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OH
Br
( )-12 (0% e.e.)
100% conversion of 11
11 (99% e.e.)
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Br
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~35% conversion of 11
b
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Ru(bpy)3Cl2 (1.0mol%)
CBr4 (2.0 equiv.)
NaBr (2.0 equiv.)
Br
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12
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25
0
0
3
6
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12
Time (h)
Figure 4 | Degenerate SN2 reaction results in racemization with optically
enriched alcohol 11. a, Racemic bromide product 12 was obtained when
optically pure alcohol 11 was treated with Ru(bpy)3Cl2 (1.0 mol%), CBr4
(2.0 equiv.) and NaBr (2.0 equiv.) in DMF for 12 h. However, highly optically
enriched bromide 12 was isolated at low conversion of 11, which indicates
that the racemization may be due to a concomitant Finkelstein reaction. This
hypothesis was confirmed by the isolation of chiral formate ester 13 at high
conversion of 11 indicating the reactive intermediate is configurationally
stable. b, Time-dependent loss of optical activity of chiral bromide 12 implies
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photocatalysis. The reaction is highlighted by its exceptional func-
tional group tolerance, avoidance of stoichiometric oxidized phos-
phine by-products, and generation of the halogenated compounds
in high yields under mild reaction conditions. Moreover, the cost-
effective nature of Ru(bpy)3Cl2, low catalyst loadings, operational
simplicity and activity under visible-light irradiation makes this
transformation industrially valuable.
Methods
A flame-dried 10 ml Schlenk flask with a rubber septum and magnetic stir bar was
charged with tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate (5.0 mmol,
0.010 equiv.), the corresponding alcohol (0.50 mmol, 1.0 equiv.), carbon tetrabromide
(1.0 mmol, 2.0 equiv.) and sodium bromide (1.0 mmol, 2.0 equiv.). The flask was
purged with a stream of nitrogen, and dry DMF (5.0 ml) was added with a syringe. The
mixture was degassed by the freeze–pump–thaw procedure (three cycles), and placed in
a 250 ml beaker with blue LEDs wrapped inside (the reaction reaches temperatures
between 25 and 30 8C upon exposure to the blue LEDs). The reaction mixture was
stirred until it was complete (as judged by thin layer chromatography analysis). The
mixture was poured into a separatory funnel containing 25 ml Et2O and 25 ml H2O.
The layers were separated and the aqueous layer was extracted with Et2O (2 × 25 ml).
The combined organic layers were washed with saturated Na2S2O3 solution, brine,
dried (MgSO4) and concentrated in vacuo. The residue was purified by
chromatography on silica gel to afford the desired product.
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Received 8 July 2010; accepted 11 November 2010;
published online 9 January 2011
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