Communications
ometry, and thus the carbon atom is approximatively bound
an achiral additive was investigated. In regards to the high
[10]
trans to the oxygen atom [C(33)ÀScÀO 154.49(11)8]. As expect-
sensitivity of rare-earth alkyl complexes to protic sources, it
was assumed that controlled addition of a simple Brønsted
acid such as hydrochloric acid would release the chiral ligand
from the alkyl complex and afford the corresponding protonat-
ed salt of the ligand. To probe the viability of such an ap-
proach, the reactivity of 2b and hydrochloric acid was evaluat-
ed. The room-temperature addition of 4 equivalents of HCl
(1m in diethyl ether) in apolar solvents led to the formation of
a precipitate, which was identified as the monoprotonated
amine (R)-H L ·HCl from an independent synthesis
ed, the ScÀN
[2.125(3), 2.105(3) ꢃ] bond lengths are signifi-
amido
cantly shorter than the ScÀN
[2.360(3), 2.350(3) ꢃ] bond
pyridyl
lengths but fall in the range of those found in similar six-coor-
[
12]
[13]
dinated amido- and pyridine-bound scandium complexes,
respectively. The ScÀC [2.279(3) ꢃ] distance is close to the aver-
age value (2.287 ꢃ) reported for related six-coordinated alkyl
[
12b,14]
scandium complexes.
Twisting between the naphthyl
rings results in a torsion angle of 83.6(3)8. To fully characterize
the metal center and its surrounding four nitrogen atoms in
2
1
1
15
[16,17]
1
15
solution, heteronuclear H– N NMR correlations through
HMBC experiments were next undertaken. The latter technique
was crucial to evaluate the coordination modes and to deter-
mine the first lines of the structure–catalytic activity relation-
(Scheme 4).
Overlapping of heteronuclear H– N NMR cor-
[
15]
ships of the Pd complexes. The effect of metal chelation was
investigated by comparing the chemical shifts of the (R)-H2L1
ligand with those of complex 2b for each nitrogen atom
2 1
Scheme 4. Evolution of 2b into (R)-H L ·(HCl)n (n=1, 4) in the presence of
HCl.
1
15
(
Figure 2). Overlapping H– N NMR correlations can clearly es-
relations of H L and H L ·(HCl) revealed a powerful tool that
2
1
2 1
gave a clear fingerprint of the evolution of H L under acidic
2
1
[17]
conditions. Indeed, the chemical shifts of the pyridine nitro-
+
gen atoms were strongly impacted by the presence of H . A
+
large deshielding of Dd=À88 ppm describes the N ÀH in-
pyr
teraction. In contrast, the binaphthylamine nitrogen atoms
+
were barely affected by the presence of H (Dd=À2 ppm).
Complex 2b could also be converted into the fully protonated
[17]
salt (R)-H L ·(HCl) by the addition of 7 equivalents of HCl.
2
1
4
After demonstrating the viability of tuning scandium alkyl
complex 2b into the protonated salt (R)-H L ·(HCl) (n=1 or 4),
2
1
n
their catalytic efficiencies were independently evaluated in the
targeted CÀN and CÀC bond-formation reactions of the
tandem sequence. To our delight, 5 mol% of the monoproto-
nated salt (R)-H L ·HCl promoted the alkylation of 2-methylin-
2
1
dole (3a) and ethyl 3,3,3-trifluoropyruvate at 08C with full con-
version, which afforded FC product 4a in 86% yield with 34%
enantiomeric excess (ee) (Table 1, entry 1). A control experi-
ment highlighted the importance of the protonation step in
1
15
Figure 2. Overlapping heteronuclear H– N NMR correlations for H
blue and complex 2b in red. Focus in the aromatic region (chemical shifts
are referred to external pure CH NO and D is expressed in ppm).
2 1
L in
3
2
[18]
the enantioinduction in the transformation (Table 1, entry 2).
The tetraprotonated salt (R)-H L ·(HCl) led to a product with
2
1
4
tablish whether or not Sc is coordinated to the nitrogen atoms
and can further confirm the coordination modes. First, the pyri-
a lower ee value, probably as a result of a racemic background
[19]
reaction (Table 1, entry 3). As previously observed,
but in
dine nitrogen atoms (N ) experience deshielding, and thus
sharp contrast to our earlier findings with monoprotonated
pyr
[7e]
the corresponding signal is shifted nearly Dd=À30 ppm on
coordination to the metal center. However, the difference is
smaller than typically observed for NÀPd complexes (ranging
pyridylalkylamines, the presence of an N-methyl substituent
on the indole core provided racemic products (Table 1, en-
tries 4 and 5).
[
15]
from Dd=À88 to À93 ppm). Within this family of ligands,
deshielding is characteristic of complexation to pyridine nitro-
gen atoms. In contrast, a large shielding of Dd= +97 ppm is
observed for the amido nitrogen atoms (Namido). In this case,
a large positive Dd value plausibly results from the contribu-
tion of the amido coordination mode to Sc, which is in sharp
contrast to the small Dd value observed (from Dd=0 to
À20 ppm) for NHÀPd bonds.
The ability of novel binaphthylamido rare-earth complexes
to catalytically promote intramolecular hydroamination reac-
tions was first demonstrated for the cyclohydroamination of
[17]
well-known benchmark alkenyl and alkynyl substrates. From
the outcome of this preliminary work, scandium catalysts 2b
and 2c were chosen for the rest of the study. The aptitude of
2b to catalyze indole-ring synthesis starting from 2-(alk-1-yn-1-
yl)aniline derivatives was then assessed, and the results are
gathered in Table 2. Among the various methods reported for
the construction of the indole core, the metal-catalyzed cycli-
Subsequently, the potential transformation of chiral scandi-
um complex 2b into the monoprotonated salt (R)-H L ·HCl by
2
1
ChemCatChem 2016, 8, 1 – 7
3
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&
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