Bromine as the Ortho-Directing Group in the Aromatic Metalation/Silylation of Substituted Bromobenzenes

Article abstract of DOI:10.1021/jo034790h

The one-pot metalation/disilylation of selected bromobenzenes bearing electron-withdrawing substituents p-, m-, o-XC₆H₄Br (X = F, Cl, I, CN, CF₃) using 2 equiv of lithium diisopropylamide (LDA) and 2 equiv of chlorotrimethylsilane (TMSCl) was investigated. The best results of disilylation were obtained for para-substituted bromobenzenes, but the regioselectivity of the reaction is strongly influenced by the ortho-directing power of the substituent. On the contrary, the disilylation of metasubstituted bromobenzenes was not efficient or even failed in some cases and hence monosilylated derivatives were isolated as major or sole products. Diverse reactivity was observed for orthosubstituted bromobenzenes, e.g., 2-bromobenzonitrile and 2-bromochlorobenzene, were converted into corresponding disilylated derivatives in a high and moderate yield, respectively, whereas 1-bromo-2-(trifluoromethyl)benzene underwent only monosilylation.

Full text of DOI:10.1021/jo034790h

Br om in e a s th e Or th o-Dir ectin g Gr ou p in th e Ar om a tic
Meta la tion /Silyla tion of Su bstitu ted Br om oben zen es
Sergiusz Lulin´ski and J anusz Serwatowski*
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
serwat@ch.pw.edu.pl
Received J une 9, 2003
The one-pot metalation/disilylation of selected bromobenzenes bearing electron-withdrawing
substituents p-, m-, o-XC6H4Br (X ) F, Cl, I, CN, CF3) using 2 equiv of lithium diisopropylamide
(LDA) and 2 equiv of chlorotrimethylsilane (TMSCl) was investigated. The best results of disilylation
were obtained for para-substituted bromobenzenes, but the regioselectivity of the reaction is strongly
influenced by the ortho-directing power of the substituent. On the contrary, the disilylation of meta-
substituted bromobenzenes was not efficient or even failed in some cases and hence monosilylated
derivatives were isolated as major or sole products. Diverse reactivity was observed for ortho-
substituted bromobenzenes, e.g., 2-bromobenzonitrile and 2-bromochlorobenzene, were converted
into corresponding disilylated derivatives in a high and moderate yield, respectively, whereas
1-bromo-2-(trifluoromethyl)benzene underwent only monosilylation.
In tr od u ction
arynes6,12-14 or are susceptible to isomerization, via the
well-known Bunnett’s “halogen dance” process.5 These
undesired reactions could be successfully avoided by
trapping the organolithium species in situ using the
appropriate electrophile (TMSCl) as was proved recently
for the metalation of 1,2- and 1,4-dibromobenzene.7 In
this paper, we present the significant extension of our
previous work, providing new and interesting results
concerning the metalation/silylation of substituted mono-
bromobenzenes.
The ortho-directed metalation of substituted arenes can
be regarded nowadays as the classical synthetic method
used for the functionalization of these compounds.1
Among halogen substituents, fluorine possesses the
strongest ortho-directing ability, and this property has
been extensively studied and has also found many
applications to organic synthesis.2,3 The metalation of
brominated arenes is not so straightforward since the
ortho-directing ability of bromine is relatively weak.4 An
important limitation is the competitive bromine-lithium
exchange that occurs preferentially to the ortho-metala-
tion if alkyllithium reagents are employed. Hence, only
non-carbon-based strong bases such as lithium amides,
e.g., LDA or lithium 2,2,6,6-tetramethylpiperidide (LTMP),
can be considered as effective deprotonating agents. Until
now, only a few reports concerning the metalation of
oligobromobenzenes5-7 and brominated heteroarenes8,9
have been published. On the other hand, ortho-bromo-
aryllithium intermediates are significantly less thermally
stable than their fluorine and chlorine analogues10,11 and
may decompose under metalation conditions to form
Resu lts a n d Discu ssion
It was found earlier that fluorinated bromobenzenes
are metalated preferentially in the position ortho to the
fluorine due to the stronger ortho-directing ability of
fluorine with respect to bromine.3,4 In fact, the disilylation
of 1-bromo-4-fluorobenzene proceeds smoothly in posi-
tions 3 and 5 adjacent to the fluorine as demonstrated
by the high-yield isolation of 1-bromo-4-fluoro-3,5-bis-
(trimethylsilyl)benzene (1a ) as shown in Scheme 1.
