Fixed stereochemical control in the synthesis of new mono- and disubstituted 2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononenes

Article abstract of DOI:10.1139/v02-109

Five new boronates of the type 2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononenes (6a-e) were prepared from substituted 2-[(2-hydroxyethylamino)methyl]phenols (4a-e) and phenylboronic acid (5) in benzene-EtOH (4:1) mixtures. Tridentate ligands 4a-e and boronates 6a-e were characterized by ¹H, ¹³C, ¹⁵N, and 2D-NMR (HETCOR, NOESY, and COLOC) experiments, FT-IR, mass spectra, and elemental analysis, as well as ¹¹B NMR for the boron derivatives. Suitable monocrystals of 2-[(2-hydroxyethylamino)methyl]phenol hydrochloride (4a), cis-2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononene (6a), (2S,5R,6S)-2,5-diphenyl-6-aza-1,3-dioxa-2-borabenzocyclononene (6b), and (2S,4R,5R,6S)-2,4-diphenyl-5-methyl-6-aza-1,3-dioxa-2-borabenzocyclononene (6e) were obtained and their structures are discussed. The X-ray structures of 6a, 6b, and 6e, as well as the NMR data established that the configurations at the nitrogen and boron atoms are both "S" and the transannular fusion is cis. A semi-empirical (SAM1) study was used to calculate the energy for all possible stereoisomers, showing that the stabilization increases as the THC (tetrahedral character of the boron atom) increases and also as the N→B bond distance decreases, in agreement with the experimental results and previous work related to amino acid boronates.

Full text of DOI:10.1139/v02-109

801
Fixed stereochemical control in the synthesis of
new mono- and disubstituted 2-phenyl-6-aza-1,3-
dioxa-2-borabenzocyclononenes
Hiram I. Beltrán, S. Jesús Alas, Rosa Santillan, and Norberto Farfán
Abstract: Five new boronates of the type 2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononenes (6a–e) were prepared
from substituted 2-[(2-hydroxyethylamino)methyl]phenols (4a–e) and phenylboronic acid (5) in benzene–EtOH (4:1)
1
mixtures. Tridentate ligands 4a–e and boronates 6a–e were characterized by H, 13C, 15N, and 2D-NMR (HETCOR,
NOESY, and COLOC) experiments, FT-IR, mass spectra, and elemental analysis, as well as 11B NMR for the boron
derivatives. Suitable monocrystals of 2-[(2-hydroxyethylamino)methyl]phenol hydrochloride (4a), cis-2-phenyl-6-aza-1,3-
dioxa-2-borabenzocyclononene (6a), (2S,5R,6S)-2,5-diphenyl-6-aza-1,3-dioxa-2-borabenzocyclononene (6b), and
(2S,4R,5R,6S)-2,4-diphenyl-5-methyl-6-aza-1,3-dioxa-2-borabenzocyclononene (6e) were obtained and their structures
are discussed. The X-ray structures of 6a, 6b, and 6e, as well as the NMR data established that the configurations at
the nitrogen and boron atoms are both “S” and the transannular fusion is cis. A semi-empirical (SAM1) study was used
to calculate the energy for all possible stereoisomers, showing that the stabilization increases as the THC (tetrahedral
character of the boron atom) increases and also as the N→B bond distance decreases, in agreement with the experi-
mental results and previous work related to amino acid boronates.
Key words: boron compounds, X-ray structures, NMR, boronates, synthesis.
Résumé : On a préparé cinq nouveaux boronates du type 2-phényl-6-aza-1,3-dioxa-2-borabenzocyclononènes (6a–e) en
faisant réagit des 2-[(2-hydroxyéthylamino)méthyl]phénols substitués (4a–e) avec de l’acide phénylboronique (5) dans
des mélanges 4:1 de benzène–éthanol. Les ligands tridentates (4a–e) et les boronates (6a–e) ont été caractérisés par les
1
analyses élémentaires, par infrarouge à transformée de Fourier, par des expériences de RMN du H, du 13C, du 15N et
par RMN-2D (HETCOR, NOESY et COLOC) ainsi que par RMN du 11B pour les dérivés du bore. On a obtenu des
monocristaux appropriés du chlorhydrate du 2-[(2-hydroxyéthylamino)méthyl]phénol (4a), du cis-2-phényl-6-aza-1,3-
dioxa-2-borabenzocyclononène (6a), du (2S,5R,6S)-2,5-diphényl-6-aza-1,3-dioxa-2-borabenzocyclononène (6b) et du
(2S,4R,5R,6S)-2,4-diphényl-5-méthyl-6-aza-1,3-dioxo-2-borabenzo-cyclononène (6e) et on discute de leurs structures.
Les structures des composés 6a, 6b et 6e déterminées par diffraction des rayons-X ainsi que les données de la RMN
ont permis d’établir que les configurations au niveau des atomes d’azote et du bore sont toutes les deux S et que la fu-
sion transannulaire est cis. On a fait appel à des calculs semi-empiriques (SAM1) pour calculer l’énergie des tous les
stéréoisomères possibles; ils montrent que la stabilisation augmente avec une augmentation du caractère tétraédrique de
l’atome de bore ainsi qu’avec une diminution de la longueur de la distance N→B, en accord avec les données
expérimentales obtenues ici et avec celles obtenues dans un travail antérieur sur des boronates apparentés d’acides aminés.
Mots clés : composés du bore, structures par diffraction des rayons X, RMN, boronates, synthèse.
[Traduit par la Rédaction] Beltrán et al. 812
Introduction
all cases, the boron atom is tetracoordinated and forms a
coordinative bond with the nitrogen atom (6), which is re-
sponsible for the self-assembly of the macrocycles, whereby
the oxygens build up five- or six-membered rings as well as
intermolecular boron–oxygen bonds. Moreover, the reaction
product is mainly dependent on the structure of the ligand
and, to a lesser extent, on the steric and electronic factors.
