Mechanism and selectivity of radical alkylation of 3,4-Dichloro-2,5- dihydrofuran-2,5-dione

Article abstract of DOI:10.1134/S1070428007060024

The mechanism of radical alkylation of 3,4-dichloro-2,5-dihydrofuran-2,5- dione with cyclohexane and 2,3-dimethylbutane follows an addition-elimination pattern with reversible formation of alkyl radicals. The proposed kinetic scheme takes into account the

Full text of DOI:10.1134/S1070428007060024

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2007, Vol. 43, No. 6, pp. 801–811. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © A.V. Trukhin, A.V. Messorosh, V.S. Karavan, E.V. Eliseenkov, 2007, published in Zhurnal Organicheskoi Khimii, 2007, Vol. 43,
No. 6, pp. 806–816.
Mechanism and Selectivity of Radical Alkylation
of 3,4-Dichloro-2,5-dihydrofuran-2,5-dione
A. V. Trukhin, A. V. Messorosh, V. S. Karavan, and E. V. Eliseenkov
St. Petersburg State University, Universitetskii pr. 26, St. Petersburg, 198504 Russia
e-mail: atrukhin@yandex.ru
Received April 28, 2006
Abstract—The mechanism of radical alkylation of 3,4-dichloro-2,5-dihydrofuran-2,5-dione with cyclohexane
and 2,3-dimethylbutane follows an addition–elimination pattern with reversible formation of alkyl radicals. The
proposed kinetic scheme takes into account the possibility for isomerization of primary 2,3-dimethylbutane
radicals into tertiary and is consistent with the experimental data. The regioselectivity of the process is linearly
related to the concentration of hydrogen chloride, so that the rate constant for the addition of primary 2,3-di-
methylbutane radical to 3,4-dichloro-2,5-dihydrofuran-2,5-dione may be estimated. Effective procedures for
the synthesis of 3-chloro-4-(2,3-dimethylbut-2-yl)-, 3-chloro-4-cyclohexyl-, and 3,4-dicyclohexyl-2,5-dihydro-
furan-2,5-diones have been proposed.
DOI: 10.1134/S1070428007060024
At present, radical reactions attract increased in-
terest from the viewpoint of building up new carbon–
carbon bonds, and they are extensively used in syn-
thetic organic chemistry [1]. Widely used reagents for
radical reactions are organotin compounds; examples
are radical addition or allylation (Scheme 1, reaction a,
SO R) [4, 5], vinyl sulfones (reaction b, X = SO Alk)
2 2
[6], and nitrostyrenes (reaction b, R′ = Ar, X = NO2)
[7, 8] and addition of S,O-dialkyl dithiocarbonates to
olefins [9, 10] have been proposed, and mechanisms of
these reactions have been studied in detail. Addition–
elimination processes involving 2,3-dichloro-2,5-di-
hydrofuran-2,5-dione [11–13], tetrachloroethylene
[14, 15], trichloroethylene [11, 14, 15], 1,2-dichloro-
ethylene [14–16], chloranil, and 2,4,6-trichloro-1,3,5-
triazine [11] as chlorovinyl substrates were also
studied (reaction b, X = Cl). However, these studies
were concerned with synthetic aspects of reactions of
chlorovinyl compounds, while we were aimed at eluci-
dating detailed mechanism of the radical addition–
elimination process. As model substrate we selected
X = SnBu ) [2]. However, toxicity and high cost of
3
organotin compounds, difficulties in their preparation
and purification, and complex procedures for treatment
of reaction mixtures strongly restrict application of
such procedures on an enlarged scale [3]. Therefore,
approaches that are free from the above disadvantages
are extensively developed in the recent years. Among
these, the most promising are based on the known
radical addition–elimination sequence (Scheme 1).
3
,4-dichloro-2,5-dihydrofuran-2,5-dione (I), taking
For example, synthetic procedures for radical
into account that potential reaction products, 3,4-dial-
kyl-2,5-dihydrofuran-2,5-diones, are important inter-
mediate compounds in the synthesis of biologically
active furan-2(5H)-ones (e.g., maculalactones) [17]. In
addition, alkylchlorofurandiones are readily converted
into the corresponding alkylchloropyrrolediones that
are convenient intermediate products in the synthesis
of various disubstituted pyrrole-2,5-diones [18].
alkylation of allyl sulfones (Scheme 1, reaction a, X =
Scheme 1.
·
·
R
a
X
–
R
X
H C
2
R
·
CH
2
X
R
·
R
R'
Scheme 2 shows the mechanism proposed by
Araneo et al. [11] for alkylation of compound I in the
presence of calcium carbonate. It was presumed that
X
R'
·
b
X
R'
R
·
·
–
X
the process is chain-like and that alkyl radicals R are
801

8
02
TRUKHIN et al.
Scheme 2.
2-chloro-2,3-dimethylbutane to 1-chloro-2,3-dimethyl-
butane) [19].
·
·
RH
+
Init
R
+
InitH
O
O
R
Scheme 3.
Cl
Cl
Cl
Cl
O
·
O
+
R
O
Cl
Cl
·
Me
Me
Me
O
+
O
O
Me
O
O
R
I
II
·
+ Cl
O
Me
Me
Cl
Me
O
O
Me
O
Me
·
·
+ HCl
O
+
O
RH
+
Cl
R
Me
Me
–HCl
Cl
III
Cl
InitH is initiator.
