Penn 241 alkyne mechs

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CHEM 241-601 
Alkynes – How Things Work II 
 
Hydration (addition of H2O) of alkynes: 
 
OXYMERCURATION HYDRATION
Markovnikov
CH3CH2C CH �����
HgSO4, H2SO4(aq)
C
Hg
H
CH3CH2C
Hg+2
�����
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vinyl carbocation
���� �����
CH3CH2C C
Hg
HO
HHH2O
���
���
CH3CH2CCH3
O
����� �����
����
����
OH
C
Hg
H
CH3CH2C
H2O
CH3CH2C C
H
HOH
tautomerization��
��
��
H3O
+
SO42- SO4
2-
��
��
��
��
��
��
��
O
H
C
H
H
CH3CH2C
O
H
C
H
H
CH3CH2C
O
H
C
H
H
CH3CH2C
�����
�����
�����
�����
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����
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����� ����
CH3CH2CCH3
O
enol - keto tautomerization (intermolecular)
enol
 
 
 
 
 
 
 
 
 
HYDROBORATION HYDRATION
Anti-Markovnikov
CH3CH2C CH
�����
CH3CH2CH2CH
O
1) disiamyl borane
2) H2O2, NaOH
B
H
���
���
(sia)2BH
CH3CH2C
H
CH
B(sia)2 �����
�����H2O2, NaOH CH3CH2CH=CH2
OH
enol
��
��
��
tautomerization
 
 
 
 
Some problems: 
 
C CH
1) disiamylborane, THF
2) H2O2, NaOH
�����
�����
HgSO4, H2SO4 (aq) �����
 
 
 
 
 
 
 
 
 
 
 
 
 
 
HYDROGENATION 
Standard catalytic hydrogenation reduces all C-C π bonds (except aromatics
�����
�����H2 - 1%Pt on C
75 psi
"Poisoned" catalyst results in reduction of alkyne to alkene (syn addition)
5-decyne
�����
�����
H2 / Pd on BaSO4
quinoline
Lindlar's catalyst H H
Dissolved metal (Na or Li) - ammonia reduction of alkyne to trans alkene
CH3C CCH3
�����Li in NH3(l)
-78oC
C=C
CH3H
CH3 H
Li
��
���
���
CH3C=CCH3
��
���
NH
H
H
���
����� ����
����
�����
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C=CCH3
H
CH3
�
�
�����
�����Li
��
�����
����
CH3
H
C=CCH3
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NH2H������
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trans more stable