Synthesis and reactivity of novel Schiff bases containing boronate esters

Article abstract of DOI:10.1139/v01-188

Condensation of 2-aminophenol with boronate ester derivatives of benzaldehyde afforded the corresponding boron-containing Schiff bases, 2-HOC₆H₄N=C(H)C₆H₄R (1a: R = 2-Bpin; 1b: R = 3-Bpin; 1c: R = 4-Bpin; pin = 1,2-O₂C₂Me₄). Crystals of 1b were triclinic, space group P1, a = 11.9420(6), b = 13.0871(7), and c = 13.2720(7) A, α = 70.983(1), β = 67.793(1), and γ = 78.380(1)°, Z = 2. Reaction of 2-aminophenol with 2-HC(O)C₆H₄B(OH)₂ in EtOH, however, gave a macrocyclic dimer 2 with a OBOBO structural unit. The molecular structure of this dimer has been confirmed by an X-ray diffraction study. Crystals of 2 were monoclinic, space group P2₁/c, a = 10.0447(8), b = 21.0894(15), and c = 12.6214(9) A, β = 105.301(2)°, Z = 4. Further reaction of these Schiff bases with manganese triacetate in toluene afforded 2-arylbenzoxazoles 3a-c via an oxidative cyclization pathway. The molecular structure of the 4-Bpin derivative (3c) was characterized by an X-ray diffraction study. Crystals of 3c were monoclinic, space group P2₁/n, a = 6.5392(3), b = 16.3330(8), and c = 16.1942(8) A, β = 97.9620(10)°, Z = 4.

Full text of DOI:10.1139/v01-188

31
Synthesis and reactivity of novel Schiff bases
containing boronate esters
David W. Norman, Janet P. Edwards, Christopher M. Vogels, Andreas Decken,
and Stephen A. Westcott
Abstract: Condensation of 2-aminophenol with boronate ester derivatives of benzaldehyde afforded the corresponding bo-
ron-containing Schiff bases, 2-HOC6H4N=C(H)C6H4R (1a: R = 2-Bpin; 1b: R = 3-Bpin; 1c: R = 4-Bpin; pin =
1,2-O2C2Me4). Crystals of 1b were triclinic, space group P1, a = 11.9420(6), b = 13.0871(7), and c = 13.2720(7) Å,
a = 70.983(1), b = 67.793(1), and ꢀ = 78.380(1)°, Z = 2. Reaction of 2-aminophenol with 2-HC(O)C6H4B(OH)2 in
EtOH, however, gave a macrocyclic dimer 2 with a OBOBO structural unit. The molecular structure of this dimer has
been confirmed by an X-ray diffraction study. Crystals of 2 were monoclinic, space group P21/c, a = 10.0447(8), b =
21.0894(15), and c = 12.6214(9) Å, b = 105.301(2)°, Z = 4. Further reaction of these Schiff bases with manganese tri-
acetate in toluene afforded 2-arylbenzoxazoles 3a–c via an oxidative cyclization pathway. The molecular structure of
the 4-Bpin derivative (3c) was characterized by an X-ray diffraction study. Crystals of 3c were monoclinic, space group
P21/n, a = 6.5392(3), b = 16.3330(8), and c = 16.1942(8) Å, b = 97.9620(10)°, Z = 4.
Key words: boron heterocycles, Schiff bases, arylbenzoxazoles.
Résumé : La condensation du 2-aminophénol avec des dérivés boronates de benzaldéhyde conduit à la formation des
bases de Schiff correspondantes contenant du bore, 2-HOC6H4N=C(H)C6H4R (1a: R = 2-Bpin; 1b: R = 3-Bpin; 1c:
R = 4-Bpin; pin = 1,2-O2C2Me4). Les cristaux du composé 1b sont tricliniques, groupe d’espace P1, avec a =
11,9420(6), b = 13,0871(7) et c = 13,2720(7) Å, a = 70,983(1), b = 67,793(1) et ꢀ = 78,380(1)° et Z = 2. La réaction
du 2-aminophénol avec le 2-HC(O)C6H4B(OH)2 dans l’éthanol conduit toutefois à la formation du dimère macrocy-
clique 2 qui comporte une unité structurale OBOBO. La structure moléculaire de ce dimère a été confirmée par diffrac-
tion des rayons X. Les cristaux du composé 2 sont monocliniques, groupe d’espace P21/c, avec a = 10,0447(8), b =
21,0894(15) et c = 12,6214(9) Å, b = 105,301(2)° et Z = 4. Des réactions subséquentes de ces bases de Schiff avec du
triacétate de manganèse dans le toluène conduisent à la formation des 2-arylbenzoxaoles (3a–c) par le biais d’une voie
réactionnelle de cyclisation oxydante. La structure moléculaire du dérivé 4-Bpin (3c) a été caractérisée par diffraction
des rayons X. Les cristaux du composé 3c sont monocliniques, groupe d’espace P21/n, avec a = 6,5392(3), b =
16,3330(8) et c = 16,1942(8) Å, b = 97,9620(10)° et Z = 4.
Mots clés : hétérocycles du bore, bases de Schiff, arylbenzoxazoles.
