Unusual products in reactions using ethyldimesitylborane, mesityllithium, and carbonyl compounds

Article abstract of DOI:10.1002/hc.1054

Unusual carbonyl adducts, Mes₂BCH = CHCHCH₃CRR¹OH, were obtained by sequential treatment of ethyldimesitylborane with mesityllithium (<1.0 equiv.) and carbonyl compounds instead of the normal adducts, Mes₂BCHCH₃CRR¹OH. A mechanistic study was carried out.

Full text of DOI:10.1002/hc.1054

Heteroatom Chemistry
Volume 12, Number 5, 2001
Unusual Products in Reactions Using
Ethyldimesitylborane, Mesityllithium, and
Carbonyl Compounds
Takayuki Kawashima, Naoko Yamashita, Tohru Kannabe,
and Renji Okazaki
Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo 7-3-1,
Bunkyo-ku, Tokyo 113-0033, Japan; E-mail: takayuki@chem.s.u-tokyo.ac.jp
Received 14 December 2000; Revised 26 February, 2001
carbonyl compounds as well as the mechanistic
study of the reactions.
ABSTRACT: Unusual carbonyl adducts, Mes2BCH
סCHCHCH3CRR1OH, were obtained by sequential
treatment of ethyldimesitylborane with mesityllithium
(ꢀ1.0 equiv.) and carbonyl compounds instead of the
normal adducts, Mes2BCHCH3CRR1OH. A mechanis-
tic study was carried out. ᭧ 2001 John Wiley & Sons,
Inc. Heteroatom Chem 12:354–357, 2001
INTRODUCTION
In the course of our studies on heteracyclobutanes 1
containing a highly coordinate main group element
at the position adjacent to the heteroatom [1], we
have reported the synthesis of tetracoordinate 1,2-
oxaboretanides 2a, namely intermediates of the Bo-
ron–Wittig reaction under basic conditions [2,3].
During investigation on the synthesis of b-hydroxy-
alkyldimesitylboranes, we have found that unusual
products were obtained when less than one equiva-
lent of mesityllithium was used. We report the de-
tails of the reactions using ethyldimesitylborane and
RESULTS AND DISCUSSION
Sequential treatment of ethyldimesitylborane (3)
with mesityllithium, carbonyl compounds such
as benzophenone, 1,3-diphenyl-2-propanone, and
benzaldehyde, and then aqueous NH4Cl gave un-
usual adducts 4 and 5 and a diastereomeric mixture
of 6a and 6b [4], respectively, instead of normal ad-
ducts 7. The results are shown in Scheme 1.
1H and 13C NMR and 2D NMR spectroscopy re-
vealed that compounds 4–6 have the CHסCH group
between the boron and methine carbon atoms of 7.
It is known that use of excess (1.5–2.0 equiv.) mesi-
tyllithium gave an orange solution, and the following
reactions with carbonyl compounds gave usual
products 7 [2,5]. On the other hand, the solution
color after the reaction with less than one equivalent
of mesityllithium became green or brown [6], and
Dedicated to Prof. Naoki Inamoto on the occasion of his 72nd
birthday.
Correspondence to: Takayuki Kawashima.
Contract Grant Sponsor: Ministry of Education, Science, Sports
and Culture of Japan.
Contract Grant Number: Grant-in-Aid for Scientific Research
No. 12304037.
᭧ 2001 John Wiley & Sons, Inc.
354

Unusual Carbonyl Compounds 355
no dimesitylpropylborane was obtained when the
green solution was treated with iodomethane, indi-
cating that the expected Mes2BCHLiCH3 (8) does not
remain in the solution [7]. In order to clarify the for-
mation mechanism of 4–6, we performed the 11B
NMR measurement of the green solution and
quenched the reaction mixture with water.
likely that the signal at d 36 is attributable to 11,
which causes the change of the color, judging from
the results of water-quenching experiments.
Thus, it can be concluded that the origin of the
CHסCH group of 4–6 is the ethyl group of 3 and that
the precursor of 4–6 is 11. Borates 9 and 10 were
treated with water to give Mes2BH and 3, respec-
tively, the former of which was oxidized to afford
Me2BOH (13) during the workup. The addition of 11
to carbonyl compounds took place at the c-position
rather than the ␣-position, because of steric hin-
drance by bulky mesityl groups [10].
