Deprotonation from an N-methyl group in 2-[1-(dimethylamino)-1-methylethyl]phenylborane derivatives

Article abstract of DOI:10.1021/om990724n

Reaction of 2-[1-(dimethylamino)-1-methylethyl]phenyllithium (Ar*Li) with a trialkyl borate, B(OR)₃, in the 3:1 ratio gave 1-Ar*-3,4,4-trimethyl-1,2,3,4-tetrahydro-3,1-benzazaborin as a major product together with the corresponding protonated compound and the boronic acid. The structure of the hetelocyclic compound was determined by X-ray analysis and NMR spectroscopy. This compound is formed via the deprotonation from one of the N-Me groups in Ar*₂B(OR) by the remaining Ar*Li followed by the facile intramolecular cyclization between the boron and carbon atoms.

Full text of DOI:10.1021/om990724n

206
Organometallics 2000, 19, 206-208
Notes
Dep r oton a tion fr om a n N-Meth yl Gr ou p in
2-[1-(Dim eth yla m in o)-1-m eth yleth yl]p h en ylbor a n e
Der iva tives
Mitsuhiro Asakura, Michinori Oh ki, and Shinji Toyota*
Department of Chemistry, Faculty of Science, Okayama University of Science, Ridaicho,
Okayama 700-0005, J apan
Received September 14, 1999
Ch a r t 1
Summary: Reaction of 2-[1-(dimethylamino)-1-methyl-
ethyl]phenyllithium (Ar*Li) with a trialkyl borate, B(OR)3,
in the 3:1 ratio gave 1-Ar*-3,4,4-trimethyl-1,2,3,4-tet-
rahydro-3,1-benzazaborin as a major product together
with the corresponding protonated compound and the
boronic acid. The structure of the hetelocyclic compound
was determined by X-ray analysis and NMR spectros-
copy. This compound is formed via the deprotonation
from one of the N-Me groups in Ar*2B(OR) by the
remaining Ar*Li followed by the facile intramolecular
cyclization between the boron and carbon atoms.
Sch em e 1
In tr od u ction
Recently, we revealed that the amine ligand in 2-[1-
(dimethylamino)-1-methylethyl]phenyl group 1 (Ar*)
coordinated to the attaching boron atom more tightly
than that in 2-[1-(dimethylamino)methyl]phenyl group
2 (Ar) (Chart 1), on the basis of kinetic data.1-3 By using
the bulky C,N-bidentate ligand 1, we attempted to
prepare sterically congested organoboron complexes, for
instance BAr*2X and BAr*3, from the structural and
stereodynamic interests.4 However, the reaction of a
trialkyl borate with an appropriate amount of Ar*Li
yielded no desired compounds but an unexpected com-
pound (7), in which one of the N-Me carbons was bonded
to the boron atom, as a major product: this type of
product has never been reported in the series of studies
of the intramolecular organoboron complexes to our
knowledge.1-3,5 This paper is to report the identification
of the new heterocyclic product and a plausible mech-
anism of its formation, in which deprotonation from an
N-Me group is the key step.
Resu lts a n d Discu ssion
The organolithium compound Ar*Li (4), prepared
from Ar*Br (3) and BuLi by the literature method,1 was
treated with a 1:3 molar ratio of triisopropyl borate in
ether. After the workup, three major products were
found in the reaction mixture, Ar*H (5), Ar*B(OH)2 (6),
and another product, 7, in 46, 9, and 40% yields,
respectively (Scheme 1). The reactions with other tri-
alkyl borates, B(OMe)3, B(OEt)3, and B(OPr)3, gave the
same product, 7, although the yields were low (ca. 5%).6
The structure of 7 was established by X-ray analysis
(Figure 1). A molecule consists of one boron atom and
two Ar* groups, and an N-Me carbon in one of the Ar*
(1) Toyota, S.; Asakura, M.; Futawaka, T.; Oh ki, M. Bull. Chem. Soc.
J pn. 1999, 72, 1879.
(2) Toyota, S.; Oh ki, M. Bull. Chem. Soc. J pn. 1990, 63, 1168.
