The Role of an Acid in the Decomposition of Mixed Benzoyl-Substituted Phosphonium–Iodonium Ylide

Article abstract of DOI:10.1134/S0023158419010105

Abstract—Mixed phosphonium–iodonium ylides allow the synthesis of not easily accessible and novel heterocyclic compounds. Photoinitiated reactions of phosphonium–iodonium ylides with acetylenes proceed with induction time and are catalyzed by acids, the acids formed in the reaction among them. The kinetic peculiarities of the reaction between the benzoyl-substituted phosphonium–iodonium ylide and trifluoroacetic acid were studied by UV-visible spectrophotometry. The kinetic parameters of the reaction were determined. The mechanism of the autocatalysis by acids has been proposed, which involves the formation of a protonated form of the ylide active in the decomposition into radical cations. The more active decomposition of the protonated ylide is confirmed by theoretical thermochemical calculations.

Full text of DOI:10.1134/S0023158419010105

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 1, pp. 44–51. © Pleiades Publishing, Ltd., 2019.
Russian Text © T.D. Nekipelova, T.A. Podrugina, D.S. Vinogradov, P.I. Dem’yanov, V.A. Kuzmin, 2019, published in Kinetika i Kataliz, 2019, Vol. 60, No. 1, pp. 45–53.
The Role of an Acid in the Decomposition
of Mixed Benzoyl-Substituted Phosphonium–Iodonium Ylide
a,
b
b
b
a
T. D. Nekipelova *, T. A. Podrugina , D. S. Vinogradov , P. I. Dem’yanov , and V. A. Kuzmin
a
Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, 119334, Russia
b
Chemical Faculty, Moscow State University, Moscow, 119991 Russia
*
e-mail: nekip@sky.chph.ras.ru
Received May 18, 2018; revised August 17, 2018; accepted September 14, 2018
Abstract—Mixed phosphonium–iodonium ylides allow the synthesis of not easily accessible and novel het-
erocyclic compounds. Photoinitiated reactions of phosphonium–iodonium ylides with acetylenes proceed
with induction time and are catalyzed by acids, the acids formed in the reaction among them. The kinetic
peculiarities of the reaction between the benzoyl-substituted phosphonium–iodonium ylide and trifluoro-
acetic acid were studied by UV-visible spectrophotometry. The kinetic parameters of the reaction were deter-
mined. The mechanism of the autocatalysis by acids has been proposed, which involves the formation of a
protonated form of the ylide active in the decomposition into radical cations. The more active decomposition
of the protonated ylide is confirmed by theoretical thermochemical calculations.
Keywords: phosphonium–iodonium ylides, acid catalysis, radical decomposition, theoretical thermochemi-
cal analysis
DOI: 10.1134/S0023158419010105
INTRODUCTION
The phenyliodonium group in the composition of
mixed ylides determines their ability to decomposition
under the action of light to afford active ionic and rad-
ical ionic transient species as was observed for dia-
ryliodonium salts [4, 5]. Recently, two new photo-
chemical reactions of mixed phosphonium–iodonium
ylides with compounds with triple bond (С≡N and
С≡С) were developed. As a result, substituted oxazoles
Currently, mixed phosphonium–iodonium ylides,
the compounds with several reactive centers, which
general structure is given below, are studied in order to
synthesize on their basis not easily accessible and
novel heterocyclic compounds [1–3]:
R1
BF4−
+
(
with nitriles) and furan derivatives and phosphor-
Ph2P
containing heterocycles (with acetylenes) are formed.
The reaction with acetylenes is general for the phos-
phonium–iodonium ylides of different structure [6–
C− R2
+
PhI
1
1
1] and is given in Scheme 1 for the benzoyl-substi-
where R = Ph or five-membered aromatic heterocy-
2
1
2
cle, and R = CO(Ph), CO(OMe), CO(OEt), CN, tuted ylide 1 (R = Ph, R = CO(Ph)) used in this
PO(OEt) , S(O) (п-MeC H ).
investigation.
