Organic Reactions Summary Alkenes, alkynes and variations ...

Alkenes, Alkynes & Variations

Beauchamp

1

Organic Reactions Summary Alkenes, alkynes and variations

For Use as a Study Guide

Beauchamp

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Alkenes, Alkynes & Variations

Beauchamp

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Making alkenes and alkynes

a. mechanism using potassium t-butoxide, KOC(CH3)3, SN2 at and E2 at 1o, 2o and 3o RBr,

H3C H3C

O C

CH3

H3C H3C

O C

CH3

H3C H3C

O C

CH3

H

C Br H

H

D

H Ha

C Br R

C CH3

Hb

H

C Br

HC H2

Ha

C CH3

Hb

SN2 E2

E2

H3C

H

OC

Br

H

H

H CD

H CD

Hb

C

Z

H3C C

E

Br

CH3

Ha

E2 (-CH2-H) Br

E2 (-CHa-H)

E2 (-CHb-H)

Example reactions

Br

KOC(CH3)3

E2 > SN2 anti

elimination

Br

KOC(CH3)3

E2

R

Br R Br

S Br

KOC(CH3)3 KOC(CH3)3

KOC(CH3)3

E2

S

H3C

CH3

C

O

CH3

E2

SN2 no other option

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Alkenes, Alkynes & Variations

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b. Double elimination from dibromoalkanes to form alkynes and terminal acetylides used in many additional reactions (SN2 with RBr, C=O addition to aldehyses and ketones, and reaction with epoxides)

Br Br 2 eqs. Br2

h H

b

Na

N

R

R

H

O

c

CH R

Br H Na

H

N

R

H

a

H2 C

b

R

H2O H

O

CH

R

H

R

H a

O

H

H

2. workup

2.

H2 C

b

X

R

SN2

H 2.

c

C

O

R

N

R

R

3rd equivalent

most stable anion in mixture

Na

Na

Na

b HO

d

H2O H

O

2.

d

O

Na

The zipper reaction moves a triple bond in an unbranched linear chain to the end and allows all of the above

reactions.

H2 C

C

H

C

R

N

Na

R

R

CH2 C C R

CH2 C

C

R H

R

R

N

Na

H

C

R

C C

HH

R

R

N

H

H

C

C

R

C

C

R

C

C

H

H

N

H

R

R

N

R

R

H

C

R

C

H

C

H

Na N

R

R

C

R

C C

HH

H

2. workup H

O

H

H

H

C

R

C C

H2

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Alkenes, Alkynes & Variations

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c. mechanism using NaCC-R to make a bigger alkyne, SN2 at methyl, 1o and 2o RBr and only E2 at 3o RBr,

C C R

H

H

SN2

C Br H

RCCC

Br

H

H

H

C C R

D

H H3C

C Br R

CH2

C C R

H

C Br

HC

H2

Ha

C CH3

E2 > SN2

Hb

SN2 R

E2 (-CH2-H)

D

CCC

Br

H H2C

CH3

E2 (-CHa-H)

H3C O

H

C

Br

E2 (-CHb-H)

H

H2C

SN2

CH3

Example reactions

Br Br R

NaCC-R NaCC-R

SN2 inversion of R configuration

E2 > SN2

Br R Br

S Br

NaCC-R NaCC-R NaCC-R

E2

E2 > SN2 S

C

achiral

C

R SN2

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Alkenes, Alkynes & Variations

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d. mechanism using triphenylphosphine to make triphenylphosphonium salt, SN2 at methyl, 1o and 2o RBr and

only E2 at 3o RBr, used to make a triphenylphosphonium ylid to make Z and E alkenes with aldehydes and ketones.

