Expected and unexpected photochemical reactions of acyl iodides

Article abstract of DOI:10.1134/S1070428009110062

Photolysis of acyl iodides RCOI (R = Me, Me₂CH, Ph) under UV irradiation in toluene environment for 20-55 h proved to be a simple and efficient method of preparation of symmetrical α-diketones RCOCOR. In contrast, the photolysis under the same

Full text of DOI:10.1134/S1070428009110062

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 11, pp. 1616−1620. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © M.G. Voronkov, N.N. Vlasova, A.V. Vlasov, L.I. Belousova, O.Yu. Grigor’eva, A.I. Albanov, G.F. Myachina, T.I. Vakul’skaya, 2009,
published in Zhurnal Organicheskoi Khimii, 2009, Vol. 45, No. 11, pp. 1627−1631.
Expected and Unexpected Photochemical Reactions of Acyl
Iodides
M. G. Voronkov, N. N. Vlasova, A. V. Vlasov, L. I. Belousova, O. Yu. Grigor’eva,
A. I. Albanov, G. F. Myachina, and T. I. Vakul’skaya
Faworsky Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences,
Irkutsk, 664033 Russia
e-mail: grigoreva_o@irioch.irk.ru
Received December 26, 2008
Abstract—Photolysis of acyl iodides RCOI (R = Me, Me2CH, Ph) under UV irradiation in toluene environment for
20–55 h proved to be a simple and efficient method of preparation of symmetrical α-diketones RCOCOR. In
contrast, the photolysis under the same conditions of acyl iodides RCOI [R = Me(CH2)3, Me3C] did not lead to the
formation of the corresponding diacyls, and the reaction products were unexpected 1,1-bis(4-methylphenyl)pentane
and a mixture of isomeric 3- and 4-methyl(tert-butyl)benzenes respectively. The most probable mechanism of their
formation is the primary photochemical acylation of toluene in the aromatic ring followed by the photochemical
reduction of the arising butyl 4-methylphenyl ketone in the case of the valeroyl iodide or the photochemical
Norrish type I cleavage of isomeric 3- and 4-methylphenyl (tert-butyl) ketones in event of the pivaloyl iodide. In
the photolysis of acetyl iodide (R = Me) in benzene or toluene alongside the diacetyl formation polyarylation
process was observed of acylated and iodinated into the aromatic ring solvents with the formation of polymeric
products with semiconductor and paramagnetic properties.
DOI: 10.1134/S1070428009110062
It was formerly reported that on the photolysis of
acetyl iodide the diacetyl MeCOCOMe did not form [1].
This fact was ascribed to the short lifetime of MeCO·
radical (10–11 s) decomposed further under the UV
irradiation into Me· radical and carbon monoxide CO.
the diacyl black nonmelting polymers formed insoluble in
organic solvents. The polymer formed at the photolysis
of MeCOI in benzene has the elemental composition close
to the formula C14H8I2O (VII), and in toluene, to
C16H15IO (VIII). The formation of these polymers shows
the proceeding of concurrent processes of photochemical
iodination and acylation with acetyl iodide of the aromatic
ring of solvents with subsequent polyarylation, a char-
acteristic photochemical reaction of such aromatic
compounds substituted in the ring [5] (Scheme 1).
Yet in the course of the systematic research on acyl
iodides application as reagents and synthons in organic
and organoelemental synthesis [2] we established a pos-
sibility to obtain symmetric diacyls by photochemical
dissociation of acyl iodides RCOI (R = Me, Me2CH, Ph)
[3].
The polymers obtained are paramagnetic semi-
conductors. In their ESR spectra singlet signals are
observed with the characteristics inherent to the polymers
with a well-developed conjugation system (see the table).
Polymer VII is apparently built from the regularly
alternating blocks IC6H2COMe and IC6H3, and polymer
VIII, from fragments IC6H2CH3 and CH3C6H2COMe.
The concentration of the paramagnetic centers in polymer
VIII (2.1 × 1019 spin/g) is by an order of magnitude larger
than in polymer VII (2.9 × 1018 spin/g), and the con-
ductivity of the former is 2.6 times lower [7.7×10–8 (VII),
We report here on the search for the optimum condi-
tions of the photochemical conversion of acyl iodides
RCOI [R = Me (I), Me2CH (II), Me3C (III), Me(CH2)3
(IV), Ph (V)] into α-diketones.
It was established that the photolysis of MeCOI (I)
under the irradiation with a mercury-quartz lamp OKN-
11 in the medium of benzene or toluene that were
sensitizers of photochemical reactions [4] within 19–
21 h led to the formation of diacetyl (VI) in amounts of
62 and 49% from the theoretical respectively. Alongside
1616

