First example of the chemical, oxidative cleavage of the C-P bond in aminophosphonate chemistry. The oxidation of 1-amino-1-(3,4-dihydroxyphenyl)methylphosphonic acid by NaIO4.

Article abstract of DOI:10.1039/b401633e

Oxidative, chemical cleavage of the C-P bond in 1-amino-1-(3,4-dihydroxyphenyl)methylphosphonic acid upon the action of NaIO(4) have been the subject of the NMR, EPR and UV-Vis investigations in acidic and basic conditions.

Full text of DOI:10.1039/b401633e

First example of the chemical, oxidative cleavage of the C–P bond in
aminophosphonate chemistry. The oxidation of
1-amino-1-(3,4-dihydroxyphenyl)methylphosphonic acid by NaIO4†
Marcin Dra˜g,a Adam Jezierskib and Pawel Kafarski*a
a Institute of Organic Chemistry, Biochemistry and Biotechnology, University of Technology, Wyb.
Wyspianskiego 27, 50-370 Wroclaw, Poland. E-mail: pawel.kafarski@pwr.wroc.pl; Fax: +48 71 328 40
64; Tel: +48 71 320 36 82
b Faculty of Chemistry, University of Wroclaw, ul. Joliot-Curie 14, 50-383 Wroclaw, Poland
Received (in Cambridge, UK) 3rd February 2004, Accepted 16th March 2004
First published as an Advance Article on the web 8th April 2004
Oxidative, chemical cleavage of the C–P bond in 1-amino-
1-(3,4-dihydroxyphenyl)methylphosphonic acid upon the ac-
tion of NaIO4 have been the subject of the NMR, EPR and UV-
Vis investigations in acidic and basic conditions.
oxidation by tyrosinase, in the formation of two products: inorganic
phosphate (31P NMR) and 3,4-dihydroxybenzaldehyde (1H
NMR).4,7 It is worthy of note that this process is accompanied by a
change of pH (to 4.2 in the case of acidic solution and to 7.5 under
basic conditions). The reaction was always very fast, which is
demonstrated by the immediate change of the color of the rection
mixture (from transparent to dark brown) upon addition of
periodate to the phosphonic substrate 1. After 10 min of the reaction
there were almost only dominant signals coming from the
3,4-dihydroxybenzaldehyde 8 in the 1H NMR spectra, accom-
panied by only minute signals coming from unidentified com-
pounds. Most probably they are products of polymerization of this
aldehyde, as already observed in oxidation reactions of structurally
similar compounds.8,9 After 20 min the aldehyde 8 started to
disappear and finally only the signals resulting from the products of
its decomposition were found in the spectra. This is in contrast to
the biological oxidation of 1 by tyrosinase, which had finished with
the production of 3,4-dihydroxybenzaldehyde 8. In the 31P NMR
spectra, only the presence of phosphate was observed, demonstrat-
ing complete cleavage of the C–P bond.
In order to detect whether the reaction goes via a radical pathway
EPR measurements were done. The lack of signals in EPR spectra
during the experiment in acidic pH suggests that the observed
reaction does not have a radical character. This is in a good
agreement with observations reported for other polyphenols.
However, under basic conditions one should consider the possibil-
ity of an auto-oxidation of 1 by O2 from the air9 because a doublet
of doublets (g = 2.0048) was found in the EPR spectra (pH 8.5),
demonstrating the presence of free radicals under these conditions.
This signal remains stable even after one hour of the reaction, i.e.
time where NMR monitoring of the process showed total
decomposition of the phosphonic substrate and production of
phosphate and 3,4-dihydroxybenzaldehyde, and therefore it has
been ascribed to this aldehyde. The independent oxidation of a
commercial sample of 3,4-dihydroxybenzaldehyde with an equi-
molar amount of NaIO4 at pH 8.5 resulted in the appearance of an
identical signal (g = 2.0048). Summing up, the studied cleavage of
the C–P bond under both acidic and basic conditions has rather non-
radical in character.
In order to confirm the above observations we have perforrmed
the oxidation of 1-amino-1-(3,4-dihydroxyphenyl)methylphos-
phonic acid 1 using the one-electron oxidizing agent, CuCl2. The
addition of the oxidant to the phosphonic substrate 1 under basic
conditions resulted in the change of the solution color to bright
green, whereas in acidic conditions no such change was observed.
The NMR (1H and 31P) spectra at acidic pH showed the lack of
resonance signals (indicating the presence of paramagnetic compo-
nents of the mixture), whereas at basic pH only the phosphonic acid
substrate 1 could be observed. The EPR spectra under both pH
conditions revealed only the presence of a signal coming from
Cu(II), which seems to indicate that in the presence of CuCl2 the
oxidation of 1 did not proceed in an observable manner. The
obtained results also suggest the formation of a coordination
complex between copper ions and aminophosphonate 1, similarly
as was described earlier for this class of compounds.10
Organophosphorus compounds are of importance because of their
growing applications in medicine and agriculture, as well as use in
industry (mostly as corrosion inhibitors or plasticizers).1 The
covalent, carbon-to-phosphorus (C–P) bond present in these
molecules is considered to be extremely resistant to chemical
cleavage (hydrolytical, thermal, photochemical and oxidative) and
there are only very few reports describing its decomposition.1,2
Interestingly, microorganisms (fungi, bacteria) are able to degrade
this bond quite efficiently.1,3
Recently we have found that the copper-containing enzyme
tyrosinase (E.C.1.14.18.1), responsible for melanization in animals
and browning in plants, promotes cleavage of a C–P bond in the
phosphonic analogue 1 of 3,4-dihydroxyphenylglycine via oxida-
tion of the catecholic part of this molecule.4 This observation has
stimulated us to undertake more detailed studies on the chemical
nature of this process, in the hope that it may improve the
understanding of biological C–P bond cleavage. The observed
reaction is the first example of non-enzymatic oxidative cleavage of
the C–P bond in aminophosphonate chemistry. In contrast to the
reaction catalyzed by tyrosinase, the chemical oxidation gives the
possibility to modify the reaction conditions (by variation of pH
and type of oxidant) and to compare this process with already
known oxidative decomposition of polyphenolic amino acids.5
In this study, the oxidation of 1-amino-1-(3,4-dihydroxyphe-
nyl)methylphosphonic acid 1 was monitored by NMR (1H and 31P),
EPR and UV-Vis spectroscopy at two pH values (3.0 and 8.5). In
the NMR and EPR measurements D2O was used as a solvent (to
obtain the desired pD, NaOD was applied), whereas the appropriate
buffer was applied when using UV-Vis. Sodium periodate (NaIO4)
was chosen as the chemical oxidant because it is routinely used for
the investigation of oxidative processes in polyphenols.5,6 The
concentration of the phosphonic substrate was set at 2.3 3 1022 M.
To initiate the reaction an equimolar amount of NaIO4 (as a 9.35 3
1022 M solution) was added. In order to observe whether an
inorganic phosphate was formed, the 31P NMR measurements were
carried out without the addition of H3PO4 as a standard.
The chemical oxidation carried out both in acidic (pH = 3.0) and
basic conditions (pH = 8.5) resulted, as in the case of biological
† Electronic supplementary information (ESI) available: experimental
details, NMR, EPR and UV-Vis spectra. See http://www.rsc.org/suppdata/
cc/b4/b401633e/
1132
C h e m . C o m m u n . , 2 0 0 4 , 1 1 3 2 – 1 1 3 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

