Tautomerization of α,β-and β,γ-unsaturated phosphonium salts resulting from the reaction of triphenylphosphine with β-bromomethacrylic acid. Nucleophilic addition at the γ position of the allyl group in phosphonium salt

Article abstract of DOI:10.1134/S1070363206100070

Temperature factor was found to be determining in the isomerization of 2-carboxyprop-1-en-1-yl-and 2-carboxyprop-2-en-1-yl(triphenyl)phosphonium bromides. Elevated temperature favors formation of isomer with the double bond in the β,γ position with respec

Full text of DOI:10.1134/S1070363206100070

ISSN 1070-3632, Russian Journal of General Chemistry, 2006, Vol. 76, No. 10, pp. 1550 1553.
Original Russian Text R.Dzh. Khachikyan, N.V. Tovmasyan, G.A. Panosyan, M.G. Indzhikyan, 2006, published in Zhurnal Obshchei Khimii, 2006,
Vol. 76, No. 10, pp. 1617 1620.
Pleiades Publishing, Inc., 2006.
Tautomerization of , - and , -Unsaturated Phosphonium
Salts Resulting from the Reaction of Triphenylphosphine
with -Bromomethacrylic Acid. Nucleophilic Addition
at the Position of the Allyl Group in Phosphonium Salt
R. Dzh. Khachikyan, N. V. Tovmasyan, G. A. Panosyan, and M. G. Indzhikyan
Institute of Organic Chemistry, National Academy of Sciences of Armenia,
ul. Zakaria Kanakertsi 167A, Erevan, 375091 Armeniya
fax: (374+1) 28-35-11
e-mail: nazotov2004@mail.ru
Received December 30, 2005
Abstract Temperature factor was found to be determining in the isomerization of 2-carboxyprop-1-en-1-yl-
and 2-carboxyprop-2-en-1-yl(triphenyl)phosphonium bromides. Elevated temperature favors formation of
isomer with the double bond in the , position with respect to the phosphonium group. Alkaline hydrolysis at
room temperature promotes the reverse isomerization. The isomerization of 2-carboxyprop-1-en-1-yl(tri-
phenyl)phosphonium bromide is hampered by addition of hydrobromic acid, as well as by carrying out its
synthesis in the presence of aqueous HBr. Alkaline hydrolysis of 2-carboxyprop-1-enyl(triphenyl)phosphonium
bromide and (E)-(2-carboxyvinyl)triphenylphosphonium chloride is accompanied by phenyl group migration
to the -position with formation of 2-methyl-3-(diphenylphosphoryl)-3-phenylpropionic acid and 3-(diphenyl-
phosphoryl)-3-phenylpropionic acid, respectively. The possibility for nucleophilic addition at the position of
the allyl group in 2-carboxyprop-2-enyl(triphenyl)phosphonium bromide was demonstrated using the reaction
with triphenylphosphine as an example.
DOI: 10.1134/S1070363206100070
We previously found that -bromomethacrylic acid
reacts with triphenylphosphine in benzene at room
temperature to give 2-carboxyprop-1-en-1-yl(tri-
phenyl)phosphonium bromide (I) in almost quantita-
tive yield. When the reaction was carried out in boil-
ing acetonitrile, we isolated a mixture of two salts,
one of which was isomeric 2-carboxyprop-2-enyl(tri-
phenyl)phosphonium bromide (II, yield 40%). In
order to elucidate the structure of the second product,
the isolated mixture was subjected to alkaline hydro-
lysis by the action of a 16% solution of NaOH in boil-
ing acetonitrile. The hydrolysis products were tri-
phenylphosphine oxide and 3-(diphenylphosphoryl)-
2-(diphenylphosphorylmethyl)propanoic acid (65 and
34%, respectively).
These data led us to presume that the compound
formed together with 2-carboxyprop-2-en-1-yl(tri-
phenyl)phosphonium bromide (II) is 2-carboxypro-
pane-1,3-diylbis(triphenylphosphonium)
dibromide
(III) arising from nucleophilic addition of triphenyl-
phosphine to salt II.
