Transcript Document 7627264

4I-11 Case studies in Inorganic Chemistry
Imperial College
London
Lecture 8
Biorenewable Polymers 2:
The Stereoselective Polymerisation of Lactide
Dr. Ed Marshall
Rm: M220, Mezzanine Floor, RCS 1
[email protected]
www.ch.ic.ac.uk/marshall/4I11.html
4I-11 - Lecture 8 - Slide 1
Recap of Lecture 7: Mechanism of propagation
Imperial College
London
Every step is reversible.
Coordinative-insertion Mechanism
4I-11 - 8 - 2
Imperial College
London
Last lecture
(salen)Al(OR) and derivatives convert
rac-lactide into isotactic poly(lactide)
Product is believed to be a stereoblock copolymer, with short sequences of all
R alternating with short sequences of all S (as opposed to a stereocomplex,
formed from complete all R chains and all S chains)
correct structure
incorrect structure
4I-11 - 8 - 3
Remaining learning outcomes
Imperial College
London
Over these two lectures you should acquire the knowledge to allow you to:
1. Describe why the polymerisation of lactide is so intensely researched.
2. Explain how chiral and achiral (salen)-supported Al complexes may be used
to prepare isotactic and syndiotactic polylactide.
3. Explain how b-diketiminate supported complexes of Zn and Mg may be
used to prepare heterotactic polylactide.
4. Understand how computational chemistry may be used to investigate
polymerisation mechanisms and to shed light onto the causes of
stereoselectivity.
4I-11 - 8 - 4
b-Diketiminate ligands
Imperial College
London
diketimine
canonicals:
Deprotonation results in a monoanionic bidentate ligand - known as NacNac or BDI.
e.g.
Ar = 2,6-diisopropylphenyl
Dalton Trans. 2003, 3088 - WebCT Gibson2003.pdf
[(BDI)MgiPr]
4I-11 - 8 - 5
Imperial College
London
First report of heterotactic PLA
Coates
rac-LA
(R)
(S)
CH2Cl2
25 °C
(S)
(R)
Heterotactic PLA
• 100 equiv rac-LA consumed in 20 mins
• Highly stereoselective - Pr = 0.90 (0.94 at 0 °C)
R = iPr, Pr = 0.90
R = nPr, Pr = 0.76
R = Et, Pr = 0.79
J. Am. Chem. Soc. 2001, 123, 3229 - WebCT Coates2001.pdf
steric bulk of iPr groups is
essential for stereocontrol
4I-11 - 8 - 6
Imperial College
London
The Mg analogue
Under the same conditions
- i.e. CH2Cl2, 25 °C [(BDI)Mg(m-OiPr)2] gives atactic PLA
But the Mg initiator is heteroselective in coordinating solvents:
Chisholm
rac-LA
THF
25 °C
Inorg. Chem. 2002, 41, 2785 - WebCT Chishiolm2002.pdf
Heterotactic PLA, Pr = 0.90
4I-11 - 8 - 7
However, magnesium BDI initiators can be heteroselective
Imperial College
London
NMR studies reveal that in THF, the Mg-propagating species is mononuclear, but
in CH2Cl2 it is dimeric. The Zn analogue is monomeric even in CH2Cl2:
Propagating Mg
species in THF
Propagating Zn
species in CH2Cl2
Heterotactic PLA formed when the
propagating species are mononuclear
J. Am. Chem. Soc. 2005, 127, 6048 - WebCT Rzepa2005.pdf
4I-11 - 8 - 8
Computational studies
Imperial College
London
Goals of this project:
• to understand the mechanism of ring-opening better.
• to explain why the Mg and Zn initiators give heterotactic PLA.
• to explain why reduction in the N-aryl ortho substituents (e.g. from iPr to Et)
leads to a loss in stereoselectivity.
Method employed:
(i) Reaction coordinate mapped out for the insertion of two LA units (LA1 and LA2)
using (BDI)Mg(OMe)(THF) as the initiator.
(ii) Free energies of competing transition states (i.e. R,R or S,S-lactide insertion)
calculated.
All calculations performed at a very high level [B3-LYP 6-311G(3d)] - many of the
calculated geometries took 7 - 10 days to converge.
