Synthesis of boron macrocycles from 1,2-aminoalcohols and 2-formylphenylboronic acid

Article abstract of DOI:10.1002/jhet.5570450526

(Chemical Equation Presented) Condensation of aminoalcohols with 2-HC(O)C₆H₄B(OH)₂ afforded the macrocyclic compounds 1 - 7 with an OBOBO structural unit. Crystals of 1 were triclinic, space group P-1, a = 9.9818(12) A, b = 12.8302(15) A, c = 13.1663(15) ?, α = 105.503(2)°, β = 94.860(2)°, γ = 92.585(2)°, Z = 2.

Full text of DOI:10.1002/jhet.5570450526

Sep-Oct 2008
1415
Synthesis of Boron Macrocycles from 1,2-Aminoalcohols and 2-
Formylphenylboronic Acid
Francis A. Appoha, Stephanie S. Barnesa, Marcy J. Manninga, Courtney S. Turnera,
Christopher M. Vogelsa, Andreas Deckenb and Stephen A. Westcotta,*
a Department of Chemistry, Mount Allison University, Sackville, NB E4L 1G8, Canada.
b Department of Chemistry, University of New Brunswick, Fredericton, NB E3B 5A3, Canada.
swestcott@mta.ca
Received March 8, 2008
H
OH
R
NH2
O
R
N
O
B
O
+
O
B
B
N
R
HO
OH
H
Condensation of aminoalcohols with 2-HC(O)C6H4B(OH)2 afforded the macrocyclic compounds 1 - 7
with an OBOBO structural unit. Crystals of 1 were triclinic, space group P-1, a = 9.9818(12) Å, b =
12.8302(15) Å, c = 13.1663(15) Å, ꢀ = 105.503(2)°, ꢁ = 94.860(2)°, ꢂ = 92.585(2)°, Z = 2.
J. Heterocyclic Chem., 45, 1415 (2008).
INTRODUCTION
ꢂ-phenyl-ꢂ-amino alcohols [3a] and Höpfl has prepared
air-stable cyclophane-type macrocycles from salicylidene-
aminoaryl alcohols and arylboronic acids [3c]. In this
study, we found that 2-formylphenylboronic acid adds to
1-amino-2-naphthol in ethanol to give the Schiff base
macrocycle 1 (Scheme 1). Similar boron macrocycles
containing a NꢀB-O fragment have shown significant
antimicrobial properties [4]. While the imine proton is
observed in the 1H NMR spectra at ꢃ 8.61 ppm, the
boronic acid hydroxyl groups are absent. The diagnostic
C=N stretching band is observed in the FT-IR spectra at
ca. 1633 cm-1 and a peak at ꢃ 7 ppm in the 11B NMR
spectra indicates that the boron atom is four coordinate
[5].
Boronic acids [RB(OH)2] and boronate esters
[RB(OR')2] are remarkably versatile functional groups.
Their usefulness extends from synthetic applications such
as Suzuki-Miyaura cross-coupling reactions [1] to their
potent biological activities [2].
For example, an
antifungal agent, 5-fluoro-1,3-dihydro-1-hydroxy-2,1-
benzoxarole, was recently developed where the boron
group is a key element in the compound's toxicity [2a].
As well, serine proteases, a diverse group of proteolytic
enzymes responsible for the generation of most disease
processes, are inhibited by certain aminoboronic acids
[2c]. Simple aminoboronic acid derivatives are also being
examined for application in boron neutron capture
therapy, a binary form of cancer treatment that relies on
delivering a compound containing boron-10 selectively to
tumour tissues prior to irradiation by neutrons [2d]. Due
to their synthetic utility and the wealth of biological
activities demonstrated with boron-containing compounds
we decided to examine the reactivity of 1,2-amino-
Scheme 1
H
OH
R
NH2
O
R
N
O
B
O
+
O
alcohols
and
2-formylphenylboronic
HC(O)C6H4B(OH)2]. Results of our study are presented
herein.