+
+
(C6H5)3P + BrCH=CCOOH
CH3
(C6H5)3P CH=C COOH
Br
(C6H5)3P CH2 C COOH
Br
CH2
CH3
I
II
(C6H5)3P
+
+
+
HBr
(C6H5)3P CH2 C COOH
(C6H5)3P CH2 CH CH2 P(C6H5)3
+
Br
Br
Br
COOH
CH2 P(C6H5)3
III
1550

TAUTOMERIZATION OF , - AND , -UNSATURATED PHOSPHONIUM SALTS
1551
The source of HBr may be either salt II or, what
is more probable, initial -bromomethacrylic acid.
We previously obtained data indicating elimination of
hydrogen bromide and formation of carbene from
-bromomethacrylic acid by the action of triethyl-
amine and tris(dimethylamino)phosphine [1]. In keep-
ing with the above scheme, we found that heating of
phosphonium salt II with triphenylphosphine in boil-
ing hydrobromic acid yields 75% of salt III. Alkaline
hydrolysis of the latter gave 67% of 3-(diphenylphos-
phoryl)-2-(diphenylphosphorylmethyl)propanoic acid
(IV).
+
+
OH
(C6H5)3P CH2 CH CH2 P(C6H5)3
(C6H5)2P CH2 CH CH2 P(C6H5)2.
Br
Br
COOH
COOH
IV
O
O
III
Thus we have demonstrated the possibility for
addition of triphenylphosphine at the position of the
allyl group in quaternary phosphonium salt.
An analogous result was obtained by heating
-bromomethacrylic acid and triphenylphosphine
hydrobromide in boiling acetonitrile. No isomeriza-
tion of I into II was observed when the synthesis was
performed in the presence of hydrobromic acid. It
should be noted that we failed to reproduce the results
described in [2]. In all cases, the reaction of triphenyl-
phosphine with prop-2-yn-1-yl bromide gave a mix-
ture of the corresponding allene and -acetylenic
isomers.
We presumed in [1] that salt II is formed on heat-
ing in boiling acetonitrile as a result of unusual migra-
tion of the double bond in the primary product, salt I,
from the
, to , position by the action of triphe-
nylphosphine. In the present work, we observed the
same isomerization on heating of salt I in boiling
acetonitrile or water in the absence of triphenylphos-
phine. In both cases, the yield of II was 70%; and
the remaining amount of salt I was recovered from the
reaction mixture.
While studying alkaline hydrolysis of salt II at
room temperature, we revealed the reverse isomeriza-
tion, i.e., II I. Treatment of salt II a 10% solution
of sodium hydroxide gave 60% of 3-(diphenylphos-
phoryl)-2-methyl-3-phenylpropionic acid (V, product
of -migration of the phenyl group) and 36.8% of tri-
phenylphosphine oxide. We failed to isolate a phos-
phorus-free product which should be formed together
with triphenylphosphine oxide. Compound V was also
obtained in 62% yield from salt I under analogous
conditions. When the alkaline hydrolysis was carried
out at the boiling point, the yields of compound V and
triphenylphosphine oxide were, respectively, 46 and
53% from salt I and 10 and 60% from salt II.
According to published data [2], pure triphenyl-
(prop-2-yn-1-yl)phosphonium salt is formed from tri-
phenylphosphine and prop-2-yn-1-yl bromide in the
presence of aqueous HBr. Although no reasonable
explanation was given in [2], we believe that the role
of hydrobromic acid is to suppress the prototropic
isomerization. In fact, when -bromomethacrylic acid
was heated in a boiling solution of triphenylphosphine
in hydrobromic acid we isolated salt I as the only
product.
+
+
CH3CN or H2O,
(C6H5)3P CH=C COOH
(C6H5)3P CH2 C COOH
Br
Br
CH3
CH2
I
II
OH ,
OH ,
room temperature
room temperature
C6H5
(C6H5)2P CH CH COOH + (C6H5)P=O
CH3
O
V
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006

1552
KHACHIKYAN et al.
