Reaction of (Z)-1-bromoalk-1-enyldialkylboranes with DMSO: Regio- and stereo-selective formation of internal (E)-alkenyl bromides

Article abstract of DOI:10.1039/a900652d

Treatment of (Z)-1-bromoalk-1-enyldialkylboranes 1 with DMSO results in 1,2-migration of an alkyl group from the boron to the α-carbon without elimination of bromine to give internal (E)-alkenyl bromides 2 with excellent stereoselectivity (>99%).

Full text of DOI:10.1039/a900652d

Reaction of (Z)-1-bromoalk-1-enyldialkylboranes with DMSO: regio- and
stereo-selective formation of internal (E)-alkenyl bromides
Masayuki Hoshi,* Hideyuki Tanaka, Kazuya Shirakawa and Akira Arase
Department of Applied and Environmental Chemistry, Kitami Institute of Technology, 165 Koen-cho, Kitami
090-8507, Japan. E-mail: HOSHI-Masayuki/chem@king.cc.kitami-it.ac.jp
Received (in Cambridge, UK) 25th January 1999, Accepted 2nd March 1999
Table 1 Reaction of (Z)-1-bromoalk-1-enyldialkylboranes 1 with DMSOa
Treatment of (Z)-1-bromoalk-1-enyldialkylboranes 1 with
DMSO results in 1,2-migration of an alkyl group from the
boron to the a-carbon without elimination of bromine to give
internal (E)-alkenyl bromides 2 with excellent stereoselectiv-
ity ( > 99%).
Entry
R1
R2
Product Yield (%)
(
Z)-1-Haloalk-1-enylboranes, which can be prepared by hydro-
1
2
3
4
5
6
7
8
9
a
Cyclohexyl
Hexylb
Bu
Bu
Bu
Bu
Bu
Ph
Ph
Bu
2a
2b
2c
2d
2e
2f
2g
2h
2i
73
69
70
70
68
59
45
28
60
boration of 1-haloalk-1-ynes with borane derivatives, are
versatile and potential precursors of alkenes. For example, the
1
b
Pr(Me)CHCH
2
reaction of (Z)-1-haloalk-1-enyldialkylboranes with MeONa
followed by protonolysis with carboxylic acids offers regio- and
stereo-selective syntheses of internal (E)-alkenes where one of
the alkyl groups on the boron atom is introduced into the a-
t
b
2 2
Bu CH CH
b
Cyclopentyl
Cyclohexyl
Hexylb
alkenyl carbon atom with elimination of a halogen atom [eqn.
Hexyl
Cyclohexyl
Reactions were carried out in ClCH
b
t
2
(1)]. This strategy could be applied to the synthesis of
(CH
2
)
2
CH
2
Cl
2
2
CH Cl using 2 equiv. of DMSO
unless otherwise noted. b Reactions were carried out in CCl
.
4
with those of an authentic sample prepared according to the
literature.8
3
prostaglandin analogues. On the other hand, we reported that a
similar alkyl group migration was performed without elimina-
tion of the halogen atom by treatment of (Z)-1-haloalk-
The above example demonstrates that the relatively hindered
alkyl group on the boron atom is introduced to the alkenyl
moiety of 2 as one of the alkyl substituents in a regio-and stereo-
selective manner. We next explored the reaction of 1, which was
prepared using a variety of dialkylboranes derived from less
1
-enyldialkylboranes, whose alkyl groups were derived from a
relatively hindered alkene, with lead(iv) acetate or (diacetox-
yiodo)benzene giving internal (Z)-alkenyl halides in a highly
4
9
stereoselective manner (95–98% for Z) [eqn. (2)]. Ster-
hindered alkenes through hydroboration with BH Br·SMe ,
2
2
10
followed by hydridation with LiAlH
4
.
For example, (Z)-
-bromohex-1-enyldihexylborane 1b reacted with DMSO
under similar conditions§ to afford isomerically pure (E)-
1
6
-bromododec-5-ene 2b¶ in 69% yield (entry 2).
In all cases listed in Table 1, internal (E)-alkenyl bromides 2
eodefined alkenyl halides are important substrates, especially as
coupling partners of transition metal mediated cross-coupling
were obtained in > 99% isomeric purity. Accordingly, the
present reaction can be applied not only to the cases of relatively
hindered dialkylboranes (entries 1, 6 and 9) but also to the cases
of less hindered dialkylboranes (entries 2–5, 7 and 8),
demonstrating that a wide variety of compounds 2 can be
formed using this strategy.
A plausible mechanism for the formation of internal (E)-
alkenyl bromides 2 is shown in Scheme 3. It was observed that
the yields of 2 depended considerably on the solvent employed.
In addition, the addition of an equimolar amount of pyridine, a
more polar compound than THF, to 1 inhibited completely the
present reaction. These results suggest that the reaction is
initiated by nucleophilic attack of DMSO. It appears that a
concerted reaction, which involves the transfer of a methyl
group from sulfur to oxygen and the concomitant formation of
methylthio cation, occurs through two-electron oxidation. Then
electrophilic addition of the methylthio cation to the carbon–
carbon double bond followed by 1,2-migration of an alkyl group
from boron to the a-carbon would take place to form
intermediate A. Finally, intermediate A would undergo trans-
5
reactions to give alkenyl units stereoselectively. For example,
6
7
(
11Z)-retinal and rapamycin were synthesized using the above
coupling reaction. In these reactions the purity of the haloalkene
is important. This prompted us to investigate the alkyl group
transfer reaction in the hope of obtaining highly pure internal
alkenyl halides. We report here a new type of synthesis of
internal (E)-alkenyl bromides 2 from (Z)-1-bromoalk-1-en-
yldialkylboranes 1 with excellent stereoselectivity ( > 99%), on
which DMSO has a decisive influence.
The reactions of 1 with DMSO were carried out under
reaction conditions optimised using (Z)-1-bromohex-1-enyldi-
cyclohexylborane 1a as a typical substrate. Thus, 1a was treated
with 2 equiv. of DMSO in ClCH
temperature for 18 h (Scheme 1). After work-up† (E)-1-bromo-
-cyclohexylhex-1-ene 2a‡ was obtained in 73% yield and in
99% isomeric purity (entry 1, in Table 1). The ster-
eochemistry of 2a was assigned the E configuration by
2 2
CH Cl at 0 °C to room
1
>
1
13
comparing GC and H and C NMR analyses of 3a (Scheme 2)
Scheme 1
Scheme 2
Chem. Commun., 1999, 627–628
627