[Traduit par la Rédaction] Norman et al. 40
Introduction
cedure requiring several complex organic transformations (1,
7). As a result, the range of compounds containing boronic
acids is surprisingly small. As part of our investigation into
generating novel aminoboron compounds (22–24), we de-
cided to examine the synthesis and reactivity of Schiff bases
derived from 2-aminophenol and benzaldehydes containing
boronate esters. Results of our study are presented herein.
Compounds containing boronic acids [RB(OH)2] or
boronate esters [RB(OR>)2] have been used extensively as
synthons in the Suzuki–Miyaura cross-coupling reaction for
a variety of applications (1–11). Interest in these compounds
also arises from their potent biological activities (12–21).
For instance, aminoboronic acids are among the most potent
inhibitors of serine proteases, a diverse group of proteolytic
enzymes responsible for the generation of most disease pro-
cesses (13–18). The synthesis of aminoboronic acid-
containing compounds, however, is often a complicated pro-
Results and discussion
Schiff bases are important intermediates in organic chem-
istry and have been used to prepare a number of pharmaco-
Received 24 August 2001. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 10 January 2002.
D.W. Norman, J.P. Edwards, C.M. Vogels, and S.A. Westcott.1 Department of Chemistry, Mount Allison University, Sackville,
NB E4L 1G8, Canada.
A. Decken.2 Department of Chemistry, University of New Brunswick, Fredericton, NB E3B 5A3, Canada.
1Corresponding author (telephone: (506) 364-2351; fax: (506) 364-2313; e-mail: swestcott@mta.ca).
2Author to whom correspondence about crystallography may be addressed (telephone: (506) 453-4875; fax: (506) 453-4981; e-mail:
adecken@unb.ca).
Can. J. Chem. 80: 31–40 (2002)
DOI: 10.1139/V01-188
© 2002 NRC Canada

32
Can. J. Chem. Vol. 80, 2002
Table 1. Crystallographic data collection parameters for 1b, 2, and 3c.
1b
2
3c
Formula
FW
C38H44B2N2O6
646.37
C26H18B2N2O3·1.5CH2Cl2
555.43
C19H20BNO3
321.17
Crystal system
Space group
Triclinic
P1
Monoclinic
P21/c
Monoclinic
P21/n
a (Å)
b (Å)
c (Å)
a (°)
b (°)
ꢀ (°)
V (Å3)
11.9420(6)
13.0871(7)
13.2720(7)
70.983(1)
67.793(1)
78.380(1)
1808.29(16)
2
10.0447(8)
21.0894(15)
12.6214(9)
90
105.301(2)
90
2578.9(3)
4
1.431
6.5392(3)
16.3330(8)
16.1942(8)
90
97.9620(10)
90
1712.94(14)
4
1.245
Z
ꢁcalcd (g cm–3)
Crystal size (mm3)
T (K)
1.187
0.40 × 0.35 × 0.20
173(1)
0.30 × 0.30 × 0.35
173(1)
0.18 × 0.23 × 0.55
173(1)
Radiation
Mo Ka (ꢂ = 0.71073)
0.079
19912
12584
8560
Mo Ka (ꢂ = 0.71073)
0.390
28199
9224
7072
Mo Ka (ꢂ = 0.71073)
0.083
18770
6156
4173
(mm–1)
Total reflectionsa
Total unique relections
I > 4ꢃ(F)
No. of variables
Rint
521
0.0152
432
0.0211
302
0.0304
Theta range (°)
Largest difference peak/hole (e Å–3)
S (GoF) on F2 a
R1 (I > 2ꢃ(I))b
wR2 (all data)c
1.65–32.50
0.451/–0.281
1.038
0.0573
0.1736
1.93–32.50
0.715/–0.646
1.132
0.0.527
0.1725
1.78–32.50
0.346/–0.137
0.966
0.0488
0.1379
aS = (S[w(F2o – F2c )2/(n – p))1/2
.
bR1 = S||(Fo| – |Fc||/S|Fo|.
cwR2 = (Sw(Fo2 – F2c )2/SwF4o )1/2, where w–1 = ꢃ2(F2o) + (aP)2 + bP, where P = (max(Fo2, 0) + 2Fc2 )/3. For 1b, a = 0.1068, b = 0; for 2, a = 0.1088,
b = 0.0676; for 3c, a = 0.0859, b = 0.
Scheme 1.
Fig. 1. Molecular structure of 1b with ellipsoids drawn at the
30% probability level. Hydrogen atoms were omitted for clarity.
HO
O
C
N
C
NH
2
H
H
+
OH
O
B
O
B
FPBpin
O
1a: 2-Bpin
1b: 3-Bpin
1c: 4-Bpin
O
logically important compounds (29–34). For instance,
Whiting and co-workers (29) have recently used imines con-
taining boronate esters to make enantio-enriched ꢀ-phenyl-ꢀ-
amino alcohols. In this study, we found that pinacol-
protected derivatives of formylphenylboronic acid [(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)benzaldehyde (FPBpin)]
add to 2-aminophenol in organic solvents to give crystalline
compounds having spectroscopic data consistent with the
Schiff bases 1a–c (Scheme 1). As expected, a shift for the
30 ppm in the 11B NMR spectra indicates that the boron
atom lies in a three-coordinate environment (35), even for
the 2-Bpin derivative 1a. This result is somewhat surprising
in light of boron’s propensity to form coordinative NJB
bonds (36).