EXPECTED AND UNEXPECTED PHOTOCHEMICAL REACTIONS
1617
Scheme 1.
MeCOI
MeCOCOMe
_I2
I
hv,
hv
_HI
2
hv
VI
RC6H5 + I2
RC6H3I2
RC6H4I
MeCOI
_HI
RC6H5
_HI
I
RC6H4COMe
hv, RC6H3I2
hv,
RC6H4COMe
_HI
hv
RC6H3I(R)C6H2COMe(R)C6H3I
RC6H3IC6H3(R)COMe
....
_HI
R = H, Me
1.2 × 10–10 (VIII) Ohm/cm]. These differences may be
due to the higher iodine content in polymer VII. On
additional doping of the polymers with iodine vapor for
24 h the conductivity of polymer VIII considerably
increased (to 1.9 × 10–6 Ohm/cm), and the concentration
of unpaired spins increased two-fold (to 3.8 × 1019 spin/g),
whereas in polymer VII these parameters changed insig-
nificantly (see the table). The sharp increase in the
conductivity of polymer VIII at the relatively small
increase in the concentration of the paramagnetic centers
after its doping with iodine may be due to the formation
of diamagnetic bipolarons governing the conductivity. The
broadening of signals and increased g-factor in the doped
polymers manifest the interaction between the unpaired
electron and the orbitals of the heavy iodine atom.
polymeric products in these processes formed in
insignificant amounts. The studied potochemical
dissociation of acyl iodides I, II, and V is now the simplest
and the most efficient preparation procedure of the
corresponding α-diketones VI, IX, and X (Scheme 2).
Scheme 2.
hv
RCOCOR
2RCOI
VI, IX, X
I, II, V
R = Me (I, VI), Me2CH (II, IX), Ph (V, X).
Unexpectedly the photolysis under analogous
conditions of acyl iodides III and IV did not yield the
corresponding diacyls.After UV irradiation of the solution
of acyl iodide IV in toluene for 26 h only trace amounts
of decane-5,6-dione (XI) were identified. The main
product of this reaction proved to be 1,1-bis(4-methyl-
phenyl)pentane (XIV). It formed along the following most
probable mechanism. In the first stage toluene is acylated
into the para-position of the benzene ring analogously to
the reaction with H-donors (to which belongs also the
toluene [6]) of acyl chlorides [7]. The obtained in this
stage 4-butyl methylphenyl ketone (XII) further adds
The fact that the residual content of acyl iodide after
the studied reaction in toluene was only 16% compared
with 33% in benzene was the reason of the choice of
toluene as the solvent for further investigation of the
photochemical transformations of acyl iodides.
For instance, the increase of the duration of the acetyl
iodide photolysis in toluene to 55 h resulted in nearly
quantitative yield of diacetyl (VI) (93%). Similarly, in the
photolysis in toluene environment of acyl iodides II and
V for 20 h the symmetric diacyls RCOCOR IX and X
formed in 49 and 89% yield respectively. The side
Parameters of electroconductivity and ESR spectra of polymers VII and VIII
Ohm/cm
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VORONKOV et al.
1618
Scheme 3.
Me(CH2)3COI
_I2
Me(CH2)3COCO(CH2)3Me
XI
hv
Me
Me
Me
Me(CH2)3COI
IV
*
.
ICO(CH2)3Me
Me
.
O
I
C
H
(CH2)3Me
Me
Me
C
Me
Me
C
hv, 2HI
_H2O, _I2
hv,
hv
_HI
hv
.
OH
(CH2)3Me
O
.
C
(CH2)3Me
HC
(CH2)3Me
(CH2)3Me
O
XII
Me
Me
XIII
XIV
Scheme 4.
Me
Me
Me
Me
Me
.
O
*
hv
hv
+ Me3CCOI
III
ICOCMe3
.
+
O
CH
CMe3
I
I
C
H
CMe3
Me
Me
Me
O
hv
_2HI
hv
_2CO
+
+
C
CMe3
CMe3
C
CMe3
CMe3
O
XVIII
XVI
XVII
a toluene molecule to the carbonyl group followed by
the reduction of the formed 1,1-bis(4-methylphenyl)
pentan-1-ol (XIII) with the liberated hydrogen iodide
(Scheme 3).
After the photolysis of acyl iodide III a mixture was
isolated of isomeric 3- and 4-methyl(tert-butyl)benzenes
XVII and XVIII in a ratio 1.5:1 (overall yield 45%).
Alongside these products trace amount of tert-butyl
methylphenyl ketone was identified. The formation of the
latter indicates that in this case also the toluene is acylated
A similar mechanism of benzophenones reduction was
formerly described [8].
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 11 2009

