Formation of 1,3,5-Trilithiobenzene
Organometallics, Vol. 16, No. 23, 1997 5031
determined by GC (see Table 1). 4: MS (EI, m/z, relative
intensity) 456 (100, M+•), 329 (43, [M - I]+), 238 (7), 202 (16,
[M - 2I]+), 75 (16), 74 (11). 13: MS (EI, m/z, relative intensity)
205 (7), 204 (100, M+•), 128 (1), 127 (13, [M - C6H5]+), 78 (2),
77 (25, [M - I]+), 76 (2), 75 (3), 74 (5), 73 (2), 51 (13), 50 (8).21
Qu en ch in g w ith Ca r bon Dioxid e. The preparation of 1
was carried out according to the general procedure, using 3
(0.7869 g, 2.5 mmol) in Et2O (100 mL) and a LiDBB solution
in THF (35 mL, 0.52 M, 18.2 mmol). Then, a flask with solid
CO2 and P2O5 was equipped with a septum and connected via
a capillary to the reaction vessel. On bubbling the CO2
through the reaction mixture, a tough white precipitate was
formed, which was acidified (2 M HCl) and extracted with
Et2O. The combined organic layers were extracted with
aqueous NaOH; the water layer was neutralized with HCl,
extracted with Et2O, and dried (MgSO4). Evaporation of the
solvent and azeotropic distillation with benzene yielded 0.4976
g of a mixture of 14:15:16 (70:25:5) (1H NMR). 14: 1H NMR
(DMSO-d6, ppm) δ 8.64 (s, CH). 15: 1H NMR (DMSO-d6, ppm)
were identified by GC/MS, and the yields were determined
with GC (see Table 2). The responses for 23, 24, and 25 with
regard to the internal standard were supposed to be 1:1. 23:
23
MS (EI, m/z, relative intensity) 566 (1, M+•, Sn3 pattern),
551 (100, [M - CH3]+, Sn3 pattern), 521 (9, [M - 3CH3]+, Sn3
pattern), 491 (6, [M - 5CH3]+, Sn3 pattern), 389 (5.5, Sn2
pattern), 24:24 MS (EI, m/z, relative intensity) 404 (1, M+•, Sn2
pattern), 387 (100, [M - CH3]+, Sn2 pattern), 359 (18, [M -
3CH3]+, Sn2 pattern), 342 (4, [M - 4CH3]+, Sn2 pattern), 329
(12, [M - 4CH3]+, Sn2 pattern), 312 (5, [M - 5CH3]+, Sn2
pattern), 240 (2, Sn pattern), 223 (2, Sn pattern), 209 (4, Sn
pattern), 187 (5, Sn pattern), 165 (5, Sn pattern), 135 (8, Sn
pattern), 120 (5, Sn pattern). 25: MS (EI, m/z, relative
intensity) 231 (18), 227 (100, [M - CH3]+, Sn pattern), 197
(35, [M - 3CH3]+, Sn pattern), 145 (5), 135 (11, [CH3Sn]+),
120 (17).21,25
Qu en ch in g w ith Mer cu r y Dibr om id e. The preparation
of 1 was carried out according to the general procedure, using
3 (0.3143 g, 1 mmol) in Et2O (10 mL) and a LiDBB solution in
THF (14.6 mL, 0.48 M, 7.0 mmol). Subsequently, HgBr2 (in
THF) (10 mL, 0.40 M, 4 mmol) was added dropwise to the
reaction mixture. Instantly, a white precipitate formed, which
turned grey on completion of the addition; the red supernatant
decolorized simultaneously. After the reaction mixture was
stirred for another 3 days at -40 °C, an excess of MeOD was
added. The precipitate was filtered off and washed with water
3
3
δ 7.63 (t, 5-CH, J ) 7.73 Hz), 8.17 (d, 4-CH, J ) 7.73 Hz),
8.48 (s, 2-CH). 16: 1H NMR (DMSO-d6, ppm) δ 7.46-7.62 (m,
2-CH, 3-CH), 7.92 (m, 1-CH).
Qu en ch in g w ith MeOD. The preparation of 1 was carried
out according to the general procedure, using 3 (0.0779 g, 0.247
mmol) and a LiDBB solution in THF (3.5 mL, 0.48 M, 1.68
mmol). Subsequently, MeOD (40 µL, excess) was added to the
reaction mixture, which was stirred for another 15 min (the
solution decolorized fast) and warmed to room temperature.
After the internal standard (hexamethylbenzene) and water
were added, the reaction mixture was extracted with Et2O,
washed with water and a NaCl solution, and dried (MgSO4).
After filtration, the amount of ethereal solvent was diminished
carefully by distillation at atmospheric pressure, using a long
Vigreux column. The percentage of deuteration was deter-
mined by GC/MS of the residue (see Table 2). 17, 18, 19
(mixture): MS (EI, m/z, relative intensity) 82 (7), 81 (100), 80
(34), 79 (22), 78 (19, M+•), 77 (6), 76 (4), 75 (2).