The UV-Vis spectra monitoring the sodium periodate oxidation
of 1 revealed two dominant absorption bands (at 320 and 390 nm)
for experiments carried out under basic (pH = 8) conditions, and
one absorption band (390 nm) in acidic solutions (pH = 5). Based
on the results observed during enzymatic oxidation of this
substrate, we can affirm that the band at 320 nm to be
3,4-dihydroxybenzaldehyde and the one appearing at 380 nm to be
phosphonic o-quinone 4 (Scheme 1).4 Under basic conditions the
intensity of the band at 380 nm decreased quickly and disappeared
after 5 min. The difference in time-scale between UV-Vis and
NMR spectroscopy explains why we were unable to observe signals
corresponding to o-quinone 4 by NMR. Practical lack of the
presence of the band at 380 nm shows that the reaction in acidic
conditions is faster, but this supposition was not confirmed by other
spectroscopic methods. In order to confirm the presence of this
intermediate, we have carried out the experiment in the presence of
5 equivalents of HI, a known o-quinone reducing agent. Total lack
of the reaction progress confirms the supposition that the oxidation
proceeds via o-quinone 4.
To prove that the cleavage of the C–P bond in 1-amino-
1-(3,4-dihydroxyphenyl)methylphosphonic acid 1 is preceded by
the oxidation of hydroxy groups in the aromatic ring, we have
carried out this reaction using the analogue of 1 containing methoxy
groups in place of hydroxylic ones (compound 2) and compound 3,
being a formal analogue of phenylglycine. The lack of oxidation of
these two compounds by periodate was confirmed by the fact that
only the phosphonic substrates were observed in the NMR
spectra.
Summing up, we postulate that the oxidation of 1-amino-
1-(3,4-dihydroxyphenyl)methylphosphonic acid 1 by NaIO4 pro-
ceeds by intermediate formation of phosphonic o-quinone 4, which
immediately undergoes C–P bond cleavage and via corresponding
quinone methide 5. This intermediate may isomerize to imine 6 or
react directly with water yielding compound 7. The latter
compound, upon the loss of ammonia, provides 3,4-dihydroxy-
benzaldehyde 8 (Scheme 1, D2O conditions). This mechanism is
similar to the pathway already proposed for the oxidation of
3,4-dihydroxymandelic acid and 3,4-(dihydroxyphenyl)acetic
acid.11 This is somewhat surprising since the dephosphonylation is
far more difficult than decarboxylation. Therefore, the mechanism
of C–P bond cleavage still remains an open question. We speculate
that the very probable path of the reaction relies upon dissociative
cleavage of this bond yielding the unstable metaphosphate. The
dissociative C–P bond cleavage may be expected to take place by
a preassociation mechanism: metaphosphate ion is released pre-
associated with a water molecule. The reaction probably takes place
most rapidly when a fully resonance stabilized metaphosphate may
be formed, i.e. through the dianionic phosphonic acid. This
suggestion seems to be confirmed by the fact that the benzylic
proton was not exchanged with deuterium when monitoring the
oxidation reaction by means of NMR. This observation excludes
the alternative mechanism (Scheme 1), namely transformation of
the a-amino group of 1 and the formation of intermediate a-
ketophosphonate 11 (path via compounds 9–11). The studies of this
reaction using other phosphonic and phosphinic analogues of
DOPA as substrates are in progress.
This work was supported by Komitet Badan Naukowych.
Authors would like to acknowledge Professor H. Wojtasek for
helpful discussions and Miss B. Gasowska for technical assis-
tance.
Notes and references
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3
formyl proton), 7.27 (d, 1H, JH–H = 8.2 Hz, aromat.), 7.21 (s, 1H,
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Scheme 1
C h e m . C o m m u n . , 2 0 0 4 , 1 1 3 2 – 1 1 3 3
1133