Thermal decomposition of triphenylphosphonium alkyl ester salts

Article abstract of DOI:10.1080/10426500701613030

In thermolyses of molten triphenylphosphonium alkyl ester bromides and chlorides, alkyl = methyl, ethyl, isopropyl, at 130 and 225°C, initial attack of the halide ion on the methyl group gives the methyl halide and ylid 1, Ph3P = CH2, which can be methylated, or is protonated by the phosphonium salt with transylidation giving Ph3P+-CH3X-, X = Br, Cl. The initial reactions of the ethyl or isopropyl esters are with the halide ion, X-, as a base giving ylid, 1, which can be protonated by HX or by transylidation. The t-butyl ester generates Ph3P+-CH3X-but no products of transylidation. The first-formed ylid1, can be trapped by reactive alkyl and acyl halides, and the transient ylidic esters decompose thermally to triphenyl phosphine oxide, Ph3P = O, react further with unreacted phosphonium ester, or are trapped by added aldehyde in a Wittig reaction. The final product compositions are affected by a decrease in pressure, due to escape of volatile intermediates, and by replacement of the X- halide ion by the less nucleophilic and basic tosylate ion. Reactions under reflux, in solution in chloroform, or in suspension in benzene, are similar to those of the molten salts, but yields are generally lower at the lower temperatures. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500701613030

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Thermal Decomposition of
Triphenylphosphonium Alkyl
Ester Salts
Fernando Castañeda a , Christian Aliaga a , Cristina
Acuña a , Paul Silva a & Clifford A. Bunton b
a University of Chile, Santiago, Chile
b University of California, Santa Barbara, California,
USA
Available online: 09 May 2008
To cite this article: Fernando Castañeda, Christian Aliaga, Cristina Acuña, Paul
Silva & Clifford A. Bunton (2008): Thermal Decomposition of Triphenylphosphonium
Alkyl Ester Salts, Phosphorus, Sulfur, and Silicon and the Related Elements, 183:5,
1188-1208
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Phosphorus, Sulfur, and Silicon, 183:1188–1208, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701613030
Thermal Decomposition of Triphenylphosphonium Alkyl
Ester Salts
1
1
1
˜
˜
Fernando Castaneda, Christian Aliaga, Cristina Acuna,
Paul Silva,1 and Clifford A. Bunton2
1University of Chile, Santiago, Chile
2University of California, Santa Barbara, California, USA
In thermolyses of molten triphenylphosphonium alkyl ester bromides and chlorides,
alkyl=methyl, ethyl, isopropyl, at 130 and 225◦C, initial attack of the halide ion
on the methyl group gives the methyl halide and ylid 1, Ph P=CH , which can be
3
2
methylated, or is protonated by the phosphonium salt with transylidation giving
Ph P+-CH X−, X=Br, Cl. The initial reactions of the ethyl or isopropyl esters are
3
3
with the halide ion, X− , as a base giving ylid, 1, which can be protonated by HX
or by transylidation. The t-butyl ester generates Ph3P+-CH X−but no products of
3
transylidation. The first-formed ylid 1, can be trapped by reactive alkyl and acyl
halides, and the transient ylidic esters decompose thermally to triphenyl phosphine
oxide, Ph P=O, react further with unreacted phosphonium ester, or are trapped by
3
added aldehyde in a Wittig reaction. The final product compositions are affected by
a decrease in pressure, due to escape of volatile intermediates, and by replacement
of the X− halide ion by the less nucleophilic and basic tosylate ion. Reactions under
reflux, in solution in chloroform, or in suspension in benzene, are similar to those
of the molten salts, but yields are generally lower at the lower temperatures.
Keywords Carboalkoxy phosphonium salts; thermolyses; solvent free
INTRODUCTION
Thermolyses of carboxylic and xanthate esters give the alkene
through a spontaneous syn elimination.1,2 The phosphonium salts,
R3P+-CH2CO2RꢀX−, decompose at temperatures between 130 and
225◦C to give the alkene derived from Rꢀ.3−5 The course of these and sim-
ilar reactions is understandable in terms of mechanistic treatments of
reactions in solution. The corresponding reactions of carboalkoxy phos-
phonium salts, shown as triphenyl derivatives, are complex, because the
Received 18 June 2007; accepted 17 July 2007.
Thanks are due to the CEPEDEQ of the Faculty of Chemical and Pharmaceutical
Sciences, University of Chile, for instrumental facilities.
Address correspondence to Fernando Castan˜eda, Departamento de Qu´ımica Orga´nica
y Fisicoqu´ımica, Facultad de Ciencias Qu´ımicas y Farmace´uticas, Universidad de Chile,
Casilla 233, Santiago, Chile. E-mail: fcastane@ciq.uchile.cl
1188

