Free radical addition of α-telluroesters to alkenes

Article abstract of DOI:10.1002/hc.20716

α-(Phenyltelluro)esters 2 reacted with electron-rich alkenes 3 in the presence of a catalytic amount of azobisisobutyronitrile (AIBN) as an initiator to give the corresponding addition products 4. This carbotelluration of alkenes proceeded via a radical c

Full text of DOI:10.1002/hc.20716

Heteroatom Chemistry
Volume 22, Number 3/4, 2011
Free Radical Addition of α-Telluroesters
to Alkenes
Nobuaki Kambe,1 Li-Biao Han,1 Shin-ichi Fujiwara,2
and Noboru Sonoda1
1Department of Applied Chemistry, Graduate School of Engineering, Osaka University,
Suita, Osaka 565-0871, Japan
2Department of Chemistry, Osaka Dental University, Hirakata, Osaka 573-1121, Japan
Received 19 October 2010; revised 11 January 2011
ful synthetic tools in organic chemistry because
ABSTRACT: α-(Phenyltelluro)esters 2 reacted with
electron-rich alkenes 3 in the presence of a catalytic
amount of azobisisobutyronitrile (AIBN) as an ini-
tiator to give the corresponding addition products 4.
This carbotelluration of alkenes proceeded via a rad-
ical chain mechanism involving the bimolecular ho-
molytic substitution (SH2) reaction on tellurium as
the key step, wherein tellurium plays an important
role in a kinetic fashion to realize the rapid group
transfer process resulting in suppressing the compet-
new carbon–carbon and carbon–tellurium bonds are
formed in one step giving rise to vinyl tellurides,
which can allow further synthetic manipulation [6].
If alkene inserts between Te R bond similarly as in
the case of alkynes, alkyl tellurides can be formed as
shown in Eq. (2). However, this reaction, carbotellu-
ration of alkenes, does not seem to be an easy process
to proceed efficiently (vide infra).
R'Te
+
R'Te
R
ꢀ
C
ing olefin polymerization. 2011 Wiley Periodicals, Inc.
R
Heteroatom Chem 22:518–522, 2011; View this article on-
line at wileyonlinelibrary.com. DOI 10.1002/hc.20716
(1)
R′Te
+
R′Te
R
R
INTRODUCTION
(2)
Ever since we disclosed free radical addition of
diorganyl tellurides to alkynes to form alkenyl tel-
lurides in 1992 [1], as exemplified in Eq. (1), sev-
eral examples of carbotelluration of alkynes have
been reported so far by us [2,3] and other groups
[4,5]. Carbotelluration of alkynes would provide use-
Plausible mechanisms of carbotelluration of alkenes
(Eq. (2)) are outlined in Scheme 1 assuming that
the reaction proceeds by a similar mechanism as
the reaction of alkynes. Initiation steps are com-
posed of the addition of 1-cyano-1-methylethyl rad-
ical (In•), generated by thermal decomposition of
azobisisobutyronitrile (AIBN), to alkene to form an
alkyl radical 1ꢀ, and the abstraction of the PhTe
group from telluride PhTeR 2 by 1ꢀ to provide in-
termediate radical R• (steps 1 and 2). Steps 3 and
4 constitute the propagation sequence. Addition of
R• to alkene 3 and the following abstraction of the
PhTe group from the starting telluride 2 affords
the addition product 4 with regeneration of the key
Dedicated to Professor Kin-ya Akiba on the occasion of his 75th
birthday.
Noboru Sonoda is an emeritus professor of Osaka University.
Correspondence to: N. Kambe; e-mail: kambe@chem.eng.osaka
-u.ac.jp.
