Synthesis and theoretical studies of gallium complexes back-shielded by a cage-shaped framework of tris(m-oxybenzyl)arene

Article abstract of DOI:10.1039/c0cc00253d

Cage-shaped gallium complexes with a back-shielding framework of tris(m-oxybenzyl)arene were synthesized, their bottom arene rings tuned their characteristic Lewis acidity, which was supported by theoretical calculation as well as catalytic application in a hetero Diels-Alder reaction.

Full text of DOI:10.1039/c0cc00253d

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Synthesis and theoretical studies of gallium complexes back-shielded
by a cage-shaped framework of tris(m-oxybenzyl)arenew
Hideto Nakajima,a Makoto Yasuda,*a Kouji Chibab and Akio Baba*a
Received 6th March 2010, Accepted 28th April 2010
First published as an Advance Article on the web 25th May 2010
DOI: 10.1039/c0cc00253d
Cage-shaped gallium complexes with a back-shielding frame-
work of tris(m-oxybenzyl)arene were synthesized, their bottom
arene rings tuned their characteristic Lewis acidity, which was
supported by theoretical calculation as well as catalytic applica-
tion in a hetero Diels–Alder reaction.
gallium complex 3aGaꢀPy.7 In the 1H NMR of 3aGaꢀPy, the
upfield-shift of the proton at the m- and p-positions of the oxygen
moiety relative to that of 3aH3 was confirmed (m-position,
7.15 - 7.07 ppm; p-position, 6.78 - 6.68 ppm). Analogous
results for spectral analysis were obtained for 3b–3dGaꢀPy.
The structure of gallium complex 3bGaꢀPy was analyzed by
X-ray crystallography.8 Considering the occupancy factor of
the gallium atom was 0.44, the refinement of the crystal
structure was carried out successfully by using disorder
The chemistry of Lewis acids has been developed quite extensively
because of their effective activation of reactant reagents.1
A representative class of Lewis acids contains group 13
elements because they have a vacant p-orbital to contribute
to an accepting basic substrate.2 Recently, we designed tris-
(o-hydroxyphenyl)methane 1H3 and tris(o-hydroxyphenyl)-
silane 2H3 as ligands to synthesize cage-shaped borates 1B
and 2B (Scheme 1a).3,4 The cage-shaped geometry created a
highly accessible vacant molecular orbital (MO) on boron in
1B and 2B and enhanced catalytic activity. We expected this
cage-shape concept to lend a greater advantage for larger
group 13 metal compounds. In contrast to boron complexes,
the open shape of ML3, having either a Ga or an In center, has
four, or more, coordinated spheres5 and easily accepts an
external ligand (Lex) to lower the Lewis acidity (Scheme 2a).
That is a disadvantage for activation of the substrate as a
Lewis acid. It was suggested that the cage-shaped framework
could overcome this problem by inhibiting the external ligand
(Lex) coordination (Scheme 2b). Herein, we report a new type
of cage-shaped gallium complex, 3aGa, with a back-shieiding
framework (Scheme 1b) and interesting properties.
modeling. We think that
a part of the complex was
decomposed by pyridine hydrochloride during recrystalliza-
tion because it took several months to obtain a crystal which
was suitable for X-ray analysis. The structure shown in Fig. 1
supports the generation of 3bGaꢀPy. The top view shows that
the Ga atom lies above the center of the benzene ring (Fig. 1b).
The average lengths of the Ga–O bond, 3.06 A, and of the
Ga–N1 bond, 3.28 A, were considerably longer than usual
(1.94 A and 2.02 A, respectively),9 because of a deficiency of
ca. half the gallium atoms in the crystal.
Theoretical calculations were performed to investigate the
properties of the cage-shaped gallium complex, 3bGa, in
comparison with the open-shaped one, Ga(OPh)3 7, using
the hybrid density functional theory B3PW91/6-31+G(d,p)
method with the Gaussian 03 program.10 The stabilization
energies in pyridine complexation reveal that the back-shielding
effect of the cage-shaped structure should be effective for
keeping Lewis acidity high (Scheme 4). The open-shaped
gallium complex 7ꢀPy coordinated by pyridine has a stabiliza-
tion energy of 39.2 kcal molꢁ1 (path a), and coordination
of the second pyridine leads to additional stabilization of
14.2 kcal molꢁ1 (path b), as shown in Scheme 4a. These results
show that the second ligand coordination-dissociation step
(path b) mainly contributes to Lewis acidity. However, the
cage-shaped gallium complex, 3bGa, disturbs the second
coordination to get one stabilization energy of 39.3 kcal molꢁ1
that contributes to Lewis acid-mediated reactions (path c), as
shown in Scheme 4b. This is the reason the cage-shaped
complex has high potential as a Lewis acid.