(3) For recent other papers in the series: (a) Toyota, S.; Futawaka,
T.; Ikeda, H.; Oh ki, M. J . Chem. Soc., Chem. Commun. 1995, 2499. (b)
Toyota, S.; Futawaka, T.; Asakura, M.; Ikeda, M.; Oh ki, M. Organome-
tallics 1998, 17, 4155. (c) Toyota, S.; Oh ki, M. Bull. Chem. Soc. J pn.
1991, 64, 1554.
(4) (a) Blount, J . F.; Finocchiaro, P.; Gust, D.; Mislow, K. J . Am.
Chem. Soc. 1973, 95, 7019. (b) Brown, H. C.; Dodson, V. H. J . Am.
Chem. Soc. 1957, 79, 2302.
(5) (a) Lauer, M.; Wulff, G. J . Organomet. Chem. 1983, 256, 1. (b)
Kalbarczyk, E.; Pasynkiewicz, S. J . Organomet. Chem. 1984, 262, 11.
(c) Horner, L.; Kaps, U.; Simons, G. J . Organomet. Chem. 1985, 287,
1. (d) Lauer, M.; Bo¨hnke, H.; Grotstollen, R.; Salehnia, M.; Wulff, G.
Chem. Ber. 1985, 118, 246. (e) Schlengermann, R.; Sieler, J .; Hey-
Hawkins, E. Main Group Chem. 1997, 2, 141.
(6) It is difficult to explain the substituent effect of trialkyl borates
on the yield of 7 from available data. We consider that the stability of
the alkoxy boranes, 8 and 9 in Scheme 2, plays a role in determining
the yield of 7. The reactions with BF3 or BCl3 gave a complicated
mixture, in which no 7 was detected spectroscopically.
10.1021/om990724n CCC: $19.00 © 2000 American Chemical Society
Publication on Web 12/29/1999

Notes
Organometallics, Vol. 19, No. 2, 2000 207
then Ar*2B(OiPr) (9). The tight coordination and the
steric hindrance prevent another molecule of Ar*Li from
attacking the boron atom in 9. Instead, Ar*Li works as
a base to abstract a proton from one of the N-Me groups.
It is known that the acidity of R-protons in tert-amines
is considerably increased by the coordination of Lewis
acids such as BF3.9,10 Therefore, the deprotonation from
an N-Me group at the coordinating amine ligand (9f10)
is likely under the conditions. Thus formed 10 readily
undergoes intramolecular substitution of the anionic
carbon at the boron atom to give 7.
In the reaction mixture, we could not find any
products derived from 9 such as Ar*2BOH by spectro-
scopic analyses, this indicating that the deprotonation
takes place faster than the substitution of the second
Ar* group at the boron atom. The proposed mechanism
explains the formation of a moderate amount of 5, which
is produced by the deprotonation of 9 by Ar*Li and
partly by hydrolysis of the unreacted Ar*Li during the
workup. Taking into account the reaction yields, ca. 60%
of the original organolithium compound is consumed by
the formation of 7 as a source of Ar* groups (40%) and
as a base (20%).
To see the influence of the benzylic methyl groups on
the reactivity, 2-[1-(dimethylamino)methyl]phenyllith-
ium (ArLi) was treated with triisopropyl borate under
the same conditions. The reaction did not give a product
like 7 but triarylborane, BAr3 (12), together with the
corresponding boronic acid. The presence of the gem-
dimethyl group strongly hinders the substitution of the
third aryl group at the boron atoms.
F igu r e 1. ORTEP drawing of 7 (50% probability el-
lipsoids). Selected structural parameters: N(1)-B 1.723-
(5), C(1)-B 1.579(6), C(12)-B 1.608(6), C(22)-B 1.599(6)
Å; C(1)-B-C(12) 115.8(4), C(1)-B-C(22) 115.8(4), C(12)-
B-C(22) 105.2(4)°; tetrahedral character7 73%.
Sch em e 2
Exp er im en ta l Section
1H NMR spectra were measured on a Varian Gemini-300
and a Bruker AMX-R400 spectrometer operating at 300 and
400 MHz, respectively. 13C NMR were measured by the Varian
machine at 75 MHz. 11B NMR were measured on the Bruker
machine at 128.4 MHz with external reference of BF3:OEt2 at
0 ppm. Melting points are uncorrected. Mass spectra were
measured on a J EOL MStation-700 spectrometer.
