tacticity isotactic and syndiotactic polymers are crystalline, atactic is amorphous. nmr...
TRANSCRIPT
Tacticity
m
r
Dyad Tacticity
isotactic (meso, m)
syndiotactic (racemic, r)
• Isotactic and syndiotactic polymers are crystalline, atactic is amorphous.
• NMR spectroscopy is a powerful tool for studying polymer stereochemistry.
• Ziegler/Natta catalysts allow control of tacticity.
•Tacticity of polymer is determined by % m or r dyads. (Perfectly isotactic polypropylene has 100% m dyads)
isotactic
syndiotactic
atactic
m m
m r
r r
Triad Tacticity
isotactic(mm)
atactic (mr)
syndiotactic (rr)
Site vs. chain-end control
Site control mechanism: the catalytic center determines the stereospecificity of the polymer.
Chain-end control mechanism: the polymer chain determines the stereospecificity of the final polymer.
Ti
Cl
ClCl
Cl
CH3
Ti
Ti
Ti
P
re face
si face
12
MP*
+
MP*
*
MP*
*
Chain-end control
TiP*
*
VP**
1
2
V
P*
* VP
Ti
P**1
2Ti
P
Chain-end control through 2,1-insertions leading to syndiotactic polymer
Chain-end control through 1,2-insertions leading to isotactic polymer
trans
trans
trans
syntranstrans
trans
V = VCl4 / AlClEt2 or V(acac)3 / AlClEt2
Ti = Cp2TiPh2 / MAO at -45 oC; atactic at 25 oC
13C NMR spectroscopy
Atactic: 9 signals (10 expected, 2 coincidental) based on pentads
# # # # ###o o
#
m m m m m m m m mr
o
mmmm
mmmrmmrm
Chain-end control
# # # # ##o
#
m m m m m r m m mr
o
mmmm
mmmr mmrr
Site control
o
mrrm
Late transition metal catalysts
N N
M
Pol
+
ArAr N
N NM Ar
Cl Cl
MAO
EthenePropene
Polymers
M = Ni, Pd
N N
M
S Pol
+
ArAr
M = Ni, Pd
COOMe
COOMe
MeOOC
Brookhart M = Fe, Co
Ethylene polymerization catalysts
Ethylene-acrylate copolymerization catalysts
M=C complexes
Textbook H: Chapter 13.1 – 13.2
Textbook A: Chapter 9
2005 Nobel prize in chemistry
Yves ChauvinInstitut Français du Pétrole Rueil-Malmaison, France
Robert GrubbsCalifornia Institute of Technology
(Caltech) Pasadena, CA, USA
Richard SchrockMassachusetts Institute
of Technology (MIT) Cambridge, MA, USA
http://nobelprize.org/chemistry/laureates/2005/index.html
"for the development of the metathesis method in organic synthesis"
Outline Carbene complexes
Bonding Structural and spectroscopic features Synthesis Reactivity
Olefin metathesis Mechanism Reaction overview
ROMP RCM
Grubbs catalysts Stereochemistry Catalyst decomposition
Alkyne metathesis Reactions related to olefin metathesis
M=C complexes: comparison
Property Fischer-type
(carbene)
Schrock-type
(alkylidene)
Nature of carbene C
Typical R groups
Typical metal
Typical ligands also attached to the metal
Electrophilic
donor (e.g. OR)
Mo(0), Fe(O), Cr(0)
Good acceptor (e.g. CO)
Nucleophilic
Alkyl, H
Ti(IV), Ta(V), W(VI)
Good or donor (Cl, Cp, Alkyl)
C
R
R
M C
R
R
M
Fischer carbene Schrock carbene
(OC)5WOR
R
W(0), 18e
Cp2(Me)TaH
H
Ta(V), 18e
Fischer carbenes: synthesis Nucleophilic attack at a carbonyl ligand: most common method
Activation of a neutral acyl complex
From activated olefins
(OC)5Cr
OLi
Ph
Cr(CO)6 + PhLi (OC)5Cr
OMe
Ph
Me3OBF4
Re
OCPh3P R
O
FSO3CH3Re
OCPh3P R
OCH3
FSO3-
NR
RN
RN
NR
+ RuCl2(PPh3)2 Cl2Ru
RN
NR 4
TaCl5 + 3Li
tBu
Ta
Cl
Cl
tButBu
tBu
Zn
tBu
2Ta
tButBu
tButBu
tBuHHH
H
Ta
tButButBu
tBu
-abstraction: induced by steric bulk
Rearrangement of coordinated ligands
From activated olefins
Schrock carbenes: synthesis
LnM R + H+ LnM C
R
Hvinylidene