Phosphonium Alkyl Ester Salts
1189
initial ylidic products can react further or be trapped by added reagents,
and these reactions control formation of the final products. In the two
probable initial reactions of the molten phosphonium halide salts, the
halide ions act as either nucleophiles or bases in the aprotic media, as
expected in terms of the known chemistry of ylidic phosphonium salts6
and general mechanistic concepts.
Ph3P+ − CH2CO2 − CH3X−−→ Ph3P = CH2 + CO2 + CH3X (1)
A
1
Ph3P+ − CH2CO2Et X− −→ 1 + CO2 + CH2 = CH2 + HX
(2)
B
1
We identify the phosphonium esters used in thermolysis alphabetically
(A–F) and intermediates, products, and added reagents are identified
by Arabic numerals.
The initially formed ylid 1 reacts rapidly with any electrophile, e.g.,
CH3X, HX or unreacted phosphonium ion, and a variety of products
are formed depending on the reaction conditions, for example, volatile
species may be removed under reduced pressure, or ylid 1 may be
trapped by an added electrophile.
Reactions were generally carried out at temperatures slightly above
the melting points of the salts, and thermolyses are them initially of the
solid phosphonium salt, which soon melts and initial products decrease
the melting point. The phosphonium salts have sharp melting points,
with no indication of rapid decomposition, but they gradually decom-
pose on heating in bulk. There is, therefore, some initial reaction of the
crystalline salt, but reaction is largely in the melt, which is a viscous
ionic liquid, and should accomodate the charge transfers occurring in
the course of decomposition. There is uncertainty in the conditions of
the overall reaction, which change during the decomposition. The high
viscosity of the molten salt means that initially formed species will not
diffuse apart rapidly, for example, adjacent cationic phosphonium es-
ters, e.g., A or B, should react effectively with initially formed ylid in
competition with added trapping agents. We used methyl, ethyl, iso-
propyl, and t-butyl derivatives in view of extensive mechanistic work
on the effects of α-substituents on elimination and nucleophilic substi-
tution. We performed a few experiments with an allylic ester group, and
with introduction of a phenyl group, and we show that these structural
modifications do not significantly affect the course of reaction.

1190
F. Castan˜eda et al.
The reaction can be carried out at atmospheric or reduced pressure,
which affects secondary reactions of volatile species. The halogen acids
formed in initial reactions of some substrates should not be strong acids
in the molten salts and will probably exist largely as ion pairs or clus-
ters; but, halide ions should be strong bases and nucleophiles towards
compounds with which they can react readily in the melt.
The first formed ylid can be rapidly protonated or methylated giving
phosphonium ions:
Ph3P = CH2 + HX −→ Ph3P+CH3X−
(3)
1
2
Ph3P = CH2 + CH3X −→ Ph3P+CH2CH3X−
(4)
1
3
Alternatively, ylid 1 can react with its precursor, the cationic phospho-
nium ester, by transylidation,6generating a new ylid, e.g.
1 + Ph3P+ − CH2CO2MeX− −→ 2 + Ph3P = CH-CO2Me
↓ 4a
(5)
HC ≡ C-OCH3 + Ph3P = O
Ylidic esters formed in this and similar reactions decompose ther-
mally giving the triphenyl phosphine oxide, which is thermally stable
and is a major product in these reactions. Except in special cases, ylides
are not final products.
Formation of most of the thermolysis products can be explained in
terms of the above reactions, plus those with added electrophiles, e.g.,
acyl and alkyl halides, and activated aldehydes in a Wittig reaction.
In some conditions, an ylidic ester may react with a phosphonium ion
with transylidation rather than by thermal decomposition, as in the
reverse of the first step in Eq. (5). In the viscous ionic liquid, these
reactions of intermediates may behave as if they are intramolecular and
compete against the intermolecular trapping of the ylidic intermediates
by added electrophiles, which give stable products.
The phosphonium ylides are labile compounds and the final products
of their reactions depend on the physical conditions, including the re-
moval of volatile intermediates under reduced pressure. The reactions
that take place in the ionic liquids can also occur to some extent in inert
organic liquids, although at lower temperatures and generally with
lower yields than in the thermolyses. In these conditions, solubilities
of the phosphonium esters play a key role, for example, the salts
are soluble in chloroform, but not in benzene, where they react in

Phosphonium Alkyl Ester Salts
1191
suspensions. In addition, escape of volatile intermediates should be
slower at the relatively low temperatures in solvents than in molten
salts.
These thermolysis are reactions, which can be carried out efficiently
without a solvent, which is currently highly desirable.7 The work is
not directed to synthesis of particular products but has the potential to
show how relatively small changes in reaction conditions of tempera-
ture or pressure, for example, can generate significant changes in prod-
uct compositions. Organophosphorus compounds, particularly phospho-
nium salts and ylides, are essential building blocks in organic synthesis
which makes understanding of their reactions practically important.8
RESULTS AND DISCUSSION
Various operating conditions were used, the temperature was varied but
was generally above the melting point of the phosphonium salt, which
decreased in the course of the reaction. Initial conditions are in Tables I–
IV, but, as noted, they change during reaction and it is therefore difficult
to treat medium effects on the products quantitatively. Probable courses
of reaction are shown in Schemes 1 and 2 and final products of reaction
in various conditions are shown in Tables I–IV.
Most experiments were made with the bromide salts, but the chlo-
rides and tosylates (p-toluenesulfonates) were also examined and anion
effects are considered separately.
Decarboxylation in the course of reaction was confirmed by formation
of BaCO3, precipitation of silver halide confirmed formation and escape
of a halogen acid or an alkyl halide, and decolorization of Br2 confirmed
formation of volatile alkenes or acetylenes.
TABLE I Thermolysis of Phosphonium Methyl Ester Salt Aa
Ph3P+-CH2-CO2CH3 X−
Ph3P+-CH3X−+ Ph3P+-CH2-CH3X− + Ph3PO
A
29
44%
39
44%
a) X− = Br− (5 mmol)
12%
24%
b) X− = Br− (4.8 mmol)
c) X− = Br− (7.2 mmol)
48%
16%
28%
39%
d) X− = Br− (4.8 mmol)
e) X− = Cl− (4 mmol)
52%
50%
8%
40%
50%
aCO2 formation is omitted for clarity.