2
2
6
4
Ph
O
C
+
Ph
+
O−
Ph3P
+ (b)
Ph3P
O
CH Cl
-aH+
-PhI
2
2
P
+
C
C + R1 C CH
Ph
(
a) Ph
+ (c) Ph P CH C
3 2
R1
+
PhI
Ph
R1
Ph
O
1
2
3
4
5
Scheme 1.
44

THE ROLE OF AN ACID
45
5
λ -Phosphinoline (3), furan derivative (4), and
The kinetics of the reaction of ylide 1 with TFA in
−
CH Cl were registered on a Shimadzu UV3101 PC
2
2
phosphonium salt (5) (Scheme 1, counter ion BF is
4
spectrophotometer (Japan) in quartz cells with an
optical pathlength of 1.0 cm in kinetic operation mode
at the wavelength of 270 nm.
omitted in this and following schemes) are major reac-
tion products. Photochemical reactions of acetylenes
with phosphonium–iodonium ylide 1 proceeds only
in dichloromethane at the ylide concentration exceed-
Mass-spectra were obtained on a 7T LTQ FT Ultra
mass-spectrometer (Thermo Finnigan, Germany)
with direct probe injection and a Finnigan Ion Max-
Source universal ionization source in ESI regime (see
for details [14]). Spectrophotometric investigations in
the UV-visible range and mass-spectrometric measure-
ments were carried out in the Shared Research Facilities
of IBCP New Materials and Technologies.
Programs GAMESS US [19] with a PBE0 func-
tional [20], 6-311++G(d,p) basic sets (involved in the
GAMESS US program) for the H, C, O, and P atoms,
and an aug-cc-pVDZ-pp basic set with an efficient
core potential for the I atom [21] were used for the
quantum chemical calculations. The geometry of ylide 1,
its protonated form, and all other particles formed in
the decomposition of both ylide forms were optimized
taking into account the solvation by dichlorometane
using a model of polarization continuum [22]. All the
studied systems were optimized without any restric-
–1
ing 0.01 mol L with an induction time [12, 13]. An
acid is formed during the photolysis of the ylides and
their mixtures with acetylenes in CH Cl . Therefore, it
2
2
was proposed that the acid could play the role of a cat-
alyst [13, 14]. It has been shown that the induction
time of the reaction of ylide 1 with phenylacetylene
decreases upon addition of strong organic acids (tri-
fluoroacetic acid and toluenesulfonic acid) and the
reaction becomes thermal, i.e., occurs without irradi-
ation [13]. It is important that, to obtain the target
product 3, the concentration of the added acid should
not exceed the ylide concentration.
It is known that dichloromethane undergoes pho-
toinduced radical reactions yielding HCl and phos-
gene [15–17]. Therefore, the reaction scheme of the
ylide decomposition with implication of the solvent
(
CH Cl ) in radical reactions was suggested [14, 18]. It
2 2
was assumed that generated hydrogen chloride could cat- tions except the fact that only doublet states were cal-
alyze the photolysis and accelerate the reaction [14].
Therefore, here we studied the interaction of ylide
culated for the particles with unpaired electron.
Vibrational analysis also performed with consider-
ation for the solvation with dichloromethane con-
firmed that real minima on the potential energy sur-
face corresponded to all optimized systems. The ther-
mochemical analysis for all theoretically studied
systems was carried out at 298.15 K (25°C). The Gibbs
1
with trifluoroacetic acid (TFA) and considered the
mechanism of autocatalysis with participation of acids
with intermediate formation of the protonated ylide,
i.e., a phosphonium–iodonium dication active in the
decomposition on radicals and radical cations. The
easier decomposition of the protonated form of ylide 1
yielding radical cations is confirmed within the frame-
work of DFT calculations.
free energy changes (ΔG ) at 25°C corresponding to
298
the decomposition of ylide 1 and its protonated form
were determined as a difference between the sum of
the decomposition products free energies and the ini-
tial ylide free energy. To model the vertical excitations
of 1 and its protonated form to their singlet states, a
TD-DFT method [23] was used in combination with a
EXPERIMENTAL
Ylide 1 was synthesized according to the procedure PCM simulation of dichloromethane solvation.
developed in [1, 7, 8]. Ylide was thoroughly washed by
ether from the acid (HBF ) used in the synthesis.
