Bromide ions and methyltrioxorhenium as cocatalysts for hydrogen peroxide oxidations and brominations

Article abstract of DOI:10.1021/jo9817164

Oxidation of alcohols by hydrogen peroxide is negligible; even when catalyzed by methyltrioxorhenium (MTO), the process requires a long reaction time. The addition of a catalytic quantity of bromide ions, as HBr or NaBr, greatly enhances the rate. Some of the reactions were carried out on a larger scale in glacial acetic acid, and others at kinetic concentrations. The data establish that Br₂ is the active oxidizing agent in the system, because the catalytic rates under suitable circumstances match those for the independently measured Br₂ reaction with alcohol (benzyl alcohol, in particular). At much lower levels of MTO, however, Br₂ formation plays a role in the kinetics. Certain other reluctant transformations are conveniently carried out with the MTO/H₂O₂/Br^- combination: aldehydes to methyl esters; 1,3-dioxolanes to glycol monoesters; and ethers (with cleavage) to ketones (mostly), but in fair yield only. When Br^- was used in stoichiometric quantity, certain bromination reactions occur. Thus, phenyl acetylenes (PhC₂R, R = H, Me, Ph) are converted to dibromoalkenes that are entirely or largely formed as the trans isomer, and phenols are brominated. The latter reaction shows the preference para > ortho > meta. Kinetic studies of benzyl alcohol oxidation with MTO/H₂O₂Br^- were carried out in aqueous solution. With sufficient (normal) levels of MTO, the rate constant for the formation of benzaldehyde agreed with the independently determined value for Br₂ + PhCH₂OH, k = 4.3 x 10^-3 L mol^-1 s^-1 at 25.0 °C; for sec- phenethyl alcohol, k = (9.8 ± 0.4) x 10^-3 L mol^-1 s^-1. Bromine is formed from the known oxidation of Br^- with H₂O₂, catalyzed by MTO. This reaction results in BrO^-/HBOr, which is then rapidly converted to Br₂. However, with substantially lower concentrations of MTO, the buildup of benzaldehyde is ca. 4-fold slower, reflecting the diminished rate of Br^- oxidation.

Full text of DOI:10.1021/jo9817164

J . Org. Chem. 1999, 64, 1191-1196
1191
Br om id e Ion s a n d Meth yltr ioxor h en iu m a s Coca ta lysts for
Hyd r ogen P er oxid e Oxid a tion s a n d Br om in a tion s
J ames H. Espenson,* Zuolin Zhu, and Timothy H. Zauche
Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 50011
Received August 24, 1998
Oxidation of alcohols by hydrogen peroxide is negligible; even when catalyzed by methyltrioxorhe-
nium (MTO), the process requires a long reaction time. The addition of a catalytic quantity of
bromide ions, as HBr or NaBr, greatly enhances the rate. Some of the reactions were carried out
on a larger scale in glacial acetic acid, and others at kinetic concentrations. The data establish
that Br2 is the active oxidizing agent in the system, because the catalytic rates under suitable
circumstances match those for the independently measured Br2 reaction with alcohol (benzyl alcohol,
in particular). At much lower levels of MTO, however, Br2 formation plays a role in the kinetics.
Certain other reluctant transformations are conveniently carried out with the MTO/H2O2/Br-
combination: aldehydes to methyl esters; 1,3-dioxolanes to glycol monoesters; and ethers (with
cleavage) to ketones (mostly), but in fair yield only. When Br- was used in stoichiometric quantity,
certain bromination reactions occur. Thus, phenyl acetylenes (PhC2R, R ) H, Me, Ph) are converted
to dibromoalkenes that are entirely or largely formed as the trans isomer, and phenols are
brominated. The latter reaction shows the preference para > ortho > meta. Kinetic studies of benzyl
alcohol oxidation with MTO/H2O2/Br- were carried out in aqueous solution. With sufficient (normal)
levels of MTO, the rate constant for the formation of benzaldehyde agreed with the independently
determined value for Br2 + PhCH2OH, k ) 4.3 × 10-3 L mol-1 s-1 at 25.0 °C; for sec-phenethyl
alcohol, k ) (9.8 ( 0.4) × 10-3 L mol-1 s-1. Bromine is formed from the known oxidation of Br-
with H2O2, catalyzed by MTO. This reaction results in BrO-/HOBr, which is then rapidly converted
to Br2. However, with substantially lower concentrations of MTO, the buildup of benzaldehyde is
ca. 4-fold slower, reflecting the diminished rate of Br- oxidation.