The use of a tridentate ligand such as 2,6-pyridinedi-
methanol lead to a tetrameric compound containing four 5-
membered- and one 20-membered ring by self-assembly (1).
The stabilization of all these compounds is due to tetraco-
ordination of the boron atom.
In previous studies we described the syntheses of a series
of mono-, di-, tri-, and tetrameric boronates derived from
tridentate ligands (1–8). These ligands possess one nitrogen
and two oxygen donor atoms with several variations in the
main skeleton, which allowed for the evaluation of the steric
and electronic effects (Fig. 1). The studies showed that, in
Received 26 January 2002. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 28 June 2002.
H.I. Beltrán, S.J. Alas, R. Santillan, and N. Farfán.1
Departamento de Química, Centro de Investigación y de
Estudios Avanzados del Instituto Politécnico Nacional, Apdo.
Postal 14–740, 07000 México, México, D.F.
It has been reported that the formation of dimeric and
monomeric structures is highly dependent on reaction conditions
as well as the structure of the salicylidenimino alcohol. It is
therefore possible in certain cases to obtain either the
1Corresponding author (e-mail: jfarfan@mail.cinvestav.mx).
Can. J. Chem. 80: 801–812 (2002)
DOI: 10.1139/V02-109
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802
Can. J. Chem. Vol. 80, 2002
Fig. 1. Mono-, di-, tri-, and tetrameric boronates.
R1
H
O
R1
O
R1
O
O B
N
H
H
N
O
Ph
H
B
Ph
N
B
N
O
N
B
O
B
O
O
O
O
B
Ph
O
Ph
B
O
R2
OMe
N
O
N
n
n
R1=H, Me, Ph
R2=Aryl
n=1,2
R1=H, Me
n=3, 4
R1=Alkyl, Aryl
Fig. 2. Formation of monomers and dimers.
Fig. 3. Stereochemistry for the fused rings in amino acid boronates.
R1
H
H
H
H
O
O
H
Me
N
O
N
N
O
O
N
O
B
N
O
B
B
B
B
O
O
O
O
O
R1=Alkyl, Aryl
dimeric or monomeric derivatives in high yields by using ki-
netic or thermodynamic control (Fig. 2) (7). The use of long
reaction times leads to monomeric structures (7, 8). In gen-
eral, the N→B bond distances are between 1.6 and 1.7 Å,
characteristic for dative nitrogen–boron bonds (1–13), and
the boron atom shows a distorted tetrahedral geometry.
Regarding the stereochemistry of these compounds, a lit-
erature search showed preference for cis-fused transannular
N→B bond (8–30). Moreover, the presence of a stereogenic
center in the five-membered ring induces formation of the
cis–cis isomer (8, 12) (Fig. 3).
Continuing in our investigations, we describe herein the
preparation and complete characterization of five new 4,5-
disubstituted-2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononenes
(6a–e) prepared from 2-[(2-hydroxyethylamino)methyl]phe-
nols (4a–e) and phenylboronic acid (2). Some of the phenols
have found application as chelating agents for Cu(II) (31)
and in catalytic enantioselective hydrogenation of ketones
with ruthenium (32). Analogous compounds have been pre-
pared by reduction of the corresponding Schiff base (33) and
hydrogen-bonded structures for these derivatives have been
reported (31, 34).
and (2S,4R,5R,6S)-2,4-diphenyl-5-methyl-6-aza-1,3-dioxa-2-
borabenzocyclononene (6e) were obtained. The theoretical
calculations (SAM1) support that compounds 6a–e are ener-
getically favored and indeed all the products obtained show
cis–cis stereochemistry.
Results and discussion
Synthesis of 2-[(2-hydroxyethylamino)methyl]phenol
hydrochlorides (4a–e)
The synthetic route to prepare 2-[(2-hydroxyethyl-
amino)methyl]phenols 4a–e involved the reaction of β-
aminoalcohols 1a–e with an equimolecular ratio of salicyl-
aldehyde 2 under reflux of THF–MeOH (1:1) for 4 h to give
corresponding Schiff bases 3a–e, followed by reduction with
sodium borohydride (Scheme 1). The products were ob-
tained in moderate to high yields.
Synthesis of 4,5-disubstituted 2-phenyl-6-aza-1,3-dioxa-
2-borabenzocyclononenes (6a–e)
The condensation of 2-[(2-hydroxyethylamino)methyl]phen-
ol hydrochlorides 4a–e with phenylboronic acid 5 in the
presence of an excess of sodium bicarbonate was carried out
at reflux in benzene–EtOH mixtures to yield the correspond-
ing (2S,5S,6R)-2-phenyl-6-aza-1,3-dioxa-4,5-(R,R′)-2-bora-
benzocyclononenes 6a–e in yields from 69 to 89%. These
tricyclic derivatives show transannular N→B bonds with a
tetrahedral geometry for the boron and nitrogen atoms (35)
as well as five- and six-membered fused rings. In all cases,
the configurations at positions 2, 5, and 6 are S, R, and S, re-
spectively (Scheme 1).
The aim of this work is to investigate the influence of ni-
trogen hybridization and the acidity of the amino alcohol to
selectively form monomeric or dimeric boronates, since in-
creased flexibility in the ligand should lead to release of
strain at the fused bicyclic structure. In addition, it was de-
sirable to establish the stereochemistry of these new
boronates, which possess two fixed chiral atoms (nitrogen
and boron) and to compare the theoretical and experimental
results.
Tridentate ligands 4a–e and monomeric boronates 6a–e
were characterized by 1H, 13C, 15N, and 2D-NMR
(HETCOR, NOESY, and COLOC) experiments, FT-IR, mass
spectra, and additionally for boronates, the 11B NMR spectra
were recorded. Suitable monocrystals of 2-[(2-hydroxy-
ethylamino)methyl]phenol hydrochloride (4a), cis-2-phenyl-
6-aza-1,3-dioxa-2-borabenzocyclononene (6a), (2S,5R,6S)-
2,5-diphenyl-6-aza-1,3-dioxa-2-borabenzocyclononene (6b)
Spectroscopic data
The 1H NMR data of 2-[(2-hydroxyethylamino)methyl]
phenol hydrochlorides 4a–e is summarized in Table 1; 13C
and 15N NMR are given in Table 2. For 4,5-disubstituted-2-
phenyl-6-aza-1,3-dioxa-2-borabenzocyclononenes 6a–e, the
© 2002 NRC Canada

Beltrán et al.
