BORON TRIBROMIDE 1
OMe O
Boron Tribromide1
MeO
BBr3, CH2Cl2
 70 °C
BBr3
N CO2Et
H
OMe
[10294-33-4] BBr3 (MW 250.52)
Br Br
InChI = 1/BBr3/c2-1(3)4
B H
InChIKey = ILAHWRKJUDSMFH-UHFFFAOYAA
O O O O
MeO MeO
(3)
(Lewis acid used for deprotection of OH and NH groups; cleaves 80%
ethers or esters to alkyl bromides; bromoborates allene and
N CO2Et N CO2Et
alkynes) H H
OMe OMe
ć% ć%
Physical Data: mp -45 C; bp 91.7 C; d 2.650 g cm-3.
Form Supplied in: colorless, fuming liquid; a 1.0 M solution in
The cleavage of mixed dialkyl ethers occurs at the more substi-
dichloromethane and hexane; BBr3·Me2S complex is available
tuted carbon oxygen bond. Methyl ethers of secondary or tertiary
as either a white solid or a 1.0 M solution in dichloromethane.
alcohols give methanol and secondary or tertiary alkyl bromides
Purification: by distillation.
selectively by the reaction with BBr3,8 although the addition of
Handling, Storage, and Precautions: BBr3 is highly moisture
Sodium Iodide and 15-Crown-5 ether can change this selecti-
sensitive and decomposes in air with evolution of HBr. Store
vity (eq 4).9 In contrast, methyl ethers of primary alcohols are
under a dry inert atmosphere and transfer by syringe or through
generally cleaved at the Me O bond, as demonstrated in Corey s
a Teflon tube. It reacts violently with protic solvents such as
prostaglandin synthesis (eq 5).10
water and alcohols. Ether and THF are not appropriate solvents.
Original Commentary
Akira Suzuki & Shoji Hara
Hokkaido University, Sapporo, Japan
MeO
Removal of Protecting Groups. BBr3 is highly Lewis acidic. (4)
BBr3, CH2Cl2 BBr3, NaI, 15-crown-5
It coordinates to ethereal oxygens and promotes C O bond cleav- 25 °C CH2Cl2,  30 °C
high yield 100%
age to an alkyl bromide and an alkoxyborane that is hydrolyzed
to an alcohol during workup (eq 1).2
BBr3 H2O
Br HO
R1OR2 R1Br + Br2BOR2 R1Br + R2OH (1)
BBr3 has been widely used to cleave ethers because the reac-
tion proceeds completely under mild conditions. In a special case,
O O
BBr3 has been used to cleave acetals that cannot be deprotected O O
BBr3, CH2Cl2
by usual acidic conditions.3 Because alkyl aryl ethers are cleaved
(5)
at the alkyl oxygen bond to give ArOH and alkyl bromides, BBr3 0 °C
>90%
has been most generally used for the demethylation of methyl
HO CH2OMe HO CH2OH
aryl ethers,2,4 for example as the final step of zearalenone synthe-
sis (eq 2).5 Problems are sometimes encountered in attempts to
deprotect more than one nonadjacent methoxy group on one aro-
BBr3 has been also used for the deprotection of carbohydrate
matic ring, and when stable chelates are formed.6 The presence
derivatives11 and polyoxygenated intermediates in the synthesis
of a carbonyl substituent facilitates the selective deprotection of
of deoxyvernolepin,12 vernolepin,13 and vernomenin.13 Although
polymethoxyaryl compounds (eq 3).7
one of the model compounds is deprotected cleanly (eq 6),14 appli-
cation of BBr3 to more highly functionalized intermediates leads
OMe O
to cleavage of undesired C O bonds competitively (eq 7).12,13
BBr3, CH2Cl2
O
0 °C, 59%
MeO O
OH O
BBr3, CH2Cl2
MeO O
(6)
O
 78 °C
(2) O
88%
H H
MeO2C
HO O
Avoid Skin Contact with All Reagents
2 BORON TRIBROMIDE
the C O bond to give alkyl bromides.25 Alcohols can be converted
OAc
BBr3, CH2Cl2
MeO to alkyl bromides by this method.