O
O
IV
generated by abstraction of hydrogen from the RH
hydrocarbon with chlorine atom. The last chain pro-
pagation step is the same as in the radical chlorination
of alkanes. Therefore, the regioselectivity in the
alkylation of furandione I should coincide with that
observed in the chlorination of the corresponding
alkane RH. According to the data of [11], the reaction
of I with 2,3-dimethylbutane (II) in the presence of
calcium carbonate gives a mixture of 3-chloro-4-
With a view to examine the mechanism of radical
alkylation of dichloride I in more detail, first of all we
tried various initiation versions using the reaction of I
with cyclohexane as an example (Table 1). The reac-
tions were carried out at room temperature or under
reflux in the presence of the most commonly used
chemical initiators and initiating systems. The progress
of reactions was monitored by GLC following accum-
ulation of 3-chloro-4-cyclohexyl-2,5-dihydrofuran-
2,5-dione; the latter was preliminarily isolated in the
reaction initiated by benzoyl peroxide. The kinetic
chain length was estimated as the yield of the alkyla-
tion product per mole of initiator, taking into account
(
1,1,2-trimethylpropyl)- and 3-chloro-4-(2,3-dimethyl-
butyl)-2,5-dihydrofuran-2,5-diones III and IV at
a ratio of about 1:1 (Scheme 3), whereas the regio-
selectivity in the chlorination of 2,3-dimethylbutane
(
II) with molecular chlorine is equal to 0.66 (ratio of
Table 1. Reaction of 3,4-dichloro-2,5-dihydrofuran-2,5-dione (I) with cyclohexane under different initiation modes
Run
no.
a
Temperature, °C
(irradiation)
b
I, mmol
Substrate (ml)–solvent (ml)
Initiator (mol %)
(BzO) (10)
Time, h
Yield, %
1
2
3
4
5
6
7
8
2.0
1.5
1.5
1.5
1.0
1.0
1.0
2.0
C
6
C
6
C
6
C
6
C
6
C
6
C
6
C
6
H
H
H
H
H
H
H
H
12 (10)
2
80 (hν)
80 (hν)
80 (hν)
20 (hν)
20 (hν)
20 (hν)
20 (hν)
20 (hν)
1.0
1.0
59.0
27.0
c
12 (4)
AIBN (10)
[(CH NCS
12 (7)
3
)
2
2
]
2
(5)
1.0
0000.042
12.0
12 (3)–CHCl
12 (3)–CHCl
12 (3)–CHCl
12 (3)–CHCl
12 (4)–CHCl
3
3
3
3
3
(3)–CH
(3)
3
CN (4) [c-HexCo(dmgH)
2
Py] (5)
1.5
d
PhICl
PhICl
PhICl
2
2
2
(10)
1.0
07.5
d
(3)
(5×2)
5×0.3
20×0.25
2.5
23.0
d
(3)
(20×0.5)
23.0
e
(4)
Cl
2
(250)
65.0
a
b
c
d
e
Relative to compound I.
Calculated on the initial compound I from the GLC data.
AIBN is 2,2′-azobis(isobutyronitrile).
3
(
Dichloro-λ -iodanyl)benzene was added in 10, 2, and 0.5% portions, respectively.
Chlorine was slowly passed through the reaction mixture in a stream of argon (the latter was bubbled through a saturated solution of
chlorine in carbon tetrachloride).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 6 2007

MECHANISM AND SELECTIVITY OF RADICAL ALKYLATION
803
that the half-decomposition period of benzoyl peroxide
is 3.03 h in cyclohexane at 80°C [20], and of azobis-
Table 2. Effect of the conversion on the selectivity of the
reaction of 3,4-dichloro-2,5-dihydrofuran-2,5-dione (I) with
a
2
,3-dimethylbutane (II)
(
[
isobutyronitrile) (AIBN) in toluene at 80°C, 1.20 h
20]. We found that, despite chain character of the
Reaction Conver- Selec-
time, h sion, % tivity
Solvent
Additive
b
alkylation of I, the chains are rather short: their length
is ~10. The most effective thermal initiators were
benzoyl peroxide and AIBN, in keeping with published
data [11, 12]. For comparison, the kinetic chain length
in the radical addition–elimination reaction of cyclo-
hexane with cyanogen chloride, initiated by benzoyl
peroxide (98°C), is ~11 [21]. Published data on the
alkylation of I under photochemical initiation are very
scanty: only biacetyl was used as photosensitizer, and
the reaction time varied from 24 to 120 h. Our experi-
ments showed (Table 1, run nos. 4–8) that photoinitia-
tion may be effective in the presence of both bis-
CHCl
3
3
–
1
2
3
4
1
2
3
4
11.0
19.0
29.0
36.0
14.0
22.0
34.0
43.0
01.9
02.6
03.2
04.0
01.1
01.1
01.1
01.2
10.8
c
c
c
d
CHCl
2 3
K CO
c
(
CH
2
Cl)
2
e
–
HCl (0.47 M) 00.0.25 001.85
CH CN
3
(
[
dimethylglyoximato)(pyridine)cyclohexylcobalt(III)
c-HexCo(DmgH) Py] and chlorinating agents [(di-
0.0.5
03.3
06.3
11.6
16.1
10.0
10.5
10.4
10.9
2
3
chloro-λ -iodanyl)benzene or molecular chlorine]. In
the latter case, the concentration of the chlorinating
agent should be maintained at a low level to minimize
side transfer of chlorine to alkyl radical R. This is
achieved by as slow as possible addition of the
chlorinating agent to the reaction mixture (Table 1,
run nos. 5–7).
1
2
3
a
Reactant concentrations, M: I, 0.25, II, 2.88, (BzO)
CHCl , 1 ml; 80°C.
Hereinafter, the conversion is defined as the overall amount of
alkylation products (primary and tertiary) in mol % relative to
dichloride I.
2
, 0.02 (8%);
3
b
Important results were obtained while studying
factors affecting the selectivity in the alkylation of
compound I with 2,3-dimethylbutane (II). All reac-
tions with alkane II were carried out in sealed ampules
in the presence of an internal standard. The selectivity
was calculated as the product ratio determined from
the GLC data; the products were preliminarily identifi-
ed by NMR spectroscopy. Compound III was isolated
as individual substance. We revealed an important
relation: in the absence of potassium carbonate, the
selectivity in the alkylation of I increases in parallel
with its conversion (Table 2). Therefore, we concluded
that the selectivity depends on the concentration of
hydrogen chloride in the reaction system and that rise
in the HCl concentration favors formation of tertiary
radical addition product III. It is known that, in the
general case, the existence of a dependence of selec-
tivity on the conversion indicates complex mechanism
of the process (some parameters of the reaction sys-
tem, affecting the selectivity, are not fixed). An analo-
gous effect of HCl on the regioselectivity of radical
addition–elimination was described in [21] for the
reaction of cyanogen chloride with alkanes (Scheme 4)
c
d
e
An additional 0.1 mmol of (BzO)
00 mg.
