Structure and properties of 3-(Diethylboryl)pyridines

Article abstract of DOI:10.1021/jo960825k

3-(Diethylboryl)pyridine (1a), a versatile starting material for the preparation of arylpyridines, is notable for its stability under ambient conditions, in spite of little steric hindrance on the boron atom. ¹H and ¹³C spectra of 1a indicated that the boryl group does not act as a mere π-acceptor and that the boron atom is shielded by ca. 50 ppm even when compared with trivalent boron atoms conjugated with the π-donor. A single-crystal X-ray crystallographic study for 1a revealed formation of a cyclic-tetramer with a void via the intermolecular boron-nitrogen coordination bond. Vapor pressure osmometry in various solvents suggested that 1a comprises the tetramer in these solutions. In order to know the actual structure, synthesis of 3-(2-methoxyethoxy)-5-(diethylboryl)pyridine (1b) and its scrambling experiment with 1a were carried out. Heating at 100 °C for 24 h was required to attain the equilibrium of the scrambling of the component molecules in the tetramers. This means that 3-(diethylboryl)pyridines 1a and 1b comprise the rigid cyclic-tetramer in solution at ambient temperature. Compound 1b is stable in aerated tetrahydrofuran containing up to 33 % water.

Full text of DOI:10.1021/jo960825k

J . Org. Chem. 1996, 61, 6829-6834
6829
Str u ctu r e a n d P r op er ties of 3-(Dieth ylbor yl)p yr id in es1
,
†
,‡
†
‡
Yoshikazu Sugihara,* Katsuto Takakura,* Toshihiro Murafuji, Ryuta Miyatake,
‡
§
§
Kazuhiro Nakasuji, Masako Kato, and Shigenobu Yano
Department of Chemistry, Faculty of Science, Yamaguchi University, Yamaguchi City 753, J apan,
Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560, J apan, and
Department of Chemistry, Faculty of Science, Nara Women’s University, Narashi, Nara 630, J apan
Received May 6, 1996X
3
-(Diethylboryl)pyridine (1a ), a versatile starting material for the preparation of arylpyridines, is
notable for its stability under ambient conditions, in spite of little steric hindrance on the boron
1
13
atom. H and C spectra of 1a indicated that the boryl group does not act as a mere π-acceptor
and that the boron atom is shielded by ca. 50 ppm even when compared with trivalent boron atoms
conjugated with the π-donor. A single-crystal X-ray crystallographic study for 1a revealed formation
of a cyclic-tetramer with a void via the intermolecular boron-nitrogen coordination bond. Vapor
pressure osmometry in various solvents suggested that 1a comprises the tetramer in these solutions.
In order to know the actual structure, synthesis of 3-(2-methoxyethoxy)-5-(diethylboryl)pyridine
(1b) and its scrambling experiment with 1a were carried out. Heating at 100 °C for 24 h was
required to attain the equilibrium of the scrambling of the component molecules in the tetramers.
This means that 3-(diethylboryl)pyridines 1a and 1b comprise the rigid cyclic-tetramer in solution
at ambient temperature. Compound 1b is stable in aerated tetrahydrofuran containing up to 33
%
water.
In tr od u ction
Ch a r t 1
Synthesis of 3-(diethylboryl)pyridine (1a ) was initially
reported by Terashima et al.,2 who then developed a
coupling reaction using 1a as the starting material.3
Compound 1a , now commercially available from Aldrich
Chemical Co., is notable for its stability under ambient
conditions, in spite of little steric hindrance on boron. The
authors ascribed the stability to the depression of both
the nucleophilicity of the nitrogen atom and the electro-
philicity of the boryl group, caused by the donation of π
electrons in the pyridine ring to the vacant p-orbital of
the boron atom. However, since pyridine is known to be
highly deactivated toward electrophilic substitution, the
pyridine ring in 1a (Chart 1) would not act effectively as
a π donor. Furthermore the boron is electropositive, and
thus, significant stabilization would not arise from de-
Sch em e 1a
4
localization of the π electron onto this atom. Our studies
support this consideration: the boron atom in 6-phenyl-
borepino[4,5-c]thiophene (2), 6-phenylborepino[4,5-b]-
thiophene (3), or 2-methyl-6-phenylborepino[4,5-c]pyrrole
a
Reagents (a) NaOCH2CH2OCH3 (1.5 equiv), DMF, 65 °C,
overnight; (b) n-BuLi (1.2 equiv) then Et2BOMe (2.1 equiv), ether,
5% in two steps.
3
(4) forms the coordination bond with the nitrogen atom
of various amines in spite of formal conjugation with
strong π-electron donors, such as heteroles.
Hence, it would be suggested that the boron and
nitrogen atoms in 1a should act as a Lewis acid and
Lewis base, respectively, to form the intermolecular
boron-nitrogen coordination bond. This paper describes
the structure of 3-(diethylboryl)pyridines in both solution
and solid state.
Resu lts a n d Discu ssion
†
Yamaguchi University.
Osaka University.
Nara Women’s University.
Abstract published in Advance ACS Abstracts, August 15, 1996.
