Synthesis of binucleating ligands of pyridylphenol

Article abstract of DOI:10.1081/SCC-100103995

The synthesis of pyridylphenol-type dinucleating tri- and penta-dentate ligands has been accomplished by cross-couplings.

Full text of DOI:10.1081/SCC-100103995

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SYNTHESIS OF BINUCLEATING
LIGANDS OF PYRIDYLPHENOL
Huichang Zhang a , Man Kin Tse a & Kin Shing Chan
b
a Department of Chemistry , The Chinese
University of Hong Kong , Shatin, N.T., Hong Kong
b Department of Chemistry , The Chinese
University of Hong Kong , Shatin, N.T., Hong Kong
Published online: 09 Nov 2006.
To cite this article: Huichang Zhang , Man Kin Tse & Kin Shing Chan (2001)
SYNTHESIS OF BINUCLEATING LIGANDS OF PYRIDYLPHENOL, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 31:8, 1129-1139, DOI: 10.1081/SCC-100103995
To link to this article: http://dx.doi.org/10.1081/SCC-100103995
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SYNTHETIC COMMUNICATIONS, 31(8), 1129–1139 (2001)
SYNTHESIS OF BINUCLEATING
LIGANDS OF PYRIDYLPHENOL
Huichang Zhang, Man Kin Tse, andKin Shing Chan *
Department of Chemistry, The Chinese University of
Hong Kong, Shatin, N.T. Hong Kong
ABSTRACT
The synthesis of pyridylphenol-type dinucleating tri-
and penta-dentate ligands has been accomplished by cross-
couplings.
Dinucleating ligands and bimetallic complexes play important roles in
the studies of bioinorganic chemistry and homogeneous catalysis, and have
continued to arouse wide interest.1–3 Dinuleating ligands capable of binding
two metal ions in close proximity serve as essential precursors for the for-
mation of homo- and hetero- bimetallic complexes. The design of a dinu-
cleating system determines the nature of metal ions to be incorporated,
coordination environment, and metal–metal separation of the bimetallic
complexes. Phenoxy-bridged Schiff-base ligands have been used as binu-
cleating ligands, and some of the bimetallic complexes of these ligands
show catalytic activities for oxidation of organic substrates.1–3 To enhance
the oxidative and hydrolytic robustness of the imine functional groups of the
Schiff base type complexes, we have attempted to aromatize the imine as
part of a pyridine ring4–6 and we report the synthesis of pyridylphenol type
dinucleating tri- and penta-dentate ligands.
* Corresponding author.
1129
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

1130
ZHANG, TSE, AND CHAN
Scheme 1. Syntheses of dipyridylphenols 5 and 6.
4-Substituted 2,6-dibromophenols7 1a–c and 1a–d underwent smooth
Stille cross-couplings8 with tributylstannylpyridine (2) and 2-(tributylstan-
nyl)-6-methylpyridine (3), respectively, catalyzed by Pd(PPh3)4 in THF
under N2 at 90ꢀ–100ꢀC for two days to yield 4-substituted tridentate dipyr-
idylphenols 4a–c and 5a–d (Scheme 1). The yield of 4b was very
poor, possibly due to low solubility of both the starting material and final
product. The yields of 5a–d were higher than that of 4a–c, likely due to the
enhancement of nucleophilicity in 3 and the solubility of the resultant
ligands.
A more lipophilic, pentadentate ligand 11 was also prepared, which
resembles the structural feature of a dimanganese Schiff base complex.9 3,5-
Di-tert-butyl-2-[20-(60-bromo)pyridyl]anisole 8 was synthesized by modified
Suzuki coupling of 2,6-dibromopyridine 7 and arylboronic acid 6, catalyzed
t
by Pd(PPh3)4 with BuOK in 68% yield (Scheme 2).10 Lithiation of 9 with
nBuLi,11 followed by addition of excess DMF, gave the aldehyde 10 in 82%
yield. 10 reacted with lithiated 8 to yield 10 in 74% yield. It should be noted
that 10 could not be prepared directly from the reaction of lithiated 8 and
DMF. 10 was then deprotected with 48% HBr in refluxing acetic acid for
12 h to give pentadentate ligand 11 in 65% yield.
