Mechanism and remarkable features of photoinduced cycloaddition of phenylacetylene to mixed phosphonium-iodonium ylide

Full text of DOI:10.1134/S0012500812110092

ISSN 0012ꢀ5008, Doklady Chemistry, 2012, Vol. 447, Part 1, pp. 262–265. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © T.D. Nekipelova, M.A. Taranova, E.D. Matveeva, T.A. Podrugina, V.A. Kuzmin, N.S. Zefirov, 2012, published in Doklady Akademii Nauk, 2012, Vol. 447,
No. 3, pp. 296–299.
CHEMISTRY
Mechanism and Remarkable Features
of Photoinduced Cycloaddition of Phenylacetylene
to Mixed Phosphonium–Iodonium Ylide
a, b
b
b, c
T. D. Nekipelova , M. A. Taranova , E. D. Matveeva ,
b
a, b
b, c
T. A. Podrugina , V. A. Kuzmin , and Academician N. S. Zefirov
Received May 14, 2012
DOI: 10.1134/S0012500812110092
In recent years, the investigation of the structure tion [4–7]. The direction of the photolysis depends on
and reactivity of an almost uninvestigated class of the structures of the compounds with a triple bond.
mixed phosphonium–iodonium ylides allowed us to Nitriles
create novel reagents for organic synthesis on their being the leaving group [4]. When the reaction occurs
basis. This class of compounds with several reactive between mixed phosphonium–iodonium ylide ( ) and
), furan derivative
), and phosphonium salt ( ) are formed as a
result of the photolysis (Scheme 1; the counterion BF4
R CN afford oxazoles with high yield, PhI
1
1
5
centers exhibits new reactions and opens new routes to acetylenes (
the synthesis of not easily accessible and novel heteroꢀ
cyclic compounds [1–3]. Recently, we described two
2),
λ
ꢀphosphinoline (4
(
5), PhI (
3
6
−
unknown photochemical reactions of mixed phosphoꢀ is omitted in Scheme 1 and further on for simplicity)
nium–iodonium ylides in the presence of compounds [6, 7]. The relative yield of products
depends subꢀ
4–6
with triple bond to give products of pseudoꢀcycloaddiꢀ stantially on the structure of acetylene [7].
2
R C≡CH
2
COR1
+
Ph
+
+
–
Ph3P
+ (α) H + (β)
Ph3P
O
Ph P
+
hν
+
1
PhI + (α)
+ (γ)Ph PCH –C–R
C
CH Cl
2
3
2
+
PhI
2
R2
R1
R2
R1
3
O
O
1
4
5
6
(α) + (β) + (γ) = 1,
where
α,
β
, is the relative yield of products.
γ
Scheme 1.
From examination of Scheme 1, the following pound and its formation from ylide
1
(the cation!) is
points should be emphasized: (1) all reaction products accompanied by elimination of proton; and (3) two
do not contain iodine, which is evolved in the form of extra hydrogen atoms are necessary for the formation
iodobenzene
consumed; (2)
3
in the amount equimolar to ylide
1
of phosphonium salt
The mechanism of the formation of the furan
derivative is probably similar to that of the oxazole
formation in the photolysis of in the solutions of
nitriles [4, 5], whereas the regularities of the formation
6 (the СН2 group of the salt).
5
λ
ꢀphosphinoline is a neutral comꢀ
5
1
a
Emanuel Institute of Biochemical Physics,
Russian Academy of Sciences,
5
of
λ
ꢀphosphinoline, a relatively exotic heterocycle,
deserve special attention. This is particularly necessary
ul. Kosygina 4, Moscow, 119991 Russia
to take into account because sometimes, for example,
b
Moscow State University, Moscow, 119991 Russia
Institute of Physiologically Active Compounds,
1
in the reaction of
1
(R = Ph), we omit the designation of
c
2
this group further on) with phenylacetylene
2
(R
= Ph),
Russian Academy of Sciences, Chernogolovka,
Moscow oblast, 142432 Russia
5
λ
the corresponding
ꢀphosphinoline is a major prodꢀ
262