acid
[2-
B
B
N
R
HO
OH
H
R = 2-naphthyl, 1
= CO2H, 2
= Cl, 3
= CH3, 4
= Ph, 5
RESULTS AND DISCUSSION
Salicylaldimines are versatile intermediates in organic
synthesis and have been used to prepare numerous
The molecular structure of 1 is shown in Figure 1 and
crystallographic data given in Table 1. The two frag-
ments form seven-membered rings and are connected via
a B-O-B bridge [B(1)-O(1)-B(2) = 116.1(2)°] with B-O
pharmacologically important compounds [3].
instance, Whiting and co-workers have used imines
containing boronate esters to make enantio-enriched
For

1416
F. A. Appoh, S. S. Barnes, M. J. Manning, C. S. Turner, C. M. Vogels,
A. Decken and S. A. Westcott
Vol 45
Table 1.
bond distances of 1.405(3) Å. The B-O-B angle is
somewhat smaller in 1 than that observed in a related
heterocycle prepared through the reaction of 2-salicyl-
ideneamino hydroxyethane and tris-dimethylamine-
borane, 124.4(2)° [6]. The two phenolic B-O distances in
1 are slightly longer (1.494(3) and 1.501(3) Å) than the
Crystallographic data collection parameters for 1.2CH2Cl2
Complex
Chemical formula
fw
Crystal system
Space group
a, Å
b, Å
c, Å
ꢀ, °
ꢁ, °
ꢂ, °
V, Å3
Z
1
C34H22B2Cl4N2O3
698.01
triclinic
P-1
9.9818(12)
12.8302(15)
13.1663(15)
105.503(2)
94.860(2)
92.585(2)
1615.0(3)
2
1.435
0.60 x 0.30 x 0.15
183(1)
MoKꢀ (ꢄ = 0.71073)
0.408
8190
5271
453
bridged B-O-B bonds (avg. 1.405(3) Å).
Similar
distances and angles have been reported for other O-B-O
ring systems [3b,7]. For instance, a related partial ester of
trimetaboric acid derived from salen-type compounds and
boric acid has distances of 1.414(6) Å (avg.) for the
heterocyclic B-O bonds and 1.466(6) and 1.454(6) Å for
the terminal B-O bonds [3b]. The aldimine functionality
in 1 is stabilized by NꢀB interactions with average B-N
bonds of 1.647(3) Å, which are characteristic of strong
coordination [3c]. Similar bond distances have been
reported in related oxime, salen, and salicylaldehyde
derivatives [8].
ꢃcalcd, mg m-3
Crystal size, mm3
Temperature, K
Radiation
μ, mm-1
Total reflections
Total unique relections
No. of variables
Rint
0.0208
Theta range, deg
Largest difference peak/hole, e Å-3
GoF on F2
R1a (I>2ꢅ(I))
wR2b (all data)
1.61-24.99
0.995/-0.780
1.090
0.0528
0.1394
a R1 = ꢆ ||Fo|–|Fc||/ꢆ|Fo|. b wR2 = (ꢆ[w(Fo2ꢇFc2)2]/ꢆ[Fo4])1/2, where
w = 1/[ꢅ2(Fo2) + (0.0564*P)2 + (1.6623*P)], where P = (max (Fo2, 0)
+ 2*Fc2)/3.
Figure 1. Molecular structure of 1 showing 30% probability ellipsoids
with hydrogen atoms omitted for clarity. Selected bond angles (°) and
distances (Å): B(1)-O(1) 1.405(3), B(1)-O(3) 1.494(3), B(1)-N(26)
1.644(3), B(2)-O(1) 1.405(3), B(2)-O(2) 1.501(3), B(2)-N(8) 1.649(3);
O(1)-B(1)-O(3) 111.5(2), O(1)-B(1)-N(26) 107.9(2), O(3)-B(1)-N(26)
99.41(18), O(1)-B(2)-O(2) 111.1(2), O(1)-B(2)-N(8) 107.3(2), O(2)-
B(2)-N(8) 98.77(18), B(2)-O(1)-B(1) 116.1(2).