In preliminary studies, ligands 4 and 5 complexed with the first row
transition metal ions in both 1:1 and 1:2 ratio.
In conclusion, bidentate and pentadentate ligands of pyridylphenol
were synthesized by Stille and Suzuki cross-coupling reaction in moderate
yield and coordination chemistry is being studied.

PYRIDYLPHENOL LIGANDS
1131
Scheme 2. Synthetic route of pentadentate ligand 11.
EXPERIMENTAL
General Procedures
All materials were obtained from commercial suppliers and used
without further purification unless otherwise specified. Tetrahydrofuran
(THF) was freshly distilled from sodium benzophenone ketyl under N2.
Chromatography was carried out on silica gel.
1H and 13C NMR spectra were recorded in CDCl3 on a Bruker DPX-
300 spectrometer at 300 and 75.47 MHz, respectively. Proton chemical shifts
were referenced with the residual solvent protons in CDCl3 (ꢀ 7.24 ppm) or
with tetramethylsilane (ꢀ 0.00 ppm) as the internal standard. Carbon chemi-
cal shifts were referenced to the residual solvent peak of CDCl3 (ꢀ 77.0 ppm).
Mass spectra were recorded on either a VG7070F mass spectrometer,
Hewlett Packard 5989B mass spectrometer, or a Bruker APEX 47e FT-ICR
mass spectrometer. Fast atom bombardment spectra were obtained using
3-nitrobenzyl alcohol (NBA) as the matrix. Electrospray ionization spectra
were obtained with a solvent mixture of acetone with 3% of acetic acid. IR
spectra were obtained on either a Nicolet Magne-IR 550 FT-IR spectro-
photometer, Perkin Elmer Paragon 500 FT-IR spectrophotometer, or a
Perkin Elmer 1600 FT-IR spectrophotometer. Samples were prepared

1132
ZHANG, TSE, AND CHAN
either as neat film on KBr plates or as a KBr disk. UV-vis spectra were
recorded on a Hitachi U-3300 spectrophotometer.
Preparation of Methyl 3,5-Dibromo-4-hydroxybenzoate (1a)12
To the solution of methyl 4-hydroxybenzoate (8.0 g, 53 mmol) in
CHCl3 (35 mL) at 0ꢀC, Br2 (6.4 mL, 125 mmol) in CHCl3 (20 mL) was
slowly added for 2 h 15 min. The reaction mixture was then raised to
room temperature and stirred for 1 day. Then sat.NaS2O5 was added.
The crude product was extracted with CH2Cl2, washed with water, brine,
dried (MgSO4), filtered, and rotary-evaporated to dryness. A white solid
was obtained after recrystallization from CH2Cl2/hexanes. Rf ¼ 0.35
(hexanes:E.A. ¼ 3:1); 1H NMR ꢀ 3.91 (s, 3 H), 6.29 (br s, 1 H), 8.16 (s, 2 H).
Preparation of 2,6-Dibromo-4-nitrophenol (1b)13,14
Br2 (8.7 mL, 169 mmol) in glassical acetic acid (27 mL) was added
dropwise to 4-nitrophenol (10.0 g, 72 mmol) in glassical acetic acid
(30 mL) for 1 h 15 m. The reaction mixture was then stirred at room tem-
perature for 9 h. Then the reaction mixture was heated to 80ꢀC for 1 h to
remove HBr and excess Br2. Cold water (33 mL) was then added and the
reaction mixture was stirred in an ice bath for 6 h. The resulting pale yellow
solid was then filtered and washed with 50% acetic acid and then water.
A slightly pale yellow solid (20.6 g, 69 mmol, 96%) was obtained after
being dried under high vacuum. Rf ¼ 0.14 (hexanes/E.A. ¼ 3:1); m.p.
d 140ꢀC (Lit.13 d ¼ 138ꢀ–140ꢀC); H NMR ꢀ 6.54 (br s, 1 H), 8.41 (s, 2 H).
