HBr–DMPU: The First Aprotic Organic Solution of Hydrogen Bromide

Article abstract of DOI:10.1002/chem.201703457

HBr and DMPU (1,3-dimethyl-3,4,5,6-tetrahydro-2-pyrimidinone) form a room-temperature-stable complex that provides a mild, effective, and selective hydrobrominating reagent toward alkynes, alkenes, and allenes. HBr–DMPU could also replace other halogenating reagents in the halo-Prins reaction, ether cleavage, and deoxy-bromination reactions.

Full text of DOI:10.1002/chem.201703457

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Accepted Article
Title: HBr-DMPU: the First Aprotic Organic Solution of Hydrogen
Bromide
Authors: Gerald Hammond, Zhou Li, Bo Xu, rene ebule, and jessica
kostyo
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HBr-DMPU: the First Aprotic Organic Solution of Hydrogen
Bromide
Zhou Li,[a] Rene Ebule,[a] Jessica Kostyo,[a] Gerald B. Hammond,[a]* and Bo Xu[b] *
Dedication ((optional))
Abstract: HBr and DMPU form a room temperature-stable complex
that provides mild, effective and selective hydrobrominating
between hydrohalogenation and undesired hydroacetoxylation is
poor,[15] because of the weak, but still nucleophilic, acetate anion
and the reduced nucleophilicity of halide anions in protic
medium.[16] In addition, the rearrangement of the intermediate
a
reagent toward alkynes, alkenes and allenes. HBr-DMPU could also
replace other halogenating reagents in the halo-Prins reaction, ether
cleavage and deoxy-bromination reactions.
carbocation
is
not
uncommon.[17]
Triphenylphosphine
hydrobromide is known as an alternative HBr source, but
triphenylphosphine tends to further react with formed alkyl
bromides to produce phosphonium salts.[18] Kropp et al[19] have
highlighted a few applications of HBr with the aforementioned
challenges. In this regard, HBr solution in a neutral aprotic
medium would be the optimal choice to prevent competitive
solvolysis reaction and to achieve high reactivity. However, the
excess reactivity of HBr toward ethers and esters excludes
ethereal and ester solvents. Inspired by the concept of hydrogen
bond basicity (pKBHX) that was first systematically summarized
by Laurence,[20] we previously succeeded in a new formulation of
Alkyl and alkenyl bromides are common substrates in a wide
scope of transformations, including nucleophilic substitution
reactions,[1] metallation reactions[2] and transition metal-
catalyzed cross-coupling reactions.[3] Highly efficient synthetic
approaches toward organic halides are in high demand because
of their crucial role in synthesis. Electrophilic addition of
hydrogen halides to alkenes and alkynes provides a uniformed
approach toward branched alkyl and alkenyl halides, and is a
model reaction in most textbooks of organic chemistry.[4]
Compared
with
other
approaches,
including
hydrogen fluoride, HF-DMPU,[21] which exhibits
reactivity than its predecessors[22] in
wide scope of
a higher
deoxyhalogenation,[5] Hunsdiecker reaction,[6] hydrometallation-
electrophilic halogenation,[7] olefin metathesis[8] and transition
metal-catalyzed hydrohalogenation,[9] the electrophilic addition of
hydrogen halides is atom economical. However, to succeed in
such transformation, handling gaseous hydrogen halides[10] with
special and expensive equipment[11] is almost always required.
While there maybe other methods to achieving hydrobromination
of alkynes and alkenes, these usually involve the use less
environmentally beneign precursors, conditions and low atom
economy.[12] Moreover, HBr addition to alkynes and akenes in
solution also results in uncontrollable radical-chain additions that
give the anti-Markovnikov products.[13] We herein report the first
room temperature stable solution of HBr in a neutral aprotic
organic solvent, DMPU that expands the utility of HBr. This
complex yielded a mild, effective and selective hydrobrominating
reagent toward alkynes, alkenes and allenes. HBr-DMPU could
also replace other less atom economical halogenating reagents
in the halo-Prins reaction and deoxy-bromination reactions.
a
transformations. We hypothesized that DMPU could effectively
form hydrogen bond complexes with HBr. In addition, DMPU is
inert toward alkylation reactions, thus it should not compete with
HBr. We were pleased to find that, by passing HBr gas through
DMPU, we obtained a solution of HBr-DMPU in the form of a
room temperature-stable liquid (Error! Reference source not
found., entry 3) with a higher mass percentage compared to
other HBr solutions.
