upfield shift of the methine proton [Py‚B(OC6H4)3CH]
relative to that of 4H3 was confirmed (6.07 f 5.19 ppm).
This shift is observed in similar cage-shaped compounds.10
The pyridine signals which appear in lower field relative to
those of free pyridine indicate strong coordination of the
pyridine to the boron in 5.11 The ORTEP drawings of 5 are
shown in Figure 1.12 Boron has a tetrahedral coordination
Scheme 1. Triphenolic Methane and Cage-Shaped Borate
which are less sterically hindered and reduces π-electron
overlap between boron and its ligand oxygens. Here, we
report cage-shaped borates which show high catalytic activity
as compared with normal open-shaped ones. Furthermore,
the ab initio calculations revealed highly accessible vacant
MO and its lowered energy level caused by the cage-shaped
framework.
Figure 1. ORTEP drawing of 5 (aryl hydrogens are omitted for
clarity): (a) side view and (b) top view (pyridine is omitted for
clarity). Selected bond lengths (Å): B-O(1) 1.457(5), B-O(2)
1.432(4), B-O(3) 1.441(4),B-N(1) 1.630(4). Selected bond angles
(deg): O(1)-B-O(2) 113.3(3), O(2)-B-O(3) 115.7(3), O(3)-
B-O(1) 113.7(3), O(1)-B-N 104.3(3), O(2)-B-N 105.8(3),
O(3)-B-N 102.2(3).
Based on the concept of cage-shaped Lewis acid, we
intended to prepare a borate 4B as a structurally strained
Lewis acid, that could be derived from borane and tris(2-
hydoxyphenyl)methane 4H3 (Scheme 1).4 It has been reported
that organic components including preorganized phenoxy
moieties nicely act as ligand systems for metal complexes.5,6
Although some substituted derivatives of 4H3 are known,7
its unsubstituted version has not been reported. We were able
to synthesize 4H3 in three steps (Scheme 1). Ortho-lithiation
of anisole followed by treatment with ethyl chloroformate
gave triarylcarbinol 2. The treatment of 2 with p-toluene-
sulfonic acid in THF/MeCN directly gave the reduced
compound 3. The in situ-generated carbenium cation stabi-
lized by electron-donating groups8 can be reduced by THF
probably in either an ionic or SET mechanism.9 The desired
compound 4H3 was obtained after treatment of 3 with BBr3.
The reaction of 4H3 with BH3‚THF generated the cage-
shaped borate 4B‚THF. It was easily decomposed in air and
thus confirmed by NMR under nitrogen. The ligand-free 4B
was not observed under the conditions. The pyridine complex
5 () 4B‚Py) was formed by treatment with pyridine and
well analyzed by X-ray crystallography. NMR of 5 shows
reasonable spectra for the closed cage-shape: The significant
sphere with the average of bond angles (O-B-O, 114.2-
(9)° and N-B-O, 104.1(9)°). This is the first example of a
triphenolic methane-based mononuclear complex which can
be a Lewis acid.13 The top view (pyridine is omitted) clearly
shows C3 symmetry. Interestingly, the aromatic rings deviate
from a perpendicular plane to that of three oxygens (ca.
19.2°), and thus, the complex 5 has chirality that is caused
by the cage-shape.14 A similar borate structure whose
phenolic rings are connected to nitrogen was reported but
its coordination to boron causes almost perpendicular
aromatic rings.15
Ab initio calculations were performed to investigate the
ligand-free borate 4B comparing with the open-shaped
B(OPh)3 6 (Table 1). We had expected that 4B had distorted
geometry around boron from the planar structure which is
typical for open-shaped borate like 6. However, both species
contain planar geometry around boron. The notable differ-
ence is the dihedral angles C-O-B-O (2.0° in 6, 48.4° in
4B). The diagram of the lowest unoccupied MO16 of 4B to
which boron pz orbital contributes (corresponding to next-
(4) A triphenolic methane-based complex with aluminum does not give
a mononuclear cage-shaped complex but a multinuclear one with the
methane hydrogen inside. Cottone, A., III; Morales, D.; Lecuivre, J. L.;
Scott, M. J. Organometallics 2002, 21, 418-428.
(10) Dinger, M. B.; Scott, M. J. Inorg. Chem. 2001, 40, 856-864.
(11) See the Supporting Information.
(12) The X-ray crystallographic data of 5 is given in the Supporting
Information.
(13) The Lewis basic similar cage-shaped complex has been reported
using P or As (ref 10). During the reviewing process of this manuscript, a
titanium complex with a similar cage-shape ligand was reported. Akagi,
F.; Matsuo, T.; Kawaguchi, H. J. Am. Chem. Soc. 2005, 127, 11936-11937.
(14) Linear triphenolic compounds shows chirality when complexed with
Al. Appiah, W. O.; DeGreeff, A. D.; Razidlo, G. L.; Spessard, S. J.; Pink,
M. Young, Jr. V. G.; Hofmeister, G. E. Inorg. Chem. 2002, 41, 3656-
3667.
(5) Recent review; Matsuo T.; Kawaguchi, H. Chem. Lett. 2004, 33, 640-
645.
(6) (a) Verkerk, U.; Fujita, M.; Dzwiniel, T. L.; McDonald, R.; Stryker,
J. M. J. Am. Chem. Soc. 2002, 124, 9988-9989. (b) Fujita, M.; Qi, G.;
Verkerk, U. H.; Dzwiniel, T. L.; McDonald, R.; Stryker, J. M. Org. Lett.
2004, 6, 2653-2656.
(7) Dinger, M. B.; Scott, M. J. Eur. J. Org. Chem. 2000, 2467-2478.
(8) Wada, M.; Kirishima, K.; Oki, Y.; Miyamoto, M.; Asahara, M.; Erabi,
T. Bull. Chem. Soc. Jpn. 1999, 72, 779-785.
(9) Wada, M.; Mishima, H.; Watanabe, T.; Natsume, S.; Konishi, H.;
Hayase, S.; Erabi, T. J. Chem. Soc., Chem. Commun. 1993, 1462-1463.
(15) Livant, P. D.; Northcott, J. D.; Shen, Y.; Webb, T. R. J. Org. Chem.
2004, 69, 6564-6571.
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Org. Lett., Vol. 8, No. 4, 2006