Synthesis and structural characterization of the bis(diisopropylamino)boron enolate of tert-butyl methyl ketone

Article abstract of DOI:10.1021/om700676w

Lithium, sodium, and potassium enolates reacted with bisaminoboron halides to give bisaminoboron enolates 1a-5c. Specifically, the potassium enolate of tert-butyl methyl ketone reacted with bis-(diisopropylamino)boron chloride in THF at room temperature within 1h to give the bis(diisopropylamino)-boron enolate of tert-butyl methyl ketone (3b) in 61-84% isolated yields. Under similar conditions, the reactivity is highly dependent on the metal employed to generate enolates (K > Na > Li) as well as the nitrogen substituents in the bisaminoboron halides (iPr > Et > TMS). This latter observation is a compromise between the boron-nitrogen resonance and the steric effect. The structural information of enolate 3b was studied in detail. Unlike the aggregated alkali-metal enolates, this boron enolate exists exclusively as a monomer in both the solid and solution states, as identified by X-ray and diffusion-ordered NMR.

Full text of DOI:10.1021/om700676w

5834
Organometallics 2007, 26, 5834-5839
Synthesis and Structural Characterization of the
Bis(diisopropylamino)boron Enolate of tert-Butyl Methyl Ketone
Lili Ma,† Russell Hopson,† Deyu Li,† Yong Zhang,‡ and Paul G. Williard*,†
Department of Chemistry, Brown UniVersity, ProVidence, Rhode Island 02912 and Department of
Chemistry, Boston UniVersity, 590 Commonwealth AVenue, Boston, Massachusetts 02215
ReceiVed July 5, 2007
Lithium, sodium, and potassium enolates reacted with bisaminoboron halides to give bisaminoboron
enolates 1a-5c. Specifically, the potassium enolate of tert-butyl methyl ketone reacted with bis-
(diisopropylamino)boron chloride in THF at room temperature within 1 h to give the bis(diisopropylamino)-
boron enolate of tert-butyl methyl ketone (3b) in 61-84% isolated yields. Under similar conditions, the
reactivity is highly dependent on the metal employed to generate enolates (K > Na > Li) as well as the
nitrogen substituents in the bisaminoboron halides (iPr > Et > TMS). This latter observation is a
compromise between the boron-nitrogen resonance and the steric effect. The structural information of
enolate 3b was studied in detail. Unlike the aggregated alkali-metal enolates, this boron enolate exists
exclusively as a monomer in both the solid and solution states, as identified by X-ray and diffusion-
ordered NMR.
Table 1. Conditions for Bisaminoboron Enolates from
Bisaminoboron Chlorides and Metal Enolates
Introduction
Boron enolates enjoy widespread use in synthetic organic
chemistry due to the relatively mild conditions used to generate
these intermediates and the stereoselectivity observed in both
the enolization reaction and the subsequent aldol reactions.1 The
most widely utilized asymmetric synthetic methodology involv-
ing boron enolates developed from the extensive studies of
Evans and co-workers beginning 30 years ago.2 H. C. Brown
also reported extensively on the stereoselectivity of enolboration
and aldol reaction of the corresponding boron enolates starting
from dialkyl boron halides.3 In light of our previous structural
characterizations of Li, Na, K, and Mg enolates of simple
ketones4 and the dearth of structural information on simple boron
enolates,5 we report the preparation of several bisaminoboron
* To whom correspondence should be addressed. Fax: 01-401-863-9368.
E-mail: pgw@brown.edu.
† Brown University, Providence.
‡ Boston University, Boston.
(1) (a) Ishihara, K.; Yamamoto, H. Modern Aldol Reactions; Wiley-VCH
Verlag GmbH: Weinheim, FRG, 2004; Vol. 2, pp 25-68. (b) Mukaiyama,
T.; Matsuo, J.-I. Modern Aldol Reactions; Wiley-VCH Verlag GmbH:
Weinheim, FRG, 2004; Vol. 1, pp 127-160. (c) Cowden, C. J.; Paterson,
I. Organic Reactions; John Wiley & Sons: New York, 1997; Vol. 51, pp
1-200.