Table 1. Comparison of Different Formulation of Hydrogen
Halide Solutions
Mole ratio
(HX:solvent)
pKBHX of
Stabilizer
entry
Formulation
1
2
3
HBr-water, 48%[a]
HBr-AcOH, 33%[a]
HBr-DMPU, 58%
0.21
0.36
2.2
0.65
N/A
2.79
Although several easy-to-handle HBr reagents are available
commercially or have been recently made,[14] none of them are
good choices for a more general hydrobromination reactions due
to scope limitations. An acetic acid solution of HBr is capable of
hydrobrominating unsaturated hydrocarbons, but selectivity
[a] Commercially available.
We tested the reactivity of HBr-DMPU in hydrohalogenation
reactions. HBr-DMPU showed a high reactivity toward alkyne 1a.
Under neat conditions, it completely converted 1a into the
hydrobromination product 1b in 83% yield, without the formation
of the 6-exo-trig cyclization product (eq 1).[23] Gram-scale
reaction gave the same product in 85% isolated yield (eq 1). A
small amount of gem-dibromide, as a result of a two-fold
hydrobromination, was observed after extended reaction times.
HBr-DMPU eliminated the use of gaseous HBr,[10b] the
competitive hydroacetoxylation reaction using HBr-HOAc[15d] and
eliminated the use of a catalyst.[9a, 24]
[a]
[b]
Dr. Zhou Li, Rene Ebule, Jessia Koysto and Prof. Dr. Gerald B.
Hammond, Department of Chemistry
University of Louisville
Louisville, Kentucky 40292, United States
E-mail: gb.hammond@louisville.edu
Dr. B. Xu
College of Chemistry, Chemical Engineering and Biotechnology
Donghua University, 2999 North Renmin Lu
Shanghai 201620 China
Email: bo.xu@dhu.edu.cn
Supporting information for this article is given via a link at the end of
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a Reaction conditions: 1 (0.3 mmol), 0.15 mL of HBr-DMPU (58% w/w), room
temperature, yields are isolated. b Reaction run at 70 oC.
Alkenes are also suitable substrates for HBr-DMPU
hydrobromination (Table 3). Allylbenzenes 3a-3c and tridecene
3e afforded the corresponding Markovnikov hydrobromination
products 4a, 4b, 4c and 4e in excellent yields. The reaction time
of 3b should be limited to 1 hour, since prolonged reaction times
caused the demethylation of 4b. Styrene 3d was completely
hydrobrominated, without the formation of commonly observed
polymers.[27] Homoallyl amide 3f, imide 3g and ester 3h afforded
the corresponding hydrobromination products in good yield and
with functional group tolerance. 4f could be directly converted
into azetidine 5 upon a treatment with sodium hydride in DMF
(eq. 2). The hydrobromination of internal alkenes was partially
successful. β-Methyl styrene 3i and cyclohexene could be
hydrobrominated in quantitative yield but several substrates
could not be effectively hydrobrominated. For example,
hydrobromination of α-methyl styrene 3k could be only partially
achieved, even after an extended reaction time or after adding
extra loading of HBr-DMPU. Hydrobromination of methyl
cinnamate 3l and stilbene 3m was not successful due to their
slow conversion, competition with undesired demethylation or
easy loss of HBr during purification.
A wide scope of alkynes reacted smoothly with HBr-DMPU
under ambient conditions (Table 2). Homopropargyl ester 1b,
amide 1c and imide 1d afforded the corresponding 3-bromo-but-
3-enyl ester 2b, amide 2c and imide 2d in 82%, 83 % and 80%
yields respectively, without any ester cleavage. Hex-5-ynoic acid
1e and its nitrile 1f also yielded products 2e and 2f in 78 % and
89%
bromovinyl)benzene and (2-bromovinyl)benzene in a 97:3 ratio
with a combined 92 % yield. With internal alkynes, 1-
respectively.
Phenylacetylene,
1h,
yielded
(1-
phenylpropyne 1i yielded a 3:2 mixture of α- and β-bromides 2i
with exclusive Z-configuration. The exclusive formation of Z-
isomers (trans addition) indicated to us that this transformation
follows an Ad3 mechanism[25] and that the bromide
concentration in the reaction medium is not low.[17]
Hydrobromination of decyne 1j provided 2j in a similar yield. 1k
and 1l yielded the corresponding hydrobromination product 2k
and 2l in regioselective manner, albeit with a lower selectivity for
2k. This is probably due to the diminished steric difference
26]
between
methyl
and
bromomethyl
moieties.[25a,
Table 3. Hydrobromination of Alkenesa
Diphenylacetylene (1m) yielded the Z-product in a modest 68 %.
