Transcript Alkene Preparation Reactions Alkyne reductions

Alkene Preparation Reactions
 Alkyne
reductions
 Syn
orientation
 Anti orientation
 Elimination
 Alcohol
reactions
dehydration
 Alkyl halide dehydrohalogenation
 Vicinal dihalide dehalogenation
Alkenes via Alkyne Reduction
 Syn
reduction of Alkynes to Alkenes
 Catalytic
Hydrogenation using a poisoned
catalyst
 Anti
reduction of Alkynes to Alkenes
 Dissolving
Metal reduction
Alkenes via Alkyne Reduction
Syn addition
H3 C C C CH3
C CH3
H3 C C
CH3
H3 C
C C
H H
catalyst surface
H
H
H H
catalyst surface
(Z)/cis diastereomer
Anti addition
H3 C
H3 C C C CH3
H3 C
Na +
C C
CH3
a radical-anion
H
H3 C
C C
CH3
a vinyl radical
Na
Na
a vinyl anion
+ NaNH 2
CH3
a vinyl radical
H
CH3
+
H
C C
NH 3 (liq)
C C
Na
CH3
a radical-anion
H3 C
H3 C
Na
C C
+
H
H3 C
NH 3 (liq)
C C
+ NaNH 2
H
CH3
(E)/trans diastereomer
Alcohol Elimination
 E-1
 Unimolecular
kinetics
 Carbocation intermediates
 Skeletal rearrangements possible
Alkenes via Alcohol Dehydration
CH3
R C CH CH3
CH3
+
H
R C CH CH3
H OH2 +
H OH
a secondary alcohol
CH3
+
H
R C CH CH3
H
+
1,2hydride
shift
CH3
rds
H2 O
CH3
R C CH CH2 +
R C CH CH3
+
H
a secondary carbocation
H
CH3 H3 C
C C
C C
+
H
R
CH3
R
H3 C
H
CH3
R C CH2 CH3
+
CH2
a tertiary carbocation
C
R
CH2 CH3
Dehydrohalogenation of Alkyl Halides
Unimolecular versus Bimolecular elimination
CH3
E-2
R C CH CH3
E-1
X
H X
-
CH3
R C CH CH3
+
HX
H
HX
CH3
+
H3 C
R C CH CH2 +
H
a secondary carbocation
R
H
+
H
1,2-hydride
shift
R C CH2 CH3
H
+
CH2
+
C
+
a tertiary carbocation
CH3 H3 C
C C
H
CH3
HX
R
CH2 CH3
+
H
H
C C
R
CH3
Bimolecular Dehydrohalogenation of Alkyl Halides
Periplanar anti-elimination
base
H
C
C
C(CH3 )3
R
R
HX
X
R
C(CH3 )3
C C
(CH3 )3 C
R
(CH3 )3 C
trans
Erythro
base
R
H
C
C
X
R
C(CH3 )3
R
HX
R
C C
C(CH3 )3
(CH3 )3 C
(CH3 )3 C
Threo
cis
Dehalogenation of Vicinal
Dihalides
 E-2
 Bimolecular
kinetics
 Periplanar Anti-orientation
 No skeletal rearrangements possible
E-2 Dehalogenation of Vicinal Dihalides
Periplanar anti-elimination
Zn
X
C
C
C(CH3 )3
R
ZnX 2
X
R
R
C(CH3 )3
C C
(CH3 )3 C
R
(CH3 )3 C
trans
Meso
Zn
R
X
C
C
X
R
C(CH3 )3
R
ZnX 2
R
C C
C(CH3 )3
(CH3 )3 C
(CH3 )3 C
"No-name"
cis
Alkyne Preparation Reactions
 Elimination
 Vicinal
tetrahalide dehalogenation
 Vicinal dihalide dehydrohalogenation
 Substitution
 Alkynyl
anion synthesis
Vicinal Tetrahalide Dehalogenation
X X
C C
X X
a vicinal
tetrahalide
Zn
alcohol

C C
+
2 ZnX 2
Vicinal Dihalide Dehydrohalogenation
H H
strong
R C C R
base
( HX )
X X
H
R
C C
X
R
a vinyl halide
strong
base
( HX )
R C C R
Substitution
R
C CH
Na
R
+
+ H2
C C Na
Term inal
Alkyne
R
+
C C Na + R'CH 2 X
R
C C CH2 R' + NaX
Methyl
or
Primary
Alkyl
Halide
Alkynyl Anion Synthesis of Alkynes via
Bimolecular Nucleophilic Substitution