4
RESULTS AND DISCUSSION
Interaction of Ylide 1 withTtrifluoroacetic Acid
Dichloromethane (Etalon grade, Komponent Reac-
tiv, Russia) without stabilizers and HCl and acetoni-
trile (High purity grade, Komponent Reactiv, Russia)
were used without additional purification.
Upon addition of an excess of TFA (5 × 10–3 mol L–1)
into the ylide 1 solution (1 × 10– mol L ) in CH Cl
4
–1
2
2
Absorption spectra were recorded on a Shimadzu or acetonitrile, the product with an absorption spec-
UV3101 PC (Japan) spectrophotometer in quartz cells trum close to the observed spectrum of transient spe-
with an optical pathlength of 1.0 or 0.4 cm. Ylide 1 cies in the photolysis of ylide 1 of the same concentra-
–4
–1
solutions (1 × 10 mol L ) were prepared in CH Cl
tion in CH Cl is formed without irradiation (compare
2 2
2
2
or acetonitrile. Calculated aliquots of the TFA stock Fig. 1 with Fig. 1A in [14]). The rate of the reaction
–1
solutions (1 : 10, [TFA] = 1.35 mol L ) in corre- with TFA in CH Cl without irradiation is more than
2 2
sponding solvent were dropwise added by a micropipet an order of magnitude higher than that in acetonitrile
into the ylide solution, with the acid addition decreas- (Fig. 1, inset). It should be pointed out that in the ylide
ing the ylide concentration by less than 1%. In what photolysis in acetonitrile in the presence of the acid,
follows in the text, the final TFA concentration in the no derivative of oxazole, which is the major product of
reaction mixture is given. All measurements were car- photoheterocyclization of 1 with acetonitrile in the
ried out at room temperature in the air.
absence of the acid [10], was found. The product
KINETICS AND CATALYSIS Vol. 60 No. 1 2019

4
6
NEKIPELOVA et al.
A
of 1 and in the reactions of 1 with TFA without irradi-
A
3
ation are similar: the absorption increases in the wave-
length range 260–290 nm and decreases at wave-
lengths >290 nm. However, the mass-spectrometric
analysis of the photolysate of 1 in CH Cl upon pho-
0
0
0
.8
1
1.0
.6
.4
2
2
tolysis completion [14] and of the solutions obtained
0
0
0
0
.8
in the reactions of 1 with TFA in CH Cl and acetoni-
2
2
8
1
0.2
2
trile without irradiation has shown that the products
are different in these reactions. In both cases, the
products of interaction of the ylide with acids, HCl in
the photolysis and TFA upon acid addition, are major.
.6
.4
.2
4
0
20
40
60
Time, min
1
The major product of 1 with TFA, the ion with
+
m/z = 493.12 Da (1 + TFA – PhI) in the mass-spec-
trum, is, probably, compound 8b, formed as a result of
conversions shown in Scheme 2. In the mass-spectro-
metric analysis of the products of interaction of 1 with
8
TFA in CH Cl , an unidentified molecular ion with
0
2
2
2
40
280
320
360
Wavelength, nm
m/z = 721.15 Da has been found. A molecular ion with
+
m/z = 697.04 Da (1 + TFA) containing PhI in its
composition has presented in trace amounts in both
solvents. This ion belongs to the adduct of TFA to
ylide 1 (7b). Compound 7b can also give the molecular
ion with m/z = 493.12 Da in the ESI-MS analysis by
PhI ejection. It is worth noting, that no chlorine-con-
taining compounds 7a and 8a are formed in the reac-
Fig. 1. Evolution of the absorption spectra of ylide 1 ([1] =
–
4
1
× 10 mol/L) in the presence of TFA ([TFA] = 5 ×
–
3
1
0
mol/L) in acetonitrile; time, min: 0 (1), 5 (2), 10 (3),
2
0 (4), 30 (5), 45 (6), 63 (7), and 90 (8). Inset: kinetics of
absorbance changes in acetonitrile (1, 2) and CH Cl (3, 4),
2
2
tion of ylide 1 with TFA in CH Cl , although these
registration wavelength, nm: 270 (1, 3) and 300 nm (2, 4).