Although attempts to grow crystals of 1a suitable for X-
ray diffraction studies proved unsuccessful, the molecular
structure of the 3-Bpin derivative 1b (Fig. 1) confirms that
no appreciable intermolecular interactions exist between the
1
methine proton from 10 to 9 ppm is observed in the H
NMR spectra and a resonance at ca. 160 ppm in the 13C
NMR spectra corresponds to the N=CH carbon (25). Like-
wise, formation of these compounds can be monitored by
the diagnostic C=N stretching band in the IR spectra at ca.
1620 cm–1 (31). Interestingly, a broad peak at around
© 2002 NRC Canada

Norman et al.
33
Table 2. Selected bond lengths (Å) and angles (°) for 1b and 2.
Bond lengths (Å)
Bond angles (°)
Compound 1b
Molecule A
Molecule A
B(1)—O(2)
B(1)—O(5)
1.3614(15)
1.3653(14)
1.5600(16)
1.4734(14)
1.2723(14)
1.4221(13)
1.3648(13)
O(2)-B(1)-O(5)
113.52(10)
121.12(10)
125.33(10)
122.93(10)
121.36(9)
118.27(10)
121.22(9)
107.04(9)
117.78(9)
110.89(9)
110.62(9)
107.27(9)
107.14(9)
O(2)-B(1)-C(10)
O(5)-B(1)-C(10)
N(17)-C(16)-C(12)
C(16)-N(17)-C(18)
O(24)-C(19)-C(20)
O(24)-C(19)-C(18)
C(21)-B(2)-N(8)
B(2)-O(1)-B(1)
C(10)-O(2)-B(2)
C(30)-O(3)-B(1)
C(9)-N(8)-B(2)
C(29)-N(28)-B(1)
Molecule B
B(1)—C(10)
C(12)—C(16)
C(16)—N(17)
N(17)—C(18)
C(19)—O(24)
Molecule B
B(31)—O(32)
B(31)—O(35)
B(31)—C(40)
C(42)—C(46)
C(46)—N(47)
N(47)—C(48)
C(49)—O(54)
O(24)···H(39)
O(35)···H(24)
O(32)···H(21)
1.3697(14)
1.3739(13)
1.5644(15)
1.4728(14)
1.2791(13)
1.4203(13)
1.3675(14)
2.564
O(32)-B(31)-O(35)
O(32)-B(31)-C(40)
O(35)-B(31)-C(40)
N(47)-C(46)-C(42)
C(46)-N(47)-C(48)
113.35(9)
120.54(9)
126.10(9)
123.45(10)
121.25(9)
2.330
2.542
Compound 2
B(1)—O(1)
B(1)—O(3)
B(1)—C(1)
B(1)—N(28)
B(2)—O(1)
B(2)—O(2)
B(2)—C(21)
B(2)—N(8)
C(7)—N(8)
C(27)—N(28)
N(8)—C(9)
N(28)—C(29)
C(7)—N(8)
1.4194(16)
1.5075(15)
1.6493(18)
1.6544(15)
1.4154(16)
1.5162(15)
1.6476(17)
1.6581(16)
1.2958(15)
1.2976(15)
1.4196(15)
1.4226(15)
1.2958(15)
O(1)-B(1)-O(3)
O(1)-B(1)-C(1)
O(3)-B(1)-C(1)
O(1)-B(1)-N(28)
O(3)-B(1)-N(28)
C(1)-B(1)-N(28)
O(1)-B(2)-O(2)
O(1)-B(2)-C(21)
O(2)-B(2)-C(21)
O(1)-B(2)-N(8)
O(2)-B(2)-N(8)
110.67(9)
120.21(10)
108.79(10)
108.44(9)
99.42(8)
107.24(9)
111.14(9)
119.63(10)
109.36(9)
108.67(9)
98.89(8)
Scheme 2.
HO
N
O
C
H
NH2
EtOH
H
O
N
B
C
EtOH
H2O
+
B
C
H
O
H2O
OH
B(OH)2
2-FPBA
C
N
O
O
B
H
1a
O
2
Lewis acidic boron atom and the imine nitrogen. Crystallo-
graphic data are given in Table 13, selected bond distances
and angles shown in Table 2, and atomic coordinates are
provided in Table 3. The imine C(16)—N(17) bond distances
of 1.272(1) (molecule A) and 1.279(1) Å (molecule B) are
comparable to azomethine compounds derived from salicyl-
aldehyde and phenylboronic acid (31, 36). The B—O bond
distances (av = 1.368(1) Å) are also typical for three-
coordinate Bpin groups (25) and significantly shorter than
those observed in chelate complexes with diphenylborinic
3 Supplementary material may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Coun-
cil Canada, Ottawa, ON K1A 0S2, Canada. For information on obtaining material electronically go to http://www.nrc.ca/cisti/irm/
unpub_e.shtml. Crystallographic information has also been deposited with the Cambridge Crystallographic Data Centre. Copies of the data
can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Un-
ion Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2002 NRC Canada

34
Can. J. Chem. Vol. 80, 2002
Table 3. Atomic coordinates (1 × 104) and equivalent isotropic displacement parameters (Å2, 1 ×
103) for 1b.