1192
F. Castan˜eda et al.
SCHEME 1 Reaction conditions: 160◦C, 1 At., 1.5 h.
Reaction of the Phosphonium Methyl Ester Salts A
We assume that the initial reaction involves nucleophilic substitution
by halide ion with fragmentation, Eq. (1), but the initially formed ylid
can be methylated, Eq. (4), or react by transylidation, Eq. (5), gener-
ating methyltriphenylphosphonium halide 29 and the new ylidic ester
4, which spontaneously decomposes giving triphenyl phosphine oxide.
Scheme 1 and Table I show products, and the course of thermolysis of
the phosphonium methyl ester salt A under different conditions.
TABLE II Thermolysis of Phosphonium Ethyl Ester Salt Ba
Ph3P+-CH2-CO2CH2CH3X−
B
2
73%
5a
a) X = Br− (7 mmol)
27%b
48%
27%
58%
5%
b) X− = Br− (4.7 mmol)
c) X = Br− (7 mmol)c
d) X− = Br− (4.7 mmol)
e) X = Cl− (5.2 mmol)
f) X = TsO− (5.8 mmol)d
52%
55%
43%
16%
95%
12%
g) X = TsO− (3.8 mmol)e
90%
10%
aFormation of CH2 =CH2, CO2 and detection of unreacted starting material are
omitted for clarity. bPh3PO was isolated and identified by Ir spectroscopy. cUnreacted
starting material, SM, (2%) was detected by 1H NMR spectroscopy. dSM (88%) was
detected by 1H NMR spectroscopy in the crude product. eLithium p-toluenesulfonate in
the crude product was detected by 1H NMR spectroscopy.

Phosphonium Alkyl Ester Salts
1193
TABLE III Thermolysis of Phosphonium Isopropyl Ester Salt
Ca
Ph3P+-CH3Br−+
+ Ph3PO
C
2
70%
5b b
4%
a) C (4.5 mmol)c
22%
35%
b) C (4.5 mmol)d
40%
4%
aFormation of CO2, propene and the presence of non reactive starting
material (SM) is omitted for clarity. bConformation of 5b is not known, but we
assume that it is the same as that of 5a. cSM (4%) was detected in the crude
after reaction. dSM (21%) was detected after thermolysis.
Methylation of the reactive ylid 1 by the first-formed CH3Br
1
gives ethyl triphenylphosphonium bromide 3 [Eq. (4), whose H-NMR
spectrum is identical to that of a standard sample.9 In general ethylt-
riphenylphosphonium halide 3 is a significant product, except for exper-
iments c and d (Table I) where at reduced pressure CH3Br is removed
from the molten medium with less trapping of the initially formed ylid.
Examples of these reactions are shown in Scheme 1 for thermolysis at
160◦C for 1.5 h, at atmospheric pressure where phosphonium salts 2,
3, and Ph3P=O were identified by 1H NMR spectroscopy.
Reactions of the Phosphonium Ethyl Ester Salts B
The initial reaction of the ethyl ester of the phosphonium halide, B,
is apparently attack of halide ion, with elimination and fragmentation
giving ethylene and CO2. This reaction probably involves the halide ion
as an effective base in the aprotic medium. The halogen acid readily pro-
tonates the reactive ylide 1, but apparently not ethylene, because we
TABLE IV Thermolysis of Phosphonium t-Butyl Ester
Salt D
Ph3P+ CH2 CO2 CMe3X−
Ph3P+ CH3X−+ SMa
D
2
D
—
a) X− =Br− (4.4. mmol)
100%
b) X− = Cl− (4.8 mmol)
95%
5%
Formation of isobulylene and CO2 is omitted for clarity.

1194
F. Castan˜eda et al.
saw no products derived from an ethyl halide. A possible intramolecular
reaction involves cyclic proton transfer to the ylidic methylene group,
which would give the methyl phosphonium salt 2 as the only isolat-
able product. It would require the ester to take up the unfavorable E
conformation,10and this reaction is apparently unimportant.
(6)
The subsequent overall reactions of the ethyl ester are similar to
those observed with the methyl ester. The ylidic ethyl ester 4b, is formed
by transylidation and may decompose to the phosphine oxide or react
with starting material giving ylidic ester 5a.
(7)
This ylid, 5a, does not decompose during thermolysis, possibly be-
cause its high melting point, 183◦C (bromide), allows it to separate out
as a solid and be protected from further reactions. Compound 5a had
been prepared by reaction between the parent phosphonium salt, B, and
the ylidic ester, 4b, Eq. (7),11 and its crystal structure was determined.12
Scheme 2 and Table II show the thermolysis of the phosphonium
ethyl ester salt, B, with bromide, chloride or p-toluenesulfonate (to-
sylate) ion. The reaction is carried out at various pressures and tem-
peratures. A main product is always the methyltriphenylphosphonium
halide or tosylate, 2, and in only one experiment (c) was the ylid- phos-
phonium salt 5a, identified.
Under low pressure, HBr can escape from the molten reaction
medium limiting protonation of initially formed ylid 1 and favoring
formation of Ph3P=CH-CO2Et (4b) by transylidation of reactive ylid 1
with phosphonium salt B, and the subsequent formation of the ylid-
phosphonium salt 5a [eq. (7) and Scheme 2].
Examples of these reactions are shown in Scheme 2 for thermolysis
at 135◦C for 2.2 hr and 10 mm pressure, and 2 mole% residual starting

