Reaction of [(Z)-1-bromo-1-alkenyl]dialkylboranes with N-halogeno compound in THF-DMF: A novel synthesis of 1,2-disubstituted (E)-vinyl bromides

Article abstract of DOI:10.1016/S0040-4039(00)00213-6

DMF-induced reaction of [(Z)-1-bromo-1-alkenyl]dialkylboranes with an N- halogeno compound results in 1,2-migration of an alkyl group from the dialkylboryl group to the α-carbon atom without elimination of the bromine atom, followed by β-elimination to provide 1,2-disubstituted (E)-vinyl bromides stereoselectively in good yields. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00213-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 2595–2598
Reaction of [(Z)-1-bromo-1-alkenyl]dialkylboranes with
N-halogeno compound in THF–DMF: a novel synthesis of
1,2-disubstituted (E)-vinyl bromides
Masayuki Hoshi ∗ and Kazuya Shirakawa
Department of Applied and Environmental Chemistry, Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan
Received 21 December 1999; revised 24 January 2000; accepted 28 January 2000
Abstract
DMF-induced reaction of [(Z)-1-bromo-1-alkenyl]dialkylboranes with an N-halogeno compound results in 1,2-
migration of an alkyl group from the dialkylboryl group to the α-carbon atom without elimination of the bromine
atom, followed by β-elimination to provide 1,2-disubstituted (E)-vinyl bromides stereoselectively in good yields.
© 2000 Elsevier Science Ltd. All rights reserved.
Keywords: [(Z)-1-bromo-1-alkenyl]dialkylboranes; DMF; N-halogeno compound; 1,2-disubstituted (E)-vinyl bromides;
1,2-migration.
Alkenylboranes are known to be quite useful intermediates in organic synthesis, especially in the
case of carbon–carbon bond formation such as the Zweifel-type reaction1 and Suzuki–Miyaura coupling
reaction.2 Among various alkenylboranes, [(Z)-1-halo-1-alkenyl]dialkylboranes have been usually used
as precursors of [(Z)-1-alkenyl]dialkylboranes3 and internal (E)-alkenylboranes4 which are unavailable
via hydroboration of alkynes. Thus, the reaction of [(Z)-1-halo-1-alkenyl]dialkylboranes with nucleo-
philes, such as hydride3 and methoxide,4a causes elimination of the halogen atom and simultaneous
migration of the nucleophile or the alkyl group on the boron atom to the α-carbon atom with inversion
of configuration via the corresponding ate-complexes. However, there are very few reports of organic
synthesis using [(Z)-1-halo-1-alkenyl]dialkylboranes without elimination of the halogen atom.4a,5 Very
recently, we have reported that treatment of [(Z)-1-bromo-1-alkenyl]dialkylboranes (1) with DMSO,
which would act as a nucleophilic activator, in a nonpolar solvent such as ClCH2CH2Cl or CCl4,
gives 1,2-disubstituted (E)-vinyl bromides (2) stereoselectively.6 We now report here a new and general
access to 2 using DMF-induced reaction of 1 with an N-halogeno compound where DMF would play an
important role as a polar solvent and the N-halogeno compound would act as an electrophilic promoter.
∗
Corresponding author: Tel: +81 157 26 9403; fax: +81 157 24 7719; e-mail: hoshi-masayuki/chem@king.cc.kitami-it.ac.jp
(M. Hoshi)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00213-6
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We chose [(Z)-1-bromo-1-hexenyl]dicyclohexylborane (1a), prepared by hydroboration of 1-bromo-1-
hexyne with dicyclohexylborane, as a substrate and examined the reaction with N-halogeno compounds.
When the reaction with N-chlorosuccinimide (NCS) was carried out in a mixed solvent of THF–DMF, the
formation of (E)-1-bromo-1-cyclohexyl-1-hexene (2a)7 was observed (Eq. (1)). The results performed
under several reaction conditions are summarized in Table 1. The highest yield was obtained in the
reaction carried out in THF–DMF (1:1) at −50°C to room temperature. Thus, the reaction of 1a with
1.75 equiv. of NCS gave 84% yield of 2a with high stereoselectivity (E:Z=98:2) (entry 4). It should be
noted that in the absence of DMF the reaction gave only a trace amount of 2a (entry 1). Although the
desired reaction also proceeded in the presence of other co-solvents under similar conditions (entries
8–10), DMF was superior to other co-solvents employed in terms of both yield and stereoselectivity
(entry 3).
(1)
Table 1
Effects of co-solvent and amount of NCSa
The reaction of a variety of 1 with an N-halogeno compound was carried out in the presence of DMF,
and the results are summarized in Table 2. The present reaction could be applied to various alkyl groups
(R1) ranging from the relatively hindered 1,2-dimethylpropyl group to the less hindered hexyl group,
and is tolerant of functional groups such as chloro, ether, ester and silyl function (entries 4–7 and 15).
The yields of 2 were improved by making a choice of the N-halogeno compound, for example, using
N-bromosuccinimide (NBS) instead of NCS (entry 3) and N-bromoacetamide (NBA) instead of NBS
(entries 6 and 9). In the case of 1j, however, changing NCS for NBA did not lead to a significant
increase in yield. In all cases examined, 1,2-disubstituted (E)-vinyl bromides (2) were formed in a highly
stereoselective fashion and obtained in good yields. The reaction of 1d, 1e and 1f with NBA proceeded
smoothly to give the corresponding products 2d, 2e8 and 2f, respectively (entries 6, 7 and 9). On the
other hand, the reaction of 1d and 1e with DMSO gave 2d and 2e in poor yields, respectively, and the
reaction of 1f with DMSO could hardly proceed under the conditions described in Ref. 6. The result of