Syn th esis. 3-(2-Methoxyethoxy)-5-(diethylboryl)py-
ridine (1b) was prepared by nucleophilic substitution of
‡
§
X
5
3,5-dibromopyridine (5) with 1.5 equiv of sodium 2-meth-
(
1) A preliminary communication has appeared: Sugihara, Y.;
Miyatake, R.; Takakura, K.; Yano, S. J . Chem. Soc. Chem. Commun.
994, 1925.
2) Terashima, M.; Kakimi, H.; Ishikura, M.; Kamata, M.; Chem.
Pharm. Bull. 1983, 31, 4573.
3) Ishikura, M.; Kamada, M.; Terashima, M. Heterocycles 1984, 22,
65; Idem. Synthesis 1984, 936. Ishikura, M. Kamada, M.; Ohta, T.;
Terashima, M. Heterocycles 1984, 22, 2475.
4) For the Lewis acidity of the trivalent boron conjugated to the
6
oxyethoxide in dimethylformamide at 65 °C overnight,
followed by treatment of the resulting crystalline 3-bromo-
5-(2-methoxyethoxy)pyridine (6) with n-butyllithium and
with 2.1 equiv of diethylmethoxyborane in ether in 35%
yield (Scheme 1).
1
(
(
2
Nu clea r Ma gn etic Reson a n ce Sp ectr oscop y. As
(
already reported,2 only one set of the signals as a
electron-rich aromatic ring, see: Sugihara, Y.; Yagi, T.; Murata, I.;
Imamura, A. J . Am. Chem. Soc. 1992, 114, 1479. Sugihara, Y.;
Miyatake, R.; Yagi, T.; Murata, I.; J inguji, M.; Nakazawa, T.; Imamura,
A. Tetrahedron 1994, 50, 6495. Sugihara, Y.; Miyatake, R.; Murata,
I.; Imamura, A. J . Chem. Soc., Chem. Commun. 1995, 1249.
(5) Garcia, E. E.; Greco, C. V.; Hunsberger, I. M. J . Am. Chem. Soc.
1960, 82, 4430.
(6) Comins, D. L.; Killpack, M. O. J . Org. Chem. 1990, 55, 69.
S0022-3263(96)00825-0 CCC: $12.00 © 1996 American Chemical Society

6
830 J . Org. Chem., Vol. 61, No. 20, 1996
Sugihara et al.
Ta ble 1. 1H a n d 13C (in P a r en th eses) Ch em ica l Sh ifts of
The same trends are shown with respect to the chemi-
cal shifts of all the atoms of 1b and the distinct nuclear
Overhauser effect.
1
a a n d Rela ted Com p ou n d s
Ma ss Sp ectr oscop y a n d Va p or P r essu r e Osm om -
etr y. In the spectrum of EIMS (30 eV), in addition to
+
the M - Et peak (118, 100) and parent peak as a
+
+
monomer (M , 147, 45), peaks of 2 × M - Et (265, 47),
1Ha (13Ca)
1a b,c
7c
8c
9c
+
+
3
× M - Et (412, 67), and 4 × M - Et (559, 39) are
H2 (C2)
H4 (C4)
H5 (C5)
H6 (C6)
7.58 (149.2) 8.91 (152.5) 8.04 (137.2) 8.60 (149.8)
7.72 (143.8) 8.00 (139.3) 6.91 (121.4) 7.64 (135.7)
7.23 (123.4) 7.47 (123.7) 6.99 (123.9) 7.25 (123.6)
8.01 (140.9) 8.84 (153.0) 7.95 (139.2) 8.60 (149.8)
observed. FABMS displays the same peaks at intensities
of 14, 19, 100, 41, and 2%, respectively, together with
+
+
the new peaks of 2 × M (293, 7) and 3 × M (441, 1). A
+
FABMS spectrum of 1b only displays a peak of 3 × M
a
Chemical shifts in CDCl3 are given in δ an ppm. b Reference
. c This work.
- Et (634, 31) as a heaviest ion. The strength of the
coordination bonds in 1a and 1b is indicated to be close
to the scope of mass spectroscopy.
2
monomer is observed both in H and 13C NMR spectra of
1
1
1
a . The H NMR spectrum is essentially independent
Vapor pressure osmometry by Knauer digital vapor
pressure osmometer with benzil as a standard gives 3.73,
.92 and 3.91 with various concentrations of 1a in
of both temperature ranging from -90 to 80 °C and
-
3
-1
concentration ranging from 6 × 10 to 3 × 10 M. As
shown in Table 1, the well-known downfield or upfield
shift, such as in 3-cyanopyridine (7) or in 3-aminopyridine
3
-
3
-1
benzene (from 3.78 × 10 to 2.96 × 10 M) at 60 °C,
-
3
-2
chloroform (from 6.39 × 10 to 9.98 × 10 M) at 40 °C,
(8), when compared with pyridine (9), is not seen in
-
2
-1
and tetrahydrofuran (from 1.17 × 10 to 1.82 × 10 M)
signals due to both the proton and carbon at the ortho
and para positions of the boryl group in 1a . Shielding
of R-carbons of the nitrogen atom, therefore, implies the
formation of a coordination bond at the nitrogen atom
rather than the consideration that the boryl group
behaves as a mere electron-withdrawing group in 1a .