x
y
z
Ueq
B(1)
O(2)
C(3)
C(4)
O(5)
C(6)
C(7)
C(8)
7513(1)
6389(1)
6550(1)
7814(1)
8412(1)
5508(2)
6545(3)
7787(2)
8574(1)
7705(1)
6690(1)
6794(1)
7940(1)
8955(1)
8842(1)
5695(1)
5721(1)
4645(1)
4829(1)
3846(1)
2680(1)
2486(1)
3460(1)
5964(1)
–536(1)
–66(1)
–1100(1)
–2127(1)
–1764(1)
–748(2)
–1337(2)
–2138(1)
–3395(1)
299(1)
1498(1)
2304(1)
1900(1)
718(1)
8388(1)
8904(1)
10035(1)
10186(1)
9075(1)
10737(1)
10104(2)
10526(1)
10934(1)
7127(1)
6527(1)
5395(1)
4841(1)
5420(1)
6552(1)
4819(1)
3790(1)
3262(1)
2127(1)
1505(1)
2019(1)
3141(1)
3764(1)
1599(1)
2119(1)
1931(1)
1903(2)
1576(1)
2027(1)
1077(3)
3050(2)
355(1)
2074(1)
2426(1)
2642(1)
2928(1)
2997(1)
2790(1)
2512(1)
3174(1)
3417(1)
3665(1)
3879(1)
4142(1)
4189(1)
3992(1)
3732(1)
3833(1)
1906(1)
1967(1)
1315(2)
1285(1)
1419(1)
1944(3)
142(2)
33(1)
54(1)
59(1)
38(1)
38(1)
121(1)
128(1)
68(1)
58(1)
31(1)
32(1)
30(1)
38(1)
43(1)
36(1)
32(1)
33(1)
30(1)
31(1)
37(1)
40(1)
41(1)
38(1)
41(1)
30(1)
46(1)
58(1)
38(1)
31(1)
120(1)
97(1)
61(1)
49(1)
29(1)
31(1)
30(1)
41(1)
48(1)
38(1)
32(1)
32(1)
29(1)
34(1)
40(1)
41(1)
41(1)
36(1)
51(1)
2297(2)
204(1)
C(9)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
N(17)
C(18)
C(19)
C(20)
C(21)
C(22)
C(23)
O(24)
B(31)
O(32)
C(33)
C(34)
O(35)
C(36)
C(37)
C(38)
C(39)
C(40)
C(41)
C(42)
C(43)
C(44)
C(45)
C(46)
N(47)
C(48)
C(49)
C(50)
C(51)
C(52)
C(53)
O(54)
2332(1)
2767(1)
3145(1)
3092(1)
2673(1)
2296(1)
3570(1)
3847(1)
4232(1)
4464(1)
4844(1)
5001(1)
4774(1)
4387(1)
4323(1)
4135(1)
4981(1)
6028(1)
5771(1)
4528(1)
7001(1)
6157(2)
6003(1)
6330(1)
2865(1)
2605(1)
1492(1)
605(1)
842(1)
–82(1)
1960(1)
1310(1)
333(1)
3528(1)
4348(1)
5509(1)
6313(1)
7492(1)
7872(1)
7085(1)
5909(1)
5925(1)
201(1)
–927(1)
–1203(1)
–351(1)
763(1)
1039(1)
–1756(1)
Note: Ueq is defined as one third of the trace of the orthogonalized Uij tensor.
acid (ca. 1.5 Å), where the boron atom is four coordinate
(37).
Compounds 1b and 1c could also be prepared in ethanol
using a catalytic amount of formic acid. However, we found
that reactions of 2-FPBpin with 2-aminophenol in ethanol
containing adventitious water gave the novel Schiff base
dimer 2 as the only new boron-containing product. It is pos-
sible that the formation of 2 proceeds via initial cleavage of
the pinacol groups in 2-FBpin to give 2-formylphenyl-
boronic acid. We found that the reaction of this aldehyde
with 2-aminophenol in wet ethanol does indeed give conden-
sation product 2 (Scheme 2). Another possible route to this
© 2002 NRC Canada

Norman et al.
35
Table 4. Atomic coordinates (1 × 104) and equivalent isotropic displacement parameters (Å2, 1 × 103)
for 2.
