A new palladium-catalysed synthesis of 1,1-dialkylbuta-1,3-dienes via organoboron intermediates

Article abstract of DOI:10.1039/b104449b

Iodobenzene and tetrakis(triphenylphosphine)palladium(0) [(C6H5)3P]4Pd catalyse a new synthesis of 1,1-dialkylbuta-1,3-dienes, starting from 1,1-diethoxybut-2-ene and trialkylboranes, in the presence of Schlosser's superbase LIC-KOR.

Full text of DOI:10.1039/b104449b

A new palladium-catalysed synthesis of 1,1-dialkylbuta-1,3-dienes via
organoboron intermediates†
Paolo Balma Tivola,a Annamaria Deagostino,a Cristina Prandi*b and Paolo Venturello*a
a Dipartimento di Chimica Generale ed Organica Applicata dell’Università, Corso M. D’Azeglio, 48, I
10125 Torino, Italy. E-mail: venturello@ch.unito.it; Fax: 0039 0116707642; Tel: 0039 0116707646
b Dipartimento di Scienze e Tecnologie Avanzate dell’Università, Corso Borsalino, 54, I 15100
Alessandria, Italy
Received (in Cambridge, UK) 21st May 2001, Accepted 26th June 2001
First published as an Advance Article on the web 31st July 2001
Iodobenzene and tetrakis(triphenylphosphine)palladium(0)
[(C6H5)3P]4Pd catalyse a new synthesis of 1,1-dialkylbuta-
1,3-dienes, starting from 1,1-diethoxybut-2-ene and trialk-
ylboranes, in the presence of Schlosser’s superbase LIC–
KOR.
a,b-unsaturated ketones 5 (Scheme 1: path c). Moreover,
derivatives 5 can be directly obtained from 3 upon treatment
with a Lewis acid (F3B·OEt2) in an oxidative milieu (Scheme 1:
path d). More interestingly, a new and intriguing result comes
from the reaction with iodobenzene in the presence of catalytic
amounts of [P(C6H5)3]4Pd, according to the procedure of the
Suzuki–Miyura cross-coupling reaction. Thus, when inter-
mediate 3 was warmed to room temperature and treated with a
THF solution of iodobenzene and [P(C6H5)3]4Pd no coupling
product 8 was detected, while the gem-dialkyl substituted buta-
1,3-diene 7 was isolated in good yield (Scheme 1: path e).7,8 In
contrast, treatment of 3 with [P(C6H5)3]4Pd in the absence of
iodobenzene afforded only traces of product 7.9 Moreover, also
when intermediate 3 undergoes the iodination reaction6 or
treatment with iodobenzene in the absence of a palladium
complex no trace of product 7 was detected and only ketone 5
was recovered. The results obtained with different alkylboranes
are reported in Table 1.
The synthesis of polienic structures has been of great interest to
organic chemistry owing not only to their presence in natural
products,1 but also to their importance as useful chemicals in the
perfume industry and other fields.2 Our interest in the synthesis
of stereodefined substituted dienes required the development of
diverse organometallic reagents and protocol for the preparation
of key building blocks.3 In the course of these studies we have
set up a new synthesis of 1,1-dialkyl substituted buta-
1,3-dienes, resorting to the reactivity of organoboron com-
pounds.4 Treatment of crotonaldehyde diethyl acetal (1) at
295 °C with LIC–KOR base (LIC: butyllithium, KOR:
potassium tert-butoxide)5 readily gives a-metalated 1-ethoxy-
buta-1,3-diene (2). Subsequent reaction with trialkylboranes
leads to the immediate disappearance of the deeply red colour of
the metalated diene. An intermediate ‘ate’ complex 36 (Scheme
1) is probably formed that undergoes different reactions
according to experimental conditions.
Table 1 Synthesis of 1-alkyl-1-ethoxybuta-1,3-dienes and 1,1-dialkylbuta-
1,3-dienesa
1-Alkyl-1-ethoxybuta-1,3-dienes
1,1-Dialkylbuta-1,3-dienes
Treatment of complex 3 at 295 °C with H2O–THF leads to
the corresponding 1-alkoxy-1-alkylbuta-1,3-dienes 4, that can
be, like all enol ethers, smoothly converted to the corresponding
R
Product
Yield (%)
R
Product
Yield (%)
n-C4H9
c-C5H11
n-C6H13
c-C6H11
4a
4b
4c
4d
86
87
82
71
n-C4H9
c-C5H11
n-C6H13
c-C6H11
7a
7b
7c
7d
79
86
68
77
a For experimental details, see note 7. The spectral data for new compounds
4a–4d and 7a–7d are consistent with the structures proposed.
The formation of compound 7 suggests the intermediate
formation of the rearranged borate complex 6, and in Scheme 2
a possible reaction mechanism is reported that may account for
the formation of a gem substituted diene. According to this
hypothesis vinylborate interacts with the Pd catalyst and
undergoes transfer of an alkyl group, which promotes the
Scheme 1 Reagents and conditions: a: LIC–KOR, THF, 295 °C; b: BR3,
THF, 295 °C; c: 295 °C, H2O; d: BF3·OEt2, H2O2, 295 °C; e: C6H5I,
[(C6H5)3P]4Pd, THF, 25 °C.
† Dedicated to Professor Iacopo Degani on the occasion of his 70th
birthday.
Scheme 2 Proposed reaction mechanism.
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Chem. Commun., 2001, 1536–1537
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104449b