In tr od u ction
ing agents for Br- are the two peroxorhenium species A,
CH3Re(O)2(η2-O2), and B, CH3Re(O)(η2-O2)2(H2O).
Reactions of hydrogen peroxide for which MTO is an
efficient catalyst are nearly always those in which one
oxygen atom can be added immediately to the substrate.1-3
A rare exception is alcohol oxidation, which occurs quite
slowly even with MTO present.4,5 Alcohols show an
appreciable C-H/C-D kinetic isotope effect, which is one
reason to suggest a mechanism involving C-H activation
and, in particular, hydride abstraction.5
The oxidation of organic compounds by hypohalite salts
or halogens is an important method in organic syn-
thesis.12-15 Halogenation reactions are widely used in the
synthesis of flame retardants, pharmaceuticals, agro-
chemicals, and specialty chemicals.16 Because they are
high-risk oxidants, it is useful to develop catalytic routes
that minimize the introduction of halogens and the
production of halogenated wastes.17 The development of
regiospecific reactions is one area needing investigation.
Alcohols were one target for oxidations, in that the
reactions without bromide ions are so sluggish despite
the acceleration provided by MTO. We also sought to
discover the conditions under which alcohols would be
oxidized, to evaluate bromide vs chloride as cocatalyst,
and to examine the oxidations of 1,3-dioxolanes and
ethers. In a separate aspect of this research, we set out
Because MTO catalyzes the peroxide oxidation of bro-
mide to hypobromite,6 we chose to examine whether Br-
would promote hydrogen peroxide oxidations that occur
by hydride ion abstraction. Catalytic functionalization of
organic compounds by mimicking metalloenzymes is one
of the important methods for mild and highly selective
syntheses.7 Haloperoxidases are enzymes that catalyze
the oxidation of halide ions by hydrogen peroxide.8 Cer-
tain of their reactions have been mimicked by ammonium
metavanadate.9-11 In the case of MTO, the active oxidiz-
(10) Conte, V.; Furia, F. D.; Moro, S. Tetrahedron Lett. 1994, 35,
7429.
(11) Conte, V.; Furia, F. D.; Moro, S.; Rabbollini, S. J . Mol. Catal.
1996, 113, 175.
(12) Stevens, R. V.; Chapman, K. T.; Weller, H. N. J . Org. Chem.
1980, 45, 2030.
(13) Katritzky, A. R.; Meth-Cohn, O.; Rees, C. W. In Comprehensive
Organic Functional Group Transformations; Pergamon Press: New
York, 1995; Vol. 5.
(14) Larock, R. C. Comprehensive Organic Transformations; VCH
Publishers, Inc.: New York, 1989.
(15) Palou, J . Chem. Soc. Rev. 1994, 357-361.
(16) Taylor, R. Electrophilic Aromatic Substitution; J ohn Wiley &
Sons: New York, 1990.
(17) Nelson, D. W.; Gypser, A.; Ho, P. T.; Kolb, H. C.; Kondo, T.;
Kwong, H.-L.; McGrath, D. V.; Rubin, A. E.; Norrby, P.-O.; Gable, K.
P.; Sharpless, K. B. J . Am. Chem. Soc. 1997, 119, 1840.
(1) Espenson, J . H.; Abu-Omar, M. M. Adv. Chem. Ser. 1997, 253,
99-134.
(2) Herrmann, W. A.; Ku¨hn, F. E. Acc. Chem. Res. 1997, 30, 169-
180.
(3) Gable, K. P. Adv. Organomet. Chem. 1997, 41, 127-161.
(4) Murray, R. W.; Iyanar, K.; Chen, J .; Wearing, J . T. Tetrahedron
Lett. 1995, 36, 6415.