x
y
z
Ueq
B(1)
B(2)
O(1)
O(2)
O(3)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
N(8)
C(9)
C(10)
C(11)
C(12)
C(13)
C(14)
C(21)
C(22)
C(23)
C(24)
C(25)
C(26)
C(27)
N(28)
C(29)
C(30)
C(31)
C(32)
C(33)
C(34)
C(35)
Cl(1)
Cl(2)
C(36)
Cl(3)
Cl(4)
9593(1)
7404(1)
8796(1)
6647(1)
11096(1)
9466(1)
10229(2)
10353(2)
9737(2)
8986(1)
8809(1)
7956(1)
7362(1)
6546(1)
6152(1)
5304(1)
4868(2)
5265(2)
6122(1)
6440(1)
5095(1)
4121(1)
4456(1)
5761(1)
6761(1)
8068(1)
9256(1)
10526(1)
11552(1)
12920(1)
13221(1)
12192(1)
10815(1)
9221(2)
7883(1)
10769(1)
5563(7)
6687(2)
3872(2)
1517(1)
1500(1)
1702(1)
1812(1)
1669(1)
790(1)
666(1)
61(1)
–461(1)
–364(1)
255(1)
263(1)
756(1)
744(1)
1370(1)
1491(1)
975(1)
351(1)
229(1)
1536(1)
1294(1)
1301(1)
1570(1)
1835(1)
1818(1)
2119(1)
2010(1)
2302(1)
2078(1)
2278(1)
2692(1)
2907(1)
2717(1)
1628(1)
1469(1)
1232(1)
236(3)
9052(1)
9527(1)
9776(1)
10282(1)
9538(1)
8550(1)
7778(1)
7355(1)
7723(1)
8486(1)
8879(1)
9655(1)
9939(1)
10702(1)
10859(1)
11555(1)
12076(1)
11916(1)
11230(1)
8251(1)
8075(1)
7049(1)
6145(1)
6284(1)
7318(1)
7308(1)
7995(1)
7973(1)
8887(1)
9048(1)
8273(1)
7359(1)
7200(1)
5013(2)
3830(1)
4970(1)
4741(4)
5931(1)
4638(2)
19(1)
19(1)
20(1)
22(1)
24(1)
20(1)
27(1)
32(1)
31(1)
25(1)
20(1)
21(1)
19(1)
20(1)
21(1)
25(1)
30(1)
31(1)
26(1)
19(1)
24(1)
29(1)
29(1)
25(1)
20(1)
20(1)
19(1)
20(1)
21(1)
25(1)
28(1)
27(1)
23(1)
53(1)
59(1)
70(1)
68(1)
86(1)
112(1)
6(1)
–2(1)
Note: Ueq is defined as one third of the trace of the orthogonalized Uij tensor.
Fig. 2. Molecular structure of 2 with ellipsoids drawn at the
Fig. 3. Molecular structure of 3c with ellipsoids drawn at the
30% probability level. Hydrogen atoms were omitted for clarity.
30% probability level. Hydrogen atoms were omitted for clarity.
© 2002 NRC Canada

36
Can. J. Chem. Vol. 80, 2002
Scheme 3.
O
B
O
O
Mn(OAc)3
B
N
O
N
O
H
toluene / ∆
OH
3a-c
Scheme 4.
O
B
O
B
O
O
MnIII(OAc)3
H
MnIII(OAc)3
-MnII(OAc)2
N
N
H
N
O
H
H
-AcOH
O
B
O
O
MnIII(OAc)2
AcO MnIII(OAc)2
O
-MnII(OAc)3
+
N
N
H
O
O
B
O
B
O
-H+
O
O
Table 5. Selected bond lengths (Å) and angles (°) for 3c.
Bond lengths (Å)
membered rings and are connected via a BOB bridge (B(1)-
O(1)-B(2) = 117.8°) with average B—O bond distances of
1.417(2) Å. Interestingly, the two phenolic B—O distances
are slightly longer (1.508(2) and 1.516(2) Å) than the
bridged BOB bonds. Similar distances and angles have been
reported for other BOB ring systems (38–47). The aldimine
functionality in 2 is stabilized by NJB interactions with av-
erage B—N bonds of 1.651(2) Å. Similar bond distances
have been reported in related oxime (48) and salen deriva-
tives (49–51). The 11B NMR spectra of 2 show a peak at
7 ppm, which also suggests that the boron atom remains four
coordinate in solution (52). Although blue electrolumines-
cence has been observed in similar organoboron compounds
(40–42), this behavior is not observed for 2.
As Schiff bases are ubiquitous in transition metal chemis-
try, we decided to investigate the use of these compounds as
ligands for biologically active metals (such as Fe, Mo, Cu).
Although initial attempts failed in this regard, we found that
reaction of these Schiff bases gave the corresponding boron-
containing benzoxazoles in varying yields (Scheme 3).
Benzoxazoles are an important family of compounds that are
found in a variety of natural products with potent biological
activities (53–62). As a result, these compounds have at-
tracted a considerable amount of attention for their medici-
nal and agrochemical uses.
B(1)—O(2)
B(1)—O(5)
B(1)—O(2A)
B(1)—C(10)
C(16)—O(17)
C(16)—N(24)
O(17)—C(18)
C(23)—N(24)
1.3366(15)
1.3430(12)
1.4681(19)
1.5635(13)
1.3229(12)
1.3407(12)
1.4015(12)
1.3927(11)
Bond angles (°)
O(2)-B(1)-O(5)
O(2)-B(1)-O(2A)
O(5)-B(1)-O(2A)
O(2)-B(1)-C(10)
O(5)-B(1)-C(10)
O(2A)-B(1)-C(10)
O(17)-C(16)-N(24)
O(17)-C(16)-C(13)
N(24)-C(16)-C(13)
112.08(9)
33.74(8)
111.56(10)
122.89(10)
123.52(8)
120.81(10)
115.88(8)
122.50(8)
121.45(8)
dimer involves initial formation of 1a with subsequent
cleavage of the pinacol group to give a boronic acid Schiff
base. This transient imine could rapidly dehydrate to form 2.
Indeed, we observed that addition of ethanol to preformed
1a gave bright yellow crystals of dimer 2.
The molecular structure of 2 is shown in Fig. 2 and crys-
tallographic data are given in Table 1. Selected bond dis-
tances and angles shown in Table 2, and atomic coordinates
are provided in Table 4. The two fragments form seven-
Although the synthesis of benzoxazoles can be accom-
plished via a number of different methods, oxidative intra-
molecular cyclization of readily prepared phenolic Schiff
bases provides a general route to these important organic
compounds (62–67). Indeed, recent work by Varma et al.