 78 °C
BBr3, CH2Cl2
H
MeO2C OAc CO2Me
Br CO2H
rt
98%
O
OAc OAc
O
O O (10)
+ (7)
O O
H H
OAc CO2Me Br CO2Me
BBr3, NaI, MeCN
40% 40% I CO2H
rt
96%
For the complete cleavage, 1 mol of BBr3 is required for each
ether group and other Lewis-basic functional groups. Sometimes
In a special case, BBr3 is used for the bromination of hydro-
it is difficult to find reaction conditions for the selective cleavage
carbons. Adamantane is brominated by a mixture of Bromine,
of the desired C O bond. Recently, modified bromoboranes such
BBr3, and Aluminum Bromide to give 1,3-dibromoadamantane
as B-Bromocatecholborane,15 dialkylbromoboranes,16 Bromo- selectively.26 Tetrachlorocyclopropene27 and hexachlorocyclo-
bis(isopropylthio)borane,17 and 9-Bromo-9-borabicyclo[3.3.1]
pentadiene28 are substituted to the corresponding bromides by
nonane,18 have been introduced to cleave C O bonds more
BBr3 and, in the latter case, addition of AlBr3 and Br2 is effective
selectively under milder conditions. BBr3·SMe2 is also effective
to improve the result.29
for ether cleavage and has the advantage of being more stable than
BBr3. It can be stored for a long time and handled easily. However,
Reduction of Sulfur Compounds. Alkyl and aryl sulfox-
a two- to fourfold excess of the reagent is necessary to complete
ides are reduced by BBr3 to the corresponding sulfides in good
the dealkylation of alkyl aryl ether.19
yields.30 Addition of Potassium Iodide and a catalytic amount
Amino acid protecting groups such as benzyloxycarbonyl and
of Tetrabutylammonium Iodide is necessary for the reduction of
t-butoxycarbonyl groups are cleaved by BBr3. However, the
sulfonic acids and their derivatives.31
hydrolysis of the ester function also occurs under the same
reaction conditions.20 Debenzylation and debenzyloxymethyla- Transesterification of Esters or Conversion to Amides.
tion of uracils proceed successfully in aromatic solvents, but
Transesterification reactions of carboxylic esters or conversion
demethylation is more sluggish and less facile (eq 8).21
into the amides is promoted by a stoichiometric amount of BBr3.32
O O
Removal of Methyl Sulfide from Organoborane Methyl
Bn
BBr3, xylene
N HN Sulfide Complexes. Methyl sulfide can be removed from
(8)
BrBR2·SMe2 or Br2BR·SMe2, which are prepared by the hydro-
138 °C
O N O N
80%
boration reaction of alkenes or alkynes with BrBH2·SMe2 or
H
Bn
Br2BH·SMe2, by using BBr3.33 The resulting alkenyldibromo-
boranes are useful for the stereoselective synthesis of bromodienes
(eq 11).34
Substitution Reactions. BBr3 reacts with cyclic ethers to give
tris(É-bromoalkoxy)boranes which provide É-bromoalkanols or
Et Et Et Et
BBr3, CH2Cl2
É-bromoalkanals when treated with MeOH or Pyridinium + BBr3SMe2
BBr2SMe2 0 °C to rt BBr2
Chlorochromate, respectively (eq 9).22 Unfortunately, unsym-
metrically substituted ethers such as 2-methyltetrahydrofuran are
BuCa"CH
cleaved nonregioselectively. Generally, ester groups survive under CH2Cl2
 78 °C to rt
the reaction conditions for ether cleavage, but the ring opening of
lactones occurs under mild conditions to give É-halocarboxylic
Et Et
Et
I2, AcOK, THF
acids in good yields (eq 10).23
Br
(11)
Br BBr
Et
 78 °C to rt
O
BBr3, CH2Cl2
Bu
B
3
Bu
Br O
63%, (Z,Z) = 98%
3
0 °C "
MeOH PCC
(9)
86% 72%
Bromoboration Reactions. BBr3 does not add to isolated
CHO
3 Br 3 Br double bonds, but reacts with allene spontaneously even at low
OH
temperature to give (2-bromoallyl)dibromoborane,35 which pro-
In the reaction with methoxybenzaldehyde, bromination of vides stable (2-bromoallyl)diphenoxyborane by the addition of
the carbonyl group takes place more rapidly than demethy- anisole.36 The diphenoxyborane derivative reacts with carbonyl
lation; therefore benzal bromide formation is generally ob- compounds to give 2-bromohomoallylic alcohols in high yields