2 3
(CH Cl) ] = 3.17, [CH CN] = 7.19 M.
2
was added.
2
[
2
than 150. The authors proved that increase in the regio-
selectivity with rise in HCl concentration is related
to reversibility of hydrogen transfer from alkane to
chlorine.
Scheme 4.
·
·
+
RH
+
Cl
R
HCl
R
·
·
+ Cl
R
+
ClCN
RCN
·
N
Cl
We analyzed the dependence of the regioselectivity
in the alkylation of compound I on the concentration
of HCl in two ways: (1) in terms of a radical ion mech-
anism and (2) in terms of reversibility of elementary
steps in the assumed radical mechanism.
It is known that dichloride I is a strong one-electron
oxidant (EA = 1.90 eV [22]). Therefore, it is quite
probable that the addition of nucleophilic tertiary alkyl
radical to molecule I involves a polar transition state
(radical ion intermediate in the limiting case); by con-
[
21]. For example, the selectivity in the reaction of
cyanogen chloride with hexane (ratio of the secondary
and primary alkyl cyanides) attained a value of more
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 6 2007

8
04
TRUKHIN et al.
Scheme 5.
Me
Me
Me
O
Me
Me
O
Cl
Me
·
Cl
Me
Me
Me
Me
Me
Me
·
O
O
III
·
Cl
Cl
O
O
I
·
Me
Me
CH
2
O
Me
Me
Cl
Me
O
IV
·
Me
Cl
O
trast, primary radical reacts with I following a com-
mon radical addition pattern through a relatively
weakly polar transition state (Scheme 5). Then, added
acid should solvate more polar transition state in a spe-
cific mode, and polar solvents, in a nonspecific mode;
i.e., addition of tertiary radical to compound I should
be accelerated, and the selectivity of the alkylation
should increase.
philic chlorine atom toward C=C bond [24, 25] in
combination with the high rate of the subsequent reac-
tion of chlorine atom with alkane molecule II suggests
that the β-decomposition step is also irreversible
tert
prim
(k , k ).
3
3
The isomerization of primary alkyl radicals into
·
·
tertiary (R
+ RH → R + RH) was included in
prim
tert
Scheme 6, for in some rare cases this process affects
the kinetics of radical transformations of alkane II.
We examined the effect of acids and polar solvent
The absolute rate constant k of the isomerization step
(
CH CN) on the selectivity in the reaction of dichlo-
i
3
was measured by ESR spectroscopy using decomposi-
ride I with alkane II (Table 3). The results showed that
addition of acetonitrile or trifluoroacetic acid only
slightly affects the selectivity (cf. the data in Table 2).
These data are not consistent with the radical ion
mechanism of addition of alkyl radicals to compound
I. Another interpretation of the dependence of the
selectivity on the HCl concentration is based on the
kinetic scheme shown in Scheme 6, which assumes
both reversibility of all steps of the process and
possible isomerization of primary radicals into tertiary.
Taking into account published data, the step of addi-
tion of nucleophilic alkyl radicals to electrophilic
tion of pentanoyl peroxide in 2,3-dimethylbutane (II)
3
–1 –1
as model system; it is 4×10 l mol s at 353 K (cal-
culated on a single hydrogen atom) [26]. If [II] >> [I],
the contribution of the isomerization process may
become quite appreciable.
There are no published data on absolute rate con-
stants (k ) and equilibrium constants for liquid-phase
–
1
reactions of alkyl radicals with HCl. The use of the
corresponding data for gas-phase reactions is inappro-
priate because of considerable disagreement between
the experimental and calculated constants obtained by
different authors [27]. We estimated the absolute rate
double carbon–carbon bond at moderate temperatures
tret
tert
prim
can be regarded as irreversible [23]; therefore, k >>
constants k–1 and k–1 on the basis of the H–Cl and
3
tert
prim
prim
k
and k3 >> k–2 . The low reactivity of electro-
C–H bond dissociation energies: E (H–Cl) = 432,
–2
dis
Scheme 6.
Me
O
Cl
Me
tert
tert
tert
3
Me
Me
Me
k
I, k2
Me
k
k
1
·
+ Cl
·
+
+
HCl
HCl
O
III
·
tert
k–
tert
–2
tert
–3
k
Me
1
Me
Cl
O
·
+ Cl
II
ki
O
Me
Me
Cl
·
prim
1
prim
prim
3
Me
Me
CH
k
2
I, k2
k
k
·
+ Cl
O
IV
·
prim
k–
prim
prim
–3
k
–2
Me
Cl
1
Me
O
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 6 2007

MECHANISM AND SELECTIVITY OF RADICAL ALKYLATION
805
E (R
–H) = 417.2 (propane), E (Rtert–H) =
Let us analyze the selectivity in the reaction be-
tween compounds I and II on a quantitative level.
Application of the quasistationary concentration prin-
ciple [Eqs. (1), (2)] gives expressions (3) and (4) for
the stationary concentrations of alkyl radicals. Substi-
tuting [R] into the expression for the apparent selec-
tivity (Sapp) gives (after algebraic transformations)
dis
prim
dis
3
90.2 kJ/mol (2-methylpropane) [28]. Then, the heat
effects of hydrogen abstraction from the primary and
tertiary carbon atoms in molecule II by chlorine atom
are ΔHtert = –41.8 and ΔHprim = –14.8 kJ/mol. Assum-
ing (as a rough approximation) that the main contribu-
tion to ΔG for the step of hydrogen transfer by chlorine
is given by the enthalpy factor, ΔG ≈ ΔH = –RTlnK
formula (5). If the reaction system contains hydrogen
–
1
–1
prim
(
R = 8.314 J mol K , T = 353.15 K), we calculated
chloride traps ([HCl] = 0) or inequalities k [I] +
2
6
prim
1
tert
1
prim
the equilibrium constants Ktert = 1.52×10 and K
=
2k [RH] + 12(k
k /k )[RH] >> k–1 [HCl] and
prim
i
i
2
tert
2
tert
–1
1
.55×10 (353.15 K). The absolute rate constants for
k [I] >> k [HCl] are fulfilled, the apparent selec-
chlorination of alkane II were measured in [19] by
tivity should tend to
tert
8
prim
1
laser flash photolysis at 300 K: k = 6.5×10 , k
=
1
tert
tert
1
8
–1 –1
1
k
1
k
i
[RH]
k
1
.67×10 l mol s (calculated on a single hydrogen
S
app
=
+
2 +
.