EXPECTED AND UNEXPECTED PHOTOCHEMICAL REACTIONS
1619
into meta- and para-positions followed by the photo-
chemical Norrish type I cleavage of the obtained
isomeric ketones [9] (Scheme 4).
2.0 g of the fraction boiling at 70–80°C and containing
33% of the initial acetyl iodide and 67% of diacetyl (VI),
yield on initial acetyl iodid 62%. IR spectrum, ν, cm–1:
1795 (CO in MeCOI), 1700 (CO in MeCOCOMe).
Found, %: I 24.65. C2H3OI. Calculated, %: I 74.07.
After the photolysis of acyl iodides III and IV in
benzene or in its absence but in the presence of elemental
copper or ittrium chloride whose photolytic activity has
been discovered before [10] no the corresponding α-di-
ketones or other products were found, the initial reagents
were recovered intact.
The filtered off solid precipitate was washed with
ethanol, ether, and it was kept for 24 h in a vacuum-
desiccator. We isolated 6.5 g of black polymer, nonmelting,
insoluble in ethanol, ether, aliphatic and aromatic hydro-
carbons, acetone, chloroform, and tetrachloromethane.
IR spectrum, ν, cm–1: 1695 (CO), 1600, 1545 (C–C of
benzene ring), 1010, 860, 760 (C–H of ring), 420 (C–I).
Found, %: C 33.02; H 2.38; I 59.92. C14H8I2O.
Calculated, %: C 37.66; H 1.79; I 56.95.
The differences in the photolysis directions of acyl
iodides I, II, V, on the one hand, and III, IV, on the
other hand, are due to the sharp distinctions in their
dissociative ability, and also to the difficulties arising in
further recombination of bulky acyl radicals RCO· with
substituents R = Me3C (III) and Me(CH2)3 (IV)
(Scheme 2).
Photolysis of acetyl iodide (I) in toluene. a.
A solution of 17.0 g (100 mmol) of acetyl iodide in 10 ml
of toluene was kept under UV irradiation for 19 h. On
completion of the reaction the reaction mixture was
filtered and distilled to separate 2.5 g of a fraction with
bp 80–100°C containing diacetyl (VI) with an impurity
of 16% of acetyl iodide (yield of diacetyl on initial acetyl
iodid 48%). IR spectrum, ν, cm–1: 1795 (CO in MeCOI),
1700 (CO in MeCOCOMe). Found, %: I 12.15. C2H3OI.
Calculated, %: I 74.67.
EXPERIMENTAL
As a source of UV radiation a mercury-quartz lamp
of low pressure OKN-11 was applied. IR spectra were
recorded on a spectrophotometer UR-20 from thin films.
1H and 13C NMR spectra were registered on a spec-
trometer Bruker DPX-400 at operating frequency 400
MHz from solutions in CDCl3 (internal references TMS
or HMDS). ESR spectra were measured at room
temperature on a radiospectrometer SE/X-2547
(Radiopan) of sensitivity 5 × 1010 spin/g equipped with
a magnetometer and the high frequency meter. The
concentration of the paramagnetic centers was calculated
by procedure [11] using calibrated standards of diphenyl-
picrylhydrazyl and a nomogram of the double integration
of the derivative of the absorption line [12]. The electro-
conductivity of polymers VII and VIII was measured
on a standard teraohmmeter E6-13A. The studied samples
were prepared as pellets pressurized at 700 kg/cm2. Iodine
doping of polymers was performed by diffusion method
in the gas phase.
The filtered off solid precipitate was washed with
methanol and ether. We isolated 7.8 g (46%) of black
polymer, nonmelting, insoluble in water, methanol, ether,
acetone, toluene, deuterochloroform, and tetrachloro-
methane. IR spectrum, ν, cm–1: 1700 (CO), 1610, 1550
(C–C of benzene ring), 1000, 850, 780 (C–H of ring),
450 (C–I). Found, %: C 54.31; H 4.20; I 36.22. C16H15IO.
Calculated, %: C 54.87; H 4.32; I 36.24.
b. A solution of 17.0 g (100 mmol) of acetyl iodide in
10 ml of toluene in a quartz flask of 50 ml capacity
equipped with a reflux condenser and isolated from the
access of moisture was kept under UV irradiation for
55 h. On completion of the reaction the reaction mixture
was filtered and distilled under the atmospheric pressure.
Yield of diacetyl (VI) 4.0 g (93%), bp 88–90°C (88°C
[15]). IR spectrum coincided with the standard. Found,
%: C 55.02; H 6.67. C4H6O2. Calculated, %: C 55.81;
H 7.02.
Initial acyl iodides I, III–V were obtained by treating
the corresponding acyl chlorides with anhydrous sodium
iodide [13], isobutyryl iodide (II) was prepared by the
reaction of isobutyric acid with acetyl iodide [14].
Photolysis of acetyl iodide (I) in benzene
solution. A solution of 8.5 g (50 mmol) of acetyl iodide
in 6 ml of benzene charged into a quartz flask of 50 ml
capacity equipped with a reflux condenser was kept under
UV irradiation for 21 h. On completion of the reaction
the liquid phase of the reaction mixture was filtered off
and distilled at the atmospheric pressure. We isolated
Photolysis of isobutyryl iodide (II). A solution of
9.0 g (45 mmol) of isobutyryl iodide in 8 ml of toluene
was kept under UV irradiation for 20 h. The reaction
mixture was distilled and treated with copper powder.
Yield of α-diketone IX 1.6 g (49%), bp 140–145°C,
nD20 1.5310. IR spectrum, ν, cm–1: 1705 (CO). Found, %:
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 11 2009