served in the reaction with aromatic aldehydes.24 Cleavage of (eq 12). Bromoboration of 1-alkynes provides (Z)-(2-bromo-1-
t-butyldimethylsilyl ethers or t-butyldiphenylsilyl ethers occurs at alkenyl)dibromoboranes stereo- and regioselectively (eq 13),37
A list of General Abbreviations appears on the front Endpapers
BORON TRIBROMIDE 3
which are applied for the synthesis of trisubstituted alkenes,38 First Update
Ä…,²-unsaturated esters,39 and Å‚,´-unsaturated ketones,40 bromo-
Xianhai Huang
dienes,41 1,2-dihalo-1-alkenes,42 2-bromoalkanals,43 and
Schering-Plough Research Institute, Kenilworth, NJ, USA
²-bromo-Ä…,²-unsaturated amides.44
Protecting Group Removal. Boron tribromide is one of the
Br
most commonly used reagents for the deprotection of hydroxyl
1. BBr3,  20 °C PhCHO, THF
H2C=C=CH2
B(OPh)2 moieties and other active hydrogen heteroatoms. The phenolic
2. 2 PhOMe  78 °C to rt
demethylation or dealkylation reactions of this reagent are widely
 78 °C to rt
used in the preparation of pharmaceuticals.51 57 Although this
Br
OH
reagent is a strong Lewis acid, the dealkylation conditions are
(12)
Ph usually mild and selectivity can be achieved. In the synthesis of
89%
ventilone A, the methyl group is selectively deprotected in the
presence of an acetal group: selective monodemethylation can
also be achieved (eq 18).58
Although BBr3 can react with a lactone to give the bromoalkyl
Br
BBr3, CH2Cl2
carboxylic acid at room temperature,23 selective demethylation
RCa"CH (13)
BBr2
R
 78 °C can be carried out in the presence of lactones59,60 and pyran rings
H
(eq 19).61,62
OH O
1. BBr3, -78 °C
O
2. Na2CO3, H2O
Chiral Bromoborane Reagents. Complexes made from
O
chiral 1-alkyl-2-(diphenylhydroxymethyl)pyrrolidines and BBr3 17%
O
are effective catalysts for asymmetric Diels Alder reactions.45
OH O
Bromoboranes prepared from chiral 1,2-diphenyl-1,2-bis(arene-
sulfonamido)ethanes46,47 are used to prepare chiral allylic
OMe O
boranes,47,48 allenylic borane,49 propargylic boranes,49 and
O
(18)
enolates.46,47,50 The B-bromodiazaborolidinene (1), prepared
O
from 1,2-diphenyl-1,2-bis(p-toluenesulfonamido)ethane, is par-
O
ticularly effective in these applications. The reagents prepared
OMe O
OMe O
from (1) are highly effective for the enantioselective synthe-
1. BBr3, -78 °C
sis of homoallylic alcohols (eq 14),48 homopropargylic alcohols O
2. Na2CO3, H2O
O
(eq 15),49 propadienyl carbinols (eq 16),49 and aldol condensation
28%
O
products (eq 17).46
OH O
SnBu3
Ph Ph
1. Cl
OH
CH2Cl2, 0 °C to rt
OMe O
H
Me
(14)
TolSO2N NSO2Tol
Cl Ph
2. PhCHO,  78 °C
Br
B
O
84% ee
BBr3
Br
H
(1)
82%
H
O O
Ph Ph
1. HCa"CCH2SnPh3
O
OH
CH2Cl2, 0 °C to rt
(15)
TolSO2N NSO2Tol
Ph
2. PhCHO,  78 °C
B OH O
H
Me
96% ee
Br
Br
(1) O
H
(19)
Ph Ph H
1. H2C=C=CHSnPh3
OH
O O
CH2Cl2, 0 °C to rt
" (16)
O
TolSO2N NSO2Tol
Ph
2. PhCHO,  78 °C
B
>99% ee
Br
A bridged phenolic lactone can also survive the BBr3 demethy-
(1)
lation reaction even when the reaction is carried out at room tem-
perature (eq 20).63
Ph Ph
1. 3-pentanone, i-Pr2NEt
Selective demethylation can also be achieved in the presence of
O OH
CH2Cl2,  78 °C
several methoxy groups. Cooke et al. reported that unsymmetrical
(17)
TolSO2N NSO2Tol
2. EtCHO,  78 °C
B monohydroxypentaalkoxytriphenylenes could be prepared from
Br
symmetrical hexaalkyloxytriphenylenes using BBr3-mediated
98% ee, 98% syn
(1)
ether cleavage.64 Zhu and Zhang showed that the methoxy group
Avoid Skin Contact with All Reagents
4 BORON TRIBROMIDE
NMe
could be selectively deprotected with BBr3 in the presence of
imines.65 Although a phenoxy methyl group can generally be
OH
deprotected selectively in the presence of a methyl ester in the
KOH/glycol
Me
BBr3 demethylation reaction, demethylation of the methyl ester