(5) Zauche, T. H.; Espenson, J . H. Inorg. Chem., 1998, 37, 6827.
(6) Espenson, J . H.; Pestovsky, O.; Huston, P.; Staudt, S. J . Am.
Chem. Soc. 1994, 116, 2869.
(7) Barton, D. H. R.; Doller, D. Acc. Chem. Res. 1992, 25, 504-512.
(8) Butler, A.; Walker, J . V. Chem. Rev. 1993, 93, 1937-1944.
(9) Dinesh, C. U.; Kumar, R.; Pandey, B.; Kumar, P. J . Chem. Soc.,
Chem. Commun. 1995, 611.
10.1021/jo9817164 CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/22/1999

1192 J . Org. Chem., Vol. 64, No. 4, 1999
Espenson et al.
Ta ble 1. Com p a r ison of Br om id e to Ch lor id e Ion s a s
Coca ta lysts in Oxid a tive Tr a n sfor m a tion s w ith
Hyd r ogen P er oxid e Ca ta lyzed by MTO
Ta ble 2. Yield s a n d Con ver sion s Obta in ed for th e
Oxid a tion of Secon d a r y Alcoh ols w ith Hyd r ogen
P er oxid e w ith Meth yltr ioxor h en iu m a n d Br om id e Ion s
a s Coca ta lysts
a
a
b
b
b
c
b
c
a
b
In MeOH. In HOAc. c All halide ions were consumed and
organic halides were formed.
to learn whether phenols and acetylenes could be bro-
minated, which of course requires a stoichiometric and
not a catalytic level of bromide. The experimental effort
was divided between synthetic and mechanistic investi-
gations. Several “active bromine” intermediates, includ-
ing BrO-, HOBr, and Br2, as well as 1O2, have been
identified as a part of the reaction sequence when
bromide is oxidized by hydrogen peroxide with MTO
catalyst.6 Our results point to Br2 as the only active
species under the conditions employed.
Ta ble 3. Oxid a tion of P r im a r y Alcoh ols w ith Hyd r ogen
P er oxid e Usin g Meth yltr ioxor h en iu m a n d Br om id e Ion s
a s Coca ta lysts
Resu lts a n d In ter p r eta tion
Net Oxid a tion s. Acetic acid was chosen as the solvent
for certain experiments because acidic conditions prolong
the lifetime of the catalyst against irreversible decom-
position.18 The overall reactions of secondary and primary
alcohols can be represented by this chemical equation:
RCH2OH/R2CHOH +
H2O2 MTO/NaBr (5 mol %)8 RCHO/R2CO + 2H2O (1)
HOAc
Hydrogen peroxide is consumed in stoichiometric amounts.
Both bromide6 and chloride19 ions were tried as cocata-
lysts. The former reacted more efficiently and gave a
purer product, as presented in Table 1. The products from
a representative sampling of secondary alcohols with the
cocatalysts NaBr/CH3ReO3 are given in Table 2. In all of
the cases examined, the conversions and selectivities
were nearly quantitative.
Data for primary alcohols are given in Table 3. The
conversions in 10 h ranged from 32% to 100%, the lower
values being found for aliphatic alcohols. Benzylic alco-
hols reacted satisfactorily, giving aldehydes; they are all
p-CH3OC6H4CH2Br as more Br- was added; with Br-
maintained at the catalytic level, the reaction stopped
completely once the Br- had been converted to the benzyl
bromide. This is an example of a bromination reaction,
which is the subject of the next section.
The reaction time of 10 h was chosen as the uniform
standard. The larger primary aliphatic alcohols gave
lower yields because of the concurrent decomposition of
MTO and hydrogen peroxide at longer times: CH3ReO3
1
known compounds and were identified by their H NMR
spectra. The one exception to a high yield of oxidation is
the highly activated substrate p-CH3OC6H4CH2OH, which
gave 1% yield of the aldehyde and 99% of the bromide
(18) Abu-Omar, M.; Hansen, P. J .; Espenson, J . H. J . Am. Chem.
Soc. 1996, 118, 4966-4974.