VORONKOV et al.
1620
of dibenzoyl X, mp 95°C (95–96°C [15]). IR spectrum
coincided with the standard. Found, %: C 79.52; H 5.06.
C14H10O2. Calculated, %: C 79.98; H 4.79.
C 67.00; H 9.88. C8H14O2. Calculated, %: C 67.57;
H 9.92.
Photolysis of pivaloyl iodide (III). A solution of
9.6 g (94 mmol) of pivaloyl iodide (III) in 10 ml of toluene
was kept under UV irradiation for 22 h. We isolated from
the reaction mixture 5.0 g (52%) of a fraction with bp
75–80°C containing mainly a mixture of 3- and 4-methyl-
(tert-butyl)benzenes (XVII, XVIII) in a ratio 1.5:1. IR
spectrum, ν, cm–1: 3960–2860 (C–H), 1600, 1480, 1460
(C–C of ring), 800, 760, 700 (C–H of ring). 1H NMR
spectrum, δ, ppm: 1.35 s [9H, p-(CH3)3C], 1.353 s [9H,
m-(CH3)3C], 2.35 s (3H, p-Me), 2.39 s (3H, m-Me), 7.14,
The study was carried out under a financial support
of the Council of Grants of the President of the Russian
Federation (NSh-255.2008.3).
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10.0 g (47 mmol) of acyl iodide IV in 5 ml of toluene was
kept under UV irradiation for 26 h. On completion of the
reaction we isolated from the reaction mixture 5.0 g (45%)
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30.40
(CH3CH2CH2),
35.65
2,6
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kept under UV irradiation for 20 h. From the reaction
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 11 2009