H
N
O
B
R
O
N
O
It is possible that the formation of 1 proceeds via
initial formation of
a Schiff base followed by
R
dehydration of the hydroxyl groups of the boronic acid
group to form the anhydride dimer 1. Anhydride
formation is also observed with other 2-aminophenol
derivatives to give the corresponding heterocycles 2-5.
Similar reactivity has been reported previously for
analogous Schiff bases derived from 2-aminophenol and
2-(4,4,5,5,-tetramethyl-[1,3,2]-dioxaborolan-2-yl)benz-
aldehyde [5a]. Heterocyclic formation was also observed
when 2-HC(O)C6H4B(OH)2 was added to aliphatic 1,2-
amino-alcohols; ethanolamine and 2-amino-1-phenyl-
ethanol (6 & 7, respectively, Figure 2).
B
R = H, 6
= Ph, 7
H
Figure 2. Boron macrocycles derived from aliphatic 1,2-
aminoalcohols.
functionality in these dimers is stabilized by intra-
molecular NꢀB interactions. All new boron-containing
macrocycles will be tested for their potential antifungal
and antibacterial activities, the results of which will be
reported in due course.
CONCLUSION
Reactions of 2-HC(O)C6H4B(OH)2 with 1,2-amino-
alcohols gave novel boron macrocycles containing seven-
membered rings with a B-O-B bridge. The aldimine

Sep-Oct 2008
Synthesis of Boron Macrocycles from 1,2-Aminoalcohols and 2-Formylphenylboronic Acid
1417
160.4, 158.7, 152.7 (br, C-B), 136.4, 134.3, 133.7, 133.5, 133.0,
132.2, 128.3, 122.4, 116.2, 115.6. FTIR (Nujol): 2943, 2897,
2864, 1622, 1599, 1550, 1464, 1377, 1302, 1265, 1097, 964, 798,
688. Anal. Calcd. for C26H16B2Cl2N2O3 (497.06): C, 62.82; H,
3.25; N, 5.64. Found: C, 62.69; H, 3.11; N, 5.42.
EXPERIMENTAL
Reagents and solvents were purchased from Aldrich
Chemicals and used as received. NMR spectra were recorded on
a JEOL JNM-GSX270 FT NMR (1H 270 MHz; 11B 87 MHz; 13C
68 MHz) spectrometer. Chemical shifts (ꢀ) are reported in ppm
[relative to residual solvent peaks (1H and 13C) or external
Compound 4. An EtOH (5 mL) solution of 2-formylphenyl-
boronic acid (303 mg, 2.02 mmol) was added to an EtOH (45
mL) solution of 2-amino-4-methylphenol (252 mg, 2.05 mmol).
The reaction was heated at reflux for 2 h and then stored at RT.
A yellow precipitate was collected by suction filtration and
washed with cold EtOH (2 x 2 mL) to give 4. Yield: 140 mg (30
%). Mp = 210 - 214 °C. 1H NMR (DMSO-d6) ꢀ: 9.34 (s, 2H,
Ar), 7.82 (s, 2H, C(H)N), 7.67 (d, J = 6.7 Hz, 2H, Ar), 7.32 -
7.21 (ov m, 4H, Ar), 7.17 (d, J = 8.2 Hz, 2H, Ar), 7.06 (d, J =
6.7 Hz, 2H, Ar), 6.76 (d, J = 8.2 Hz, 2H, Ar), 2.36 (s, 6H, CH3);
11B NMR (CH2Cl2) ꢀ: 7 (sharp); 13C{1H} NMR (DMSO-d6) ꢀ:
158.1, 157.8, 153.4 (br, C-B), 135.8, 134.6, 133.4, 133.3, 132.9,
131.8, 127.9, 127.7, 115.7, 114.0, 21.2. FTIR (Nujol): 2945,
2896, 2864, 1622, 1550, 1502, 1460, 1377, 1281, 1105, 921,
805, 733. Anal. Calcd. for C28H22B2N2O3 (456.14): C, 73.72; H,
4.87; N, 6.14. Found: C, 73.49; H, 4.67; N, 6.03.