(19) Hansen, P. J .; Espenson, J . H. Inorg. Chem. 1995, 34, 5839.

Bromide and MTO as Cocatalysts
J . Org. Chem., Vol. 64, No. 4, 1999 1193
Ta ble 4. Con ver sion of Ald eh yd es to Meth yl Ester s w ith
Hyd r ogen P er oxid e Usin g Meth yltr ioxor h en iu m a n d
Br om id e Ion s a s Coca ta lysts
Ta ble 5. Oxid a tive Clea va ge of 1,3-Dioxola n es to Glycol
Mon oester s by MTO/H2O2/Br - in Gla cia l Acetic Acid
a
a
The balance is the carboxylic acid.
+ H2O2 f CH3OH + HReO4.18 Direct detection of
1
Ta ble 6. Oxid a tive Clea va ge of Eth er s w ith Hyd r ogen
P er oxid e Usin g Meth yltr ioxor h en iu m a n d Br om id e Ion s
a s Coca ta lysts
CH3ReO3, A, and B by H NMR allowed us to monitor
catalyst stability. Most of the alcohols had been largely
oxidized during the time in which catalyst deactivation
had occurred. The oxidation of the secondary alcohols,
on the other hand, was complete while most of the CH3-
Re bonds still remained intact.
F u r th er Oxid a tion s: Ald eh yd es, Dioxola n es, a n d
Eth er s. The further oxidation of aldehydes to carboxylic
acids and, in acetic acid, to acetate esters, signals
complications. The other products found in Tables 2 and
3 indicate the extent to which the side reactions consti-
tute a major difficulty. When the reactions are run in
methanol, the aldehydes can be further oxidized to their
methyl esters. A minor amount of carboxylic acid is also
found. Table 4 summarizes the findings. Unlike the
literature method20 that uses >1.5 equiv of NaOCl at 0-5
°C to attain an acceptable yield, this method with only
5% NaBr gives an excellent yield of the esters.
The oxidation of 1,3-dioxolanes is of some interest,
given the importance of glycol monoesters. Existing
methods that start with the diol require a tedious
separation.21 Alternatively, the oxidative cleavage of
cyclic acetals by tert-butyl hydroperoxide with a Pd(II)
catalyst can be used.22 The MTO/H2O2/Br- combination,
under the same conditions as for the alcohols, was used
for three 1,3-dioxolanes. A palladium-catalyzed acetal
hydrolysis/oxidation leads to similar products.22 These
results are reported in Table 5.
mination of methylphenyl acetylene in acetic acid with
bromine itself gives a 42:58 trans:cis ratio, which in-
creases to (an extrapolated) 100% trans at higher bro-
mide concentrations, where the mechanism has changed.
The bulk of these oxidations were carried out with
bromide present at fairly high concentration, resulting
in the predominance of the trans product.25
The trans configuration of dibromostilbene, a com-
pound that had not been well-characterized previously,
was confirmed by an X-ray determination. The structure
is displayed in Figure 1. The stoichiometry of the net
transformation is given in eq 2.
Ethers, generally quite difficult to cleave oxidatively,
can be cleaved in fair yield by this combination. The
yields, however, are <60%, as summarized in Table 6.
These results are presented only because this is an
unusual transformation.
Phenols are brominated rapidly by H2O2, Br-, and
MTO. These reactions (Table 8) exhibit a preference for
bromination at the para position. Bromination did occur,
however, when only the meta position was available. The
selectivity in general follows the order para > ortho >
meta. This ordering is sufficiently pronounced that only
a single product was obtained in every case when compet-
ing sites were available. The stoichiometry of the overall
reaction is
Ca ta lytic Br om in a tion s. MTO catalyzes the bromi-
nation of alkynes by the combination of H2O2 and Br-.
Diphenylacetylene, methyl phenylacetylene, and phen-
ylacetylene were brominated. As presented in Table 7,
the trans-dibromoalkene is favored, increasingly so as the
group R increases in size. The products were identified
by 1H NMR and MS, and the isomer ratio was determined
1
by H integration.23-25 The literature reports that bro-
(20) Stevens, R. V.; Chapman, K. T.; Stubbs, C. A.; Tam, W. W.;
Albizati, K. F. Tetrahedron Lett. 1982, 23, 4647.