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the former reaction is due to the decomposition of 1d and 1e caused by removal of THF, and in the latter
reaction is probably due to the steric hindrance between the 1,2-dimethylpropyl group on the boron atom
and DMSO.9 The present reaction thus overcame the disadvantages of the protocol using DMSO.
Table 2
DMF-induced reaction of [(Z)-1-bromo-1-alkenyl]dialkylboranes with N-halogeno compounda
We envisage the reaction mechanism for the formation of 2 as shown in Scheme 1. DMF probably
plays an important role in the initial stage of the present reaction. The reaction appears to be initiated by
the electrophilic attack of the N-halogeno compound. It is assumed that the formation of a halonium ion
across the carbon–carbon double bond followed by 1,2-migration of an alkyl group from the boron atom

2598
to the α-carbon atom would take place to form intermediate A, which would undergo trans-elimination
of the alkylaminoboryl and halogeno groups to yield 2.
Scheme 1. Proposed mechanism for the formation of disubstituted (E)-vinyl bromides
In conclusion, we have demonstrated for the first time that DMF induces the reaction of [(Z)-1-
bromo-1-alkenyl]dialkylboranes (1) with an N-halogeno compound to provide 1,2-disubstituted (E)-vinyl
bromides (2), where a wide range of alkyl substituents are permitted to participate in the conversion of 1
into 2, as compared to the reaction with DMSO.
References
1. For example, see: (a) Pelter, A.; Smith, K.; Brown, H. C. In Borane Reagents; Academic: London, 1988. (b) Matteson, D. S.
In Stereodirected Synthesis with Organoboranes; Springer: Berlin, 1995.
2. For example, see: (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483. (b) Suzuki, A. In Metal-Catalyzed Cross-
Coupling Reactions; Diederich, F.; Stang, P. J., Eds.; Wiley-VCH: Weinheim, 1998; pp. 49–97.
3. (a) Negishi, E.; Williams, R. M.; Lew, G.; Yoshida, T. J. Organomet. Chem. 1975, 92, C4–C6. (b) Campbell Jr., J. B.;
Molander, G. J. Organomet. Chem. 1978, 156, 71–79.
4. (a) Zweifel, G.; Arzoumanian, H. J. Am. Chem. Soc. 1967, 89, 5086–5088. (b) Negishi, E.; Yoshida, T. J. Chem. Soc., Chem.
Commun. 1973, 606–607. (c) Arase, A.; Hoshi, M.; Masuda, Y. Bull. Chem. Soc. Jpn. 1984, 57, 209–213. (d) Hoshi, M.;
Masuda, Y.; Arase, A. Bull. Chem. Soc. Jpn. 1985, 58, 1683–1689. (e) Brown, H. C.; Basavaiah, D.; Kulkarni, S. U.; Lee, H.
D.; Negishi, E.; Katz, J.-J. J. Org. Chem. 1986, 51, 5270–5276. (f) Brown, H. C.; Imai, T.; Bhat, N. G. J. Org. Chem. 1986,
51, 5277–5282.
5. (a) Masuda, Y.; Arase, A.; Suzuki, A. Chem. Lett. 1978, 665–668. (b) Masuda, Y.; Arase, A.; Suzuki, A. Bull. Chem. Soc.
Jpn. 1980, 53, 1652–1655. (c) Brown, H. C.; Blue, C. D.; Nelson, D. J.; Bhat, N. G. J. Org. Chem. 1989, 54, 6064–6067.
6. Hoshi, M.; Tanaka, H.; Shirakawa, K.; Arase, A. Chem. Commun. 1999, 627–628.
7. The spectral data of the product 2a were consistent with those reported in Ref. 6.
8. To a solution of 1e (4 mmol) in THF was added dry DMF (10 ml) at 0°C under argon atmosphere. The mixture was stirred
at the same temperature for 0.5 h and cooled to −50°C. To this mixture was added NBA (1.379 g, 10 mmol), and then
the reaction mixture was allowed to warm to room temperature without removing the bath and was stirred overnight. The
resulting mixture was treated with NaBO3·4H2O (1.23 g, 8 mmol) and H2O (2.67 ml) at room temperature with vigorous
stirring for 2 h. Water (20 ml) was added, and the organic products were extracted with n-hexane. The organic layer was
separated, dried with Na2SO4 and concentrated in vacuo. The residue was purified by column chromatography on silica gel
(n-hexane:CH2Cl2=9:1) to give 2e (0.918 g, 88% yield) as a colorless liquid. Compound 2e: IR (neat) 2931, 2856, 1743,
1
1639, 1448, 1379, 1363, 1226, 1122, 1026, 893, 798 cm–1; H NMR (500 MHz, CDCl3) δ 1.18 (dtt, J=12.8, 12.8, 3.7 Hz,
1H), 1.34 (dtt, J=12.8, 12.8, 3.7 Hz, 2H), 1.48–1.61 (m, 4H), 1.65–1.73 (m, 1H), 1.76–1.83 (m, 2H), 2.06 (s, 3H), 2.52 (tt,
J=10.7, 4.0 Hz, 1H), 4.57 (d, J=7.3 Hz, 2H), 5.97 (t, J=7.3 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 20.86, 25.52, 25.60 (2C),
31.61 (2C), 42.36, 60.55, 124.56, 140.11, 170.70; MS m/z (EI) 181 (M+−79 or 81, 22%), 139 (100), 121 (21), 93 (15), 79
(25), 67 (19), 44 (90), 42 (15).
9. Hoshi, M.; Shirakawa, K., unpublished result.