Anionotropic Rearrangement of Chloromethyl Borate Species
gation reactions carried out in the presence of Lewis acids
such as magnesium and zinc halides.
CH
Na
2
Cl
SO
2
(3 × 10 mL). The combined organic layers were dried
4
), filtered, and evaporated to dryness to afford a
(
2
residue that was purified by distillation (bp 69-70 °C, 25
mmHg) to afford 2.1 g (14.2 mmol, 47%) of 2-chlorooctane: MS
(
c) The migration barrier increases on passing from
borate to boronate-derived systems. This is due to an
increase of the B-C bond strength.
d) When “ate” species A deriving from boranes are
(70 eV) m/z 112 (18), 97 (10), 83 (71), 84 (47), 71 (17), 70 (100),
1
5
5 (79), 56 (56).
(
A small portion of pure 2-chlorooctane was added to a
involved, theoretical data are consistent with both our
experimental results and literature reports (Table 1)
pointing out the following migratory aptitude: primary
mixture of magnesium turnings (0.34 g, 14 mmol) in diethyl
ether (3 mL). When the reaction had started, a solution of
2
-chlorooctane (2.08 g, 14 mmol) in diethyl ether (17 mL) was
slowly added at a rate that maintained a gentle reflux. After
the addition was complete, the reaction mixture was stirred
at 40 °C for 1 h and allowed to reach rt. The solution so
obtained was transferred via cannula in a separatory funnel
and slowly added at -78 °C to a solution of trimethoxyborane
(4 mL, 35 mmol) in THF (30 mL). The reaction mixture was
stirred at -78 °C for 1 h and quenched in an ice-cooled solution
=
secondary > tertiary.
e) When “ate” species B deriving from boronates are
(
considered, the experimental evidence obtained in the
present work agrees with the results of model b, which
includes the counterion effect (migratory aptitude
trend: tertiary > secondary > primary).
According to our simple theoretical model, points d and
e are the result of a delicate interplay between two
opposite factors. The first factor (steric factor) favors the
most sterically demanding migrating groups. A more
of aq NaHCO
pentane, and the combined organic layers were dried (Na
3
. The aqueous layers were extracted with
SO ),
2
4
filtered, and concentrated at reduced pressure to a small
volume (∼5 mL). To this solution of 2-(1-methylheptyl)-
dihydroxyborane in pentane was added ethylene glycol (0.63
mL, 11 mmol), and the reaction mixture was stirred at rt
1
pronounced lengthening of the B-C bond marks its
effect in structural terms. The second factor (charge
factor) concerns the carbanionic nature of the migrating
group. It favors the less substituted migrating carbons
and its effect mainly resides in the local charge density
4
overnight. MgSO was added, and the resulting suspension
was stirred for 30 min, filtered, and evaporated to dryness to
afford 1.92 g (10.4 mmol, 74%) of 4 as a clear oil that was used
1
without further purification: H NMR (CDCl
(
3
) δ 0.80-1.00
m, 3H), 1.00-1.10 (m, 3H), 1.20-1.35 (m, 10H), 1.35-1.50
m, 1H), 4.2 (s, 4H); 13C NMR (CDCl
) δ 14.1, 15.5, 22.7, 28.8,
9.4 (broad m), 29.6, 31.9, 33.3, 65.4. Anal. Calcd for
: C, 65.25; H, 11.50. Found: C, 65.33; H, 11.42.
-(1,1,2-Tr im eth ylp r op yl)[1,3,2]d ioxa bor ola n e (5). A
solution of 2,3-dimethyl-2-butene (2.4 mL, 20 mmol) in THF
1
found on C .
(
3
2
Exp er im en ta l Section
10 2
C H21BO
2
Gen er a l Meth od s. H and 13C NMR were recorded at 300
and 75 MHz, respectively, using tetramethylsilane as an
internal standard. Chemical shifts are reported in ppm (δ)
downfield from TMS. GC-MS analyses (70 eV) were per-
formed with a quadrupole instrument. Solvents were dried
using standard methods: THF was distilled from lithium
1
(
(
2
8 mL) was slowly added at -15 °C to a solution of BH
2 mL, 20 mmol) in THF (8 mL). The reaction was stirred for
h at 0 °C and quenched with ice. The aqueous layer was
extracted with diethyl ether, and the combined organic layers
were dried (Na SO ), filtered and concentrated at reduced
3
‚SMe
2
2 2 2 5
aluminum hydride (LAH), CH Cl from P O , and pentane
2
4
from sodium benzophenone immediately prior to use. All
reactions were carried out in oven-dried glassware under an
atmosphere of dry argon. All reagents were commercially
available and were used without further purification, unless
otherwise stated.