R ea ct ion of 2-[1-(Dim et h yla m in o)-1-m et h ylet h yl]-
p h en yllith iu m (4) w ith Tr iisop r op yl Bor a te. A solution
of 500 mg (2.1 mmol) of 1-bromo-2-[1-(dimethylamino)-1-
methylethyl]benzene (3)1 in 5 mL of dry ether was cooled to
-78 °C, and to the solution was added 1.3 mL (2.1 mmol) of
15% butyllithium solution in hexanes under nitrogen atmo-
sphere. The solution of 4 was allowed to warm to room
temperature and then stirred for 2 h. To the solution was
slowly added a solution of 132 mg (0.70 mmol) of triisopropyl
borate in 2 mL of dry ether at -78 °C. After the removal of
the cooling bath, the mixture was stirred for 15 h at room
temperature. The volatile materials were removed by evapora-
tion, and the residue was treated with 5 mL of wet dichlo-
romethane. After the removal of insoluble materials by
filtration, the solvent was evaporated to give a mixture of the
products as an oil. The product distributions were determined
groups is attached to the boron atom. Further coordina-
tion of the amine ligand in the other Ar* group allows
the boron atom to take a tetrahedral structure. The
B-N coordination bond in 7 (1.723 Å) is a little shorter
than those in similar 9-BBN complexes (ca. 1.75 Å).7
The NMR data are consistent with the structure. The
1H NMR spectrum of 7 showed seven methyl signals
and an AB quartet in the aliphatic region, this signal
pattern being expected from the structure with a chiral
center at the boron atom. The 11B NMR chemical shift
of 3.3 ppm is typical for tetracoordinated boron atoms.8
A plausible mechanism of the formation of 7 is
illustrated in Scheme 2. At first, the substitution of Ar*
groups at the boron atom affords Ar*B(OiPr)2 (8) and
1
by the integral intensities of H NMR signals. [1-(Dimethyl-
amino)-1-methylethyl]benzene (5) and 2-[1-(dimethylamino)-
1-methylethyl]phenylboronic acid (6)1 were identified by com-
(9) (a) Kessar, V. S.; Singh, P. Chem. Rev. 1997, 97, 721, and
references therein. (b) Kesser, V. S.; Singh, P.; Vohra, R.; Kaur, N. P.;
Singh, K. N. J . Chem. Soc., Chem. Commun. 1991, 568.
(10) Theoretical calculations of a model reaction (NH3 + L:NMe3
f
+
(7) Toyota, S.; Oh ki, M. Bull. Chem. Soc. J pn. 1992, 65, 1832.
(8) Kidd, R. G. In NMR of Newly Accessible Nuclei; Laszlo, P., Eds.;
Academic Press: London, 1983; Vol. 2, Chapter 3.
NH4 + L:NMe2CH2-) also support this phenomenon. The energy
required for the process is decreased by 31.7 kcal/mol by changing L
from none to BF3 at the HF/3-21G* level.

208 Organometallics, Vol. 19, No. 2, 2000
Notes
Ta ble 1. Cr ysta l a n d Str u ctu r e An a lysis Da ta of
7 a n d 12‚H2O
7
12‚H2O
C27H38BN3O
formula
fw
C
22H31BN2
334.31
431.43
crystal size (mm3) 0.30 × 0.13 × 0.10
0.50 × 0.25 × 0.15
monoclinic
P21/n
15.2299(9)
8.5721(6)
20.3491(6)
107.730(3)
2530.4(2)
4
1.132
5.25
1.84 + 0.30 tan θ
12.0
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
8.160(2)
12.021(2)
20.478(2)
93.50(1)
2005.1(6)
4
1.107
4.76
1.31 + 0.30 tan θ
â (deg)
V (Å3)
Z
Dc (g/cm3)
µ(Cu KR) (cm-1
)
scan width (deg)
scan rate (deg/min) 8.0
no. of unique data 3161 (2° < 2θ < 120°) 3762 (2° < 2θ < 120°)
no. of data used
R(F)
Rw(F)
2506 (Fo > 2.0σ(F))
0.066
0.072
3762 (Fo > 0.0σ(F))
0.053
0.061
F igu r e 2. ORTEP drawing of 12‚H2O (30% probability
ellipsoids).