(61, 62) has shown that manganese triacetate can be used as
a relatively benign oxidizing agent for reactions involving
© 2002 NRC Canada

Norman et al.
37
Table 6. Atomic coordinates (1 × 104) and equivalent isotropic displacement parameters (Å2, 1 × 103)
for 3c.
x
y
z
Ueq
B(1)
O(2)
O(2A)
C(3)
C(4)
O(5)
C(6)
C(6A)
C(7)
C(7A)
C(8)
4684(2)
2796(2)
2680(2)
2719(2)
5067(2)
6087(1)
1977(4)
1449(5)
1355(8)
1484(10)
5904(4)
5905(5)
5722(3)
5542(5)
5264(1)
3740(1)
4243(1)
6303(1)
7843(1)
7320(2)
6870(2)
5523(1)
5523(1)
6730(2)
6175(3)
7762(3)
9782(3)
10355(2)
8756(2)
8831(1)
8831(1)
7560(1)
7448(1)
7151(1)
6560(1)
6415(1)
7064(1)
6059(2)
5775(2)
6587(2)
6850(3)
5605(1)
5666(2)
6742(2)
6212(2)
8316(1)
8722(1)
9349(1)
9574(1)
9195(1)
8572(1)
10180(1)
10475(1)
10475(1)
10960(1)
11395(1)
11795(1)
11766(1)
11333(1)
10932(1)
10424(1)
10424(1)
5615(1)
5823(1)
5408(1)
6103(1)
6430(1)
6043(1)
5430(2)
5614(2)
6820(3)
6717(4)
6373(2)
5859(3)
7397(1)
7268(2)
5102(1)
4557(1)
4039(1)
4053(1)
4612(1)
5129(1)
3454(1)
2841(1)
2841(1)
2383(1)
1648(1)
1327(1)
1716(1)
2450(1)
2769(1)
3463(1)
3463(1)
39(1)
41(1)
42(1)
45(1)
48(1)
48(1)
59(1)
55(1)
51(1)
50(1)
56(1)
64(1)
52(1)
64(1)
34(1)
38(1)
36(1)
31(1)
35(1)
36(1)
37(1)
44(1)
44(1)
43(1)
63(1)
70(1)
67(1)
57(1)
41(1)
39(1)
39(1)
C(8A)
C(9)
C(9A)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
N(17)
O(17)
C(18)
C(19)
C(20)
C(21)
C(22)
C(23)
N(24)
O(24)
Note: Ueq is defined as one third of the trace of the orthogonalized Uij tensor.
sensitive functional groups. We found that Mn(OAc)3 can
also be used with 1a–c to give the corresponding boron-
containing benzoxazoles 3a–c in low to moderate yield. A
proposed mechanism for the manganese promoted intra-
molecular cyclization of Schiff bases 1a–c is shown in
Scheme 4 (61, 62). An alternate mechanism exists whereby
initial coordination of the Schiff base to the metal centre oc-
curs via the imine nitrogen atom followed by nucleophilic
attack of the oxygen atom to the imine carbon. The forma-
tion of these heterocycles can be monitored by the disap-
pearance of the aldimine N=CH functionality in the 1H
NMR spectra. The 11B NMR spectra show a broad peak at
31 ppm for the three-coordinate boron atom. The molecular
structure of 3c is shown in Fig. 3 and crystallographic data
provided in Table 1. Selected bond distances and angles are
shown in Table 5, and atomic coordinates provided in Ta-
ble 6. Bond distances are similar to those observed for 1b
and 2 and once again no considerable intermolecular interac-
tion is observed between the imine nitrogen and the boron
atom. As expected, the imine C(16)=N(24) bond distance
(1.341(1) Å) is somewhat shorter than the C(23)—N(24)
distance (1.393(1) Å). Unfortunately, attempts to generate
the 2-Bpin derivative 3a using either 1a or 2 gave only mi-
nor amounts of the desired product (by 1H NMR
spectroscopy) along with a number of unidentified products.
Experimental
General
All reagents and solvents used were obtained from
Aldrich. NMR spectra were recorded on a JEOL JNM-
1
GSX270 FT NMR spectrometer. H NMR chemical shifts
are reported in ppm and referenced to residual protons in
deuterated solvent at 270.1 MHz. 11B{1H} NMR chemical
shifts are referenced to external F3B·OEt2 at 86.6 MHz.
13C{1H} NMR chemical shifts are referenced to solvent car-
bon resonances as internal standards at 67.8 MHz. Multiplic-
ities are reported as singlet (s), doublet (d), triplet (t), quartet
(q), multiplet (m), broad (br), and overlapping (ov). Infrared
spectra were obtained using a Mattson Genesis II FT-IR
spectrometer and reported in cm–1. Melting points were
measured uncorrected with a Mel-Temp apparatus. Micro-
analyses for C, H, and N were carried out at Desert Analyt-
ics (Tucson, AZ). The synthesis of pinacolated derivatives of
formylphenylboronic acids has been described elsewhere
© 2002 NRC Canada

38
Can. J. Chem. Vol. 80, 2002
(25). All reactions were carried out in the air and products
are stable indefinitely under such conditions.
