DOI: 10.1002/chem.201502205
Communication
&
Homogeneous Catalysis
A Well-Defined Aluminum-Based Lewis Acid as an Effective
Catalyst for Diels–Alder Transformations
[a]
´
Zhizhou Liu, Jazreen Hui Qi Lee, Rakesh Ganguly, and Dragoslav Vidovic*
by the work of Yamamoto and Ishihara, as a very reactive
Abstract: A catalytically active aluminum-based system
for Diels–Alder transformations is reported. The system
was generated by mixing a b-diketiminate-stabilized alu-
minum bistriflate compound with Na[BArCl4] (ArCl =3,5-
Cl2C6H3). Solid-state analysis of the catalytic system reveals
a unique structure incorporating a two-dimensional coor-
dination polymer. According to the experimental results
obtained from several Diels–Alder transformations, the
aluminum-based system appears to be a more practical
and more robust alternative to the recently reported com-
pounds based on carbon and silicon cations.
system for various Diels–Alder reactions. In fact, the authors of
that work refer to the examined transformations as being
AlCl3-catalyzed cycloadditions.[5] We believe that this could not
be further from the truth, simply because all experimental pro-
cedures in this work were performed without the use of glove-
box or Schlenk techniques, resulting in AlCl3 being subjected
to severe hydrolysis. This assessment is additionally supported
by observations made by Cowley and co-workers, who empha-
sized that molecular AlCl3·2THF is an extremely air- and mois-
ture-sensitive material.[6a] Additionally, mixing AlCl3 (i.e., Al2Cl6)
with THF, in different ratios and under dry conditions, could
form several different products, including [AlCl2(THF)4][AlCl4],
adding to the overall uncertainty when attempting to identify
the active species.[6b,c] Therefore, there is a great probability
that, in the work by Fringuelli and co-workers, AlCl3 was hydro-
lyzed, producing an HBA (presumably HCl) that was then re-
sponsible for the observed catalytic activity. In fact, this study,
as was the case for many other reports, did not include any of
the control experiments suggested by Hintermann and co-
workers to minimize, if not completely eliminate, the possibility
of HBA activity.[2e]
Lewis acids (LAs) have been known to act as stoichiometric or
catalytic reagents in various organic transformations, including
allylic substitutions, aldol transformations, CÀH activation, cycli-
zation, and cycloaddition reactions.[1] As aluminum is not only
inherently electron deficient but is also a highly electron-posi-
tive element, it is not surprising that reagents based on this
metal have been widely used for organic transformations in-
cluding Diels–Alder reactions. However, on examining a recent
book by Yamamoto and Ishihara[1a] on Al-based reagents in-
volved in Diels–Alder cycloadditions, it was evident that cata-
lytic transformations, especially those involving less reactive
dienes and dienophiles (see below), were not well developed.
In fact, the reactions that could be classified as catalytic em-
ployed predominantly cyclopentadiene, which is among the
more reactive dienes. Furthermore, the vast majority of these
Al-based systems were prepared and used in situ without any
information on the structural properties of the active species.
This could be quite problematic because several recent reports
indicated that the observed catalytic activity was due not to
the Lewis acid properties of the investigated compound(s) but
to their ability to generate hidden Brønsted acids (HBAs).[2,3,4]
For example, Fringuelli and co-workers[5] identified
AlCl3·2THF, classified as an aluminum-based Lewis acid system
Considering all the evidence and being inspired by the work
on silylium cations[7] involved in catalytic Diels–Alder reactions,
our goal was to prepare a well-defined and well-characterized
triflate-containing aluminum species LAl(OTf)2 (L=HC(MeC-
NAr)2; Ar=2,6-iPr2C6H3) that is capable of acting as a Lewis
acid catalyst. We synthesized a b-diketiminate-supported alu-
minum bistriflate compound that, when mixed with Na[BArCl
]
4
(ArCl =3,5-Cl2C6H3), catalyzed several Diels–Alder transforma-
tions. Additional experiments were also performed to gain evi-
dence against HBA activity.
The overall synthetic pathway for the preparation of the
target bistriflate compound LAl(OTf)2 is summarized in
Scheme 1. Even though LAlCl2 had been previously synthe-
sized,[8] for this work the precursor has been prepared by in-
´
[a] Z. Liu, J. H. Q. Lee, Dr. R. Ganguly, Dr. D. Vidovic
School of Physical and Mathematical Sciences, Department of Chemistry
and Biological Chemistry
Nanyang Technological University
21 Nanyang Link, Singapore 637371
Fax: (+65)6791-196
Supporting information for this article, including synthetic procedures, spec-
troscopic data for new compounds, copies of multinuclear NMR spectra,
and details regarding single crystal X-ray analysis, is available on the
Scheme 1. General synthetic procedures. Reaction conditions: a) nBuLi
(1.0 equiv.), 2 h, AlCl3 (1.0 equiv.), overnight, toluene; b) AlOTf (2.1 equiv.),
overnight, 1,2-difluorobenzene.
Chem. Eur. J. 2015, 21, 11344 – 11348
11344
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