Present address of Li-Biao Han: National Institute of Advanced
Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-
8565, Japan.
c
ꢀ
2011 Wiley Periodicals, Inc.
518

Free Radical Addition of α-Telluroesters to Alkenes 519
Then we examined the reaction of α-
(phenyltelluro)esters 2 with electron-rich alkenes
due to the following reasons: (i) PhTeCH2CO2Et 2b
is known to undergo rapid bimolecular homolytic
substitution on Te with n-octyl radical in a rate
of 2.3 × 107 M−1 s−1 at 50◦C [12]. (ii) Radicals 1
(X = n-C6H13, R• = CH2CO2Rꢀ) are not stabilized
by conjugation and are considered to have similar
reactivity as s-octyl radical toward α-telluroesters
2. (iii) We already developed a convenient synthetic
method of α-telluroesters [13]. As expected, treat-
ment of α-(phenyltelluro)esters 2a with the same
equivalent of 1-octene 3a in the presence of 10 mol%
of AIBN at 80◦C for 6 h resulted in the formation of
carbotelluration product 4a in 63% yield (Eq. (4)).
Initiation steps
In
(step 1)
In
+
X
X
1′
3
3
PhTe
X
In
In
(step 2)
X
+
PhTeR
R
+
1′
2
Propagation steps
R
(step 3)
+
R
X
X
R
1
PhTe
X
R
(step 4)
+
X
PhTeR
R
+
1
2
4
SCHEME 1 Carbotelluration of alkenes.
intermediate R•. Thus, both steps 3 and 4 should
proceed rapidly to achieve efficient radical chain
sequence giving carbotelluration product 4. Since
alkyl radicals are known to add to alkenes more
rapidly than to alkyne [7], the key process of this
transformation is a bimolecular homolytic substi-
tution (SH2) reaction on Te (step 4); i.e., if this pro-
cess is slow, radical addition to olefins predominates
to afford polymerization or telomerization prod-
ucts [8]. As a successful example of carbotellura-
tion of alkenes, Crich and coworkers [9] reported an
intramolecular version employing tellurobenzoates
carrying olefinic moiety at the ortho position. How-
ever, there are no examples of intermolecular car-
botelluration of alkenes.
AIBN
(10 mol%)
PhTe
n-C6H13
PhTe
CO Et
CO2Et
4a, 63%
+ n-C6H13
3a
2
neat
80°C, 6 h
2a
(4)
Several examples of carbotelluration of alkenes are
summarized in Table 1. α-Telluroesters having an H
atom or a methyl substituent on the α-carbon added
to the terminal double bonds regioselectively to give
the corresponding adducts 4 in good yields. How-
ever, as generally seen in radical reactions, the reac-
tion was sluggish toward internal alkenes such as
4-octene. The yields of adducts were significantly
improved when an excess amount of alkene was
used. For example, when 5 equiv of the alkene was
used, the corresponding adduct was obtained almost
quantitatively based on the telluride (run 3). Alkenes
having a functional group such as Br, OH, or CO2H
also gave the products in good yields. It is worth
to note that bromoalkane moiety of 3c is tolerant
to the present reaction because bromoalkanes can
also undergo atom transfer reaction under radical
conditions (run 4). Similar addition reactions of α-
bromoesters to alkenes in the presence of peroxide,
known as the Kharasch reaction, are well recognized
[14]. However, under the conditions of run 4, only
27% of the expected adduct was obtained by the re-
action of ethyl 2-bromopropioate with 1-octene. The
higher reactivity of α-telluroester over α-bromoester
would be ascribable to more facile SH2 process on
Te than on Br via stable hypervalent radical inter-
mediates of tellurium (it was suggested that the SH2
reaction on Te is about 10 times faster than that
on I; see ref. 1a.) and also to smaller bond disso-
ciation energy of C Te bonds than that of C Br
bonds.
RESULTS AND DISCUSSION
First, we carried out a reaction of PhTePr-i with
acrylonitrile at 80◦C using AIBN (10 mol%) as
an initiator; however, acrylonitrile polymerized
completely and PhTePr-i was recovered unreacted
(Eq. (3)). This result suggests that 1-cyanoalkyl radi-
cal attacked acrylonitrile predominantly rather than
telluride probably because alkyl radical conjugated
with a CN group formed by radical addition to
acrylonitrile is more stable than i-Pr radical, which
can be generated by SH2 on Te (Eq. (3)). The SH2
reaction on Te is an equilibrium process in which
the most stable radical eliminates from hypervalent
intermediates [10]. Yamago and Yoshida applied
similar systems to radical polymerizations of styrene
derivatives [11].
