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DIELS- ALDER REACTION. The Diels-Alder Reaction. concerted. ring-forming reaction. three pairs of electrons move at one time. diene + dienophile cyclohexene. = gain of bond order. NOTE: . = loss of bond order. THE DIENE MUST BE ABLE TO ADOPT

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Presentation Transcript
slide2

The Diels-Alder Reaction

concerted

ring-forming reaction

three pairs of electrons

move at one time

diene + dienophile cyclohexene

= gain of bond order

NOTE:

= loss of bond order

slide3

THE DIENE MUST BE ABLE TO ADOPT

THE S-CIS CONFORMATION

rotate

+

reacts

s-trans

conformation

s-cis

conformation

does not

react

reacts

normally

slide4

push

R

pull

W

DIELS-ALDER REACTION

HOMO

(diene)

The reaction is

concerted - all

of the orbitals

are aligned in a

6-ring.

2

The HOMO

of the diene

donates elec-

trons into the

LUMO of the

dienophile.

1

LUMO

(dienophile)

slide5

BUTADIENE

ETHYLENE

p4

The HOMO of the diene

donates electrons into

the LUMO of the

dienophile.

p3

LUMO

HOMO

p2

p*

p1

p

slide6

EXAMPLE - WITH ELECTRONIC FACTORS

pull

push

D

Diene

Dienophile

A Cyclohexene

Works best if the dienophile

has electron-withdrawing

groups, and the diene has

electron-donating groups.

The HOMO of the diene

donates PUSHES electrons

into the LUMO of the dieno-

phile PULLS.

slide8

THE REACTION IS USUALLY STEREOSELECTIVE

endo

exo

The endo product

is usually preferred.

It has been suggested

that the pi systems

like to establish

maximum overlap

during the reaction.

slide10

OR

Retrosynthesis

slide11

ACETYLENES (ALKYNES)

If the dienophile is an alkyne a cyclohexadiene is formed.

slide13

a-ELIMINATIONS

REVIEW

( also covered in Section 9.13 )

Alkyl halides which have no b-hydrogens will

undergo a-elimination to give carbenes.

b-hydrogen

no b-hydrogens

in these groups

a

a

b

a-hydrogen

E1 or E2

carbene

-

alkene

slide14

MECHANISM

Some halides without

a-hydrogen atoms

Step 1

fast

-

Step 2

-

..

slow

+

..

-

carbene

is a strong base such as NaOH, NaOEt,

KOH, NaNH2, or nBuLi

slide15

:

CARBENES ARE ELECTRON-DEFICIENT

Carbenes are neutral, no charge, but have an incomplete octet.

They react with alkenes (electron donors) to form cyclopropanes.

carbene

R

R

syn

addition

cis

cis

R

R

The reaction is thought to

be concerted.

a cyclopropane

STEPWISE ANALYSIS OF THE CONCERTED REACTION

:

..

-

+

slide17

SYNTHESIS OF NORCARANE

phenyl sodium

-

+

:

+

+

-

strong

base

:

:

“ylide”

-

-

a -elimination

tetramethylammonium

chloride

+

+

:

+

carbene

norcarane

(bicyclo[ 4.1.0 ] heptane

slide18

two

methods

various solvents

are used

explosive if not

handled carefully

DIAZOMETHANE

(gas)

UNSTABLE

easily decomposed

by light or heat

-

..

..

-

+

+

:

:

light heat

photolysis:

:pyrolysis

=

hnD

:

:

:

nitrogen is a very

thermodynamically

stable molecule,

and forms readily

nitrogen

methylene

( the specific name for this carbene )

slide19

DIAZOMETHANE REACTIONS

pyrolysis

D

+

trans

trans

D

+

cis

cis

photolysis

hn

slide21

SIMMONS-SMITH REACTION

“a carbenoid reaction”

Zn-Cu

+

Zn-Cu is a “zinc-copper couple” which is more reactive

than either would be alone. Copper activates the zinc.

R

R

+

cis

R

R

cis

concerted

stereospecific

carbenoid reactions give a result as

if a carbene is involved, but it is not.

slide22

WHY USE THE SIMMONS-SMITH ?

1) not hazardous like diazomethane

2) milder reaction conditions

3) doesn’t require strong base like a-elimination

4) fewer side products

5) higher yields

….. what else could you want ?