chemistry 125: lecture 52 february 16, 2011 transition metal catalysis: hydrogenation &...
TRANSCRIPT
Chemistry 125: Lecture 52February 16, 2011
Transition Metal Catalysis:Hydrogenation & Polymerization
Additions by Radicals & Electrophilic Carbon;
Isoprenoids;Tuning Polymer Properties This
For copyright notice see final page of this file
Other “Simultaneous” ReagentsCl2C: (Carbene)
R2BH (Hydroboration)
CH2I2 Zn/Cu (Carbenoid)
O3 (Ozonolysis)
H-metal (Catalytic Hydrogenation)
R-metal (Metathesis, Polymerization)
RC (Epoxidation)OOH
O
OsO4 or MnO4- (Dihydroxylation)
OsO4 and Permanganate
Os analogueof
cyclic acetal
H-O-H
OsO4 is poisonous and expen$ive!Use as a 1% catalyst by adding oxidant.
H2O2 (1936) “NMO” (1976 - Upjohn)
ChiralAmineLigand
e.g. J&F Sec. 10.5c p. 443
Osmate Ester
H
HC C
H3C
CH3
HH
C C
H3CCH3
O O
OO
Os
OO
Os
OOOH H
Sharpless Asymmetric Dihydroxylation(1988)
SAD
C C
K+
O-O
Mn
OO
97% ee
C C
O O
O-O
Mn
OH HH2O
K+
OH
OH
85%yield
KMnO4
MeOH / H2ONaOH, 20°C
all syn
(S,S) + (R,R)
syn additionto trans 2-butene
H H H H*LUMO HOMO
orthogonal
Catalytic Hydrogenation
HOMO/LUMO : Concerted
H HH H*LUMOHOMO
C C
H H
C C
H H
C CC C
*LUMOHOMO
(“works” with metal catalysts!)
HOMO-HOMO repulsive emptyPd
e.g. J&F Sections Sec 4.9A, 168ff., 10.2a (410-413), 10.10 (452)
Orbital Varietyfrom Metals
Pd HOMO (4d )
Ethylene LUMO ()HOMO ()
HOMO-4Ethylene-PdComplex
…(4d)10 (5s) 0 (5p)0
13%
40% 4dxy
47% C-H
x2-y2z2xyxzyz
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” Addition (crummy PM3 calculation)
H-H+Pd
10
5
0
kcal/mole
H2 dissociates on bulk Pd surface, then hydrides move.(entropy help)
bondingH2 to Pd
splitting H2
kcal/mole
H
Catalytic Hydrogenation “oxidative addition” “oxidative addition”
Pd
C C
Pd
C C
Pd
H H
Pd
H H
“reductive elimination” “reductive elimination”
Pd
HCC
H Pd
HCC
H
Pd addition concerted (syn)
Pd
HCC
H Pd
HCC
Experts discuss the extent of bonding in this “-complex”
H atoms replace Pd frontside syn hydrogenation product
Catalytic HydrogenationStereochemistry
syn addition
e.g. J&F pp. 412
Stereochemistry
No yields
specified!
No literature
reference!
A general elementary texte.g. Loudon, Sec. 7.9 E p. 313
pp. 20-22 of H. O. HouseModern Synthetic Chemistry (1972)
(a graduate-level text)
J. Chem. Soc., 1354 (1948)
H2 / Pt
R’ = Ac R’ = Ac
allylic isomers
Suppose there is an allylic H in the alkene:can lead to allylic rearrangement
H
Catalytic Hydrogenation
Pd
H
Pd
HCC
Pd
HCC
H
Pd
HCC
H
CC CH
Pd
H
Pd
H CC CH
CC
alkene isomerized
symmetric
C C CH H H
10
12
34 5
6
7
89
10
12
34 5
6
7
89
??
VII VIII
Alkene Metathesis
metallacyclobutane
C
C
Grubbs Catalyst
Ru
C
C
C
Ru
CC
C
Ru
C
C
C
Ru
C
C
C
Ru
C
Nobel Prize 2005
a metal alkylidene complex
Tall Prof. F. Ziegler(not Prof. Karl Ziegler)
with Prof. R. Grubbs
TouristsZiegler Grubbs
HostProf. S.-I. Murahashi
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 = 800-250,000
Ziegler-Natta Polymerization
45 x 106 tons(2007)
-(CH2-CH)n-CH3
n up to 105
isotactic
Heterogeneous Catalysthard to study mechanism
RR R
Et3Al + TiCl4
Stereochemistry: Tacticity
All head-to-tail, but stereorandom (atactic)
All head-to-tail, and stereoregular (isotactic)
All head-to-tail, and stereoregular (syndiotactic)
How do you know which is which?
NMR(coming soon)
R2B-HC=C-CH3
R’
RC-C-CH3
Stereochemistry: Tacticity
All head-to-tail, but stereorandom (atactic)
All head-to-tail, and stereoregular (isotactic)
All head-to-tail, and stereoregular (syndiotactic)
axisHomogeneous“Kaminsky” catalysts
activated by MAO(“methyaluminoxane”)
homotopicfaces
mirror
enantiotopicfaces
achiralfaces
C=C-CH3
R’C-C-CH3
RR
C=C-CH3
R’
C-C-CH3+
Alkenes approachfrom alternate faces
Radical Polymerization(e.g. J&F Sec 11.5 pp.487-489)
R
H
Occasional butyl side-chains inhibit close packing.
Cl CCl3CCl3R Cl
Controlling Polymer Chain LengthCCl4 is a “Chain-Transfer Agent”
shortens polymer molecules without terminating chain reaction
Properties like viscosity and melting point depend on
chain length.etc.
Cl
ktransfer/kpolymerization ~ 0.01 for styrene polymerization
When other termination is negligible, molecular length
~ kp[styrene] / kt[CCl4]“dispersity”
Alkene/Diene Oligomerization and Polymerization Using
Carbon Electrophiles
R+ (SN1)
R-L*
(SN2)
(“oligo”, a few)
CH3
CH3
H2C C
R+ Electrophile in Formation of 2,2,4-Trimethylpentane, “Isooctane”
CH3
CH3
CH3 C H
CH3
CH3
H2C C
CH3
CH3
CH3 C CH2
CH3
CH3
HCH2SO4+
CH3
CH3
CH3 C+
CH3
CH3
CH3 C +
CH3
CH3
CH2 C
H+
(defined as “100 octane”)
inter molecular hydride shift(Bartlett, 1944)
chainpoly(isobutylene)
“butyl rubber” air-tight
+
CH3
CH3
CH2 C
CH3
CH3
CH3 C
CH3
CH3
CH2 Cetc. etc.
CH3
CH3
H2C C
H
CH3
CH3 C CH3
+
e.g. J&F Sec. 12.13 pp. 554-562
R-L and R+ Electrophiles in*
Terpene/Steroid Biogenesis
End of Lecture 52February 16, 2011
Copyright © J. M. McBride 2011. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).
Use of this content constitutes your acceptance of the noted license and the terms and conditions of use.
Materials from Wikimedia Commons are denoted by the symbol .
Third party materials may be subject to additional intellectual property notices, information, or restrictions.
The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0