similar mechanism to that we proposed in the nickel-catalysed
hydroarylation of alkynes,6 seems to be less probable because
internal adduct 3 has to be produced through insertion of 2a to
the carbon–nickel bond of oxidative adduct 7 in an unfavorable
regioselective manner giving alkylnickel complex 8, which
cannot take stabilization by p-coordination found in p-
allylnickel 9 leading to 4. The probability of Path B must rely on
the reactivity of H–X with nickel(0) complexes, H–N bonds in
conditions B and D being unlikely to add oxidatively to
nickel(0) complexes. Consequently, it seems to be more
plausible that a nickel(0) catalyst first reacts with a 1,3-diene
(Path C), the resulting complex, e.g., nickelacyclopentene 10,8
accepting the addition of an organoboronate, though we do not
have any evidence to solve the mechanistic details, including
the role of proton sources.
In conclusion, we have demonstrated that organoboronates
activated by a nickel catalyst in combination with a stoichio-
metric amount of a proton source added to 1,3-dienes. Further
studies on details of the reaction mechanism, as well as
application of the activation method of carbon–boron bonds to
development of new reactions, are in progress.
Ni(acac)2/Ph3P (1+2) or Ni(OAc)2/Ph3P (1+2) as a catalyst
precursor was totally ineffective. The reaction did not take place
in the absence of a proton donor. Aniline as a proton donor was
effective to improve the yields and the selectivity for internal
adduct 3a (Table 1, entry 2), whereas the reaction using NMP as
a solvent in combination with water or N,N-dimethylethylene-
diamine as an additive proceeded in a reversed regioselectivity
to give 4a as the major product (entries 3 and 4 of Table 1). Use
of a more acidic proton donor like acetic acid and phenol or a
simple aliphatic amine such as butylamine was much less
effective.
The scope of the addition reaction was next examined using
these four sets of conditions as summarized in entries 5–24 of
Table 1. The regioselectivities with conditions A–D observed in
the reaction of 1a with 2a were conserved, on the whole, also in
the reaction with phenylboronates having an electron-donating
(entries 5–8), electron-withdrawing (entries 9–12) or protic
(entries 13 and 14) substituent. A naphthylboronate (entries
15–18), a heteroarylboronate (entries 19 and 20) and alke-
nylboronates (entries 21–24) also underwent the addition
reaction.
Besides 2a, 2,3-diphenyl-1,3-butadiene (2b) accepts the
addition of organoboronates [eqn. (2)]. Under both conditions A
and C, the addition proceeded regioselectively to give 1,4-addi-
tion products 6 predominantly, though a small amount of the
(E)-isomer was produced in some cases. Under similar
conditions, the reaction of less substituted 1,3-dienes such as
1,3-butadiene and isoprene unfortunately gave complex mix-
tures including the dimerization–hydroarylation products of the
1,3-dienes.
Notes and references
‡ General procedure for the nickel-catalysed addition of organoboronates
to 1,3-dienes: An additive (0.30 mmol) was added to a solution (0.45 mL)
of an organoboronate (0.30 mmol), a 1,3-diene (0.90 mmol), Ni(cod)2 (4.1
mg, 15 mmol) and triphenylphosphine (7.9 mg, 30 mmol). After the mixture
was stirred at 100 °C for 2 or 24 h, the resulting solution was treated with
saturated NaHCO3 aqueous solution (3 mL) and extracted with ether (5 mL
3 3). The combined organic layer was washed with brine (10 mL) and dried
over anhydrous magnesium sulfate. Bulb-to-bulb distillation or evaporation
of the solvent followed by purification with silica gel chromatography
(hexane–ethyl acetate) gave 3 and 4 (5 and 6).
1 J. Tsuji, Transition Metal Reagents and Catalysts: Innovations in
Organic Synthesis, Wiley, Chichester, 2000, pp. 169–197.
2 S. Akutagawa and S. Otsuka, J. Am. Chem. Soc., 1975, 97, 6870–6871;
F. Barbot and Ph. Miginiac, J. Organomet. Chem., 1978, 145,
269–276.
(2)
3 E. Shirakawa, Y. Nakao, H. Yoshida and T. Hiyama, J. Am. Chem. Soc.,
2000, 122, 9030–9031.
4 The palladium-catalysed three-component coupling of 1,3-dienes, dis-
ilanes and acyl chlorides affords net carbosilylation products, see: Y.
Obora, Y. Tsuji and T. Kawamura, J. Am. Chem. Soc., 1993, 115,
10414–10415; Y. Obora, Y. Tsuji and T. Kawamura, J. Am. Chem. Soc.,
1995, 117, 9814–9821.
5 The activation of carbon–boron bonds by rhodium complexes has been
applied to the addition of organic groups to unsaturated compounds. For
a,b-unsaturated carbonyl compounds: M. Sakai, H. Hayashi and N.
Miyaura, Organometallics, 1997, 16, 4229–4231; T. Hayashi, M.
Takahashi, Y. Takaya and M. Ogasawara, J. Am. Chem. Soc., 2002, 117,
5052–5058 and references cited therein; for aldehydes: M. Sakai, M.
Ueda and N. Miyaura, Angew. Chem., Int. Ed., 1998, 37, 3279–3281; M.
Ueda and N. Miyaura, J. Org. Chem., 2000, 65, 4450–4452; for alkynes:
T. Hayashi, K. Inoue, N. Taniguchi and M. Ogasawara, J. Am. Chem.
Soc., 2001, 123, 9918–9919.
Although the reaction mechanism is not clear at present, the
fact that nickel(II) complexes did not catalyse the reaction
should imply that a nickel(0) complex is an active catalyst,
which reacts first with an organoboronate (Path A in Scheme 1),
a proton source (Path B) or a 1,3-diene (Path C). Path A, a
6 E. Shirakawa, G. Takahashi, T. Tsuchimoto and Y. Kawakami, Chem.
Commun., 2001, 2688–2689.
7 The cobalt(I)-catalysed hydroalkenylation of 1,3-dienes using terminal
alkenes, whose methyne C–H bond inevitably undergoes 1,4-addition to
1,3-dienes, has been reported, see: G. Hilt and S. Lüers, Synthesis, 2002,
609–618.
8 Generation of this type of metallacycle from a transition metal and a
1,3-diene is widely known, see: R. H. Crabtree, The Organometallic
Chemistry of the Transition Metal, Wiley & Sons, New York, 3rd edn.,
2001, pp. 125–128.
Scheme 1
CHEM. COMMUN., 2002, 2210–2211
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Shirakawa, Eiji