CN
NC
In
NC
In
In
+
NC
NC
PhTe
NC
PhTePr-i
i-Pr
+
In
Tellurides having a C C double bond at an
appropriate position undergo a cyclization reac-
tion. For example, when s-butyl 4-pentenyl telluride
(3)
Heteroatom Chemistry DOI 10.1002/hc

520 Kambe et al.
TABLE 1 Radical Addition of α-(Phenyltelluro)esters to Alkenesa
Run
Telluride
Alkene
Product
PhTe
n-C6H13
Yield (%)b
PhTe
CO2Et
CO2Et
n-C6H13
3a
4b
2b
2a
1
2
49(72)c
CO2Et
PhTe
PhTe
CO2Et
CO2Et
54d
3b
TePh
PhTe
4c
X
CO2Et
63e (97)c,e
61e
X
3
4
5
6
2a
2a
2a
3c: X = Br
3d: X = OH
3e: X = CO2H
4d
4e
4f
67e
66e
O
O
O
O
HO
PhTe
TePh
HO
65e
3d
2c
4g
aConditions: telluride (1.0 mmol), alkene (1.0 mmol), AIBN (0.1 mmol), 80◦C, 6 h.
bIsolated yield.
c5.0 mmol of alkene was used.
dA 52/48 mixture of (exo, endo)/(exo, exo) isomers.
eA ca. 1/1 mixture of two isomers.
5 was heated at 80◦C in the presence of AIBN,
cyclization took place to afford the corresponding
telluracyclopentane 6 in 46% yield (Eq. (5)). In this
reaction, s-butyl radical adds to a C C double bond
to form 7 and a subsequent intramolecular SH2 reac-
tion on Te affords telluracyclopentane 6. This result
demonstrates that the intramolecular SH2 reaction
on Te can take place efficiently even when substan-
tial energy gain is not expected as in the present case
of secondary alkyl–secondary alkyl exchange.
electron-rich alkenes. The structures of tellurides
and alkenes exert crucial effects on the thermody-
namics of each reaction process to achieve this car-
botelluration of alkenes efficiently.
EXPERIMENTAL
General
Melting points were determined on a Yanagimoto
micro melting point apparatus. 1H and 13C NMR
spectra were recorded on a JEOL (JEOL Ltd.,
Akishimashi, Tokyo, Japan) JNM-GSX-270 (270 and
68 MHz, respectively) spectrometer using Me4Si (in
CDCl3) as an internal standard. Infrared (IR) spectra
were determined on a Perkin-Elmer (PerkinElmer,
Inc., Waltham, MA) model 1600 spectrometer. Mass
spectra were obtained on a JEOL (JEOL Ltd.)
JMS-DX303. Elemental analyses were performed on
PerkinElmer 240C apparatus.
AIBN
(10 mol%)
Te
Te
neat
80°C, 30 h
5
6, 46%
s-Bu
s-Bu
Te
7
(5)
General Procedure
In summary, free radical addition of tellurides to
alkenes was attained by the use of α-telluroesters and
Into a glass tube (φ = 6 mm), α-telluroester 2
(1 mmol), alkene 3 (1 mmol), and AIBN (0.1 mmol)
Heteroatom Chemistry DOI 10.1002/hc

Free Radical Addition of α-Telluroesters to Alkenes 521
were placed under argon. The tube was sealed and
heated at 80◦C for 6 h. The reaction mixture was
washed out using chloroform, and the product was
purified by column chromatography on silica gel us-
ing n-hexane/Et2O (20/1) as an eluent or by recycling
preparative high-performance liquid chromatogra-
phy (HPLC).
