References and Notes
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Alkene 1a (0.2 mmol), LiBr (0.4 mmol), NBS (0.4 mmol), and THF (2.0 mL) were added to a Schlenk
tube successively under an ambient atmosphere. The mixture was stirred at r.t. for
the appropriate time. Sat. aq Na2S2O3 was added to quench the reaction, the organic layer was washed, then dried over anhyd
Na2SO4. Flash column chromatogra-phy gave the pure product 2a as a white solid. 1H NMR (CDCl3, 300 MHz, TMS): δ = 3.98-4.11 (m, 2 H), 5.14 (dd, 1 H, J = 5.4, 9.9 Hz), 7.34-7.42 (m, 5 H, Ar). 13C NMR (CDCl3, 75 MHz, TMS): δ = 34.99, 50.84, 127.62, 128.83, 129.15, 138.56. MS: m/z (%) = 262 (M+, 1), 185 (86), 183 (90), 104 (100).
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<A NAME="RW31405ST-11">11</A> For the dibromination of MCPs in the presence of TiBr4 and DEAD/DIAD, or by molecular Br2, see:
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<A NAME="RW31405ST-12">12</A>
MCP 1a (0.2 mmol), LiBr (0.4 mmol), and THF (2.0 mL) were placed in a Schlenk tube under
an ambient atmosphere and the mixture was stirred for about 10 min at r.t. Then NBS
(0.4 mmol) was added and the mixture was stirred at r.t. for the appropriate time.
Sat. aq Na2S2O3 was added to quench the reaction. The mixture was washed and dried over anhyd Na2SO4. The solvent was removed under reduced pressure and the residue was purified by flash
column chromatogra-phy to give product 8a as a colorless liquid. IR (CH2Cl2): 3933, 3054, 2985, 1486, 1443, 1265, 896, 746 cm-1.
1H NMR (CDCl3, 300 MHz, TMS): δ = 3.07 (t, 2 H, J = 6.9 Hz), 3.62 (t, 2 H, J = 6.9 Hz), 7.20-7.32 (m, 10 H, Ar).
13C NMR (CDCl3, 75 MHz, TMS): δ = 31.04, 40.76, 123.20, 127.42, 127.57, 128.06, 128.51, 128.72,
128.83, 140.31, 142.68, 144.87. MS: m/z (%) = 368 (29), 366 (60), 364 (M+, 34), 192 (100). HRMS: m/z calcd for C16H14Br2: 363.9457; found: 363.9470.
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We found that the reaction mixture quickly changed from colorless to deep orange as
soon as NBS was added. After the reaction was complete, the reaction solution changed
to pale yellow. When the reaction was quenched with a sat. solution of aq Na2S2O3, the reaction mixture immediately changed from pale yellow to colorless. These phenomena
suggest that molecular Br2 is involved in the equilibrium.
<A NAME="RW31405ST-15">15</A>
We were unsuccessful in obtaining a crystal structure, however, the white solid obtained
from the reaction of NBS and LiBr in THF decomposed to pyrrolidine-2,5-dione, which
was confirmed by X-ray diffraction.
<A NAME="RW31405ST-16">16</A>
The yields in the presence of TEMPO and BHT were 99% and 96%, respectively.