Fujita et al.
Hz, 1H), 7.60 (t, J ) 7.8 Hz, 1H), 6.75 (s, 1H), 2.72-2.68 (m,
2H), 2.47-2.35 (m, 2H), 1.88-1.82 (m, 2H), 1.64-1.60 (m, 1H),
1.18-0.97 (m, 2H), 0.89 (d, J ) 6.3 Hz, 3H); 13C NMR (100
MHz, CDCl3) δ 165.9, 138.1, 133.9 (q, J ) 33 Hz), 132.6, 130.7,
128.8, 122.4 (q, J ) 273 Hz), 110.1, 92.1, 36.6, 36.2, 35.8, 35.2,
31.4, 20.9; MS (FAB+) m/z (relative intensity, %) 381 (100);
participation to give the rearranged, ring-expanded,
chiral cation I2 can compete with the external nucleo-
philic attack under poorly nucleophilic conditions. The
rearranged products completely maintain the chirality
of the substrate due to the high barrier for the hydride-
shift racemization of the chiral cation I2.
HRMS (FAB+) calcd for
C15H17F3I (1b) 381.0327, found
381.0343. The R-enriched 1b‚BF4- was prepared by the same
procedure using the R isomer of the vinylsilane (65% ee).
Crystallization of the crude mixture from dichloromethane-
Exp er im en ta l Section
-
ether-hexane gave (R)-1b‚BF4 of 37% ee in 17% yield as a
1-Meth yl-4-(tr im eth ylsilylm eth ylen e)cycloh exa n e. To
a solution of bromide 2Br 30 (1.34 g, 7.1 mmol) in ether (14
mL) cooled at -65 °C was added tert-butyllithium (7.8 mL of
a 1.57 M solution in pentane, 12.2 mmol) dropwise for 90 min.
Then, trimethylsilyl chloride (1.0 mL, 7.9 mmol) was added
to the mixture at -65 °C. The mixture was allowed to warm
to room temperature, quenched by water, and extracted with
ether two times. The combined organic extracts were dried
(MgSO4) and concentrated in vacuo. The residue was purified
by chromatography (SiO2; eluent, hexane) to give the title
compound (0.86 g, 4.7 mmol, 66% yield) as a colorless oil. When
(R)-2Br (85% ee) and (S)-2Br (82% ee) were employed, the R
and S isomers were obtained at 78% and 71% ee, respectively.
The ee was determined by a chiral GC (DEX-CB, 80 °C, 32
cm s-1). The retention times of the S and R isomers are 15.5
white solid and a filtrate of 83% ee. The following crystalliza-
-
tion of the filtrate gave (R)-1b‚BF4 of 63% ee in 4% yield as
an oily solid: [R]21 ) -4.4 (CHCl3, c ) 0.91) (63% ee).
D
4-Met h ylcycloh exylid en em et h yl(4-m et h oxylp h en yl)-
iod on iu m Tetr a flu or obor a te (1c‚BF4-). The titled com-
-
pound was prepared in the same way as 1a ‚BF4 using
4-methoxy(diacetoxyiodo)benzene.41 Crystallization from dichlo-
-
romethane-hexanes-ether gave 1c‚BF4 as a white solid in
41% yield: mp ) 104.0-105.0 °C; 1H NMR (400 MHz, CDCl3)
δ 7.83 (d, J ) 8.8 Hz, 2H), 6.95 (d, J ) 8.8 Hz, 2H), 6.62 (s,
1H), 3.82 (s, 3H), 2.76-2.72 (m, 1H), 2.66-2.62 (m, 1H), 2.42-
2.33 (m, 2H), 1.91-1.88 (m, 1H), 1.82-1.79 (m, 1H), 1.65-
1.57 (m, 1H), 1.31-1.01 (m, 2H), 0.90 (d, J ) 6.8 Hz, 3H); 13
C
NMR (100 MHz, CDCl3) δ 163.6, 162.8, 136.9, 118.2, 98.3, 92.7,
55.8, 36.4, 36.1, 35.7, 35.2, 31.5, 21.0; MS (FAB+) m/z (relative
intensity, %) 343 (100); HRMS (FAB+) calcd for C15H20OI (1c)
343.0559, found 343.0574.
20
and 16.3 min, respectively. S isomer (71% ee): [R]D ) +10.6
1
(CHCl3, c ) 0.36); H NMR (400 MHz, CDCl3) δ 5.09 (s, 1H),
2.46-2.42 (m, 1H), 2.21-2.09 (m, 2H), 1.99-1.91 (m, 1H),
1.76-1.72 (m, 2H), 1.57-1.47 (m, 1H), 1.19-0.92 (m, 2H), 0.89
(d, J ) 6.8 Hz, 3H), 0.06 (s, 9H); 13C NMR (100 MHz, CDCl3)
δ 159.5, 120.7, 39.9, 37.2, 36.8, 33.8, 32.4, 21.8, 0.4; MS (EI)
m/z (relative intensity, %) 182 (M+, 19), 167 (70), 73 (100);
HRMS (EI) calcd for C11H22Si (M) 182.1491, found 182.1482.