First, we attempted to synthesize cage-shaped gallium
complexes with the previously reported ligands 1H3 and 2H3,
but no desired complexes were obtained due to their narrow
space. Therefore, we designed new ligands 3H3, as shown in
Scheme 3, in which three oxybenzyl moieties were linked to a
benzene ring.6 The Cu(I)-catalyzed coupling between the
Grignard reagent 4a and 1,3,5-tris(bromomethyl)benzene 5a
gave the compound 6a bearing three m-methoxybenzyl moieties.
Treatment of 6a with BBr3 afforded the compound 3aH3. Three
more types of 3b–dH3 derivatives bearing substituents on
benzene rings were similarly synthesized. The reaction of 3aH3
with GaCl3 in the presence of pyridine gave the cage-shaped
The optimized structures and the MO diagrams of 3bGa, 7
and 7ꢀPy are shown in Scheme 5.11 The complex 3bGa has a
a Department of Applied Chemistry and Center for Atomic and
Molecular Technologies, Graduate School of Engineering, Osaka
University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
E-mail: yasuda@chem.eng.osaka-u.ac.jp,
baba@chem.eng.osaka-u.ac.jp;
Fax: +81-06-6879-7387; Tel: +81-06-6879-7386
b Science and Technology System Division, Ryoka Systems Inc.,
1-28-38 Shinkawa, Chuo-ku, Tokyo 104-0033, Japan
w Electronic supplementary information (ESI) available: Experimental
details and spectral data. CCDC 764736. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0cc00253d
Scheme 1 Cage-shaped metal complexes and their ligands.
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Scheme 2 Concept of the back-shielding effect.
Scheme 5 Comparison of open- and cage-shaped gallium complexes
(3bGa,
7
and 7ꢀPy) by first principles calculations (B3PW91/
6-31+G(d,p)).
We examined the catalytic activity of the gallium complexes
during the hetero Diels–Alder reaction of Danishefsky’s diene
8 with benzaldehyde 9 to give pyran 10 (eqn (1)).13,14 The
open-shaped gallium complex 7ꢀnPy (n = 1 or 2) afforded 10
in only 34% yield. In contrast, the cage-shaped gallium
complex 3bGaꢀPy gave 10 in a higher yield of 61%. The
cage-shaped structure apparently enhanced catalytic activity
of the complex.
Scheme 3 Preparation of ligands 3H3 and their gallium complexes
3GaꢀPy.
ð1Þ
Next, we examined the effect of the substituents on the cage-
shaped gallium complexes 3a–dGa by theoretical calculation
(Scheme 6 and 7). The substituents, R, on the bottom benzene
ring changed the geometry around the gallium as shown in
Scheme 6. In the complex with bulky substituents, both the
benzene rings of the arm and the bottom benzene ring
approach a perpendicular state because of the steric hindrance
between the substituents (dihedral angle (C1–C2–C3–C4):
3aGa, 68.61; 3bGa, 78.01; 3cGa, 85.71). The large dihedral
angle of 3cGa creates a more concave geometry around
gallium than those of 3aGa and 3bGa (sum of +OGaO: 3aGa,
353.21; 3bGa, 352.01; 3cGa, 351.51). These geometric changes
produce differences in the energy levels of the LUMOs and
LUMOs+n12 of 3a–cGa, as shown in Scheme 7.15 These
orbitals are the unoccupied orbitals to which gallium pz
orbitals contribute. LUMOs+n have upward lobes on
gallium, which contribute to Lewis acidity, while LUMOs
have downward lobes that are not suitable for accepting a
nucleophile. The order of energy levels for LUMOs+n,
3aGa o 3bGa o 3cGa, is reverse to that of LUMOs,
3aGa > 3bGa > 3cGa. Although the reason for this relation-
ship is not yet clear, the bulkiness of the substituents on the
bottom benzene ring would finely tune the eigenvalue of
the orbital to which the gallium pz orbitals contribute. The
pyridine-complexation energy also showed the same order for
the eigenvalue of LUMO+n. On the other hand, the electron
withdrawing character of the fluoro groups on the benzene
rings of the arm led to larger pyridine-complexation energy
and lower energy levels for LUMO and LUMO+n than for
3bGa. These results reveal that the steric substituents on the
Fig. 1 ORTEP drawing of 3bGaꢀPy with disorder modelled in
(thermal ellipsoids are at 50% probability level. Hydrogens are
omitted for clarity. The occupancy factor of the gallium atom is 0.44):
(a) side view and (b) top view (pyridine is omitted for clarity).