pressure to a small volume. The residue was dissolved in
pentane, ethylene glycol was added (0.92 mL, 16 mmol), and
the reaction mixture was stirred at rt overnight. MgSO was
4
added, and the resulting suspension was stirred for 30 min,
filtered, and evaporated to dryness to afford 2.31 g (15 mmol,
2
-Decyl[1,3,2]d ioxa bor ola n e (3). A 1 M solution of di-
bromoborane in CH Cl (25 mL, 25 mmol) was slowly added
to a stirred solution of 1-decene (5 mL, 25 mmol) in 12.5 mL
of CH Cl . The resulting solution was refluxed for 3 h, allowed
7
5%) of 5 as a clear oil that was used without further
2
2
1
purification: H NMR (CDCl
.96 (m, 6H), 1.14-1.20 (m, 1H), 4.2 (s, 4H). Anal. Calcd for
: C, 61.58; H, 10.98. Found: C, 65.64; H, 11.03.
In ter m olecu la r Com p a r a tive Exp er im en ts w ith Bor -
3
) δ 0.87 (d, J ) 6.9 Hz, 6H), 0.91-
0
2
2
8 2
C H17BO
to reach rt, and transferred via cannula in a ice-cooled flask
filled with water (4.5 mL) and ether (12.5 mL). The aqueous
layer was extracted with pentane (3 × 10 mL), and the
on a tes 3-5. An equimolar THF solution of 3 (0.27 M), 4 (0.27
M), and undecane (used as internal standard) was prepared,
and a small aliquot was quenched with alkaline hydrogen
peroxide and analyzed (GC/GC-MS) in order to characterize
2 4
combined organic layers were dried (Na SO ) and concentrated
at reduced pressure to afford a white solid that was purified
by recrystallization from pentane to afford 2.47 g (13.3
the reaction at the initial time t
0
. The solution was cooled at
2
7
mmol,53%) of pure decyldihydroxyborane: mp 75-77 °C (lit.
-
78 °C, and ICH Cl (neat, 2 equiv) was added followed by
2
mp 76-77 °C).
n-BuLi (2 M in hexanes, 2 equiv). After 10 min, the acetone-
dry ice bath was replaced with an ice bath, and the reaction
was stirred at 0 °C for 4 h and then quenched with alkaline
hydrogen peroxide. The crude extract in diethyl ether was
again analyzed by GC and by GC-MS to obtain peak areas at
Ethylene glycol (0.34 mL, 6.65 mmol) was added to a
solution of boronic acid (1.24 g, 6.65 mmol) in pentane (5 mL),
and the reaction mixture was stirred at rt overnight. MgSO
4
was added, and the resulting suspension was stirred for 30
min, filtered, and evaporated to dryness to afford 1.36 g (6.42
1
final time t .
mmol, 97%) of 3 as a clear oil that was used without further
The same procedure was applied to the competitive analysis
of 3 and 5.
1
purifications: H NMR (CDCl
(
9
3
3
) δ 0.8-0.95 (m, 3H), 1.20-1.35
1
3
m, 16H), 1.35-1.50 (m, 2H), 4.2 (s, 4H); C NMR (CDCl
3
) δ
.2 (broad m), 12.6, 21.2, 22.5, 27.85, 27.95, 28.08, 28.16, 30.4,
0.9, 63.8. Anal. Calcd for C10 : C, 67.94; H, 11.88.
Ack n ow led gm en t. We thank the University of
Bologna (funds for selected topics) and MURST (Rome,
fondi ex-40%) for financial support.
H
23BO
2
Found: C, 67.91; H, 11.82.
-(1-Meth ylh eptyl)[1,3,2]dioxabor olan e (4). Thionyl chlo-
2
ride (2.2 mL, 30 mmol) was slowly added at 0 °C to a solution
of 2-octanol (4.8 mL, 30 mmol) in pyridine (2.7 mL, 33 mmol),
and the reaction mixture was stirred overnight. Water was
added (10 mL), and the aqueous layer was extracted with
Su p p or tin g In for m a tion Ava ila ble: Geometrical param-
eters and detailed representation for the structures 11-17,
TS11-17 and P 11-17. This material is available free of
charge via the Internet at http://pubs.acs.org.
(27) Thaisrivongs, S.; Wuest, J . D. J . Org. Chem. 1977, 42, 3.
J O026733E
J . Org. Chem, Vol. 68, No. 9, 2003 3405