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Fig. 1 Oxidation of 2-octanol using 0.5 mol% (ligand)Pd(OAc)2 in
DMSO : water (50 : 50 vol%). General conditions: 100 1C, 50 bar
O2 : N2 (8 : 92), 5 ml of solvent (DMSO : water solvent (1 : 1)),
Na[OAc] (1 Â 10À4 mol) and 2-octanol (0.002 mol).
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formed nanoparticles in aqueous solvent mixtures such as
DMSO : water at this catalyst loading.5h Under these con-
ditions, it may well be that (19)Pd(OAc)2 does not form
nanoparticles as readily compared to (1)Pd(OAc)2.11
Of course it is not generally necessary to achieve full
conversion in under 1 hour. We have still to fully determine
the optimal conditions for (19)Pd(OAc)2, but it is worth
mentioning that 100% conversion of 2-octanol could also be
achieved with lower catalyst loadings (e.g. 0.1–0.2 mol%).
Obviously extended reaction times were required but they were
still very viable (i.e. several hours). Preliminary studies also
indicate that the catalyst can operate efficiently in a range of
organic solvents (e.g. toluene and ethyl acetate), and can
deliver useful rates at lower temperatures (e.g. 60 1C).
In summary, we have shown that a number of N,O-ligands
are excellent ligands for Pd(II) catalysed oxidation of challenging
aliphatic alcohols. (19)Pd(OAc)2 compares very favourably to
other catalysts that have been reported to date,3 including
(1)Pd(OAc)2 which is seen as one of the yardsticks in the field.
An additional advantage of 19 is that it should be more
oxidatively stable than 1; as under some reaction conditions,
the methyl groups of 1 are oxidised, resulting in an inactive
catalyst.5g Future work will examine N,O-ligated complexes in
more detail, examining a wider range of substrates for alcohol
oxidation, as well as other types of oxidative reactions.
Endeavours will be made to better understand the reasons
behind some of the observed ligand effects, with the ultimate
aim of designing improved catalysts for selective oxidations.
We would like to acknowledge The Nuffield Foundation for a
scholarship (D.S.B), the Centre for the Theory and Application
of Catalysis, the Queen’s University Ionic Liquid Laboratories
and the RCUK for funding. We also thank Prof. Roger
Sheldon, Dr Mathew Cook, Dr Nicholas Fletcher and Dr Stuart
James for insightful discussions.
8 S. Tanaka, H. Saburi, T. Hirakawa, T. Seki and M. Kitamura,
Chem. Lett., 2009, 38, 188.
9 Ligand 19 was part of a 96-member pyridine type ligand library
which was screened in a study of Ullmann ether formation using
copper catalysis. In this study 19 was not found to be particularly
effective for this reaction compared to other ligands screened and
was therefore not highlighted to any degree: P. J. Fagan,
E. Hauptman, R. Shapiro and A. Casalnuovo, J. Am. Chem.
Soc., 2000, 122, 5043.
10 This was also the case with when studying the oxidation of
2-octanol, for example in the case of 19, when 3.4 Â 10À7 mol of
catalyst was used as opposed to 1.6 Â 10À6 mol (the amount of
catalyst used in Table 1) the TOF obtained was 1840 hÀ1. Non-
linear dependence on catalyst concentration has been reported
before for Pd(II) catalysed alcohol oxidations4c,5c and there are a
number of potential reasons for this behaviour. Consequently,
determining the exact reasons in this instance will require further
study.
11 Visually comparing the solutions after the reactions, there appears
to be significant catalyst aggregation in the case of 1, while
aggregation is not evident with 19. However, we feel that in order
to properly examine and compare the aggregation between the two
catalysts, we should carry out a more detailed study in the future.
In Sheldon’s study,5h TEM was used to examine the nanoparticles,
and this involves allowing the solvent to evaporate, before analysis
of the particles. We felt that this raises the possibility of nano-
particles forming as the solvent evaporates. Therefore, we intend to
investigate this area further using techniques, such as Environ-
mental SEM and XAFS that will allow in situ analysis of palladium
aggregation.
Notes and references
1 J. S. Carey, D. Laffan, C. Thomson and M. T. Williams, Org.
Biomol. Chem., 2006, 4, 2337.
2 Review of catalytic systems that utilise electron-transfer mediators:
J. Piera and J.-E. Backvall, Angew. Chem., Int. Ed., 2008, 47, 3506.
¨
3 Reviews of Pd(II) catalysts that undergo direct O2 coupled turn-
over: (a) S. S. Stahl, Angew. Chem., Int. Ed., 2004, 43, 3400;
(b) M. S. Sigman and D. R. Jensen, Acc. Chem. Res., 2006, 39,
221; (c) K. M. Gligorich and M. S. Sigman, Chem. Commun., 2009,
3854.
c
7240 Chem. Commun., 2010, 46, 7238–7240
This journal is The Royal Society of Chemistry 2010