(2) (a) Evans, D. A.; Cote, B.; Coleman, P. J.; Connell, B. T. J. Am.
Chem. Soc. 2003, 125, 10893. (b) Duffy, J. L.; Yoon, T. P.; Evans, D. A.
Tetrahedron Lett. 1995, 36, 9245. (c) Evans, D. A.; Nelson, J. V.; Vogel,
E.; Taber, T. R. J. Am. Chem. Soc. 1981, 103, 3099. (d) Evans, D. A.;
Bartroli, J.; Shih, T. L. J. Am. Chem. Soc. 1981, 103, 2127. (e) Evans, D.
A.; Taber, T. R. Tetrahedron Lett. 1980, 21, 4675. (f) Evans, D. A.; Vogel,
E.; Nelson, J. V. J. Am. Chem. Soc. 1979, 101, 6120. (g) Evans, D. A.;
Thomas, R. C.; Walker, J. A. Tetrahedron Lett. 1976, 1427. (h) Evans, D.
A.; Crawford, T. C.; Thomas, R. C.; Walker, J. A. J. Org. Chem. 1976, 41,
3947.
(3) (a) Ramachandran, P. V.; Zou, M.-F.; Brown, H. C. HelV. Chim.
Acta 2002, 85, 3027. (b) Ganesan, K.; Brown, H. C. J. Org. Chem. 1994,
59, 7346. (c) Ganesan, K.; Brown, H. C. J. Org. Chem. 1993, 58, 7162. (d)
Ganesan, K.; Brown, H. C. J. Org. Chem. 1994, 59, 2336. (e) Brown, H.
C.; Ganesan, K.; Dhar, R. K. J. Org. Chem. 1993, 58, 147. (f) Brown, H.
C.; Ganesan, K.; Dhar, R. K. J. Org. Chem. 1992, 57, 3767. (g) Brown, H.
C.; Dhar, R. K.; Ganesan, K.; Singaram, B. J. Org. Chem. 1992, 57, 2716.
(h) Brown, H. C.; Dhar, R. K.; Ganesan, K.; Singaram, B. J. Org. Chem.
1992, 57, 499.
a Yields before recrystallization. Their purity was confirmed by 1H and
11B NMR spectra.
enolates derived from simple ketones and the crystal structure
of the bis(diisopropylamino)boron enolate of tert-butyl methyl
ketone.
Results and Discussion
Synthesis of Bisaminoboron Enolates. As is true for many
aldol reactions, the role of boron enolates is often crucial. As a
starting point to study the mechanism of boron-mediated
reactions, we concentrate our investigation on the boron enolates
listed in Table 1, a compound class that performs aldol-type
and Mannich-type reactions with aldehydes. These compounds
10.1021/om700676w CCC: $37.00 © 2007 American Chemical Society
Publication on Web 10/18/2007

Synthesis of the Bis(diisopropylamino)boron Enolate
Organometallics, Vol. 26, No. 24, 2007 5835
Figure 1. B3LYP/6-31G*-optimized geometries of (Et2N)2BCl, (iPr2N)2BCl, and (TMS2N)2BCl. Hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Distances, Torsion Angles, and
Partial Charges for Bisaminoboron Chlorides
on N increase with the increase of bond length. All this evidence
suggests that the amount of double bond decreases with
increasing steric effect.
compound
(Et2N)2BCl (iPr2N)2BCl (TMS2N)2BCl
A second yet simple piece of evidence for the B-N π
bonding in these complexes comes from the 11B chemical shift.
In similar structures, higher field chemical shifts are indicative
of stronger resonance interaction.7 The 11B spectra for (TMS2N)2-
BCl, (iPr2N)2BCl, and (Et2N)2BCl shift upfield, -32.0, -30.2,
and -27.9 ppm, respectively, as the resonance effect increases.
As the B-N π bond character decreases in the order (Et2N)2-
BCl > (iPr2N)2BCl > (TMS2N)2BCl, the Lewis acidity on boron
increases and the reactivity increases correspondingly. Alter-
natively, as the steric hindrance of the nitrogen substituents
increases in the order (Et2N)2BCl < (iPr2N)2BCl < (TMS2N)2-
BCl, the accessibility of boron to nucleophiles decreases and
the reactivity decreases correspondingly. On the basis of this
reasoning, the substituent effects observed in Table 1 were
interpreted as the interplay of steric effects and B-N π bonding.