Cp2Ti(PMe3)2 + Cp2(PMe3)Ti
-unsaturated carbene
M=C complexes: comparison
Property Fischer-type
(carbene)
Schrock-type
(alkylidene)
Nature of carbene C
Typical R groups
Typical metal
Typical ligands also attached to the metal
Electrophilic
donor (e.g. OR)
Mo(0), Fe(O), Cr(0)
Good acceptor (e.g. CO)
Nucleophilic
Alkyl, H
Ti(IV), Ta(V), W(VI)
Good or donor (Cl, Cp, Alkyl)
C
R
R
M C
R
R
M
Fischer carbene Schrock carbene
(OC)5WOR
R
W(0), 18e
Cp2(Me)TaH
H
Ta(V), 18e
Carbene complexes: Fischer type
x
z
y *
*
dz2 ( )
dyz ( )dxz ( )
dxy
COH
H
COH
H
COH
H
(OC)5Mo=CH(OH) CH(OH)Mo(CO)5
Alkylidene complexes: Schrock type
x
z
y *
*
dz2 ( )
dyz ( ) CH
H
CH
H
Cp(Cl)2Nb=CH2 CH2NbCpCl2
Structural and spectroscopic features
1H NMR: 5 – 15 ppm13C NMR: 220 – 260 ppmJCH typical: 150 – 160 HzJCH agostic: 90 – 120 Hz
Schrock typeFischer type
MCC angle: 160 – 170°
MC distance: longer than M=C, but shorter than M-CCX distance: shorter than C-X
(OC)5Cr
OEt
NMe2
2.13 Å 1.35 Å
1.33 Å
LnM
OR
R'
LnM
OR
R'
C
R'
OR
M
LnM
R
H
LnM
R
H
Fischer carbenes: reactivity Heteroatom substitution: related to the aminolysis of esters to give amides
Cyclopropanation
The Dötz reaction
(OC)5Cr
OMe
Ph
:NH2R (OC)5Cr
OMe
PhNHR
H - MeOH(OC)5Cr
NHR
Ph
(OC)5Cr
OMe
Ph
or h(OC)4Cr
OMe
Ph
X
Y(OC)4Cr
OMePh
XY MeO
Ph
X
Y- "Cr(CO)4"
(OC)5Cr
OMe R1
R2
+50 oC
- CO
OH
R1
R2
OMe Cr(CO)3
+
OH
R1
R2
OMe(OC)3Cr
Mechanism of the Dötz reaction
(OC)5Cr
OMe R
R
+ (OC)4Cr
OMe
R
R
(OC)4Cr
OMe
R
R
CO insertion
(OC)4Cr
OMe
RR
O
Cr(CO)4
OMe
RR
C
OO
R
R
OMe Cr(CO)4
H
- COproducts
enolize
- CO
+ CO
Schrock carbenes: reactions with electrophiles
(tBuH2C)3Ta CH
tBu+ O
R
R'
Wittig-like reactivity:
[(tBuCH2)3TaO]n +R
R'
H
tBu
Reactions with Bronsted acids:
Mo C CH
R
CpMe3P
Br PMe3
HBF4Mo C HC
H
R
CpMe3P
Br PMe3
+
BF4-
WMe3P
ClPMe3
CR
CO
PMe3
HCl
- PMe3
WMe3P
ClPMe3
CHR
CO
Cl
Other reactions
(R3P)2Cl3Ta CH
tBu+ O W(OtBu)4
Alkylidene transf er:
TaCl(OtBu)4 + WCl
ClO
CHtBu
PR3
PR3
C-C bond f ormation:
Cp2Ta
CH2
+ Me3P CH2
Me
Ta + PMe3
Me
IrR3PBr
PR3
CH2
CH3
PR3
+
Br- IrR3PBr
PR3
CH2CH3
Br
PR3
Cp2
Olefin metathesis
Reference: Chem. & Eng. News 2002, Dec 23, 34-38http://nobelprize.org/nobel_prizes/chemistry/laureates/2005/animation.html
Mechanism:
Such cycloaddition reactions between two alkenes to give cyclobutanes are symmetry forbidden and occur only photochemically. However, the presence of d-orbitals on the metal alkylidene fragment breaks this symmetry and the reaction is quite facile.
Normally, the products are statistical, unless the reaction can be driven in some way or the two alkenes have different reactivities.
R1
R1
R1
R1
R2
R2
R2
R2
+catalyst R2
R2
R1
R1
2
LnMR
R'R'
LnMR
R'R'
MLnR
R'R'
R1
R1
+catalyst R1
R1
R1
R1
2 C2H4
Timeline
Titanium
Tungsten
Molybdenum
Ruthenium
AcidsAlcohols, water
AldehydesKetones
Esters, amidesOlef ins
AcidsAlcohols, water
AldehydesKetonesOlef ins
Esters, amides
AcidsAlcohols, water
AldehydesOlef insKetones
Esters, amides
Olef insAcids
Alcohols, waterAldehydesKetones
Esters, amides
1950
1960
1970
1980
1990
2000
discovery of olef in metathesis
RuCl3 performs ROMP
Chauvin proposes metal alkylidene
-based mechanismevidence for Chauvin's
mechanism found
single-componentcatalysts developed
synthesis of Mo-alkylidene
synthesis of Ru-alkylidene(Cy3P)2Cl2Ru=CHPhmechanism
of (Cy3P)2Cl2Ru=CHPh investigated mono(NHC) Ru
catalysts developed