4I-11 - 8 - 9
The Reaction Coordinate - calculated for LA1 = (R,R) & LA2 = (S,S)
Imperial College
London
Two transition states, TS1 and TS2
TS2 is higher in energy than TS1
4I-11 - 8 - 10
Revised mechanism
Imperial College
London
TS1:
Formation of new M-O bond
and cleavage of M-OR bond
TS2:
Formation of new M-O bond
and cleavage of heterocycle
Both transition states involve bond breaking / forming
4I-11 - 8 - 11
Imperial College
London
Computing the origin of stereocontrol
Although initially calculated for LA1 = R,R-lactide and LA2 = S,S-lactide, we have
to consider several other possible assemblies.
Total number of assembly modes:
LA1 = R,R or S,S;
LA2 = R,R or S,S;
LA2 may approach either face of the ring-opened LA1
8 possibilities
However, the 8 possible assembly modes exist as 4 enantiomeric pairs:
LA2 S
R
S
Mg
R
S
Mg
Mg
R
S
S
Mg
Mg
R
S
S
S
R
S
R
S
R
S
LA1
S
R
Mg
R
R
R
S
R
S
Mg
Mg
S
R
R
S
R
R
mirror plane
mirror plane
4I-11 - 8 - 12
Imperial College
London
e.g. Consider the approach of S,S-LA2 to R,R-LA1
S
R
S
R
S
R
S
R
mirror image
8 possible assembly modes = 4 enantiomeric pairs
∴ only 4 calculations required
4I-11 - 8 - 13
Imperial College
London
Curtis-Hammett Principle
Since every stage of the ring-opening mechanism is reversible, the product
distribution (i.e. whether R,R or S,S lactide is inserted) depends only on the
competing geometries for the rate-determining step.
Calculated transition state free energies (kcal mol-1):
LA1
LA2
TS1
TS2
RR
RR
13.5
20.2
SS
SS
6.7
25.4
RR
SS
10.5
18.9
SS
RR
12.5
28.1
lowest barrier for
heterotactic PLA
In every case TS2 is rate-determining
Therefore, the reason for heterotactic stereocontrol must lie within the four
competing geometries for TS2 - RR,RR - SS,SS - RR,SS - and SS,RR.
4I-11 - 8 - 14
Competing TS2 geometries - the origin of stereocontrol
Imperial College
London
RR,SS:
18.9 kcalmol-1
4I-11 - 8 - 15
Competing TS2 geometries - the origin of stereocontrol
Imperial College
London
RR,SS:
18.9 kcalmol-1
RR,RR:
20.2 kcalmol-1
4I-11 - 8 - 16
Competing TS2 geometries - the origin of stereocontrol
Imperial College
London
SS,RR:
18.9 kcal mol-1
RR,RR:
20.2 kcal mol-1
SS,SS:
25.4 kcal mol-1
4I-11 - 8 - 17
Competing TS2 geometries - the origin of stereocontrol
Imperial College
London
SS,RR:
18.9 kcal mol-1
RR,RR:
20.2 kcal mol-1
SS,SS:
25.4 kcal mol-1
SS,RR:
28.1 kcal mol-1
Heterotactic PLA formed via LA1= R,R and LA2 = S,S. Next R,R
then inserts via the enantiomer of the SS,RR transition state
4I-11 - 8 - 18
Summary of the origin of stereocontrol
Imperial College
London
Heterotactic PLA
formed via LA1 = R,R
and LA2 = S,S.
R,R-LA3 then inserts via
the enantiomer of the
SS,RR transition state
4I-11 - 8 - 19
Conclusions
Imperial College
London
• Heterotactic PLA may be prepared using b-diketiminate Zn and Mg
alkoxides, but the Mg initiators must be used in THF.
• The propagating species responsible for heterotactic PLA formation is
mononuclear.
• Computational analysis reveals that the rate determining step is TS2, i.e.
the cleavage of the monomer heterocycle.
• Heterotactic PLA arises because of the minimisation of Me - Me steric
clashes in the competing geometries of TS2.
4I-11 - 8 - 20