The steric effect is more important than the π bonding effect
since the TMS derivatives reacted much slower and often needed
elevated temperatures and longer reaction times.
Structural Features of Enolate 3b. In addition to the
synthesis of boron enolates, we were also keen to know their
structure and aggregation state. Crystals of enolate 3b8 suitable
for X-ray diffraction analysis were prepared from heptane at
low temperature. A computer-generated plot of this crystal
structure is depicted in Figure 2,9 and the related crystallographic
data are summarized in Table 3.
This boron enolate is a covalent monomer unlike the
corresponding alkali metal pinacolone enolates, all of which
exist as ionic aggregates. Significant structural features are as
follows. The B atom is trigonal planar and only 0.03 Å above
the plane formed by N(1), N(2), and O. A torsion angle of
approximately 18° is observed for B-O-C(2)-C(1). Both
nitrogen atoms are also relatively planar; however, there is an
observable difference in the two B-N bond lengths. These bond
lengths correlate to an observable difference in dihedral angles.
Hence, the shorter B-N(1) bond, 1.417 Å, is associated with a
B(1)-N(2) (Å)
1.42353
1.42492
1.83861
17.065
1.41903
1.44174
1.85500
18.211
1.43736
1.43736
1.86900
36.685
B(1)-N(3) (Å)
B(1)-Cl(4) (Å)
Cl(4)-B(1)-N(2)-C (deg)
Cl(4)-B(1)-N(3)-C (deg)
B(1) (e)
27.645
37.734
36.684
+0.378
-0.357
-0.362
-0.258
+0.402
-0.368
-0.413
-0.259
+0.497
-0.790
-0.790
-0.258
N(2) (e)
N(3) (e)
Cl(4) (e)
were prepared by reaction of preformed Li, Na, or K enolates
of the corresponding ketones with bisaminoboron halides.6 The
yield of boron enolate formation is highly dependent on the
alkali-metal cation in the expected order K > Na > Li and also
on the substituents on nitrogen iPr > Et > TMS. Two factors
might account for the latter observation: boron-nitrogen π
bonding and steric hindrance.
The B-N π bond character arises from delocalization of the
nitrogen lone pair into the empty 2p orbital on boron. Several
lines of evidence support a B-N π bonding increase in the
bisaminoboron chlorides in the following order: (TMS2N)2BCl
< (iPr2N)2BCl < (Et2N)2BCl. Hence, a density functional theory
(DFT) study was undertaken. The optimized geometries are
shown in Figure 1.
Calculated bond distances and partial charges on these atoms
(B3LYP/6-31G*) are summarized in Table 2. The two B-N
bond lengths for (Et2N)2BCl are both short with Cl-B-N-C
torsion angles of about 17.0° and 27.6°, indicating formation
of partial double bonds. For (iPr2N)2BCl, the B(1)-N(2) is short,
only 1.419 Å with a 18.2° torsion angle, while the other is
longer, 1.442 Å with a 37.7° torsion angle, indicating the
existence of two unequal bonds. In the case of (TMS2N)2BCl,
both B-N bonds are long and both torsion angles are large.
Additionally, the positive charge on B and the negative charge
(4) (a) Sun, C.; Williard, P. G. J. Am. Chem. Soc. 2000, 122, 7829. (b)
Williard, P. G.; MacEwan, G. J. J. Am. Chem. Soc. 1989, 111, 7671. (c)
Williard, P. G.; Hintze, M. J. J. Am. Chem. Soc. 1987, 109, 5539. (d)
Williard, P. G.; Salvino, J. M. J. Chem. Soc., Chem. Commun. 1986, 153.
(e) Williard, P. G.; Carpenter, G. B. J. Am. Chem. Soc. 1986, 108, 462.
(5) The closest analogs include structures reported for derivatives of
oxazaborolidines: Weber, L.; Schnieder, M.; Maciel, T. C.; Wartig, H. B.;
Schimmel, M.; Boese, R.; Blaeser, D. Organometallics 2000, 19, 5791.