(21) Babler, J . H.; Coghlan, M. J . Tetrahedron Lett. 1979, 1971.
(22) Hosokawa, T.; Imada, Y.; Murahashi, S.-I. J . Chem. Soc., Chem.
Commun. 1983, 1245.
(23) Berthelot, J .; Fournier, M. Can. J . Chem. 1986, 64, 603.
(24) Uemura, S.; Okazaki, H.; Okano, M. J . Chem. Soc., Perkin
Trans. 1 1978, 1278.
Kin etics a n d Mech a n ism . The steps in this catalytic
process are presumed to be as follows. MTO binds
(25) Pincock, J . A.; Yates, K. Can. J . Chem. 1970, 48, 3332.

1194 J . Org. Chem., Vol. 64, No. 4, 1999
Espenson et al.
Ta ble 8. P r od u cts of th e MTO-Ca ta lyzed Br om in a tion of
P h en ols by Hyd r ogen P er oxid e a n d Sod iu m Br om id ea
F igu r e 1. Molecular structure of trans-R,R′-dibromostilbene
prepared from the MTO-catalyzed bromination of diphenyl-
acetylene with hydrogen peroxide; this provides definite proof
for the exclusive formation of the trans isomer.
Ta ble 7. P r od u cts of th e MTO-Ca ta lyzed Br om in a tion of
P h en yla cetylen es by Hyd r ogen P er oxid e a n d Sod iu m
Br om id e
a
Yields were quantitative. b 2,4-Dibromophenol was also formed
after ∼1 h.
Sch em e 1. Rea ction s in th e Cycle of Alcoh ol
Oxid a tion by Hyd r ogen P er oxid e w ith
Meth yltr ioxor h en iu m a n d Br om id e Ion s a s
Coca ta lysts
hydrogen peroxide11 in two reversible and reasonably
rapid equilibria.13,17 Each peroxorhenium species, A and
B, converts Br- to BrO-.6 This, too, is reasonably rapid.
In an acidic medium, BrO-/HOBr then becomes Br2
nearly instantly. The alcohol is subsequently attacked
by species that are known to abstract hydride from the
OH-bearing carbon. That reagent is probably bromine15,26
or, less likely, hypobromite.27,28 This sequence of reactions
is depicted in Scheme 1.
Bromide ions are central to this process. Without Br-,
the peroxorhenium complexes are unable to proceed
further. Although numerous examples can be cited in
which peroxorhenium complexes are efficient donors of
an oxygen atom,1 they are for the most part unable to
abstract hydride. Thus, the synergistic effect of the
cocatalysts: the one excises an oxygen atom from per-
oxide, and the other, after accepting that oxygen, in turn
is converted to a species capable of abstracting hydride
ion from the alcohol, which is the key to its oxidation.
The only rate constant in Scheme 1 that was not
previously known in this medium is k5. Our determina-
tions for benzyl alcohol were carried out with [Br2]0 )
1.58 mM and [PhCH2OH]0 ) 64-450 mM. The absor-
bance increase accompanying benzaldehyde formation
was monitored. The data were fit by nonlinear least
squares to first-order kinetics. The resulting second-order
rate constant is k5 ) (4.3 ( 0.3) × 10-3 L mol-1 s-1; for
sec-phenethyl alcohol, k5 ) (9.8 ( 0.4) × 10-3 L mol-1
s-1
.
Two series of experiments were performed, each with
these initial concentrations: 0.296 M benzyl alcohol, 4.7
mM H2O2, and 1.0 M HClO4. In the first set of four
determinations, [MTO] and [Br-] were varied in the
ranges 2.1-5.0 mM and 2.5-9.9 mM, respectively. With
this fairly high level of MTO, it seemed that Br2 forma-
tion would be considerably faster than its consumption.
