good yields of 1-bromo-2-substituted 1,4-dienes.5 Whereas
the regioselectivity of the reaction originated from the less-
hindered site addition of bromine radicals to alkynes; it is
known that, in the free-radical addition of HBr to allenes,
bromine radicals selectively add to the allene center carbon
to give stable 2-bromo-substituted allyl radicals.6,7 If the
resulting allyl radicals would add to allylbromides, 2-bromo-
substituted 1,5-dienes would be formed (Scheme 2).
Table 1. Regioselective Radical Bromoallylation Reaction of
5-Ethenylidenenonane (1a) and Allylbromides 2a
Herein we report the free-radical-mediated addition of
allylbromides to allenes proceeds regioselectively to give
excellent yields of the desired 2-bromo-substituted 1,5-
dienes (Scheme 2). The obtained bromo-dienes could then
be successfully converted to substituted alkenes and car-
bonyl compounds via the subsequent manipulation of
vinyl-bromine bonds by Pd-catalyzed reactions.
entry
2 (equiv)
C6H6 (M)
time (h)
yieldb (%)
1
2
3
4
5
6
7c
2a (2)
2a (58)
2b (2)
2b (2)
2b (1)
2b (1.5)
2b (2)
0.1
ꢀ
6
6
6
1
1
1
1
0
86
72
66
40
61
84
0.1
0.1
0.1
1.0
1.0
Scheme 2. Regioselective Radical Bromoallylation of Allenes
a Conditions: 1a (1.0 mmol), AIBN (30 mol %), 80 °C under an argon
atmosphere. b Isolated yield based on 1a. c 20 mol % of AIBN was used.
Having identified the optimal reaction conditions with
2b, a variety of allenes were examined in order to investi-
gate the scope of the reaction, and the results are shown in
Table 2. Monosubstituted allenes such as cyclohexylallene
(1b) and phenylallene (1c), underwent bromoallylation
with 2b efficiently to give excellent yields of the corre-
sponding addition products3cand 3d(entries 2 and 3). The
reaction using an electron-deficient allene 1d with an
ethoxycarbonyl substituent also worked well with the
identical regioselectivity (entry 4). Products were generally
obtained as an E/Z mixture. However, in this case, only
the E-form of 3e was obtained. The reaction using 2-
(bromomethyl)acrylonitrile (2c) with vinylidenecyclo-
hexane (1e) wasalso successful, whichgave the correspond-
ing bromoallylated product 3f in 88% yield (entry 5). A
variety of geminally disubstituted allenes 1fꢀ1j reacted
with 2b smoothly to form the corresponding bromo-dienes
3gꢀ3k in excellent yields (entries 6ꢀ10). Functional
groups, such as OTIPS, NPhth, and CO2Et, were tolerated
in the present radical bromoallylation reaction. The bro-
moallylation of 1,3-disubstituted allenes also worked well.
For example, similar reactionsof allene 1k or 1l with2 gave
the corresponding products in 87% and 77% yields, re-
spectively (entries 11 and 12).
In the present study, 5-ethenylidenenonane (1a) was
selected as a test allene substrate for the control experi-
ments (Table 1). Thus, when a mixture of allene 1a (1 mmol),
allylbromide 2a (2 mmol), and AIBN (30 mol %) was
stirred for 6 h at 80 °C under an argon atmosphere, no
anticipated bromoallylated product was obtained (Table 1,
entry 1). On the other hand, with the use of a large excess of
2a (58 equiv), the desired reaction proceeded to give
5-bromo-1,5-diene (3a) in 86% yield (entry 2). This reac-
tion was completely regioselective with no other iso-
mers detected. In the hope of accelerating the CꢀC
bond-forming step, methyl 2-(bromomethyl)acrylate
(2b) was tested, in which a methoxycarbonyl group
was introduced into the β-position of allylbromide.
This proved to markedly increase bromoallylation even
with 2 equiv, which gave the expected 2-bromo-sub-
stituted diene 3b in 72% yield (entry 3). A shorter
reaction time (1 h) sufficed to complete the reaction
(entry 4). A higher concentration of the solution and a
lesser amount of AIBN improved the yield to 84%
(Table 1, entries 6 and 7).
A proposed reaction mechanism for the present radical
bromoallylation of allenes is shown in Scheme 3. Thus,
initiating cyanopropyl radicals are formed by the thermal
decomposition of AIBN, which add to 2 to generate
bromine radicals. The bromine radicals regioselectively
add to the allene center carbon to form allyl radicals, and
then an addition to allylbromide 2 takes place in an SH20
manner to produce the desired 2-bromo-substituted 1,5-
dienes and regenerate the bromine radicals, thus achieving
(6) (a) Taylor, D. R. Chem. Rev. 1967, 67, 317. (b) Pasto, D. J.
Tetrahedron 1984, 40, 2805. (c) Krause, N.; Hashmi, A. S. K. Modern
Allene Chemistry; Wiley-VCH: Weinheim, 2004; Vols. 1 and 2. (d) Ma, S.
Chem. Rev. 2005, 105, 2829. (e) Ma, S. Aldrichmica Acta 2007, 40, 91.
(7) For radical hydrobromination of allenes, see: (a) Kovachic, D.;
Leitch, L. C. Can. J. Chem. 1961, 39, 363. (b) Griesbaum, K.; Oswald,
A. A.; Hall, D. N. J. Org. Chem. 1964, 29, 2404. (c) Abell, P. I.;
Anderson, R. S. Tetrahedron Lett. 1964, 5, 3727. (d) Tien, R. Y.; Abell,
P. I. J. Org. Chem. 1970, 35, 956. (e) Moorthy, S. N.; Singh, A.;
Devaprabhakara, D. J. Org. Chem. 1975, 40, 3452.
(8) For ditin/hν induced radical addition of alkyl halides to allyl
chlorides, see: Huval, C. C.; Singleton, D. A. Tetrahedron Lett. 1993, 34,
3041.
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