MECHANISM AND REMARKABLE FEATURES OF PHOTOINDUCED CYCLOADDITION
263
1
2
3
4
5
6
7
8
Fig. 1. Development of the reaction between ylide
1
and phenylacetylene
2
in CH Cl2 upon irradiation by the light with
λ
irr
400 nm.
2
Irradiation time, min: (
1) 0, (2) 7.0, (3) 7.25, (
4
) 7.5, (
5) 7.75, (6) 8.00, (
7
) 8.5 and (
8) 9.0.
uct of the photolysis (the yields of
less than 7%, respectively [6]).
4
and
5
are 60%, and off, and the reaction proceeds without irradiation. As
one can see in Fig. 2, the absorption spectrum of the
colored product registered in the visible spectral range
In this communication, we present the more
detailed study of the reaction between and in the
CH Cl solution carried out by steadyꢀstate photolyꢀ
5
corresponds to the absorption spectrum of
λ
ꢀphosꢀ
1
2
phinoline . One more peculiarity of the reaction is
4
2
2
the observation that the sampling should be carried out
carefully without mixing the solution. If the solution
does be mixed, the reaction ceases and occurs with a
new induction time. The addition of the stable reacꢀ
sis. It has been found that the reaction between
1
and
2
of
to give 4 occurs only at relatively high concentrations
1 (0.02 mol L and more in CH Cl2). At these conꢀ
2
–1
centrations, ylide
1 is not completely soluble in
CH Cl and gives weakly opalescent suspension. The
2
2
solution is light yellow, whereas the parent ylide
crystalline form is white powder. The reaction was carꢀ
ried out at a fivefold excess of acetylene and was
induced by irradiation with the light with from 350 to
50 nm, where ylide in concentrations usually used
1 in
A
.8
A
.050
1
0
2
λ
4
1
1.5
0
.035
0.020
.005
–4
–1
in the spectrophotometric measurements (10 mol L )
does not absorb noticeably. However, upon the
1
.2
.9
enhancement of the concentration of
1
or, the use of
2
0
1
cells with an optical path length of 5 cm, the tail of the
UV absorption is observed in the visible spectral range
up to 460 nm. The reaction kinetics was controlled by
spectrophotometry by sampling with a syringe in a
definite times and 100ꢀfold dilution of the samples by
ethanol.
−
0.005
400
450
500
, nm
0
λ
0.6
5
The development of the reaction is remarkable.
The induction time (from 7 to 20 min in dependence
on the irradiation intensity) is observed. During this
4
3
0
.3
1
2
time, there are no changes in the absorption spectrum
0
250
5
350
450
550
, nm
in the visible range, where
λ
ꢀphosphinoline
4
absorbs.
λ
Then the reaction develops spontaneously beginning
locally either on a heterogeneous particle of ylide , or
1
Fig. 2. Changes in the absorption spectra of the mixture of
on a cell wall, or on the solution surface that manifests
itself by the appearance of a bright brown spot (Fig. 1).
During 1–2 min, the reaction develops over the whole
volume of the solution and is completed. The final
solution is brown and transparent. Interestingly, when
the reaction develops, the irradiation can be switched
(0.024 mol L–1) +
2 (0.1 mol L ) in CH Cl2 during the
–1
1
2
photolysis ( 400 nm), photolysis time, min: (
λ
1
) 0, (
5ꢀphosphinoline
10 mol L ) in EtOH; inset, photolysis time, min:
) 6.0 and (2) 7.0. For recording of the spectra, samples
2) 7.0,
irr
(
3
) 8.0, (
4
) 9.0, and (
5) the spectrum of
λ
–
5
–1
(5.6
×
(
1
were diluted 100ꢀfold with EtOH.
DOKLADY CHEMISTRY Vol. 447
Part 1
2012