6.3%
O
is observed in some cases.66
MeO
OMe
NMe
OMe
OH
OH
O
O
Me
(23)
BBr3, rt
O
(20)
MeO
75%
O
OH
O
H
H
H
H
When several phenolic alkyl groups are present, better selectiv-
ity may sometimes be achieved with lithium 4,4 -di(tert-butyl)-
biphenylide (LDBB) (eqs 24 and 25).75
BBr3 is not only effective in the demethylation reactions, but
also is powerful in cleavage of other phenolic alkyl groups, such
as the octyl group,67 the hexyl group,68 ethylene groups,69 and
Bu O
BBr3
Bu OH
the benzyl group.70 Press showed that deethylation of ethyl ethers
(24)
Bu O
79%
Bu OH
of naphthols and phenols by BBr3 gave naphthols and phenols
in >92% yields, whereas demethylation of comparable methyl
ethers gave naphthols and phenols in 13 95% yields. Thus, ethyl
ethers may be used as protective groups for the hydroxy group of
phenols and naphthols.71 Finn et al. demonstrated that aryl propar-
(25)
gyl ethers and esters were cleaved selectively in the presence of
O O
O OH
LDBB
aryl methyl ethers and esters by BBr3 in dichloromethane (eq 21). (CH2)n
(CH2)n
O O
88 97% O OH
Under the same conditions, allyl ethers undergo very rapid Claisen
rearrangement, and benzyl ethers are also cleaved more rapidly
than propargyl ethers. A mechanism involving intramolecular
delivery of bromide to the propargyl terminus is proposed.72
Costero showed that in the case of demethylation of polyether
2, neither BBr3, TMSI, nor SnCl4 were appropriate. The desired
product, polyether phenol 3, was obtained only when demethyla-
1 equiv BBr3
OH
tion was carried out under nucleophilic conditions (EtSNa/DMF)
O
CH2Cl2, rt
with an unreported yield (eq 26).76
MeO (21)
MeO
89 99%
Br
HO
BBr3
HO
Gauvry and Mortier showed that BBr3 cleanly and quanti-
99% Br
tatively converted dimethyl-, diethyl-, diisopropyl-, and di-tert-
butyl phosphonates RP(O)(OR )2 into the corresponding phos-
phonic acids RP(O)(OH)2 via methanolysis. The use of BBr3
is compatible with a variety of functionalities in the R group
(eq 22).73
I
O O
MeO
MeO
TMSI
(26)
O
MeO
MeO 60%
I
O
1. BBr3, toluene/hexane
O
R-P(O)(OR2 )2 2. MeOH
R-P(O)(OH)2 (22)
2
77 95%
O O
BBr3 is very powerful at removing phenolic methyl and alkyl
NaSEt/DMF
HO
groups, but it is not always the optimal choice. When the sub- O
HO
O
strate contains another strong Lewis basic site, KOH/glycol can
O
be used instead of BBr3, for example, in the synthesis of substi-
3
tuted ethenoisomorphinans and ethenomorphinans (eq 23).74
A list of General Abbreviations appears on the front Endpapers
BORON TRIBROMIDE 5
In some cases, trimethylsilyl chloride/sodium iodide and diben- Bjłrnholm et al. reported the use of the base-resistant tert-butyl
zyl diselenide/sodium borohydride can be used instead of BBr3.77 group for the protection of an aromatic thiol. This tert-butyl group
AlCl3/pyridine,78 NbCl5,79 and methane sulfonic acid/methio- was replaced with a base-labile acetyl group using BBr3 and AcCl
nine80 also can be used as BBr3 substitutes on some occasions in one pot (eq 30). This protection-reprotection process was suc-
for the demethylation of phenolic ethers. cessfully applied in the synthesis of oligo(phenylenevinylene)s,
Other than the deprotection of ether linkages, BBr3 can be used new organic materials with electrical and optical properties.84,85
for the cleavage of nitrogen-protecting groups. Sotelo reported that
a methoxymethyl group at the 2-position of acid-sensitive pyri-
O
dazinones is deprotected with BBr3 without affecting the double
S
bond (eq 27).81 S
AcCl/BBr3
(30)
O 61%
O
MOM
BBr3
N
HN
(27)
N 85%
N
OH
OH
Ph
Ph
Deprotection/Bromination and Cascade Deprotection/
Cyclization Reactions. The B Br bond strength is weak com-
Paliakov and Strekowski established that the treatment of 2- or