Qu en ch in g w ith Ma gn esiu m Dibr om id e. The prepara-
tion of 1 was carried out according to the general procedure,
using 3 (0.2516 g, 0.799 mmol) in Et2O (20 mL) and a LiDBB
solution in THF (7.2 mL, 0.80 M, 5.76 mmol). Subsequently,
a solution of MgBr2 (3.196 mmol) in THF (18.8 mL) was added
dropwise. The solution decolorized very slowly to yield a
faintly red solution with a white precipitate. After 20 h of
stirring at -40 °C, 2 h at -30 °C, 2 h at -25 °C, 12 h at -15
°C, 12 h at -10 °C, 12 h at 0 °C, and 8 h at room temperature,
an excess of 1,2-diiodoethane was added. Subsequently, the
internal standard (hexamethylbenzene) and water were added,
and the reaction mixture was extracted with Et2O, washed
with water and a NaCl solution, and dried (MgSO4). After
filtration, the ethereal solution showed the presence of 4, 8,
and 13 (GC/MS). The yields were determined by GC (see Table
2). In a second identical experiment, the precipitate was
filtered off under argon and washed with THF prior to
quenching. This solid residue was suspended in Et2O, and 1,2-
diiodoethane was added; GC analysis indicated the presence
of 4 and 8 exclusively in a ratio of 7.9:2.1. Extraction of the
precipitate with 2-MeTHF (in which 21 is soluble) prior to
reaction with 1,2-diiodoethane did not furnish the presence of
pure 20, the ratio of 4:8 had deteriorated to 5.5:4.5.
and MeOH to yield 0.3910 g of a grey powder.
A
1H NMR
spectrum in DMSO-d6 indicated the presence of 26, as con-
cluded from the signal at δ 7.66 ppm (s, CH). However, the
main part of the precipitate was insoluble in DMSO and
seemed to consist of symmetrized products. Elemental analy-
sis of the grey precipitate confirmed this assumption: Found:
C, 9.99; H, 0.73; Hg, 80.9; Br, 4.49. Anal. Calcd for a mixture
of 26:27 (74:26; vide infra): C, 8.75; H, 0.41; Hg, 64.97; Br,
25.88. Anal. Calcd for 28 (a mixture of 74% C6H3Hg1.5:26%
C6H4Hg): C, 20.93; H, 0.97; Hg, 78.10.
The percentage of di- and trisubstituted products was
determined by addition of bromine (in the dark) to an NMR
tube containing a suspension of the precipitate in DMSO-d6,
which gave an immediate exothermic reaction. The clear
solution contained exclusively 3 and 29 in a ratio of 74:26. 3:
1H NMR (DMSO-d6, ppm) δ 7.69 (s, CH). 29: 1H NMR
(DMSO-d6, ppm) δ 7.19 (t, 5-CH), 7.45 (d, 4,6-CH), 7.67 (s,
2-CH).
In contrast, bromination in other solvents, such as pentane,
were still incomplete after stirring for 1 day. However, GC/
MS measurements of the supernatant (after washing with
aqueous NaHSO3, followed by drying on MgSO4) showed the
presence of 3 and 29 as well. 3: MS (EI, m/z, relative
intensity) 312/314/316/318 (34/100/97/32, M+•, Br3 pattern),
233/235/237 (19/37/18, [M - Br]+, Br2 pattern), 154/156 (10/
10, [M - 2Br]+, Br pattern), 75 (20, [M - 3Br]+), 74 (27).25 29:
MS (EI, m/z, relative intensity) 234/236/238 (51/100/50, M+•
,
Br2 pattern), 155/157 (31/30, [M - Br]+, Br pattern), 76 (12,
[M - 2Br]+), 75 (19), 74 (16), 50 (15).21
Determination of the absolute yields by addition of an
internal standard to the brominated products in DMSO was
unsuccessful due to insolubility (e.g., hexamethylbenzene) and/
or bromination (e.g., CHI3) of the standard.
Ack n ow led gm en t. This work was supported in part
by the Netherlands Foundation for Chemical Research
(SON) with financial aid from the Netherlands Orga-
nization for Scientific Research (NWO).
Qu en ch in g with Tr im eth ylstan n yl Ch lor ide. The prepa-
ration of 1 was carried out according to the general procedure,
using 3 (0.0786 g, 0.25 mmol) and a LiDBB solution in THF
(3.5 mL, 0.48 M, 1.68 mmol). Subsequently, a solution of Me3-
SnCl in THF (1 mL, 1.72 M, 1.72 mmol) was added. After the
mixture was stirred for another 15 min, MeOD (30 µL) was
added and the reaction mixture was allowed to warm to room
temperature. After the internal standard (hexamethylben-
zene) and water were added, the reaction mixture was
extracted with Et2O and the organic layer washed with water
and brine and dried (MgSO4). After filtration, the amount of
solvent was reduced by pumping off in vacuo. The products
OM9705620
(23) The mass spectrum was identical with that of an authentic
sample obtained via a different synthetic approach (from the reaction
of 3 with trimethylstannyl sodium, paper to be published).
(24) The mass spectrum was identical with that of an authentic
sample obtained from 2 and trimethylstannyl chloride.
(25) Wiley Registry of Mass Spectral Data; Palisade (6th edition),
1995.