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Diborane derivatives from N-alkyloxazaborolidines.
Tmahedron: Asymmetry Vol.3.No.9,pp.1145-1148.1992 Printed inGreat Britain
DIBORANE
0957-4166/[email protected] PergamonPressLId
DERIVATIVES
Hugo Tlahuext
FROM N-ALKYLOXAZABOROLIDINES.
and Rosalinda
Contreras*
Departamento de Quimica, Centro de Investigacibn y de Estudios Avanzados de1 Instituto Politknico National. Apartado Postal 14-740, MBxico, D. F. 07000, Mexico.
(Received 27 July 1992)
Abstract. to
N-methyl
afford
N-BH3
considered
or N-ethyl adducts.
as diborane
oxazaborolidines
N-methyl
derivatives.
derived
derivatives Borane
from
dimerize.
addition
ephedrines
Monomers
was found
react
and
with
dimers
BzH6
can
to be stereoselective
be for
8 and 10.
The
high
enantioselectivity
converted high
them
into
of
valuable
stereoselectivity
to add experimental have
already
N-acyl
into the nature
We
have
(erythro)
donor
a
of the borane
N-methyl
and pseudoephedrine
atoms give electronic atoms
Therefore,
lose
density
some
of
it was interesting
1 2 3 4
R (erythro) (three) (erythro) (three)
R R R R
= = = =
with
to the theoretical
new
of ephedrines.
synthesized
reaction
has
in the study of the nature
information
published
derivatives
reducing
A mechanism
of the reduction
of BH3 la. We are interested
as
oxazaborolidines
reagents'.
Et Et Me Me
method
of
we present
complexes
and
of this borane
preparation
to explain
results
of
have the
in the presence complex,
in order
its behavior'.
the
We from
investigation
of these oxazaborolidines.
N-ethyl
(threoJ3.
basic
to explore
5 6 7 8
catalysts to explain
oxazaborolidines3
of
our
oxazaborolidines In these
l-4
heterocycles
derived
(erythro) (three) (erythro) (three)
properties
and
the coordination
the
from
the oxygen
to the boron by retrocoordination. their
or
proposed
oxazaborolidines
efforts
Herein,
agents
been
and
It is expected boron
ability
atom
its
Figure 1.
1145
nitrogen that the acidity.
of these heterocycles.
R = Et R = Et R = Me R = Me
ephedrine
9 (erythro) R = Me 10 (threo) R = Me
1146
H. TLAHUEXTand R. CONTRERAS
Adding N-Eta to oxazaborolidines spectra
indicating
the
The nitrogen
basicity
in a mixture
of THF
compounds
5
(12%)
spectra.
Two
new
doublet (Table).
near
was evaluated and
and 6
of
figure
diborane
followed
by
nitrogen detected
with
high
because
was converted NMR
boron
Both
because
the chemical
of
the
each
for
NMR
compound:
a N-BHa
a
adduct,
shifts
can be explained
increased,
is
of 6 are
were observed
concentration
the N-B
promoting
This behavior similar
of
to that
for compound 5
(12%)
in
an
can also be
for 6 indicating
atom. Diasteromers
small
by the
of the N-BHa coordination,
to the boron atom.
chiral
"B
in their
for
ppm
changes
acidity
Two diastereoisomers
into a stable
-23
to new
of oxazolidines
field.
of one hydride
NMR
bond.
NMR of compounds 1 and 2 (6 = +28 and 4b (as compound 11 6= -12.3
"B
and
broken
conversion
observed
appeared
near
As a consequence
is
13C NMR
13C
a quartet
bond
N-BHs adducts'.
by
compared
at abnormally
coordination
the
oxazolidine
when
and
"B
retrocoordination
partial
clearly
integration
of the complexes.
retrocoordination intramolecular
function
2 suffered
as was
same
of the corresponding
to a strong
BzH6 in CDCla to 1, 2 and 4 and BzH6 to 3
respectively, the
due
1 and
and with N-BHa adducts
2) appear
character
by adding
with
B-H
resonances
+29 ppm respectively31 ppm.
(43%)
of boron
Compounds
CHCla.
resonances
1 ppm
Both
l-4 showed no modification
inert behavior
of the
that the
5 could not the
reaction
mixture.
Compound
3 reacts
and adduct detected
at -17.8
with
the N-methyl
from
comparison
spectrum atoms
with
9 (87%).
was
an excess
The doublet
ppm. The
interpreted
Reaction
of compound
to dimer
10 (20%I.13C
nitrogen
atom
evidenced
from
derivatives4.
that
as
a result
indicated
and only
the presence
adducts
derived
of a fast
from
is
showed,
shown
in
equilibrium
with
signal
7 is
of only one N-epimer as was deduced
that
The
exchanges
"B
NMR
the boron
structure.
again, figure
overHauser
compared
of compound
broad
ephedrines4.