A different regioselectivity of disilylation was observed
for 1-bromo-4-chlorobenzene since 1-bromo-4-chloro-2,5-
bis(trimethylsilyl)benzene (2a ) was isolated as the prod-
uct in a good yield, which resembles the behavior of 1,4-
dibromobenzene.7 Apparently, the introduction of the
TMS group reduces the ortho-directing power of the
neighboring halogen atom depending on the first position
metalated, i.e., ortho to chlorine or bromine (Scheme 1).
Hence, the second metalation occurs predominantly in
* Fax: 48-22-628-2741.
(1) (a) Snieckus, V. Chem. Rev. 1990, 90, 879. (b) Beak, P.; Snieckus,
V. Acc. Chem. Res. 1982, 15, 306.
(2) Bridges, A. J .; Lee, A.; Maduakor, E. C.; Schwartz, C. E.
Tetrahedron Lett. 1992, 33, 7495. Bridges, A. J .; Lee, A.; Maduakor,
E. C.; Schwartz, C. E. Tetrahedron Lett. 1992, 33, 7499.
(3) Moyroud, J .; Guesnet, J .-L.; Bennetau, B.; Mortier, J . Tetrahe-
dron Lett. 1995, 36, 885.
(4) Mongin, F.; Schlosser, M. Tetrahedron Lett. 1996, 37, 6551.
(5) Mongin, F.; Marzi, E.; Schlosser, M. Eur. J . Org. Chem. 2001,
14, 2771.
(6) Leroux, F.; Schlosser, M. Angew. Chem., Int. Ed. 2002, 41, 4272.
(7) Lulin´ski, S.; Serwatowski, J . J . Org. Chem. 2003, 68, 5384.
(8) Van, P. C.; Macomber, R. C.; Mark, H. B., J r.; Zimmer, H. J .
Org. Chem. 1984, 49, 5250.
(11) Chen, L. S.; Chen, G. J .; Tamborski, C. J . Organomet. Chem.
1980, 193, 283.
(12) Gilman, H.; Gaj, B. J . J . Org. Chem. 1957, 22, 447.
(13) Dougherty, T. K.; Lau, K. S. Y.; Hedberg, F. L. J . Org. Chem.
1983, 48, 5273.
(9) Effenberger, F.; Daub, W. Chem. Ber. 1991, 124, 2119.
(10) Gilman, H.; Gorsich, R. D. J . Am. Chem. Soc, 1956, 78, 2217.
(14) Pellissier, H.; Santelli, M. Tetrahedron 2003, 59, 701 and
references therein.
10.1021/jo034790h CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/31/2003
9384
J . Org. Chem. 2003, 68, 9384-9388

Aromatic Metalation/ Silylation of Substituted Bromobenzenes
SCHEME 1 16
the position para to the first introduced TMS group,
which reflects a small difference of acidifying properties
of chlorine and bromine.15
literature reports,17 we have might suppose the disilyla-
tion of 4-bromobenzonitrile to occur predominantly in
positions 2 and 6 adjacent to the nitrile group due to the
stronger acidifying effect of this group with respect to
bromine. However, this was apparently not the case since
4-bromo-2,5-bis(trimethylsilyl)benzonitrile (5a ) was iso-
lated as the only product in a good yield. Such a reaction
course provides evidence for a similar ortho-directing
power of the nitrile group when compared to bromine and
chlorine.