(6)
prim
1
prim
prim
1
atom). On the basis of these data we estimated the rate
6 k
k
2
[I]
3k
tert
2
prim
6
–1 –1
constants k–1 = 4.3×10 and k–1 = 1.1×10 l mol s
on a single hydrogen atom). Comparison of the above
Under the additional constraint [RH] → 0 or
(
prim
2
4
8
–1 –1
ki[RH] << k
[I] it attains its minimal value:
estimates of k–1 with k values (10 –10 l mol s
2
[
11]) shows that the reaction of primary alkyl radical
tert
1
6
k
1
with HCl, leading to regeneration of initial alkane II
and chlorine atom, should successfully compete with
the addition to compound I; on the other hand, the
reaction of tertiary alkyl radical with HCl may be
neglected. This means that addition of hydrogen chlo-
ride to the reaction mixture will favor more effective
back transformation of primary radicals into compound
II, the concentration of tertiary radicals remaining
unchanged; as a result, the selectivity in the alkylation
of dichloride I should increase.
S
app
=
.
(7)
prim
1
k
This limiting expression is similar to the formula
reflecting the selectivity of radical chlorination of
alkane II with molecular chlorine. Thus at early steps
of the reaction of compound I with alkane II (i.e., at
a low conversion), as well as in the presence of hy-
drogen chloride acceptor (potassium carbonate), the
apparent selectivity should be similar to the selectivity
in the photochemical chlorination of alkane II with
molecular chlorine in an open system, where the
hydrogen abstraction step is irreversible (Sapp = 0.66 at
·
∂
[Rprim
]
prim
1
·
prim
·
=
12k
[RH][Cl ] – k–1 [Rprim][HCl]
∂
τ
prim
2
·
·
3
00 K) [19]. The real selectivity in the alkylation of I
in the presence of HCl acceptor is 0.9–1.2, according
to both our data (Table 2) and those given in [11]. In
–
k
[Rprim][I] – 2k
i
[Rprim][RH] = 0;
(1)
·
∂
[Rtert]
tert
1
·
tert
·
=
2k
[RH][Cl ] – k–1 [Rtert][HCl]
∂
τ
tert
2
·
·
Table 3. Effect of acids on the selectivity of the reaction
of 3,4-dichloro-2,5-dihydrofuran-2,5-dione (I) with 2,3-di-
–
k
[Rtert][I] + 2k
i
[Rprim][RH] = 0;
(2)
(3)
a
prim
1
·
methylbutane (II)
1
2k
[RH][Cl ]
·
[
R
prim] =
;
prim
prim
k
–1 [HCl] + k
2
[I] + 2k
i
[RH]
+
Reaction Conver- Selec-
time, h sion, % tivity
Solvent
Additive
prim
tert
1
1
2k
1
[RH]
k
·
CH
CH
CH
3
3
3
CN–(CH
CN–(CH
2
Cl)
Cl)
2
2
–
b
1
3
1
2
24
26
22
11
2.60
0.94
6.00
2.30
[
R
prim] =
prim
prim
2
k
–1 [HCl] + k
[I] + 2k
i
[RH]
k
i
2
K
2
CO
3
·
c
2
k
i
[RH][Cl ]
2 2
CN– (CH Cl) HCl
×
;
(4)
tert
tert
2
d
k
–1 [HCl] + k
[I]
Alkane II
3
CF COOH
a
Amounts of the reactants: I, 1 mmol; benzoyl peroxide, 0.1–
.2 mmol; K CO , 200 mg; compound II, 1 ml; solvent, 2 ml;
temperature 80°C.
tert
2
·
tert tert
[
III]
IV]
k
[Rtert][I]
1
2 1
k k
0
2
3
S
app
=
=
=
prim
·
prim prim
[
k
2
[Rprim][I]
6 k
2
k
1
b
c
d
2
00 mg.
HCl] = 0.4 M.
[CF COOH] = 0.6 M.
prim
prim
2
prim
1
tert
1
k
–1 [HCl] + k
[I] + 2k
i
[RH] + 12(k
k
i
/k
)[RH]
[
×
. (5)
tert
tert
2
k
–1 [HCl] + k
[I]
3
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 6 2007

8
06
TRUKHIN et al.
prim
tert
tert
Table 4. Effect of the concentration of compound I on the
selectivity of its reaction with 2,3-dimethylbutane (II) in the
absence of hydrogen chloride
[RH] << k–1 [HCl] and k [I] << k [HCl] are ful-
2
–1
filled. Then, the expression for the selectivity is sim-
plified to formula (9):
[
I], M
[II], M
Conversion, %
Selectivity
tert tert
1
6
1 2
K k
0
0
0
.3000
.0100
.0025
6.9
6.9
6.9
12.5
40.0
15.0
0.9
1.1
1.9
Sapp
=
.
(9)
prim prim
K
1
k
2
Equation (9) characterizes the highest possible (or
equilibrium) selectivity. On the basis of our estimates
of the equilibrium and rate constants, the maximal
a
(
[
BzO)
2
, 10 mol % relative to I; 80°C, 1 h at [I] = 0.3 M or 6 h at
was used as HCl acceptor, and
I] = 0.01 and 0.0025 M; CaCO
acetonitrile was added.