Weber, L.; Rausch, A.; Stammler, H.-G.; Neumann, B. Z. Anorg. Allg.
Chem. 2005, 631, 1633. 2-Oxa-1-boranaphthalene: Arcus, V. L.; Main,
L.; Nicholson, B. K. J. Organomet. Chem. 1993, 460, 139. Unpublished
diphenylborane enolate: CCDC 216036.
(7) (a) Haberecht, J.; Krummland, A.; Breher, F.; Gebhardt, B.; Rueegger,
H.; Nesper, R.; Gruetzmacher, H. Dalton Trans. 2003, 2126. (b) Chavant,
P. Y.; Vaultier, M. J. Organomet. Chem. 1993, 455, 37.
(8) A certain amount of anhydrous heptane was added to the crude
product of 3b to make a ∼1.6 M solution of boron enolate. After centrifuging
briefly, the clear solution was transferred to flamed-dried glass tube and
placed in a freezer at -50 °C during which time the boron enolate crystals
formed.
(9) Crystals of 3b were mounted on a Bruker Apex I diffractometer under
a stream of dry nitrogen at -70 °C. Structure solution and refinement was
carried out using Shelxtl programs. Crystallographic data was deposited
with the Cambridge Crystallographic Data Centers#647918.
(6) (a) Brown, H. C.; Ravindran, N. Inorg. Chem. 1977, 16, 2938. (b)
Chavant, P. Y.; Vaultier, M. J. Organomet. Chem. 1993, 455, 37. (c)
Geymayer, P.; Rochow, E. G. Monatsh. Chem. 1966, 97, 429.

5836 Organometallics, Vol. 26, No. 24, 2007
Ma et al.
determined by diffusion-ordered NMR spectroscopy (DOSY),
the relative ratio of the hydrodynamic radii of 3b and the internal
standard perylene is 1.186. For comparison, the geometry of
these compounds was optimized by DFT calculations, and the
theoretical ratio of the hydrodynamic radii was determined to
be 1.155, which agrees with the experimentally derived value
determined by NMR. This DOSY spectrum unambiguously
indicates that 3b exists exclusively as a monomer in solution,
although dimeric aminoboranes are known.13 The strong interac-
tion of the lone pairs on the attached heteroatoms with the B
precludes formation of higher aggregates.14
From the 1H NMR and 13C NMR spectra it can be seen that
all of the isopropyl groups are equivalent at room temperature.
A consequence of the relative planarity of the N, N, O, B core
of this compound is the extent of bonding interaction between
the B and the covalently attached heteroatoms. Additionally, it
is useful to utilize dynamic NMR spectroscopy to gain insight
into the rotational barriers about the B-N bonds. Corresponding
studies of monoaminoboranes, R2N-BR′2, bisaminoboranes,
(R2N)2-BR′, and trisaminoboranes, (R2N)3B, suggest rotational
Figure 2. X-ray crystal structure of bis(diisopropylamino)boron
enolate of pinacolone (3b). Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg) with estimated standard
deviations: B(1)-N(1) 1.417(2); B(1)-N(2) 1.458(2); B(1)-O(1)
1.414(2); O(1)-B(1)-N(1) 115.68(16); O(1)-B(1)-N(2) 121.71-
(15); N(1)-B(1)-N(2) 122.52(17).
Table 3. Crystallographic Parameters for 3b
barriers in the ranges of 17-24 kcal mol-1, 10-11 kcal mol-1
,
formula
fw
C18H39BN2O
310.32
and complicated, correlated rotation barriers, respectively.15 In
all cases increasing the number and availability of lone pair
electrons on covalently attached heteroatoms lowers the rota-
tional barrier and the corresponding bond order of the B-het-
eroatom interaction. For enolate 3b, we chose to monitor the
methine carbon in the isopropyl group. The NMR spectra were
recorded in 10 K increments from room temperature to 123 K
in freon-12/THF-d8 (80:20 v:v). A small peak begins to appear
at 149 K in a ratio of 3:1 relative to a larger peak. A chemical
shift difference of 302 Hz between these two peaks combined
with the coalescence temperature of 159 K implies a rotational
barrier of 7.05 kcal‚mol-1. This value compares favorably with
the values reported for bisaminoboranes.