On the contrary, the metalation and subsequent sily-
lation of 1-bromo-4-(trifluoromethyl)benzene was con-
trolled exclusively by bromine. 1-Bromo-4-(trifluorome-
thyl)-2,6-bis(trimethylsilyl)benzene (6a ) was isolated as
the major product (48%), but the small amount (ca. 15%)
of the monosilylated compound, i.e., 1-bromo-4-(trifluo-
romethyl)-2-(trimethylsilyl)benzene (6b), was also de-
tected in the reaction mixture. Presumably, the relatively
weak ortho-directing properties of the CF3 group may be
atrributed to a considerable extent to the steric repulsions
with the bulky base. This explanation is strongly sup-
ported by the diverse regioselectivity of the metalation
of 1,3-bis(trifluoromethyl)benzene, which is strongly de-
pendent on the size of the base used.19
The disilylation of 1-bromo-4-iodobenzene was also
efficacious but not so regioselective. A mixture of 1-bromo-
4-iodo-2,5-bis(trimethylsilyl)benzene (3a , 57%) and 1-bro-
mo-4-iodo-2,6-bis(trimethylsilyl)benzene (3b, 19%) was
obtained. The main product could be isolated from the
mixture by fractional crystallization from ethanol in ca.
30% yield. The synthesis of compound 3a is the first
example of the successful trapping of the ortho-iodoaryl-
lithium species that is not stabilized by the adjacent
electronegative substituents such as fluorine.2 On the
other hand, it should be noted that the formation of a
significant amount of 3b gives evidence for distinctly
weaker ortho-directing properties of iodine with respect
to bromine. Nevertheless, 1,4-diiodobenzene could also
be disilylated without difficulty under standard condi-
tions to produce 1,4-diiodo-2,5-bis(trimethylsilyl)benzene
(4a ) in 77% yield.
Results obtained for halogen-substituted bromoben-
zenes have prompted us to investigate the reactivity of
their analogues bearing carbon-based electronegative
functions, i.e., the nitrile and trifluoromethyl groups. It
was found earlier that these substituents are good ortho
directors for aromatic lithiation.17,18 On the basis of
Meta-substituted bromobenzenes were generally sus-
ceptible to metalation and subsequent monosilylation in
(15) Mongin, F.; Schlosser, M. Tetrahedron Lett. 1997, 38, 1559.
(16) Yields of isolated pure products are given in parentheses.
(17) Krizan, T. D.; Martin J . C. J . Am. Chem. Soc. 1983, 105, 6155.
(18) Bigwood, M. P.; Corran, P. J .; Zuckerman, J . J . J . Am. Chem.
Soc. 1981, 103, 7643.
(19) Schlosser, M.; Katsoulos, G.; Takagishi, S. Synlett 1990, 747.
J . Org. Chem, Vol. 68, No. 24, 2003 9385

Lulin´ski and Serwatowski
SCHEME 2
SCHEME 3
the most activated position 2, i.e., between bromine and
the electronegative substituent. However, the introduc-
tion of the second TMS group was not straightforward
in most cases (Scheme 2). The attempted disilylation of
1-bromo-3-fluorobenzene resulted in the formation of the
mixture containing mainly the monosilylated product,
i.e., 1-bromo-3-fluoro-2-(trimethylsilyl)benzene (7a ). The
yield of 1-bromo-3-fluoro-2,4-bis(trimethylsilyl)benzene
was rather poor, and this compound could not be satis-
factorily purified (7b). 1-Bromo-3-chlorobenzene20 and
1-bromo-3-iodobenzene were converted exclusively into
expected monosilylated products, which were found to be
resistant against further metalation and silylation. Clearly,
the activation of remaining hydrogen atoms by the
adjacent heavier halogen substituents is not sufficient
to enable deprotonation. A different behavior was found
for 3-bromobenzonitrile, as the disilylation of this com-
pound was observed. Unfortunately, a mixture of prod-
ucts was formed and it was not possible to isolate them
in a pure state. However, when 3-bromobenzonitrile was
treated with only 1.1 equiv of LDA and 1.1 equiv of
TMSCl, then the pure 3-bromo-2-(trimethylsilyl)benzo-
nitrile (10a ) was prepared in a good yield. Finally,
1-bromo-3-(trifluoromethyl)benzene was converted into
the mixture of monosilylated derivatives. Contrary to
previous examples, the product expected for electronic
reasons, i.e., 1-bromo-3-(trifluoromethyl)-2-(trimethylsi-
lyl)benzene (11a ), was formed in a rather low yield (ca.