The formation of benzyldehyde was again monitored. The
data from each experiment fit first-order kinetics. The
average value of kψ was divided by [PhCH2OH]. The
resulting quotient was a constant, with the average value
(4.6 ( 0.5) × 10-3 L mol-1 s-1. This agrees with the value
directly determined for k5, thus confirming that the
reaction between Br2 and PhCH2OH was indeed rate-
controlling under these circumstances. It further con-
(26) March, J . Advanced Organic Chemistry; 4th ed.; Wiley: New
York, 1992.
(27) Puttaswamy; Mahadevappa, D. S.; Made Gowa, N. M. Int. J .
Chem. Kinet. 1991, 23, 27-35.
(28) Wolfe, S.; Hasan, S. K.; Campbell, J . R. Chem. Commun. 1970,
1420.

Bromide and MTO as Cocatalysts
J . Org. Chem., Vol. 64, No. 4, 1999 1195
The mechanism by which Br2 oxidizes alcohols involves
hydride abstraction. The considerable body of evidence
for a hydride abstraction mechanism has been sum-
marized.29
Discu ssion
The use of hydrogen peroxide with a molybdenum
catalyst has been reported: secondary alcohols reacted,
primaries did not, and aldehydes were converted to
carboxylic acids.10,11 The principle evinced here, however,
is entirely different because of the involvement of bro-
mide, hypobromite, and bromine. One report has ap-
peared concerning the oxidation of benzylic alcohols by
hydrogen peroxide with an HBr catalyst; as in our work,
the active intermediate was shown to be Br2.30 This is
consistent with the known fact that H2O2 and Br- react
to generate Br2, even in the absence of MTO.31,32 The use
of a somewhat higher level of Br- and an elevated
temperature (60 °C) allowed the net conversion even
without MTO. Oxygen is not effectively activated by
methylrhenium peroxide and cannot be substituted for
hydrogen peroxide. These features are just those pre-
sented by vanadium bromoperoxidase.8
F igu r e 2. Absorbance-time recordings for the oxidation of
PhCH2OH (0.296 M) with hydrogen peroxide (4.7 mM) in
solutions containing the cocatalysts Br- (4.94 mM) and MTO
(A, 5.02 mM; B, 0.05 mM). The experiments were carried out
in aqueous solution at 25.0 °C.
firms, of course, that Br2 is the active catalytic interme-
diate responsible for alcohol oxidation. Under these
reaction conditions, no further oxidation of benzaldehyde
occurred; this was confirmed by an experiment with
independent aldehyde that confirmed no reaction in
>4000 s.
The second set of experiments was performed with
much lower concentration of MTO, 50-100 µM. This was
done deliberately to reduce the rate of Br- oxidation by
A, thus making Br2 formation more competitive with its
consumption. The rate constants in this series were much
less, about 25% of the values of those cited previously,
substantiating the contribution of the k3 reaction to the
kinetic barrier. Figure 2 shows the catalytic buildup of
benzaldehyde in two experiments with different levels
of MTO. The two curves vary somewhat in shape, but
the effect of catalyst variation is not very great because
MTO is not involved in the step between bromine and
alcohol.
Exp er im en ta l Section
The experiments on alcohol oxidations entailed the use of
15 mL of acetic acid, to which were added 2 mmol of alcohol,
4 mmol of hydrogen peroxide, 0.1 of mmol sodium bromide,
and (in three portions over 10 h) 0.1 mmol of MTO. The
solution was stirred at room temperature over this time.
For the conversion of an aldehyde to an ester, the aldehyde
(2 mmol) was dissolved in 15 mL of methanol containing HOAc
(3 mmol), sodium bromide (0.1 mmol), and hydrogen peroxide
(>4 mmol). A solution of MTO (0.1 mmol in all) was added
over 10 h, while the solution was stirred at room temperature.
The reaction products were checked by GC-MS before separa-
tion.
A typical procedure for the cleavage of an ether is as follows,
using dibenzyl ether as the example. Acetic acid (25 mL) was
added to a solution of (PhCH2)2O (19 mmol), followed by NaBr
(1.9 mmol) and hydrogen peroxide (38-45 mmol). The solution
was maintained at room temperature while the MTO (2 mmol)
was added in three portions over 12 h with stirring. The
solvent was removed by distillation, and the product was
extracted with diethyl ether and dried over anhydrous sodium
sulfate.