Thus, the decrease in π bond order of the C
results from the formation of the coordination bond,
which causes the upfield shift of C
at 45 °C, respectively. Osmometry values of 1b in
-
3
-1
-3
benzene (from 2.0 × 10 to 1.1 × 10 mol dm ) at 60
C and in tetrahydrofuran (from 5.0 × 10 to 9.4 × 10
-
3
-2
°
-3
mol dm ) at 45 °C are 3.6 and 4.1, respectively. In each
experiment a clear linear relationship is found between
the concentration and the reading. Thus, these esti-
mated values and the clear linear relationship suggest
that each compound 1a or 1b forms a tetramer in these
solvents. Independence of the values on solvent, even
in tetrahydrofuran of high affinity with the trivalent
boron atom, shows that this atom in 1a and 1b is
protected from the coordination of an oxygen atom of the
solvent by the head-tail mutual intermolecular interac-
tion.
2
-N bond
2
and C
6
like the case
7
of pyridinium ions.
The 11B spectrum of 1a displays a signal at -0.35 ppm
Table 2). This is a farther upfield than the signals of
(
compounds 2, 3, and 4 and 1-(dimesitylboryl)-4-(di-
methylamino)benzene (10), each possessing the strong
8
π-electron-donating heterole ring or dimethylamino moi-
eties, respectively. Though the chemical shift of the
boron atom is believed to correlate well with π-electron
Str u ctu r e in th e Cr ysta llin e Sta te. A single-crystal
X-ray crystallographic study reveals that 1a forms a
cyclic tetramer with a C
2
symmetry, as illustrated in
Figure 1 and outlined in Table 3 and 5. The framework
of one side is almost superimposable on the other. An
outstanding structural feature is the short distance
between the boron and nitrogen atoms (B1-N2*, 1.639-
9
density, the value of 1a is beyond the criterion. Hence,
the marked shielding of this atom, comparable to that of
amine adducts of triethylborane or trimethylborane (11-
1
0,11
1
3),
should be ascribed to the formation of the
oligomer via a head-tail coordination bond. The chemi-
cal shift of the nitrogen atom in 1a also supports our
consideration.12 Thus, while the value of pyridine (9) is
(
5) Å). The short length of this coordination bond is
comparable to that of the B1-C2 σ bond (1.630(4) Å),
symmetrizing the framework of the tetramer. The bond
angles constituted by the boron atom and three substitu-
ent carbons are 113.9(3)° (C2-B1-C6), 107.2(3)° (C2-
B1-C8), and 113.7(3)° (C6-B1-C8), respectively. Fur-
thermore, the coordination bond (N2*-B1) forms angles
of 104.1(3)° (N2*-B1-C2), 106.9(3)° (N2*-B1-C6), and
-59 ppm, the nitrogen atom in the trialkylborane-amine
adducts (11, 12) is shielded by over -40 ppm. It should,
furthermore, be pointed out that the chemical shifts of
the boron and nitrogen atoms in 1a , similar to those of
the adducts, clarify that the equilibrium is to the side of
oligomers.
The same conclusion is derived from the nuclear
Overhauser effect. Namely, upon irradiation of the
signals due to the methyl groups in 1a , the differential
NOE spectrum exhibited the distinct and comparable
1
10.5(3)° (N2*-B1-C8) with these substituents, clearly
showing the pyramidal structure of the boron atom (B1).
The coordination bond (N2*-B1) lies in the plane of the
pyridine ring with a deviation of only 1.6-3.0°. Further-
more, the angles between C10*-N2*-B1 and C14*-
N2*-B1 are 117.9(2)° and 125.1(3)°, respectively, indi-
cating that the coordination bond is almost an extension
of the line which binds N2* and C12*. A similar situation
is found around the B2-N1 coordination bond. Of the
four pyridine rings two rings (P1 and P3) face each other
and are perpendicular to the plane of the tetramer
(dihedral angle between least-squares planes of the
pyridine rings, 18.96°). The distance C1-C1* is 5.119-
2 4 6
enhancement of the signals due to H , H , and H .
(
7) Pugmirc, R. J .; Grant, D. M. J . Am. Chem. Soc. 1968, 90, 699.
J ones, A. J .; Grant, D. M.; Russell, J . G.; Fraenkel, G. J . Phys. Chem.
969, 73, 1624.
8) Doty, J . C.; Babb, B.; Grisdale, P. J .; Glogowski, M.; Williams,
J . L. R. J . Organomet. Chem. 1972, 38, 229.
9) N o¨ th, H.; Wrackmeyer, B. Chem. Ber. 1974, 107, 3089. Spielvo-
1
(
(
gel, W. R. Nutt, R. Izydore, R. A. J . Am. Chem. Soc. 1975, 97, 1609.
N o¨ th, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spectroscopy
of Boron Compounds, Spring-Verlag, New York, 1978.
(10) N o¨ th, H.; Wrackmeyer, B. Chem. Ber. 1974, 107, 3070.
(11) N o¨ th, H.; Vahrenkamp, H. Chem. Ber. 1966, 99, 1049.
(12) Levy, G. C.; Lichter, R. L. Nitrogen-15 Nuclear Magnetic
(
4) Å and C4-C4* is 4.228(7) Å. The other set of the
pyridine rings (P2 and P4) virtually lies on this plane
Resonance Spectroscopy; J ohn Willey & Sons, Inc.: New York, 1979.