Migration of the phenyl group in salt I seems to be
the sp2-hybridized carbon atom neighboring to the
carboxy group). The hydrolysis with 10% sodium
hydroxide at room temperature involved mainly
-migration of the phenyl group to give 3-(diphenyl-
phosphoryl)-3-phenylpropionic acid (VII) (58%), and
triphenylphosphine oxide was formed as minor pro-
duct (13%). When the hydrolysis was carried out at
elevated temperature, 29% of compound VII and 15%
of triphenylphosphine oxide were obtained, while no
products resulting from elimination of phenyl group
or -elimination were detected.
somewhat surprising, taking into account hypercon-
jugation with the methyl group. Our results indicate
the crucial role of temperature factor in the isomeriza-
tion of phosphonium salts I and II.
We also examined alkaline hydrolysis of (E)-(2-
carboxyvinyl)triphenylphosphonium chloride synthe-
sized previously [1]. In this case, apart from concur-
rent paraffin cleavage and phenyl group migration,
-elimination is possible (involving hydrogen atom at
(C6H5)2P CH=CH COOH
O
(C6H5)3P=O + [CH2=CH COOH]
+
(C6H5)3P CH=CH COOH OH
(C6H5)2P CH CH2 COOH
Cl
VI
O C6H5
VII
HC C COOH + (C6H5)3P
EXPERIMENTAL
Cm), 130.90 (Cp), 132.93 d (1JPC = 98.3) and 133.14 d
(1JPC = 98.3, Ci), 173.52 t (3JPC = 8.7, CO). Found, %:
C 68.2; H 5.1; P 12.5. C28H26O4P2. Calculated, %: C
68.9; H 5.3; P 12.7.
The H, 31P, and 13C NMR spectra were recorded
1
on a Varian Mercury-300 spectrometer at 300.08,
121.75, and 75.46 MHz, respectively (303 K). The
chemical shifts were measured relative to tetramethyl-
silane (1H and 13C, internal reference) and orthophos-
phoric acid (31P, external reference).
An analogous experiment was carried out by heat-
ing in 0.78 ml of 20% sodium hydroxide. We isolated
0.23 g (88%, calculated on salt II) of triphenylphos-
phine oxide and 0.27 g (71%, calculated on salt III)
of 3-(diphenylphosphoryl)-2-(diphenylphosphoryl-
Alkaline hydrolysis of a mixture of salts II and
III. A mixture of salt II and unidentified product III
(obtained as described in [1]), 1 g and 1.2 ml of a
16% solution of sodium hydroxide in acetonitrile was
heated for 5 10 min at the boiling point. The mixture
was poured into water, and the precipitate was filtered
off and thoroughly washed with water. We thus iso-
lated 0.17 g (65%, calculated on salt II) of triphenyl-
phosphine oxide with mp 154 C. The aqueous filtrate
was acidified, and the precipitate was filtered off,
washed with water until neutral reaction, and dried
under reduced pressure. Yield of compound IV 0.13 g
(34%, calculated on salt III), mp 230 231 C. 1H
NMR spectrum (DMSO-d6 CCl4, 1:3), , ppm, (J,
Hz): 2.59 2.79 m (3H, CHCH2), 2.88 2.99 m (2H,
CH2), 7.39 7.51 m (12H) and 7.64 7.79 m (8H,
C6H5), 12.4 br.s (1H, COOH). 31P NMR spectrum
1
methyl)propanoic acid (IV). The H NMR spectrum
of IV coincided with that of a sample obtained as
described above.
