

M

Nucleophilic attack on a Felkin-Ahn conformation 2.

O

H

Ph

Me

H

Et

H

Ph

Me

H

Et

OH

Ph

Et

Me

OH

Ph

H

O

Me

EtMgCl

Ph

Et

Me

OH

Major (Minor)

Another example

Unstable

conformer

Stable

conformers

Product

Redrawing the product Newman projection into a Newman projection

with the main substituents (Me, tBu in this case) opposite to each other

often makes it easier to translate back into a normal zig-zag structure

Me

O

Et

NaBH 4

O

Me H

tBu

Et

O

Me

tBu

Et

H

O

Me

tBu

H

Et

H

Me

tBu

Et

H

OH

H

OH

tBu

Et

H

Me

Et

OH

Electronegative α -substituents occupy the

perpendicular position: example

Homework: Check which product would have formed if you put

“R” in the perpendicular position instead of NBn

2

O

H

NBn 2

O

H

NBn 2

R

H

O

H NBn 2

R

H

R

O

H

Bn 2

N

H

R

OMe

OLi

Nu

OH

H

NBn 2

R

H

Nu

OH

H

R

H

Bn 2

N

Nu

O

OH OMe

NBn 2

Nu

R

OH

NBn 2

Nu =

Chelation-controlled carbonyl conformations

Alpha substituents with lone pairs can coordinate divalent (or higher valency) metal

ions together with the carbonyl lone pairs.

The chelation ring becomes the dominant factor in determining the conformation,

and gives VERY high selectivity for nucleophilic attack.

Common chelating metals:

Zn

2+

, Cu

2+

, Ti

4+

, Ce

3+

, Mg

2+

(MgCl

is not as good)

Non-chelating metals:

Li

, Na

, K

O

R'

OR

O

R'

NR 2

O

R'

SR

O

R'

L

M

2 +

O

R'

Et

L

H

M

2 +

M

2 +

or or

Chelation control can reverse selectivity: example

Ph

O

OMe

Ph

OMe

H

OH

Ph

OMe

HO

NaBH H

4

Ph

O

OMe

Ph

OMe

Me

OH

Ph

OMe

HO

Me

2

Mg

Work these problems to make sure you can predict the right products

Attack on α -substituted carbonyls: summary

Your choices for predicting the reactive conformation:

1. Normal Felkin-Ahn model (No α-heteroatoms)

2. Electronegative heteroatom perpendicular

3. Electonegative heteroatom chelated and in the plane of the carbonyl

(p. 895, CGWW)

Explaining cis -enolate— syn -aldol product

selectivity using a cyclic chair-like T.S.

O

R

Me

H

Li

O

H

Ph

O

H

Ph

R

Me

H

Li

R

O

Li

Me

O

H Ph

R

O

Me

Ph

OH

R

O

Li

O

H Ph

Explaining trans -enolate— anti -aldol product

selectivity using a chair-like T.S.

O

R

Me

Li

O

H

Ph

H

O

H

Ph

R

H

Me

Li

R

O

Li

Me

O

H Ph

R

O

Me

Ph

OH

Diastereoselectivity in Organic Chemistry: Felkin-Ahn Model and Carbonyl Conformations, Exercises of Organic Chemistry

Related documents

Partial preview of the text

Download Diastereoselectivity in Organic Chemistry: Felkin-Ahn Model and Carbonyl Conformations and more Exercises Organic Chemistry in PDF only on Docsity!

Chapter 34 — Diastereoselectivity

diastereoselective nucleophilic attack

stereoselective formation of syn and anti isomers

review of reactions)

The conformations of acyclic carbonyls

The Felkin-Ahn model for carbonyl conformations

Ph•••O

eclipsed

H•••O

eclipsed

nothing eclipsed,

largest substituent

perpendicular

Alpha-substituted carbonyls assume conformations that:

1) avoid all eclipsed interactions

2) have the largest substituent perpendicular to the plane of

the carbonyl

Nucleophilic attack on a Felkin-Ahn conformation 2.

Major (Minor)

Another example

Unstable

conformer

Stable

conformers

Product

Redrawing the product Newman projection into a Newman projection

with the main substituents (Me, tBu in this case) opposite to each other

often makes it easier to translate back into a normal zig-zag structure

O

O

O

H

O

H

H

H

OH

H

OH

H

H

OH

Electronegative α -substituents occupy the

perpendicular position: example

Homework: Check which product would have formed if you put

“R” in the perpendicular position instead of NBn

Chelation-controlled carbonyl conformations

Alpha substituents with lone pairs can coordinate divalent (or higher valency) metal

ions together with the carbonyl lone pairs.

The chelation ring becomes the dominant factor in determining the conformation,

and gives VERY high selectivity for nucleophilic attack.

Common chelating metals:

Zn

, Cu

, Ti

, Ce

, Mg

(MgCl

is not as good)

Non-chelating metals:

Li

, Na

, K

Chelation control can reverse selectivity: example

Ph

O

OMe

Ph

OMe

H

OH

Ph

OMe

HO

NaBH H

Ph