Structure and Properties of 3-(Diethylboryl)pyridines
J . Org. Chem., Vol. 61, No. 20, 1996 6831
Ta ble 2. 11B a n d 14N Ch em ica l Sh ifts of 1a ,b a n d Rela ted Com p ou n d s
1
a a-c
-0.35
-133
1ba,c
2a,d
10a,c,e
11a,f
12a,f
0.0
-108
13a,g
7a,h
9a,h
1
1
1
4
Bi
N
-0.50
-156
50.8
71.5
-2.2
-108
0.0
j
-60
-59
a
Chemical shifts in CDCl3 are given in δ and ppm. Reference 1. c This work. d Reference 4. e Reference 8. f Reference 10. g Reference
b
i
1
1. h Reference 12. BF3‚OEt2 as standard. j Aqueous NaNO3 as standard.
F igu r e 1. Computer-generated thermal ellipsoid of the tet-
ramer of 1a .
(dihedral angle between least-squares planes of pyridine
rings, 125.65°) and the distance C10-C10* is 5.797(4)
Å.
F igu r e 2. Crystal structure of the tetramer of 1b.
The B1-C2 bond is much longer than the correspond-
ing boron-carbon bond in triphenylborane (1.577 Å),13
indicating the low double bond character of this bond in
angles of 105.6(7)° [N(2)*-B(1)-C(2)], 108.3(9)° [N(2)*-
B(1)-C(11)], and 105.3(9)° [N(2)*-B(1)-C(9)].
According to the criterion proposed by Toyota and
Oki,18 the tetrahedral character (THC) of a boron atom
correlates well to the the barrier of dissociation rather
than the length of the coordination bond. The THC
values for 1a are estimated to be 79.7% (B1) and 84.2%
1
a . As regards the length of the coordination bond, an
early X-ray crystallographic study revealed an N-B bond
distance of 1.56 ( 0.05 Å14 for the prototype adduct H
NBH in a solid state, and microwave spectroscopy
3
-
3
revealed the longer length of 1.658(2) Å for this bond in
(B2) and those for the two boron atoms in the tetramer
the gas phase.1 For the B-N bond of the alkylated
5
1
b are 72.4% (B1) and 77.6% (B2), respectively. When
adduct Me NBMe , an elongated distance of 1.70 ( 0.01
3 3
the weak basicity of pyridine 8 is taken into account,
these values indicate that each tetramer is formed via
strainless coordination bonds. This will be helpful in
prediction of the structure of the cyclic oligomer com-
prised by the isomer of 3-(diethylboryl)pyridines.
16
Å was revealed by means of microwave spectroscopy.
Accordingly, the distance between the boron and the
nitrogen in the tetramer is considered appropriate for the
formation of the head-tail coordination bond of the
(diethylboryl)pyridine.
Scr a m blin g a n d Mixin g Beh a vior in Solu tion .
Both spectroscopic and vapor pressure osmometric data
for 1a and 1b suggest that each equilibrium is exclusively
to the side of the cyclic oligomer. A scrambling experi-
ment using 1a and 1b gives further information concern-
ing the structures in solution. Thus, when a mixture of
equimolar amounts of 1a and 1b in deuteriotoluene was
Though, due to the disorder around the ethyl groups,
the R value is not lowered, the single-crystal X-ray
crystallographic study of 1b confirms formation of a cyclic
tetramer with a center of symmetry (Figure 2 and Table
4
). In contrast to a cone conformation of 1a , the tetramer
1
7
occupies a 1,2-alternate conformation.
A distance
slightly elongated by 0.024 Å is found between the boron
and the nitrogen atoms [B(1)-N(2)*, 1.66(1) Å] compared
with 1a . Furthermore, this coordination bond makes
1
allowed to stand at 60 °C for 10 h, the H NMR spectrum
did not show any change. However, upon heating the
mixture to 100 °C for 24 h, the component molecules were
scrambled into tetramers. Isolation of the newly formed
tetramers, followed by spectroscopic analysis, showed
that the least- and most-polar tetramers (Mp 121-122
(
13) Zettler, F.; Hausen, H. D.; Hess, H. J . Organomet. Chem. 1974,
7
7
2, 157.
(
(
14) Hughes, E. W. J . Am. Chem. Soc. 1956, 78, 502.
15) Thorne, L. R.; Suenram, R. D. Lovas, F. J . J . Chem. Phys. 1983,
°
C, 25%, and Mp 131-133 °C, 17%, respectively) consist
8, 167.
of 1a and 1b in 3:1 and 1:3 ratio, respectively. Though
(
(
16) Kuznesof, P. M.; Kuczkowski, R. L. Inorg. Chem. 1978, 17, 2308.
17) Stewart, D. R.; Krawiec, M.; Kashyap, R. P.; Watson, W. H.;
Gutsche, C. D. J . Am. Chem. Soc. 1995, 117, 586.
(18) Toyota, S.; Oki, M. Bull. Chem. Soc. J pn. 1992, 65, 1832.