3
3
tert
2
prim
selectivity should be ~1.6×10 (at k ≈ k2 ). How-
ever, such a high selectivity is unattainable because of
limited solubility of hydrogen chloride in the reaction
mixture.
addition, we found that Sapp does not depend on the
concentration of alkane II over a wide range (Table 4)
at a typical concentration of I (0.25–0.3 M) in the
presence of calcium carbonate. This means that the
Analysis of Eq. (5) in the hydrogen chloride con-
prim
inequality k [RH] << k [I] is valid within the above
centration range from 0 to 0.7 M makes it possible to
i
2
prim
concentration range of dichloride I and that the reac-
estimate k
2
in an alternative mode. Formula (5) can
tert
2
tion selectivity in the presence of CaCO (K CO ) is
be simplified as follows. Insofar as k–1 (4.28×10 l×
mol s ) << k
3
2
3
–
1
–1
tert
2
4
8
–1 –1
described by Eq. (7); i.e., the mechanism of alkylation
(10 –10 l mol s ) and the concen-
of compound I corresponds to simplified Scheme 2
tert
2
tert
[
11]. Slightly increased selectivity compared to the
chlorination of alkane II is likely to be related to partic-
ipation of phenyl radicals arising from decomposition
of the initiator at the hydrogen abstraction step. The
selectivity Sapp for the reaction of phenyl radicals with
compound II is equal to 7.6 (318 K) [29]. As noted
above, the kinetic chain length in the examined process
is about 10; therefore, a 5–10% contribution of phenyl
radicals at the hydrogen abstraction step should give
rise to an overestimated value of Sapp (~1) which corre-
sponds to the experimental data. Increased selectivity
in the alkylation of I can also result from limited ef-
ficiency of carbonates as heterogeneous HCl acceptor.
tion of tertiary radical with HCl can be neglected.
Additional exclusion of the contribution of isomeriza-
tion of alkyl radicals at [I] = 0.25 M (in this case, the
selectivity does not depend on [II]; see Table 4) leads
(after some transformations) to Eq. (10).
tert prim
prim
2
tert
1
1 k
1
(k–1 [HCl] + k
[I])
1 k
S
app
=
=
prim prim
prim
1
6
k
2
k
1
[I]
6 k
tert prim
1
1
k k–1 [HCl]
+
.
(10)
prim prim
6
k
2
k
1
[I]
Thus the selectivity should be linearly related to the
concentration of HCl at a fixed concentration of I (i.e.,
at low conversions). Figure shows the plot of selec-
tivity versus HCl concentration over a wide range of
the latter. From the good linear correlation between
Sapp and [HCl] (Sapp = a[HCl] + b, a = 21.1±1.2, b =
As follows from Eq. (6), the selectivity depends on
the concentration of alkane II, and this dependence
should be given by a straight line with a slope de-
scribed by Eq. (8):
tert
k
prim
i
k
1
tert
8
prim
=
1
0
1
.74±0.43, R = 0.98) and k = 6.5×10 , k
1
tanα =
2 +
.
(8)
prim
1
8
prim
6
–1 –1
k
2
[I]
3k
.67×10 , k
= 1.1×10 l mol s , and [I] = 0.25 M
–
1
we obtained the rate constant of the addition of pri-
In order to determine tanα, we performed experi-
ments at reduced concentrations of dichlorde I. The
data in Table 4 show that decrease in [I] leads to
increased selectivity. However, when the concentration
of I is smaller than 0.01 M, the error in the determina-
prim
2
5
mary alkyl radicals to molecule I: k
mol s . This value well fits the range of rate con-
stants for addition of alkyl radicals to electron-defi-
= 1.4×10 l×
–1 –1
prim
2
4
8
cient double bonds, given in [11]: k
~10 –10 l×
–1 –1
prim
2
mol s . However, the k
value obtained by us
tion of selectivity (and hence of tanα) is large; there-
should be regarded only as a rough estimate, for the
prim
fore, the obtained data cannot be used to calculate k2
.
prim
k
–1 value was determined on the assumption that the
Another limiting case of Eq. (5) is achieved when
entropy in the corresponding elementary step of the
process does not change.
prim
prim
tert
the inequalities k2 [I] + 2k [RH] + 12(k k /k ) ×
i
1
i
1
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 6 2007

MECHANISM AND SELECTIVITY OF RADICAL ALKYLATION
807
S
app
According to Eq. (10), increase in the concentration
of I should lead to reduced selectivity. In fact, as
follows from the data in Table 5, the selectivity sharply
decreases as the concentration of I rises, the concen-
tration of HCl being constant. At a fixed concentration
of hydrogen chloride, [HCl] = 0.51 M, Eq. (11) should
be fulfilled:
1
2
8
4
0
tert prim
1
6
1
k k–1 [HCl]
=
2.7.
(11)
prim prim
k
2
k
1
[I]
Therefore, the selectivity should be related to the
concentration of I through the equation Sapp = 0.66 +
2
.7/[I]. We showed that the experimental selectivities
(
Table 5) well fit the above dependence. Thus our
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
results are fully consistent with Scheme 6 which in-
cludes reversible transfer of hydrogen from primary
carbon atom in alkane II and isomerization of primary
radicals into tertiary.
[
HCl], M
Plot of the selectivity in the reaction of 3,4-dichloro-2,5-di-
hydrofuran-2,5-dione (I) with 2,3-dimethylbutane (II) versus
HCl concentration. The conversion of I is 0.6–14%.