pared to B F and B Cl bonds. Consequently, BBr3 is particularly
4-benzylamino-substituted quinolines, 9-benzylaminoacridine, 2-
prone to halide displacement chemistries by a range of nucleo-
benzylaminopyridine, 4-benzyloxyquinoline, and N-benzyloxy-
philes. It is believed that BBr3 complexes with the lone pair of
amidine with BBr3 yields the corresponding amino or hydroxy-
electrons of the ether oxygen in a classic Lewis acid/base inter-
substituted compounds (eq 28).82
action, the complex then loses a bromide anion which can act
Theodoridis showed that the reaction of the 2-fluoroethyl pro-
as a nucleophile.86 Press et al. reported that treatment of vinyl
tecting group for amines with BBr3 resulted in the replacement
ketone (4) with BBr3 resulted in demethylation as well as hydro-
of fluorine by bromine, to give the corresponding 2-bromoethyl
group, which was easily removed by known methods (eq 29).83 bromination of the enone to give bromide (5) in addition to the
demethylated enone product (eq 31).87
R
Br
N
O
O
BBr3
S
S
(31)
R
20%
N N
N
OMe
OH
4
5
F
Tsukayama et al. demonstrated that demethylation of a phe-
R
nolic methyl group in the benzoate (6) with BBr3 also resulted
N
in replacement of the tertiary alcohol by a bromide group (7)
R
N N
(eq 32).88 In a recent publication Waldwan described a one-pot
H
demethylation/primary alcohol bromination with BBr3, although
N
primary alcohols are usually more difficult to convert to bromides
with BBr3.89
BBr3
R = Bn R = H BzO OBz
(28) BzO OBz
(32)
54 88%
BBr3
79%
Ac
Ac
OH
Br
OMe
OH
F O
7
6
F O Br
F
N
N Cl N
Tsukayama et al. reported that cyclization of propargyl alcohol
BBr3
Cl N
N
(8) in the presence of BBr3 provided cyclized product (9) (eq 33).90
N 97%
N
N
OMe
O
SO2Me
SO2Me BBr3
(33)
54%
F O
Br
1. K2CO3/NaI
OH
2. KMnO4
NH
8 9
Cl N
(29)
68%
N
Kim and Lee described the selective benzylic C O bond cleav-
N
age with concomitant bromination of a pyrano[3,4-c]isoxazole
SO2Me
with BBr3 (eq 34).91
Avoid Skin Contact with All Reagents
6 BORON TRIBROMIDE
i-Pr O OH
i-Pr OMe
N
Br MeO
BBr3
BBr3
(34)
O
N
O
MeO 68%
O 98% OMe
H
OH
O
OH
Nordvik and Brinker reported the conversion of the dimethyl
O
ketal of a cyclobutanone to the corresponding geminal dibromo-
OH
(38)
H
cyclobutane using BBr3 (eq 35).92
CO2Me
O
OMe Br
Cotelle and Dupont used BBr3 to demethylate and cyclize
BBr3
OMe Br
(35)
diarylethanoid molecules (epoxide or alcohol) in one pot to give
70%
polyhydroxylated 2-arylbenzo[b]furans (eq 39).96
OMe
Recently, it was reported that treatment of Cp*(CO)2Fe
1. m-CPBA
OMe
SiMe2SiMe3 with 1 equiv of BBr3 at room temperature led to
2. BBr3
the selective bromodemethylation at the Ä…-silicon atom to pro-
OMe 66%
duce Cp*(CO)2FeSiBrMeSiMe3. This product was converted to
OMe
Cp*(CO)2FeSiBr2SiMe3 on heating with another equivalent of
ć%
OH
BBr3 at 40 C in quantitative yield (eq 36).93
OH
(39)
O
SiMe3 OH
SiMe3 BBr3
Fe Si Br
Fe Si Me
99%
A convenient and simple two-step method for the synthesis
Me
Me OC
OC CO
CO
of coumestrol has been established by Al-Maharik and Botting,
which involves a base-catalyzed condensation of phenyl acetate
with benzoyl chloride, followed by demethylation and subsequent
tandem cyclization (eq 40). This method was employed for the
SiMe3
BBr3 13
efficient synthesis of multiply C-labelled coumestrol.97
(36)
Fe Si Br
99%
Br
OC HO O O
CO
OMe
MeO
(40)
CO2Me BBr3
BBr3 is also an effective halogen exchanging reagent for prepa-
82%
O
ring metallocene dibromides from the corresponding dichlorides
O
in high yields (eq 37).94
OMe OH
MeO
Treatment of 9-{2-[1-(1,3-dioxalan-2-yl)ethyl]phenyl}-3,4,5,
Cl
Cp
Br
ć%
Cp 6-tetrabenzyloxy-9H-xanthen-9-ol with excess BBr3 at -78 C,
Ti Cl
Ti Br
followed by warming to room temperature, provides a facile route
Cp
R1 OMe BBr3
Cp
R1 OMe heat