The 11B NMR spectrum of the green solution
1
showed four signals at d מ23.5 (t, JBH ס 73 Hz),
1
מ12.8 (d, JBH ס 78 Hz), 36 (br s), and 84 (br s),
מ
ם
which were assigned to [Mes2BH2] Li (9), [Mes2
מ
ם
מ
ם
BHCH2CH3] Li (10), [Mes2BCH-CH-CHCH3] Li
(11), and 3, respectively. This signal assignment was
carried out by comparison with those of authentic
samples, except for the signal at d 36, as follows: bo-
rates 9 and 10 were prepared by sequential treat-
ment of Mes2BF with LiAlH4 and then LiH, and treat-
ment of 3 with t-BuLi, respectively [8,9].
As shown in Scheme 2, quenching the green so-
lution with H2O gave 3 (43%), 12 (5%), and 13 (20%).
Although quenching with D2O gave 12-d1, 3-d1 was
not obtained, also indicating the absence of 8. Al-
though a borylallylic anion is known [10], its 11B
NMR spectrum has not been reported. It is most
The plausible mechanism of the formation of 4–
6 is shown in Scheme 3. The deprotonation of 3 with
mesityllithium gives 8. Hydride shift from 8 to un-
reacted 3 [11] affords dimesitylvinylborane (14) and
borate 10. Michael type addition of 8 to 14 takes
place to give diborylcarbanion 15 that affords 11
with elimination of Mes2BH (16) [12]. The reaction
of 11 with carbonyl compounds gives 4–6 and 16
acts as a hydride acceptor of 8 and/or 10 to give 9.
In summary we have demonstrated that unusual
adducts are formed in the reactions using close to
one equivalent of MesLi and that key steps of this
reaction are a hydride shift from an ␣-lithioalkyl-
borane to an unreacted alkylborane and the forma-
tion of ␣-borylallylic anion.
SCHEME 1
SCHEME 3 Plausible mechanism for the formation of 4–6
(Cation is omitted for clarity).
SCHEME 2

356 Kawashima et al.
EXPERIMENTAL
(d, 3JHH ס 7 Hz, 2H, o-H of Ph); 13C NMR (CDCl3, 125
Hz) d 14.7 (q, CH3), 21.1 (q, p-CH3 of Mes), 23.0 (q,
o-CH3 of Mes), 48.0 (d, CHCH3), 79.8 (s, Ph2COH),
125.7 (d, o-C of Ph), 125.9 (d, o-C of Ph), 126.5 (d, p-
C of Ph), 126.6 (d, p-C of Ph), 128.0 (d, m-C of Mes),
128.1 (d, m-C of Ph), 128.2 (d, m-C of Ph), 138.1 (s,
p-C of Mes), 140.3 (s, o-C of Mes), 141.9 (br s, B-
CHסCH), 146.0 (s, ipso-C of Ph), 146.4 (s, ipso-C of
Ph), 158.7 (d, B-CHסCH). The signal due to ipso-C
of Mes was not observed. HRMS (70 eV) m/z calcd
All solvents used in the reactions were purified by
the reported methods. Tetrahydrofuran (THF) was
purified by distillation from diphenylketyl before
use. All reactions were carried out under an argon
atmosphere unless otherwise noted. Dry column
chromatography (DCC) was performed with ICN sil-
ica DCC 60A. Preparative gel permeation liquid
chromatography (GPLC) was performed by LC-908
with JAIGEL 1H ם 2H columns (Japan Analytical
for C35H3711B (M -H2O) 433.2515, found 433.2493.
ם
1
Industry) with chloroform or toluene as solvent. H
Anal. Calcd for C35H39BO•1/4 H2O: C, 85.62; H, 8.11.
Found: C, 85.77; H, 8.12.
NMR spectra were recorded on a JEOL JNM-A500,
or a Bruker AM-500 spectrometer, operated at 500
MHz. 13C NMR spectra were recorded on a Bruker
AM-500 or a JEOL JNM-A500 spectrometer at 126
MHz. 11B (86 MHz) NMR spectra were recorded on
a JEOL EXcalibur270 spectrometer, whose chemical
shifts were relative to Et2O•BF3. High resolution
mass spectra were obtained with a JEOL JMS-
SX102L spectrometer.
