A novel chemiluminescence from the reaction of dioxiranes with alkanes. Proposed mechanism of oxygen-transfer chemiluminescence

Article abstract of DOI:10.1039/b008315l

Oxidation of adamantane and 2,3-dimethylbutane by methyl(trifluoromethyl)dioxirane is accompanied by chemiluminescence (CL); formation of the emitter of CL, triplet excited trifluoropropanone, is proposed to occur via a concerted oxenoid mechanism of oxygen insertion into C-H bond of the hydrocarbons.

Full text of DOI:10.1039/b008315l

A novel chemiluminescence from the reaction of dioxiranes with alkanes.
Proposed mechanism of oxygen-transfer chemiluminescence
Dmitri V. Kazakov,* Anna B. Barzilova and Valeri P. Kazakov
Institute of Organic Chemistry, Ufa Scientific Center of the RAS, 71 Pr. Oktyabrya, 450054 Ufa, Russia.
E-mail: chemlum@ufanet.ru
Received (in Cambridge, UK) 16th October 2000, Accepted 7th December 2000
First published as an Advance Article on the web 8th January 2001
Oxidation of adamantane and 2,3-dimethylbutane by me-
thyl(trifluoromethyl)dioxirane is accompanied by chemi-
luminescence (CL); formation of the emitter of CL, triplet
excited trifluoropropanone, is proposed to occur via a
concerted oxenoid mechanism of oxygen insertion into C–H
bond of the hydrocarbons.
Indeed, under [TFD]o < < [Ad]o, the plots ln(ICL) vs. time
were found to be linear, yielding reproducible values of k1.
From this k2 values were estimated at different temperatures
(Table 1). The activation parameters for k2 calculated from the
data of Table 1 are equal to Ea = 8.2 ± 0.7 kcal mol21 and logA
= 6.5 ± 0.5.
Under pseudo-first order conditions the rate of the reaction
(W) is connected with the CL intensity by the following
expression:
Chemiluminescence (CL) is a promising rapidly developing
area of chemistry of new class of hyperenergetic molecules—
dioxiranes.1,2,3a,b,4 In 1981 it was suggested1 that isomerization
of dioxirane into the corresponding ester should lead to the
electronic excitation of the latter. Recently it was revealed that
decomposition of dioxiranes in the absence of reactive sub-
strates under certain conditions really results in production of
the light emission.2,3a,b Apart from the study of CL of isolated
dioxiranes, dioxirane intermediate has been also postulated to
explain luminescence in biochemical systems.5
ICL = oCL W = oCLk2[Ad][TFD] = k1[TFD]
(1)
where oCL is yield of CL.
The value of oCL was estimated from eqn. (1) using k2 values
taken from Table 1. Thus, at 18 °C oCL is equal to 1.2 3 10211
Einstein mol21. The yield of CL calculated from the ratio
S/[TFD]o ≈ 1.16 3 10211 Einstein mol21 is in agreement with
that obtained from eqn. (1).
On the other hand, dioxiranes are known to be highly
effective yet selective oxidants in respect to various classes of
organic compounds.3 Some of these oxidation reactions are
exothermic enough6 to generate excited species and conse-
quently to produce CL. The area of oxidative CL of dioxiranes
seems to be highly perspective from the viewpoint of funda-
mental questions of chemistry of excited states since it could
provide new valuable information not only on mechanisms of
CL, but also on some aspects of generation of light in
biochemical reactions as it has been suggested that dioxirane
intermediates may be involved in biochemical oxidations.7a
However, in spite of these promising perspectives, so far the
literature contained only a single report on generation of CL in
the course of oxidative reactions of isolated dioxiranes. In 1994
CL occurring during oxidation of olefin (9-arylmethylene-