6
832 J . Org. Chem., Vol. 61, No. 20, 1996
Ta ble 3. Selected Geom etr ica l P r op er ties of th e Tetr a m er of 1a a
Sugihara et al.
distances (Å)
bond angles (deg)
N(2)-B(1)-C(2)
N(2)-B(1)-C(6)
N(2)-B(1)-C(8)
C(2)-B(1)-(C6)
C(2)-B(1)-C(8)
C(6)-B(1)-C(8)
B(1)-N(2)-C(10)
B(1)-N(2)-C(14)
C(10-N(2)-C(14)
N(1)-B(2)-C(11)
N(1)-B(2)-C(15)
N(1)-B(2)-C(17)
C(11)-B(2)-C(15)
C(11)-B(2)-C(17)
C(15)-B(2)-C(17)
B(2)-N(1)-C(1)
B(2)-N(1)-C(5)
C(1)-N(1)-C(5)
B(1)-N(2)
1.639(5)
1.630(4)
1.608(6)
1.626(5)
1.636(4)
1.641(5)
1.622(6)
1.620(5)
104.1(3)
106.9(3)
110.5(3)
113.9(3)
107.2(3)
113.7(3)
117.9(2)
125.1(3)
117.0(3)
103.6(3)
111.3(3)
108.1(3)
107.8(3)
111.9(3)
113.7(3)
124.7(3)
118.2(3)
117.1(3)
B(1)-C(2)
B(1)-C(6)
B(1)-C(8)
B(2)-N(1)
B(2)-C(11)
B(2)-C(15)
B(2)-C(17)
dihedral angles (deg)
C(11)-C(10)-N(2)-B(1)
C(13)-C(14)-N(2)-B(1)
C(2)-C(1)-N(1)-B(2)
C(4)-C(5)-N(1)-B(2)
-177.0(3)
178.4(4)
177.8(3)
-178.8(4)
a
Numerals in parentheses are estimated standard deviations.
Ta ble 4. Selected Geom etr ica l P r op er ties of th e Tetr a m er of 1ba
distances (Å)
bond angles (deg)
B(1)-N(2)
1.66(1)
1.63(1)
1.61(2)
1.67(2)
1.66(1)
1.63(1)
1.61(1)
1.62(1)
N(2)-B(1)-C(2)
N(2)-B(1)-C(9)
N(2)-B(1)-C(11)
C(2)-B(1)-C(9)
C(2)-B(1)-C(11)
C(9)-B(1)-C(11)
B(1)-N(2)-C(13)
B(1)-N(2)-C(17)
C(13)-N(2)-C(17)
N(1)-B(2)-C(16)
N(1)-B(2)-C(21)
N(1)-B(2)-C(23)
C(16)-B(2)-C(21)
C(16)-B(2)-C(23)
C(21)-B(2)-C(23)
B(2)-N(1)-C(1)
B(2)-N(1)-C(5)
C(1)-N(1)-C(5)
105.6(7)
105.3(9)
108.3(9)
112.3(9)
107.7(9)
117.0(10)
118.5(8)
123.3(7)
117.9(7)
104.6(7)
111.0(8)
105.5(8)
110.3(9)
111.5(8)
113.5(8)
123.4(9)
119.1(8)
117.5(8)
B(1)-C(2)
B(1)-C(9)
B(1)-C(11)
B(2)-N(1)
B(2)-C(16)
B(2)-C(21)
B(2)-C(23)
dihedral angles (deg)
C(14)-C(13)-N(2)-B(1)
C(16)-C(17)-N(2)-B(1)
C(2)-C(1)-N(1)-B(2)
C(4)-C(5)-N(1)-B(2)
-174(1)
173.6(1)
-178.0(8)
179.6(9)
a
Numerals in parentheses are estimated standard deviations.
Ta ble 5. Cr ysta llogr a p h ic Da ta for th e Tetr a m er s of 1a
a n d 1b
of the coordination bonds shows the reliability of vapor
pressure osmometry for determination of the actual
structure of 3-(diethylboryl)pyridines in solution, though
the method generally displays only average values of
aggregated molecules.
1
a
1b
molecular formula
molecular weight
crystal size (mm)
crystal shape, color
C
36
H
56
B
4
N
4
48 80 8 4 4
C H O B N
884.42
588.11
0.30 × 0.30 × 0.15 0.22 × 0.16 × 0.08
1
The H NMR spectra of the mixture of 1a and bases
prismatic,
colorless
prismatic,
colorless
Cu KR
(λ ) 0.71069 Å)
triclinic
P1(#2)
11.815(2)
12.024(2)
11.046(2)
107.42(2)
102.43(2)
64.12(1)
1340.6(5)
1
such as pyridine (9) (as solvent), piperidine (14) (in
deuteriodichloromethane), or quinuclidine (15) (in deu-
teriodichloromethane) left standing at room temperature
for several days, remained unchanged. The fact shows
that the concentration of the open-chain oligomer, which
has a boron atom free from the coordination, is quite low,
since the basicity of the nitrogen atom in pyridine (9)
should be comparable to that of 1a . The spectrum of the
mixture of equimolar amounts of 1a and N,N-dimethy-
laminopyridine (16) displayed proton signals due to the
tetramer of 1a and some sharp doublets having the same
coupling constants as that of 16 at higher positions,
indicating that some molecules of 16 are coordinated with
the boron atom and, namely, several open-chain oligo-
mers of 1a involving the coordination bond with the
pyridine are formed. Integration of the signal showed
that 17.5% of the tetramer of 1a had not decomposed.