Scheme 4 Stabilization energy of 7 and 3bGa in a pyridine complexation
(Gaussian03 D.01, B3PW91/6-31+G(d,p), gas phase).
small concave geometry around gallium while 7 has a planar
geometry (sum of +OGaO: 3bGa, 352.01; 7, 360.01). The
diagram of the unoccupied orbital of 3bGa, which contributes
to Lewis acidity (corresponding to LUMO+n),12 shows a
large and accessible lobe on gallium, while the corresponding
lobes in 7 and 7ꢀPy are small and buried. These results indicate
that the MO of 3bGa is more suited to accepting a reagent
than that of either 7 or 7ꢀPy.
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Table 1 Hetero Diels–Alder reaction using cage-shaped derivatives
Entry
Catalyst
Yield/%
1
2
3
4
3aGaꢀPy
3bGaꢀPy
3cGaꢀPy
3dGaꢀPy
42
61
57
46
a
All reaction were performed under the same conditions with eqn (1).
Scheme 6 Structural change caused by the substituents on the
bottom benzene ring.
for their valuable advice regarding X-ray crystallography.
H. N. thanks the Yoshida Scholarship Foundation.
bottom benzene ring and the strongly electronegative
substituents on the benzene arm affect the Lewis acidity of
the gallium complexes.
Notes and references
1 Lewis Acids in Organic Synthesis, ed. H. Yamamoto, Wiley-VCH,
Weinheim, Germany, 2000, vol. 1 and 2.
The results of the hetero Diels–Alder reaction using the
cage-shaped derivatives 3b–dGaꢀPy are summarized in
Table 1. The unsubstituted 3aGaꢀPy gave a lower yield than
the methyl-substituted 3bGaꢀPy despite having almost the
same pyridine complexation energy. This result suggests that
methyl groups on the bottom benzene ring disturbed the
inversion of the benzene rings on the arm to keep them on
the same side as the gallium metal even when the Ga–O bond
was cleaved, consequently preventing the decomposition of the
complex. The bulkier ethyl group provided no improvement,
likely due to the high energy level of its LUMO+n. The fluoro
derivative 3dGaꢀPy showed lower catalytic activity than
3bGaꢀPy. The larger stabilization energy in the pyridine
complexation of 3dGa probably inhibits the release of an
original pyridine and a product from the gallium metal at
the start and at the end of the reaction. The generation and
regeneration of an active catalyst are important in this case,
and 3bGaꢀPy was the best catalyst among 3a–dGaꢀPy.
In summary, we synthesized gallium complexes with Lewis
acidity enhanced by a cage-shaped structure. Theoretical
calculations and application to a hetero Diels–Alder reaction
suggest that the back-shielding framework shows promise for
the high activation of carbonyl compounds. The substituents
on the bottom benzene ring finely tuned the energy level and
controlled the stability of the complexes.
2 Acid Catalysis in Modern Organic Synthesis, ed. H. Yamamoto,
and K. Ishihara, Wiley-VCH, Weinheim, Germany, 2006, vol. 1,
pp. 187–467.
3 Similar Lewis base cage-shaped complexes have been reported.
(a) M. B. Dinger and M. J. Scott, Inorg. Chem., 2001, 40, 856–864.
A
titanium complex with a similar cage-shaped ligand was
reported. (b) F. Akagi, T. Matsuo and H. Kawaguchi, J. Am.
