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Efficient aerobic oxidation of alcohols using CuBr2 and
a series of α- pyridyl-imine terminated
polydimethylsiloxane ligands
Zhenzhong Hu<[email protected]>, Fran M. Kerton*<[email protected]>
Department of Chemistry, Memorial University of Newfoundland,
St. John’s, NL, Canada, A1B 3X7
Introduction
Ligand
Synthesis
From the point of view of Green Chemistry, polymerbased catalysts are very environmentally friendly. These
catalysts can be used and recycled in green solvents
such as liquid polymers and supercritical carbon dioxide.
They can also be very selective.
In our recent studies, a series of α- pyridyl-imine
terminated polydimethylsiloxane ligands have been
synthesized and characterized. It is interesting that these
carbon dioxide miscible ligands form metallocyclic
complexes when coordinated with copper(I) salt.
Meanwhile, they provide perfect binding sites for copper
(II) ions which then work as catalysts for more efficient
aerobic oxidations of primary and secondary alcohols.
Pressure / psi
Solubility in Supercritical CO2
Coordination Chemistry
[Cu(CH3CN)4](PF6) + L
e / Lmol-1cm-1
0.2
0.4
0.6
15000
0.8
70
90
110
Aerobic oxidation of selected alcohols with
in situ made TEMPO/[Cu]/L1 system
OH
L1
R1
R2
700
600
500
400
300
200
• L2 titrated with Cu(I) reagent (L1 behaves similarly)
2
• MLCT band max. intensity reached at a ratio CM/CP= 1
• Metallocyclopolymer is characterised by NMR, FT-IR,
GPC, CHN, MALDI-TOF MS
Summary
α- Pyridyl-imine terminated PDMS compounds have
been prepared and characterised.
Coordination chemistry with Cu(I) shows the formation
of [1+1] metallocycles.
The TEMPO/[CuBr2]/L1 catalytic system is very
effective for the aerobic oxidation of alcohols.
Future studies will involve the study of catalytic
reaction mechanism and the recycling of the catalyst.
Attempts will also be made to obtain further evidence
for the structures of these metallocyclic polymers.
Product
OH
O
OH
O
OMe
4
R2
t[h]
Conversion
[%][a]
6
100
6
98
6
86
8
100
6
93
12
70
12
88[b]
6
100/0
6
0[c]
OMe
O
OH
3
R1
CH3CN/H2O (2:1), air, 25 ºC
Entry Substrate
1
O
5 mol% CuBr2, 2.5 mol% L1
5 mol% TEMPO, 5 mol% t-BuOK
Cu
 / nm
•
4000
Temperature / oC
1.0
10000
0
•
5000
Cloud point data for L1 ( ) and [Cu(L2)](PF6) (▲) in carbon dioxide. L2 is
miscible in liquid CO2. [Cu(L1)](PF6) is insoluble at Ts and Ps studied.
5000
•
6000
[Cu(L)](PF6)
20000
•
7000
50
25000
•
8000
O 2N
O2N
O
OH
O
HO
O
OH
5
OH
6
OH
O
O
7
OH
8
OH
O
+
OH
9
no product
[a] GC conversion, selectivity > 99%; [b] Reaction
45 oC, using compressed air; [c] Reaction time =
temperature =
48 h
Acknowledgements; Prof. Christopher M. Kozak; Green Chemistry and Catalysis Group; Dr. Celine Schneider ; C-CART (Brent, Linda & Julie); Chemistry
Department, Memorial University