In an attempt to explain the observed 3:1 ratio of isopropyl
methine peaks in the NMR, we performed chemical shift
calculations using Gaussian03. The geometry and electronic
structure of the molecule were investigated at the B3LYP level
with the cc-pvdz basis set. The NMR data were calculated by
means of the individual gauges for atoms in molecules (IGAIM)
and continuous set of gauge transformations (CSGT) methods.16
The optimized geometry is in excellent agreement to the X-ray
conformation. The chemical shift calculations from IGAIM and
CSGT are almost identical for compound 3b. On an absolute
scale, the 13C chemical shifts for the methine carbons are 139.71,
139.32, 139.28, and 138.55 ppm and the chemical shift
differences between adjacent carbons are 0.39, 0.04, and 0.73
ppm. Even though these data are not calibrated, a clear trend
can be seen that three methine carbons are very close to each
other while the fourth methine carbon is distinguishable, leading
to the observed 3:1 ratio of peaks. It is interesting to note that
the distinguishable methine carbon is in the isopropyl group
cryst size (mm)
cryst syst
space group
a (Å)
b (Å)
c (Å)
0.3 × 0.2 × 0.1 mm
monoclinic
P21/c
9.718(2)
18.126(3)
12.198(2)
109.965(3)
2019.6(6)
4
1.021
0.061
Mo KR 0.71073
212
â (deg)
V (Å3)
Z
D
calc (g cm-3
)
µ/mm-1
radiation
T/K
2θmax
F(000)
28.24
696
R1 (I > 2σ(I))
wR2 (all data)
goodness of fit
0.0669 (I > 2)
0.1764(I > 2)
0.933
dihedral angle of approximately 19° between the methine
C-N(2)-C plane and the N-B-N plane. A longer B-N(2)
bond, 1.458 Å, is observed with a corresponding dihedral angle
of nearly 48°. In the solid state, a geometry that favors overlap
of the empty orbital on boron with the lone pair on N clearly
leads to a shorter B-N bond length.
1H, 13C, and 11B NMR spectra of 3b are shown in Figure 3.
From the extensive compilation of NMR data on organoboron
compounds compiled by No¨th and Wrackmeyer, trialkoxybo-
ranes exhibit an 11B chemical shift of approximately -18 ppm
whereas the bis(alkyl) monoalkoxyboranes appear at about -55
ppm.10 The 11B chemical shift of -26 ppm (C6D6 solution) for
3b is between these two values but closer to that of a
trialkoxyborane. The 11B chemical shift found is identical to
that reported by Suginome and Murakami for the bis(diethy-
lamino)boron enolate of acetophenone.11
We also carried out a pulsed field gradient, diffusion-oriented
NMR experiment on a solution of this enolate. Diffusion-ordered
NMR spectroscopy (DOSY) has received increasing attention
recently for its ability to identify reaction intermediates and
aggregation status in solution.12 The DOSY NMR spectrum is
shown in Figure 4. On the basis of the diffusion coefficients
(12) (a) Keresztes, I.; Williard, P. G. J. Am. Chem. Soc. 2000, 122, 10228.
(b) Schlorer, N. E.; Cabrita, E. J.; Berger, S. Angew. Chem., Int. Ed. 2002,
41, 107. (c) Cohen, Y.; Avram, L.; Frish, L. Angew. Chem., Int. Ed. 2005,
44, 520.
(13) (a) Coates, G. E.; Livingstone, J. G. J. Chem. Soc. 1961, 1000. (b)
Wiberg, E.; Hertwig, K.; Bolz, A. Z. Anorg. Allg. Chem. 1948, 256, 177.
(14) Lappert, M. F.; Sanger, A. R.; Srivastava, R. C.; Power, P. P. Metal
and Metalloid Amides: Synthesis, Structure, and Physical and Chemical
Properties; John Wiley & Sons: New York, 1980.
(15) (a) Beswick, Y. F.; Wisian-Neilson, P.; Neilson, R. H. J. Inorg.
Nuclear Chem. 1981, 43, 2639. (b) Imbery, D.; Jaeschke, A.; Friebolin, H.