2
2
compounds were identified in the photolysis of diluted
solutions of ylide 1 in CH Cl [14].
2
2
formed in the decomposition of 1 in acidic solutions is
stable without irradiation, but it is subjected to the
The following scheme for the interaction of acids
(
HA) with ylide 1 (Scheme 2) has been suggested on
photolysis during the irradiation of the reaction mix- the basis of experimental results of this study on the
ture by the light with the wavelength λ < 365 nm dark interaction of 1 with TFA and the results obtained
irr
affording salt 5. The spectral changes in the photolysis recently in the photolysis of 1 in CH
Cl [14]:
2
2
(
Ph)3P+
O−
Ph
(Ph) P+
OH
+ A−
Ph
(Ph)3P+
C
PhI
A
OH
Ph
Ph3P+
OH
Ph
HA
3
-
PhI
C
C
C
C
C
C
A
C
+
PhI
K
+
PhI
k1
k2
1
6
7
8
A = Cl
(7a, 8a)
A = CF COO (7b, 8b)
3
Scheme 2.
Phosphonium–iodonium dication 6 is a product of tion of compounds similar to 7 and 8 was suggested in
reversible addition of proton to ylide 1. The dication in [24] in the study of S 2 substitution in alkylvinyl(phe-
N
–
the ionic pair [6+ A ] enters into the reaction with an acid nyl)iodonium salts. Here we have shown that the
–
anion to give compound 7. The presence of the traces of product of addition of the anion (A ) to the iodonium
+
group 7 is unstable, ejects PhI, and fast coverts into 8.
Based on these results, we propose that the reaction of
dication 6 with the acid anion to give 7 is the limiting
step of conversion of dication 6 into final product 8.
the molecular ion with m/z = 619.10 Da (1 + HCl) in the
photolysis of 1 in CH Cl [14] (compound 7а) and
with m/z = 697.04 Da (1 + TFA) (compound 7b) in
the reaction of 1 with TFA confirms this. It follows
from our studies that ions formed as a result of detach-
2
2
+
According to spectrophotometric investigation of
ment of PhI are identified mainly in the ESI-MS anal- the kinetics of the ylide 1 reaction with TFA excess
ysis of the phenyliodonium compounds. Compounds 8 (registration at 270 nm) (Fig. 2a), the absorption
are the products of further conversion of 7. The forma- changes are approximated adequately by exponential
KINETICS AND CATALYSIS
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THE ROLE OF AN ACID
47
(а)
(b)
kobs, s–1
ΔA
0.16
5
0.020
4
0
.016
.012
3
0.12
0
2
0.08
0.008
1
0.04
0.004
0
200
400
600
Time, s
0
0.004
0.008
0.012
[TFA], mol/L
Fig. 2. (a) Kinetics of absorbance changes ΔA = A – A at λ = 270 nm in the mixture of ylide 1 ([1] =1 × 10–4 mol/L) and TFA
t
0
reg
in CH Cl as a function of the TFA concentration (mol/L): 0.00045 (1), 0.00135 (2), 0.0027 (3), 0.0045 (4), and 0.0135 (5);
2
2
(b) dependence of the rate constant of absorbance changes (kobs) on the TFA concentration.
functions, with the rate constant for the product for- (Fig. 3). If the acid concentration in the mixture at
mation (k ) being a linear function of the TFA con- zero time does not exceed the ylide concentration, the
obs
centration (Fig. 2b), and the bimolecular efficient rate absorption spectrum practically does not vary during
constant for the formation of the products of the ylide 15 min confirming the low concentration of dication 6.