1
Preparation of 2-Tributylstannylpyridine (2)
nBuLi (1.6 M in hexanes, 33 mL, 53 mmol) was added dropwise to a
solution of 2-bromopyridine (5.0 mL, 52 mmol) in THF (20 mL) at À78ꢀC
under N2. After addition of about 20 mL nBuLi, a brown precipitate
appeared, together with a brown solution. After the addition of nBuLi,
the solution was stirred for 2 h at À78ꢀC under N2. Then nBu3SnCl
(15 mL, 55 mmol) was added by syringe for 30 m. It was then stirred at
À78ꢀC for 1 h and stood at room temperature for 12 h. A brown solution
resulted with some white precipitate. The reaction mixture was then filtered
through celite and the brown solution was rotary-evaporated to dryness
and a viscous brown liquid obtained. It was then vacuum-distillated

PYRIDYLPHENOL LIGANDS
1133
and a colorless liquid (14.3 g, 39 mmol, 75%) was obtained. Rf ¼ 0.74
(hexanes:E.A. ¼ 6:1); b.p. 120ꢀ–122ꢀC (0.04 mm Hg); 1H NMR ꢀ 0.88
(t, 9 H, J ¼ 7.3 Hz), 1.10–1.15 (m, 6 H) 1.32–1.39 (m, 6 H), 1.51–1.62
(m, 6 H), 7.08–7.13 (m, 1 H), 7.38–7.44 (m, 1 H), 7.45–7.53 (m, 1 H), 8.73
(ddd, 1 H, J ¼ 1.0, 1.6, 4.8 Hz); IR (KBr, cmÀ1) n 664, 690, 1376, 1448, 1567,
2927, 3392.
Preparation of 2-(Tributylstannyl)-6-methylpyridine (3)
nBuLi (1.6 M, 29 mL, 47 mmol) was added dropwise to a solution of
2-bromo-6-methylpyridine (3) (8.0 g, 47 mmol) in THF (20 mL) at À78ꢀC
under N2. Then, the solution was stirred for 2 h at À78ꢀC under N2 and
nBu3SnCl (13.2 mL, 49 mmol) was added by syringe in 30 m. The mixture
was then stirred at À78ꢀC for 1 h and stood at room temperature for 12 h.
A yellow solution with some white precipitate was resulted. The reaction
mixture was then filtered through celite and the yellow solution was
rotary-evaporated to dryness to yield a viscous yellow liquid. The residue
was vacuum-distillated to give a colorless liquid (13.4 g mL, 35 mmol, 74%).
Rf ¼ 0.70 (hexanes/E.A. ¼ 6:1); b.p. 126ꢀ–128ꢀC (0.04 mm Hg); H NMR
1
ꢀ 0.88 (t, 9 H, J ¼ 7.3 Hz), 1.07–1.13 (m, 6 H) 1.27–1.39 (m, 6 H), 1.51–
1.62 (m, 6 H), 2.53 (s, 3 H), 6.94 (dd, 1 H, J ¼ 0.7, 7.8 Hz), 7.18 (dd, 1 H,
J ¼ 0.5, 7.3), 7.33–7.38 (unresolved dd, 1 H); 13C NMR ꢀ 9.81, 13.67, 24.86,
27.31, 29.07, 121.45, 133.23, 158.55, 172.99; IR (KBr, cmÀ1) n 665, 691, 772,
1437, 1557, 2925, 3436; FABMS: m/z (relative intensity) 384 {[(MþH)þ]þ4,
100}, 382 {[(MþH)þ]þ2, 75}, 380 [(MþH)þ, 42]; HRMS (ESIMS): Calc. for
(C18H35NSn)þ: m/z 385.1791. Found: m/z 385.1789.
Preparation of 4-Substituted-2,6-di-(20-pyridyl)phenol
General Procedure. The preparation of methyl 4-hydroxy-3,5-di-(20-
pyridyl)benzoate (5a) was described as a typical example for the preparation
of 4-substituted-2,6-di-(20-pyridyl)phenol.