Synthesis of 2
2-Formylphenylboronic acid (253 mg, 1.69 mmol) was
added to a solution of 2-aminophenol (184 mg, 1.69 mmol)
in anhydrous ethanol (15 mL). The clear yellow solution was
heated at reflux for 3 h whereupon the solution was allowed
to cool to room temperature. After 12 h a bright yellow pre-
cipitate formed, which was filtered, washed with hexane
(3 × 5 mL), and dried under vacuum to give 2. Yield:
226 mg (62%) of a bright yellow solid; mp 180°C (dec). IR
(Nujol) (cm–1): 2912, 2857, 1628, 1460, 1375, 1270, 1164.
1H NMR (CDCl3) d: 8.71 (s, 2H, CH=N), 7.54 (d, J = 8 Hz,
2H, Ar), 7.43 (m, 2H, Ar), 7.25 (ov m, 8H, Ar), 6.95 (m,
4H, Ar). 11B NMR d: 7 (br). 13C NMR d: 160.1 (C=N),
158.4, 135.9, 134.5, 133.4, 133.0, 132.7, 132.1, 130 (br, C-
B), 128.0, 118.7, 115.7, 114.4. Anal. calcd. for
C26H18B2N2O3: C 72.95, H 4.24, N 6.54; found: C 72.49,
H 4.20, N 6.37.
Synthesis of 1a
2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)benzaldehyde
(2-FPBpin) (524 mg, 2.27 mmol) was added to a solution of
2-aminophenol (248 mg, 2.27 mmol) in THF (15 mL) and
magnesium sulfate (10.0 g). After four days at room temper-
ature the yellow solution was concentrated to 5 mL and
stored at 0°C for three days. A brown precipitate was fil-
tered, washed with diethyl ether (3 × 10 mL), and dried un-
der vacuum to give 1a. Yield: 429 mg (59%) of a brown
solid; mp 120–122°C. IR (Nujol) (cm–1): 3413, 3049, 2918,
2858, 1699, 1620, 1587, 1562, 1481, 1462, 1375, 1346,
1
1317. H NMR (CDCl3) d: 9.04 (s, 1H, CH=N), 7.95 (d, J =
8 Hz, 1H, Ar), 7.78 (d, J = 8 Hz, 1H, Ar), 7.46 (m, 2H, Ar),
7.18 (m, 2H, Ar), 7.02 (d, J = 8 Hz, 1H, Ar), 6.92 (ov d of d,
J = 8 Hz, 1H, Ar), 1.34 (s, 12H, pin). 11B NMR d: 30 (br).
13C NMR d: 161.2 (C=N), 152.0, 140.1, 136.0, 134.4, 133
(br, C-B), 131.0, 130.2, 128.7, 127.9, 120.1, 117.5, 115.4,
83.7 (BOC), 24.9 (BOCCH3).
Reaction of 1a with Mn(OAc)3
Manganese triacetate (934 mg, 3.48 mmol) was added to a
solution of 1a (563 mg, 1.74 mmol) in toluene (20 mL) and
the dark brown solution was heated at reflux for 2 h. The
precipitated manganese salts were removed by suction filtra-
tion and the toluene was removed under vacuum to afford a
Synthesis of 1b
1
3-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)benzaldehyde
(3-FPBpin) (508 mg, 2.16 mmol) was added to a solution of
2-aminophenol (238 mg, 2.16 mmol) in anhydrous ethanol
(10 mL). The clear yellow solution was heated at reflux for
1 h, after which the reaction mixture was stored at 5°C for
2 days. The resultant precipitate was filtered, washed with
hexane (2 × 10 mL), and dried under vacuum to give 1b.
Yield: 609 mg (87%) of a pale yellow solid; mp 116–117°C.
IR (Nujol) (cm–1): 3408, 2924, 2856, 1626, 1462, 1365,
dark brown solid. The mixture was analyzed by H NMR
spectroscopy and showed the presence of minor amounts of
3a along with a number of unidentified products. Attempts
to isolate 3a proved unsuccessful.
Synthesis of 3b
Manganese triacetate (1.36 g, 5.07 mmol) was added to a
solution of 1b (819 mg, 2.54 mmol) in toluene (20 mL) and
the dark brown solution was heated at reflux for 2 h. The
precipitated manganese salts were removed by suction filtra-
tion and the toluene was removed under vacuum to afford a
dark red solid. The solid was dissolved in hexane (25 mL),
filtered through alumina, and allowed to crystallize at 5°C
over 72 h. The resultant solid was filtered, washed with cold
hexane (3 × 5 mL), and dried to give 3b. Yield: 314 mg (39%)
of a red-brown solid; mp 131–132°C. IR (Nujol) (cm–1): 2933,
1
1252, 1140. H NMR (CDCl3) d: 8.70 (s, 1H, CH=N), 8.30
(s, 1H, Ar), 8.05 (d, J = 8 Hz, 1H, Ar), 7.95 (d, J = 8 Hz,
1H, Ar), 7.51 (ov d of d, J = 8 Hz, 1H, Ar), 7.29 (d, J =
8 Hz, 1H, Ar), 7.22 (ov d of d, J = 8 Hz, 1H, Ar), 7.03 (d,
J = 8 Hz, 1H, Ar), 6.93 (ov d of d, J = 8 Hz, 1H, Ar), 1.37
(s, 12H, pin). 11B NMR d: 32 (br). 13C NMR d: 157.2 (C=N),
152.3, 138.0, 135.7, 135.5, 135.1, 131.0, 130 (br, C-B),
128.8, 128.3, 120.1, 115.8, 115.0, 84.1 (BOC), 24.8
(BOCCH3).