803
Scheme 1. Synthetic route for the preparation of 4,5-disubstituted-2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononenes (6a–e).
R2
R2
R3
R1
R3
R1
R1
O
R2
R3
H
1
+ Cl-
2
1)NaBH /1h.
2)HCl/10min.
H
THF/MeOH
4hr.
HO
N
10
HO
N
5
4
+
H
HO
NH2
9
H
HO
HO
6
HO
8
7
2
1
3
R1
R2
R3
H
H
H
H
H
4a
4b
4c
4d
4e
13
12
11
H
14
12
13
13
15
11
11
14
Me
17
16
12
15
H
14
18
13
12
15
11
14
Me
H
R2
R1
R3
R2
H
R3
+ Cl-
R1
1
2
HO
B
1)NaHCO3
H
4
O
11
+
N
HO
N
3
2)C6H6/EtOH
12h.
B
HO
H
5
5
O
10
9
HO
14
6
12
13
4a-4e
7
8
R1
R2
R3
H
H
H
6a
6b
17
16
15
H
H
H
18
16
17
17
19
15
15
18
18
6c
6d
6e
Me
21
20
16
19
H
22
17
16
15
15
18
Me
H
1H and 13C NMR data are given in Tables 3 and 4, respec-
tively. Table 5 summarizes the 15N and 11B NMR data.
constants of 13.2 Hz; for the remaining compounds these
protons give rise to broad signals. Hydrogens in positions 1
and 2 are shifted according to substitution; for example, the
signals for 4b give an ABX system for protons H-1a, H-1b,
and H-2 with a geminal coupling constant of 11.4 Hz and
AX and BX couplings of 5.1 and 7.0 Hz, respectively.
The 13C NMR spectra show the following trends: the reso-
nance for C-6 always appears at higher frequencies (156.6
and 155.7 ppm) followed by C-10 (132.1–131.4 ppm), and
C-8 (131.2–130.2 ppm); C-9 shows chemical shifts between
120.4 and 119.0 ppm, C-5 between 118.6 and 116.5 ppm,
and C-7 is the most shielded at 116.6 to 115.4 ppm. The
chemical shifts for C-1 and C-2 depend on substitution: a
phenyl group at position 1 shifts the signal to higher
frequencies (∆δ = 18.2 and 13.5 ppm, respectively) and a
NMR data of 2-[(2-hydroxyethylamino)methyl]phenol
hydrochlorides (4a–e)
The 1H NMR spectra show the phenolic OH between
10.42 and 9.68 ppm, followed by the nitrogen protons (H-3a
and H-3b) in the range from 10.10 to 8.05 ppm — these pro-
tons are diastereotopic in 4b–e and the proton antiperiplanar
to the substituent is shifted to higher frequencies and labeled
as H-3a. The salicylidene fragment shows an ABCD system,
with H-10 observed as a double of doublets at higher fre-
quency, followed by H-8 (ddd or t), H-7 (ddd), and H-9 (ddd
or t). The protons in position 4 are diastereotopic for com-
pounds 4b and 4c, showing an AB system with coupling
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Beltrán et al.
805
Table 2. 13C and 15N NMR data of 2-[(2-hydroxyethylamino)methyl]phenol hydrochlorides (4a–e).
Compd
C-1
C-2
C-4
C-5
C-6
C-7
C-8
C-9
C-10
15N
4a
56.3
62.5
69.8
70.6
74.5
48.4
63.8
58.5
66.6
58.7
45.0
44.5
43.3
44.1
46.0
118.0
118.4
118.6
118.0
116.5
156.2
156.5
156.6
156.1
155.7
115.5
115.8
116.1
115.4
116.6
130.3
130.7
130.7
130.2
131.2
119.0
119.4
119.5
119.0
120.4
131.7
131.9
132.1
131.5
131.4
–335.2
–326.1
–324.0
–324.3
–327.4
4ba
4cb
4dc
4ed
Note: 13C NMR at 100 MHz, 15N NMR at 40.515 MHz. Solvent: DMSO-d6 (ꢂ in ppm).
a4b: 134.0 (C-11), 129.2 (C-12), 129.2 (C-13), 129.4 (C-14).
b4c: 141.7 (C-11), 126.2 (C-12), 128.6 (C-13), 127.7 (C-14), 9.8 (C-15).
c4d: 131.1 (C-11), 129.9 (C-12), 127.7 (C-13), 128.6 (C-14), 140.6 (C-16), 126.0 (C-16), 127.7 (C-17), 127.2 (C-18).
d4e: 139.5 (C-11), 127.2 (C-12), 128.8 (C-13), 128.7 (C-14), 12.1 (C-15).
and there is an extra anisotropic contribution (4.32 ppm).
For compound 6a, the couplings for these signals were diffi-
cult to assign and selective decoupling experiments were
carried out. Hydrogens at the 3 and 4 positions appear as an
ABC system, with an additional coupling between H-4a and
H-3.
Fig. 4. Molecular structure of 4a.
The difference in chemical shifts with respect to the
Schiff bases is approximately 1 ppm and the most shielded
signal is C-6. The ipso (to boron) carbon (C-11) is observed
in the range between 146.2 and 144.6 ppm, followed by the
ortho (C-12), meta (C-13), and para carbons (C-14), which
were assigned using HETCOR experiments.
The 15N NMR chemical shifts of 6a–e (–331.8 to –312.1)
and those of tridentate ligands 4a–e (Table 2), are in the
characteristic range for tetracoordinated nitrogen atoms (36).
