MASS SPECTROMETRY OF 1-PHENYL-1,2-DIBROMOPROPYLPHOSPHONIC ACID AND ITS METHYL ESTERS

Article abstract of DOI:10.1016/S0040-4020(01)96880-2

erythro-1-Phenyl-1,2-dibromopropylphosphonic acid is known to decompose rapidly into the monomeric metaphosphate anion(PO3(1-)), 1-phenyl-1-bromopropene, and bromide ion in protic or aprotic solvents in the presence of a hindered tertiary amine.The present study compares the behavior of this phosphonic acid in solution and in the gas phase.The gas-phase thermolysis of the acid involves mainly the loss of bromine and of the phosphoryl group, -P(O)(OR)(OR') (R=R'=H).The same type of pathway is followed in the ionization/excitation processes ("cationic reactions") that follow from electron impact in mass spectrometry.The thermal and cationic reactions of the monomethyl and dimethyl esters of the phosphoric acids (R=H, R'=CH3 and R=R'=CH3, respectively, in -P(O)(OR)(OR')) are entirely analogous to those of the free dibasic acid.

Full text of DOI:10.1016/S0040-4020(01)96880-2

+
Ltd.
Tetrahedron Vol. 40, No. 14, pp. 2633 to 2642,
Printed in the U.S.A.
MASS SPECTROMETRY OF
AND ITS METHYL ESTERS
ACID
SEYMOUR
and EUGENE S. KUHN
Research Department, Standard Oil Company (Indiana),
Box 400, Naperville, IL 60566
F AU S T0
and
F.
Department of Chemistry, State University of New York at Stony Brook,
Stony Brook, NY 11794
(Received USA 29 September 1983)
erythro-1-Phenyl-1,2-dibromopropylphosphonic acid-is known to
Abstract --
decompose rapidly into the monomeric metaphosphate anion
and bromide ion in
1-phenyl-1-bromopropene,
the presence of a hindered tertiary amine.
or aprotic solvents in
The present study compares the
beh a vior of t h is
ga s-ph a se
of t h e ph osph or yl gr ou p,
acid in solution and in the gas phase. The
of the acid involves mainly the loss of bromine and
The same type of
(R R’ H).
pa t h wa y is followed in t h e ion iza t ion /excit a t ion pr ocesses (“cationic
Th e
reactions”) that follow from electron impact in mass spectrometry.
thermal and
ph osph or ic a cids (R
reactions of the monomethyl and
H, and R
a r e en t ir ely a n a logou s t o t h ose of t h e fr ee diba sic a cid.
esters of the
CH ,, r espect ively, in
We h a ve st u died t h e beh a vior of er yt h r o-1-ph en yl-1,2-dibr om opr opylph osph on ic a cid,
and its
mono- and
esters,
and DHBP, in the gas phase by the technique of electron-impact mass
We have focused attention on both
spectrometry.
the reactions that precede and those that follow
ion iza t ion /excit a t ion by elect r on im pa ct ,
on
both purely thermal and
r ea ct ion s. We
O R ’
reactions of
find that the gas-phase thermal and
MBPH, and DMBP are all very similar, and result mainly in the loss of bromine
atoms and of the phosphoryl group,
r a dica l-ca t ion .
from the molecule or the molecular
The present study reveals the inherently different pathways that are available to molecules of
the type of the
acid,
and its methyl esters,
and DMBP, in the
u n der goes r a pid
gas phase and in solutions.
Previous research has shown that the free acid,
loss of the monomeric metaphosphate anion in solution in aprotic solvents according to equation (1)
=
diisopr opylet h yl a m in e).’
Under the same experimental conditions, the monomethyl ester
The ester
and related systems have been
decomposes into monomeric methyl metaphosphate, according to equation
under similar conditions is stable.
Other reactions of
2-9
descr ibed in t h e lit er a t u r e.
+
+
+
0 ’
2633