to 3,4,5,6-tetrahydroxy-spiro(1H-indene-1,9-[9H]-xanthene-3-
R2
15 20%
R2
30 54%
(2H)-one, a stable mimic for the ring-closed form of galleon
(eq 41).98
Br
Cp O
O
Ti O
Cp
R1 O
(37)
R2
OH BBr3
(41)
54%
BnO O OBn HO OOH
OBn OBn OH OH
In the synthesis of some aryldihydrobenzofuran systems, BBr3
was used as a deprotecting and cyclization reagent. This transfor-
mation is highly user-friendly and does not require any specially Lewis Acid-mediated Reactions. BBr3 is widely used
dried solvent or protecting atmosphere (eq 38).95 as a Lewis acid to mediate processes such as Diels-Alder
A list of General Abbreviations appears on the front Endpapers
BORON TRIBROMIDE 7
reactions,99,100 Lewis acid-catalyzed cyclopropanation The methyl ester moieties are also presumably hydrolyzed under
reactions,101 [4 + 2]/[3 + 2] cycloadditions,102 and the synthesis these conditions. The final product (11) results from lactonization
of various porphyrins and sapphyrins.103 In a recent publication, and reesterification of the carboxy residues with diazomethane
6-bromo-3-oxoalkanoates, benzofurans, and 1,7-dibromoheptan- (eq 46).113
4-ones were chemo- and regioselectively prepared by reaction of
CO2Me
CO2Me
2-alkylidenetetrahydrofurans with BBr3 as Lewis acid, allowing
1. BBr3/Bu4NI
an efficient synthesis of a variety of carbonyl compounds with CO2Me
2. CH2N2
CO2Me
remote halide functionality (eq 42).104,105
O
O
O BBr3
O
O
Br3B
O
1. BBr3
O b
a
O O
2. H2O
10
I-
(42)
Br
OEt OEt
76%
O
CO2Me
75%
A series of hydroxylated 3-hydroxycoumarins was synthesized
(46)
O
by the reaction of 3-aryl-2-hydroxypropenoic derivatives with O
MeO2C
BBr3 (eq 43).106
11
OMe
Davies showed that the transformation of vinyl cyclopropane
CO2H
HO OH
ć%
(12) to lactone (E-13) could be carried out with BBr3 at -78 C
BBr3
(43) (56 72% yield): under these conditions the isomeric butyrolac-
OH
15%
O O
tone (Z-13) was not observed (eq 47).114
OMe
Dupont and Cotelle reported that treatment of arylacetones with
BBr3 gives 1,3-dimethyl-2-arylnaphthalenes in fair to good yields
CO2tBu
BBr3
(47)
by a tandem aldol condensation-cyclization (eq 44). The cycliza-
EtO
56% EtO O
O
tion also takes place with o-methoxyphenylacetones leading to
E-13
intramolecular acetals or to 5-, 6-, or 7-hydroxy-2-methylbenzo
12
[b]furans (eq 44).107
Gao and Portoghese discovered a novel boron tribromide-cata-
O
lyzed rearrangement of ketones to allylic alcohols in the 7-phenyl-
morphinan-6-one system (eq 48). The reaction involved the stereo-
BBr3
(44)
specific migration of an axial phenyl (or a hydrogen) to the
90%
carbonyl carbon, followed by the elimination of the proton leading
to the generation of allylic alcohols.115
Shi and Wang used BBr3 to promote Baylis-Hillman reactions
N Br
of arylaldehydes with but-3-yn-2-one (eq 45).108
Ph
BBr3, 2 h
CHO
OH O Ph
39%
BBr3
O
Et
(45)
Me
81%
OH
(48)
+
Et Br
N
COMe N
Ph
Ph
BBr3, 10 min
Ph
BBr3 can be used to effect cationic rearrangement reactions,
Ph
43%
for example, the Lewis acid-mediated ring-opening reaction and O
O
rearrangement of cyclobutyl ketones.109 The Lewis acid-mediated O
O
dimerization of 1,3-diarylpropargylic alcohols to give cyclobu-
OMe
tane derivatives,110 the conversion of a trans-hexahydronaphtho-
OH
xazine to the cis-isomer have all been achieved under the aegis of
N
BBr3.111 The BBr3-mediated conversion of 3-methoxypiperidines
BBr3, 30 min
Ph
to 2-(bromomethyl)pyrrolidines, which proceeds via an interme-
diate bicyclic aziridinium ion, is an example of a rare conversion 52% Ph
of piperidines into pyrrolidines.112 Renaud showed lactone ester
OH
(11) can be obtained upon treatment of bridged bicyclic lactone
O
(10) with BBr3. This reaction consists of the simultaneous acid-
OH
catalyzed opening of the bicyclic ether (a) and of the lactone (b).