–
1 –1
The further increase in the acid concentration
increases the absorbance in the range 260–286 nm,
with the solution absorbance at λ > 286 nm being
almost the same for several minutes up to the tenfold
increase in the acid concentration. (Fig. 3a). In
1
reaction with TFA (k ) being 1.5 ± 0.1 L mole s .
eff
Within the framework of the proposed mechanism of
the interaction of mixed phosphonium–iodonium
ylide 1 with acids (Scheme 2), the linear dependence
of the rate constant vs. the acid concentration implies
that K[HA] ! 1, and the experimental k = k K. The
1
5 min, the absorption spectrum varies in dependence
eff
1
on the TFA concentration: when the TFA concentra-
tion increases, the absorbance at λ > 286 nm decreases
similar to that observed in time after addition of the
large TFA excess (compare Fig. 3b and Fig. 1). When
the observation time increases, the isosbestic points
shift (Figs. 3a and 3b) also indicating the complex
character of the reaction between ylide 1 and the acid.
highest concentration of the acid in our experiments
–1
–1
was [TFA] = 0.014 mol L , i.e., K! 70 L mol . The low
value of the equilibrium constant means that the equilib-
rium concentrations of dication 6 in the mixture are low.
–1
In fact, if K = 5 L mol , the equilibrium concentration of
is ~5 × 10–6 mol L at [1] = 1 × 10 mol L and
–1
–4
–1
6
[
–2
–1
TFA] = 1 × 10 mol L , i.e., only 5% of the initial
ylide 1 concentration. This is probably caused by the
fact that dication 6 with double positive charge is
formed instead of cation 1. It is likely that the stability
of this dication should not be very high. Probably, this
is the reason why 6 either converts back into 1, or
Theoretic Thermochemical Analysis of Decomposition
of Ylide 1 and Its Protonated Form 6
Recently, it has been assumed that phosphonium–
iodonium dication 6, the protonated ylide 1, is more
active in the decomposition onto radical intermediates
than the parent ylide [14] (here and further on we
−
•
+
decomposes on 10 and PhI (see further reaction
(
(
II)), or stabilizes by conversion into 7 with a lower
+1) positive charge.
The complex character of the ylide 1 reaction with neglect the influence of the counterion BF4 on the
an acid and its subsequent transformations is con- properties of 1 and dication 6), and its formation in situ
firmed by the dependence of the absorption spectra of in the reaction of 1 with HCl (generated from the sol-
the ylide mixture with TFA on the TFA concentration vent, dichloromethane) or with the added acid accel-
KINETICS AND CATALYSIS Vol. 60 No. 1 2019

4
8
NEKIPELOVA et al.
(а)
(b)
A
A
A
0
0
0
0
0
.6
1
′
1
.0
1.0
.5
.4
.3
.2
1
0
0
0
0
.8
0.8
0.6
0.4
0.2
0
6
2
.6
.4
.2
1–3
0
.1
2′
6
0
1
0.0005
0.0015
[TFA], mol/L
, 2
1
, 2
1
–5
6
6
0
2
30
260
290
320
350
380
230
260
290
320
350
380
Wavelength, nm
Wavelength, nm
–4
Fig. 3. Absorption spectra of ylide 1 ([1] =1.0 × 10 mol/L) in 1 (а) and 15 (b) min after addition of TFA in concentrations
4
(
10 , mol/L): 0 (1), 0.6 (2), 2.0 (3), 5.0 (4), 10 (5), and 20 (6). Inset: dependence of absorbance on concentration of TFA at wave-
length (nm): 270 (1, 1'), 300 (2, 2') in 1 (1, 2) and 15 min (1', 2'); CH Cl solvent.
2
2
erates the synthesis of final products due to an increase 2.094 Å in 6 and shortening of the bond length
in the initiation rate. To confirm the assumption that between the iodine atom and the carbon atom of the
dication 6 decomposes on radical and radical cation phenyl group from 2.137 Å in 1 to 2.128 Å in 6 are sup-
species more readily than ylide 1, the theoretic ther- plementary evidences of higher tendency of 6 to
mochemical analysis of reactions (I) and (II) was car- decomposition (i.e., to the ejection of the radical cat-
•
+
ried out taking into account the solvation with ion (PhI) ). The Mulliken charges on the iodine atom
dichloromethane with the use of the model of polar-
ized continuum.
•+
in 6 and 1 are 0.594 and 0.606, respectively. In (PhI)
the С–I bond length is 2.038 Å, and the charge on the
iodine atom is 0.578. Thus, the calculations show that
+
O−
+
O−
Ph3P
Ph3P
•+
(PhI)+
6 is more prepared to the (PhI) ejection than 1. Pos-
+
+
C
C
C
C
+
PhI
sibly, this is the reason for the easy decomposition of
(
I)
Ph
Ph
the protonated form of 1 onto two radical cations.