2634
S.
et al.
=
+
+
+
Haterials.
(Scheme I,
R
R'
H),
39 Hz (in
0), was prepared by the method of
gave
positive chemical shifts are to the low-field side of
and Westheimer.
acid
Bromination of
in chloroform
from
at
R
R'
H; mp
Reaction of the acid with diaxomethane in ether/methanol at
provided the
ester
R
R'
as an oil.
R' = H; mp
was kept at
from
The monomethyl ester
R
was obtained when an acetone solution of the diester and
by acidification of an aqueous solution of the salt.
for 2 days, followed
Electron-impact mass spectra were measured on a CEC Model
instrument
were injected directly into the
source via probe, and the spectrum was scanned repeatedly as the probe temperature was gradually
raised.
with electron energy of 70
and the source at
In each case volatilization occurred almost solely during one or two such scans. Spectra
were recorded in digital form via the AEI
data system at a resolution of 10,000, sufficient
Metastable scans, to define
to allow unequivocal identification of elemental compositions.
decomposition paths,
were made by focusing on a selected fragment ion, decoupling the
10
and scanning the latter to increasing values.
sector voltage from the ion-accelerating voltage,
In view of the propensity of phosphate esters and related species to undergo thermal reactions,11
11
we scrutinized the data closely for clues to identify products of such reactions.
RESULTS
We have omitted peaks due to hydrocarbon
Table I shows the spectra of the three bromophosphonates.
+
ions smaller than
those containing
and to naturally occurring heavy-isotopic ion contributions other than
The metastable peaks observed, coupled with ion
as the only heavy isotope.
formulas inferred from high-resolution exact-mass measurements, furnished the data needed to
construct networks, Reaction steps
broken arrows identify steps deemed
which are displayed in Figures 1, 2 and 3.
estastable peaks;
denoted by solid arrows are supported by
q
probable but not so supported. In a few instances, energy resolution was not good enough to allow
Thus, in DMBP, the fragment ions of masses
or of or both; and in the fragment
In each such case, we have
a clear choice between two possible precursors.
and 226 may derive from precursor of mass
ion of mass 117 may derive from precursor of mass 211 or 212 or both.
Parentheses around an ion formula indicate
included both possibilities in the pertinent figure.
that no normal peak for that ion was detected;
the probable formula in such a case is deduced from
the associated daughter-ion formula(s) and the nominal mass difference(s) between parent- and
daughter-ion masses.
(i) purely thermal
The spectra can be interpreted in terms of two types of reactions:
decompositions,
and (ii) ionization/dissociation by electron impact on the initial molecules and
whether solely ionic or a mix
None of the
their thermal decomposition products.
The dominant reaction paths,
of thermal and ionic,
are those that effect loss of the phosphoryl groups and bromine.
spectra shows a molecular-ion peak.
However, the DMBP and MBPH data define fragment-ion
compositions and reaction paths that, together, leave little doubt that the bulk of the ion yield
+
a
q
etastable scan of the
arises from the respective parent compounds. Even in BPH,,
(mass 197) peak identifies as a precursor
(mass
which most likely arises by