Phosphonium Alkyl Ester Salts
1195
SCHEME 2 Thermolysis conditions are 135◦C, 2.2 h, and 10 mm Hg.
material. Trapping of 1 by benzoyl chloride and benzyl bromide is shown
later. Formation of ethylene and CO2 are omitted for clarity.
Thermolysis of the tosylate salt of the ethyl ester phosphonium salt,
B, gives only a low yield of Ph3P+-CH−3 OTos and no other identifiable
product, probably because even in an ion pair tosylate ion is an ineffec-
tive base. However, lithium tosylate and equimolar bromide salt of the
ethyl ester give a high yield of Ph3P+-CH3 Br− because of the higher
basicity of Br− as compared with that of −OTos in the reaction medium.
Reactions of the Phosphonium Isopropyl Ester Salt C
The reactions outlined for thermolysis of the ethyl ester B control the
decomposition of the isopropyl ester C, although there are minor differ-
ences in product ratios.
Table III shows the products of pyrolysis of the phosphonium iso-
propyl ester bromide C. At atmospheric and reduced pressure 1H NMR
spectroscopy indicates formation of the phosphonium ylide bromide 5b
as a minor product, from addition of the nucleophilic isopropylester ylid
4c to the electrophilic carbonyl group of isopropyl ester phosphonium
bromide C. As is general methyl triphenyl phosphonium bromide, 2, is
a major product in reactions of the bromide salt.

1196
F. Castan˜eda et al.
Reactions of the Phosphonium t-Butyl Ester Salt D
Decomposition of the t-butyl ester D is very simple in that there is
no indication of formation of ylidic compounds as final products, and
only the phosphonium salt 2 and unreacted D were identified (Table
IV). Volatile materials were passed into Br2-CCl4, which was partially
decolorized and removal of the solvent left a residue which had CH3
and CH2 1H NMR signals and is probably polymeric.
We saw no evidence for transylidation with Ph3P=CH2, which is
consistent with an intramolecular concerted reaction was considered
for decomposition of the ethyl ester B, Eq. (6). However, Ph3P=CH2, 1,
is trapped by added Ph-COCl, indicating that the reaction cannot be
wholly intramolecular. The intermolecular reaction probably involves
the halide ion as a base, and steric hindrance by the bulky t-butyl group
should preclude transylidation. We note that addition of Ph-COCl may
affect the composition of the reaction region, and therefore, the course
of the reaction.
Structural Modifications
Reactions of the phosphonium benzyl ester salt, E, are similar to those
of the methyl ester salts A, except that products are insensitive to pres-
sure, probably because of the low volatility of the intermediate benzyl
bromide which effectively traps Ph3P=CH2, 1, Table V, to form Ph3P+-
CH2-CH2-PhBr−, 8, detected by 1H NMR spectroscopy in the crude
product.
In a separate experiment the intermediate ylidic ester 4 was trapped
by 4-nitrobenzaldehyde in a Wittig reaction, Eq. (9). Reaction of E
in CHCl3, under reflux, gave no product of trapping of Ph3P=CH2
by benzyl bromide generated in situ, whose concentration is probably
too low for it to compete with transylidation, and subsequent forma-
tion of triphenyl phosphine oxide by decomposition of the ylidic ester
(Table VI).
Thermolysis of the allylic ester, F, involves attack of Br− giving the
allyl bromide, 6, bp. 70◦C, which was isolated by distillation. Although
TABLE V Thermolysis of Phosphonium Benzyl Ester Salt E
Ph3P+-CH2-CO2CH2-Ph Br−
E (4.1 mmol)
Ph3P+-CH3Br−
+
Ph3P+-CH2-CH2-Ph Br− + Ph3P=O
2
8
43%
42%
28%
27%
29%
31%
E (4.1 mmol)

Phosphonium Alkyl Ester Salts
1197
TABLE VI Thermolyses of Phosphonium Alkyl Ester Salts in organic
liquidsa
Ph3P+-CH2-CO2CH3Br−
Ph3P+-CH3Br− + Ph3P+-CH2-CH3Br− + Ph3P=O
A (7.2 mmol)b
29
29%
39
10 %
13.7 %
Ph3P+-CH2-CO2-CH2-CH3Cl−
B (2.6 mmol)c
Ph3P+-CH3Cl−
2
100%
Ph3P+-CH2-CO2-C(CH3)3Br−
D (1.8 mmol)d
Ph3P+-CH3Br−
+ SM
2
10 %
90%
+ SM +
Ph3P+-CH2-CO2CH2PhBr−
Ph3P+-CH3Br−
Ph3P=O
E (4.1 mmol)e
2
E
17%
69%
14%
aAlkenes and CO2 are omitted for clarity. bThermolysis is carried out in solution;
SM: starting material (47%) was detected. cMethyl phosphonium chloride 2 was
identified by Ir spectroscopy and comparison with a standard for reaction in benzene
suspension . dBenzene suspension . eReaction in CHCl3 solution with no formation of
Ph3P+-CH2-CH2-Ph Br−.
allyl bromide is a reactive electrophile it did not react with Ph3P=CH2
and Ph3P+-CH3Br−, 2, formed by transylidation, was a final product,
eq. (8). This system was not examined in detail.
(8)
Decompositions in Organic Liquids
Reactions were carried out in suspension in benzene, or in solution
in chloroform under reflux at temperatures much lower than those
used for thermolyses and reactions generally do not go to completion
(Table VI).