The unsymmetrical diaryl acetylene (1n) gave a 4.6:1 mixture of
regioisomers. The reaction with derivatized amino acid (1o) and
estrone (1p) also proceeded in 71 % and 95 % yields
respectively, showing the wide applicability of this method.
Table 2. Hydrobromination of Alkynesa
a Reaction conditions: 3 (0.3 mmol), 0.15 mL of HBr-DMPU (58% w/w), room
temperature, yields are isolated. Numbers in parenthesis are NMR yields
using CH2Br2 as an internal standard.
Alkynes are known to be less reactive than alkenes in a wide
scope of electrophilic addition reactions,[28] because of the
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anticipated highly energetic vinyl cation intermediate.[25a]
However, using HBr-DMPU, alkyne 1b showed
a faster
conversion than alkene 1m in both, parallel and competition
experiments (Scheme 1). This inversed reactivity could be
attributed to the high concentration of HBr in the reaction
medium, signaling a reaction profile that is different from that in
diluted reaction medium. [29]
Scheme 1. Reactivity comparision
HBr-DMPU was shown to be very useful in other bromination
reactions. For example, typical halogenating reagents employed
in organic synthesis are gaseous hydrogen halides, metal
halides (e.g. AlX3, InX3, NbX5, GaX3, FeX3, SnX4), sulfonyl
halides, boron halides, phosphorus halides or silicon halides
(Scheme 2),[30] which either require special handling or produce
large amounts of side products after the reaction, and each
transformation requires a different halogenating reagent. HBr-
DMPU, on the other hand, provided a uniform solution to all
these transformations. It could substitute niobium and gallium
halides in the bromo-Prins reaction (Scheme 2a). It could also
convert alkyl alcohols 7 into alkyl bromides 8, thus replacing the
corrosive PBr3 (Scheme 2b).[31] Hydrobromination of allenenoic
ester 9a occurred with the regioselective protonation of the sp2-
hybridized α-carbon, producing a β-vinylic cation that was further
trapped by bromide to provide a vinyl bromide 9b in 54% yield
(Scheme 2c).[32] The reduced yield was primarily caused by
cleavage of the benzyl group, since formation of benzyl bromide
was observed on GC-MS.
Scheme 2. Other synthetic reactions using DMPU-HBr.
Experimental Section
Preparation of HBr-DMPU. HBr-DMPU was prepared by dropping
hydrobromic acid to phosphorus pentoxide, passing evolved gas
thruough a phosphorus pentoxide drying tower, and trapping dried HBr
gas by DMPU under an argon atmosphere of ambient pressure.
General Procedure of Hydrobromination of Alkenes and Alkynes;
To an argon-flushed 2-dram vial loaded with 0.3 mmol of alkene or
alkyne, 0.15 mL of HBr-DMPU (58 w/w%) was added under stirring at
room temperature. GC-MS and TLC monitored reaction progress. Once
complete, the reaction mixture was quenched by adding 2 mL of
saturated sodium bicarbonate solution and extracted 3 times with 2 mL of
ethyl acetate. To the combined extract, 1 gram of silica gel was added,
and the solvent was removed under a reduced pressure. The final
product was purified by flash column chromatography using ethyl
acetate-hexane eluent
In conclusion, HBr-DMPU is a good alternative to existing of
hydrogen bromide solutions. Its advantages lie in the ease of
handling, high HBr concentration, high reactivity towards the
hydrobromination of alkenes and alkynes without competition
reactions caused by rearrangement or solvolysis. It also served
as an effective substitution to other bromides in bromination
transformations.
Acknowledgements
We are grateful to the National Institutes of Health for financial
support (1R01GM121660-01). We also acknowledge the
National Science Foundation (CHE-1401700) for funding a
postdoctoral fellowship to Dr. Zhou Li, who participated in the
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usually above 1500 USD.
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Keywords: hydrogen halide, hydrohalogenation, organic halide
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Zhou Li, Rene Ebule, Jessica Kostyo,
Gerald B. Hammond* and Bo Xu
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HBr-DMPU: an Aprotic Organic
Solution of Hydrogen Bromide
HBr-DMPU is a room temperature-stable complex with HBr. It is an effective and
selective hydrobrominating reagent for alkynes and alkenes.
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