An important tool for controlling the regioselec-
tivity in radical chlorination of alkenes is proper choice
of a solvent capable of forming complexes with
chlorine atoms. Such solvents are simple homologs of
aromatic hydrocarbons [30], carbon disulfide [31],
pyridine [32], alkyl halides [33], etc. As shown in [11],
the use of benzene as solvent increases the regioselec-
tivity in the reaction of compound I with alkane II in
the presence of calcium carbonate from ~1 to ~6. We
found conditions which ensured advantages of the
effects of a complex-forming solvent and hydrogen
chloride to be combined. In the alkylation of com-
pound I with 2,3-dimethylbutane in benzene in the
presence of HCl, the selectivity of the process attained
a value of 40 (Table 6), which is very important from
the viewpoint of synthetic applications.
reaction was aroused by the data of [12], according to
which the reaction of hexane with dichloride I in the
presence of biacetyl as photosensitizer gave a mixture
of 3-chloro-4-hexyl-, 3-chloro-4-(1-methylpentyl)-,
and 3-chloro-4-(1-ethylbutyl)-2,5-dihydrofuran-2,5-di-
ones V–VII at a ratio of ~5:3:1. The regioselectivity
observed in [12] sharply differs from that found for the
chlorination of hexane with molecular chlorine, where
the ratio of 1-, 2-, and 3-chlorohexanes was ~1.0:
1.6:1.7 (20°C) [34]. We examined the reaction of I
with hexane (Scheme 7) under the conditions most
typical for the reaction of I with 2,3-dimethylbutane
(
II) (Table 7). The progress of the reaction was
monitored by GLC, and the products were identified
by comparing the GLC and NMR data. The results
showed that the ratio of the primary and secondary
hexyl-substituted products follows the same relations
as those observed in the alkylation with 2,3-dimethyl-
butane (II). The selectivity values obtained by us dis-
While studying the alkylation of I we also ex-
amined the effect of phenyl iodide and ethyl iodide on
the selectivity. According to Russell [30], no interpret-
able results on the effect of iodine-containing solvents
on the selectivity of radical chlorination of alkanes
with molecular chlorine were obtained because of
concurrent oxidation of iodides [30]. We have found
that both ethyl iodide and phenyl iodide considerably
increase the selectivity in the reaction of compound I
with alkane II (Table 6) and that no appreciable
oxidation of the iodides occurs (when the reaction was
complete, iodine color did not appear). We were the
first to observe increased selectivity in ethyl iodide.
Table 5. Effect of the concentration of compound I on the
selectivity of its reaction with 2,3-dimethylbutane (II) at
a
a fixed concentration of hydrogen chloride
b
[
I], M
Conversion, %
Selectivity
0.05
0.25
60.0
18.0
05.7
35.3
12.6
03.6
1
.25
a
Apart from 2,3-dimethylbutane (II), the alkylation
of I with hexane was studied. Our interest in this
Reactant concentrations, M: (BzO)
2 2 2
, 0.02; II, 2.88; (CH Cl) ,
3.17; CH CN, 7.19; HCl, 0.51; 80°C, 1 h.
3
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 6 2007

8
08
TRUKHIN et al.
Scheme 7.
Me
Me
O
O
O
Me
Me
Me
I
+
Me(CH ) Me
O
+
O
+
O
2
4
–
HCl
Cl
Cl
Cl
VII
O
O
O
V
VI
prove the data of [12] indicating preferential formation
ed furandiones. In the reaction with 2,3-dimethyl-
of compound V.
butane (II), regardless of the III-to-IV ratio, tertiary
2
-chloro-2,3-dimethylbutane was always formed,
While studying factors affecting the selectivity in
the alkylation of I, initiated by benzoyl peroxide, we
also detected formation of up to 20% of the corre-
sponding alkyl chlorides together with alkyl-substitut-
while no primary chloride was detected. Presumably,
the source of chlorine for the formation of 2-chloro-
2
from the reaction of benzoyl peroxide with HCl [35].
At high HCl concentrations (>0.7 M), the fraction of
the tertiary chloride exceeded 50%. Therefore, while
studying the dependence of the selectivity upon HCl
concentration (see figure), the latter was lower than
,3-dimethylbutane is benzoyl hypochlorite arising
Table 6. Effect of the solvent on the selectivity of the
reaction of 3,4-dichloro-2,5-dihydrofuran-2,5-dione (I) with
a
2
,3-dimethylbutane (II)
Solvent (additive)
CHCl
Conversion, %
22.0
Selectivity
0
.7 M to avoid errors in the determination of selec-
3
01.1
05.4
40.0
45.0
25.0
tivity owing to side processes.
b
b
Benzene
50 .0
The information obtained in the present work on
the alkylation of dichloride I was used to develop
preparative procedures for the synthesis of some 3-al-
kyl-4-chloro- and 3,4-dialkyl-2,5-dihydrofuran-2,5-di-
ones (Table 8). The syntheses of 3-chloro-4-cyclo-
hexyl- and 3,4-dicyclohexylfurandiones VIII and IX
were described previously; however, our procedures
are characterized by considerably shorter reaction
time, the product yield being comparable. 3-Chloro-4-
c
Benzene–HCl
Phenyl iodide
Ethyl iodide
06.9
03.1
9.
a
Amounts of the reactants: I, 1 mmol; (BzO)
00 mg; II, 1 ml; solvent, 2 ml; 80°C, 2 h.
BzO) , 0.2 mmol.
The same as in , but without potassium carbonate; 0.5 ml of
CH CN and HCl (0.4 M) were added.
2 2 3
, 0.1 mmol; K CO ,
2
(
b
c
2
a
3
(
1,1,2-trimethylpropyl)-2,5-dihydrofuran-2,5-dione
Table 7. Selectivity in the reaction of 3,4-dichloro-2,5-di-
hydrofuran-2,5-dione (I) with hexane
(III) was isolated by us for the first time. Studies on
possible synthetic extensions of the alkylation of com-
pound I will be the main line of our research activity.
a
Selectivity
Conver-
sion, %
Solvent (additive)
(
[VI]+[VII])/[V] [VI]/[VII]
EXPERIMENTAL
b
Hexane (CaCO
3
)
14
08
03.9
08.0
1.10
0.94
Thermal radical reactions were initiated with the
Hexane–benzene
C
b
(
CaCO
Hexane
HCl, 0.5 M)
Hexane
HCl, 1 M)
Hexane–benzene
3
)
aid of an MLW U15 flow thermostat. Photochemical
initiation was performed using a DRL-400 high-pres-
sure mercury lamp (400 W). GLC analysis was per-
formed on a Model 3700 gas chromatograph (manu-
factured by Khromatograf, Moscow, Russia) equipped
with a flame ionization detector and an HP-1 30-m
capillary column (polydimethylsiloxane); carrier gas
nitrogen. The chromatograms were quantitated using
a CI-100 integrator. The calibration coefficients of iso-
meric chloro(alkyl)furandiones were assumed to be
equal. Column chromatography was performed on
a column (2.5 cm i.d.) charged with 10 g of silica gel
17
16
32
20.0
33.0
86.0
1.20
1.30
1.30
c
(
c
(
c
(
HCl, 0.5 M)
a
Amounts of the reactants: I, 1 mmol; (BzO)
2
, 0.1 mmol; hexane,
1
ml; solvent, 2 ml; 80°C, 2 h.