10-methyl-9,10-dihydroacridine) with isolated dimethyldioxir-
ane (DMD) was reported.4
In this communication we report the observation of a novel
type of CL of dioxiranes occurring during oxidation of saturated
hydrocarbons, such as adamantane (Ad) and 2,3-dimethyl-
butane (DMB), with isolated methyl(trifluoromethyl)dioxirane
(TFD).7a We also suggest a plausible mechanism of lumines-
cence generation. To the best of our knowledge, CL in the
reaction of alkanes with dioxiranes is unprecedented.
CL occurring under interaction of dioxiranes with alkanes
appeared to be a more general phenomenon. In fact, we have
found that interaction of DMB with TFD is also accompanied
by light emission. The yield of CL (estimated as S/[TFD]o ratio)
is equal to 2.6 3 10212 Einstein mol21 (at 18 °C). It is important
to note that the region of CL appeared to be the same as for the
reaction of TFD with AD, i.e. lmax = 430–470 nm.
What is the mechanism of CL occurring under reaction of
TFD with Ad and DMB? Interaction of alkanes with dioxiranes
is known3,7 to result in the formation of the corresponding
alcohol and ketone–dioxirane reduction product. In the course
of previous studies it was shown3,7 that oxidation of alkanes
(including Ad and DMB) by TFD is a bimolecular reaction
implying involvement of high-ordered (so-called butterfly)
transition state (Scheme 1 (path (a)).†
Therefore, it seems to be reasonable to attribute the light
emission to the mechanism depicted in Scheme 1. According to
the proposed mechanism only corresponding alcohol R–OH and
trifluoropropanone (TFP) could be regarded as potential CL
emitters. However, we can exclude alcohols from the considera-
tion since they emit in the shorter wavelength region of
spectrum than that recorded for CL in our system. Indeed, the
range of CL observed upon oxidation of DMB and AD by TFD
corresponds to the phosphorescence (PS) of TFP, testifying that
We have found that interaction of Ad and TFD is accom-
panied by light emission in the visible spectrum region (lmax
=
Table 1 Dependence of the rate constant of oxidation of Ad by TFD on
430–470 nm). Under air atmosphere, the maximum intensity
of CL and the total amount of light (S) evolved in the reaction
(18 °C, CCl4, [Ad]o = 0.063 mol l21, [TFD]o = 0.0019 mol
l21) were equal to 2 3 106 photon s21ml21 and 1.4 3 107
photon ml21 respectively. However, significant increase in CL
intensity (ICL) was observed when the reaction was carried out
under nitrogen atmosphere. This effect testifies in favour of the
triplet nature of the CL emitter since it is known that oxygen
quenches triplet excited states.
temperature (solvent-CCl4, N2 atmosphered)
T/K
251
262
272
279.5 283.5 288.5 291.5
ak2 (l mol21 s21
)
0.23c 0.47 0.73 1.42 1.57 1.83 2.25
0.51b
a Estimated as k2 = (k1/[Ad]o); k1 values were obtained from pseudo-first
order kinetic plots of CL damping with [TFD]o = 1.8–3.8 3 1023 mol L21
and [Ad]o = 4–6.3 3 1022 mol L21; most data are averages (±5%) from
duplicate or more runs. b Value of the rate constant obtained previously7a (in
CH2Cl2–TFP mixture) under second-order rate conditions by non-CL
method (GLC). c Extrapolated to 251 K from Arrhenius equation.
d Replacing N2 with O2 atmosphere does not affect the rate constant of the
reaction.
Reaction of Ad with TFD is known7a,b to obey a second order
low (first order on each substrate):
W = k2[Ad][TFD]
or, under pseudo-first order conditions:
W = k1[TFD], where k1 = k2[Ad].
DOI: 10.1039/b008315l
Chem. Commun., 2001, 191–192
This journal is © The Royal Society of Chemistry 2001
191

