42
S.A. Patil et al. / Journal of Molecular Structure 1015 (2012) 41–45
H3C
Si
CH3
CH3
CH3
Si
H3C
CH3
+ H3C
H3C
Mg
Br
[PdCl2(dppf)]
THF, Reflux
H3C
H3C
H3C
2
H3C
3
Br
1
THF
Br2
H3C
H3C
Br
H3C
4
Fig. 1. Synthesis of 1-bromo-4-(3,7-dimethyloctyl)benzene.
analysis was done with an Exeter Analytical CE-440 Elemental
Analyzer and was within 0.4% of the theoretical values.
yield of 85%. 1H NMR (CDCl3) d (ppm) 7.37 (d, J = 8.7 Hz, 2H,
ArH), 7.04 (d, J = 8.1 Hz, 2H, ArH), 2.64–2.45 (m, 2H, -CH2), 1.62–
1.04 (m, 10H, -CH2, CH, CH2, CH2, CH2, CH), 0.91 (d, J = 6.3 Hz, 3H,
CH3), 0.86 (d, J = 6.3 Hz, 6H, CH3). 13C NMR (CDCl3) d (ppm)
142.1, 131.2, 130.1, 119.1, 39.3, 39.2, 38.7, 37.0, 32.8, 32.3, 27.9,
24.6, 22.6, 19.5. Mass data (TOF MS EI+): calculated for C16H25Br
[M+] 296.11, found: 296.01. Compound was analyzed for
2.1. Synthesis (see Fig. 1)
The synthetic pathway for the 1-bromo-4-(3,7-dimethyloctyl)-
benzene GNR precursor, 4, described in this work is outlined in
Fig. 1. The synthesis of 4 starts with commercially-available 4-
bromo-trimethylsilyl benzene. After a palladium catalyzed Kumad-
a cross-coupling reaction with 3,7-dimethyloctyl bromide, the
compound was transformed into 1-trimethylsilyl-4-(3,7-dimethy-
loctyl) benzene, 3. The TMS group was further converted into a
bromo moiety through bromination as detailed below.
C
16H25Br (calculated C 64.64, H 8.48, Br 26.88%; found C 64.59, H
8.45, Br 26.72%). IR absorptions (KBr, cmÀ1): 2953 (s), 2867 (m),
1716 (m), 1488 (s), 1463 (m), 1403 (m), 1377 (w), 1364 (m),
1259 (w), 1218 (m), 1072 (s), 1011 (s), 801 (s).
3. Computational details
2.1.1. Synthesis of 1-trimethylsilyl-4-(3,7-dimethyloctyl)benzene (3)
In a 300-mL round bottom flask, 22 mL of a 1 M solution of 3,7-
dimethyloctanyl-1-magnesiumbromide was added dropwise to a
solution of 2.5 g (10.9 mmol) 1-bromo-4-trimethylsilylbenzene
dissolved in 150 mL of dry THF and 250 mg [PdCl2(dppf)] catalyst.
The resulting mixture was stirred under reflux in an inert atmo-
sphere overnight. The reaction was quenched with methanol and
the solvent removed under reduced pressure. Purification using
column chromatography on silica gel with petroleum ether as
the eluent afforded 2.8 g 1-trimethylsilyl-4-(3,7-dimethyloc-
tyl)benzene as a colorless oil. Yield: 90%. 1H NMR (CDCl3) d
(ppm) 7.46 (d, J = 8.4 Hz, 2H, ArH), 7.20 (d, J = 7.5 Hz, 2H, ArH),
2.71–2.52 (m, 2H, -CH2), 1.70–1.04 (m, 10H, -CH2, CH, CH2, CH2,
CH2, CH), 0.95 (d, J = 6.0 Hz, 3H, CH3), 0.89 (d, J = 6.0 Hz, 6H, CH3),
0.28 (s, 9H, CH3). 13C NMR (CDCl3) d (ppm) 143.9, 137.0, 133.3,
127.8, 39.3, 38.8, 37.1, 33.5, 32.6, 27.9, 24.6, 22.7, 22.6, 19.6,
À1.0. Mass data (TOF MS EI+): calculated for C19H34Si [M+]
290.24, found: 290.24. Compound was analyzed for C19H34Si (cal-
culated C 78.54, H 11.79, Si 9.67%; found C 78.42, H 11.67, Si
9.65%). IR absorptions (KBr, cmÀ1): 2953 (s), 2925 (m), 2868 (w),
1716 (m), 1601 (w), 1462 (m), 1396 (w), 1364 (m), 1247 (s),
1218 (m), 1108 (s), 850 (s), 836 (s), 804 (m), 754 (m), 720 (w),
691 (w).