In 11B NMR, the signals are observed from 5.1 to 6.0 ppm,
characteristic for tetracoordinated boron atoms (37).
H1n_3
Cl1_6
H2o_5
IR and MS data of 4a–e and 6a–e
The IR spectra of compounds 4a–e show strong N–H and
O–H stretching bands, while the IR spectra of compounds
6a–e show only the N–H band. The mass spectra of 4a–e
show the molecular ion peak, along with the loss of hydro-
chloric acid, and the hydroxytropylium and cycloheptatri-
enone ions. In most cases M+, [M – C6H5]+, and [M – C6H6]+
are observed.
H1o_6
similar behavior is observed for C-2. The shift induced by a
methyl group is less than for phenyl substituents, thus C-4
appears between 46.0 and 43.3 ppm, showing no significant
variations due to substitution.
Molecular structures of 4a, 6a, 6b, and 6e
Since all 15N NMR spectra were determined on HCl de-
rivatives, their chemical shifts are shifted to lower frequen-
cies. Substituents at the ꢀ position show an additive effect
with respect to compound 4a. The 15N chemical shift for 4a
is shifted to lower frequencies with respect to compounds
4b–e, which show ꢁꢂ W 10 ppm.
Crystallographic data for 4a, 6a, 6b, and 6e are listed in
Table 6; selected bond lengths, bond angles, torsion angles,
and plane deviations are summarized in Table 7. The molec-
ular structures are shown in Figs. 4–7.
For 4a all distances are in agreement with reported values
(31, 34). The salicylidene and ꢀ-amino alcohol fragments
have gauche conformations, with C(2)-N(1)-C(3)-C(4) and
O(1)-C(1)-C(2)-N(1) torsion angles of 60.63(21)° and
58.17(22)°, respectively. There are two hydrogen-bonding
interactions in the asymmetric unit, one intramolecular con-
tact between H(1n) and O(1) with a magnitude of 2.359(23) Å
(which is mostly responsible for the gauche conformation in
this fragment), and a second intramolecular contact between
Cl(1) and H(1o) with a value of 2.295(29) Å. There are
also important interactions with the neighboring molecules.
The first is for both hydrogens attached to nitrogen
(H(1n)···Cl(1) = 2.376(25) Å, H(2n)···Cl(1) = 2.241(24) Å),
and the second one between chlorines at opposite sides of
the molecule and the phenolic hydrogen (H(2o)···Cl(1) =
2.202(33) Å).
NMR data of 4,5-disubstituted 2-phenyl-6-aza-1,3-dioxa-
2-borabenzocyclononenes (6a–e)
The H NMR data for compounds 6a–e show that H-12 is
1
the most deshielded signal (7.72–7.48 ppm), followed by H-
13 and H-14 (7.34 and 7.20 ppm, respectively). The chemi-
cal shift for the benzylidene fragment and the hydrogens at
positions 1, 2, and 4 are very similar to those of the Schiff
bases, while that of H-3 depends on the anisotropy of the
vicinal group (7.29 (6a), 7.27 (6b), 6.87 ppm (6c)). The ex-
istence of two phenyl groups in 6d shifts the signal to
4.07 ppm. In 6e, methyl substitution at the opposite side of
H-3 modifies the conformation at the five-membered ring in
such a way that both phenyl groups affect its environment
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Beltrán et al.
807
Table 4. 13C NMR data of 2-phenyl-6-aza-1,3-dioxa-4,5-(R,R′)-2-borabenzocyclononenes (6a–e).
Compd
C-1
C-2
C-4
C-5
C-6
C-7
C-8
C-9
C-10
C-11
C-12
C-13
C-14
6a
62.1
68.9
77.4
78.7
82.4
48.1
63.0
59.1
68.0
61.1
46.7
44.8
46.1
46.3
45.1
119.8
118.5
118.8
117.5
117.9
157.5
156.8
157.6
157.1
156.2
118.5
118.7
118.2
118.4
119.4
130.0
129.9
128.6
129.8
127.9
118.8
119.0
118.6
118.8
119.2
128.8
128.9
129.7
128.8
128.1
146.2
145.7
145.9
145.3
144.6
132.7
133.0
132.7
133.0
132.3
127.4
127.4
127.5
127.6
127.9
127.2
127.3
127.3
127.6
130.3
6ba
6cb
6dc
6ed
Note: 13C NMR at 100.535 MHz. Solvent: DMSO-d6 (δ in ppm).
a6b: 134.6 (C-15), 129.2 (C-16), 129.2 (C-17), 129.2 (C-18).
b6c: C141.7 (C-15), 127.2 (C-16), 128.3 (C-17), 127.5 (C-18), 14.4 (C-19).
c6d: 134.7 (C-15), 129.1 (C-16), 128.3 (C-17), 128.1 (C-18), 141.1 (C-19), 127.2 (C-20), 127.8 (C-21), 127.2 (C-22).
dIn CDCl3. 6e: 141.1 (C-15), 126.9 (C-16), 128.4 (C-17), 128.0 (C-18), 13.2 (C-19).
Table 5. 15N and 11B NMR data for 4,5-disubstituted-2-phenyl-6-
aza-1,3-dioxa-2-borabenzocyclononenes (6a–e).
Fig. 5. Molecular structures of 6a and 6b.
Compd
15N
11B
6aa
6ba
6cb
6db
6ec
–331.8
–318.8
–320.7
–324.0
–312.1
5.1
5.7
5.3
5.3
6.0
Note: 15N NMR data at 30.423 MHz. 11B NMR data at 128.266 MHz.
Solvent: DMSO-d6 (δ in ppm).
a15N NMR at 40.515 MHz.
b11B NMR at 128.7 MHz.
6a
6b
cIn CDCl3.
With respect to the boronates, compounds 6a and 6b are
shown in Fig. 5, while compound 6e is shown in Fig. 6.
