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
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, alkenes, -ynes, with mechs.doc
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
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, alkenes, -ynes, with mechs.doc
Alkenes, Alkynes & Variations
Beauchamp
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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
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, alkenes, -ynes, with mechs.doc
Alkenes, Alkynes & Variations
Beauchamp
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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
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, alkenes, -ynes, with mechs.doc
Alkenes, Alkynes & Variations
Beauchamp
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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
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, alkenes, -ynes, with mechs.doc
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
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, alkenes, -ynes, with mechs.doc
Alkenes, Alkynes & Variations
Beauchamp
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
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, alkenes, -ynes, with mechs.doc
Alkenes, Alkynes & Variations
Beauchamp
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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
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, alkenes, -ynes, with mechs.doc
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