Methyl 3,5-dibromo-4-hydroxybenzoate (1a) (500 mg, 0.65 mmol) and
Pd(PPh3)4 (386 mg, 0.32 mmol) were charged into a side-armed, rotaflow,
screw-stoppered flask. 2-Tributylstannylpyridine (2) (1.31 g, 3.35 mmol) in
THF was then added. It was then degassed by freeze-pump-thaw method
(3 cycles) and refilled with N2. The reaction mixture was then heated to
90ꢀ–100ꢀC. After 16 h, 2-tributylstannylpyridine (1.31 g, 3.55 mmol) was
added again and the reaction mixture was heated to 90ꢀ–100ꢀC for 1 d.
After removal of solvent by rotary evaporation, the product was isolated

1134
ZHANG, TSE, AND CHAN
by column chromatography on silica gel using a solvent mixture of hexanes/
E.A. (1:1) as the eluent. A yellow crystalline solid (101 mg, 0.33 mmol, 20%)
was obtained after recrystallization from CH2Cl2/hexanes. Rf ¼ 0.29
(hexanes:E.A. ¼ 1:1); m.p. 178ꢀ–179ꢀC; 1H NMR ꢀ 3.94 (s, 3 H),
7.30–7.35 (m, 2 H) 7.85–7.91 (m, 2 H), 8.12 (d, 2 H, J ¼ 8.2 Hz), 8.61
(s, 2 H), 8.65 (d, 2 H, J ¼ 4.4 Hz); 13C NMR ꢀ 51.98, 120.42, 122.22,
122.44, 131.61, 137.45, 146.95, 155.89, 162.53, 166.78, 179.47; UV-visible
(CH2Cl2) ꢁmax, nm (log e) 334 (4.40); IR (KBr, cmÀ1) n 747, 802, 1247, 1456,
1599, 1714, 3418; FABMS: m/z 307 [(MþH)þ]; HRMS (ESIMS): Calc. for
(C18H15N2O3)þ: m/z 307.1077. Found: m/z 307.1063.
Preparation of 4-Nitro-2,6-di-(20-pyridyl)phenol (5b)
2,6-Dibromo-4-nitrophenol (1b) (1.0 g, 3.4 mmol) was reacted with
2-(tributylstannyl)pyridine (2) (2.7 g, 7.4 mmol) and Pd(PPh3)4 (806 mg,
0.68 mmol) in THF under N2 at 90ꢀ–100ꢀC for 2 days. It was purified by
column chromatography (70–230 mesh) using hexanes/E.A. (3:1) as the
eluent. A yellow solid (71 mg, 0.24 mmol, 7%) was obtained after recrystal-
lization from CH2Cl2/hexanes. Rf ¼ 0.12 (hexanes/E.A. ¼ 3:1); m.p.
205ꢀ–206ꢀC; H NMR ꢀ 7.26–7.39 (m, 2 H) 7.89–7.94 (m, 2 H), 8.13–8.16
1
(m, 2 H), 8.66 (d, 2 H, J ¼ 4.9 Hz), 8.86 (s, 2 H); 13C NMR ꢀ 122.44,
122.84, 125.37, 137.48, 139.64, 147.38, 154.88, 164.19 (due to low
solubility of 53b, only 8 peaks was found out of the expected 9); UV-visible
(CH2Cl2) ꢁmax, nm (log e) 338 (4.67); IR (KBr, cmÀ1) n 739, 802, 1339, 1599,
3436; FABMS: m/z 294 [(MþH)þ]; HRMS (ESIMS): Calc. for
(C16H12N3O3)þ: m/z 294.0873. Found: m/z 294.0888.