1198
F. Castan˜eda et al.
Reaction of the methyl ester, A, under reflux, in chloroform solution
did not go to completion but gave the expected phosphonium ion, 2, the
phosphine oxide and the ethyl triphenylphosphonium bromide 3 formed
by C-alkylation of the reactive Ph3P=CH2 by CH3Br.
Reactions of the ethyl and t-butyl esters, B and D, as suspensions in
benzene under reflux at 80◦C, gave only the methyl phosphonium salt,
2 (Table VI). Apparently for the ethyl ester the first formed ylid 1 reacts
with HCl or HBr in the solid, and intermediates do not separate. There
are no products of transylidation and, depending on conditions, some
reactions only go to a limited extent. In these experiments. the overall
reactions could be intramolecular or behave as such.
Ylid Trapping
We assume that the initial reaction is formation of the very labile
Ph3P=CH2 which reacts by transylidation, or with other electrophiles,
e.g., HX or CH3X formed in the initial reaction.
Added electrophiles should also trap Ph3P=CH2, and other ylides,
but they have to be sufficiently reactive to compete with starting ma-
terial, or with products of the initial reaction. The initial products and
starting material will diffuse apart very slowly in viscous molten phos-
phonium salt, which favors their reaction, while an added reagent has
to diffuse through this medium before it can trap an intermediate. As a
result, only reactive electrophiles, e.g., acyl chlorides, or benzyl bromide,
trap Ph3P=CH2, 1, in some conditions, but there is no reaction with the
less reactive succinic anhydride. Products of the Wittig reaction with
intermediate ylidic esters 4 are observed with p-nitrobenzaldehyde, but
not with p-methoxybenzaldehyde. This reaction, Eq. (9), confirms the
transient existence of these ylidic ester intermediates and the Wittig
reaction competes with their spontaneous decomposition to the phos-
phine oxide.
(9)

Phosphonium Alkyl Ester Salts
1199
TABLE VII a Trapping of Intermediate 1, Ph3P=CH2, by Benzyl
Bromide
Ph3P+-CH2- CO2R Br−
Ph3P+-CH3Br−
+
Ph3P+-CH2-CH2-Ph Br−
+ Ph3P=O
2
8
R=Me, A (4.8 mmol)
R=Et, B (4.7 mmol)
29%
51%
20%
18%
82%
Traces
There was no indication of trapping of Ph3P=CH2 in a Wittig reac-
tion, probably because it reacts very rapidly with HBr or the phospho-
nium salt.
Trapping results depend on relative amounts of substrate and the
trapping agent; if the latter is in significant excess, it can dissolve the
substrate, and the reaction intermediates will form in intimate contact
with the trapping agent. Then, the reaction will progress as if in an
organic medium, with concentrated reagents, rather than as in an
ionic liquid.
The competition between the added reagent and an electrophile gen-
erated in the initial reaction is illustrated by trapping by benzyl bro-
mide in thermolysis of the methyl and ethyl esters (Table VII). There
is extensive trapping by benzyl bromide with the methyl, but almost
no trapping with the ethyl, ester. The initial step of decomposition of
the methyl ester, A, generates MeBr which can escape and does not
compete with the stronger alkylating agent, benzyl bromide, for the
initially formed Ph3P=CH2. Phenyl ethyl phosphonium salt 8 is gener-
ated as the major product, which is identified by 1H NMR spectroscopy
and comparison with a standard, (Table VII a). In the reaction of the
ethyl ester B, HBr is generated adjacent to the ylid and, as an effective
electrophile, competes strongly with benzyl bromide by protonating the
initially formed ylid. Methylphosphonium salt 2 was formed (82%), iso-
lated by crystallization from CHCl3/AcOEt (3:1) and identified by IR
spectroscopy and comparison with a standard (Table VII a). Benzoyl
TABLE VII b Trapping of Thermolysis Intermediate 1, Ph3P=CH2,
by Benzoyl Chloride
Ph3P+-CH2-CO2RBr−
Ph3P+-CH3Br− + Ph3P+-CH2-CO-Ph Cl− + Ph3P=O
2
9
R = CH3, A (3.6 mmol)
10%
22%
80%
78%
10%
R = C(CH3)3, D (3.5 mmol)

1200
F. Castan˜eda et al.
chloride is a very effective electrophile and its trapping of the first-
formed ylid 1 is a significant reaction. Phenacylphosphonium chloride
9 was formed as the main reaction product (Table VII b).
Physical Conditions of the Thermolysis
Reactions were carried at atmospheric or reduced pressure, where gen-
erally protonation or methylation of the first formed Ph3P=CH2 is re-
duced by increasing the escape of the halogen acid or methyl halide.
However, reaction conditions are not reproducible, because they depend
on the rate of decomposition, which changes as the medium changes in
the course of reaction. In addition isolatable nonvolatile products are
derived from both nonvolatile intermediates (Schemes 1 and 2) and
volatile species which may escape during reaction.
Reactions in the melt were generally continued until bubbling had
apparently ended indicating essentially complete reaction. The reaction
medium was not stirred but bubbling generates some mixing.
Carrying out reaction under reduced pressure generally has a major
effect on product composition because of escape of some reactive inter-
mediates, especially at the higher temperatures, however, even volatile
species such as CH3X, can remain as bubbles in the viscous melt. The
loss of volatile electrophiles apparently depends upon the physical state
of the ionic liquid reaction medium. In some experiments small bubbles
escaped freely, but sometimes there was little escape until gas built
up and large bubbles formed, and the volatiles escaped. Therefore, al-
though changes in the products are usually understandable in these
simple terms, we cannot quantify the results.
Formation of Triphenylphosphine Oxide
An intermediate ylidic ester, 4, can form triphenyl phosphine oxide,
probably by transfer of an acyl oxygen to phosphorus. Formation of
acetylene derivatives has been observed in other thermolyses and may
occur here.13
(10)