4 with BzH6 in CHC13 shows a transformation NMR
traces
a very
on the same face of the molecule
N-BHa
of the dimer
a nuclear when
groups
oxazolidines
in both positions
displacements,
13C NMR spectra
and C-methyl
with
of BaH6 in THF and CDC13, giving of the B-H disappears
2.
The
experiment those
to compound
that only one N-configuration
of
nitrogen
and
from
oxazolidine
configuration values N-BH3
of
8 (60%) and
appeared
the
adducts
was methyl and
at the clearly groups
boraspiro
Diborane derivatives
9
8 Figure The different
behavior
detected
for
1147
from N-alkyloxazaborolidines
114
2
N-ethyl and N-methyl oxazaborolidines is attributed
to the volume of the ethyl substituent which inhibits dimer formation for compounds 1 and 2.
Solvent evaporation of the solutions of S-10 makes the complexes to dissociate to the free oxazaborolidines 1-4 except for 6
which does not completely lose the BH3,
indicating a more stable coordination bond.
Addition of BzD6 in CDC13 to the free oxazaborolidines l-4 produces the same results as with BzH6, but all boron atoms were found to lose their proton coupling indicating fast exchange between them. Triethylamine addition to the BH3 complexes 5-10 promotes a decrease in the diborane complexes concentration, increasing oxazaborolidine as well as the N-BH3 adduct of triethylamine [6= -13.0 ppm). These results confirm the equilibrium between l-4 and its borane complexes. N-tertbutyl-oxazaborolidinesfailed to give N-BH3 adducts and N-benzyl give similar results to the N-ethyl derivatives.
Despite the delocalized and planar structure of oxazaborolidine, it is remarkable that they can coordinate to BH3 giving stable adducts in solution. It is also interesting that N-BH3 coordination makes acidic the boron
in
the
ring and
coordination occurs from a hydride stabilizing the boron atom.
that a
second
H. TLAHUEXTand R. CONTRERAS
1148
13C and
"B
NMH data
c-4
Compd 5 6
(6, ppm; J, Hz)
C-4-CH3
c-5
N-R
65.14 64.64
13.52 11.40
82.30 83.23
50.43 50.97
8
69.03
9.82
83.06
44.55
9 10
71.90
9.32
80.00
44.40
10.89 10.89
B-H 2.18, d (173) -0.41, d (164)
7
6.0,
We
Acknowledgements, helpful
thank
Jonathan
d (173)
1.47, d (161)
N-BHa -22.0, q (931 -23.8, q (95) -20.8, q (100) -23.1, q (95)
- 17.8 (br) - 18.0 (brl
Guthrie
for
reading
the
manuscript
and
for
his
comments.
References. 1. a) Corey E.J., b) Corey 1987,
Bakshi
E.J.,
R. K., Shibata
Bakshi
S.,
R. K., Shibata
J. Am. Chem. Sot., 1987, 109, 5551.
S., Chen
C.-P.,
V. K. Singh,
J. Am. Chem.
Sot..
109, 7925.
c) Corey E.J., Reichard
G. A., Tetrahedron
d) Mathre
T. K., Xavier
D. J., Jones
J. > Jones E. T. T., Hoogsteen
K.,
Lett.,
1989, 30, 5207.
L. C., Blacklock
T. J.,
Baun M. W., Grabowski
Reamer
E.J.J.,
R. A., Mohan
J. Org. Chem.
J.
1991,
56, 751 e) Jones Reamer
T. K..
Mohan
R. A., Jones
J. J., Xavier
L. C., Blacklock
E. T. T., Roberts
F.E.,
T. J.,
Grabowski
Mathre
E.J.J.,
D.J.,
J. Org.
Sohar
Chem.
P.,
1991,
56, 763. 2. a) Nevalainen
V., Tetrahedron
Asymmetry,
1991, 2, 63
b) Nevalainen
V., Tetrahedron
Asymmetry,
1991, 2, 429.
c) Nevalainen
V., Tetrahedron
Asymmetry,
1991, 2, 827.
d) Nevalainen
V., Tetrahedron
Asymmetry,
1991, 2, 1133.
3. Tlahuext
H., Contreras
4. a) Santiesteban Comm. b)
1983,
R., Tetrahedron
F., Grimaldo
Asymmetry,
C.. Contreras
1992, 3,727.
R., Wrackmeyer
B., J. Chem. Sot. Chem.
1486.
Contreras
R.,
Santiesteban
F.,
Paz-Sandoval
M.
A.,
Wrackmeyer
B.,
Tetrahedron,
1984, 40, 3829. cl Paz-Sandoval d)
F.
M.A., Santiesteban
Santiesteban,
Polyhedron,
M.
1984. 3, 1984.
A.
F., Contreras
Campos,
H.
R., Magn. Reson.
Morales,
R.