Compound 6e crystallized with a molecule of benzene and
there are three different molecules in the unit cell. The con-
cluding remarks of the obtained X-ray structures are as fol-
lows: (i) the N→B distances found are 1.647(2) Å for 6a,
1.635(3) Å for 6b, and 1.615(6), 1.643(3), 1.620(6) Å for
6e, in agreement with the distances found in similar deriva-
tives described in the literature (1–30). (ii) The B—Oaliphatic
distances are 1.465(2) Å for 6a, 1.455(3) Å for 6b, and
1.447(6), 1.454(6), 1.457(5) Å for 6,which are also in agree-
ment with values found for similar compounds (7, 8).
(iii) The B—Ophenolic distances are 1.462(2) Å for 6a,
1.463(3) Å for 6b, and 1.475(6), 1.459(6), 1.472(6) Å for
6e, in agreement with the literature values for the same frag-
ment (8). The structures possess similar dihedral angles for
the H-N(1)→B(1)-C(10) fragment; the differences are due to
the nature of the R3 substituent in position 2 and, to a lesser
extent, to the R1 and R2 substituents in position 1, resulting
in dihedral angles of 18.16(126)° for 6a, –38.72(172)° for
6b, and −47.60(47)°, −48.79(44)°, −49.85(45)° for 6e. Ad-
ditionally, there are intermolecular hydrogen bonds between
the NH proton and the aliphatic oxygen O(1) with magni-
tudes of 1.906(20) Å for 6a, 2.256(24) Å for 6b, and
2.313(3) and 2.327(3) Å for 6e. In all cases, the six-
membered ring containing the B-N-C(3)-C(4)-C(5)-O(3)
fragment has a twisted boat conformation, the five-
membered ring formed by the B-N-C(2)-C(1)-O(1) moiety
(6a and 6b) are in the half-chair conformation, with C(1)
and C(2) out of the plane, and the three molecules in 6e have
envelope conformations with N(1) out of the plane.
Fig. 6. Unit cell for compound 6e.
The tetrahedral character (THC) of the boron atom (38) is
a useful parameter to evaluate the geometry of the boron
atom for crystalline boronates (6a, 6b, and 6e). The THC
was calculated from the boron-bond angles obtained by X-
ray analysis as described by Höpfl (38), whereby a value of
100% corresponds to a perfect tetrahedral structure for bo-
ron. The values of THC (%) are 80.13 for 6a, 76.86 for 6b,
and 74.44, 70.08, and 71.39 for 6e. It is important to men-
tion that the lower bond angle measured corresponds to the
O(1)-B(1)-N(1) angle with THC values of 100.37(14)° for
6a, 101.23(19)° for 6b, and 101.1(4)°, 99.64(28)°, and
99.89(29)° for 6e. This can be attributed to ring fusion.
© 2002 NRC Canada

808
Can. J. Chem. Vol. 80, 2002
Table 6. Crystallographic data of 4a, 6a, 6b, and 6e.
Compd
4a
6a
6b
6e
C9H14ClNO2
203.66
Orthorhombic
Pcab
C15H16BNO2
253.10
Monoclinic
P21/n
C21H20BNO2
329.19
Orthorhombic
P212121
C24H24BNO2·1/3C6H6
369.25
Orthorhombic
P212121
Molecular formula
Molecular Weight
Crystal Lattice
Space Group
Cell Dimensions
a (Å)
b (Å)
c (Å)
β (°)
Volume (Å3)
Z
7.7036(2)
15.8161(4)
16.9502(5)
90
2065.23(10)
8
8.639(5)
9.038(5)
16.745(5)
91.061(5)
1307.2(11)
4
9.149(19)
10.885(5)
18.133(2)
90
1805.9(42)
4
13.255(5)
18.261(5)
26.097(5)
90
6317.0(30)
12
Dcalcd. (g m–3)
µ (mm–1)
Parameters
Collected reflections
Independent reflections
R (int.)
1.310
0.339
174
4362
2363
0.0261
1.073
1.286
0.084
237
2454
2293
0.0206
1.024
1.211
0.076
280
3520
3150
0.0762
1.048
1.165
0.073
827
6822
6822
0.0000
1.135
GoF
R
Rw
0.0378
0.1111
0.0378
0.0779
0.0369
0.0970
0.0514
0.1218
1/2
Note: R = Σ**Fo*-*Fc**/Σ*Fo*. Rw(Fo)2 = [Σw(F 2o – F 2c )2/ΣwF 4o
] .
Table 7. Selected distances, angles, and plane deviation for 6a, 6b, and 6e.