CaCO , 200 mg.
The same as in , and a required amount of a saturated solution of
HCl in CH CN was added.
b
c
3
a
3
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 6 2007

MECHANISM AND SELECTIVITY OF RADICAL ALKYLATION
809
1
(
0.035–0.070 mm; from Acros Organics). The H and
Table 8. Preparative syntheses of some 3-alkyl-4-chloro-
and 3,4-dialkyl-2,5-dihydrofuran-2,5-diones by alkylation of
compound I
1
3
C NMR spectra were recorded at 20°C from solu-
tions in CDCl on a Bruker DPX-300 spectrometer at
3
3
00.130 and 75.468 MHz, respectively.
Hydrocarbon
(reaction time, h)
b
Yield, % (reaction
time, h) [12]
a
Product Yield, %
c
1
,2-Dichloroethane, 2,3-dimethylbutane (from Lan-
caster Synthesis), and cyclohexane were used without
additional purification. Hexane was purified by wash-
ing with concentrated sulfuric acid, followed by dis-
tillation. Acetonitrile was freed from traces of water
by distillation over phosphoric anhydride. A saturated
solution of hydrogen chloride in acetonitrile was
prepared by passing dry hydrogen chloride until the
latter no longer absorbed. Hydrogen chloride was
generated by heating of concentrated hydrochloric acid
and was dried by passing through a layer of concen-
trated sulfuric acid. The concentration of hydrogen
chloride was determined by titration with a standard
solution of sodium hydroxide (a portion of acetonitrile
solution of HCl was preliminarily diluted with water).
The concentration of hydrogen chloride was on the
average 2.7–3.0 M.
C
6
6
H
H
12 (1.3)
VIII
IX
80
91 (20)0
76 (120)
–
d
C
12 (6)
e
74 (55)
78
II (3)
III
a
b
c
d
e
(
BzO)
Isolated product.
Biacetyl, 80°C, hν.
In parentheses is given the yield after recrystallization.
HCl in benzene.
2
, 80°C.
an alkane–acetonitrile–benzene mixture (1:1:3, by
volume) was added. The ampule was sealed in such
a way that the ratio between the volumes of the liquid
and gas phases was about 1:1 and heated for 1 h at
0°C (on a water bath). The ampule was then opened,
and the mixture was analyzed by GLC.
8
General procedure for kinetic experiments in the
presence of hydrogen chloride acceptors. The corre-
sponding alkane, 2 ml (if necessary, as a mixture with
added solvent), was added to a mixture of 100 mg
3
,4-Dichloro-2,5-dihydrofuran-2,5-dione (I) was
synthesized by reaction of maleic anhydride with
thionyl chloride in the presence of pyridine according
to the procedure described in [36]. Yield 75%. The
product was purified by recrystallization from hexane
and was stored in a tightly capped vessel in a refrig-
erator (~0°C). Compound I decomposes on prolonged
storage, and even small amounts of impurities strongly
inhibit radical reactions. If the yield of the radical
alkylation products decreased appreciably, dichloride I
was subjected to repeated recrystallization.
(
0.6 mmol) of compound I, 12.5 mg (0.06 mmol) of
benzoyl peroxide (as a 10% solution in 1,2-dichloro-
ethane), 10.4 mg (0.06 mmol) of 2,4-dichlorobenzo-
nitrile (as a 20% solution in 1,2-dichloroethane;
internal standard for GLC), and 200 mg of calcined
K CO or CaCO . The reaction vessel was tightly
2 3 3
capped with a glass stopper and heated for 1 h at 80°C
under stirring using a magnetic stirrer. The mixture
was then filtered and analyzed by GLC.
All kinetic experiments were performed at least in
triplicate, and the relative error in the given selectivity
values did not exceed 15%.
3-Chloro-4-cyclohexyl-2,5-dihydrofuran-2,5-di-
one (VIII). A mixture of 500 mg (3 mmol) of com-
pound I, 37 mg (0.15 mmol) of benzoyl peroxide, and
General procedure for kinetic experiments in the
presence of hydrogen chloride. A ~2-ml ampule was
charged with 50 mg (0.3 mmol) of dichloride I, 6.2 mg
5
ml of cyclohexane was heated for 80 min at 80°C in
a flask equipped with an air-cooled reflux condenser.
The progress of the reaction was monitored by GLC.
The product was isolated by column chromatography
using hexane–1,2-dichloroethane (5:1) as eluent. A 70
to 350-ml fraction was collected, and removal of the
solvent gave 520 mg (80%) of compound VIII as
(
1
0.03 mmol) of benzoyl peroxide (as a 10% solution in
,2-dichloroethane), and 5.2 mg (0.03 mmol) of 2,4-di-
chlorobenzonitrile (as a 20% solution in 1,2-dichloro-
ethane; internal standard for GLC). A saturated solu-
tion of HCl in acetonitrile with a required concentra-
tion, 1 ml, in a mixture with alkane–acetonitrile–
1
1
,2-dichloroethane (3:3:2, by volume; 0.375 ml of
a transparent liquid. Purity 95% (GLC). H NMR spec-
alkane, 0.375 ml of acetonitrile, and 0.25 ml of 1,2-di-
chloroethane) was then added; the solvent volume ratio
was maintained by adding anhydrous acetonitrile. In
studies on the joint effect of HCl and benzene, 1 ml of
a solution of HCl with a required concentration in
trum, δ, ppm: 1.20–1.45 m (2H, CH ), 1.65–1.95 m
2
1
3
(8H, CH
), 2.65–2.85 m (1H, CH). C NMR spec-
), 26.1 (CH ), 29.3 (CH ),
2
trum, δ , ppm: 25.7 (CH
C
2
2
2
36.7 (CH), 134.7 (C=C), 147.6 (C=C), 160.4 (C=O),
162.6 (C=O).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 6 2007

8
10
TRUKHIN et al.