Phosphonium Alkyl Ester Salts
1201
This reaction competes with trapping of the ylid ester by an electrophile,
e.g., an activated electrophile.
Decomposition of ylid, 4, producing triphenylphosphine oxide, was
demonstrated in a control reaction: Thermolysis of Ph3P=CH-CO2Et,
4b, at 152◦C, 1 At, over 2 h, gave a 16% yield of Ph3P=O which was
isolated and identified by 1H-NMR and Ir spectroscopy.
The t-butyl ester, D, behaves differently from esters of primary and
secondary alcohols in giving no phosphine oxide on thermolysis, where
the only nonvolatile product is Ph3P+-CH3X−. Ylid, 1, could be formed
but reacts only with the halogen acid, or Ph-COCl, and not by transyl-
idation, which would be sterically hindered by the bulky t-butyl group.
Effect of the Halide Ion
Most experiments were made with the bromide salts, but they and the
corresponding chlorides, generally behaved similarly, although ther-
molysis of the methyl ester is an exception to this generalization be-
cause while we assume that the initial reactions generate ylid 1 and
the methyl halide, only methyl bromide traps the ylid.
(11)
Several factors contribute to this difference, viz. methyl chloride is a
weaker methylating agent than the bromide and, being more volatile,
should escape more readily.
The situation is more complicated for the ethyl ester because the
initially formed halogen acid can trap Ph3P=CH2 , 1, giving the phos-
phonium salt, 2, but transylidation of 1 with the starting material also
gives2 which complicates comparison of the roles of the halide ion in
these reactions.
In thermolysis of the chloride and bromide salts of the t-butyl ester,
the methyl triphenylphosphonium ion 2 is the only nonvolatile product
and we have no evidence regarding the role of the halide ion. As noted,
ion pair interaction would favor a concerted fragmentation, as shown
in Eq. (6).

1202
F. Castan˜eda et al.
Thermal Decomposition of Ylidic Esters
The initial decomposition of the phosphonium esters, possibly excepting
the t-butyl ester, forms Ph3P=CH2 which can be methylated, proto-
nated, or undergo transylidation to give an ylidic ester which decom-
poses spontaneously. The final major nonvolatile products of decomposi-
tion of the methyl, ethyl and isopropyl phosphonium esters are therefore
Ph3PO, Ph3P+-CH3X− and, for methyl ester, Ph3P+-CH2CH3X−.
Except in a few special cases, the ylidic ester formed by transylida-
tion decomposes thermally giving the phosphine oxide and probably a
volatile acetylene derivative (Eq. 5),13 which, like an alkene, should de-
colorize bromine. The driving force for this reaction, as in the Wittig
reaction, is oxygen transfer to a cationoid phosphorus.
Substituted acetylenes are acids in solution and in the gas phase,14
and could compete with a halogen acid in protonating ylid 1. The acety-
lene derivative should escape readily from the molten reaction medium
but may survive long enough to protonate 1, or other ylides. In that
event, the product balance will depend upon the competition between a
volatile halogen acid and a transient acetylene derivative formed in the
last step of a reaction sequence which may also involve transylidation.
It appears that most intermediate ylidic esters 4 do not survive
thermolysis, but although these compounds have sharp melting points,
without decomposition, and properties of the molten salts, as ionic liq-
uids, may promote decomposition of the ylidic esters.
Reaction Paths
Some of the reaction steps in thermolysis of the phosphonium esters
are chemically similar to other overall fragmentations.15 The initial
decarboxylation is similar to thermal decompositions of carboxylate,1
xanthate,2 and protonated amino acid esters,16 except that the initial
reaction gives a transient ylid.
The first formed phosphorus ylid 1 reacts with any available elec-
trophile with proton transfer from a halogen acid, or a phosphonium
salt in transylidation, or with alkylation or acylation, and these subse-
quent reactions may involve added reagents.
Transylidation generates an ylidic ester 4, which is unstable at the
reaction temperatures and generally decomposes to a phosphine oxide
and probably a substituted acetylene, in a reaction akin to the Wit-
tig reaction, although reaction with a phosphonium ester can give the
thermally stable ylid-phosphonium salt, 5.11
Intermediates in thermolyses of the cationic phosphonium esters
should be very short-lived and, in the viscous ionic liquid, tend to react

Phosphonium Alkyl Ester Salts
1203
with their nearest neighbor, which therefore competes effectively with
more reactive added reagents.
Reaction of the t-butyl ester does not fit the above generalizations
because its thermolysis does not involve transylidation and formation
of phosphine oxide.
The reaction paths in these thermolysis are generally similar to those
observed in decompositions in solution or suspension, although temper-
atures differ.
CONCLUSIONS
Products of thermal decompositions of molten salts of triphenylphos-
phonium esters have been examined. Reactions of methyl, ethyl and
isopropyl esters occur with initial formation of Ph3P=CH2 , which can
react with the starting material by transylidation, and, for the methyl
ester, by reaction with methyl halide formed in the initial reaction, and,
for the ethyl and isopropyl esters, by reaction with initially formed halo-
gen acid, but decomposition of the t-butyl ester gives only the methyl
phosphonium salt. Ylidic intermediates can be trapped by alkyl and
acyl halides and by an activated aldehyde, but the reaction media are
viscous ionic liquids and these added electrophiles have to be in suffi-
cient concentration to compete with electrophiles formed in the initial
decompositions in contact with the ylides. An important exception to
this generalization is that with excess trapping agent it, rather than
the molten ester, may become the reaction medium, with considerable
trapping of the ylid. Ylidic esters, formed by transylidation, generally
decompose intramolecularly giving triphenyl phosphine oxide in the
ionic liquid, although isolated esters have sharp melting points. The
initial decomposition gives volatile products, which may escape or re-
act with the ylidic intermediates, and overall products depend on pres-
sure. Changes in the anion of the phosphonium ester salt affect product
composition. Ions are probably paired in the ionic liquid but changes in
product composition are understandable in terms of the nucleophilicity
or basicity of the anion and electrophilicity of the undissociated conju-
gate acid. Products of reaction in chemically inert solvents are similar
to those in the molten salts, except for temperature differences and the
fact that reactive ionic intermediates mix more freely in moderately po-
lar solvents, e.g., CHCl3, in which reactants are soluble, than in benzene
where reaction is in a dispersion of the solid.
Product formation depends on reactant properties and concen-
trations, but also on physical conditions, and therefore on physical
properties which change during the reaction. The reactions that occur
in these thermolysis are as expected in terms of the known chemistry