4-Meth ylcycloh exyliden em eth yl(ph en yl)iodon iu m Tet-
r a flu or obor a te (1a ‚BF 4-). To a solution of 1-methyl-4-
(trimethylsilylmethylene)cyclohexane (1.0 g, 5.5 mmol) and
PhIO40 (1.25 g, 5.7 mmol) in dichloromethane (20 mL) was
added BF3‚Et2O (0.7 mL, 5.7 mmol) dropwise at 0 °C under
nitrogen, and the reaction mixture was stirred for 100 min at
0 °C. A saturated aqueous sodium tetrafluoroborate solution
was added to the reaction mixture. The mixture was stirred
vigorously for 20 min and extracted with dichloromethane
twice. The organic layer was concentrated in vacuo to give an
oil. Crystallization of the crude mixture from dichloromethane-
-
The R-enriched 1c‚BF4 was prepared by the same proce-
dure from the (R)-vinylsilane of 72% ee. Crystallization of the
crude mixture from dichloromethane-ether-hexane gave (R)-
1c‚BF4- of 70% ee in 54% yield as a white solid: mp ) 84.0-
85.0 °C; [R]20 ) -18.3 (CHCl3, c ) 0.91) (70% ee).
D
Deter m in a tion of th e ee of Vin yliod on iu m Sa lts. A
-
typical procedure for (R)-1a ‚BF4 follows. (R)-1-Methyl-4-
(trimethylsilylmethylene)cyclohexane of 78% ee was converted
to the iodonium salt (R)-1a ‚BF4- by the established procedure
described above. A part of the crude (R)-1a ‚BF4- was converted
to 2Br by treating it with tetrabutylammonium bromide (0.1
M) in chloroform. The 2Br obtained was 66% ee in the S form,
determined by the chiral GC. This indicates that the substitu-
tion proceeds with 92% [)100 × (66/78 + 1)/2] inversion. A
sample of (R)-1a ‚BF4- obtained by crystallization of the crude
mixture was also converted under the same conditions to 2Br ,
which was found to be of 58% ee. From these results, the ee of
-
ether-hexane gave 1a ‚BF4 (1.1 g, 2.75 mmol, 50% yield) as
-
a white solid: mp ) 93.7-94.2 °C; 1H NMR (400 MHz, CDCl3)
δ 7.90 (d, J ) 7.8 Hz, 2H), 7.61 (t, J ) 7.8 Hz, 1H), 7.47 (t, J
) 7.8 Hz, 2H), 6.65 (s, 1H), 2.75-2.66 (m, 2H), 2.47-2.34 (m,
2H), 1.92-1.83 (m, 2H), 1.65-1.58 (m, 1H), 1.15-1.00 (m, 2H),
0.91 (d, J ) 6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 164.6,
134.5, 132.3, 132.2, 110.4, 91.8, 36.5, 36.1, 35.7, 35.1, 31.4, 21.0;
MS (FAB+) m/z (relative intensity, %) 313 (100); HRMS
(FAB+) calcd for C14H18I (1a ) 313.0453, found 313.0445.
the sample of (R)-1a ‚BF4 is calculated to be 69% [)58/(2 ×
-
0.92 - 1)]. If the conversion of the vinylsilane to (R)-1a ‚BF4
proceeded with a partial loss of ee, the ee of the crystallized
(R)-1a ‚BF4 should have been lower than 69%. Nonetheless,
-
the solvolysis product 3C has 69% ee, the ee of the substrate
(R)-1a ‚BF4- should not be lower than 69%, unless enantiomeric
enrichment could occur during the reaction. These results are
also consistent with complete retention of the configuration
during the preparation of vinyliodonium salts from the corre-
sponding vinylsilanes. Determination of the ee of 2Br was
performed using a gas chromatograph equipped with a chiral
column (DEX-CB, i.d. 0.25 mm × 25 m), at a column temper-
ature of 100 °C and a linear velocity of 32 cm s-1. The retention
times of (R)- and (S)-2Br are 9.0 and 9.8 min, respectively.
The ee values of the other salts were determined in the same
way. For details, see the Supporting Information.
Sta n d a r d P r oced u r e for Solvolysis of 1. The tetrafluo-
roborate salt of 1 (1 mg) was dissolved in 1 mL of alcohol or
acetic acid and kept at 60 °C. After the addition of 1 mL of an
ether solution containing tetradecane (5 µmol), the products
were extracted with ether and washed with water. The yields
of the products were determined by GC with tetradecane as
an internal standard and the ee values were found by chiral
GC. Details are given in the Supporting Information.
-
The R-enriched 1a ‚BF4 was prepared by the same proce-
dure using the R isomer of the vinylsilane (78% ee, 0.40 g, 2.2
mmol) to give a crude mixture (0.66 g). Crystallization of a
part of the crude mixture (0.4 g) from dichloromethane-ether-
-
hexane gave (R)-1a ‚BF4 of 69% ee (0.15 g) as a white solid:
mp ) 91.8-93.0 °C; [R]20 ) -20.4 (CHCl3, c ) 0.91) (69%
D
ee).
-
(S)-1a ‚BF4 (79% ee) was also prepared from the (S)-
vinylsilane.
4-Met h ylcycloh exylid en em et h yl(3-t r iflu or om et h yl-
p h en yl)iod on iu m Tetr a flu or obor a te (1b‚BF 4-). The titled
-
compound was prepared in the same way as 1a ‚BF4 using
3-trifluoromethyl(diacetoxyiodo)benzene.41 Crystallization from
-
dichloromethane-hexanes-ether gave 1b‚BF4 as a white
1
solid in 45% yield: mp ) 77.8-78.0 °C; H NMR (400 MHz,
CDCl3) δ 8.22 (d, J ) 7.8 Hz, 1H), 8.08 (s, 1H), 7.81 (d, J ) 7.8
(41) (a) McKillop, A.; Kemp, D. Tetrahedron 1989, 45, 3299-3306.
(b) Sharefkin, J . G.; Saltzmann, H. Org. Synth. 1973, Coll. 5, 660-
663.
(40) Saltzmann, H.; Sharefkin, J . G. Org. Synth. 1973, Coll. 5, 658-
659.
8136 J . Org. Chem., Vol. 67, No. 23, 2002