Chemistry, ed. V. Snieckus, JAI Press Inc., Greenwich CT, 1992, vol. 1,
pp. 1–45; M. Schlosser, F. Faigl, L. Franzini, H. Geneste, G. Katsoulos
and G. Zhong, Pure Appl. Chem., 1994, 66, 1439; L. Lochmann, Eur. J.
Inorg. Chem., 2000, 1115.
6 A. B. Levy and S. J. Schwartz, N. Wilson and B. Christie, J. Organomet.
Chem., 1978, 156, 123; H. C. Brown, J. A. Sinclair and M. M. Midland,
J. Org. Chem., 1986, 51, 4507.
7 Typical run: to a cooled solution (295 °C) of t-BuOK (1.4 g, 12.5
mmol) in anhydrous THF (10 ml) acetal 1 (0.72 g, 5.0 mmol) and BuLi
(7.8 ml, 12.5 mmol) were consecutively added dropwise under stirring.
After 2 h the purple–red solution was treated with tributylborane (5.0
mmol, 5 ml, 1.0 M in THF). The solution was allowed to warm to 25 °C
and then added to a THF solution of C6H5I (4.0 mmol)–[(C6H5)3P]4Pd
(0.058 g, 1 mol%). The reaction mixture was allowed to react for 8 h.
After standard work up the crude product was purified by column
chromatography to afford pure 7a. dH (400 MHz; CDCl3) 0.87 (t, J 6.5,
3 H), 0.90 (t, J 6.5, 3 H), 1.2–1.5 (m, 8 H), 2.05 (t, J 6.2, 2 H), 2.15 (t,
J 6.2, 2 H), 4.95 (dd, J 10.0, 1.0, 1 H), 5.10 (dd, J 16.0, 10.0, 1 H), 5.85
(d, J 10.0, 1 H), 6.60 (dt, J 16.0, 10.0, 1 H); dC (100.4 MHz; CDCl3)
14.1, 22.7, 22.9, 29.8, 30.4, 30.5, 31.1, 37.0, 114.4, 125.3, 128.3, 133.4,
144.6.
formation of a new p allyl complex. At this stage a new ‘ate’
complex forms that undergoes the elimination reaction much
faster than the phenyl transfer process.
In conclusion, these results indicate that depending on the
experimental conditions 1,1-dialkyl substituted buta-
1,3-dienes, 1-ethoxy-1-alkylbuta-1,3-dienes and a,b-unsatu-
rated ketones can be selectively prepared. Extension to other
organoboron intermediates (asymmetric ones, in particular) and
synthetic applications for these functionalized synthons, as well
as mechanistic studies to check the role of the palladium
complex and iodobenzene, are currently in progress in our
group.10
This work was supported by grants from Italian MURST. We
thank Dr Arnaud Gauthier and Dr Jacques Maddaluno (Uni-
versité de Rouen) for helpful criticism, and CNR-CNRS for a
grant for cultural exchange.
Notes and references
1 R. Baker and J. W. S. Bradshaw, in Aliphatic and Related Natural
Product Chemistry, ed. F. D. Gunstone, Specialist Periodical Report,
Royal Society of Chemistry, London, 1983, vol. 3; K. C. Nicolau, J. Y.
Ramphal, N. A. Petasis and C. N. Serhan, Angew. Chem., Int. Ed. Engl.,
1991, 30, 1100; V. Launay, I. Beaudet and J.-P. Quintard, Bull. Soc.
Chim. Fr., 1997, 134, 937; B. Dominguez, B. Iglesia and A. R. de Lera,
J. Org. Chem., 1998, 63, 4135; B. H. Lipshutz, B. Ullman, C. Lindsley,
S. Pecchi, D. J. Buzard and D. Dickson, J. Org. Chem., 1998, 63,
6092.
2 M. Goldbach, E. Jäkel and M. P. Schneider, J. Chem. Soc., Chem.
Commun., 1987, 1434.
3 P. Venturello, J. Chem. Soc., Chem. Commun., 1992, 1032; P. Balma
Tivola, A. Deagostino, C. Prandi and P. Venturello, J. Chem. Soc.,
Perkin Trans. 1, 2001, 437, and refs. therein.
8 Organoboron reagents have been used in the preparation of alkylidene-
cycloalkanes and trans disubstituted alkenes. See, G. Zweifel and R. P.
Fisher, Synthesis, 1972, 557, and E. Negishi, J.-J. Katz and R. C. Brown,
Synthesis, 1972, 555, respectively. Moreover, to our knowledge
substituted and gem-dibutyl substituted buta-1,3-dienes have only been
prepared in aqueous medium by the reaction of aldehydes and ketones
with 1-chloro-3-iodopropene in the presence of zinc powder. See, T.-H.
Chan and C.-J. Li, Organometallics, 1990, 9, 2649.
9 We have carried out some control reactions (R = n-C4H9 and R = c-
C6H11) in the presence of catalytic amounts (ranging from 5 to 10% with
respect to acetal 1) of PhI. The reactions proceed in good yields, but
longer reaction times are needed.
10 In order to assess the role of the catalyst, in accordance with the
suggestion of one of the referees, we have carried out a reaction in the
presence of a Pd(II) complex. In particular we have repeated the
synthesis of compound 7d (R = c-C6H11) using Pd(AcO)2 as a catalyst.
The reaction takes place, but the product was obtained with a quite lower
yield (by GC).
4 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457; A. Suzuki,
J. Organomet. Chem., 1999, 576, 147.
5 M. Schlosser, J. Organomet. Chem., 1967, 8, 9; M. Schlosser, Mod.
Synth. Methods, 1992, 6, 227; A. Mordini, Advances in Carbanion
Chem. Commun., 2001, 1536–1537
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