S.
et al.
2636
loss of a bromine atom from the
molecular ion; hence some
apparently volatilized and
The most
remained intact long enough to undergo ionization/dissociation by electron impact.
abundant products in all three spectra are
hydrocarbon ions.
we show in Scheme I the three staggered
and the unsaturated compounds,
To facilitate description of possible reactions,
conformations,
E-I-phenyl-1-bromopropene,
would result from elimination reactions via trans-coplanar transition states
and
of the
and the Z-1-phenyl-2-bromopropenylphosphonate derivatives,
mechanism).
decompositions occurring under the conditions of our
that
-
-
Evidently, the thermal and
hence, product stereochemistry.
experiments need not conform to this concerted mechanism and,
Molecular models suggest that conformers
and
are very crowded, and there seems to be
considerable steric hindrance to rotation around the single C-C bond that is needed to interconvert
the conformers.
models.
It is virtually impossible to rotate the phenyl ring around its C-C bond in the
H
Q
‘OR
4
phenyldibromopropylphosphonate.
Figure 1 shows a proposed partial decomposition network
under electron impact for DMBP and associated thermal reaction products.
The data show two well
loss of a bromine atom and
and of the phosphoryl
defined primary processes in decomposition of the DMBP molecular ion:
loss of the phosphoryl radical.
These same processes, and losses of
group in conjunction with either a hydrogen or a bromine atom,
secondary and tertiary reaction steps.
comprise a large part of the
The data reveal also contributions from chemical species other than DMBP. Metastable scans of
the
(mass 226) and
(mass
peaks identify a
precursor of
mass 3061308, which we picture as the molecular ion of
the molecule is most probably formed by thermal loss of
phenylmonobromopropylphosphonate;
from DMBP followed by a
(mass 212) peaks probably
abstraction step.
indicate a little
The weak
(mass 211) and
(see below), possibly present as an impurity from the synthesis.
Additional
molecular species apparently present in the vapor in the ionization chamber are:
mol wt presumably dimethoxyphosphoryl bromide, possibly formed by coupling of a bromine
atom and a dimethoxyphosphoryl radical;
mol wt 224, presumably formed by loss of
of which supports the suggested reaction
2BBr from DMBP; and
mol wt
(see below).
the
path leading to
The only metaphosphate species indicated in this spectrum is
with
the neutral product formed
mass 117) from the
fragment ion
(mass 211).

Mass spectrometry of
acid
2631
H 0
0
3
224
\
\
q
,’
\
.
.
.
.
Phenyldibromopropylphosphonic acid.
electron impact for
Figure 2 shows a proposed partial decomposition network under
and associated thermal reaction products.
274
q
,
\
197
\
2.

Except for a small peak at mass 540 the spectrum obtained from BPH,
shows nothing at mass values
beyond the prominent grouping at
Precise-mass measurements on these peaks identify
the major contributing ions as containing phosphorus but no bromine,
accompanied by ions retaining
one bromine atom but free of phosphorus.
Formation of the corresponding neutral molecules from
BPH, by thermal processes is pictured in equations (3)-(5).
Metastable scans of the peaks of masses 196 and 198 reveal precursors of masses 374, 376, and
378. These masses can be accounted for by loss of
units each of mass 196 or 198.
of the phosphorus-containing species of masses 196 and 198, as shown in equations
The four metastable peaks observed in these
from the three possible combinations of two
We therefore assign them to anhydrides -- pyrophosphonates --
q
C
q
scans are very weak,
decompositions of
suggesting that
and make only a small
5, mol
196
contribution to the ion yields at 196 and 198.
(mass 196) and
Probably the bulk of
(mass 198) are the molecular ions of
thermal decomposition products which arise,
Br;
6,
respectively, by loss of 2HBr and
as shown in equations Possible
mechanisms for these reactions will be suggested
that also
appears at masses 196 and 198 may arise by thermal
from BPH,
=
3.
in the Discussion section. The
loss of
1-propenyl bromide
ionization.
between this sequence and an alternative one in
which and are lost successively from
and
from BPH, to yield
(Scheme I), followed by
The data do not permit a distinction
OH
OH
the molecular ion of BPH,.
The compound of mol
540 may be the cyclic
as shown in
triphosphonate
derived from the
I-propenylphosphonic acid
equation (9). Exact-mass measurement supports the
and hence is consistent
peaks at
composition
with this structure.
Metastable scans of the
0
CH
As in previous
masses
reveal no precursors.
11
378
work,
we take this negative finding as evidence
that the ion formed via electron impact on
derived from a thermal reaction, such as (3).
0
0
II
In Figure 2,
the formation of
(mass 198) by
0
0
(mass 196) and
0
”
decomposition of the molecular ions of the
symmetrical allenephosphonic anhydride
mixed allenepropenylphosphonic anhydride
the
and
the symmetrical propenylphosphonic anhydride 9 implies loss as neutral products of three-coordinate
phosphorus compounds, derived from phosphonates in the same way that metaphosphates are derived
from orthophosphates. Similarly, the loss of metaphosphoric acid as a neutral product is implicit
in the decomposition step
(mass 197)
+ (mass 117).
Monomethyl phenyldibromopropylphosphonate.
in the case of the phosphoacetoins,11
The data on this ester are summarized in Figure 3. As
the spectrum of the monomethyl ester exhibits the most