Preparation of 4-Chloro-2,6-di-(20-pyridyl)phenol (5c)
2,6-Dibromo-4-chlorophenol (1c) (1.0 g, 3.49 mmol) was reacted with
2-(tributylstannyl)pyridine (2) (2.83 g, 7.68 mmol) and Pd(PPh3)4 (835 mg,
0.70 mmol) in THF under N2 at 90ꢀ–100ꢀC for 2 days. The reaction mixture
was purified by column chromatography using hexanes/E.A. (4:1) as the
eluent. A yellow solid (253 mg, 0.89 mmol, 26%) was obtained after
recrystallization from CH2Cl2/hexanes. Rf ¼ 0.22 (hexanes/E.A. ¼ 3:1);
m.p. 127ꢀ–128ꢀC; H NMR ꢀ 7.23–7.28 (m, 2 H), 7.77–7.82 (m, 2 H) 7.84
1
(s, 2 H), 8.00 (d, 2 H, J ¼ 8.2 Hz), 8.59 (d, 2 H, J ¼ 4.1 Hz); UV-visible
(CH2Cl2) ꢁmax, nm (log e) 347 (4.41); IR (KBr, cmÀ1) n 735, 794, 1264,
1456, 1598, 3418; HRMS (ESIMS): Calc. for (C16H12N2OCl)þ: m/z
283.0633. Found: m/z 283.0625.

PYRIDYLPHENOL LIGANDS
1135
Preparation of Methyl 4-Hydroxy-3,5-di-[20-(60-methyl)pyridyl]-
benzoate (5a)
Methyl 3,5-dibromo-4-hydroxybenzoate (1a) (200 mg, 0.65 mmol) was
reacted with 2-(tributylstannyl)-6-methylpyridine (3) (543 mg, 1.42 mmol)
and Pd(PPh3)4 (154 mg, 0.13 mmol) in THF under N2 at 90ꢀ–100ꢀC
for 33 h. It was purified by column chromatography on silica gel
(70–230 mesh) using hexanes/E.A. (3:1) as the eluent. A yellow solid
(71 mg, 0.21 mmol, 33%) was obtained after rotary evaporation. Rf ¼ 0.21
(hexanes/E.A. ¼ 3:1); m.p. 142ꢀ–144ꢀC; H NMR ꢀ 2.62 (s, 6 H), 7.13 (d, 2
1
H, J ¼ 7.5 Hz), 7.69–7.75 (unresolved dd, 2 H), 7.85 (d, 2 H, J ¼ 8.0 Hz), 8.53
(s, 2 H); 13C NMR ꢀ 24.01, 51.83, 119.18, 120.13, 121.65, 131.27, 137.26,
155.50, 156.23, 162.54, 166.86; UV-visible (CH2Cl2) ꢁmax, nm (log e) 334
(4.54); IR (KBr, cmÀ1) n 768, 1258, 1574, 1714, 2951, 3408; FABMS: m/z
335 [(MþH)þ]; HRMS (ESIMS): Calc. for (C20H19N2O3)þ: m/z 335.1390.
Found: m/z 335.1403.
Preparation of 4-Nitro-2,6-di-[20-(60-methyl)pyridyl]phenol (5b)
2,6-Dibromo-4-nitrophenol (1b) (200 mg, 0.67 mmol) was reacted with
2-(tributylstannyl)-6-methylpyridine (3) (566 mg, 1.48 mmol) and Pd(PPh3)4
(161 mg, 0.14 mmol) in THF under N2 at 90ꢀ–100ꢀC for 17 h. It was purified
by column chromatography on silica gel (70–230 mesh) using hexanes/E.A.
(3:1) as the eluent. A yellow solid (70 mg, 0.22 mmol, 32%) was obtained
after recrystallization from CH2Cl2/Hexanes. Rf ¼ 0.22 (hexanes/E.A. ¼
3:1); m.p. 204ꢀ–206ꢀC; H NMR ꢀ 2.64 (s, 6 H), 7.19 (d, 2 H, J ¼ 7.5 Hz),
1
7.74–7.79 (unresolved dd, 2 H), 7.88 (d, 2 H, J ¼ 7.9 Hz); 8.75 (s, 2 H); 13C
NMR ꢀ 23.93, 119.39, 122.53, 125.30, 137.84, 139.32, 154.15, 156.43, 164.68;
UV-visible (CH2Cl2) ꢁmax, nm (log e) 340 (4.43); IR (KBr, cmÀ1) n 808,
1328, 1471, 1574, 3501; FABMS: m/z 322 [(MþH)þ]; HRMS (ESIMS):
Calc. for (C18H16N3O3)þ: m/z 322.1186. Found: m/z 322.1160.