DFT calculations of 1-bromo-4-(3,7-dimethyloctyl)benzene and
1-trimethylsilyl-4-(3,7-dimethyloctyl)benzene have been carried
out using the Gaussian 09 program [11]. We have employed the
widely used [12] B3LYP/6-311+G(2d,2p) level of theory for geom-
etry optimizations as well as IR frequencies and chemical shifts
(additional functionals can be found in Supporting information).
NMR parameters were evaluated using the gauge-including atomic
orbitals (GIAOs) formalism [13]. Values of d are presented with re-
spect to TMS, obtained at the same level of theory.
4. Result and discussion
Fig. 2 shows the basis set effect on selected 13C chemical shifts
with the deshielding trend of the larger basis sets and the opposite
behavior for smaller basis sets (for complete chemical shifts of 13
C
and 1H see Supporting Figs. S-9 and S-10). We have evaluated the
effect of chloroform as a solvent on the NMR chemical shifts using
a PCM model [14]. The solvent effect was found to be negligible in
both the geometrical parameters as well as the NMR chemical
shifts (shown in Fig. 2) and will therefore not be discussed further.
IR frequencies have been obtained at the B3LYP/6-311+G(2d,2p) le-
vel of theory and scaled as reported previously (with a scaling fac-
tor of 0.96) [15].
2.1.2. Synthesis of 1-bromo-4-(3,7-dimethyloctyl)benzene (4)
Selected bond lengths and bond angles of the DFT optimized
structure of these compounds are shown in Fig. 3. The optimized
bond lengths [C3–Si = 1.89, C6–C24 = 1.51, C3–Br = 1.92 and C6–
C11 = 1.51 Å] and angles [C2–C3–Si = 120.55, C4–C3–Si = 122.69,
C2–C3–Br = 119.54 and C4–C3–Br = 119.53°] are in good agree-
ment with the bond lengths and bond angles found in the similar
compounds 4-(trimethylsilyl)benzoic acid and 2-(4-bromoben-
zyl)-1,3-diphenylpropane-1,3-dione reported in literature [16,17].
Calculated values of d1H and d13C agree well with their experi-
mental values, within the accuracy of the theoretical framework
employed. These results are shown in Fig. 4, wherein we observe
To obtain 1-bromo-4-(3,7-dimethyloctyl)benzene, 1-trimethyl-
silyl-4-(3,7-dimethyloctyl) benzene (2.9 g, 10.0 mmol) and sodium
acetate (2.72 g, 20.0 mmol) were dissolved in THF (60 mL) at 0 °C.
Bromine (1.59 g, 20.0 mmol) was then added dropwise via a drop-
ping funnel, and the mixture was stirred for 20 min. The reaction
was quenched with saturated sodium sulfite aqueous solution
(40 mL). The organic layer was then extracted with CH2Cl2
(65 mL Â 3) and dried over MgSO4. After being filtered, the solvent
was removed in vacuo and the crude product was purified by col-
umn chromatography (eluent petroleum ether), affording 2.5 g of
1-bromo-4-(3,7-dimethyloctyl)benzene as transparent oil in a
a
good correlation between fist-principles calculations and