Compound 5. An EtOH (5 mL) solution of 2-formylphenyl-
boronic acid (303 mg, 2.02 mmol) was added to an EtOH (45
mL) solution of 2-amino-4-phenylphenol (371 mg, 2.00 mmol).
The reaction was heated at reflux for 3 h then stirred at RT for
an additional 16 h. Following removal of the solvent under
vacuum the residual solid was redissolved in MeOH (10 mL)
and stored at RT. A dark yellow precipitate was collected by
suction filtration and washed with MeOH (2 x 2 mL) to afford 5.
Yield: 216 mg (37 %). Mp = 220 - 222 °C. 1H NMR (DMSO-
d6) ꢀ: 9.60 (s, 2H, Ar), 8.42 (s, 2H, C(H)N), 7.78 - 7.72 (ov m,
8H, Ar), 7.51 - 7.48 (ov m, 4H, Ar), 7.37 - 7.33 (ov m, 6H, Ar),
7.17 (m, 2H, Ar), 6.98 (m, 2H, Ar); 11B NMR (DMSO-
d6/CDCl3) ꢀ: 7 (br); 13C{1H} NMR (DMSO-d6/CDCl3) ꢀ: 159.7,
158.3, 152.6 (br, C-B), 140.4, 135.7, 134.5, 133.6, 132.8, 132.4,
131.5, 131.0, 129.1, 127.8, 127.2, 126.8, 114.6, 113.6. FTIR
(Nujol): 2951, 2929, 2922, 2912, 2863, 2852, 1630, 1606, 1552,
1514, 1481, 1464, 1377, 1306, 1265, 1195, 1124, 1105, 960,
908, 758. Anal. Calcd. for C38H26B2N2O3 (580.28): C, 78.65; H,
4.53; N, 4.83. Found: C, 79.02; H, 4.27; N, 4.65.
Compound 6. An EtOH (10 mL) solution of 2-formylphenyl-
boronic acid (450 mg, 3.00 mmol) was added to an EtOH (10
mL) solution of ethanolamine (183 mg, 3.00 mmol). The
reaction was heated at reflux for 3 h, at which point the solvent
was removed under vacuum and the residual solid dissolved in
CHCl3 (5 mL). A white precipitate was collected by suction
filtration to afford 6. Yield: 102 mg (20 %). Mp = 196 - 198
°C. 1H NMR (CD3OD) ꢀ: 8.71 (s, 2H, C(H)N), 7.64 (d, J = 7.4
Hz, 2H, Ar), 7.52 - 7.50 (ov m, 4H, Ar), 7.40 - 7.32 (ov m, 2H,
Ar), 3.87 (t, J = 5.5 Hz, 4H, CH2), 3.74 (t, J = 5.5 Hz, 4H, CH2);
11B NMR (CD3OD) ꢀ: 10 (sharp); 13C{1H} NMR (CD3OD) ꢀ:
170.3, 150.1 (br, C-B), 138.2, 132.5, 129.7, 127.7, 126.7, 59.1,
51.9. FTIR (Nujol): 2972, 2945, 2881, 2839, 1631, 1562, 1460,
1377, 1170, 1107, 1070, 968, 746, 723. Anal. Calcd. for
C18H18B2N2O3 (332.00): C, 65.11; H, 5.48; N, 8.44. Found: C,
64.81; H, 5.27; N, 8.39.