Chem. Soc., 2005, 127, 11936–11937.
4 (a) M. Yasuda, S. Yoshioka, S. Yamasaki, T. Somyo, K. Chiba
and A. Baba, Org. Lett., 2006, 8, 761–764; (b) M. Yasuda,
S. Yoshioka, H. Nakajima, K. Chiba and A. Baba, Org. Lett.,
2008, 10, 929–932.
5 Five coordinate monomeric gallium(III) complexes: (a) S. Zanias,
C. P. Raptopoulou, A. Terzis and T. F. Zafiropoulos, Inorg. Chem.
Commun., 1999, 2, 48–51 and references cited therein;
(b) A. Crispini, I. Aiello, M. L. Deda, I. D. Franco, M. Amati,
F. Lelj and M. Ghedini, Dalton Trans., 2006, 5124–5134;
(c) M. Rajeswaran, D. W. Place, J. C. Deaton, C. T. Brown and
W. C. Lenhart, Acta Crystallogr., Sect. E: Struct. Rep. Online,
2007, 63, m550–m552; (d) J. C. Deaton, D. W. Place, C. T. Brown,
M. Rajeswaran and M. E. Kondakova, Inorg. Chim. Acta, 2008,
361, 1020–1035.
6 The ligands 3bH3 and 3cH3 were synthesized but were not applied
to metal complexes. J. Kim, Y. K. Kim, N. Park, J. H. Hahn and
K. H. Ahn, J. Org. Chem., 2005, 70, 7087–7092.
7 We were not able to remove pyridine hydrochloride from the
gallium complexes (see ESI for further details).
8 The X-ray crystallographic data of 3bGaꢀPy is given in the ESI.
9 The average value of 1.94 A and 2.02 A is based on a statistical
analysis of all of the Ga–O and Ga–N covalent bonds described for
compounds in the Cambridge Structural Database, respectively.
10 Gaussian 03, Revision D.01; Gaussian, Inc.: Wallingford CT,
2004. See ESI for the full list of authors.
11 In 3bGa, there is no interaction between Ga and the bottom
benzene (the Ga–C distances are 4.9 A). This length is much longer
than that of a usual gallium–arene interaction. Review of arene
complexes of gallium; M. Gorlov and L. Kloo, Coord. Chem. Rev.,
2008, 252, 1564–1576.
This work was supported by Grants-in-Aid for Scientific
Research on Priority Areas [No. 18065015 (‘‘Chemistry of
Concerto Catalysis’’) and No. 20036036 (‘‘Synergistic Effects
for Creation of Functional Molecules’’)] and for Scientific
Research (No. 21350074) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. We thank
Dr Nobuko Kanehisa and Dr Masato Ohashi (Osaka University)
12 The LUMOs+n are most important for Lewis acid because the
LUMOs+n have upward lobes and are suitable for accepting a
reagent: 3aGa, LUMO+3; 3bGa, 3cGa and 3dGa, LUMO+5; 7,
LUMO+10; 7ꢀPy, LUMO+7.
13 (a) S. Danishefsky, J. F. Kerwin and S. Kobayashi, J. Am. Chem.
Soc., 1982, 104, 358–360; (b) K. Hattori and H. Yamamoto,
Synlett, 1993, 129–130.
14 Although the catalyst contains pyridine hydrochloride (ref. 7),
pyridine hydrochloride did not act as a catalyst; hetero Diels–Alder
reaction of 8 with 9 resulted in no reaction (see ESI).
15 (a) S. E. Denmark, B. D. Griedel, D. M. Coe and M. E. Schnute,
J. Am. Chem. Soc., 1994, 116, 7026–7043; (b) S. G. Nelson,
B.-K. Kim and T. J. Peelen, J. Am. Chem. Soc., 2000, 122,
9318–9319; (c) J. Kobayashi, K. Kawaguchi and T. Kawashima,
J. Am. Chem. Soc., 2004, 126, 16318–16319; (d) C. E. Wagner,
J.-S. Kim and K. J. Shea, J. Am. Chem. Soc., 2003, 125,
12179–12195.
Scheme 7 Theoretical calculation of 3a–dGa.
4796 | Chem. Commun., 2010, 46, 4794–4796
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