Mass
of
acid
2639
0
,
-HE,
It shows DMBP, suggesting, by analogy with monomethyl
complex overall chemistry of the three
phosphoacetoin, a disproportionation,
which may occur via a
intermediate as pictured in
equation
although we are not able to confirm the accompanying formation of We take
mol wt
and
C
mol wt
to be the monomethyl and
monobromophosphonates, probably arising, as
described above, loss of a bromine atom from
MBPH and DMBP, respectively, followed by hydrogen
0
OCH,
DMBP
+
HO-P
abstraction.
the
mol
and
210, is apparently
in the
OH
of
and is presumably formed by loss of 2HBr from
respectively,
spectra obtained from DMBP and
MBPH; this reaction is also the probable source of the
observed.
ion, mass 406, is equivalent to that of
The elemental composition of the
mass 212, less
Hence it would seem most readily accounted for as the molecular
ion of a pyrophosphonate arising by condensation/dehydration of two molecules that in turn form by
loss of from MBPH. If so, the observed intensity at mass 212 is most likely due in part to
the molecular ion of this debromination product and only in part to a fragment ion from a sequence
in which loss of the two bromine atoms follows ionization of MBPH, as shown in the figure.
composition mol wt 126, denotes phosphate. If the disproportionation alluded to
above proceeds via the suggested MBPH dimer, this dimer would seem to be a potential precursor
well of the phosphate. The dimer may also be a precursor of which, like
phosphate, we find only in the spectrum of MBPH.
A series of normal mass spectra interspersed as controls among the metastable scans revealed
pronounced changes in the spectrum,
The
accompanied by an increase in vapor pressure, at a probe tem-
perature of about 200°C.
but that at
Intensity at
attributed to DMBP, remained essentially constant,
and new peaks grew in at
attributed to MBPH, dropped sharply,
with an isotopic intensity profile matching that expected for a dibromo compound.
The apparent
bromine content and molecular weight are plausibly attributed to 1-phenyl-1,2-dibromopropene,
corresponding to thermal loss from MBPH of the phosphoryl radical plus a hydrogen atom, presumably
as monomethyl phosphite.
In accord with this suggestion,
the spectrum obtained from MBPH, unlike