Chem.
Contreras,
B.
1985, 23, 428. Wrackmeyer.,
DIBORANE
0957-4166/[email protected] PergamonPressLId
DERIVATIVES
Hugo Tlahuext
FROM N-ALKYLOXAZABOROLIDINES.
and Rosalinda
Contreras*
Departamento de Quimica, Centro de Investigacibn y de Estudios Avanzados de1 Instituto Politknico National. Apartado Postal 14-740, MBxico, D. F. 07000, Mexico.
(Received 27 July 1992)
Abstract. to
N-methyl
afford
N-BH3
considered
or N-ethyl adducts.
as diborane
oxazaborolidines
N-methyl
derivatives.
derived
derivatives Borane
from
dimerize.
addition
ephedrines
Monomers
was found
react
and
with
dimers
BzH6
can
to be stereoselective
be for
8 and 10.
The
high
enantioselectivity
converted high
them
into
of
valuable
stereoselectivity
to add experimental have
already
N-acyl
into the nature
We
have
(erythro)
donor
a
of the borane
N-methyl
and pseudoephedrine
atoms give electronic atoms
Therefore,
lose
density
some
of
it was interesting
1 2 3 4
R (erythro) (three) (erythro) (three)
R R R R
= = = =
with
to the theoretical
new
of ephedrines.
synthesized
reaction
has
in the study of the nature
information
published
derivatives
reducing
A mechanism
of the reduction
of BH3 la. We are interested
as
oxazaborolidines
reagents'.
Et Et Me Me
method
of
we present
complexes
and
of this borane
preparation
to explain
results
of
have the
in the presence complex,
in order
its behavior'.
the
We from
investigation
of these oxazaborolidines.
N-ethyl
(threoJ3.
basic
to explore
5 6 7 8
catalysts to explain
oxazaborolidines3
of
our
oxazaborolidines In these
l-4
heterocycles
derived
(erythro) (three) (erythro) (three)
properties
and
the coordination
the
from
the oxygen
to the boron by retrocoordination. their
or
proposed
oxazaborolidines
efforts
Herein,
agents
been
and
It is expected boron
ability
atom
its
Figure 1.
1145
nitrogen that the acidity.
of these heterocycles.
R = Et R = Et R = Me R = Me
ephedrine
9 (erythro) R = Me 10 (threo) R = Me
1146
H. TLAHUEXTand R. CONTRERAS
Adding N-Eta to oxazaborolidines spectra
indicating
the
The nitrogen
basicity
in a mixture
of THF
compounds
5
(12%)
spectra.
Two
new
doublet (Table).
near
was evaluated and
and 6
of
figure
diborane
followed
by
nitrogen detected
with
high
because
was converted NMR
boron
Both
because
the chemical
of
the
each
for
NMR
compound:
a N-BHa
a
adduct,
shifts
can be explained
increased,
is
of 6 are
were observed
concentration
the N-B
promoting
This behavior similar
of
to that
for compound 5
(12%)
in
an
can also be
for 6 indicating
atom. Diasteromers
small
by the
of the N-BHa coordination,
to the boron atom.
chiral
"B
in their
for
ppm
changes
acidity
Two diastereoisomers
into a stable
-23
to new
of oxazolidines
field.
of one hydride
NMR
bond.
NMR of compounds 1 and 2 (6 = +28 and 4b (as compound 11 6= -12.3
"B
and
broken
conversion
observed
appeared
near
As a consequence
is
13C NMR
13C
a quartet
bond
N-BHs adducts'.
by
compared
at abnormally
coordination
the
oxazolidine
when
and
"B
retrocoordination
partial
clearly
integration
of the complexes.
retrocoordination intramolecular
function
2 suffered
as was
same
of the corresponding
to a strong
BzH6 in CDCla to 1, 2 and 4 and BzH6 to 3
respectively, the
due
1 and
and with N-BHa adducts
2) appear
character
by adding
with
B-H
resonances
+29 ppm respectively31 ppm.
(43%)
of boron
Compounds
CHCla.
resonances
1 ppm
Both
l-4 showed no modification
inert behavior
of the
that the
5 could not the
reaction
mixture.
Compound
3 reacts
and adduct detected
at -17.8
with
the N-methyl
from
comparison
spectrum atoms
with
9 (87%).
was
an excess
The doublet
ppm. The
interpreted
Reaction
of compound
to dimer
10 (20%I.13C
nitrogen
atom
evidenced
from
derivatives4.
that
as
a result
indicated
and only
the presence
adducts
derived
of a fast
from
is
showed,
shown
in
equilibrium
with
signal
7 is
of only one N-epimer as was deduced
that
The
exchanges
"B
NMR
the boron
structure.
again, figure
overHauser
compared
of compound
broad
ephedrines4.
