Alkyne Reactions
I.
Reduction of Alkynes
A.
Relative Reactivity of 2 p-bonds
Alkynes react like alkenes, but twice
a) Hydrogenation of alkynes goes to alkanes
b) Electrophilic Attack
R
R R
R
A
R C C
R
AB
A
B
A B
or
R R
or
A
B
R
A
R
c)
B
B
B A
Most of the Alkene reaction we learned will work for alkynes x 2
B.
cis-Alkenes from Alkynes
1) Alkynes can be Hydrogenated under alkene conditions
a) Pd/C or PtO2, 1 atm H2
Pd/C
R C C R
b) Gives saturated alkane product
H
R R
H
2
2)
H
H H
Lindlar Catalyst gives single H2 addition
a) Lindlar Catalyst = 5% Pd-CaCO3, Pb(OAc)2, quinoline =
b) Less active catalyst surface, so only one p-bond is added to
c) H2 addition is syn (like for alkenes) and gives cis-alkene product
C C
H2, Lindlar catalyst
o
H
H
C C
25 C
C.
trans-Alkenes
1) Na is a strong reducing agent: Na
Na+ + e2) Na dissolves in NH3(l) to give Na+ + e- (solvated electron)
3) Alkynes exposed to Na/NH3(l) are selectively reduced to trans-alkenes
H
R
1. Na, NH3(l)
R C C R
2. H2O
R
H
N
4)
R C
C
R
Na
Mechanism: (see book for orbital picture)
-
R C
C
R
H NH2
Alkyne Radical Anion
R
H
C
C-
R
H NH2
H
II.
C
C
R
R
trans-Alkenyl Radical
R
C
R
H
C
H
Electrophilic Addition to Alkynes
A.
Alkynes are much like alkenes
1) p-bonds readily attacked by electrophiles
2) Terminal alkynes follow Markovnikov rule
a) Electrophile ends up on less substituted Carbon
b) Nucleophile ends up on more substituted Carbon
N
H
E N
R C
C H
R
E
Na
R
H
C
C
R
trans-Alkenyl Anion
B.
HX Additions to Alkynes
1) Single addition usually gives anti product
H
H3C C
2)
CH3
H3C
Br
Br
H
H Br
Br-
Br
H3C
CH3
H
Br
Terminal Alkynes give Marknovnikov Addition Products
H3C C
4)
H Br
BrH3C
The second addition gives the Geminal Dihaloalkane (Markovnikov Rule)
H
3)
C CH3
CH3
C H
HI
-70 oC
I
CH3CI2CH3 (65%) +
CH2 (35%)
H3C
Stopping the reaction after only one addition is difficult for terminal
alkynes
C.
CH3CH2
C
Halogenation of Alkynes
1) Anti addition of a single X2 molecule can be done to get vicinal
dihaloalkane anti addition product
2) The second addition gives a tetrasubstituted product
C CH2CH3
Br2, HOAc
CH3CH2
Br
C
LiBr
C
Br
Br2
CCl4
CH2CH3
CH3CH2CBr2CBr2CH2CH3
E-3,4-dibromo-3-hexene
D.
R C
Ketones from Alkynes: Mercuric Ion Catalyzed Hydration
1) Like alkenes, Alkynes can be hydrated
2) The enol product undergoes tautomerization (interconversion of isomers
by C=C, H shift) to give a ketone product
H
C
R
H2O, H+, HgSO4
H
C
R
3)
4)
O
C
tautomerization
H
H C
O
C
R
Enol
R
The Enol and Ketone are called tautomers
Ketone
2+
The Hg cation catalyzes the reaction; mechanism not understood yet
R
5)
The hydration step follows the Markovnikov Rule
OH
OH
OH
OH O
+
H2O, H , HgSO4
H
H
6)
Symmetric Alkynes give only 1 product
HO
C C
H2O, H+, HgSO4
H
O
C C
H
C C
H
+
H
OH
C C
7)
Unsymmetric Alkynes give product mixtures
HO
+
C C
H
O
C C
H2O, H , HgSO4
C C
H
+
H
H
+
OH
O
H
C C
C C
H
III. Anti-Markovnikov Additions to Alkynes
A.
Radical HBr additions
1) HBr adds to Alkynes by radical mechanism with radical initiator ROOR
2) Anti-Markovnikov Products due to need for most stable radical intermediate
ROOR + HBr
H3C C
ROH + Br
C H
Br
Br
H3C C
C
H
H Br
2
C
C
H
Aldehydes from Hydroboration-Oxidation of Alkynes
1) Like with alkenes, BHR2 adds to less substituted side of an alkyne
2) R groups prevent boration of both p-bonds
3) Oxidation results in the enolaldehyde tautomerization
HB
H3C
C
H3C
H
B.
Br
C
H
H
THF
H
C
H3C
C
BR2
H
H2O2
OH-
C
H3C
H
H
H
C
OH
Enol
C
H
C
H3C
Aldehyde
O
IV. Alkenyl Halide Chemistry
Alkenyl Halides don’t do SN1 or SN2 reactions
1) Alkenyl Halides preferentially eliminate to alkynes instead of substituting
2) Need strong base to get elimination, other nucleophiles give N.R.
3) Simple nucleophiles don’t give the substitution products
4) Alkenyl cation that would form is very unstable
A.
H
H2C C
SN 2
Br
B.
R
IH2C C
NO REACTION
R
H2O
SN 1
Br
H2C C
too high energy to exist
Alkenyl Organometallics can form and act as nucleophiles
O
H
H2C C
Br
Mg
THF
H
H2C C
1.
H
+
MgBr
2. H2O, H
H2C C
CH3
C
H3C
OH
V.
Organocuprate Chemistry
A.
Most organometallic reagents don’t react with alkyl halides
1) Alkyl and Alkenyl Metal reagents don’t attack haloalkanes fast enough
MgBr
2)
+
Alkynyl Metal reagents can react with haloalkanes
CH
nBuLi (base)
THF, HMPA
B.
NO REACTION
Br
C-
Br
SN 2
Organocuprates are more reactive
1) Formation of Organocuprates
2 RLi + CuI
H2C
CHLi + CuI
R2CuLi + LiI
(CH2=CH)2CuLi
Lithium Diethenylcuprate
2)
Organocuprates will couple their R groups with Haloalkanes
R2CuLi + R'X
R R'
C
3)
4)
CH3I
H3C
1. Li, Et2O
2. CuI
H
C
H
Either alkyl or alkenyl organocuprates work for the coupling reaction
Along with the Alkynyl Anion reactions, we now can form many different
C—C bonds
1. Li, Et2O
C
Cl
2. CuI
CH
nBuLi (base)
THF, HMPA
CH3CH3I
(CH3)2CuLi
H3C
Et2O, 0 C
H
C
H
o
CH3CH3I
C
2
CuLi
CH3CH2CH3
H3C
C
HMPA
C-
C
CH2CH3
H
Br
SN 2
H
C