(22), 121 (82), 93 (50). HRMS calcd for C18H24O2Te:
402.0838, found: 402.0841.
(exo, endo) isomer: 1H NMR (270 MHz, CDCl3) δ
1.09–1.59 (m, 12H), 1.91–1.97 (m, 1H), 2.16–2.30 (m,
3H), 3.51 (bs, 1H), 4.12 (q, J = 7.3 Hz, 2H), 7.14–7.20
(m, 2H), 7.24–7.29 (m, 1H), 7.74–7.80 (m, 2H); 13C
NMR (68 MHz, CDCl3) δ14.1, 16.2, 28.0, 29.8, 34.9,
38.5, 38.9, 42.8, 43.9, 53.2, 60.6, 112.3, 127.68, 129.0,
139.3, 175.5; IR (NaCl) 2957, 2871, 1728, 1574, 1474,
1455, 1434, 1259, 1177, 1061, 1018, 734, 693 cm−1;
MS (EI), m/e (relative intensity, %) 402 (M+, 39),
207 (18), 195 (100), 167 (27), 149 (22), 121 (90), 93
(55). HRMS calcd. for C18H24O2Te: 402.0838, found:
402.0839.
Ethyl 2-methyl-4-(phenyltelluro)decanoate (4a).
1
Obtained as a mixture of ca. 1/1 diastereomers; H
NMR (270 MHz, CDCl3) δ 0.86–2.17 (m, 21H), 2.75–
2.86 (m, 1H), 3.18–3.23 (m, 1H), 4.01–4.15 (m, 2H),
7.17–7.34 (m, 3H); 7.77–7.81 (m, 2H); 13C NMR (68
MHz, CDCl3) δ14.0, 14.1, 16.6, 17.6, 22.5, 28.7, 28.8,
29.1, 29.3, 30.2, 31.1, 31.6, 37.2, 37.9, 39.4, 39.6,
41.2, 41.6, 60.09, 60.13, 110.8, 110.9, 127.85, 127.88,
128.88, 128.91, 140.35, 140.40, 176.1; IR(NaCl) 2957,
2927, 2854, 1732, 1574, 1460, 1434, 1377, 1257,
1179, 1018, 910, 733, 693 cm−1; MS (EI), m/e (rel-
ative intensity, %) 420 (M+, 8), 213 (100), 207 (19),
167 (39), 149 (17) 139 (12), 111 (14), 97 (39), 83
(72). HRMS calcd. for C19H30O2Te: 420.1308, found:
420.1326.
Ethyl 6-Bromo-2-methyl-4-(phenyltelluro)hexa-
noate (4d). Obtained as a mixture of ca. 1/1 di-
astereomers; 1H NMR (270 MHz, CDCl3) δ 1.11–1.35
(m, 12H), 1.57–1.68 (m, 2H), 1.95–2.95 (m, 6H),
3.59 (t, J = 6.8 Hz, 2H), 3.60 (t, J = 6.8 Hz, 2H),
4.05–4.18 (m, 4H), 7.19–7.25 (m, 4H), 7.28–7.37 (m,
2H), 7.71–7.84 (m, 4H); 13C NMR (68 MHz, CDCl3)
δ14.23, 16.4, 17.6, 27.6, 28.3, 33.6, 33.8, 39.4, 39.5,
39.7, 40.4, 40.6, 41.3, 60.37, 60.40, 109.6, 109.9,
128.4, 128.5, 129.3, 140.8, 140.9, 175.9, 176.0; IR
(NaCl) 3066, 2976, 2034, 1732, 1574, 1474, 1434,
1378, 1258, 1182, 1018, 735, 694 cm−1; MS (EI), m/z
(relative intensity, %) 442 (M+, 42), 397 (11), 363
(20), 235 (98), 207 (69), 155 (93), 127 (100); HRMS
calcd. for C15H21BrO2Te: 441.9788, found: 441.9781.