With 1,3-Diphenyl-2-propanone. To a solution
of bromomesitylene (0.22 mL, 1.4 mmol) in THF (3
mL) was added t-BuLi (1.62 M pentane solution, 1.7
mL, 2.8 mmol) at מ72ꢁC, and the mixture was
stirred for 15 minutes and then allowed to warm to
room temperature. To the orange solution was added
a solution of ethyldimesitylborane (3) (409 mg, 1.5
mmol) in THF (1.5 mL), and the reaction mixture
was stirred for 1 hour. The color of the solution be-
came red brown. To the solution was added a solu-
tion of 1,3-diphenyl-2-propanone (294 mg, 1.1
mmol) in THF (1.5 mL) at מ72ꢁC, and the mixture
was stirred for 1 hour. When the temperature was
allowed to warm to מ45ꢁC, the color of the solution
became green. After addition of saturated aqueous
NH4Cl, the organic layer was extracted with CH2Cl2,
and the extracts were dried over MgSO4. After re-
moval of the solvent, the residue was subjected to
DCC (SiO2 and hexane–CH2Cl2 [1:1]) followed by
GPLC to give 3 (22%), 4-benzyl-4-hydroxy-3-methyl-
5-phenyl-1-pentenyldimesitylborane (5) (18%), and
13 (19%).
Reactions of Ethyldimesitylborane (3) with
Mesityllithium
With Benzophenone. To a solution of bromo-
mesitylene (0.23 mL, 1.5 mmol) in THF (3 mL) was
added t-BuLi (1.62 M pentane solution, 2.0 mL, 3.2
mmol) at מ72ꢁC, and the mixture was stirred for 30
minutes and then allowed to warm to room tem-
perature. To the orange solution was added a solu-
tion of ethyldimesitylborane (3) (409 mg, 1.5 mmol)
in THF (1.5 mL), and the reaction mixture was
stirred for 1 hour. The color of the solution became
brown. To the solution was added a solution of ben-
zophenone (415 mg, 1.9 mmol) in THF (1.5 mL) at
מ72ꢁC, and the mixture was stirred for 1 hour. After
addition of saturated aqueous NH4Cl, the organic
layer was extracted with CH2Cl2, and the extracts
were dried over MgSO4. After removal of the solvent,
the residue was subjected to DCC (SiO2, hexane-
CH2Cl2 [1:1]), followed by GPLC to give Ph2Cס
CHCH3 (40%), 4 (7%), and 13 (40%). The formation
of Ph2CסCHCH3 (40%) indicates that at least 40%
of Mes2BCHLiCH3 (8) remained.
Compound 5: colorless viscous oil; 1H NMR
(CDCl3, 500 MHz) d 1.16 (d, 3JHH ס 6.9 Hz, 3H, CH3),
2.19 (s, 12H, o-CH3 of Mes), 2.28 (s, 6H, p-CH3 of
Mes), 2.50 (dq, 3JHH ס 6.9, 7.1 Hz, 1H, CHCH3), 2.61
(d, 2JHH ס 13.7 Hz, 1H, CHHPh), 2.69 (d, 2JHH ס 13.7
2
Hz, 1H, CHHPh), 2.74 (d, JHH ס 13.7 Hz, 1H,
CHHPh), 2.87 (d, 2JHH ס 13.7 Hz, 1H, CHHPh), 6.61
(dd, 3JHH ס 7.1, 17.3 Hz, 1H, BCHסCH), 6.74 (d, 3JHH
ס 17.3 Hz, 1H, BCHסCH), 6.81 (s, 4H, m-H of Mes),
7.09–7.27 (m, 10H, Ph); HRMS (70 eV) m/z calcd for
Compound 4: colorless crystals, m.p. 138–139ꢁC
1
3
(hexane); H NMR (CDCl3, 500 MHz) d 1.03 (d, JHH
ס 6.8 Hz, 3H, CH3), 1.96 (s, 12H, o-CH3 of Mes), 2.24
C37H4311B (M ) 514.3407, found 514.3433.
