Sol-Gel Entrapped Pyridinium Hydrobromide Perbromide
SHORT COMMUNICATION
Table 1. Bromination of various substrates by sol-gel entrapped py- the isomeric stilbenes by non-heterogenized pyridinium hy-
ridinium hydrobromide perbromide.[a]
drobromide perbromide that transforms the (Z) and (E)
compounds to dl- and meso-dibromides, respectively,[9] the
entrapped reagent converts both stilbenes solely into the
meso product. This phenomenon may be associated with
the ability of the silica-bound reagent to reversibly intercon-
vert the isomeric stilbenes.
Upon completion of the reaction, the used heterogenized
pyridinium residue can be recharged with bromine and re-
cycled. When the bromination reagent, that had been used
in the experiments of Table 1 (6 mmol), was washed with
CH2Cl2 (3×10 mL) and sonicated followed by treatment
with Br2 and removal of the excessive halogen, the bromina-
tion ability of the reagent was renewed. For example, the
recycled PHPB@s.g., that was used under the conditions of
Table 1 for the bromination of cyclohexene, afforded in the
second, third and fourth runs of 7 h, 91, 92, and 88% of
1,2-dibromocyclohexane, respectively. Likewise, we bromi-
nated toluene with used and recharged agent and obtained
in the first four cycles of 7 h, 75, 74, 77, and 74% of benzyl
bromide.
Some of the bromination products (e.g., the dibromides)
could be dehydrobrominated by strong bases. Under homo-
geneous conditions, this operation could be carried out only
after completion of the bromination process and isolation
of the dibromides. A one-pot combined reaction is not pos-
sible because strong bases destroy the bromination reagent.
This shortcoming could be overcome by entrapment of the
base within a silica sol-gel matrix. The strong guanidine
base 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) was modi-
fied with (3-glycidoxypropyl)trimethoxysilane, hydrolyzed
and co-condensed with TMOS as shown in Scheme 2.[2b]
Application of the entrapped base TBD@s.g., does not
destroy the heterogenized reagent, PHPB@s.g., and permits
to carry out the bromination and hydrobromination in one
pot. Representative results are summarized in Table 2. Vinyl
bromides and/or acetylenes are formed in the first place
during the dehydrobromination. It seems however, that the
unsaturated products are re-brominated. Elimination of
HBr may form an olefin with a different stereochemistry
[a] Reaction conditions (except for Entry 8): 1.5 mmol of substrate
than the starting alkene. Indeed, the bromination of (Z)-
stilbene (Table 2, Entry 3) forms during a reaction period
of 15 h at 65 °C 47% of (E)-stilbene [only 2% of the (Z)
isomer remains]. Likewise, (E)-stilbene affords under the
same conditions 13% of (Z)- together with 20% of “un-
changed” (E)-stilbene. Thus, unlike under homogeneous
in n-hexane (10 mL) was stirred under N2 at 65 °C with 3 mmol of
PHPB@s.g. In Entry 8, 2.25 mmol of the bromination reagent was
used. [b] Average of at least 2 experiments that did not differ by
more than 3%. Analyses of the reaction mixtures indicated only
the presence of the bromination products and the unreacted sub-
strates. [c] If the reagent was not dried completely, the product was
contaminated with varying amounts of 2-bromocyclohexanol.
Scheme 2.
Eur. J. Org. Chem. 2006, 1396–1399
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