Ethyl 4-(phenyltelluro)decanoate (4b). 1H NMR
(270 MHz, CDCl3) δ 0.84–0.89 (m, 3H), 1.19–1.74 (m,
13H), 1.89–2.08 (m, 2H), 2.42–2.57 (m, 2H), 3.17–
3.27 (m, 1H), 4.07 (q, J = 7.3 Hz, 2H), 7.15–7.32 (m,
3H); 7.76–7.78 (m, 2H); 13C NMR (68 MHz, CDCl3)
δ14.1, 14.2, 22.6, 28.9, 29.3, 31.7, 32.3, 32.5, 34.3,
37.3, 60.3, 111.1, 128.0, 129.0, 140.3, 173.2; IR (NaCl)
3066, 2955, 2926, 2854, 1736, 1574, 1474, 1374, 1178,
734, 693 cm−1; MS (EI), m/e (relative intensity, %)
406 (M+, 14), 207 (20), 199 (100), 153 (72), 135
(51) 111 (30) 97 (55). HRMS calcd. for C18H28O2Te:
406.1151, found: 406.1148.
Ethyl 6-hydroxy-2-methyl-4-(phenyltelluro)hexa-
noate (4e). Obtained as a mixture of ca. 1/1 di-
astereomers; 1H NMR (270 MHz, CDCl3) δ 1.09 (d, J
= 6.8 Hz, 2H), 1.13 (d, J = 6.8 Hz, 3H), 1.17–1.29 (m,
6H), 1.57–1.74 (m, 1H), 1.88–2.17 (m, 1H), 2.48 (s,
1H), 2.60 (s, 1H), 3.99–4.13 (m, 2H), 7.16–7.33 (m,
3H), 7.76–7.80 (m, 2H); 13C NMR (68 MHz, CDCl3)
δ14.2, 16.5, 17.7, 25.6, 26.0, 39.5, 39.6, 40.4, 41.2,
41.3, 60.36, 60.40, 62.0, 62.1, 110.8, 111.0, 128.1,
129.1, 140.4, 176.3, 176.5; IR (NaCl) 3435, 3065,
2976, 2935, 1728, 1574, 1473, 1434, 1378, 1300, 1258,
1181, 1083, 1044, 1018, 998, 735, 694 cm−1; MS (EI),
m/z (relative intensity, %) 380 (M+, 20), 207 (27), 173
(100), 155 (38), 143 (24), 127 (50), 99 (57). HRMS
calcd. for C15H22O3Te: 380.0631, found: 380.0628.
2-(1-Ethoxycarbonylethyl)-1-(penyltelluro)norbo-
rnane (4c). The ratio of the isomers, (exo, endo) to
(exo, exo), was 52/:48. Assignment was based on the
chemical shift of exo/endo protons of norbornanes.
1
(exo, exo) isomer: H NMR (270 MHz, CDCl3)
δ 1.12–1.64 (m, 12H), 1.84–1.90 (m, 1H), 2.16–2.27
(m, 3H), 3.30 (bs, 1H), 4.12 (q, J = 7.3 Hz, 2H), 7.17–
7.19 (m, 2H), 7.26–7.32 (m, 1H), 7.78–7.81 (m, 2H);
13C NMR (68 MHz, CDCl3) δ14.2, 15.8, 28.4, 29.5,
35.1, 36.1, 40.7, 43.1, 45.0, 53.3, 60.2, 112.3, 127.8,
129.1, 139.6, 176.0; IR(NaCl) 2954, 2871, 1732, 1574,
1474, 1454, 1434, 1376, 1258, 1177, 1158, 1148, 1053,
1018, 734, 693 cm−1; MS (EI), m/e (relative intensity,
%) 402 (M+, 34), 207 (11), 195 (100), 167 (25), 149
6-(1-Ethoxycarbonylethyl)-4-(phenyltelluro)hep-
tanoate (4f). Obtained as a mixture of ca. 1/1 di-
astereomers; 1H NMR (270 MHz, CDCl3) δ 1.10–1.27
(m, 6H), 1.56–1.68 (m, 1H), 1.85–2.18 (m, 3H),
2.48–2.72 (m, 2H), 2.78–2.92 (m, 1H), 3.06–3.16
(m, 1H), 4.02–4.16 (m, 2H), 7.18–7.24 (m, 2H),
7.27–7.36 (m, 1H), 7.75–7.81 (m, 2H); 13C NMR (68
Heteroatom Chemistry DOI 10.1002/hc