264
NEKIPELOVA et al.
tion products,
salt
chloride CH Cl is used as a solvent, because ylide
λ
5ꢀphosphinoline
4
and phosphonium pH of the aqueous extracts of the starting and final
6
does not affect the reaction course. Methylene solutions shows a significant increase in the acidity of
1
is the mixture in the reaction course (pH 6.0 and 2.0,
respectively).
2
2
poorly soluble in phenylacetylene.
The unusual development of the reaction allows for
the proposal that the reaction occurs either by autoꢀ pulse photolysis of
Our experimental results obtained by the laser
1 in acetonitrile confirms the
catalytic or chain mechanism, and the principal probꢀ assumption that elimination of PhI and the formation
lem is the nature of products providing the autocatalꢀ of a transient species with the carbocationic structure
ysis or chain propagation and branching. Considering
(7) (Scheme 2, route 1) is the primary photochemical
the reaction mechanism, one should take into account reaction [5]. However, this photochemical reaction is
the following: (1) the reaction occurs under irradiation only one of the possible routes. In the literature, both
with the light with 350 <
λ
irr
< 450 nm that means that radical and radical cationic schemes of the transforꢀ
2
participates in the reaction in the ground state, and mations are considered for the photolysis of iodonium
this is ylide
1
in the excited state that is the primary salts [8, 9]. Generalization of our and the literature
active species, and (2) the reaction proceeds very data gives the following mechanism for the photodeꢀ
slowly in pure (Scheme 2): iodobenzene and
, and the rate is significantly higher in composition of ylide
methylene chloride solution. The proton elimination carbocation are formed in route , and radical and
following from Scheme 1 is confirmed by the fact that radical cation of iodobenzene are formed in route
2
1
7
1
8
9
2.
4 (R = Ph) + H+
+
1
–
O
route 1ꢀ1
2
Ph3P
PhI +
C
route 1ꢀ2
+
Ph
5
(R = Ph)
7
1
+
–
route 1
hν
Ph3P
O
+
–
+
PhI
C
route 2
Ph3P
O
+
CH2Cl2
Ph
•
•
CHCl + PhI + H+
+ PhI
•
C
2
1
Ph
8
9
Scheme 2.
The further transformation of cation
tion with phenylacetylene can occur via two routes.
Route is an electrophilic attack of the cation on the
triple bond with a consequent step of electrophilic aroꢀ
7
in the reacꢀ
As one can see in Scheme 2, route
2
affords radꢀ
. Radical
2
ical and iodobenzene radical cation
8
9
8
1ꢀ1
can abstract the hydrogen atom from the solvent,
and the subsequent protonation gives enol 11 conꢀ
5
matic substitution to give
ton elimination as we assumed in [7]. Route 1ꢀ2 is a
reaction of cation with the triple bond to give furan
similar to the formation of oxazoles in the photolysis of
in the nitrile solutions.
λ
ꢀphosphinoline 4 and proꢀ
verting into a ketone form of salt
6 (Scheme 3).
Interestingly, this process requires protons in the
amount equivalent to the amount of the forming
salt.
7
5
1
+
–
+
+
–
Ph3P
O
CH Cl
Ph3P
O
H+
Ph3P
OH
Ph
2
2
•CHCl2 +
•
C
6
C
C
Ph
H
Ph
H
8
10
11
Scheme 3.
As was mentioned above, the appearance of proꢀ both intermediates are formed in the same reaction,
tons in the medium is the result of the electrophilic and the proton is consumed in the salt formation
substitution according to route in Scheme 2. Howꢀ
10 11), the acidity of the reaction mixture as a
whole is not increased in the route
6
1ꢀ1
(
→
2.
ever, there is another route of the proton formation in
the reaction of radical cation with the solvent
One point needs special consideration. Since proꢀ
Scheme 2, route ). Since the ratio = 1, because ton is formed simultaneously with the formation of 4,
9
(
2
8
:
9
DOKLADY CHEMISTRY Vol. 447
Part 1
2012

MECHANISM AND REMARKABLE FEATURES OF PHOTOINDUCED CYCLOADDITION
265
and the higher conversion degree of
1
into
4
, the This will give a key to control the direction of this
+
higher concentration of
H
in the solution, it is natuꢀ complex photochemical–catalytic process.
rally to propose that, in this reaction, we deal with the
acidic catalysis of the initiation step. In fact, the addiꢀ
tion of acid (trifluoroacetic or pꢀtoluenesulfonic acid)
in the concentration higher than 0.15 mol/L initiates
the formation of the products even in the absence of
the irradiation. We propose that, at these acid concenꢀ
ACKNOWLEDGMENTS
This work was supported by Program no. 1 of the
Division of Chemistry and Materials Science of the
Russian Academy of Sciences.
trations, ylide
catalyzes its decomposition to give
1 is protonated, and the protonation
+
4
, PhI, and H . As
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2
3
4
5
6
7
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2
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DOKLADY CHEMISTRY Vol. 447
Part 1
2012