1
2914, 2856, 2360, 1734, 1587, 1456, 1409, 1367, 1323. H
NMR (CDCl3) d: 8.70 (s, 1H, Ar), 8.34 (d, J = 8 Hz, 1H,
Ar), 7.95 (d, J = 8 Hz, 1H, Ar), 7.76 (ov m, 1H, Ar), 7.57–
7.53 (ov m, 2H, Ar), 7.35 (ov m, 2H, Ar), 1.37 (s, 12H, pin).
11B NMR d: 31 (br). 13C NMR d: 163.2 (C=N), 150.9, 142.2,
137.8, 134.0, 133 (br, C-B), 130.3, 128.4, 126.6, 125.1,
124.6, 120.1, 110.7, 84.2 (BOC), 25.0 (BOCCH3). Anal.
calcd. for C19H20BNO3: C 71.04, H 6.29, N 4.36; found:
C 71.24, H 6.18, N 4.59.
Synthesis of 1c
4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)benzaldehyde
(4-FPBpin) (1.01 g, 4.33 mmol) was added to a solution of
2-aminophenol (477 mg, 4.33 mmol) in anhydrous ethanol
(25 mL) whereupon the solution was heated at reflux for
2 h. After 12 h at room temperature a bright yellow solid
formed from the reaction mixture. The precipitate was fil-
tered, washed with hexane (2 × 10 mL), and dried under
vacuum to give 1c. Yield: 1.27 g (91%) of a bright yellow
solid; mp 114–115°C. IR (Nujol) (cm–1): 3371, 2927, 2858,
1622, 1585, 1510, 1462, 1360, 1288, 1234, 1167, 1144,
Synthesis of 3c
Manganese triacetate (934 mg, 3.48 mmol) was added to a
solution of 1c (563 mg, 1.74 mmol) in toluene (20 mL) and
the dark brown solution was heated at reflux for 2 h. The
precipitated manganese salts were removed by suction filtra-
tion and the toluene was removed under vacuum to afford a
dark brown solid. The solid was dissolved in hexane
(25 mL), filtered through alumina, and allowed to crystallize
at 5°C over 72 h. The resultant solid was filtered, washed
with cold hexane (3 × 5 mL), and dried to give 3c. Yield:
157 mg (28%) of a pale brown solid; mp 186–188°C. IR
1
1086. H NMR (CDCl3) d: 8.64 (s, 1H, CH=N), 7.93 (ov m,
4H, Ar), 7.28 (d, J = 8 Hz, 1H, Ar), 7.20 (ov d of d, J =
8 Hz, 1H, Ar), 7.02 (d, J = 8 Hz, 1H, Ar), 6.90 (t, J = 8 Hz,
1H, Ar), 1.35 (s, 12H, pin). 11B NMR d: 31 (br). 13C NMR
d: 156.9 (C=N), 152.4, 137.9, 135.3, 135.0, 133 (br, C-B),
129.0, 127.8, 120.0, 115.8, 115.0, 84.0 (BOC), 24.8
(BOCCH3).
© 2002 NRC Canada

Norman et al.
39
(Nujol) (cm–1): 2964, 2891, 1732, 1709, 1604, 1570, 1545,
1454, 1362, 1268, 1244, 1140, 1092. H NMR (CDCl3) d:
References
1
8.28 (d, J = 8 Hz, 2H, Ar), 7.98 (d, J = 8 Hz, 2H, Ar), 7.79
(ov m, 1H, Ar), 7.60 (ov m, 1H, Ar), 7.37 (ov m, 2H, Ar),
1.38 (s, 12H, pin). 11B NMR d: 31 (br). 13C NMR d: 163.1
(C=N), 150.9, 142.2, 135.3, 132 (br, C-B), 129.4, 126.8,
125.4, 124.7, 120.2, 110.7, 84.2 (BOC), 25.0 (BOCCH3).
Anal. calcd. for C19H20BNO3: C 71.04, H 6.29, N 4.36;
found: C 71.38, H 6.39, N 4.54.
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with a scan width of 0.3ꢆ and 30 s exposure times. The de-
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(26) and corrected for absorption (SADABS) (27). The
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Conclusion
We have prepared new Schiff bases containing boronate
ester groups via condensation reactions with pinacolated
formylphenylboronic acids (FPBpin) and 2-aminophenol.
Reactions of the 2-FPBpin derivative gave a novel dimer
containing a seven-membered ring with a BOB bridge. The
aldimine functionality in this dimer is stabilized by an
intramolecular NJB interaction. Addition of metals to the
3- and 4-Bpin Schiff-bases lead to the formation of boron-
containing benzoxazoles which could be isolated in moder-
ate yield. The use of Mn(OAc)3 for these reactions provides
a gentle and efficient route to making benzoxazoles that
have the potential to be readily functionalized using Suzuki–
Miyaura cross-coupling reactions.
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Acknowledgments
Thanks are gratefully extended to the Heart and Stroke
Foundation of New Brunswick, the Research Corporation
(Cottrell College Science Award), the University of New
Brunswick, and Mount Allison University for financial sup-
port. We also wish to thank Dan Durant for his expert tech-
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