522 Kambe et al.
MHz, CDCl3) δ14.2, 16.6, 17.6, 29.1, 29.8, 32.0, 32.6,
34.0, 34.1, 39.4, 39.6, 41.0, 41.4, 60.40, 60.43, 110.0,
110.2, 128.35, 128.40, 129.21, 140.7, 176.16, 176.19,
179.16; IR (NaCl) 3066, 2977, 2934, 1732, 1710,
1446, 1434, 1378, 1257, 1181, 1018, 735, 694 cm−1;
MS (EI), m/z (relative intensity, %) 408 (M+, 25),
363 (8), 207 (20), 155 (100), 137 (38), 127 (55), 109
(30). HRMS calcd for C16H22O4Te: 408.0580, found:
408.0577.
38.2, 43.0, 43.5, 46.7, 46.9; IR (NaCl) 2958, 2871,
1460, 1437, 1377, 1245, 1136, 784 cm−1. Anal calcd.
for C9H18Te: C, 42.59; H, 7.15. Found: C, 42.55; H,
7.14.
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a
mixture of ca. 1/1
diastereomers; 1H NMR (270 MHz, CDCl3) δ
1.25 (s, 1H), 1.31 (s, 1H), 1.57–2.28 (m, 12H),
2.35–2.44 (m, 1H), 2.81–2.93 (m, 1H), 3.02–3.15
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40.26, 40.34, 61.93, 61.96, 66.5, 110.4, 110.7, 128.4,
128.5, 129.3, 129.4, 140.4, 140.5, 179.3, 179.4; IR
(NaCl) 3460, 2909, 1766, 1574, 1474, 1434, 1375,
1160, 1023, 736, 695 cm−1; MS (EI), m/z (relative
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(m, 7H), 2.14 (q, J = 7.3 Hz, 2H), 2.65 (t, J =
7.3 Hz, 2H), 3.17 (sextet, J = 7.3 Hz, 1H), 4.96–
5.08 (m, 2H), 5.72–5.87 (m, 1H); 13C NMR (68 MHz,
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137.7; IR (NaCl) 3070, 2962, 2926, 2870, 1640, 1452,
1376, 1178, 1142, 1129, 995, 912 cm−1. Anal. calcd.
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[13] Hiiro, T.; Kambe, N.; Ogawa, A.; Miyoshi, N.; Murai,
S.; Sonoda, N. Synthesis 1987, 1096.
[14] (a) Kharasch, M. S.; Skell, P. S.; Fisher, P. J Am Chem
Soc 1948, 70, 1055; (b) For a recent review of the re-
lated Kharasch reaction: Singleton, J. T. Tetrahedon
2003, 59, 1837.
2-(2-Methylbutyl)telluracyclopentane (6). Ob-
tained as a mixture of ca. 1/1 diastereomers; 1H
NMR (270 MHz, CDCl3) δ 0.83–0.90 (m, 6H), 1.02–
1.46 (m, 3H), 1.56–1.75 (m, 2H), 1.80–1.97 (m, 2H),
2.21–2.38 (m, 2H), 3.10–3.21 (m, 2H), 4.07–4.16 (m,
1H); 13C NMR (68 MHz, CDCl3) δ6.17, 6.48, 11.0,
11.4, 18.6, 19.4, 28.3, 30.3, 30.7, 31.9, 34.5, 37.1,
Heteroatom Chemistry DOI 10.1002/hc