Lanthanum triflate-catalysed allylation of aldehydes: Crucial activation by benzoic acid

Article abstract of DOI:10.1016/S0040-4039(01)02115-3

Lanthanum triflate in combination with benzoic acid catalyses the allylation of aldehydes at low catalyst loading, without the need to activate the catalyst in advance.

Full text of DOI:10.1016/S0040-4039(01)02115-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 319–321
Lanthanum triflate-catalysed allylation of aldehydes:
crucial activation by benzoic acid
Helen C. Aspinall,a James S. Bissett,a Nicholas Greevesa,* and Daniel Levinb
aDepartment of Chemistry, Donnan and Robert Robinson Laboratories, The University of Liverpool, Crown Street,
Liverpool L69 7ZD, UK
bAstraZeneca, Silk Road Business Park, Charter Way, Macclesfield, Cheshire SK10 2NA, UK
Received 20 September 2001; revised 17 October 2001; accepted 5 November 2001
Abstract—Lanthanum triflate in combination with benzoic acid catalyses the allylation of aldehydes at low catalyst loading,
without the need to activate the catalyst in advance. © 2002 Elsevier Science Ltd. All rights reserved.
The use of lanthanide triflates as Lewis acid catalysts
has been well documented.1,2 Properties such as low
toxicity, moisture and air tolerance and their recyclabil-
ity make them attractive alternatives to Lewis acids
such as AlCl3.
The yields ranged from moderate to excellent with the
lowest yields obtained for aliphatic aldehydes octalde-
hyde (55%) and the gem-dimethyl-substituted heptalde-
hyde (50%). In the case of the latter aldehyde, this
could be due to the hindered nature of the carbonyl
group.
We recently reported the use of benzoic acid to acceler-
ate the reaction between aldehydes and allyltributyltin
with ytterbium triflate as the catalyst.3 That procedure
used only 2 mol% of ytterbium triflate but required that
the catalyst be activated by drying for 2 h at 160°C
under vacuum immediately before use. The reaction
also required an inert atmosphere. We now report that
the use of benzoic acid eliminates the need to dry the
catalyst and allows the use of a significantly cheaper
and less active catalyst, lanthanum triflate.
Before the development of the unactivated catalyst
protocol, the catalyst had been dried in the usual way
and a comparison of the unactivated catalyst results
with those obtained with the activated catalyst shows
very little difference between them (Table 2).
We decided that an investigation into the mechanism
was needed to obtain a clearer understanding of the
role of the benzoic acid. A recent report commented
that the formation of triflic acid (CF3SO3H) from the
combination of the Lewis and Brønsted acids could be
catalysing the reaction.5 However, our own investiga-
tion showed that 6 mol% triflic acid did not catalyse the
formation of the product. We believed what we were
observing was a Brønsted assisted Lewis acid. A further
study of the literature revealed other examples of this
kind of Brønsted assisted Lewis acid acceleration.
Kobayashi et al.6 combined scandium triflate with HCl
in a 1:1 ratio to produce a very active catalyst for the
aldol reaction. A series of experiments confirmed that
the Brønsted acid was not simply regenerating the
catalyst and that the Brønsted and Lewis acids were
acting together as a combined catalyst for the reaction.
Individually they did not produce the same results.
Ciufolini et al. reported an acceleration in a Yb(fod)3
catalysed ene reaction.7,8 This example closely relates to
our situation. They speculated that carboxylic acid
During our study into the scope of the ytterbium
triflate–benzoic acid allylation protocol, we found that
lanthanum triflate was almost as effective a catalyst as
ytterbium triflate for a variety of aldehydes. At this
time, the catalyst was still being activated before use.
The use of unactivated lanthanide triflate catalyst was
reported by Umani-Ronchi et al. for the allylation of
imines4 and we decided to apply this to our lanthanum
triflate allylation protocol. Table 1 shows the results of
the allylation reaction between various aldehydes and
allyltributyltin in which the lanthanum triflate catalyst
and solvent were not dried.
Keywords: lanthanides; catalysis; tin; Brønsted acidity; Lewis acidity.
* Corresponding author.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02115-3

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H. C. Aspinall et al. / Tetrahedron Letters 43 (2002) 319–321
Table 1. Scope of allylation reaction with unactivated catalyst
Table 2. Comparison of activated and unactivated catalyst
La(OTf)3
OH
O
2 mol%
MeCN
SnBu3
1 eq PhCO2H
R
R
H
R
Activated catalyst
Unactivated catalyst
Time
(min)
Yield
(%)
Time
(min)
Yield
(%)
Scheme 1.
Ph
o/n
o/n
270
225
270
o/n
225
67
72
75
70
83
87
62
130
90
210
170
90
79
69
62
55
93
88
76
After carrying out various experiments both with 5 and
100 mol% lanthanide catalysts, we came to the follow-
ing conclusions. The benzoic acid together with the
Lewis acid form an enhanced Brønsted acid that cataly-
ses the reaction. This was further confirmed by observ-
ing that 100 mol% catalyst without Brønsted acid did
not catalyse the reaction in the same time, proving that
the Brønsted acid does not simply regenerate the
catalyst.
4-MeS-(C6H4)-
2-Nitrocinnamyl-
C7H15
4-NO2-(C6H4)-
3,4,5-MeO-(C6H2)-
Hydrocinnamyl-
110
110
impurities combine with the Lewis acid to produce a
‘double activation’ of the aldehyde (Scheme 1). For our
own investigation of the mechanism, we turned to GC
reaction tracking to monitor the reactions with time
and under various conditions.
Various reports on the use of lanthanide catalysts have
focused on their recyclability.1,2 We decided to demon-
strate that this was possible in our system. Our model
reaction used benzaldehyde as the model aldehyde and
10 mol% lanthanum triflate catalyst, this was to facili-

H. C. Aspinall et al. / Tetrahedron Letters 43 (2002) 319–321
321
Table 3. Catalyst recovery and re-use
OH
O
La(OTf)3
x mol%
SnBu3
H
1 eq PhCO2H MeCN
Use
Mol% cat.
Time (min)
Yield (%)
Cat. recovered (%)
1st
2nd
10
2a
120
130
83
68
88
–
a Recovered catalyst.
tate its recovery. Using our procedure, we recovered
88% of the lanthanum triflate catalyst, which was then
used in 2 mol% for its second use (Table 3). The
recovered catalyst afforded the homoallylic alcohol in
68% yield, compare this with the fresh catalyst which
affords the product in 79% yield in the same time.
(typical eluant; 10–30% ether:petroleum ether) yielded
the products.
Acknowledgements
We would like to thank EPSRC and Zeneca for finan-
cial support (CASE award to J.S.B.) during this
project.
In summary, we have reported the use of a less active
catalyst in our benzoic acid–lanthanide triflate allyla-
tion protocol. We have eliminated the need to dry this
catalyst before use, which is desirable in terms of both
reaction times and convenience. We have also carried
out an investigation into the mechanism and the role of
the benzoic acid in order to obtain a better understand-
ing of this simple allylation procedure.
References
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