30%), whereas 2-bromo-4-(trifluoromethyl)-1-(trimethyl-
silyl)benzene (11b, ca. 70%) was the major product. The
preferred formation of 11b points again to the importance
of steric factors in the metalation of CF3-substituted
arenes.19
In a series of ortho-substituted bromobenzenes, a
diverse reactivity was observed (Scheme 3). 1-Bromo-2-
fluorobenzene could be easily monosilylated in the ex-
pected position ortho to the fluorine to give 1-bromo-2-
fluoro-3-trimethylsilylbenzene (12a ) as the major product
in 60% yield.3 The subsequent silylation of this compound
(20) Effenberger, F.; Daub, W. Chem. Ber. 1991, 124, 2113.
9386 J . Org. Chem., Vol. 68, No. 24, 2003

Aromatic Metalation/ Silylation of Substituted Bromobenzenes
1H), 0.39 (s, 9H), 0.37 (s, 9H). 13C{1H} NMR: δ 144.5, 142.4,
was rather slow, and only a low yield (ca 20%) of impure
2-bromo-3-fluoro-1,4-bis(trimethylsilyl)benzene (12b) was
isolated.
140.0, 139.6, 136.7, 128.8, -0.6, -0.8. Anal. Calcd for C12H20
-
BrClSi2: C, 42.92; H, 6.00. Found: C, 42.53; H, 5.75.
1-Br om o-4-iodo-2,5-bis(tr im eth ylsilyl)ben zen e (3a). LDA
(2 M, 33 mL, 66 mmol) was added dropwise to a solution of
1-bromo-4-iodobenzene (8.5 g, 30 mmol) in THF (70 mL)
containing TMSCl (7.2 g, 8.5 mL) at -70 °C. The resultant
solution was stirred for 30 min at -75 °C and hydrolyzed with
dilute aqueous H2SO4. The reddish organic phase was sepa-
rated, and the water phase was extracted with ether. Evapora-
tion of the combined organic solutions left a crude solid that
was filtered, washed with water and cold methanol (3 × 10
mL), and dried to give a mixture of 3a and 3b in ca. 57 and
19% yields, respectively (9.7 g, combined yield ) 76%).
Fractional crystallization from ethanol (80 mL, 35 °C) afforded
In the case of 1-bromo-2-chlorobenzene, a mixture of
2-bromo-3-chloro-1-trimethylsilylbenzene (13a, 20%), 1-bro-
mo-2-chloro-3-trimethylsilylbenzene (13b, 22%), and 2-bro-
mo-3-chloro-1,4-bis(trimethylsilyl)benzene (13c, 40%) was
obtained, showing again that there is no significant
difference of acidifying properties of chlorine and bro-
mine.15 The latter compound could be isolated from this
mixture, albeit in a quite low yield (23%). The higher-
yield formation of 13c with respect to 12b should be
noted, suggesting that chlorine or bromine in the meta
position activate the hydrogen atom in compounds 13a
and 13b, respectively, more strongly than the fluorine
does in 12a .21
Interestingly, a much better result was achieved when
2-bromobenzonitrile was subjected to disilylation, as
2-bromo-3,6-bis(trimethylsilyl)benzonitrile (15a ) was syn-
thesized in a good yield (80%). On the other hand,
1-bromo-2-(trifluoromethyl)benzene gave only the monos-
ilylated product, namely, 2-bromo-3-(trifluoromethyl)-
benzene-1-trimethylsilylbenzene (16a ), which provides
the next piece of clear evidence for the better acidifying
properties of bromine when compared to the CF3 group.