Preparation of 4-Chloro-2,6-di-[20-(60-methyl)pyridyl]phenol (5c)
2,6-Dibromo-4-chlorophenol (1c) (200 mg, 0.70 mmol) was reacted
with 2-(tributylstannyl)-6-methylpyridine (3) (587 mg, 1.54 mmol) and
Pd(PPh3)4 (167 mg, 0.14 mmol) in THF under N2 at 90ꢀ–100ꢀC for 24 h. It
was purified by column chromatography on silica gel (70–230 mesh) using
hexanes/E.A. (6:1) as the eluent. A yellow solid (88 mg, 0.28 mmol, 40%)
was obtained. Rf ¼ 0.23 (hexanes/E.A. ¼ 6:1); m.p. 114ꢀ–115ꢀC; H NMR
1

1136
ZHANG, TSE, AND CHAN
ꢀ 2.60 (s, 6 H), 7.11 (d, 2 H, J ¼ 7.5 Hz), 7.66–7.71 (unresolved dd, 2 H),
7.76–7.79 (m, 4 H); 13C NMR ꢀ 24.09, 119.14, 121.66, 123.48, 125.52,
129.10, 137.17, 155.90, 156.40, 156.80; UV-visible (CH2Cl2) ꢁmax, nm
(log e) 347 (4.54); IR (KBr, cmÀ1) n 725, 1456, 1574, 2925; FABMS: m/z
311 [(MþH)þ]; HRMS (ESIMS): Calc. for (C18H16N2OCl)þ: m/z 311.0946.
Found: m/z 311.0929.
Preparation of 4-tert-Butyl-2,6-di-[20-(60-methyl)pyridyl]phenol (5d)
2,6-Dibromo-4-tert-butylphenol (1d) (200 mg, 0.65 mmol) was reacted
with 2-(tributylstannyl)-6-methylpyridine (3) (546 mg, 1.43 mmol) and
Pd(PPh3)4 (155 mg, 0.13 mmol) in THF under N2 at 90ꢀ–100ꢀC for 2 days.
The reaction mixture was purified by column chromatography on silica gel
(70–230 mesh) using hexanes/E.A. (6:1) as the eluent. A yellow viscous
liquid (44 mg, 0.13 mmol, 20%) was obtained. Rf ¼ 0.39 (hexanes/
E.A. ¼ 6:1); 1H NMR ꢀ 1.40 (s, 9 H), 2.61 (s, 6 H), 7.08 (d, 2 H,
J ¼ 7.5 Hz) 7.66–7.71 (unresolved dd, 2 H), 7.78 (d, 2 H, J ¼ 8.0), 7.82 (s,
2 H); 13C NMR ꢀ 24.14, 31.55, 34.24, 119.15, 121.00, 123.63, 126.79, 136.98,
140.63, 155.73, 156.27, 156.93; UV-visible (CH2Cl2) ꢁmax, nm (log e) 342
(4.25); IR (KBr, cmÀ1) n 666, 735, 803, 1097, 1269, 1455, 1574, 2958, 3059;
FABMS: m/z 333 [(MþH)þ]; HRMS (ESIMS): Calc. for (C22H25N2O)þ:
m/z 333.1961. Found: m/z 333.1946.
