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K. Cao et al. / Journal of Organometallic Chemistry 902 (2019) 120956
Scheme 1. Strategies for selective alkylation of o-carboranes.
meshes) was used for column chromatography, and the distillation
range of petroleum ether was 60e90 ꢀC. 1H, 13C{1H}, 11B{1H} and 11
residue was purified by column chromatography on 200e300 mesh
silica gel with petroleum ether/EtOAc ¼ 4:1 as eluent. All products
were characterized by 1H, 13C{1H}, 11B{1H} and 11B NMR spectra
(refer to electronic supporting information).
B
NMR spectra were recorded on the Bruker 600 MHz instruments.
All 1H NMR and 13C{1H} NMR spectral data were reported in ppm
relative to tetramethylsilane (TMS) as internal standard, 11B NMR
and 11B{1H} NMR spectra data were referenced to external
BF3$Et2O. HRMS data were measured with ESI techniques.
3. Results and discussion
To initiate our research, 9-benzamide-o-carborane and iodo-
methane were selected as model substrates to screen conditions,
and the results were summarized in Table 1. Just as we expected,
the B(4) methylation could take place in the presence of 10 mol%
Pd(OAc)2 and 3 equivalents of AgOAc in CH2Cl2 for 12 h (entry 1).
Further studies indicated that toluene is more favorable for this
transformation (entries 2e3), and the methylation could be
completed in 3 h with Pd(MeCN)2Cl2 (entries 4e6). Moreover, we
found that 2 equivalents of MeI are enough to fulfill this selective
methylation and generated the expected 2a with 64% yield (entry
7). On the other hand, the yield was reduced distinctly along with
decreasing the loading amount of AgOAc (entry 8). Furthermore,
we found that this transformation could proceed smoothly at 25 ꢀC,
and gave the 2a with 60% yield after 12 h (entry 9). To our surprise,
when AgBF4 and AgOTf were examined, the regioselectivity was
reversed, the B(4) methylation was almost inhibited and the B(12)
with more electrophilicity was methylated with 21% and 69%
yields, respectively (entries 10e11). It is worth noting that the
electrophilic multi-methylation was not found even with excess
amount of MeI, and the methylation could not took place in the
absence of Pd(MeCN)2Cl2 (entry 12). These results indicated the
selective methylation would not be a AgOTf promoted Friedel-
Crafts reaction.
2.2. General procedure for synthesis of 2a-2n
To a 10 mL dried flask was sequentially added 9-amide-o-car-
borane (0.1 mmol), toluene (1 mL), iodomethane (12.5 mL, 0.2 mmol),
Pd(MeCN)2Cl2 (2.6 mg, 0.01 mmol) and AgOAc (50.1 mg, 0.3 mmol)
under an argon atmosphere. After the reaction mixture was stirred at
25 ꢀC for 12 h, the reaction mixture was filtered through a short silica
gel column using ethyl acetate as eluent. After evaporation of the
solvent, the residue was purified by column chromatography on
200e300 mesh silica gel with petroleum ether/EtOAc ¼ 4:1 as
eluent. All products were characterized by 1H, 13C{1H}, 11B{1H} and
11B NMR spectra (refer to electronic supporting information).
2.3. General procedure for synthesis of 3a-3l
To a 10 mL dried flask was sequentially added 9-amide-o-car-
borane (0.1 mmol), toluene (1 mL), iodomethane (12.5 mL,
0.2 mmol), Pd(MeCN)2Cl2 (2.6 mg, 0.01 mmol) and AgOTf (77.1 mg,
0.3 mmol) under an argon atmosphere. After the reaction mixture
was stirred at 40 ꢀC for 0.5 h, the reaction mixture was cooled to
room temperature and filtered through a short silica gel column
using ethyl acetate as eluent. After evaporation of the solvent, the