The bromination of the alkynes was carried out as follows.
The alkyne (10 mmol), sodium bromide (21 mmol), and MTO
(0.2 mmol, 2 mol %) were dissolved in 20 mmol of acetic acid
and cooled to 10 °C. The hydrogen peroxide (>50 mmol) was
infused over 20 min with a syringe pump under vigorous
stirring. The low temperature was adopted to minimize
catalyst decomposition.18 Each reaction was finished within
20 min and gave a nearly quantitative yield of product. The
products were isolated by vacuum distillation (with the excep-
tion of R,R′-dibromostilbene, a solid that was recrystallized
from acetone).
Is Th er e a Role for HOBr ? One might ask whether
HOBr or a protonated species H2OBr+ is an active
brominating agent. The relative electrophilicities of the
species H2OBr+, Br2, and HOBr have been found to be
in the ratio ca. 106:103.8:1.16 Clearly, HOBr is an unat-
tractive candidate. The protonated reagent appears a
feasible candidate:
The bromination of phenols was carried out with phenol (10
mmol), sodium bromide (10.5 mmol), MTO (0.2 mmol), and
H2O2 (>30 mmol, added by syringe pump). The reactions were
finished within 30 min and proceeded in nearly quantitative
yield. The products were isolated by simple vacuum distillation
(entries 1 and 2) or filtration, after removing the solvent by
The presence of bromine is indicated by a red coloration
that rapidly develops in the reaction solutions for the
bromination of both the alkynes and phenols. However,
Br2 may not be the reactive species; it may simply exist
in labile equilibrium with H2OBr+. On the other hand,
bromine is itself a perfectly competent brominating agent
and the more likely reactant. The correspondence be-
tween the rate constants for benzaldehyde formation and
for the independently determined reaction between Br2
and benzyl alcohol support this assignment.
(29) Stewart, R. In Oxidation Mechanisms: Applications to Organic
Chemistry; W. A. Benjamin, Inc.: New York, 1964.
(30) Dakka, J .; Sasson, Y. Bull. Soc. Chim. Fr. 1988, 756.
(31) Bray, W. C. Chem. Rev. 1932, 10, 161.
(32) Bray, W. C.; Livingston, R. S. J . Am. Chem. Soc. 1928, 50, 1654.

1196 J . Org. Chem., Vol. 64, No. 4, 1999
Espenson et al.
rotary evaporation, followed by recrystallization from acetone.
Ack n ow led gm en t. This research was supported by
the U.S. Department of Energy, Office of Basic Energy
Sciences, Division of Chemical Sciences under contract
W-7405-Eng-82. We are grateful to Professor P. J . Stang
for alerting us to references on the bromination of
acetylenes.
1
The bromophenols were identified by H and MS data.33-36
Kinetics experiments on the oxidation of PhCH2OH by MTO/
H2O2/Br- were carried out by UV-visible methods. The
reaction progress was monitored by the buildup of PhCHO at
243 nm, an absorption maximum with ꢀ 1.1 × 104 L mol-1
cm-1
.
Su p p or tin g In for m a tion Ava ila ble: Summary of the
crystallographic information (data collection, structure solu-
tion, refinement details, and bond distances and angles) for
trans-dibromostilbene. This material is available free of charge
via the Internet at http://pubs.acs.org.
(33) Berthelot, J .; Guette, C.; Desbe`ne, P.-L.; Basselier, J .-J .;
Chaquin, P.; Masure, D. Can. J . Chem. 1989, 67, 2061.
(34) Fischer, A.; Henderson, G. N. Can. J . Chem. 1982, 61, 1045.
(35) Brittain, J . M.; de la Mare, P. D. B.; Newman, P. A.; Chin, W.
S. J . Chem. Soc., Perkin Trans. 2 1982, 1193.
(36) Pouchert, C. J .; Behnke, J . The Aldrich Library of 13C and 1H
FT-NMR Spectra; Aldrich Chemical Co.: Milwaukee, 1993.
J O9817164