In conclusion, the activity of bromine as the ortho-
directing group in the aromatic lithiation was found to
be comparable to the activity of chlorine and the nitrile
group and stronger with respect to the CF3 group. It
should be noted that the application of the metalation/
silylation sequence to the functionalization of bromoben-
zenes was proved in many cases, which is important
especially from the synthetic point of view. This approach
may be a suitable method for further transformations of
these compounds, e.g., due to the reactivity of the TMS
groups and/or bromine.
1
pure 3a , mp 93-95 °C. Yield: 3.8 g (30%). H NMR: δ 7.81
(s, 1H), 7.48 (s, 1H), 0.41 (s, 9H), 0.38 (s, 9H). 13C{1H} NMR:
δ 149.3, 147.2, 145.1, 140.3, 131.0, 103.1, -0.5, -0.7. Anal.
Calcd for C12H20BrISi2: C, 33.70; H, 4.72. Found: C, 33.81;
H, 4.83.
1,4-Diiodo-2,5-bis(tr im eth ylsilyl)ben zen e (4a). This com-
pound was prepared using the procedure described for 3a
starting from 1,4-diiodobenzene (9.9 g, 30 mmol. A crude solid
product was filtered, washed with water and methanol, and
dried to give 4a as white crystals, mp 132-134 °C. Yield: 10.9
g (77%). 1H NMR: δ 7.77 (s, 2H), 0.41 (s, 18H). 13C{1H} NMR:
δ 149.5, 147.4, 104.6, -0.6. Anal. Calcd for C12H20I2Si2: C,
30.39; H, 4.25. Found: C, 30.34; H, 4.20.
4-Br om o-2,5-bis(tr im eth ylsilyl)ben zon itr ile (5a ). This
compound was prepared using the procedure described for 1a
starting from 4-bromobenzonitrile (9.1 g, 50 mmol). A crude
solid product was filtered, washed with water and cold
methanol (3 × 10 mL), and dried to give colorless crystals of
1
5a , mp 107-110 °C. Yield: 10.5 g (64%). H NMR: δ 7.70 (s,
1H), 7.68 (s, 1H), 0.42 (s, 9H), 0.40 (s, 9H). 13C{1H} NMR: δ
147.9, 143.4, 140.8, 138.7, 135.7, 120.0, 115.8, -0.7, -1.40.
Anal. Calcd for C13H20BrNSi2: C, 47.84; H, 6.18; N, 4.29.
Found: C, 47.85; H, 5.95; N, 4.26.
1-Br om o-4-(t r iflu or om et h yl)-2,6-b is(t r im et h ylsilyl)-
ben zen e (6a ). This compound was prepared using the pro-
cedure described for 1a starting from 1-bromo-4-(trifluorom-
ethyl)benzene (11.1 g, 50 mmol). A crude liquid product was
distilled in vacuo to give a mixture of crystals and oil. Ethanol
(10 mL) was added, and the solution was left to stand
overnight in a -20 °C freezer. Crystals that separated from
the solution were filtered, washed with cold ethanol (2 × 10
mL), and dried to give 6a as colorless crystals, mp 78-80 °C.
Yield: 8.7 g (48%). 1H NMR: δ 7.61 (s, 2H), 0.43 (s, 18H). 13C-
Exp er im en ta l Section
1-Br om o-4-flu or o-3,5-bis(tr im eth ylsilyl)ben zen e (1a ).
LDA (2 M, 55 mL, 110 mmol) was added dropwise to a solution
of 1-bromo-4-fluorobenzene (7.8 g, 50 mmol) in THF (70 mL)
containing TMSCl (12.0 g, 14 mL) at -70 °C. The resultant
solution was stirred for 30 min at -75 °C and hydrolyzed with
dilute aqueous H2SO4. The yellow organic phase was sepa-
rated, and the water phase was extracted with ether. Evapora-
tion of the combined organic solutions left a pale yellow oil.