2-Carboxypropane-1,3-diylbis(triphenylphos-
phonium) dibromide (III). A mixture of 0.65 g of
2-carboxyprop-2-en-1-yl(triphenyl)phosphonium bro-
mide (II), 0.398 g of triphenylphosphine, and 0.37 g
of 42% hydrobromic acid was heated for 5 h on a
water bath. The mixture was extracted with chloro-
form, the extract was poured into diethyl ether, and
the precipitate was filtered off and thoroughly washed
with diethyl ether. Yield 0.87 g (87%), mp 204
1
205 C. H NMR spectrum (DMSO-d6 CCl4, 1:3), ,
2
ppm, (J, Hz): 2.83 m (1H, CH), 4.15 d.t (2H, J =
2
3
2
14.8, JPH = JHH = 11.1, CH2), 4.59 t (2H, J = 14.8,
2JPH = 14.8, CH2), 7.60 m (12H) and 7.71 7.79 m
(18H, C6H5). 31P NMR spectrum (DMSO-d6 CCl4,
1:3): P 25.2 ppm. Found, %: P 7.8; Br 20.7. C40H36
O2Br2P2. Calculated, %: P 8.0; Br 20.8.
(DMSO-d6 CCl4, 1:3):
34.3 ppm. 13C NMR spec-
P
trum (DMSO-d6 CCl4, 1:3), C, ppm, (J, Hz):
3
31.10 d.d (1JPC = 71.0, JPC = 7.0, CH2), 33.92 t
(2JPC = 2.8, CH), 127.96 d (JPC = 12.1), 128.02 d (JPC
=
3-(Diphenylphosphoryl)-2-(diphenylphospho-
rylmethyl)propanoic acid (IV). A mixture of 0.5 g
11.5), 130.24 d (JPC = 9.4), 130.36 d (JPC = 8.8, Co,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006

TAUTOMERIZATION OF , - AND , -UNSATURATED PHOSPHONIUM SALTS
1553
3
of salt III and 0.08 g of a 20% solution of sodium
hydroxide was heated for 10 min at the boiling point.
The mixture was poured into water and acidified, and
the white crystals were filtered off and washed with
water until neutral washings. Yield 0.2 g (67%), mp
230 231 C. The analytical data and H NMR spec-
trum coincided with those given above.
1:3), , ppm, (J, Hz): 1.03 d (3H, J = 7.0, CH3);
3
3
3.27 d.d.q (1H, J = 10.1, 7.0, JPH = 7.3, CHCH3);
3
2
4.05 d.d (1H, J = 10.1, JPH = 6.4, PCH); 6.94
7.07 m (3H), 7.09 7.18 m (3H), 7.28 m (2H), 7.43
7.55 m (5H), and 8.09 m (2H, C6H5); 11.80 br.s (1H,
COOH). 31P NMR spectrum (DMSO-d6 CCl4, 1:3):
P 34.8 ppm. Found, %: C 72.1; H 5.4; P 8.1. C22H20
O2BrP. Calculated, %: C 72.5; H 5.8; P 8.5. An
analogous experiment was performed using a 10%
solution of sodium hydroxide at room temperature
(reaction time 15 min). The yields of triphenylphos-
phine oxide and compound V were 36.8 and 60%,
respectively.
1
Isomerization of salt I into II. Salt I, 0.65 g, was
heated for 60 h in boiling acetonitrile. The precipitate
was filtered off and washed with acetonitrile. Yield
1
of salt II 0.45 g (70%), mp 227 228 C. H NMR
spectrum (DMSO-d6 CCl4, 1:3), , ppm, (J, Hz):
2
4
4.87 d (2H, JPH = 15.8, PCH2), 5.97 d.d (1H, JPH
=
2
4
2
5.3, J = 0.8) and 6.38 d.d (1H, JPH = 5.3, J = 0.8,
=CH2), 7.70 7.77 m (6H) and 7.82 7.91 m (9H,
C6H5), 12.0 br.s (1H, COOH). 31P NMR spectrum
Alkaline hydrolysis of salt I. A mixture of 0.77 g
of salt I and a 20% solution of sodium hydroxide
(0.144 g) was left to stand for 4 h at room temperature.
The mixture was treated as described above to isolate
0.4 g (62%) of compound V with mp 226 227 C.
When a mixture of 1.7 g of salt I and 0.32 g of so-
dium hydroxide (a 20% solution) was heated for
10 min at the boiling point, we isolated 0.58 g (53%)
of triphenylphosphine oxide and 0.45 g (46%) of
compound V.
