Synthesis, structure of borylmagnesium, and its reaction with benzaldehyde to form benzoylborane

Article abstract of DOI:10.1021/ja073037t

The reaction of boryllithium 2 with 1.0 or 0.5 equiv of MgBr2·OEt2 provided boryl Grignard reagents, borylmagnesium bromides 3 and 4, or bis(boryl)magnesium 5. Structures of 3, 4, and 5 in the crystals and solutions indicated the ionic character of the B-

Full text of DOI:10.1021/ja073037t

Published on Web 07/14/2007
Synthesis, Structure of Borylmagnesium, and Its Reaction with Benzaldehyde
to Form Benzoylborane
Makoto Yamashita,* Yuta Suzuki, Yasutomo Segawa, and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The UniVersity of Tokyo,
7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
Received April 30, 2007; E-mail: makotoy@chembio.t.u-tokyo.ac.jp; nozaki@chembio.t.u-tokyo.ac.jp
Scheme 1. Reaction of Boryllithium 2 with MgBr2‚OEt2
Boron-containing reagent made a great contribution in synthetic
organic chemistry.1 Examples are found in hydroboration chemis-
try,2 Lewis acidic boron-mediated chemistry,3 boron-enolate chem-
istry,4 and recent Suzuki-Miyaura cross-coupling chemistry.5 It
should be mentioned that the boron atom is electrophilic in all cases.
On the other hand, a boron-centered nucleophile has been a
challenging target for organic chemists.6 Although there have been
several reports about the generation of borylmetal species which
may play a role as boron nucleophiles,7,8 the application of the boron
nucleophiles is still limited.
A Grignard reagent, RMgX, is one of the most important reagents
in organic chemistry since its discovery in 1900.9 From the
viewpoint of synthetic chemistry, Grignard reagents are considered
as carbanion equivalents because of the highly polarized carbon-
magnesium bond. However, the corresponding borylmagnesium has
never been reported. Our recent discovery of boryllithium 210 by
the reduction of bromoborane 1 (eq 1, the diaminoboryl substituent
is abbreviated as B unless otherwise noted) prompted us to
synthesize borylmagnesium species. Herein, we report the chemistry
of new anionic boron nucleophiles, borylmagnesium, containing
the first example of a B-Mg single bond.11
Figure 1. ORTEP drawings of 3 (left), 4 (center), and 5 (right).
Table 1. Structural Comparison between Borylmagnesiums and
Related Compounds
3
4
5
[2
‚
DME]2
B−H (6)
B-Mg (Å)
B-N (Å)
2.281(6) 2.282(6) 2.377(4)
1.453(6)
1.453(7) 1.458(7) 1.471(5) 1.465(4) 1.418(3)
1.465(7) 1.464(7) 1.487(4) 1.467(4) 1.423(3)
1.467(7)
N-B-N (°) 100.7(4) 100.5(4) 99.3(3)
99.2(2)
105.25(16)
Borylmagnesiums were prepared by the transmetallation of
boryllithium 2 with 1 or 0.5 equiv of MgBr2‚OEt2 powder in THF
at room temperature (Scheme 1). Recrystallization of the product
from the reaction with 1.0 equiv of MgBr2 gave a separable pair
of crystals which consists of colorless 3 and pale yellow 4. On the
other hand, reaction with 0.5 equiv of MgBr2 gave the corresponding
bis(boryl)magnesium species 5 as yellow crystals. X-ray crystal-
lographic analysis revealed the structures of these three molecules
(Figure 1).
chemical shift of 3 (δLi 0.4) was identical to that of LiBr in THF-
d8. That is, 4 may have a formula of 3 in THF solution even in the
presence of LiBr. Bis(borylmagnesium) 5 could be easily distin-
guished from 3 and 4 by its 1H NMR spectra. All of 3-5 showed
a broad peak at δB 37.6 in their 11B NMR spectra. These low-field
shifted broad 11B peaks also support the ionic character of B-Mg
bonds in borylmagnesium species due to their paramagnetic
contribution to shielding as was discussed about boryllithium.10 On
the basis of the above characterization, we could estimate the
reaction conversion of 2 with MgBr2‚OEt2 as follows. The reaction
of 2 with 1.0 equiv of MgBr2‚OEt2 in THF-d8 enabled us to confirm
that 90% of 2 was converted to 3 in solution with 10% of
In the crystal structures, each of these molecules includes a nearly
ideal sp2 boron atom and slightly distorted sp3 magnesium atom
with the first structurally characterized B-Mg single bonds.11 All
B-Mg bonds (Table 1) were slightly longer than the sum (2.24 Å)
of covalent radii12 of boron and magnesium atoms as was observed
in the previously reported boryllithium [2‚DME]2.10 The elongation
of the B-Mg bond can be attributed to the weakened Coulomb
interaction due to the coordination of THF molecules.13 The B-N
bonds and N-B-N angles of borylmagnesiums were closer to those
1
hydroborane 6 as judged by H NMR spectroscopy if 100% of 4
existed as 3 in solution (Scheme 1). On the other hand, only 32%
of 2 was converted to 5 by the reaction of 2 with 0.5 equiv of
MgBr2‚OEt2 in THF-d8 accompanied with 36% of hydroborane 6
and 32% of unreacted 2.
10
in boryllithium [2‚DME]2 rather than those in the protonated
hydroborane 6. The results above indicate an ionic character of
the B-Mg single bonds.
The reaction of borylmagnesium bromide 3 generated from 2 in
situ with 1-3 equiv of benzaldehyde was performed (Table 2, runs
1-3). These reactions afforded a mixture of benzoylborane 7,
boron-substituted ester 8, and hydroborane 6. Simultaneous forma-
tion of benzyl alcohol 9 suggests intermolecular hydride transfer
from a magnesium borylbenzyloxide intermediate to an excess of
Dissolution of isolated crystals of 3 and 4 into THF-d8 showed
1
identical H and 11B NMR spectra (see Supporting Information)
with appropriate integral ratio of free THF molecules which
coordinated to magnesium in the crystal. Furthermore, the 7Li NMR
9
9570
J. AM. CHEM. SOC. 2007, 129, 9570-9571
10.1021/ja073037t CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Reaction of Borylmagnesium Bromide 3 or Boryllithium 2
with Benzaldehyde (yields are based on the amount of 1)
Scientists (B) (18750027) from MEXT, Japan, and by Takeda Phar-
maceutical Company Award in Synthetic Organic Chemistry, Japan.
Supporting Information Available: All experimental procedures,
spectroscopic data of new compounds, and CIF files of 3-5 and 7.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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In conclusion, the reaction of boryllithium 2 with 1.0 or 0.5 equiv
of MgBr2‚OEt2 provided borylmagnesium bromides 3 and 4 or bis-
(boryl)magnesium 5. Structures of 3, 4, and 5 in the crystals and
solutions indicated the ionic character of the B-Mg bonds. The
reactivity of borylmagnesium 3 with benzaldehyde was different
from that of boryllithium 2. Benzoylborane 7 was fully characterized
as the first acylborane. Further studies on reactivity of borylmag-
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Acknowledgment. This work was supported by Grant-in-Aid
for Scientific Research on Priority Areas (No. 17065005, “Advanced
Molecular Transformations of Carbon Resources”) and for Young
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