Avoid Skin Contact with All Reagents
8 BORON TRIBROMIDE
BF3 and BBr3-mediated [2,3] sigmatropic rearrangements of Boron silicon exchange is frequently used in the preparation
allylic amino amides have been developed by Somfai, affording of boronic acids,126 dibromo boron, and triaryl boron reagents.
secondary amines in good yields (eq 49).116,117 An active boron-based metallocene olefin polymerization catalyst
was synthesized using the boron-silicon exchange of BBr3.127 The
synthesis of well-defined luminescent organoboron polymers via
Bn O
O
a novel three-step procedure starting from silylated polystyrene
N
BBr3
N
is realized using the highly selective boron-silicon exchange
N
(49)
60%
reaction of poly(4-trimethylsilylstyrene) (PS-Si) (eq 53).128
NHBn
n n
Precursor for Other Borane Reagents. Many organometal-
BBr3
lic species, including Li, Hg,118 Sn, Zr, and Si reagents can be
80%
+
transmetallated with boron. Boron-lithium exchange was used to
81%
SnMe3
S
prepare B(C6F5)3 using Li(C6F5) and BBr3.119 ArBBr2 can be
BBr2
synthesized from the reaction of ArHgBr with BBr3.120 BBr3 can
SiMe3
exchange with aryl, vinyl, and allenyl tin reagents. Tris(penta-
fluorophenyl)borane and its derivatives are important Lewis acids
for olefin polymerization and organic synthesis. These reagents
n
can be prepared with a boron-tin exchange reaction (eq 50).121
1. BBr3
F F F F
(53)
2. 0.5 Cp2ZrMe2
-Cp2ZrBr2
B
S
F F
35%
S
Sn
F F
F F F F
Zr B exchange is another popular reaction of BBr3.129 The
hydrozirconation reaction has been used for the preparation of
F F (50)
organozirconium compounds. A major drawback of these types
B
F F of organozirconiums is their inability to undergo general carbon-
carbon bond forming reactions. To overcome this limitation,
transmetalation of organic groups from Zr to boron is necessary
Alkynyldihaloboranes are readily generated in situ from the
(eq 54).130,131
boron-tin exchange reaction of BCl3 or BBr3 with the corres-
ponding alkynylstannanes. These reagents were successfully
Bu Bu
applied in Diels Alder reactions with isoprene in hexanes
BBr3
Cp
(54)
(eq 51).122 Vinyl boranes are also prepared using tin-boron
Zr 53% Br B
Cp
exchange which is used for Diels-Alder reaction.123,124
Cl Br
SnBu3 BBr2
A series of boronated aluminas was prepared by the reaction
BBr3
D-A reaction
+
of BBr3 with alumina (Al2O3). These solids can catalyze the
86% for two steps
Diels-Alder reactions of methyl acrylate with cyclopentadiene
n-Bu n-Bu
and isoprene with methyl acrylate, both of which occur in high
yield and with high selectivity.132 The reaction of the anhydrides
of boronic or borinic acids with BBr3 in hydrocarbon solvents
(51)
leads to the clean exchange of the oxygen and bromide to form
n-Bu
the organodibromoboranes and diorganobromoboranes in good to
excellent yields.133 Reaction of BBr3 with triethyl phosphite led
Guillemin and Malagu reported that the allenic or propar-
to the formation of triethyl tribromoborophosphate, a complex
gylic stannanes reacted with BBr3 to give only the allenic boron
bearing a P B bond which typically is very difficult to form.134
product, which can be used in the preparation of homopropargyl
The crystalline adducts FcPBBr3 and FcPBBr2Fc, rare examples
ć%
alcohols, even at low temperature (-80 C) (eq 52).125
of P-bonded complexes between phosphaferrocene and a main-
group metal atom, were obtained by Scheibitz et al. by the re-
SnPh3
SnBu3 BBr3
action of 3,4-dimethyl-1-phosphaferrocene (FcP) with BBr3 and
or
"
H
40 42%
FcBBr2 (Fc: ferrocenyl) (eq 55).135
4-Ethyl-1-hydroxy-3-(4-hydroxyphenyl)-2-oxa-1-boranaphth-
alene is formed from the reaction of 1-(4-methoxyphenyl)-2-
OH
phenylbutan-1-one with an excess of BBr3 followed by treatment
BBr2 acetone
with water. This reagent can be used in palladium-catalyzed
"
(52)
H 45%
coupling reactions (eq 56).136
A list of General Abbreviations appears on the front Endpapers
BORON TRIBROMIDE 9
O
P BBr3
BBr3
BBr3
H
Fe +
78%
91%
F
P
(55)
Fe
F
Br
Br
P B Fe
FcBBr2
(58)
Fe
Br Br
94%
O
OH
1. excess BBr3
reaction goes through a BBr3-initiated cationic process instead
2. H2O PhBr/Pd(PPh3)4
of a bromoboration process.141,142
78% 42%
O
O
Reactions of (Z)-2-bromo-1-vinylboron dibromides, generated
B
in situ via reaction of alkynes with BBr3, with lithium propargyl-
HO
oxides in CH2Cl2 at room temperature produce the corresponding
(Z)-1-bromo-1,4-enynes in modest to good yields (eq 59).143
OH
1. BBr3
2.