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We thank Russian Foundation for Basic Research (project No
99-03-32140a) and program of Leading Scientific Schools
Support (project No 00-15-97323) for financial support of this
work. We are also grateful to Professor Curci (University of
Bari, Italy) for supplying us with the first samples of TFD and
critical reading of the manuscript.
Notes and references
† Oxidation of DMB by TFD (as 1+1 ratio) leads exclusively to the
corresponding alcohol.7a Likewise, interaction of Ad and TFD (as 0.9 ratio)
results in the formation of 94% of adamantan-1-ol (Ad-OH), 5% of
adamantane-1,3-diol and trace amounts of adamantanone (as a result of
further oxidation of Ad-OH).7b Apart from these compounds and TFP no
other products or even intermediates were detected in the course of the
reactions of alkanes with TFD.7 Kinetics evidence and the other
experimental data3,7 also testify in favour of absence of any other routes
(including those involving participation of radicals) of the reaction except
that depicted in Scheme 1 (path (a)). In particular, very low frequency factor
values (logA ~ 6.6–10) noted7a,c for oxidation of alkanes by TFD are
regarded as support for the existence of a highly ordered transition state
preceding the formation of reaction products.
Scheme 1
the latter in triplet excited state (TFP* ) is indeed an emitter of
T
CL.‡ Recent calculations5a also favor the proposed mechanism
of CL: oxygen insertion into C–H bonds of saturated hydro-
carbons was shown to be about 265–270 kcal mol21
exothermic6a so that the sum of enthalpy and activation energy
(according to our data ca. 8 kcal mol21 for the reaction of Ad
with TFD) should be sufficient for excitation (Scheme 1, path
(b)) of, at least, triplet state of TFP. In fact, the energetic level
‡ PS spectrum of TFP was recorded on Hitachi MPF-4 fluorimeter at 77K
(lexc = 320 nm): the spectrum has a broad maximum in the region 400–490
nm (spectrum of PS of TFP recorded in gas phase8 has a shift to longer
wavelength region at ca. 30 nm).
of TFP* is equal8 to ca. 75 kcal mol21, whereas the singlet
T
excited level of TFP8 is about 9 kcal mol21 higher and unlikely
to be occupied in the reaction.
*
§ Estimation of the yield of excitation of TFPT was made on the
One may note that the yield of CL arising during oxidation of
alkanes with TFD was found to be relatively low.§ This fact is
not surprising since numerous oxidation reactions are charac-
terized by rather low yields of CL (ultra-weak CL). In
particular, this is characteristic for light emission recorded in
certain biological systems.9
assumption that the quantum yield of its PS is about equal to that of acetone,
i.e. 1025. Under this approach, the yield of chemiexcitation of TFP was
estimated to be 1024 and 1025% for the reaction of TFD with AD and DMB
respectively.
¶ We have observed CL occurring upon interaction of TFD and DMD with
various types of organic compounds including some polyaromatic hydro-
carbons, dyes (rhodamine 6G, eosine), europium chelates etc. CL intensities
recorded in these reactions in many cases are significantly higher then those
observed in the reaction of dioxiranes with Ad and DMB (unpublished
results). The study of CL arising in reactions of organic compounds with
dioxiranes is under progress in our laboratory.
It is of interest that we failed to record CL upon interaction of
Ad with less reactive DMD. This is despite the fact that DMD,
similar to TFD, is known to oxidize Ad into the corresponding
alcohol.7e Probably, the slower rate of Ad oxidation by DMD,
compared with that of TFD,7b accounts for the impossibility of
recording CL due to its low intensity and the quenching of
excited acetone (formed by analogy with TFP) by molecules of
solvents or other reagents. However, introduction of 9,10-dicya-
noanthracene (DCA) in the system results in the appearance of
light emission ([DMD]o = 3.1 3 1022 mol l21, [Ad]o = 4 3
1022 mol l21, [DCA]o = 6.6 3 1025 mol l21, CCl4–acetone =
2+1, 20 °C). DCA was found to be the emitter of the CL
observed. One may suppose, that in this case DCA serves as a
CL activator and enhances CL intensity as a result of energy
transfer from excited acetone, formed by analogy with the
mechanism suggested for TFD oxidation of alkanes (Scheme 1,
path (b)). However, in contrast to the TFD case, along with
triplet ketone, significant contribution of singlet excited states
of acetone seems to have taken place since only insignificant
quenching (ca. 20%) of CL intensity by oxygen is observed in
this reaction. Consequently, excitation of DCA is likely to be
caused by transfer of energy from both singlet and triplet
excited molecules of acetone (Scheme 2).
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V. V. Shereshovets and V. P. Kazakov, Russ. Chem. Bull., 1997, 46, 456;
(b) D. V. Kazakov, A. I. Voloshin, N. N. Kabal’nova, V. V. Shereshovets
and V. P. Kazakov, Mendeleev Commun., 1998, 49; (c) D. V. Kazakov,
N. N. Kabalnova, A. I. Voloshin, V. V. Shereshovets and V. P. Kazakov,
Russ. Chem. Bull., 1995, 44, 2193; (d) D. V. Kazakov, A. I. Voloshin,
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Kricka and P. E. Stanley, John Wiley & Sons, 1999.
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Voloshin and D. V. Kazakov, Russ. Chem. Rev., 1999, 68, 253; (b) W.
Adam, L. P. Hadjiarapoglou, R. Curci and R. Mello, in Organic
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Scheme 2
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Likewise, effect of CL enhancement is observed when
reaction of TFD and AD is carried out in the presence of DCA,
obviously due to transfer of energy from the TFP* on the
T
activator with subsequent radiative deactivation of the latter.
In conclusion, oxidation of alkanes by TFD is accompanied
by CL.¶ We have proposed a plausible mechanism of CL
(Scheme 1 (path b)) to explain our observations. We suggest to
call this novel type of CL butterfly chemiluminescence or, in
more general sense, oxygen-transfer chemiluminescence. To the
best of our knowledge, this is a new mode of chemiexcitation
not only for dioxiranes but also for other liquid-phase organic
reactions with peroxide participation.
192
Chem. Commun., 2001, 191–192