BF3 OEt2 (11B)] and coupling constants (J) in Hz. Multiplicities
.
are reported as singlet (s), doublet (d), triplet (t), multiplet (m),
broad (br), and overlapping (ov). The infrared spectra were
obtained using a Mattson Genesis II FT-IR spectrometer and are
reported in cm-1. Melting points were determined using a Mel-
Temp apparatus and are uncorrected. Elemental analyses were
obtained on a Vario EL III.
Compound 1. An EtOH (5 mL) solution of 2-formyl-
phenylboronic acid (303 mg, 2.02 mmol) was added to an EtOH
(45 mL) solution of 1-amino-2-napthol (320 mg, 2.01 mmol).
The reaction mixture was heated at reflux for 3 h at which point
the reaction volume was decreased under vacuum to 20 mL and
the solution allowed to stand at RT. A reddish-brown precipitate
was collected by suction filtration and washed with cold EtOH
(2 x 2 mL) to afford 1. Yield: 218 mg (41 %). Mp = 252 - 254
°C. 1H NMR (DMSO-d6) ꢀ: 9.72 (s, 2H, Ar), 8.61 (s, 2H,
C(H)N), 7.91 (d, J = 8.2 Hz, 2H, Ar), 7.82 (d, J = 7.7 Hz, 2H,
Ar), 7.76 (d, J = 8.2 Hz, 2H, Ar), 7.47 (dd, J = 7.7 Hz, 2H, Ar),
7.39 - 7.33 (ov m, 4H, Ar), 7.28 (dd, J = 7.7 Hz, 2H, Ar), 7.18
(s, 2H, Ar), 7.13 (d, J = 7.7 Hz, 2H, Ar); 11B NMR (CDCl3) ꢀ: 7
(br); 13C{1H} NMR (DMSO-d6) ꢀ: 161.4, 157.1, 154.2 (br, C-B),
136.9, 136.8, 134.4, 134.0, 133.7, 133.6, 129.3, 128.2, 127.8,
127.5, 127.0, 124.0, 114.3, 107.3. FTIR (Nujol): 2945, 2908,
2869, 1633, 1549, 1464, 1376, 1267, 1186, 960, 860, 800, 742.
Anal. Calcd. for C34H22B2N2O3 (528.20): C, 77.31; H, 4.21; N,
5.30. Found: C, 76.85; H, 4.09; N, 5.18.
Compound 2. An EtOH (5 mL) solution of 2-formylphenyl-
boronic acid (303 mg, 2.02 mmol) was added to an EtOH (45 mL)
solution of 3-amino-4-hydroxybenzoic acid (306 mg, 2.00 mmol).
The reaction was heated at reflux for 3 h then stirred at RT for an
additional 16 h. Following removal of the solvent under vacuum
the residual solid was redissolved in MeOH (10 mL) and stored at
RT. A yellow precipitate was collected by suction filtration and
washed with MeOH (2 x 2 mL) to give 2. Yield: 228 mg (44 %).
1
Mp = 286 - 288 °C. H NMR (DMSO-d6) ꢀ: 9.63 (s, 2H, Ar), 8.65
(s, 2H, C(H)N), 7.98 (d, J = 8.4 Hz, 2H, Ar), 7.80 (d, J = 6.9 Hz,
2H, Ar), 7.33 - 7.28 (ov m, 4H, Ar), 7.07 (d, J = 6.9 Hz, 2H, Ar),
6.95 (d, J = 8.4 Hz, 2H, Ar); 11B NMR (DMSO-d6/MeOH) ꢀ: 7
(br); 13C{1H} NMR (DMSO-d6) ꢀ: 167.5, 163.4, 160.6, 152.6 (br,
C-B), 136.6, 134.4, 134.3, 133.5, 133.4, 132.2, 128.3, 121.5,
117.6, 114.0. FTIR (Nujol): 3113, 2943, 2908, 2860, 1674, 1600,
1552, 1458, 1377, 1300, 1198, 1105, 970, 918, 758, 665. Anal.