Org. Magn. Reson. 1970, 2, 271. (c) Dewar, M. J. S.; Rona, P. J. Am.
Chem. Soc. 1969, 91, 2259.
(16) (a) Baldridge, K. K.; Siegel, J. S. J. Phys. Chem. A 1999, 103, 4038.
(b) Huang, M.-J.; Lee, K. S. Mol. Phys. 2005, 103 (15-16), 2229.
(10) Noeth, H.; Wrackmeyer, B. NMR Basic Principles and Progress;
Springer-Verlag: New York, 1978; Vol. 14: Nuclear Magnetic Resonance
Spectroscopy of Boron Compounds, Tables XI and XII, pp 136-139.
(11) Suginome, M.; Uehlin, L.; Yamamoto, A.; Murakami, M. Org. Lett.
2004, 6, 1167.

Synthesis of the Bis(diisopropylamino)boron Enolate
Organometallics, Vol. 26, No. 24, 2007 5837
Figure 3. NMR spectra of bisaminoboron enolate. (a) 1H NMR (300 MHz, C6D6): Proton Ha (4.27 ppm) shows an NOE with a tert-butyl
group (1.23 ppm); Proton Hb (4.14 ppm) shows an NOE with iPr groups (1.20 ppm). (b) 11B NMR (95 MHz, C6D6). (c) 13C NMR (75 MHz,
C6D6): the chemical shift for tertiary carbon Ca (167.0 ppm) and methylene carbon Cb (87.0 ppm) is in agreement with other metal enolates.
1
Figure 4. DOSY H NMR (C6D6, 400 MHz) of boron enolate, perylene, DIPEA, and benzene-d6. The diffusion coefficients calculated
from the Stejskal-Tanner equation, ln(I/I0) ) -[γ2δ2 (∆ - δ/3)] D‚G2, are 8.796 × 10-10, 1.043 × 10-9, 1.575 × 10-9, and 1.860 × 10-9
m2‚s-1 respectively. I is the peak area, I0 is the peak area in the absence of gradients, γ is the magnetogyric ratio of the observed nucleus,
δ is the gradient duration, G is the strength of the gradient pulse in T/m, ∆ is the diffusion time, and D is the diffusion coefficient.
that exhibited a NOE with vinylic proton b (see Figure 3).
Although it is possible that the NMR spectra derive from a THF
solvated species because the solvent utilized for these NMR
experiments was a deuterated THF/freon mixture, we suggest
that solvation of the boron complex by THF is not observed by
NMR and that the chemical shift differences are a consequence
of the structural features of this enolate complex. Additionally,
the diffusion-oriented NMR spectra exhibit no evidence of THF
solvates.
We also determined the rotational barrier of the enolate 4b
to be 7.38 kcal‚mol-1 in keeping with the expected greater steric
hindrance to rotation of the methyl-substituted enolate.

5838 Organometallics, Vol. 26, No. 24, 2007
Ma et al.
coefficient was determined by fitting the peak areas to the Stejskal-
Tanner equation. For the low-temperature NMR, a mixture of boron
enolate:THF-d8:CF2Cl2 ) 20%:20%:60% was prepared. The sample
was degassed and sealed in a heavy-wall NMR tube (Wilmad 522-
PP-7) using a torch. Sample temperature was controlled using a
Bruker Eurotherm 3300 temperature controller and a liquid nitrogen/
nitrogen gas heat exchanger. Sample temperature was calibrated
with a standard methanol sample.
X-ray Structure Determinations. Single crystals of 3b suitable
for X-ray diffraction studies were grown from heptane at low
temperature. Crystallographic data and details of the refinement
are summarized in Table 3. Crystallographic calculations were
carried out using SHELXTL. Crystals were collected on a Bruker
Smart Apex I diffractometer. The structure was solved from direct
methods and Fourier syntheses and refined by full-matrix least-
squares procedures with anisotropic thermal parameters for all non-
hydrogen atoms. Hydrogen atoms bonded to carbon atoms were
included in calculated positions (C-H 0.98 Å) and refined riding
on their attached atom. CCDC-647918 contains the supplementary
crystallographic data for this paper.