Preparation of 3,5-di-tert-Butyl-2-[20-(60-bromo)pyridyl]anisole (8)
2,6-Dibromopyridine (7) (948 mg, 4.0 mmol) and Pd(PPh3)4 (231 mg,
0.20 mmol) were dissolved in DME (8 mL). The yellow suspension was
then degassed by freeze-pump-thaw method (3 cycles) and refilled with
N2. The reaction mixture was then heated at 50ꢀC for 15–20 m until a
clear yellow solution was obtained. Then 2,4-di-tert-butyl-6-methoxyphe-
t
nylboronic acid (1.06 g, 4.0 mmol) in DME (4 mL) and BuOK (898 mg,
8.0 mmol) in tBuOH (4 mL) were added. The reaction mixture was
degassed by freeze-pump-thaw method (3 cycles) and refilled with N2
and was then heated to 90ꢀC for 30 m. The reaction mixture turned
from a pale yellow solution with some white precipitate to an orange
solution with some white precipitate. The reaction mixture was then fil-
tered through celite. After rotary evaporation, the crude product was
chromatographed on silica gel (70–230 mesh) and using hexanes/CH2Cl2
(3:1) to hexanes/CH2Cl2 (3:2) as the gradient eluent. A white solid

PYRIDYLPHENOL LIGANDS
1137
(1.03 g, 2.7 mmol, 68%) was obtained after rotary evaporation. Rf ¼ 0.60
1
(hexanes/E.A. ¼ 6:1); H NMR ꢀ 1.14 (s, 9 H), 1.34 (s, 9 H), 3.63 (s, 3 H),
6.83 (s, 1 H), 7.20 (s, 1, H), 7.27 (d, 1 H, J ¼ 7.5 Hz), 7.41 (d, 1 H,
J ¼ 7.3 Hz), 7.51 (unresolved dd, 1 H); 13C NMR ꢀ 31.33, 32.46, 35.13,
36.88, 106.22, 116.50, 125.67, 126.04, 137.28, 140.40, 148.70, 151.97,
157.34, 160.23; IR (KBr, cmÀ1) n 665, 1070, 1237, 1432, 1545, 1604,
2954, 3036; FABMS: m/z (relative intensity) 378 {[(MþH)þþ2], 81}, 376
{[(MþH)þ, 100}; HRMS (ESIMS): Calc. for (C20H27BrNO)þ: m/z
376.1276. Found: m/z 376.1271.
Preparation of 6-(20,40-Di-tert-butyl-60-methoxyphenyl)pyridine-
2-carboxyaldehyde (9)
3,5-Di-tert-butyl-2-[20-(60-bromo)pyridyl]anisole
(8)
(500 mg,
1.33 mmol) and TMEDA (300 mL, 1.99 mmol) were dissolved in
n
THF (5 mL) under N2. 1.6 M BuLi (1.25 mL, 1.99 mmol( was then added
at À 78ꢀC by syringe under N2. The mixture turned from colorless to deep
yellow. After 1 h, DMF (1.0 mL, 13.3 mmol) was added. The solution deco-
lorized and some white precipitate appeared. After 1 h, water was added and
the crude product was extracted with ether (3 times). The organic layer was
then washed with water (3 times), brine, dried (MgSO4), filtered, and rotary-
evaporated to dryness. The crude product was then separated by column
chromatography on silica gel (230–400 mesh) using hexanes/E.A. (10:1) as
the eluent. A white solid (355 mg, 1.09 mmol, 82%) was obtained after
1
rotary evaporation. Rf ¼ 0.37 (hexanes/E.A. ¼ 9:1); H NMR ꢀ 1.13 (s, 9
H), 1.37 (s, 9 H), 3.64 (s, 3 H), 6.89 (d, 1 H, J ¼ 1.6 Hz), 7.26 (d, 1 H),
7.27 (d, 1 H, J ¼ 1.6 Hz), 7.54 (dd, 1 H, J ¼ 1.2, 7.3 Hz), 7.85 (unresolved dd,
1 H), 7.93 (dd, 1 H, J ¼ 1.2, 7.6 Hz), 10.11 (s, 1 H); 13C NMR ꢀ 31.35, 32.48,
35.19, 36.87, 55.76, 106.20, 116.64, 119.48, 131.68, 135.91, 148.80, 151.82,
152.11, 157.31, 160.44, 194.18; IR (KBr, cmÀ1) n 724, 1066, 1709, 2959,
3392; FABMS: m/z 326 [(MþH)þ]; HRMS (ESIMS): Calc. for
(C21H27NO2)þ: m/z 326.2120. Found: m/z 326.2112.