1
9
+
+
(
Ph)3P
OH
C
Ph3P
OH
C
The Mechanism of Acid Catalysis
(PhI)+
C
C
in the Photolysis of Ylide 1
(
II)
+
PhI
Ph
Ph
Since the theoretical analysis of the decomposition
of ylide 1 and dication 6 formed from this was carried
out for thermal reaction, the question arises about
6
10
The PBE0 calculations show that the change in the
298
Gibbs free energy (ΔG ) in reactions (I) and (II) at application of these results to the photolytic decompo-
2
98 K (25°C) is according to the thermochemical sition of ylide 1. Thoroughly purified crystalline ylide
–1 –1
analysis 42.6 kcal mol (178 kJ) and 21.2 kcal mol
1 is stable and can be stored almost without changes
(
89 kJ), respectively. The enthalpy for reactions (I) for several months. However, being dissolved in CH Cl ,
2
2
–1
and (II) at 25°C increases by 58.1 kcal mol (244 kJ) ylide 1 converts to the products, with salt 5 being the
–1
and 38.5 kcal mol (161 kJ), respectively. Thus, the major, in the dark during one to several days. The dark
calculations confirm qualitatively the distinctly lower conversion is accelerated by the traces of an acid that
stability of dication 6 (reaction (II)) to the decompo- corresponds to the calculated data for the dication 6
sition in comparison to the decomposition of parent decomposition. Under irradiation, the ylide 1 in CH Cl
2 2
–1
ylide 1 (reaction (I)). The elongation of the bond solution (0.05 mol L ) is consumed completely
between the iodine atom and the carbon atom (С2–I) during 1 h, with salt 5 being the major product simi-
linked with the с PPh group from 2.063 Å in 1 to larly to the dark process. Thus, the decomposition of
3
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2019

THE ROLE OF AN ACID
49
Table 1. TD-DFT modeling of optical vertical excitation of According to the values of E and f (Table 1), the exci-
ex
ylide 1 and its protonated form 6 in dichloromethane to cor-
tation of 6 into the singlet states 2–4 is more probable
responding singlet states*
than the similar excitation of 1.
1
6
Singlet
state
We assume that, similarly to the dark reactions (I)
and (II), the Gibbs energies for the conversion of the
λ , nm E , eV
f
λ , nm E , eV
f
ex
ex
ex
ex
•
+
singlet states of 6* into (PhI) and radical cation 10
1
2
3
4
5
355.25 3.4900 0.0038 363.71 3.4088 0.0067
316.45 3.9180 0.0159 354.65 3.4960 0.0094
293.94 4.2180 0.0861 338.58 3.6619 0.0166
292.09 4.2448 0.0029 330.69 3.7493 0.0944
276.50 4.4841 0.0136 290.32 4.2706 0.0012
will be lower than those for the conversion of the sin-
•
+
glet states of 1* into (PhI) and 9. This implies that
the photolytic decomposition of 1 in dichloromethane
also should be accelerated by acids that is confirmed
by experimental data. It is reasonable to expect that
298
ΔG for the excited singlet states of 6* and 1* should
298
*
λ
is the excitation wavelength, Eex is the excitation energy, f is be significantly lower than ΔG for decomposition of
ex
the occilator strength.
6 and 1 in the ground state, i.e., the decomposition of
1
in the photolysis should be noticeably faster. To con-
firm these assumptions, it is necessary to carry out addi-
tional quantum chemical investigation of 6* and 1* and
the thermochemical parameters of their decomposi-
tion. This is a separate problem.
ylide 1 under irradiation and in the dark occurs with
formation of radical intermediates and the same final
products, with the rate of their formation being sig-
nificantly higher in the photolysis. Both neutral radi-
cals and radical cations are meant as radical interme-
diates.
