cyclohexanol (1.7, intramolecular), all at room temperature.
These indicated a rate-determining step that involved the break-
ing of the C–H bond, with the occurrence of this concomitant
transfer of a hydride unity (for analogous interpretations, see
ref. 13a; to better interpret the intramolecular kinetic isotope
effect, it would be necessary to carry out further studies at
different temperatures, which, unfortunately, was not possible
due to the above-mentioned weak dependence of the reaction
upon the temperature). This mechanism was proposed by
Szuromi et al.,22 although in another typology of reaction
(oxidation of an alkene), for which a platinum oxo-complex was
isolated.
A zero valent metal in a colloidal form could explain the whole
mechanism, as recently observed by Maayan and Neumann for
secondary alcohols,23 although in the presence of a stabilizing
agent to obtain active nanoparticles. The alternative hypothesis
points to a resting state of the catalyst compatible to the initial
phthalocyanine molecule, and a platinum(IV) oxo derivative as
the oxidizing form (see Scheme 2). It appears highly likely that,
1-one with LiAlD4. PtPcS and PdPcS were prepared by template
synthesis starting from K2PtCl4 (or PdCl2), 4-sulfopthalic acid
and urea, following earlier reported general procedures for the
synthesis of metal-sulfophthalocyanines.26 Water was used from
a Milli Q apparatus.
In a 10 mL open tube containing 4 mL of 25 mM water
solution of alcohol, 1 mL of water solution of 5 mM PtPcS (or
PdPcS) was added, and left under stirring for the time necessary
for the experiment. One mL aliquots were sampled for a direct
NMR investigations. Experiments at different oxygen pressures
were conducted in the presence of balloons containing neat
oxygen and in an autoclave apparatus that supported an air
pressure up to 100 atm.
The NMR measurements were performed with a Bruker
Avance 300 MHz spectrometer equipped with a 5 mm BBO
probe (30–300 MHz). Water suppression was determined by a
presaturation sequence using a composite pulse (zgcppr; Bruker
sequence). A co-axial capillary tube containing a 30 mM solu-
tion of 3-(trimethylsilyl)propionic-2,2,3,3-tetradeuterium acid,
as the sodium salt (TSP) in water (D2O), was used as the
reference and for the lock procedure. The organic analyses
were also performed on aliquots withdrawn with a microsyringe
from the aqueous reaction mixtures, using an HP 6890 GLC
instrument equipped with FID, and a 30 mHP-5 capillary
column (0.32 mm i.d.; 0.25 mm film thickness), with the injection
port kept at 250◦C (carrier gas: He). The identity of each product
was confirmed by comparisons of the fragmentation patterns
of the mass spectra obtained with a mass-selective detector
(MD 800, Fisons) coupled to a gas chromatograph (GC 8000
series, Fisons) operating at 70 eV in electron ionization mode.
The integrity of metal-sulfophthalocyanines at the end of the
reactions was controlled by recording the visible spectra, 400–
800 nm, 0.2 nm using a 6505 UV/Vis Jenway spectrophotometer
and ESI-MS spectra.
while several examples of M(IV) oxo complexes are reported in
24
=
=
the literature for M Pt, this is not the case for M Pd, with
the latter, significantly, not being active in the present aerobic
reactions.
Scheme 2 Part of the catalytic cycle showing the hypothesized
b-hydride transfer.
The ESI-MS of Pd and PtPcS were performed using a
Perkin Elmer Sciex API 150 MCA single-quadrupole mass
spectrometer. Acquisitions were carried out in positive ion mode
over the mass range of 500–1100 u.
Conclusions
Here, we have studied a new selective catalytic aerobic oxidation
of alcohols to carbonyl derivatives. The reaction is worth noting
since the experimental conditions are green: room pressure
and temperature, water solution, and air as oxidant. Also, the
problem of residual catalyst at the end of the reaction can be
easily resolved by adding selective supports that act as sorbents
for polar metal complex derivatives i.e. amberlite or other
analogous inert materials.25 The reaction times are, however,
quite high and definitely need to be improved. For the reaction
mechanism, our data are not sufficient to discriminate between
the 0 valent metal species and a platinum oxo derivative. A rate-
determining step involving a b-hydride transfer where oxygen is
provided sequentially to re-oxidize the catalyst in a new active
species is fully compatible with our experimental data.
Acknowledgements
The authors are grateful to the “Consorzio di Ricerca per
l’Innovazione Tecnologica, la Qualita` e la Sicurezza degli
Alimenti S.C.R.L., L’Aquila, Italy” (CIPE fundings 20.12.04;
DM 28497) for financial support.
Notes and references
1 P. T. Anastas and M. K. Kirchhoff, Acc. Chem. Res., 2002, 35, 686.
2 H. O. House, Modern Synthetic Reactions, W.A. Benjamin,
Inc.Menlo Park, CA, 2nd edn, 1972257.
3 A. Dijksman, A. Marino-Gonza´lez, A. Mairata, I. Payeras, I.W. C. E.
Arends and R. A. Sheldon, J. Am. Chem. Soc., 2001, 123, 6826.
4 P. L. Anelli, F. Montanari and S. Quici, Org. Synth., 1990, 69, 212.
5 R. Liu, C. Dong, X. Liang, X. Wang and X. Hu, J. Org. Chem., 2005,
70, 729.
6 X. Wang, R. Liu, Y. Jin and X. Liang, Chem. Eur. J., 2008, 14, 2679.
7 R. Villa, A. Romano, R. Gandolfi, J. V. Sinisterra Gago and F.
Molinari, Tetrahedron Lett., 2002, 43, 6059.
Experimental
Chemicals were purchased from Aldrich, except shikimic acid,
which was purchased from Dayang Chemicals Co. Ltd (China).
Mono- and bis-deuterated cinnamyl alcohols (in allylic position)
and 1-deutero-2-cyclohexen-1-ol were synthesized by reduction
of trans-cinnamic acid, trans-cinnamaldehyde and 2-cyclohexen-
8 J. Hirano, K. Miyamoto and H. Ohta, Tetraedron Lett., 2008, 49,
1217.
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