Interestingly, upon heating, only the signals due to open-
chain oliogomers were broadened at 90 °C and signals
due to the tetramer of 1a remained unchanged. The
mixture of 1b and 16 exhibited similar behavior. At room
temperature 33.3% of the tetramer of 1b remained intact
radiation (graphite
monochromated)
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Cu KR
(λ ) 1.54178 Å)
orthorhombic
Fdd2(#43)
22.700(3)
45.297(4)
7.289(4)
7494(3)
8
Z
2
θ
max, deg
120.1
60.0
5261
no. of reflections collected 1598
no. of reflections refined
no. of variables
1358
198
1373
308
R
wR
0.043
0.065
0.078
0.082
the other two tetramers could not be separated (51%),
1
the H NMR spectrum was indicative of the tetramers
involving 1a and 1b in a ratio of 1:1 with divergent
sequences. The ratio of the tetramer with a sequence of
1
1
a , 1a , 1b, and 1b and the one with a sequence of 1a ,
b, 1a , and 1b was 2:1. The marked thermal stability

Structure and Properties of 3-(Diethylboryl)pyridines
J . Org. Chem., Vol. 61, No. 20, 1996 6833
and broadening of signals due to open-chain oligomers
involving 16 occurred at 90 °C. Though signals due to
open-chain oligomers partly started to coalesce at 100 °C,
the signals of both tetramers of 1a and 1b remained
unchanged.
Column chromatography over activated silica gel gave
these cyclic tetramers in the ratio that was shown by
means of thin-layer chromatography (TLC) or NMR
spectroscopy. However, the same chromatography of the
mixture containing the open-chain oligomers involving
for 0.00378, 0.0473, 0.122, 0.296 mol/L, respectively)]; 3.92 in
chloroform [benzil (15.2, 24.5, 102.5, 193.0 for 0.000 11, 0.0133,
0
0
.0525, 0.103 mol/L, respectively), 1a (1.5, 6.9, 17.2, 43.3 for
.006 39, 0.0163, 0.0395, 0.0998 mol/L, respectively)]; 3.91 in
tetrahydrofuran at 45 °C [benzil (3.1, 25.0, 86.7, 177.8 for
0
1
.000 131, 0.0163, 0.0514, 0.106 mol/L, respectively), 1a (3.3,
3.9, 26.8, 76.8 for 0.000 117, 0.0314, 0.0755, 0.182, respec-
tively)]. 1b: 4.0 in benzene at 60 °C [benzil (0, 3.2, 28.9, 115.8,
193.6 for 0, 0.0022, 0.0164, 0.1075 mol/L, respectively); 1b: 4.1
in tetrahydrofuran at 45 °C [benzil (0, 15.2, 24.5, 102.5, 193.0
for 0, 0.0052, 0.0111, 0.0521, 0.0943 mol/L, respectively), 1b
(0, 11.5, 22.2, 33.3, 52.1 for 0, 0.0211, 0.0423, 0.0634, 0.1057
1
6, formation of which was observable by TLC or NMR
mol/L, respectively)].
spectroscopy, gave only the cyclic tetramer and 16. Since
the nitrogen atom of 16 would form a stronger dative
bond19 than that of the pyridine rings in 1a and 1b, the
stability of these tetramers should be ascribable to facile
rebonding of the partly cleaved tetramer.
Scr a m blin g Exp er im en t. Compounds 1a (0.121g, 0.823
mmol) and 1b (0.182 g, 0.824 mmol) were dissolved in toluene
and heated at 100 °C for 24 h. Chromatography over silica
A
B
gel with ether gave (1a ) (1b) , (1a ) (1b) , (1a ) (1b) , and
3
1
1
1
1
1
(1a )
1
(1b)
3
, together with the starting tetramers (1a )
4
, (0.004 84
g, 0.0329 mmol) and (1b)
4
3 1
(0.0145 g, 0.0656 mmol). (1a ) (1b)
The tetramer of 1b was soluble and quite stable in
aerated tetrahydrofuran containing water up to 33% at
ambient temperature. With the change of water content
1
(
0.068 g, 0.103 mmol, 25%). Mp: 121-123 °C. H NMR (400
Mz, CDCl ): δ 0.77 (24H, m), 0.92 (16H, m), 3.05 (3H, 3), 3.19
3
(2H, m), 3.19 (2H, m), 6.65 (2H, m), 6.88 (1H, dd, J ) 7.42
1
1
in deuteriotetrahydrofuran, the signal of B did not show
any essential shift: only by δ 1.1 ppm, up to 33% water
content.
and 5.86 Hz), 7.21 (1H, dd, J ) 2.47 and 1.65 Hz), 7.61 (1H, d,
J ) 2.47 Hz), 7.43 (1H, dd, J ) 7.42 and 1.44 Hz), 7.53 (2H,
m), 7.63 (2H, s), 7.67 (1H, d, J ) 1.65 Hz), 7.70 (1H, dd, J )
In conclusion, 3-(diethylboryl)pyridines comprise the
cyclic tetramer via the coordination bonds, the cooordi-
nation bond of which is stable at ambient temperature
and in aerated organic solvents with high water content.