General Procedure for Preparation of Bisaminoboron Eno-
lates. Using Lithium Enolates. To a solution of diisopropylamine
(DIPA) (6.8 mmol) in THF (6 mL) was added n-butyllithium (6.4
mmol, 2.42 M solution in hexanes) under an argon atmosphere at
- 20 °C and stirred for 30 min. The reaction mixture was then
added dropwise the solution of ketone (6.2 mmol) in THF (1 mL).
After stirring at -20 °C for an additional 30 min, a solution of
chlorobis(dialkylamino)borane (6 mmol) in THF (2 mL) was added
to the reaction mixture and stirred at room temperature for 24 h.
The resulting solution was subject to centrifuge, and the LiCl
precipitate was filtered. Evaporation of the volatile solvents followed
by vacuum removal of diisopropylamine gave the crude product,
which was purified by distillation or crystallization.
The most commonly used enolate forming boron reagents in
organic synthesis include dibutylboron triflate and dicyclohexy-
lboron chloride, which favor Z- and E-boron enolates, respec-
tively. Initially we chose amino substituents, such as hexame-
thyldisilizane (HMDS), diisopropylamine (DIPA), and diethyl-
amine, because the resultant bisaminoboron enolates are more
stable in the solid state. As illustrated by others, bisaminoboron
enolates are used to effect aldol reactions and produce â-amino
carbonyl compounds in a Mannich-type reaction.17 The ratio
of these products is controlled by temperature, solvent, and
substituents.
Conclusion
In summary, we described the synthesis of bisaminoboron
enolates and structure investigations of compound 3b. This study
addresses the resonance and steric effects on formation of
bisaminoboron enolates. Taking all the experimental and
theoretical investigations into consideration, it can be concluded
that the greater reactivity observed for (iPr2N)2BCl is a
compromise between the boron-nitrogen resonance and the
nitrogen substituent steric effect. In addition, the structure and
aggregation states of 3b were investigated by X-ray crystal-
lography and diffusion-ordered NMR techniques. It is important
to point out that bisaminoboron enolate 3b exists exclusively
as monomer in both the solid state and solution states, indicating
the boron center is open to coordinate to an incoming Lewis
base. The methodology employed here is extendable to ad-
ditional boron enolate systems. Investigations of the transition
states in formation and subsequent reactions of boron enolates
are currently in progress.18
Using Sodium or Potassium Enolates. To a solution of sodium
hexamethyldisilazane (NaHMDS) or potassium hexamethyldisila-
zane (KHMDS) (21 mmol) in THF (15 mL) was added ketone (20.5
mmol) in THF (2 mL) dropwise at 0 °C under an argon atmosphere
for 30 min. A solution of chlorobis(dialkylamino)borane (20 mmol)
in THF (10 mL) was added to the reaction mixture and stirred at
room temperature or 60 °C for 1-12 h. The resulting solution was
subject to centrifuge, and the NaCl or KCl precipitate was filtered.
Evaporation of the volatile solvents followed by vacuum removal
of HMDS gave the crude product, which was further purified by
distillation or recrystallization.
Bis(diethylamino)boron Enolate 3a. Isolated yield: 3.66 g,
14.4mmol, 72%. Bp 90 °C/0.5 mmHg. 1H NMR (300 MHz,
C6D6): 0.95 (s, 9H), 1.05 (triplet, J ) 7.0 Hz, 12H), 2.86-2.98
(q, J ) 7.0 Hz, 8H), 4.05 (d, J ) 0.7 Hz, 1H), 4.09 (d, J ) 0.7 Hz,
1H) ppm. 13C NMR (100 MHz, C6D6): 15.1, 19.2, 40.5, 46.0, 87.6,
156.8 ppm. 11B NMR (96 MHz, C6D6): -24.4 ppm.
Bis(diisopropylamino)boron Enolate 3b. Isolated yield: 5.21
g, 16.8 mmol, 84%. It is very sensitive and readily hydrolyzes in
air. Bp 105 °C/0.5 mmHg. 1H NMR (300 MHz, C6D6): 1.20 (d, J
) 6.9 Hz, 24H), 1.24 (s, 9H), 3.38-3.47 (heptet, J ) 6.6 Hz, 4H),
4.14 (d, J ) 0.8 Hz, 1H), 4.27 (d, J ) 0.8 Hz, 1H) ppm. 13C NMR
(75 MHz, C6D6): 24.4, 29.4, 36.3, 46.3, 87.0, 167.0 ppm. 11B NMR
(95 MHz, C6D6): -26.1 ppm.
