chemistry 125: lecture 51 february 15, 2010 more addition to alkenes: organometallic reagents and...
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Chemistry 125: Lecture 51February 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
RO
ROCH2
+H
HOH
:
:
RO
ROCH2
+ H ROH
RO-CH2
+
HO
ROCH2+
H
First remove RO, and replace it by HO.
HO
ROCH2
Now remove second RO, then H (from HO) +H
:HO
ROCH2
+ H
RO=CH2
+
cation unusually stable;thus easily formed
ROH
H-O-CH2
+
O=CH2 O=CH2
ROH
ROH
RO
ROCH2 O
H
H
:Overall Transformation:
H2O + Acetal Carbonyl + 2 ROHH+
(pp. 785-787)
(hemiacetal)?
?SN1
E1
HOO=CH2CH2
HO-OO
H
H
O-O
OH2CO
H
H H
HH2C=O
Ozonide is a Double AcetalSo Double Hydrolysis
and hydrogen peroxide
Gives Two Carbonyl Compounds
which oxidizes aldehydes to carboxylic acids!
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.
3-membered ring with O-O bond is
even worse.
What Happens to HOOH + RCHO?
OC
H
R
O OH-OH
O
C
H
R
O
- O
C
H
R
O OH-
HOH
-
O
CR
OR
B
R
R
O OH
-Cf.
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.
OHOH- is a bad leaving group from C,
but O-O bond is very weak.Hydride Shift
“Nucleophilic”Additionto 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 StateMotion
Li-CH3
O=CH2
Li CH3
O CH2
Transition StateOrbital Mixing
Li-CH3
O=CH2
HOMOLUMO+2
* LUMO HOMO
Orbital Variety from Metals
overlaps with alkene *overlaps with alkene
LUMOHOMO
OsO4 / PermanganateSec. 10.5c p. 443
Os or Mn-
Os analogue of cyclic acetal
H2O
Diol + O2Os=O
OsO4 is poisonous and expen$ive!Use as a 1% catalyst by adding oxidant.
H2O2 (1936) “NMO” (1976)
ChiralAmineLigand
Sharpless(1988)
R
R
R
R
H H H H*LUMO HOMO
orthogonal
Sections 10.2a (410-413), 10.10 (452)
Catalytic Hydrogenation
HOMO/LUMO : Concerted
H HH H*LUMOHOMO
C C
H H
C C
H H
C CC C
*LUMOHOMO
(“works” with Pt/C Catalyst! Sec 4.9A, 168ff)
HOMO-HOMO repulsive emptyPd
Pd HOMO (4d)
Ethylene LUMO ()HOMO ()
HOMO-4Ethylene-PdComplex
…(4d)10 (5s) 0 (5p)0
13%
40% 4dxy
47% C-H
HOMO ()
Pd HOMO (4d)
Ethylene
UMO (5s)UMO (5p)
(4d)10 (5s) 0 (5p)0
HOMOEthylene-Pd
Complex
+6% 5s5% 5p
15% 4dz2
67%
Sigma Bond Analogue“Oxidative” Insertion (crummy PM3 calculation)
H-H+Pd
10
5
0
kcal/mole
H2 dissociates on bulk Pd surface (and moves and dissolves)(entropy help)
H
Catalytic Hydrogenation “oxidative insertion” “oxidative insertion”
Pd
C C
Pd
C C
Pd
H H
Pd
H H
“reductive elimination” “reductive elimination”
Pd
HCC
Pd
HCC
Pd
HCC
H
Pd
HCC
H
Pd addition concerted; H replaces Pd twice syn addition
Catalytic HydrogenationStereochemistry
syn addition (p. 412)
Stereochemistry(Loudon, Sec. 7.9 E p. 313)
No yields
specified!
No literature
reference!
pp. 20-22 of H. O. HouseModern Synthetic Chemistry (1972)
J. Chem. Soc., 1354 (1948)
H2 / Pt
R’ = Ac R’ = Ac
H
Catalytic Hydrogenation
Pd
H
Pd
HCC
Pd
HCC
H
Pd
HCC
H
C CC
CH H H
H
CCC
Pd
H
Pd
HCCCH
Pd
HCCCH
CC
alkene isomerized
10
12
34 5
6
7
89
10
12
34 5
6
7
89
??
VII VIII
Alkene Metathesis
C
C
Grubbs Catalyst
Ru
C
C
C
Ru
CC
C
Ru
C
C
C
Ru
C
C
C
Ru
C
Nobel Prize 2005
Tall Fred Ziegler(not Karl Ziegler)
with Robt. Grubbs
TouristsZiegler Grubbs
Host
ROMPRing-Opening Metathesis Polymerization
Ru
CRuC
RuCn
n
metathesis
metatheses
Catalytic Hydrogenation
Ti
RCC
Ti
RCC
Ti
RCC
H
Pd
HCC
Pd
HCC
Pd
HCC
H
Pd
HCC
H25 x 106 tons(2004)
-(CH2-CH2)n-
n = 800-8000
Ziegler-Natta Polymerization
45 x 106 tons(2007)
-(CH2-CH)n-CH3
n up to 105
isotactic
All head-to-tail, but stereorandom (atactic)All head-to-tail, and stereoregular (isotactic)
End of Lecture 51Feb. 15, 2010
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