Et3B-Pd-promoted allylation of benzaldehyde with allylic alcohols

Article abstract of DOI:10.1016/S0040-4039(00)00429-9

In the presence of Pd(OAc)₂ (0.05 equiv.) and PPh₃ (0.1 equiv.), triethylborane promotes a variety of allyl alcohols to undergo allylation of benzaldehyde to provide homoallyl alcohols in good yield. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00429-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3627–3629
Et B–Pd-promoted allylation of benzaldehyde with allylic
3
alcohols
∗
Masanari Kimura, Tatsuya Tomizawa, Yoshikazu Horino, Shuji Tanaka and Yoshinao Tamaru
Department of Applied Chemistry, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-Machi, Nagasaki 852-8521,
Japan
Received 15 February 2000; revised 9 March 2000; accepted 10 March 2000
Abstract
In the presence of Pd(OAc)2 (0.05 equiv.) and PPh3 (0.1 equiv.), triethylborane promotes a variety of allyl
alcohols to undergo allylation of benzaldehyde to provide homoallyl alcohols in good yield. © 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: aldehydes; allylation; boron and compounds; palladium and compounds.
Nucleophilic allylation of carbonyl compounds with allyl metal reagents is among the most useful
1
methods for the construction of desired molecules and has been studied extensively. Most of the
allyl metals have been prepared from allylic halides, acetates, carbonates, or ethers, which are in turn
derived from allylic alcohols. Therefore, if allylic alcohols themselves could be utilized directly as an
2
allylating agent, the synthetic efficiency might be enormously improved. Masuyama et al. achieved
for the first time the palladium-catalyzed allylation of carbonyl compounds with allylic alcohols, where
SnCl reduced a π-allylpalladium intermediate and bestowed it nucleophilic ability (umpolung).
2
(1)
Here we would like to report that triethylborane is also capable of promoting palladium-catalyzed
direct allylation of benzaldehyde with allylic alcohols via the umpolung of π-allylpalladium (Eq. (1)).
Thus, when a mixture of allyl alcohol (1a, 1 mmol), benzaldehyde (1.2 mmol), Pd(OAc) (0.05 mmol),
2
PPh (0.1 mmol), and Et B (2.4 mmol, 1 M hexane) in THF was stirred for 28 h at room temperature
3
3
under N , 1-phenyl-3-buten-1-ol (2a) was produced in 64% isolated yield (run 1, Table 1). The ratio
2
PPh /Pd(OAc) =2 turned out to be the best; a higher (runs 3 and 4) or a lower ratio (run 2) was
3
2
∗
Corresponding author.
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00429-9
tetl 16709

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less efficient. With this catalytic system, other allylic alcohols (1b–f) underwent allylation with similar
efficiency (Table 1).
Table 1
Palladium-catalyzed allylation of benzaldehyde with allylic alcohols promoted by triethylboranea
In accord with our previous observation of the BEt –Pd promoted allylation with allylic benzoates,3
3
the chloride ion was detrimental (run 5) and phenol (1 equiv.) significantly helped improve the yield of 2
(runs 8 and 14). Apparently, side reactions, giving rise to 1,5-hexadienes (e.g. 3, run 9) or alkene (e.g. 4,
run 15), reduced the yield of 2. Indeed, the use of an excess amount of 1 (2 equiv., relative to aldehyde)
brought about a significant improvement of the yield of 2 (runs 6, 11 and 13).
In Scheme 1 is outlined our working hypothesis, which invokes an allyl–ethyl exchange between π-
allylpalladium I and Et B as the key step, producing allylborane and ethylpalladium. Allylborane would
3
1
b
readily react with aldehyde to give homoallyl diethylborate II, and ethylpalladium might undergo β-
hydrogen elimination thereby regenerating a Pd(0) species. The facile generation of I was evidenced
4
by quantitative formation of 5,5-bis(cinnamyl)-2,2-dimethyl-1,3-dioxane-4,6-dione (82% isolated yield)
on replacing aldehyde with Meldrum’s acid: cinnamyl alcohol (2.2 mmol), Meldrum’s acid (1.0 mmol),
5,6
Et B (2.4 mmol), Pd(OAc) (0.1 mmol), and PPh (0.2 mmol) in THF at room temperature for 2 h.
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2
3

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629
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Scheme 1. A plausible catalytic cycle for allylation of benzaldehyde via umpolung of π-allylpalladium with Et B
References
1. (a) Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207; (b) Brown, H. C.; Racherla, U. S.; Pellechia, P. J. J. Org. Chem.
1
990, 55, 1868.
2
3
4
. Takahara, J. P.; Masuyama, Y.; Kurusu, Y. J. Am. Chem. Soc. 1992, 114, 2577.
. Kimura, M.; Kiyama, I.; Tomizawa, T.; Horino, Y.; Tanaka, S.; Tamaru, Y. Tetrahedron Lett. 1999, 40, 6795.
. Coordination of Et
catalyzed α-allylation of diethyl malonate with Ph
Lett. 1993, 34, 179.
3
B to the oxygen of allyl alcohol might be responsible for the facile generation of I. For palladium-
−
3
B OCH
2
2
CH_CH , see: Stary, I.; Stara, I. G.; Kocovsky, P. Tetrahedron
5
6
. Details of the palladium-catalyzed direct allylation of soft nucleophiles with allyl alcohols will be reported in due course.
. Financial support from the Ministry of Education, Science, Sports and Culture, Japanese Government, Grant-in-Aid for
Scientific Research B, is acknowledged. We also thank Mr. Y. Ohhama (NMR Facility) for his splendid assistance.