6e
Compd
6a
6b
Molecule 1
Molecule 2
Molecule 3
Bond distances (Å)
N(1)—B(1)
C(10)—B(1)
B(1)—O(1)
B(1)—O(2)
1.647(2)
1.604(3)
1.465(2)
1.462(2)
0.899(20)
1.635(3)
1.584(4)
1.455(3)
1.463(3)
0.819(23)
1.615(6)
1.589(7)
1.447(6)
1.475(6)
0.910
1.643(3)
1.589(7)
1.454(5)
1.459(6)
0.910
1.620(6)
1.596(7)
1.457(5)
1.472(6)
0.910
N(1)—H(1N)
Bond angles (°)
O(1)-B(1)-O(2)
N(1)-B(1)-C(10)
N(1)-B(1)-O(1)
N(1)-B(1)-O(2)
O(1)-B(1)-C(10)
O(2)-B(1)-C(10)
Torsion angles (°)
H(1N)-N(1)-B(1)-C(10)
N(1)-C(2)-C(1)-O(1)
Plane of deviation (Å)
B(1)-N(1)-C(2)
O(1)
112.06(15)
109.46(14)
100.37(14)
109.39(15)
114.57(16)
110.47(15)
109.0(2)
112.56(18)
101.23(19)
107.4(2)
113.9(2)
112.0(2)
109.1(3)
113.1(3)
101.1(4)
106.4(4)
114.0(4)
112.5(4)
109.95(35)
113.31(35)
99.64(28)
105.25(31)
114.54(35)
113.02(36)
109.88(35)
113.73(35)
99.89(29)
105.68(31)
114.64(35)
112.07(36)
18.16(126)
36.24(20)
–38.72(172)
–37.67(23)
−47.60(47)
–26.04(40)
−48.79(44)
–30.40(37)
−49.85(45)
–26.14(37)
0.2840(45)
0.6912(49)
–0.7222(53)
–0.9686(55)
0.8989(90)
0.9392(97)
–0.9157(84)
–0.9930(88)
–0.9454(87)
–0.9697(88)
C(1)
C(3)-C(4)-C(5)-C(6)-C(7)-C(8)-C(9)-O(2)
B(1)
N(1)
0.8019(26)
0.9649(23)
80.13
–0.1901(40)
–0.6854(30)
76.86
0.1883(70)
0.4482(57)
74.44
–0.4483(63)
0.2050(57)
70.08
0.1559(64)
–0.5134(53)
71.39
THC B (%)
The computational study for semi-empirical calculations
(AM1 method) was performed with Hyperchem (39). A full
set of semi-empirical optimizations was carried out to deter-
mine the main preferences to obtain just the 2(R), N(S), B(S)
Semi-empirical calculations of boronates 6–9
To evaluate the influence of steric factors on this reaction,
the geometry for all possible stereoisomers was determined
by using a theoretical (AM1) approach (Fig. 8).
© 2002 NRC Canada

Beltrán et al.
809
Fig. 7. Molecular structure of 6e.
Fig. 8. Possible stereoisomers relative to the R3 position.
R3
H
R3
H
R
R
R2
N
B
N
R2
S
S
R
1
R
1
R
R
B
O
O
O
O
Ph
Ph
7
6
R3
H
R3
H
R
R
R2
N
B
R2
N
S
R
R
1
R
1
R
S
B
O
O
O
O
Ph
Ph
9
8
1
2
R1
H
H
R2
H
H
R3
H
C6H5
a
b
c
d
e
C6H5
C6H5
H
H
H
CH3
CH3
C6H5
C6H5
tion is highly stereoselective yielding structures with “S”
configuration at both the boron and the nitrogen atoms (6b–e).
This was demonstrated by 2D-NMR NOESY experiments
and confirmed by the X-ray analysis of compounds 6a, 6b,
and 6e. There is an increased tension at the five-membered
ring; hence, the deviation of the boron atom from the ideal-
ized tetrahedral geometry is on average 77.5%. The theoreti-
cal calculations are in good agreement with the experimental
data. These compounds show the same behavior as those
containing tridentate ligands derived from amino acids. THC
is a powerful tool to evaluate the stabilization of these types
of molecules. The results show that the greater the stabiliza-
tion of the compounds, the greater the THC value. In addi-
tion, there is a significant influence on the configuration of
these newly formed stereogenic centers associated with the
N→B bond lengths.
3
products (6a–e); all the stationary points were classified as
minima by Hessian-matrix calculations with the same
method (AM1). The coordinates used to obtain SAM1 re-
sults were obtained from X-ray analyses.
The data for compounds 6a–e, 7a–e, 8a–e, and 9a–e have
been deposited as supplementary material.2 All possible
stereoisomers were minimized. In the case of compound 6a,
which was determined experimentally, there is only one coun-
terpart (compound 8a), while the remaining compounds have
four possible stereoisomers. The data confirm that the com-
pounds obtained experimentally are enthalpically favored, and
there is an important steric effect controlling the formation of
these fused compounds. The energy gap between stereoisomers
6 and 7 is between 5.2 and 3.1 kcal mol–1, and they show two
eclipsed interactions, while stereoisomers 8 and 9 show four
gauche interactions. The deviation of the fused structure (cis or
trans) depends directly on the substituent effect, as evidenced
by the H-N-B-Ph torsion angles.
Experimental
NMR experiments were performed on a Jeol Eclipse+400
spectrometer at room temperature. All 1H and 13C reso-
nances are reported relative to TMS, 15N resonances are rel-
ative to neat MeNO2, 11B spectra were obtained relative to
BF3·OEt2, and all the samples were determined in DMSO-d6
as solvent. The IR spectra were recorded as KBr pellets on a
PerkinElmer 16F PC FT-IR spectrometer. Mass spectra were
recorded on a Hewlett-Packard 59940-A spectrometer at
20 eV electron impact. Melting points were measured in
open capillary tubes on a Gallenkamp MFB 595 apparatus
and have not been corrected. Elemental analyses were deter-
mined on an Eager 300 equipment.
The calculated properties for each stereoisomeric series
show that both the N→B bond length and the energy are in-
versely proportional to the THC values (supplementary ma-
terial).2
Conclusions
Preparation of 2-[(2-hydroxyethylamino)methyl]phenols
(4a–e)
In conclusion, boron compounds derived from the
tridentate ligands 4a–e yield fused five- and six-membered
rings due to N→B bond formation (6a–e). In all cases, the
ring fusion for the H-N→B-Ph fragment is cis, and the reac-
2-[(2-Hydroxyethylamino)methyl] phenol hydrochloride (4a)
Compound 4a was prepared from 0.50 g (8.18 mmol) of
ethanolamine (1a) and 1.00
g
(8.18 mmol) of
2 Supplementary material may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research
Council 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 (CCDC Nos. 176548 for 4a, 176549 for 6a, 176550 for 6b, and 176551 for 6e.). 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, Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2002 NRC Canada

810
Can. J. Chem. Vol. 80, 2002
salicylaldehyde (2) under reflux in a THF–MeOH mixture
for 4 h to give 3a, followed by reduction with 0.62 g
(16.36 mmol) of sodium borohydride, and addition of
3.4 mL (40.90 mmol) of hydrochloric acid, to obtain a white
solid. Suitable crystals were obtained from methanol. Yield
94% (1.57 g, 7.71 mmol); mp 152–154°C. IR νmax
(KBr, cm–1): 3402, 3198, 3082, 2964, 2882, 2842, 2794,
2778, 1594, 1590, 1458, 1440, 1404, 1364, 1260, 1246,
1108, 1076, 922, 762. MS (20 eV) m/z (%): 167 (27), 136
(77), 107 (100), 77 (15). Anal. calcd. for C9H14ClNO2:
C 53.08, H 6.93, N 6.88; found: C 53.17, H 6.97, N 6.84.