1
in the air. Mp: 110.5-111.5 °C. H NMR (CDCl3): δ 2.09 (br,
18H), 3.24 (br, 6H), 7.00-7.18 (m, 12H). HRMS (FAB, MH+):
found 432.3212 and 413.3141, calcd for C27H37BN3‚H2O and
parison of their spectroscopic data with those of the authentic
samples, the former being commercially available. The yields
of the products 5, 6, and 7 were 150 mg (46%), 36 mg (9%),
and 140 mg (40%), respectively, based on the organolithium
reagent. The yield of 7 was decreased when the reaction
mixture was stirred for longer time or at higher temperatures.
1-{[1-(Dim et h yla m in o)-1-m et h ylet h yl]p h en yl}-3,4,4-
tr im eth yl-1,2,3,4-tetr a h yd r o-3,1-ben za za bor in (7). The
crude product was treated with an appropriate amount of
hexane-dichloromethane to afford 56 mg (16% isolated yield)
C
27H37BN3, 432.3186 and 413.3117, respectively. N,N-Dimeth-
ylbenzylamine and 2-[(dimethylamino)methyl]phenylboronic
acid11c were found in the reaction mixture.
X-r a y Cr ysta llogr a p h y. Crystals of 7 and 12 (Figure 2)
used for the X-ray measurements were grown from hexane-
dichloromethane and dichloromethane solutions, respectively.
The diffraction data were collected on a Rigaku AFC7R four-
circle diffractometer with Cu KR radiation (λ ) 1.54178 Å) at
room temperature. The scan mode was the ω-2θ method. The
structure was solved by the direct method (SIR92) and refined
by the full-matrix least-squares method by using a teXsan
program. Anisotropic thermal parameters were employed for
non-hydrogen atoms and isotropic ones for hydrogens. The
reflection data were corrected for Lorentz and polarization
effects and secondary extinction. The function minimized was
∑[w(|Fo| - |Fc|)2], where w ) [σc2|Fo|]-1. Additional crystal and
analysis data are listed in Table 1.
1
of 7 as colorless crystals. Mp: 157-158 °C. H NMR (CDCl3):
δ ) 1.27 (s, 3H), 1.52 (s, 3H), 1.58 (s, 3H), 1.59 (s, 3H), 1.94
and 2.28 (ABq, J ) 13.7 Hz, 2H), 2.34 (s, 3H), 2.36 (s, 3H),
2.62 (s, 3H), 6.83-6.93 (m, 2H), 7.09-7.22 (m, 5H), 7.30 (d,
1H, J ) 8.1 Hz). 13C NMR (CDCl3): δ ) 20.7, 25.4, 28.3, 29.8,
41.9, 43.9, 45.1, 59.6, 73.0, 120.7, 123.5, 125.9, 126.1, 126.2,
127.1, 130.1, 135.6, 148.9, 153.3. Signals due to carbons
attaching to the boron atom could not be detected. 11B NMR
(CDCl3): δ ) 3.3 (line width h1/2 193 Hz). HRMS (FAB, MH+):
found 334.2695, calcd for C22H32BN2 334.2584.
Rea ction of 2-[1-(Dim eth yla m in o)m eth yl]p h en yllith -
iu m (Ar Li) w ith Tr iisop r op yl Bor a te. A suspension of the
organolithium compound in ether was prepared from 0.62 mL
(4.1 mmol) of N,N-dimethylbenzylamine with butyllithium in
an ordinary manner.11 This compound was similarly treated
with 263 mg (1.4 mmol) of triisopropyl borate as above. From
the reaction mixture, 121 mg (20%) of BAr3 (12) was isolated
as an adduct with water. This compound slowly decomposed
Ack n ow led gm en t. This work was partly supported
by a fund from the J apan Private School Promotion
Foundation. The authors thank Messrs. T. Saijo and H.
Kuraishi for their technical assistance.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, atomic coordinates, thermal parameters, and bond
distances and angles of 7 and 12. This material is available
free of charge via the Internet at http://pubs.acs.org.
(11) (a) J ones, F. N.; Vaulx, R. L.; Hauser, C. R. J . Org. Chem. 1963,
28, 3461. (b) Manzer, L. E. J . Am. Chem. Soc. 1978, 100, 8068. (c)
Lauer, M.; Wulff, G. J . Organomet. Chem. 1983, 256, 1.
OM990724N