The oil was distilled in vacuo to give a crude product as a
colorless oil. Methanol (10 mL) was added, and the solution
was left to stand overnight in a -20 °C freezer. Crystals that
separated from the solution were filtered, washed with cold
methanol, and dried to give 1a as a white crystalline material,
mp 48-50 °C. Yield: 12.5 g (78%). 1H NMR: δ 7.46 (d, 2H,
3
{1H} NMR: δ 143.5, 142.8, 133.8 (q, J CF ) 3.8 Hz), 128.3 (q,
1
2J CF ) 31.2 Hz), 124.7 (q, J CF ) 272.6 Hz), -0.1. Anal. Calcd
for C13H20BrF3Si2: C, 42.27; H, 5.46. Found: C, 42.45; H, 5.42.
1-Br om o-3-flu or o-2-(tr im eth ylsilyl)ben zen e (7a ). This
compound was prepared using the procedure described for 1a
starting from 1-bromo-3-fluorobenzene (7.8 g, 50 mmol). A
crude liquid product was distilled in vacuo to give 7a , bp 97-
100 °C (10 mmHg), as a colorless liquid. The residue contained
7b, which could not be isolated in a pure form. Yield of 7a :
6.4 g (52%). 1H NMR: δ 7.35 (d, 1H), 7.16 (m, 1H), 6.94 (m,
5
3J HF ) 4.8 Hz), 0.32 (d, 18H, J HF ) 1.0 Hz). 13C{1H} NMR: δ
1
1H), 0.47 (dd, 9H). 13C{1H} NMR: δ 167.3 (d, J CF ) 246.5
170.9 (d, 1J CF ) 236.4 Hz), 139.0 (d, 3J CF ) 12 Hz), 128.9 (2J CF
Hz), 131.9 (d, 3J CF ) 10.1 Hz), 130.3 (d, 3J CF ) 12.1 Hz), 129.6
) 38.2 Hz), 117.8 (d, 4J CF ) 3 Hz), -0.9. Anal. Calcd for C12H20
BrFSi2: C, 45.13; H, 6.31. Found: C, 44.85; H, 6.13.
-
2
2
(4J CF ) 3.0 Hz), 127.6 (d, J CF )32 Hz), 114.5 (d, J CF ) 28.3
Hz), 1.6 (4J CF ) 4.5 Hz). Anal. Calcd for C9H12BrFSi: C, 43.73;
H, 4.89. Found: C, 43.70; H, 4.97.
1-Br om o-4-ch lor o-2,5-bis(tr im eth ylsilyl)ben zen e (2a ).
This compound was prepared using the procedure described
for 1a starting from 1-bromo-4-chlorobenzene (9.6 g, 50 mmol).
A crude solid product was filtered, washed with water and cold
methanol, and dried to give colorless crystals of 2a , mp 85-
3-Br om o-2-(t r im et h ylsilyl)ben zon it r ile (10a ). LDA (2
M, 27 mL, 54 mmol) was added dropwise to the solution of
3-bromobenzonitrile (9.1 g, 50 mmol) in THF (70 mL) contain-
ing TMSCl (6.0 g, 7 mL, 55 mmol) at -70 °C. The resultant
solution was stirred 30 min at -75 °C and hydrolyzed with
dilute aqueous H2SO4. The yellow organic phase was sepa-
rated, and the water phase was extracted with ether. Evapora-
tion of the combined organic solutions left a crude solid
product. The product was filtered, washed with water and cold
1
87 °C. Yield: 11.0 g (65%). H NMR: δ 7.54 (s, 1H), 7.34 (s,
(21) (a) Bu¨ker, H. H.; Nibbering, N. M. M.; Espinosa, D.; Mongin,
F.; Schlosser, M. Tetrahedron Lett. 1997, 38, 8519 (b) Schlosser, M.;
Marzi, E.; Cottet, F.; Bu¨ker, H. H.; Nibbering N. M. M. Chem. Eur. J .
2001, 7, 3511.