2640
S.
et al.
those from
and BPH,,
contains a peak for
(mass
the composition of which
corresponds to the molecular ion of monomethyl phosphite.
The network of Figure 3 shows two ionic reaction steps in which metaphosphate species are
eliminated as neutral products, namely, the loss of from
produce (mass and the loss of from
The latter appears to be a precise counterpart of the loss of
+ (mass
to
(mass 211) to produce
+
(mass 117).
(mass 197) to produce
from
(mass 117) in the decomposition of
as shown in
Figure 2.
DISCUSSION
The relative contributions of thermal and
reactions range from predominantly thermal in
The greater complexity of the
the acid BPH, to predominantly
in the neutral ester DMBP.
In the
spectrum of the monomethyl ester MBPH reflects its intermediate position on this scale.
11
bromophosphonates, as well as in the phosphoacetoins,
the presence or absence of acidic hydrogens
determines the temperature at which the compounds volatilize and also the pathways by which they
react at a given temperature.
the products of thermal reactions fall into two categories:
In terms of their origins,
(a) those formed by reaction sequences or concerted processes initiated by loss of a bromine atom,
and (b) those formed in processes that involve
a phosphoryl radical,
or a phosphite;
dimerization, trimerization, or disproportionation. The dimers in the (b) category may be
In
reaction paths, the
These are
pyrophosphonates or oxyphosphoranes with 5-coordinate phosphorus.
main primary steps are loss of a bromine atom and loss of a phosphoryl radical.
by further losses of one or the other of these same species, free or
followed, for the most part,
in combination with a hydrogen atom.
events, the thermal and
If we limit-ourselves to consideration of
parallel each other closely.
reactions of the
We have identified a few reaction steps, both thermal and cationic, that give rise to
metaphosphate-type products, but such processes seem to play no more than a small part in the
entire reaction networks.
In this respect, these reactions differ sharply from reactions of
In solution, acidic groups on the BP phosphorus atom are subject to
and MBPH in basic solutions.
deprotonation to form anions, which then dissociate readily to yield metaphosphate species, as
shown in equations (1) and (2).
In contrast, the neutral molecules and their corresponding
radical-cations whose behavior we have now examined react by pathways dictated by different driving
forces.
In the gas-phase thermal reactions the primary bond scission occurs at a carbon-bromine bond or,
to a lesser extent,
In both cases, we assume a
a carbon-phosphorus bond, as shown in Scheme II.
Scheme II.

2641
Mass spectrometry of
acid
homolytic cleavage of the bond with formation of a resonance-stabilized
radical. The
driving force for the subsequent loss of a second bromine atom or phosphoryl radical seems to be
An interaction between
associated with the formation of a styrene derivative or an a-bromocarbene.
the carbenoid center and the a-bromo substituent, which is represented as a neutral cyclic
12
may function to stabilize the
and to facilitate the
centered bronane or
interconversion between
depicted in Scheme II,
A second type of thermal reaction, also
and styrene structures.
is the primary loss of
The product is, presumably,
E-1-phenyl-2-bromopropylphosphonic acid or the corresponding methyl ester, E-4, an isomer of the
olefin, that would be obtained in a base-catalyzed dehydrobromination in solution
The postulated gas-phase reactions are shown in Scheme III. Again, the primary bond
scission occurs at a carbon-bromine bond and, to a lesser extent, at the carbon-phosphorus bond.
PRODUCTS
OR
N
OW the intermediate is a carbocation represented in its usual cyclic and acyclic resonance
structures. These steps are assumed to be heterolytic.
atom or a phosphoryl radical would produce the radical-cations corresponding to the styrene
derivatives shown in Scheme II.
The subsequent loss of a second bromine
Scheme III also suggests reasonable sites for charge localization
in the BP molecular ion. Charge localization in a nonbonding orbital of either bromine atom is
presumed to trigger the loss of that atom. The respective carbon-bromine bond cleavages result in
the resonance-stabilized carbocation
bromonium ion,
in which the distinction between the two
Charge localization in a non-bonding
sites considered for the initial ionization has vanished.
orbital of the doubly bonded phosphoryl oxygen would set the stage for the formation of the
observed bromostyrene radical-cation via successive losses of the phosphoryl radical and a bromine
atom.
phosphorus, as also shown in Scheme III.
molecular ion fitting this description would logically involve the cleavage of a phosphorus-oxygen
or a carbon-oxygen bond.
In conclusion, the observed loss of a phosphoryl radical in thermal, as well as cationic,
Finally,
charge localization is also possible in an alkoxy (or hydroxy) group attached to
The products to be expected from decomposition of a
Such products do not appear in any of the spectra.
decomposition of the
suggests that such a radical, rather than a
monomeric metaphosphate species,
may also be the intermediate in the formation of phosphorylated
2
products when
and
in solution are heated in a sealed tube.

S.
et al.
2642
The work done at State University of New York was supported by Grant CHE-79-04985
from the National Science Foundation (to F. R.).
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