4 with BzH6 in CHC13 shows a transformation NMR
traces
a very
on the same face of the molecule
N-BHa
of the dimer
a nuclear when
groups
oxazolidines
in both positions
displacements,
13C NMR spectra
and C-methyl
with
of BaH6 in THF and CDC13, giving of the B-H disappears
2.
The
experiment those
to compound
that only one N-configuration
of
nitrogen
and
from
oxazolidine
configuration values N-BH3
of
8 (60%) and
appeared
the
adducts
was methyl and
at the clearly groups
boraspiro
Diborane derivatives
9
8 Figure The different
behavior
detected
for
1147
from N-alkyloxazaborolidines
114
2
N-ethyl and N-methyl oxazaborolidines is attributed
to the volume of the ethyl substituent which inhibits dimer formation for compounds 1 and 2.
Solvent evaporation of the solutions of S-10 makes the complexes to dissociate to the free oxazaborolidines 1-4 except for 6
which does not completely lose the BH3,
indicating a more stable coordination bond.
Addition of BzD6 in CDC13 to the free oxazaborolidines l-4 produces the same results as with BzH6, but all boron atoms were found to lose their proton coupling indicating fast exchange between them. Triethylamine addition to the BH3 complexes 5-10 promotes a decrease in the diborane complexes concentration, increasing oxazaborolidine as well as the N-BH3 adduct of triethylamine [6= -13.0 ppm). These results confirm the equilibrium between l-4 and its borane complexes. N-tertbutyl-oxazaborolidinesfailed to give N-BH3 adducts and N-benzyl give similar results to the N-ethyl derivatives.
Despite the delocalized and planar structure of oxazaborolidine, it is remarkable that they can coordinate to BH3 giving stable adducts in solution. It is also interesting that N-BH3 coordination makes acidic the boron
in
the
ring and
coordination occurs from a hydride stabilizing the boron atom.
that a
second
H. TLAHUEXTand R. CONTRERAS
1148
13C and
"B
NMH data
c-4
Compd 5 6
(6, ppm; J, Hz)
C-4-CH3
c-5
N-R
65.14 64.64
13.52 11.40
82.30 83.23
50.43 50.97
8
69.03
9.82
83.06
44.55
9 10
71.90
9.32
80.00
44.40
10.89 10.89
B-H 2.18, d (173) -0.41, d (164)
7
6.0,
We
Acknowledgements, helpful
thank
Jonathan
d (173)
1.47, d (161)
N-BHa -22.0, q (931 -23.8, q (95) -20.8, q (100) -23.1, q (95)
- 17.8 (br) - 18.0 (brl
Guthrie
for
reading
the
manuscript
and
for
his
comments.
References. 1. a) Corey E.J., b) Corey 1987,
Bakshi
E.J.,
R. K., Shibata
Bakshi
S.,
R. K., Shibata
J. Am. Chem. Sot., 1987, 109, 5551.
S., Chen
C.-P.,
V. K. Singh,
J. Am. Chem.
Sot..
109, 7925.
c) Corey E.J., Reichard
G. A., Tetrahedron
d) Mathre
T. K., Xavier
D. J., Jones
J. > Jones E. T. T., Hoogsteen
K.,
Lett.,
1989, 30, 5207.
L. C., Blacklock
T. J.,
Baun M. W., Grabowski
Reamer
E.J.J.,
R. A., Mohan
J. Org. Chem.
J.
1991,
56, 751 e) Jones Reamer
T. K..
Mohan
R. A., Jones
J. J., Xavier
L. C., Blacklock
E. T. T., Roberts
F.E.,
T. J.,
Grabowski
Mathre
E.J.J.,
D.J.,
J. Org.
Sohar
Chem.
P.,
1991,
56, 763. 2. a) Nevalainen
V., Tetrahedron
Asymmetry,
1991, 2, 63
b) Nevalainen
V., Tetrahedron
Asymmetry,
1991, 2, 429.
c) Nevalainen
V., Tetrahedron
Asymmetry,
1991, 2, 827.
d) Nevalainen
V., Tetrahedron
Asymmetry,
1991, 2, 1133.
3. Tlahuext
H., Contreras
4. a) Santiesteban Comm. b)
1983,
R., Tetrahedron
F., Grimaldo
Asymmetry,
C.. Contreras
1992, 3,727.
R., Wrackmeyer
B., J. Chem. Sot. Chem.
1486.
Contreras
R.,
Santiesteban
F.,
Paz-Sandoval
M.
A.,
Wrackmeyer
B.,
Tetrahedron,
1984, 40, 3829. cl Paz-Sandoval d)
F.
M.A., Santiesteban
Santiesteban,
Polyhedron,
M.
1984. 3, 1984.
A.
F., Contreras
Campos,
H.
R., Magn. Reson.
Morales,
R.
Chem.
Contreras,
B.
1985, 23, 428. Wrackmeyer.,