Scheme 3
elimination of an alkylmethoxyboryl group and a methylthio
MHz, CDCl
3
) 13.8, 14.0, 22.1, 22.6, 28.0, 28.2, 29.2, 31.4, 31.6, 35.4,
2
1
1
25.9, 132.4; umax(film)/cm 2956, 2927, 2858, 1645, 1463, 1379, 842,
group∑ to yield 2. In the case of entry 8, the large steric
+
+
1
2
727, 646; m/z (EI) 248 (M , 18%), 246 (M , 17), 122 (12), 120 (12), 111
hindrance between R and R could be responsible for the low
yield of (E)-4-bromo-2,2-dimethyldec-3-ene 2h.
(28), 97 (72), 83 (43), 81 (45), 69 (82), 67 (42), 55 (100).
∑
After hydrolysis of the reaction mixture, there is a characteristic odour of
mercaptan.
In summary, we have demonstrated that a variety of
compounds 1 react with DMSO to provide the corresponding
species 2 in > 99% isomeric purity, and thus both substituents
1 For example, see A. Pelter, K. Smith and H. C. Brown, Borane
Reagents, Academic Press, London, 1988; D. S. Matteson, Stereodir-
ected Synthesis with Organoboranes, Springer, Berlin, 1995.
1
2
(
R and R ) in 2 can be selected from a wide range of possible
groups. It should be noted that the present reaction is carried out
under mild and essentially neutral conditions. Further studies on
the application of this strategy to different types of species 1
with varying functionality and a similar nucleophilic attack on
2
G. Zweifel and H. Arzoumanian, J. Am. Chem. Soc., 1967, 89, 5086; G.
Zweifel, R. P. Fisher, J. T. Snow and C. C. Whitney, J. Am. Chem. Soc.,
1
971, 93, 6309; E. Negishi, J.-J. Katz and H. C. Brown, Synthesis, 1972,
5
55.
1
are now in progress.
3
4
5
E. J. Corey and T. Ravindranathan, J. Am. Chem. Soc., 1972, 94,
013.
Y. Masuda, A. Arase and A. Suzuki, Chem. Lett., 1978, 665; Bull.
Chem. Soc. Jpn., 1980, 53, 1652.
For example, see J. Tsuji, Palladium Reagents and Catalysts, Wiley,
Chichester, 1995; N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95,
4
Notes and references
†
For work-up of the reaction mixture the use of sodium perborate was
effective.
2
457.
‡
1
Selected data for 2a: d
4H), 1.98–2.18 (m, 2H), 2.35–2.55 (m, 1H), 5.78 (t, J 7.6, 1H); d
) 13.8, 22.1, 25.6, 25.8 (2C), 29.1, 31.5, 31.6 (2C), 41.5, 130.9,
H
(200 MHz, CDCl
3
) 0.90 (m, 3H), 1.10–1.88 (m,
6
7
8
9
J. Uenishi, R. Kawahama, O. Yonemitsu, A. Wada and M. Ito, Angew.
Chem., Int. Ed., 1998, 37, 320.
K. C. Nicolaou, T. K. Chakraborty, A. D. Piscopio, N. Minowa and P.
Bertinato, J. Am. Chem. Soc., 1993, 115, 4419.
C
(50
MHz, CDCl
3
1
1
2
33.3; umax(film)/cm2 2929, 2854, 1637, 1450, 1377, 893, 648; m/z (EI)
46 (M , 22%), 244 (M , 21), 165 (32), 123 (17), 109 (100), 95 (67), 81
+
+
G. Zweifel, H. Arzoumanian and C. C. Whitney, J. Am. Chem. Soc.,
(
§
48), 67 (63), 55 (51).
After preparation of 1b, THF was removed under reduced pressure. The
residue was extracted with CCl under Ar atmosphere in order to separate
b from LiAlBr , and the extracts were transferred into another flask
through a simple filter packed with glass wool.
Selected data for 2b: d (200 MHz, CDCl ) 0.79–1.04 (m, 6H), 1.18–1.62
m, 12H), 1.90–2.04 (m, 2H), 2.41 (t, J 7.2, 2H), 5.84 (t, J 7.6, 1H); d (50
1
967, 89, 3652.
H. C. Brown, N. Ravindran and S. U. Kulkarni, J. Org. Chem., 1979, 44,
417.
0 H. C. Brown and S. U. Kulkarni, J. Organomet. Chem., 1981, 218,
99.
4
2
1
4
1
2
¶
(
H
3
C
Communication 9/00652D
628
Chem. Commun., 1999, 627–628