1204
F. Castan˜eda et al.
of phosphonium ylides and medium effects on these reactions.
Substituents in the alkyl group do not significantly affect the reactions.
EXPERIMENTAL
1H NMR spectra were obtained on a Bruker DRX300 spectrometer and
were referenced to TMS. IR spectra were recorded on a Bruker IFS 56
FT spectrometer with a KBr disk. Melting points are uncorrected. The
thermolysis products were characterized and quantified by comparison
of their 1H-NMR spectra in CDCl3 with authentic samples, and concen-
trations were estimated by signal integration, avoiding signals, which
were overlapped. Elemental analyses were performed with a Fisons EA
1108 analyzer to estimate novel compounds described in this work, Ac,
E, Be, 8, and 9. Benzyl bromoacetate; allyl 2-bromoacetate; trans-ethyl
4-nitrocinnamate, 7; 2-chloroacetophenone, were commercial samples.
Synthesis
The Synthesis of Alkylester Triphenylphosphonium Salts
A number of these compounds are known and we follow the gen-
eral procedure17 illustrated for the synthesis of methoxycarbonyl
methyl triphenylphosphonium bromide, A. An equimolar mixture
of triphenyl phosphine and methyl bromoacetate dissolved in the min-
imum amount of dry benzene, was stirred at room temperature for
4 h to produce a white solid which was removed by suction filtration
and washed with benzene. The filtrate and the washed benzene frac-
tions were stirred again at r. t. until no more solid was formed (2–8 h).
The dried solid product A, yield 97%, m.p., 161–162◦C,18 has 1H NMR
(CDCl3)δppm: 3.61 (s, 3H, CH3), 5.60 (d, 2H, J= 14 Hz, CH2), 7.66-7.94
(m, 15 H).
The following phosphonium salts were obtained by the above proce-
dure from the appropriate halo derivatives.
Methoxycarbonyl Methyl Triphenylphosphonium Chloride, Ac.
M.p. 152–153◦C., 1H NMR (CDCl3)δppm: 3.60 (s, 3H, CH3), 5.55
(d, 2H, J = 14Hz, CH2), 7.63-7.92 (m, 15 H). Anal. Calcd. for
C21H20ClO2P(371.8):C, 67.83; H, 5.69. Found: C, 67.67; H, 5.50.
Ethoxycarbonyl Methyl Triphenylphosphonium Bromide, B. Yield
97%, m.p. 155-156◦C,18 1H-NMR (CDCl3)δppm: 1.0 (t, 3H, J = 7 Hz, CH3),
4.0 (q, 2H, J= 7 Hz, CH2), 5.5 (d, 2H, J = 14 Hz, CH2), 7.7 (m, 15 H).

Phosphonium Alkyl Ester Salts
1205
Ethoxycarbonyl Methyl Triphenylphosphonium Chloride, Bc. Yield
90%, m.p. 121–122◦C,19 1H NMR (CDCl3)δppm: 1.0 (t, 3H, J = 7 Hz, CH3),
3.99 (q, 2H, J = 7 Hz, CH2), 5.48 (d, 2H, J = 14 Hz, CH2), 7.7 (m, 15 H).
Isopropyloxycarbonyl Methyl Triphenylphosphonium Bromide, C.
Yield 94%, m.p. 150–151◦C.,20 1H NMR(CDCl3)δppm:1.03 (d, 6H, CH3),
4.84 (sept, 1H, J = 6 Hz, CH), 5.50 (d, 2H, J = 14 Hz, CH2), 7.6–8.0 (m,
15 H).
t-Butyloxycarbonyl Methyl Triphenylphosphonium Bromide, D.
Yield 96%, m.p. 175–176◦C,21 1H NMR (CDCl3)δppm: 1.22 (s, 9H, CH3),
5.50 (d, 2H, J = 14Hz, CH2), 7.7–8.0 (m, 15H).
t-Butyloxycarbonyl Methyl Triphenylphosphonium Chloride, Dc.
Yield 90%, m.p. 169–170◦C,22 1H NMR (CDCl3)δppm: 1.21 (s, 9H), 5.45
(d, 2H, J = 14 Hz, CH2), 7.6–8.0 (m, 15 H).
Benzyloxycarbonyl Methyltriphenyl Phosphonium Bromide, E.
This salt was synthesized as above, by reaction of triphenylphos-
1
phine with benzyl bromoacetate, yield 84%, m.p. 99–103◦C, H NMR
(CDCl3)δppm: 5.03 (s, 2H), 5.69 (d, 2H), 7–8 (m, 20 H). Anal. Calcd. for
C27H24BrO2P (491.4): C, 66.00; H, 4.92. Found: C, 65.71; H, 4.65.
Allyloxycarbonyl Methyl Triphenylphosphonium Bromide, F. This
salt was prepared by the general procedure: reaction of triphenylphos-
phine with allyl 2-bromoacetate, yield 98%, m.p. 129◦C,23 1H NMR
(CDCl3)δppm: 4.50 (d, 2H, O-CH2), 5.20 (m, 2H, CH2 =), 5.60 (d, 2H,
P+-CH2), 5.75 (m, 1H, -CH=), 7.5–8.0 (m, 15H).
Synthesis of Ethoxycarbonyl Methyl Triphenylphosphonium p-
Toluene Sulfonate, Be. This salt was obtained by reaction of Ph3P=CH-
CO2CH2-CH3 with equimolar p-toluenesulfonic acid dissolved in a min-
imum amount of dry benzene and stirred at room temperature A white
solid formed immediately. The solid, filtered under vacuum was ben-
zene washed to yield 85% of product Be, m.p. 165–166◦C, 1H NMR (CD
Cl3)δppm: 1.01 (t, 3H, J = 7 Hz, CH3), 2.29 (s, 3H, CH3), 3.98 (q, 2H,
J = 7 Hz, CH2), 5.11 (d, 2H, J = 14 Hz, CH2), 7.04 (d, 2H, J = 8 Hz),
7.64–7.83 (m, 17 H). Anal. Calcd. for C29H29O5PS (520.6): C, 66.91; H,
5.61. Found: C, 66.60; H, 5.52.
Synthesis of Thermolysis Products as Spectroscopic
Standards
Methyl Triphenylphosphonium Bromide, 2. Prepared by the above
general procedure, yield 99%, m.p. 230◦C,9 1H NMR (CDCl3)δppm: 3.33
(d, 3H, J = 14 Hz, CH3), 7.6–8.0 (m, 15 H).