Preparation of 4,5-disubstituted 2-phenyl-6-aza-1,3-
dioxa-2-borabenzocyclononenes (6a–e)
cis-2-Phenyl-6-aza-1,3-dioxa-2-borabenzocyclononene (6a)
Compound 6a was prepared by the reaction of 0.50 g
(2.45 mmol) of 4a, 0.30 g (2.45 mmol) phenylboronic acid
(5), and 0.31 g (3.68 mmol) of sodium bicarbonate in a ben-
zene–EtOH mixture for 12 h, to give a white solid; yield
69% (0.43 g, 1.70 mmol). Suitable monocrystals were ob-
tained from a CH2Cl2–hexane mixture (4:1) ; mp 239–
241°C. IR νmax (KBr, cm–1): 3068, 2962, 2944, 2928, 2900,
2870, 2740, 2664, 2620, 2552, 1608, 1488, 1462, 1452,
1432, 1378, 1368, 1292, 1266, 1248, 1216, 1202, 1180,
1112, 1082, 1038, 1016, 994, 982, 958, 938, 874, 822, 760,
738, 704. MS m/z (%): 253 ([M+], 0.9), 222 (5.0), 176 (100),
175 (60.1), 174 (36.6), 147 (11.4), 146 (8.1), 107 (5.4), 91
(5.7), 77 (1.1). Anal. calcd. for C15H16BNO2: C 71.18,
H 6.37, N 5.53; found: C 71.45, H 6.41, N 5.76.
(R)-2-[(2-Hydroxy-1-phenylethylamino)methyl]phenol
hydrochloride (4b)
Compound 4b was prepared from 0.50 g (3.64 mmol) of
(R)-(–)-2-phenylglycinol (1b) and 0.45 g (3.64 mmol) of
salicylaldehyde (2), to obtain a highly hygroscopic yellow
powder. Yield 77% (0.79 g, 2.82 mmol); mp 83–85°C. IR
νmax (NaCl, cm–1): 2928, 1596, 1458, 1420, 1264, 758, 734,
700, 496, 460, 454. MS (20 eV) m/z (%): 212 ([M+], 41),
122 (2), 107 (29), 106 (100), 77 (4). Anal. calcd. for
C15H18ClNO2: C 64.40, H 6.49, N 5.01; found: C 64.25,
H 6.53, N 5.16.
(2S,5R,6S)-2,5-Diphenyl-6-aza-1,3-dioxa-2-borabenzocyclo-
nonene (6b)
Compound 6b was prepared from 0.30 g (1.07 mmol) of
4b, 0.13 g (1.07 mmol) of phenylboronic acid (5), and
0.14 g (1.61 mmol) of sodium bicarbonate, to give 6b as a
brown solid; yield 74% (0.26 g, 0.80 mmol). Suitable
monocrystals were obtained from a CH2Cl2–acetone–hexane
mixture (2:1:4); mp 251–253°C. IR νmax (KBr, cm–1): 3066,
3004, 2954, 2928, 2870, 1608, 1490, 1460, 1448, 1432,
1300, 1286, 1238, 1214, 1186, 1114, 1080, 1062, 1042,
1034, 1004, 994, 970, 946, 872, 754, 740, 700. MS m/z (%):
329 ([M+], 2), 298 (2), 252 (100), 251 (47), 250 (13), 174
(3), 146 (4), 107 (2), 91 (44), 77 (1). Anal. calcd. for
C21H20BNO2: C 76.62, H 6.12, N 4.25; found: C 76.33, H 6.05,
N 4.12.
(1R,2S)-2-[(2-Hydroxy-1-methyl-2-phenylethylamino)methyl]
phenol (4c)
Compound 4c was prepared from 0.50 g (3.30 mmol) of
(1R,2S)-(–)-norephedrine (1c) and 0.40 g (3.30 mmol) of
salicylaldehyde (2), to give the product as a white solid.
Yield 64% (0.62 g, 2.11 mmol); mp 164–166°C. IR νmax
(KBr, cm–1): 3398, 3274, 3072, 2956, 2846, 1598, 1462,
1404, 1352, 1266, 1244, 748, 702. MS m/z (%): 293 ([M+],
0.1), 257 (1.4), 150 (100), 122 (0.6), 107 (46.2), 106 (5.4),
77 (5.6). Anal. calcd. for C16H20ClNO2: C 65.41, H 6.86,
N 4.77; found: C 65.41, H 6.87, N 5.03.
(2S,4S,5R,6S)-2,4-Diphenyl-5-methyl-6-aza-1,3-dioxa-2-
borabenzocyclononene (6c)
Compound 6c was prepared from 0.30 g (1.02 mmol) of
4c, 0.12 g (1.02 mmol) of phenylboronic acid (5), and 0.13 g
(1.53 mmol) of sodium bicarbonate, to give 6c as a white
solid; yield 89% (0.31 g, 1.27 mmol); mp 90–92°C. IR νmax
(KBr, cm–1): 1488, 1462, 1432, 1300, 1284, 1266, 1220,
1112, 1102, 1080, 1042, 1026, 1010, 998, 956, 742, 700.
MS m/z (%): 343 ([M+], 3), 299 (1), 266 (100), 265 (40),
264 (8), 236 (9), 194 (42), 107 (2), 106 (1), 105 (10), 91 (4),
90 (8). Anal. calcd. for C22H22BNO2: C 76.99, H 6.46,
N 4.08; found: C 76.76, H 6.71, N 3.79.