Calcd. for C28H18B2N2O7 (516.10): C, 65.16; H, 3.52; N, 5.43.
Found: C, 65.43; H, 3.39; N, 5.21.
Compound 3. An EtOH (5 mL) solution of 2-formylphenyl-
boronic acid (506 mg, 3.37 mmol) was added to an EtOH (25 mL)
solution of 2-amino-4-chlorophenol (477 mg, 3.32 mmol). The
reaction was heated at reflux for 2 h and the solution stored at RT.
The resulting yellow precipitate was collected by suction filtration
and washed with cold EtOH (2 x 2 mL) to afford 3. Yield: 455
mg (55 %). Mp = 205 - 208 °C. 1H NMR (DMSO-d6) ꢀ: 9.48 (s,
2H, Ar), 8.17 (d, J = 2.0 Hz, 2H, C(H)N), 7.69 (d, J = 6.7 Hz, 2H,
Ar), 7.38 (dd, J = 8.9, 2.0 Hz, 2H, Ar), 7.33 - 7.27 (ov m, 4H, Ar),
7.08 (d, J = 6.7 Hz, 2H, Ar), 6.89 (d, J = 8.9 Hz, 2H, Ar); 11B
NMR (DMSO-d6/MeOH) ꢀ: 7 (br); 13C{1H} NMR (DMSO-d6) ꢀ:
Compound 7. An EtOH (10 mL) solution of 2-formyl-
phenylboronic acid (450 mg, 3.00 mmol) was added to an EtOH
(10 mL) solution of 2-amino-1-phenylethanol (412 mg, 3.03
mmol). The reaction was heated at reflux for 3 h at which point
the solvent was removed under vacuum. The residual solid was
dissolved in CHCl3 (5 mL) to which hexane (5 mL) was added

1418
F. A. Appoh, S. S. Barnes, M. J. Manning, C. S. Turner, C. M. Vogels,
A. Decken and S. A. Westcott
Vol 45
Chem. 1999, 576, 147.
and the solution stored at RT. An off-white solid precipitated,
was collected by suction filtration and washed with CHCl3 (2 x 1
mL) to give 7. Yield: 293 mg (40 %). Mp = 236 - 238 °C. 1H
NMR (CD3OD) ꢀ: 8.68 (s, 2H, C(H)N), 7.64 (d, J = 7.4 Hz, 2H,
Ar), 7.54 - 7.48 (ov m, 8H, Ar), 7.40 - 7.27 (ov m, 8H, Ar), 5.06
(dd, J = 9.4, 2.7 Hz, 2H, CH), 3.86 (dd, J = 12.6, 2.7 Hz, 2H,
CH2), 3.70 (dd, J = 12.6, 9.4 Hz, 2H, CH2); 11B NMR (CD3OD)
ꢀ: 9 (sharp); 13C{1H} NMR (CD3OD) ꢀ: 171.1, 150.5 (br, C-B),
142.2, 138.1, 132.6, 129.8, 128.2, 127.7, 127.6, 126.8, 125.8,
71.0, 57.2. FTIR (Nujol): 3025, 2931, 2906, 2862, 1666, 1631,
1563, 1456, 1377, 1167, 1097, 1059, 953, 746, 700. Anal.
Calcd. for C30H26B2N2O3 (484.20): C, 74.41; H, 5.42; N, 5.79.
Found: C, 74.63; H, 5.13; N, 5.61.
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Cambridge Crystallographic Data Centre, 12 Union Road,
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Acknowledgements. Thanks are gratefully extended to the
Canada Research Chair/Atlantic Innovation Fund Programs, the
University of New Brunswick, and Mount Allison University for
financial support. We also thank Dan Durant and Roger Smith
for their expert technical assistance.
[9] SAINT 6.02, Bruker AXS, Inc., Madison, Wisconsin, USA
(1997-1999).
[10] G.M. Sheldrick, SADABS, Bruker AXS, Inc., Madison,
Wisconsin, USA (1999).
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