The further transformations of radical intermedi-
•
+
ates (PhI) , 9, and 10 proceed according to reactions
(
III)–(VI):
The decomposition of 1 in the presence of the acid
traces is accelerated also in the photolysis. The irradi-
ation of 1 and dication 6 should excite them to singlet
states. TD DFT modeling of the optical vertical exci-
tation of ylide 1 and dication 6 to the corresponding
singlet states 1* and 6* in dichloromethane shows that
the probability of vertical excitation of 1 to the first
singlet state is almost twice lower than that for 6 at rel-
atively close excitation energies for 6 and 1 (Table 1).
Therefore, the generation of the first excited singlet
state and its further decomposition are more probable
for 6 upon irradiation of the mixture of 1 and 6.
+
PhI + CHCl2 + H+ (III)
(
PhI) + CH Cl
2
2
Ph3P+
HC
Ph3P+
C
O−
C
O
+ H+
C
(IV)
(V)
Ph
Ph
9
11
Ph3P+
C
OH
Ph
Ph3P+
O−
C
+ H+
C
C
Ph
10
9
Ph3P+
C
OH
Ph
Ph3P+
O−
Ph
Ph3P+
C
O (Ph) P
−
+
OH
Ph
3
C
+
C
C
C
+
C C
+
PhI
+
PhI
(VI)
Ph
1
0
1
9
6
The conversion of transient radicals 9 and 10 into
the radical cation 11 is the result of reactions (III)–
+
Ph3P
O
Ph
O
−
CHCl2
+
HC
C
+ CH Cl
2 2
Ph3PCH2C
(
VI). The PBE0 calculations with regard to the solva-
Ph
(VII)
tion by dichloromethane show that the free energy of
11
5
the radical cation 11 at 25°С is by 22.4 kcal mol–
1
The autocatalysis in the ylide 1 photolysis by
(
94 kJ) lower than that for 10. In other words, 11 is by
hydrogen chloride can be performed due to photolytic
2.4 kcal mol–1 thermodynamic more favorable than 10. and dark reactions of the solvent radical СHCl
•
2
2
(
(
[
formed in reactions (III) and (VII)) in reactions
VIII)–(X), in which hydrogen chloride is generated
15–17]:
In fact, radical cation 11 is relatively stable, and its
structure was suggested based on the quantum chemi-
cal calculation of the possible structure for the radical
cation registered by EPR in the photolysis of ylide 1
hν
CHCl2
CCl + HCl
(VIII)
(IX)
[
18]. In reaction (VII) of this radical with the solvent,
O2
H O
2
phosphonium salt 5, the major product of the ylide
transformations [18], and the radical •СHCl2 are
formed:
CHCl2
CHCl O
COCl2 + OH
2
2
COCl2 + H2O
CO2 + 2HCl
.
(X)
KINETICS AND CATALYSIS Vol. 60 No. 1 2019

5
0
NEKIPELOVA et al.
The excess of the added acid removes the ylide decomposition of other mixed phosphonium–iodo-
from the reaction, because it shifts the equilibrium nium ylides.
(
Scheme 2) to further conversion of reactive dication 6
It is shown that, at the excess of the acid, the pro-
tonated form of ylide 1 formed in its reaction with the
acid converts to compound 8, which does not partici-
pate in the subsequent ylide reactons. The mechanism
for the interaction of acids with mixed ylides is sug-
gested, the products of the interaction are character-
ized, and kinetic parameters for the reaction of ylide 1
conversion to compound 8 are estimated.
to compounds 7 and 8, which are not the initiators of
the radical reactions. The conversion of 6 to 7 occurs
in the bimolecular reaction with the acid anion. The
increase in the acid concentration increases the con-
centration of the anion and consequently accelerates
the reaction. This accounts for the experimental
observation that no target heterocyles 3 and 4 are
formed in the presence of acetylenes at the excess of
the acid [13]. The regularity of the reaction between
mixed phosphonium–iodonium ylides and acetylenes
will be considered in detail in following publications.
ACKNOWLEDGMENTS
The study was supported by the RF State Program
In theory, when ideally pure ylide 1 is used, the no. 1201253303.
decomposition of the ylide itself should be a reaction
trigger, which launches reaction sequence (I)–(X).
However, we cannot exclude that the initiation of the
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