Preparation of bis-2-[6-(20,60-Di-tert-butyl-60-methoxyphenyl)pyridyl]-
methanol (10)
3,5-Di-tert-butyl-2-[20-(60-bromo)pyridyl]anisole
(58)
(350 mg,
0.93 mmol) and TMEDA (154 mL, 1.02 mmol) were dissolved in THF
(5 mL) under N2. nBuLi (1.6 M, 0.64 mL, 1.02 mmol) was then added at
À78ꢀC via syringe. The solution turned from colorless to deep yellow.

1138
ZHANG, TSE, AND CHAN
After 1 h, the reaction mixture was transferred to a solution of 6-(20,40-
di-tert-butyl-60-methoxyphenyl)pyridine-2-carboxyaldehyde (9) (333 mg,
1.02 mmol) in THF (2 mL) under N2 at À78ꢀC via a cannular. The tempera-
ture gradually rose to room temperature. Water was added to the reaction
mixture after 1 h. The mixture was extracted with ether (3 times). The
organic layer was dried (MgSO4), filtered, and rotary-evaporated to dryness.
The residue was then separated by column chromatography on silica gel
using hexanes/E.A. (100:4) to hexanes/E.A. (9:1) as the gradient eluent. A
mixture of yellow solids (430 mg, ꢁ0.69 mmol, ꢁ74%) was obtained after
1
rotary evaporation. Rf ¼ 0.20 (hexanes/E.A. ¼ 9:1); H NMR ꢀ 1.09 (s, 12
H), 1.14 (s, 4 H), 1.15 (s,3 H), 1.39 (s, 12 H), 3.60 (s, 2 H), 3.63 (s, 2 H), 3.66
(s, 2 H), s5.89 (br s, 1 H), 6.02 (br s, 1 H) 6.89 (d, 2 H, J ¼ 2.0 Hz), 7.13–7.19
(m, 2 H), 7.24 (d, 2 H, J ¼ 5.6 Hz), 7.34–7.47 (m, 2 H), 7.55–7.60 (m, 2 H);
FABMS: m/z 623 [(MþH)þ].
Preparation of bis-2-[6-(20,60-Di-tert-butyl-60-hydroxphenyl)-pyridyl]-
methanol (11)
Crude bis-2-[6-(20,60-di-tert-butyl-60-methoxyphenyl)-pyridyl]-metha-
nol (10) (300 mg, 0.48 mmol) was dissolved in acetic acid (5 mL) with 48%
HBr (5mL) and then heated to 140ꢀC for 11 h. The reaction mixture
was then neutralized by sat. NaHCO3 and the product was extracted with
ether, dried (MgSO4), filtered, and rotary-evaporated to dryness. The
residue was then separated by column chromatography on silica gel using
hexanes/E.A. (6:1) to hexanes/E.A. (2:1) as the gradient eluent. A white
solid (187 mg, 0.31 mmol, 65%) was obtained after rotary evaporation.
1
Rf ¼ 0.06 (hexanes/E.A. ¼ 2:1); H NMR ꢀ 1.07 (s, 18 H), 1.32 (s, 18 H),
5.97 (s, 1 H), 6.86 (d, 2 H, J ¼ 1.2 Hz), 7.17 (d, 2 H, J ¼ 1.2 Hz), 7.27 (d, 2 H,
J ¼ 7.6 Hz), 7.48 (d, 2 H, J ¼ 7.8 Hz), 7.67 (unresolved dd, 2 H); 13C NMR ꢀ
31.26, 32.72, 34.85, 36.83, 75.06, 111.02, 116.66, 119.92, 123.80, 126.32,
137.36, 148.62, 152.26, 153.41, 157.03, 160.69; FABMS: m/z 595
[(MþH)þ]; HRMS (ESIMS): Calc. for (C21H27NO2)þ: m/z 595.3894.
Found: m/z 595.3924.
ACKNOWLEDGMENTS
We acknowledge the award of a postdoctoral fellowship from the
Chinese University of Hong Kong (HCZ).

PYRIDYLPHENOL LIGANDS
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1139
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Received April 3, 2000