Ph Ph P

Ph

Br

H C

Br

SN2

Ph

H

Ph P C

H CH3

Ph

CH3 H

CH2 Li

1. n-BuLi 2. H2C=O

Ph = phenyl

ylid salt

Ph Ph P O

Ph

H

H

C C

CH3

CH3 Z alkenes

Ph

O

Ph P

CH

Ph

C

CH3

H CH3 oxaphosphatane

Ph

O

Ph P

CH3 C

Ph

C

H

H

CH3

betaine

Ph

O

Ph P

CH

Ph

C

CH3

H CH3

Ph Ph P

H C

Br

SN2

Ph

H3C CH2

Ph = phenyl H3C

Br

Ph

H

Ph P C

Ph

CH2 CH3

ylid salt CH3

H

Ph

Ph

Ph

P

C O H3C C

CH3

Ph

Ph

CH2 Z alkenes

H3C

CH2 Li

1. n-BuLi 2. H2C=O

Ph

O

P

CH

C

CH3

H3C CH2 H3C oxaphosphatane

Ph H3C

O

Ph P

CH2

C

Ph

C

H

CH3

CH3

betaine

Ph

O

Ph P

CH

Ph

C

CH3

H3C CH2 H3C

Schlosser Modification of the Wittig reaction to make E alkenes

H

Ph3P O C

C HR

R

H

Ph3P O C

C HR

acid/base at -78oC

R betaine

H2C Li

extra equivalent of n-BuLi

H OH2 H

Ph3P O C

C

R

neutralize

O Ph3P

C

R

flat sp2 carbon can react from either side

The stereochemistry of the alkene is

H

determined in this step.

H C RR

O Ph3P

phosphine oxide

H

CR R C

H E alkene

H

O Ph3P

C

C RR

H

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Alkenes, Alkynes & Variations

Example reactions

P

Ph

Ph

Ph

P

Ph

Ph

Ph

P

Ph

Ph

Ph

P

Ph

Ph

Ph

P

Ph

Ph

Ph

P

Ph

Ph

Ph

Beauchamp

1.

Br

2. n-BuLi

3. CH3CH=O

1. Br R

2. n-BuLi 3. CH3CH=O

1.

Br 2. n-BuLi 3. CH3CH=O

1. R Br

S 2. n-BuLi 3. CH3CH=O

1. Br

2. n-BuLi 3. CH3CH=O

1.

Br

2. n-BuLi

3. CH3CH=O 4. -78oC, n-BuLi

5. HCl, warm

6

Normal Wittig Z alkene preferred

Normal Wittig Z alkene preferred

E2

Normal Wittig

S

Z alkene preferred

Normal Wittig Z alkene preferred

Schlosser modification of the Wittig

E alkenes preferred

e. Ohira-Bestmann modification of the Seyferth-Gilbert reaction (makes terminal alkynes from aldehydes and a special `Wittig' reagent). Overall reaction from aldehyde to the terminal alkyne ? simplified Ohira-Bestmann reaction

O

R

H

O

O

P OMe

OMe N

given

N

O R

K

H

R

terminal alkynes

N N

O

R O

O K

P

O

OMe

OMe

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Alkenes, Alkynes & Variations

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Possible mechanism ? with mechanism details

O R

O

O

P OMe

OMe N

N given

RO

OO

P OMe

OMe N

N

H C C R

N N

R

H

C

R

H

C

C

C

N resonance N

N

N

rearrangement = N2 leaves, and H migrates across

O

OP OMe

OMe

7

O R

O

O

O

P OMe

OMe N

N

R

H

O

O

R H

P OMe

OMe N

N

O

O

R H

P OMe

OMe N

N

Example reactions

O

O

P OMe

OMe N2

O

O

P OMe

OMe N2

O

O

P OMe

OMe N2

1. RO Na 2.

O H

1. RO Na 2. O

H

1. RO Na O

2. H

difficult to make alkyne at branch point

difficult to make alkyne at branch point

difficult to make alkyne at branch point

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Alkenes, Alkynes & Variations

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f. ROH with sulfuric acid / heat. Synthesis of alkenes (our only useful E1 reaction. Rearrangement is possible).

H H O SO3H

HO

H HOH

O SO3H

O H

(heat)

H O SO3H

secondary alcohol bp = +161oC

pKa = -10

H

O H slow step

water is a good leaving group

H E1

H

O SO3H

H

bp = +83oC

alcohol

alkene

Tbp = 78oC

distills out

H

H O

(heat)

H O SO3H

alcohol

pKa = -5

Example reactions

OH

H

H

H

O

CH3

H3C

C

H

HO H3C

H CH3

C

H

C

C

C

C

H rearrangement slow step CH3 H

H CH3 H

2o carbocation

3o carbocation

HO3S O

H3C

H

HOH

H3C

C

C

CH3

alkene

(E1 mechanism) CH3

H2SO4 /

probably E2

OH

H2SO4 /

E1

H2SO4 /

E1

OH

OH

H2SO4 /

E1

OH H2SO4 /

E1 rearrangement

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