1206
F. Castan˜eda et al.
Ethyl Triphenylphosphonium Bromide, 3. Prepared as above, yield
90%, m.p. 206–208◦C,9 1H NMR (CDCl3)δppm: 1.36 (d-t, 3H), 3.60 (m,
2H), 7.70 (m, 15H).
3-Ethoxycarbonyl--3--Triphenylphosphoranylidene--2-Oxopropane
Triphenyl Phosphonium Bromide, 5a. This ylid-phosphonium salt was
obtained by reaction of carbethoxymethylene triphenylphosphorane (1
g, 2.87 mmol) with the phosphonium salt 2 (1.42 g, 3.3 mmol). Yield
57%, m.p. 180–183◦C,11 1H NMR (CDCl3)δppm: 0.57 (t, 3H, J = 7 Hz,
CH3), 3.67 (q, 2H, J = 7 Hz, CH2), 5.37 (d, 2H, J = 12.1 Hz, CH2),
7.46–7.82 (m, 30 H).
2-Phenylethyl Triphenylphosphonium Bromide, 8. This compound
was prepared by C-alkylation of methylenetriphenylphosphorane24
with benzyl bromide. Methyl triphenylphosphonium bromide 2 (3.6 g,
0.01 mol) was added cautiously over several minutes to a solution of
n-butyl lithium (0.01 mol., approx. 11 ml of the commercially available
22% solution in hexane) in 20 ml of anhydrous ether. The mixture was
then stirred at room temperature under N2. After 2 h, benzyl bromide
(1.9 g, 0.01 mol) was added dropwise and the mixture was stirred for
8 h and filtered by suction. The solid was ether washed and extracted
with chloroform. The organic extract was dried (anhydrous magnesium
sulfate), and the solvent was removed under vacuum. The residue was
crystallized from CHCl3/AcOEt (1:5) to form 8 (2.9 g, 65%), m.p. 216◦C,
1H NMR (CD Cl3)δppm: 3.0 (sextuplex, 2H), 4.0 (sextuplex, 2H), 7.0–8.0
(m, 20 H). Anal. Calcd. for C26H24BrP (447.4): C, 69.81; H, 5.41. Found:
C, 69.52; H, 5.38.
Phenacyltriphenylphosphonium Chloride, 9. 2-Chloroacetophe-
none (15.5 g, 0.1 mol) was added in portions to a chloroform (70 ml.)
solution of thiphenylphosphine (26.2 g, 0.1 mol) and was stirred at
room temperature for 6 h. The solution was filtered into 0.5 l. of
petroleum ether (40-60◦C) to produce a precipitate which was collected,
1
dried and crystallized from water. M.p. 275◦C. H NMR (CDCl3)δppm
:
6.8 (d, 2H), 7.5-8.0 (m, 20 H). Anal. Calcd. for C26H22ClOP (416, 9): C,
74.91; H, 5.32. Found: C, 74.71; H, 5.10.
Thermolysis of Triphenylphosphonium Alkyl Ester Salts
General Procedure for Thermolysis at Normal Pressure
The ester-phosphonium salt (4–5 mmol) was heated in a round-
bottom flask fitted with a condenser and an internal temperature con-
trol. The vertical condenser was modified to allow collection of conden-
sate. At near to the melting point, the thermolysis starts with extensive

Phosphonium Alkyl Ester Salts
1207
bubbling in the viscous melt. Complete reaction is indicated by the end
of bubbling (1–2 h).
The crude thermolysis product, is generally a solid at room temper-
1
ature, and was analyzed by H NMR spectroscopy after dissolution in
CDCl3, eliminating product discrimination during analysis. Yields are
in mole%. Volatile products formed in the thermolysis were studied by
connecting the condenser outlet to a bubbler with aqueous solutions
of barium hydroxide or silver nitrate, or bromine/CCl4 to detect CO2,
HBrg , alkyl halides, or alkenes and acetylenes, respectively.
General Procedure for Thermolysis at Reduced Pressure
The equipment as described was connected to the vacuum system
via the condenser. Care had to be taken to avoid spattering of material
during thermolysis, and trapping of volatiles was not examined.
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