Experimental Section
General. All reactions were conducted under Ar atmosphere in
flame-dried glassware under vacuum equipped with magnetic
stirring bars. Ketones were purified by distillation over CaCl2 before
use. THF was purified by a solvent-dispensing system. Anhydrous
heptane was purchased from Aldrich. BCl3 was converted to
Me2S‚BCl3 for easier handling. CF2Cl2 was purchased from
SynQuest. IR spectra were recorded on a Perkin-Elmer 1600 series
FTIR.
NMR Measurements. 1H, 13C, and 11B NMR spectra were
recorded on a Bruker DRX spectrometer. The spectra were recorded
in C6D6 at room temperature. C6D6 solvent peaks were used as
internal standard for 1H NMR (7.15) and 13C NMR (128.02). (CH3-
CH2)2O‚BF3 was used as external standard for 11B NMR; negative
values are downfield from the standard. Data are reported as
follows: chemical shift, multiplicity (s ) singlet, d ) doublet, t )
triplet, q ) quartet, h ) heptet, br ) broad, m ) multiplet), coupling
constants (Hz). DEPT135, HSQC, and NOESY NMR spectra were
collected on a Bruker DRX-400 instrument equipped with an
Accustar z-axis gradient amplifier and a QNP probe with a z-axis
gradient coil. For the diffusion-ordered NMR (DOSY), the
LEDbpgp2s pulse sequence was used. The pulse-field gradients (g)
were incremented in 64 steps from 2% to 95% of the maximum
gradient strength in a linear ramp. A gradient length δ of 1 ms
(P30 ) 0.5 ms), a diffusion time ∆ of 0.2 s (D20), and an eddy
current delay of 5 ms (D21) were employed. The diffusion
Bis(diisopropylamino)boron Enolate 4b. Isolated yield: 5.18
1
g, 16.0 mmol, 80%. Bp 120 °C/0.2 mmHg. H NMR (400 MHz,
C6D6): 1.12 (s, 9H), 1.19 (d, J ) 6.6 Hz, 24H), 1.59 (d, J ) 6.9
Hz, 3H), 3.44-3.53 (heptet, J ) 6.8 Hz, 4H), 4.54 (quartet, J )
6.9 Hz, 1H ppm). 13C NMR (100 MHz, C6D6): 12.5, 23.8, 28.5,
35.6, 45.4, 94.6, 157.9 ppm. 11B NMR (96 MHz, C6D6): -25.6
ppm.
(17) (a) Suginome, M.; Uehlin, L.; Murakami, M. J. Am. Chem. Soc.
2004, 126, 13196. (b) Corey, E. J.; Huang, H. C. Tetrahedron Lett. 1989,
30, 5235. (c) Hoffmann, R. W.; Ditrich, K.; Froech, S. Liebigs Ann. Chem.
1987, 977. (d) Hoffman, R. W.; Ditrich, K.; Forech, S.; Cremer, D.
Tetrahedron 1985, 41, 5517.
(18) Preliminary results were presented: Ma, L.; Williard, P. G. Abstracts
of Papers, 233rd ACS National Meeting, Chicago, IL, M 25-29, 2007;
American Chemical Society: Washington, DC, 2007; ORGN-779.
Acknowledgment. This work was supported by PHS-NIH
GM-35982 and by the NSF CHE-0718275.

Synthesis of the Bis(diisopropylamino)boron Enolate
Organometallics, Vol. 26, No. 24, 2007 5839
Supporting Information Available: Cartesian coordinates of
(Et2N)2BCl, (iPr2N)2BCl, (TMS2N)2BCl, 3b, and perylene; X-ray
crystal structure of 3b in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org. Crystallographic
data have also been deposited with the Cambridge Crystallographic
Data Centre: CCDC-647918 (3b). Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: (+44)-1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
OM700676W