ם
3
(s, 6H, p-CH3 of Mes), 3.63 (dq, JHH ס 6.8, 8.2 Hz,
3
1H, CHCH3), 6.41 (dd, JHH ס 8.2, 17.5 Hz, 1H,
With Benzaldehyde. A similar reaction using
bromomesitylene (0.11 mL, 0.7 mmol), t-BuLi (1.77
M pentane solution, 0.8 mL, 1.4 mmol), 3 (176 mg,
0.63 mmol), and benzaldehyde (0.1 mL, 0.98 mmol)
was carried out. After addition of a solution of 3, the
color of the solution became green. Dry column
BCHסCH), 6.71 (d, 3JHH ס 17.5 Hz, 1H, BCHסCH),
6.71 (s, 4H, m-H of Mes), 7.11 (t, 3JHH ס 7 Hz, 1H, p-
3
H of Ph), 7.16 (t, JHH ס 7 Hz, 1H, p-H of Ph), 7.19
(t, 3JHH ס 7 Hz, 2H, m-H of Ph), 7.27 (t, 3JHH ס 7 Hz,
m-H of Ph), 7.40 (d, 3JHH ס 7 Hz, 2H, o-H of Ph), 7.47

Unusual Carbonyl Compounds 357
chromatography of the residue gave 3 (21%), less po-
lar diastereomer 6a (22%), and polar diastereomer
6b (12%) of 4-hydroxy-3-methyl-5-phenyl-1-butenyl-
dimesitylborane.
Compound 6a: colorless crystals: 1H NMR
(CDCl3, 500 MHz) d 1.11 (d, 3JHH ס 6.8 Hz, 3H, CH3),
2.01 (s, 12H, o-CH3 of Mes), 2.26 (s, 6H, p-CH3 of
Mes), 2.70 (ddq, 3JHH ס 6.7, 6.8, 7.7 Hz, 1H, CHCH3),
REFERENCES
[1] (a) Kawashima, T.; Okazaki, R. Synlett 1996, 600–608
and references cited therein; (b) Ohno, F.; Kawash-
ima, T.; Okazaki, R. J Am Chem Soc 1996, 118, 697–
698; (c) Kawashima, T.; Soda, T.; Okazaki, R. Angew
Chem Int Ed Engl 1996, 35, 1096–1098; (d) Kawash-
ima, T.; Ohno, F.; Okazaki, R.; Ikeda, H.; Inagaki, S.
J Am Chem Soc 1996, 118, 12455–12456; (e) Kawash-
ima, T.; Watanabe, K.; Okazaki, R. Tetrahedron Lett
1997, 38, 551–554; (f) Kawashima, T.; Okazaki, R.;
Okazaki, R. Angew Chem Int Ed Engl 1997, 36, 2500–
2502; (g) Ohno, F.; Kawashima, T.; Okazaki, R. Chem
Commun 1997, 1671–1672; (h) Kawashima, T.; Na-
ganuma, K.; Okazaki, R. Organometallics 1998, 17,
367–372.
3
3
4.56 (d, JHH ס 8.1 Hz, 1H, CHPh), 6.28 (dd, JHH ס
7.7, 17.3 Hz, 1H, BCHסCH), 6.56 (d, 3JHH ס 17.3 Hz,
1H, BCHסCH), 6.74 (s, 4H, m-H of Mes), 7.22–7.27
(m, 5H, Ph).
Compound 6b: colorless crystals: 1H NMR
(CDCl3, 500 MHz) d 0.88 (d, 3JHH ס 6.9 Hz, 3H, CH3),
2.14 (s, 12H, o-CH3 of Mes), 2.28 (s, 6H, p-CH3 of
[2] Kawashima, T.; Yamashita, N.; Okazaki, R. J Am
Chem Soc 1995, 117, 6142–6143.
3
[3] (a) Pelter, A.; Smith, K.; Brown, H. C. Borane Re-
agents; Academic Press: London, 1988; pp 338–339;
(b) Pelter, A.; Smith, K. In Comprehensive Organic
Synthesis: Selectivity, Strategy, and Efficiency in
Modern Synthetic Chemistry; Trost, B. M., Fleming,
I., Eds.; Pergamon: Oxford, 1991; Vol. 1, pp 487–503;
(c) Pelter, A. Pure Appl Chem 1994, 66, 223–233.
[4] Major diastereomer 6a was less polar than minor di-
astereomer 6b, but the stereochemistry of both dias-
tereomers has not yet been determined.
[5] Kawashima, T.; Yamashita, N.; Okazaki, R. Chem Lett
1995, 1107–1108.