3
,4-Dicyclohexyl-2,5-dihydrofuran-2,5-dione
IX). A mixture of 1 g (6 mmol) of compound I,
00 mg (1.2 mmol) of benzoyl peroxide, and 10 ml of
4. Quiclet-Sire, B. and Zard, S.Z., J. Am. Chem. Soc.,
996, vol. 118, p. 1209.
1
(
3
5
6
7
. Le Guyader, F., Quiclet-Sire, B., Seguin, S., and
Zard, S.Z., J. Am. Chem. Soc., 1997, vol. 119, p. 7410.
. Xiang, J., Jiang, W., Gong, J., and Fuchs, P.L., J. Am.
Chem. Soc., 1997, vol. 119, p. 4123.
. Liu, J.-T., Jang, Y.-J., Shih, Y.-K., Hu, S.-R., Chu, C.-
M., and Yao, C.-F., J. Org. Chem., 2001, vol. 66,
p. 6021.
cyclohexane was heated for 3 h at 80°C in a flask
equipped with an air-cooled reflux condenser, an addi-
tional portion of benzoyl peroxide, 300 mg (1.2 mmol)
was added, and the mixture was heated for an addi-
tional 3 h. The progress of the reaction was monitored
by GLC. The product was isolated by column chroma-
tography using hexane–chloroform (4:1). A 70 to 350-
ml fraction was collected, and removal of the solvent
gave 1.16 g (74%) of compound IX as colorless crys-
tals. The product was additionally purified by recrys-
tallization from 5 ml of cyclohexane by slowly evap-
orating the solvent (for one week). Yield 870 mg
8
9
. Jang, Y.-J., Shih, Y.-K., Liu, J.-Y., Kuo, W.-Y., and
Yao, C.-F., Chem. Eur. J., 2003, vol. 9, p. 2123.
. Gagosz, F. and Zard, S.Z., Org. Lett., 2003, vol. 5,
p. 2655.
1
0. Bertrand, F., Pevere, V., Quiclet-Sire, B., and
Zard, S.Z., Org. Lett., 2001, vol. 3, p. 1069.
1. Araneo, S., Arrigoni, R., Bjorsvik, H.-R., Fontana, F.,
Liguori, L., Minisci, F., and Recupero, F., Tetrahedron
Lett., 1996, vol. 37, p. 6897.
2. Stahlke, K.-R. and Willy, H., FRG Patent Appl.
no. 2352216, 1975; Chem. Abstr., 1975, vol. 83,
no. 79950e.
1
(
55%), large transparent crystals with mp 117–119°C;
published data [12]: mp 123–124°C; purity 99%
GLC). H NMR spectrum, δ, ppm: 1.20–1.45 m (4H,
1
(
1
CH ), 1.55–2.00 m (16H, CH ), 2.60–2.75 t.t (2H,
2
2
1
3
CH). C NMR spectrum, δ , ppm: 25.7 (CH ), 26.4
C
2
(
(
CH ), 30.4 (CH ), 36.7 (CH), 147.4 (C=C), 165.4
C=O).
13. Schmerling, L., US Patent no. 3780066, 1973; Chem.
Abstr., 1974, vol. 80, no. 70349w.
2
2
1
1
1
1
1
1
4. Schmerling, L. and West, J.P., J. Am. Chem. Soc., 1953,
3
-Chloro-4-(1,2,2-trimethylpropyl)-2,5-dihydro-
vol. 75, p. 6216.
5. Rust, F.F. and Bell, C.S., J. Am. Chem. Soc., 1970,
vol. 92, p. 5530.
furan-2,5-dione (III). A mixture of 500 mg (3 mmol)
of compound I, 150 mg (0.6 mmol) of benzoyl per-
oxide, 2 ml of 2,3-dimethylbutane (II), 2 ml of
an acetonitrile solution of HCl (cHCl = 2.7 M), and 6 ml
of benzene was heated for 2 h at 80°C, an additional
portion of benzoyl peroxide, 75 mg (0.3 mmol), was
added, and the mixture was heated for 1 h more. The
progress of the reaction was monitored by GLC. The
product was isolated by column chromatography using
hexane–1,2-dichloroethane (5:1) as eluent. A 50 to
6. Horowitz, A. and Rajbenbach, L.A., J. Am. Chem. Soc.,
1
970, vol. 92, p. 1634.
7. Kar, A., Gogoi, S., and Argade, N.P., Tetrahedron, 2005,
vol. 61, p. 5297.
8. Augustin, M., Faust, J., and Koehler, M., J. Prakt.
Chem., 1983, vol. 325, p. 293.
9. Bunce, N.J., Ingold, K.U., Landers, J.P., Lusztyk, J.,
and Scaiano, J.C., J. Am. Chem. Soc., 1985, vol. 107,
p. 5464.
3
00-ml fraction was collected, and removal of the sol-
vent gave 510 mg (78%) of compound III as a color-
less liquid with a purity of 97% (GLC). H NMR spec-
2
0. Walling, Ch., Free Radicals in Solution, New York:
Wiley, 1957; translated under the title Svobodnye
radikaly v rastvore, Moscow: Inostrannaya Literatura,
1
trum, δ, ppm: 0.90 d (6H, CH ), 1.40 s (6H, CH ),
3
3
1
3
1
960, pp. 380, 405.
2
.34 sept (1H, CH). C NMR spectrum, δ , ppm: 17.9
C
2
2
2
1. Tanner, D.D. and Bunce, N.J., J. Am. Chem. Soc., 1969,
vol. 91, p. 3028.
2. Paul, G. and Kebarle, P., J. Am. Chem. Soc., 1989,
(
CH ), 22.9 (CH ), 34.9 (CH), 42.3 (C), 135.2 (C=C),
3 3
1
49.5 (C=C), 160.6 (C=O), 162.4 (C=O).
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