The marked thermal stability and the intriguing struc-
ture should be of use both in consideration of the
reactivity of 1a and in construction of the artificial self-
assembly systems with novel functionality.20
5.86 and 1.44 Hz), 7.75 (1H, d, J ) 5.77 Hz), 7.77 (1H, d, J )
A
5
.77 Hz), 7.91 (1H, s). (1a )
1
(1b)
1
[0.154 g (estimated), 34%].
1
H NMR (400 Mz, CDCl ): δ 0.82 (24H, m), 0.95 (16H, m),
3
3
.09 (6H, s), 3.28 (4H, m), 3.70 (4H, m), 6.59 (2H, dd, J ) 7.35
and 5.84 Hz), 7.26 (2H, s), 7.41 (2H, d, J ) 7.35 Hz), 7.49 (2H,
d, J ) 2.43 Hz), 7.68 (2H, d, J ) 5.84 Hz), 7.92 (2H, d, J )
B
2
.34 Hz), 7.96 (2H, s). (1a )
1
(1b)
1
[0.0567 g (estimated), 17%].
1
H NMR (400 Mz, CDCl ): δ 0.82 (24H, m), 0.95 (16H, m),
3
3.07 (6H, s), 3.22 (4H, m), 3.54 (4H, m), 6.87(1H, dd, J ) 7.08
and 6.04 Hz), 6.87 (1H, dd, J ) 7.08 and 5.95 Hz), 7.52 (2H, d,
J ) 7.08 Hz), 7.52 (2H, s), 7.74 (1H, d, J ) 6.04 or 5.95 Hz),
Exp er im en ta l Section
7
.76 (1H, d, J ) 5.95 or 6.04 Hz), 7.77 (1H, d, J ) 2.20 Hz),
7.77 (1H, d, J ) 2.06 Hz), 7.29 (1H, d, J ) 2.20 or 2.06 Hz),
7.32 (1H, d, J ) 2.06 or 2.20 Hz), 7.82 (2H, s). (1a ) (1b)
[0.0567 g, 0.070 mmol, 17%]. Mp: 131-133 °C. H NMR (400
Mz, CDCl ): δ 0.83 (24H, m), 0.97 (16H, m), 3.05 (3H, s), 3.18
3
-Br om o-5-(2-m et h oxyet h oxy)p yr id in e (6). Sodium
(
(
0.365 g, 1.59 mmol) was dissolved in 2-methoxyethanol
distilled from sodium, 1.37 mL) under a nitrogen atmosphere.
1
3
1
After the excess of the alcohol was removed in vacuo, DMF
distilled from CaH , 1 mL) and dibromide 5 (0.250 g, 1.46
3
(
2
(2H, m), 3.47 (2H, m), 3.10 (6H, s), 3.29 (4H, m), 3.72 (4H, m),
6.93 (1H, dd, J ) 7.49 and 5.77 Hz), 7.20 (1H, d, J ) 2.20 Hz),
7.23 (2H, s), 7.42 (1H, d, J ) 7.49 Hz), 7.55 (1H, d, J ) 2.06
Hz), 7.57 (1H, d, J ) 2.19 Hz), 7.67 (1H, d, J ) 2.20 Hz), 7.71
(1H, d, J ) 5.77 Hz), 7.89 (1H, s), 7.92 (1H, d, J ) 2.19 Hz),
8.00 (1H, d, J ) 2.04 Hz).
mmol) were added. The solution stood at 65 °C overnight and,
after cooling, was extracted with ether. Product 6 was
obtained by chromatography over silica gel with ether and
recrystallized from hexane to yield 0.171g (0.74 mmol, 50.5%).
1
Mp: 30-33 °C. H NMR (400 Mz, CDCl
3
): δ 3.45 (3H, s), 3.75
(
2H, m), 4.15 (2H, m), 7.40 (1H, dd, J ) 2.57 and 1.95 Hz),
X-r a y Cr ysta llogr a p h y. 1a : a colorless prismatic crystal
8
.27 (1H, d, J ) 2.57 Hz, 1.95 Hz), 8.29 (1H, d, J ) 1.95 Hz).
of C36
H
56
B
4
N
4
, having the approximate dimensions of 0.30 ×
1
3
C NMR (100 MHz, CDCl
3
): δ 59.3, 68.1, 70.7.120.3, 124.2,
10BrNO : C, 41.40;
0.30 × 0.15 mm, was mounted on a glass fiber. All measure-
ments were made on a Rigaku AFC5R diffractmeter with
graphite monochromated Cu KR radiation and a 12 kW
rotating anode generator. Cell constants and an orientation
matrix for data collection were obtained from a least-squares
refinement using the setting angles of 25 carefully centered
reflections in the range 76.51° < 2θ < 79.79° corresponding
to an F-centered orthorhombic cell. The structure was solved
1
36.6, 143.2, 155.4. Anal. Calcd for C
8
H
2
H, 4.34; N, 6.04. Found: C, 41.35; H, 4.28; N, 6.05.
3
-(2-Meth oxyeth oxy)-5-(d ieth ylbor yl)p yr id in e (1b). To
a solution of 6 (0.10 g, 0.43 mmol) in dry ether (4.0 mL) was
added n-butyllithium (0.27 mL, 1.6 M) dropwise at -78 °C.