(1S,2R)-2-[(2-Hydroxy-1,2-diphenylethylamino)methyl]
phenol (4d)
Compound 4d was prepared from 0.50 g (2.34 mmol) of
(1S,2R)-(+)-2-amino-1,2-diphenylethanol (1d) and 0.29 g
(2.34 mmol) of salicylaldehyde (2), to afford 4d as a white
powder. Yield 93% (0.78 g, 2.19 mmol); mp 188–190°C. IR
νmax (KBr, cm–1): 3302, 3174, 3064, 3032, 2924, 2850, 1458,
758, 750, 700. MS m/z (%): 319 (0.4), 212 (87.2), 122 (0.9),
107 (32.9), 106 (100), 77 (5.9). Anal. calcd. for C21H22ClNO2:
C 70.88, H 6.23, N 3.94; found: C 70.85, H 6.25, N 3.95.
(2S,4S,5R,6S)-2,4,5-Triphenyl-6-aza-1,3-dioxa-2-
borabenzocyclononene (6d)
(1R,2R)-2-[(2-Hydroxy-1-methyl-2-phenylethylamino)methyl]
phenol (4e)
Compound 6d was prepared from 0.30 g (0.84 mmol) of
4d, 0.10 g (0.84 mmol) of phenylboronic acid (5), and
0.11 g (1.26 mmol) of sodium bicarbonate, to give 6d as a
white solid; yield 80% (0.27 g, 0.67 mmol); mp 204–206°C.
IR νmax (KBr, cm–1): 1490, 1462, 1452, 1300, 1282, 1222,
1208, 1112, 1076, 1054, 1038, 1028, 1010, 960, 744, 724,
698. MS m/z (%): 405 ([M+], 6), 328 (65), 327 (19), 300
(21), 299 (100), 222 (24), 194 (59), 193 (23), 192 (13), 106
(31), 78 (2), 77 (1). Anal. calcd. for C27H24BNO2: C 80.01,
H 5.97, N 3.46; found: C 79.96, H 5.94, N 3.45.
Compound 4e was prepared from 0.50 g (3.30 mmol) of
(1R,2R)-(–)-norpseudoephedrine (1e) and 0.40 g (3.30 mmol)
of salicylaldehyde (2), to give 4e as a white hygroscopic
solid. Yield 71% (0.69 g, 2.35 mmol); mp 50–52°C. IR νmax
(KBr, cm–1): 3190, 2940, 1610, 1598, 1458, 1388, 1352,
1262, 1040, 1024, 758, 702. MS m/z (%): 257 (1.3), 150
(100), 122 (0.7), 107 (53.7), 106 (4.4), 77 (5.7). Anal. calcd.
for C16H20ClNO2: C 65.41, H 6.86, N 4.77; found: C 65.66,
H 6.74, N 4.62.
© 2002 NRC Canada

Beltrán et al.
811
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(2S,4R,5R,6S)-2,4-Diphenyl-5-methyl-6-aza-1,3-dioxa-2-
borabenzocyclononene (6e)
Compound 6e was prepared from 0.30 g (1.02 mmol) of
4e, 0.12 g (1.02 mmol) of phenylboronic acid (5), and 0.13 g
(1.53 mmol) of sodium bicarbonate, to give 6e as a white
solid; yield 87% (0.30 g, 0.88 mmol). Suitable monocrystals
were obtained from a CH2Cl2–acetone–hexane mixture
(2:1:4); mp 160–162°C. IR νmax (KBr, cm–1): 1490, 1286,
1212, 1040, 1026, 1006, 756, 744, 704. MS m/z (%): 343
([M+], 3), 266 (100), 265 (44), 264 (10), 236 (3), 194 (14),
107 (2), 106 (2), 105 (18), 91 (5), 90 (5). Anal. calcd. for
C
22H22BNO2: C 76.99, H 6.46, N 4.08; found: C 76.81,
H 6.80, N 3.82.
X-ray structures of boronates 4a, 6a, 6b, and 6e
X-ray diffraction analyses were determined on an Enraf
Nonius-CAD4 diffractometer (λMo Kα = 0.71073 Å, graphite
monochromator, T = 293 K, ω–2θ scan mode) and on an
Enraf Nonius FR590 Kappa-CCD diffractometer (λMo Kα
=
0.71073 Å, graphite monochromator, T = 293 K, CCD rota-
tion images). The crystals were mounted in Lindeman tubes.
For compounds 6a and 6b, an absorption correction was per-
formed with SHELX-A(40); whereas for 4a and 6e, an ab-
sorption correction was not necessary due to the systematic
scaling integration of the reflections. For all structures, cor-
rections were made for the Lorentz and Polarization effects.
The X-ray data was obtained through SHELXS-97 for the
structure solution and through SHELXL-97 ver. 34 (40) for
the refinement and data output; both programs are included
in WIN-GX program set (41). The corresponding images
were prepared with ORTEP 3 (42). All non-hydrogen atoms
were found by Fourier map differences and were fully re-
fined by anisotropic thermal parameters. Most of hydrogen
atoms were determined by difference Fourier maps and re-
fined with one overall isotopic thermal parameter, the other
non-found hydrogens were added by geometrical situation
and calculation.
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Acknowledgments
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Chem. 70, 2027 (1992).
30. W. Kliegel, G. Lubkowitz, S.J. Rettig, and J. Trotter. Can. J.
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Financial support and scholarship to H.I. Beltrán from the
Consejo Nacional de Ciencia y Teconología (CONACyT,
México) is acknowledged. The authors would like to thank
Q.F.I. Dolores Castillo for IR spectra, Q.I. Victor González
and Q.I. Guillermo Uribe for NMR spectra, and I.Q. Geiser
Cuéllar for mass spectra determinations.
33. (a) J.G. Wilson. Aust. J. Chem. 43, 1283 (1990); (b) 41, 173
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© 2002 NRC Canada