[6] The color of a THF solution containing 8 is orange,
but the color changes to brown or green depending
on how much of 8 remains.
[7] For alkylation: Pelter, A.; Williams, L.; Wilson, J. W.
Tetrahedron Lett 1983, 24, 627–630.
[8] (a) Hooz, J.; Akiyama, S.; Cedar, F. J.; Bennett, M. J.;
Tuggle, R. M. J Am Chem Soc 1974, 96, 274–276; (b)
Pelter, A.; Singaram, B.; Brown, H. C. Tetrahedron
Lett 1983, 24, 1433–1436.
Mes), 2.70 (dt, JHH ס 6.9, 8.1, 8.1 Hz, 1H, CHCH3),
3
3
4.47 (d, JHH ס 8.1 Hz, 1H, CHPh), 6.44 (dd, JHH ס
8.1, 17.3 Hz, 1H, BCHסCH), 6.75 (d, 3JHH ס 17.3 Hz,
1H, BCHסCH), 6.79 (s, 4H, m-H of Mes), 7.24–7.32
(m, 5H, Ph).
With Water. A similar reaction using bromo-
mesitylene (0.06 mL, 0.36 mmol), t-BuLi (1.62 M
pentane solution, 0.44 mL, 0.72 mmol), and 3 (101
mg, 0.36 mmol) was carried out. After addition of a
solution of 3, the color of the solution became green.
After stirring was continued for 12 hours, water was
added to the solution. The organic layer was ex-
tracted with hexane, and the extracts were dried over
MgSO4. After removal of the solvent, the residue was
subjected to GPLC to 3 (43%), 1-butenyldimesityl-
borane (12) (5%), and 13 (20%).
Compound 12: Colorless oil; 1H NMR (500 MHz,
[9] Prepared by the same method reported for
3
CDCl3) d 1.04 (t, JHH ס 7.6 Hz, 3H, CH3), 2.16 (s,
מ
ם
[Mes2BHCH3] Li : Pelter, A.; Singaram, B.; Warren,
L.; Wilson, J. W. Tetrahedron 1993, 49, 2965–2978.
[10] The reaction of ␣-borrylallylic anion with carbonyl
compounds gave c-adducts, see: (a) Kow, R.; Rathke,
M. W. J Am Chem Soc 1973, 95, 2715–2716; (b) Pelter,
A.; Singaram, B.; Wilson, J. W. Tetrahedron Lett 1983,
24, 631–634.
[11] In the reaction of 3 using 1.5–2.0 equivalent of MesLi
and hexafluoroacetone, a similar unusual adduct was
formed: see Ref. [2]. Taking into consideration the
high reactivity of hexafluoroacetone as an acceptor of
hydride, this can be reasonably understood.
[12] For retro hydroboration, see: (a) Pelter, A.; Smith, K.
In Comprehensive Organic Chemistry; Barton, D. H.
R., Ollis, W. D., Eds; Pergamon: Oxford, 1973; Vol. 3,
Chapter 14.3, pp 791–881; (b) Kawashima, T.; Ya-
mashita, N.; Okazaki, R. Chem Lett 1996, 213–214.
12H, o-CH3 of Mes), 2.22–2.26 (m, 2H, CH2), 2.27 (s,
6H, p-CH3 of Mes), 6.51 (dt, 3JHH ס 17.4, 5.8 Hz, 1H,
BCHסCH), 6.64 (dt, 3JHH ס 17.4 Hz, 4JHH ס 1.2 Hz,
1H, BCHסCH), 6.79 (s, 2H, m-H of Mes); 13C{1H}
NMR(126 MHz, CDCl3) d 12.1 (s, CH2CH3), 21.1 (s,
p-CH3 of Mes), 23.2 (s, o-CH3 of Mes), 29.0 (s,
CH2CH3), 128.1 (s, m-C of Mes), 138.1, 139.1 (br s,
BCHסCH), 140.4, 142.2 (br s, ipso-C of Mes), 160.6
(s, BCHסCH); 11B NMR (86 MHz, CDCl3) d 70.7.
HRMS (70 eV) m/z calcd for C22H1911B (M )
ם
304.2362, found 304.2366.
ACKNOWLEDGMENTS
We thank Central Glass and Tosoh Finechemical
Corporation for the gifts of hexafluoroacetone tri-
hydrate and alkyllithiums, respectively.