After stirring for 15 min, diethylmethoxyborane (0.12 mL, 1.76
mmol, 1.0 M) was added dropwise. The solution was warmed
to room temperature, stood overnight with stirring, and was
extracted with ether. Chromatography over silica gel with
2
1
by direct methods (MULTAN88) and expanded using Fourier
techniques (DIRDIF92).22 The final cycle of full-matrix least-
squares refinement was based on 1358 observed reflections (I
> 3.00σ(I)) and 198 variable parameters. The Fourier map
ether and recrystallization from benzene gave 1b (0.051 g, 0.23
1
mmol, 53.5%). Mp: 139-140 °C. H NMR (400 Mz, CDCl
3
):
-
δ 0.45 (6H, m), 0.62 (4H, m), 3.45 (3H, s), 3.74 (2H, m), 4.10
2H, m), 7.20 (1H, dd, J ) 2.31 Hz), 7.32 (1H, d, J ) 1.98 Hz),
corresponded to 0.16 and -0.13 e /Å, respectively.
(
80 8 4 4
1b: a colorless prismatic crystal of C48H O B N , having
7
1
.66 (1H, d, J ) 1.98 Hz). 13C NMR (100 MHz, CDCl
4.6, 59.3, 67.9, 70.8, 128.6, 129.7, 142.2, 155.2, 156.1.
, BF ‚OEt
as standard) δ -0.5. 14N NMR
, aqueous NaNO as standard) δ -156. Anal.
: C, 65.19; H, 9.12; N, 6.33. Found: C,
4.94; H, 9.11; N, 6.40.
Va p or P r essu r e Osm om etr y Resu lts. 1a: 3.7 in benzene
at 60 °C [benzil (2.0, 26.1, 101.7, 193.9 for 0.000 085 6, 0.0148,
.0540, 0.1036 mol/L, respectively), 1a (0.9, 20.8, 55.2, 147.3
3
): δ 9.2,
approximate dimensions of 0.22 × 0.16 × 0.08 mm, was
mounted on a glass fiber. Cell constants and an orientation
matrix for data collection were obtained from a least-squares
1
1
B
NMR (128 Hz, CDCl
361 MHz, CDCl
Calcd for C12 20BNO
2
3
3
2
(
3
3
H
(21) Debaerdemaeker, T.; Germain, G.; Main, P.; Refaat, L. S.; Tate,
6
C.; Woolfson, M. M. Computer programs for the automatic solution of
crystal structures from X-ray diffraction data; University of New York,
U.K, 1988.
(22) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
0
Garcia-Granda, S.; Gould, R. O.; Smits, J . M. M.; Smykalla, C. The
DIRDIF program system, Technical Report of the Crystallography
Laboratory; University of Nijmegen: The Netherlands 1992.
(23) Gilmore, C. J . MITHRIL-an integrated direct methods computer
program; University of Glasgow, Scotland, 1990.
(
(
19) Haaland, A. Angew. Chem. Int. Ed. Engl. 1989, 28, 992.
20) Fujita, M.; Ogura, K. J . Synth. Org. Chem. J pn. 1994, 52, 839.
Hunter, C. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1079.

6
834 J . Org. Chem., Vol. 61, No. 20, 1996
Sugihara et al.
refinement using the setting angles of 21 carefully centered
reflections in the range 20.61° < 2θ < 25.66° coresponding to
a primitive triclinic cell. The structure was solved by direct
fined. The final cycle of full-matrix least-squares refinement
was based on 1373 observed reflections (I > 3.00σ(I)) and 308
variable parameters.
2
3
methods (MITHRIL90) and expanded using Fourier tech-
Neutral atom scattering factors were taken from Cromer
2
2
niques (DIRDIF92). The non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included but not re-
24
and Waber. Anomalous dispersion effects were included in
F
calc; the values for ∆f ′ and ∆f ′′ were those of Creagh and
25
McAuley. The values for the mass attenuation coefficients
are those of Creagh and Hubbel.
performed using the teXsan crystallographic software pack-
age of Molecular Structure Corp.
2
6
(
24) Cromer, D. T.; Waver, J . T. International Tables for X-ray
All calculations were
Crystallography, The Kynoch Press: Birmingham, England, 1974; Vol
IV, Table 2.2 A.
27
(
25) Creagh, D. C.; McAuley, W. L. International Tables for Crystal-
lography, Wilson, A. J . C., Ed.; Kluwer Academic Publishers: Boston,
992; Vol. C, Table 4.2.6.8, pp 219-222.
26) Creagh, D. C.; Hubbell, J . H. International Tables for Crystal-
lography; Wilson, A. J . C., Ed.; Kluwer Academic Publishers: Boston,
992; Vol. C, Table 4.2.4.3, pp 200-206.
27) Cystal Structure Analysis Package, Molecular Structure Cor-
poration, 1985 and 1992.
28) The author has deposited atomic coordinates for this structure
1
Ack n ow led gm en t. This work was supported by
Grants-in-Aid for Scientific Research on Priority Area
of Reactive Organometallics No. 05236102 and for
Developmental Scientific Research (B) (No. 06554027)
from the Ministry of Education, Science and Culture,
J apan.
(
1
(
(
with the Cambridge Crystallographic Data Centre. The coordinates
can be obtained, on request, from the Director, Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge, CB2 4E2, UK.
J O960825K