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Apparently, the increase in Lewis basicity of the 10–CBS frag-
the tetravalent Lewis acid TiCl did not show such a behaviour
4
ment leads to exceeding a threshold, which only allows effi-
as they adopted only trigonal pyramidal or in case of TiCl4
quadratic pyramidal conformation. The conformational change
could be attributed to a more efficient Lewis acid base interac-
tion upon complexation of 1,4-naphthoquinone (10) by raising
the electron density within the CBS fragment and thus allow-
ing an efficient donor–acceptor interaction of all four chlorine
cient interaction of all four chlorine atoms of SnCl with the
4
CBS fragment. In complex 13j (Figure 9d) the distortion of the
complex geometry leads to an enhanced Lewis acidity where
all four chlorine atoms show participation in electron density
delocalisation. This results in an increased Lewis acidity of
SnCl in 13j compared to 9j, which can be observed in a high
atoms of SnCl , which overcompensates the high necessary
4
4
kDA value making SnCl in the equatorial conformation a similar
preparation energy. Furthermore, interactions of the hydrogen-
atom in position 5 of the naphthoquinone with one of the
chlorine atoms could be found which further stabilises the
equatorial conformation.
4
potent Lewis acid as AlBr3.
Although the threshold in Lewis basicity for efficient interac-
tion is the main factor that influences the interplay between
both conformations, a second interaction that pushes 13j to-
wards the equatorial conformation could be found by analy-
sing the NOCV interactions in 13j. An CÀH···Cl interaction D1s,6
between the hydrogen atom of 10 at position 5 with the chlor-
ine atom Cl3 could be found, which contributes about
The dependency of the acidity of group 13 based Lewis
[
26]
acids on their structure has been known for some time, and
has inspired the design of pre-organised tetrahedral group 13
[
27]
Lewis acids. A similar behaviour for group 4 and 14 Lewis
[
23]
acids has only been briefly touched on. Through the pre-
sented study, both experimental and theoretical results could
À1
À1.9 kcalmol (Figure 10). This further stabilizes the equatorial
conformation and might be a reason for the high enantioselec-
tivity of 9j in the model Diels–Alder reaction. A similar halo-
genÀhydrogen bond has been found by Fujimoto in his de-
unravel the impact of the structural change of SnCl on its
4
acidity. Similar to the group-13-based Lewis acids, the design
of sterically fixed tin-based Lewis acids should therefore lead
to the development of novel, highly reactive catalysts.
tailed theoretical study on AlBr –CBS adduct 9e in the reaction
3
[16]
of methacrolein with cyclopentadiene.
Experimental Section
Procedure for the preparation of Lewis acid adducts [D ]9 of
1
[
D ]1b with stoichiometric amounts of Lewis acid
1
Inside
a glovebox, deuterated CBS-catalyst ([D ]1b, 32 mg,
1
9
0 mmol, 1.0 equiv) was weighed in a vial, equipped with a magnet-
ic stirring bar. The vial was sealed with a rubber septum, trans-
ferred out of the glovebox and connected to a Schlenk line. Then
CH Cl2 (0.61 mL) was added, and the solution cooled to below
2
À708C. A solution of the respective Lewis acid in CH Cl (1.0m,
2
2
9
0 mL, 90 mmol, 1.0 equiv) and 1 mL of CDCl3 were added under
constant stirring. After 15 min an aliquot of the solution (0.50 mL,
6
4 mmol adduct [D ]9) was transferred into a below À708C pre-
1
cooled NMR tube, which was sealed with a rubber/PTFE septum.
The sample was kept below À708C and analysed by NMR spectros-
copy at the desired temperature.
Figure 10. Plot of the NOCV interaction D1s,6 between the SnCl
4
chlorine
atoms and the hydrogen atom at position 5 of 10 in adduct 13j with frag-
+
ments 10–CBS and SnCl
4
at BP86/TZ2P .
Exemplary procedure for the ReactIR kinetic analysis of the
Diels–Alder reaction of 10 with 11 by using catalysts of type
9
Conclusions
A 25 mL two-neck Schlenk flask was equipped with a magnetic
stirring bar and the ReactIR probe head, and was connected to
a Schlenk line. Under argon atmosphere, the flask was cooled to
À408C and CH Cl (3.0 mL), an aliquot of stock solution of 10
2
In conclusion, by combining H NMR spectroscopic studies
with in situ-IR kinetic measurements, we could demonstrate
that several Lewis acids are able to activate CBS catalyst 1b ad-
equately for Diels–Alder reactions. This suggests the use of less
aggressive acids than the commonly employed AlBr or HNTf .
2
2
À1
(4.0 mL, 0.50 mmol, 1.0 equiv, CH Cl (c=125 mmolL )), as well as
2
2
isoprene (11, 500 mL, 4.99 mmol, 10.0 equiv) were added. Depend-
ing on the amount of catalyst solution added later, CH Cl (0.1 or
.4 mL) was added. The in situ FTIR spectroscopic measurement
3
2
2
The low experimentally quantified activation Dd( H) of 1b but
large catalytic activity of SnCl –CBS adduct 9j could be attrib-
uted to a conformational change of the chlorine atoms of
2
2
0
4
was started, as soon as there were at least 3.0 mL solution in the
flask. When the temperature (À40(Æ1)8C) as well as the intensity
SnCl4 upon coordination of 1,4-naphthoquinone (10). This
leads to a massively enhanced Lewis acidity of SnCl within the
À1
of the IR carbonyl band of dienophile 10 at 1670 cm were stable,
4
an aliquot of the respective adduct 9 in CH Cl2 (0.02 mmol,
2
1
0–SnCl –CBS complex 13j and thereby to a higher activation
of 10 in the Diels–Alder reaction. This behaviour could only be
observed for SnCl . Trivalent Lewis acids such as AlCl , but also
4
4.0 mol% active complex, preparation see below) was added
À1
under vigorous stirring (final concentrations: c(10)=61.7 mmolL
,
À1
À1
c(11)=617 mmolL , c(9)=2.5 mmolL , total volume: 8.1 mL).
4
3
Chem. Eur. J. 2016, 22, 13171 – 13180
13178 ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim