Transcript Document

Chemistry 125: Lecture 51
February 15, 2010
More Addition to Alkenes:
Organometallic Reagents
and Catalysts
This
For copyright
notice see final
page of this file
Mechanism for Acid-Catalyzed Hydrolysis of Acetal
S?1
(pp. 785-787)
:
First remove RO, and replace it by HO.
N
+
H
ROH
+ H
RO
RO
+
+
RO=CH2
CH2
RO-CH2
CH2
cation unusually stable;
RO
RO
thus easily formed
HOH
Now remove second RO, then H (from HO)
+
H
ROH
+ H
RO
RO
RO
+
CH2
H-O-CH2
CH2
+ CH2
HO
HO
HO
(hemiacetal)
H
ROH
RO
H
Overall Transformation:
O=CH
CH
O
O=CH
E1
?
2
H2O + Acetal
H+
Carbonyl + 2 ROH
2
ROH
RO
H
and hydrogen peroxide
which oxidizes aldehydes to carboxylic acids!
H
H
HO-O
O-OH
O H2C=O
CH2 O
C O=CH
H
H
HO
OH
Ozonide is a Double Acetal
So Double Hydrolysis
Gives Two Carbonyl Compounds
Sec. 10.5b pp. 440-441
Add a reducing agent like (CH3)2S (or Zn) to destroy HOOH and save RCH=O.
Or go with the flow and add more HOOH to obtain a good yield of RCOOH.
What Happens to HOOH + RCHO?
-O
OH
Hydride Shift
H
H
C
O
OH
H
O
R
O
3-membered ring C
with O-O bond is
even worse.
R
R
O
OH
C
O
R
OH
OH- is a bad leaving group from C,
but O-O bond is very weak.
Cf.
B
R
-
-
R
Problem:
Try drawing an analogous acid-catalyzed
mechanism in which HOOH attacks the protonated carbonyl,
then H+ is lost from one O of the HOOH fragment in the
product and added to the other before rearrangement.
-OH
O
-
O
HOH
R C
O
“Nucleophilic”
Addition
to C=O
The nucleophilic addition of methyl
lithium to carbonyl groups* is
formally quite different from these
additions of electrophiles
to alkenes, but the following
transition state analysis reveals
a marked mechanistic similarity.
* which will be discussed in more detail later.
Transition State
Motion
Li-CH3
Li CH3
O CH2
O=CH2
Transition State
Orbital Mixing
Li-CH3
LUMO+2
HOMO
*
 HOMO
LUMO
O=CH2
Orbital Variety from Metals
OsO4 / Permanganate
“NMO”
(1976)
H2O2 (1936)
Sec. 10.5c p. 443
Os analogue of cyclic acetal
Chiral
Amine
Ligand
R
H2O
Diol + O2Os=O
Sharpless
(1988) R
R
R
HOMO
LUMO
OsO4 is poisonous and expen$ive!
Use as a 1% catalyst by adding oxidant.
overlaps with alkene *
Os or Mn-
Sections 10.2a (410-413), 10.10 (452)
Catalytic Hydrogenation
HOMO/LUMO : Concerted (“works” with Pt/C Catalyst! Sec 4.9A, 168ff)
*HOMO
LUMO
*
LUMO
 HOMO

H H
Pd H
H
H H orthogonal H H
HOMO-HOMO repulsive
C
C
C
C
C
C
HOMO
empty
C
C
*LUMO
47% C-H
Ethylene
Ethylene-Pd
Complex
Pd
…(4d)10 (5s) 0 (5p)0
HOMO
LUMO (
())
13% 
HOMO-4
40% 4dxy
HOMO (4d)
67% 
Ethylene
HOMO ()
Ethylene-Pd
Complex
HOMO
Pd
0 (5p)0
(4d)10 (5s)
15%
4dz2
+
UMO (5p)
HOMO (4d)
UMO (5s)
6% 5s
5% 5p
Sigma Bond Analogue
“Oxidative” Insertion (crummy PM3 calculation)
H-H
+
Pd
10
0
H2 dissociates on bulk Pd surface (and moves and dissolves)
(entropy help)
kcal/mole
5
Catalytic Hydrogenation
 “oxidative insertion”
C
C
C
Pd
 “oxidative insertion”
C
H
Pd
H
Pd
“reductive elimination”
H
H
Pd
H
Pd
H
H
Pd
“reductive elimination”
H
H
H
H
Pd
Pd
Pd addition concerted; H replaces Pd twice  syn addition
Catalytic Hydrogenation
Stereochemistry
syn addition (p. 412)
Stereochemistry
(Loudon, Sec. 7.9 E p. 313)
No yields
specified!
No literature
reference!
pp. 20-22 of H. O. House
Modern Synthetic Chemistry
(1972)
J. Chem. Soc., 1354 (1948)
H2 / Pt
R’ = Ac
R’ = Ac
Catalytic Hydrogenation
H
H
H
H
H
H
H
Pd
H
H
H
Pd
H
Pd
Pd
alkene
isomerized
H
Pd
H C
H
H
H
Pd
Pd
4
3
2
VII
1
10
9
5
8
4
6
3
7
2
5
10
1
9
6
7
8
VIII
??
Alkene Metathesis
C
C
Ru
Grubbs
Catalyst
Nobel Prize
2005
Ru
C
C
C
Ru
Ru
Ru
Tourists
Ziegler
Grubbs
Host
Tall Fred Ziegler
(not Karl Ziegler)
with Robt. Grubbs
ROMP
Ring-Opening Metathesis Polymerization
metathesis
Ru
C
C
Ru
metatheses
n
C
Ru
n
isotactic CH3
Catalytic Hydrogenation
-(CH -CH) -(CH2-CH2)n-
2
n = 800-8000
H
n
n up to 105
H
6 tons
H 25 x 106 tons H
45
x
10
H
H
(2007)
(2004)
Pd
Pd
Pd
Pd
All head-to-tail, and
but stereorandom
stereoregular (isotactic)
(atactic)
Ziegler-Natta Polymerization
R
Ti
R
R
Ti
Ti
H
End of Lecture 51
Feb. 15, 2010
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