(DMSO-d6 CCl4, 1:3):
28.3 ppm. 13C NMR spec-
trum (DMSO-d6 CCl4, 1:P3), C, ppm (J, Hz): 24.58 d
(1JPC = 50.7, CH2), 117.77 d (1JPC = 86.0, Ci), 129.13
d (3JPC = 9.7, =CH2), 129.60 d (JPC = 12.6) and
133.72 d (JPC = 10.0, Co, Cm), 132.55 d (2JPC = 9.7,
=C), 134.48 d (4JPC = 3.0, Cp), 166.81 d (3JPC = 2.7,
CO). Salt II was also obtained in a similar yield by
heating 0.5 g of salt I in boiling water. In both cases,
25% of unisomerized salt I was recovered.
Alkaline hydrolysis of (E)-(2-carboxyvinyl)tri-
phenylphosphonium chloride (VI). A mixture of
0.48 g of salt VI and 0.1 g of NaOH (a 10% solution)
was left to stand for 2 h at room temperature. The
mixture was then treated as described above to isolate
0.05 g (13%) of triphenylphosphine oxide, mp 154 C,
and 0.26 g (58%) of compound VII, mp 198 199 C.
1H NMR spectrum (DMSO-d6 CCl4, 1:3), , ppm,
Reaction of -bromomethacrylic acid with tri-
phenylphosphine in hydrobromic acid. A mixture
of 0.8 g of -bromomethacrylic acid, 1.3 g of tri-
phenylphosphine, and 0.12 g of 42% hydrobromic
acid was heated for 5 h on a water bath. The mixture
was extracted with chloroform, the extract was poured
into diethyl ether, and the white crystals were filtered
off and thoroughly washed with diethyl ether. Yield of
salt I 1.63 g (75%) [1].
2
3
3
(J, Hz): 2.61 d.d.d (1H, J = 16.3, JPH = 9.4, J =
2
3
3
3.3) and 2.86 d.d.d (1H, J = 16.3, J = 11.4, JPH
=
3
2
3
5.6, CH2); 4.18 d.d.d (1H, J = 11.4, JPH = 7.7, J =
3.3, PCH); 7.05 7.16 m (3H), 7.22 7.35 m (5H),
7.52 7.58 m (5H), and 7.98 m (2H, C6H5). 31P NMR
Reaction of -bromomethacrylic acid with tri-
phenylphosphine hydrobromide. A mixture of
0.024 g of -bromomethacrylic acid and 0.05 g of tri-
phenylphosphine in acetonitrile was heated for 5 h at
the boiling point. The mixture was poured into diethyl
ether, and the precipitate was filtered off and thorough-
ly washed with diethyl ether. Yield of salt I 0.025 g
(42%) [1].
spectrum (DMSO-d6/CCl4, 1:3):
36.3 ppm. 13C
NMR spectrum (DMSO-d6 CCl4, 1P:3), C, ppm, (J,
Hz): 34.31 (CH2), 41.53 d (1JPC = 67.6, CH), 126 136
(Carom), 171.38 d (3JPC = 17.2, CO). Found, %: C
70.0; P 8.8. C21H19O3P. Calculated, %: C 72.0; P 8.9.
In an analogous experiment performed on heating at
the boiling point, we isolated 28% of VII and 15% of
triphenylphosphine oxide. In both cases, unreacted
initial salt was also isolated.
Alkaline hydrolysis of salt II. A mixture of 0.4 g
of salt II and 0.074 g of a 20% solution of sodium
hydroxide was heated for 5 10 min at the boiling
point. The mixture was poured into water, and the
white crystals were filtered off and washed with water
until neutral washings. We thus isolated 0.16 g (60%)
of triphenylphosphine oxide with mp 154 C. The
aqueous filtrate was acidified, and the precipitate was
filtered off and washed with water until neutral
washings. Yield of compound V 0.02 g (10%), mp
226 227 C. 1H NMR spectrum (DMSO-d6 CCl4,
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006