J . Org. Chem, Vol. 68, No. 24, 2003 9387

Lulin´ski and Serwatowski
methanol (3 × 10 mL), and dried to give colorless crystals of
7.49 (d, 1H), 0.43 (s, 9H), 0.41 (s, 9H). 13C{1H} NMR: δ 149.0,
144.1, 138.8, 134.5, 132.4, 121.0, 118.8, -0.5, -1.4. Anal. Calcd
for C13H20BrNSi2: C, 47.84; H, 6.18; N, 4.29. Found: C, 47.73;
H, 5.95; N, 4.33.
1
10a , mp 57-58 °C. Yield: 9.1 g (72%). H NMR: δ 7.75 (dd,
1H), 7.65 (dd, 1H), 7.26 (t, 1H), 0.58 (s, 9H). 13C{1H} NMR: δ
144.2, 137.9, 134.2, 131.7, 130.5, 119.7, 119.6, 1.9. Anal. Calcd
for C10H12BrNSi: C, 47.25; H, 4.76; N, 5.51. Found: C, 47.03;
H, 4.53; N, 5.45.
2-Br om o-3-(t r iflu or om e t h yl)-1-(t r im e t h ylsilyl)b e n -
zen e (16a ). This compound was prepared using the procedure
described for 1a starting with 1-bromo-2-(trifluoromethyl)-
benzene (11.1 g, 50 mmol). A crude product was distilled in
vacuo to give 16a as a colorless liquid, bp 101-104 °C (10 Tr).
Yield: 8.5 g (58%). 1H NMR: δ 7.68 (dd, 1H), 7.61 (m, 1H),
7.39 (m, 1H), 0.45 (s, 9H). 13C{1H} NMR: δ 145.5, 139.5, 130.5
2-Br om o-3-ch lor o-1,4-bis(tr im eth ylsilyl)ben zen e (13c).
This compound was prepared using the procedure described
for 1a starting from 1-bromo-2-chlorobenzene (9.6 g, 50 mmol).
A crude liquid product was fractionally distilled in vacuo. The
forerunner was a mixture of 13a and 13b (ca 1:1), bp 75-100
°C (2 Tr). The heavier fraction, bp 100-105 °C (2 mmHg),
which contained mainly the product, was obtained as an oil
that solidified while standing in a -20 °C freezer. The crude
solid product was mixed with cold methanol (10 mL), filtered,
and purified by recrystallization from methanol (15 mL).
Colorless crystals of 13c, mp 58-60 °C, were obtained. Yield:
4.0 g (23%). 1H NMR: δ 7.37 (d, 1H), 7.29 (d, 1H), 0.40 (s,
9H), 0.37 (s, 9H). 13C{1H} NMR: δ 145.8, 142.7, 141.1, 133.8,
133.6, 131.1, -0.2, -0.5. Anal. Calcd for C12H20BrClSi2: C,
42.92; H, 6.00. Found: C, 43.00; H, 6.13.
2
3
(q, J CF ) 30.2 Hz), 128.9 (q, J CF ) 6 Hz), 127.9, 126.7, 123.6
1
(q, J CF ) 273.6 Hz), 0.0. Anal. Calcd for C10H12BrF3Si: C,
40.42; H, 4.07. Found: C, 40.50; H, 4.11.
Ack n ow led gm en t. The support by Aldrich Chemi-
cal Co., Inc., Milwaukee, WI, through continuous dona-
tion of chemicals and equipment is gratefully acknowl-
edged.
2-Br om o-3,6-bis(tr im eth ylsilyl)ben zon itr ile (15a ). This
compound was prepared using the procedure described for 1a
starting from 2-bromobenzonitrile (9.1 g, 50 mmol). A crude
solid product was filtered, washed with water and cold
methanol (3 × 10 mL), and dried to give 15a as a white solid,
Su p p or t in g In for m a t ion Ava ila b le: Copies of the 13C
NMR spectra of compounds 1a -7a , 10a , 13c, 15a , and 16a .
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
mp 87-89 °C. Yield: 13.0 g (80%). H NMR: δ 7.56 (d, 1H),
J O034790H
9388 J . Org. Chem., Vol. 68, No. 24, 2003