Ph
OLi
Ph
(56)
68%
O
Ph Br
(59)
Ph
Synthetically useful tetraalkoxydiborane reagents were pre-
pared efficiently by Hartwig s group by the reaction of 3,5-di-
tert-butylcatechol and BBr3 on a multigram scale (eq 57).137
Although a direct reaction of BBr3 with an alkene has not
been observed, the addition of boron to a C C double bond can
1. HNMe2 t-Bu t-Bu
O O
be realized in the presence of trialkylsilanes. Trialkylsilanes or
2. Na, K
B B
dialkylsilanes react rapidly with BBr3 in the absence of ethereal
BBr3 3. t-Bu
(57)
O O
OH
solvents or other nucleophiles to form unsolvated bromoboranes,
t-Bu t-Bu
highly active hydroborating agents. With alkenes in the pres-
OH
ence of sufficient BBr3, the products are alkyldibromoboranes. An
85% for three steps
t-Bu
alkylmonobromoborane can hydroborate a second alkene to form
a dialkylbromoborane. For this purpose, differing alkyl groups
may be introduced in either order, regardless of their relative steric
Bromoboration Reactions. BBr3 reacts readily with allenes
and alkynes to give bromo vinylboranes. The reaction of BBr3 properties. With 2 equiv of trialkylsilane, alkyldibromoboranes are
converted to alkylborane dimers (eq 60).144
with alkynes usually occur in a stereo-, regio-, and chemose-
lective manner via the syn-addition of the B Br moiety to the
BBr3/HSiMe3
triple bond.138 These bromoboron adducts are synthetically useful
BBr2
building blocks. It is reported that bromoboration of l-hexyne with
>90%
BBr3, followed by treatment with 2-propanol produced alkenyl
boronic esters. Subsequent Pd0-catalyzed cross-coupling with
acetylenic zinc chlorides and iodination furnished enyne iodides,
which were then converted to enyne-allenes by a second Pd0- MeOH
(60)
HSiMe3
catalyzed reaction with allenic zinc chlorides.139 Kabalka et al.
showed that the reaction of phenylacetylene with BBr3 in the pres-
ence of benzaldehyde generates (Z,Z)-1,3,5-triaryl-1,5-dibromo-
1,4-pentadiene (eq 58).140 Alkenes also react with aromatic
BBr
aldehydes in the presence of BBr3 to produce 1,3-diaryl-1,3-
B(OMe)2
dibromopropanes in excellent yields. It is proposed that this
Avoid Skin Contact with All Reagents
10 BORON TRIBROMIDE
Chiral Bromoborane Reagents. Chiral boron reagents have Synthesis of Isocyanates from Carbamate Esters and Acyl
been playing a prominent role in asymmetric synthesis in Bromide Formation Reactions. BBr3 can facilitate the conver-
recent years and have wide application in asymmetric Diels-Alder sion of carbamate esters to isocyanates (eq 64).151
reactions145 147 and aldol reactions. Corey et al. demonstrated that
NCO
NHCO2Me
Ä…-bromo ²-hydroxy esters can be prepared on a preparative scale
by a 96:4 enantioselective aldol reaction of t-buty1 bromoacetate
BBr3, 73%
NCO
NHCO2Me
with isobutyraldehyde in the presence of a chiral boron reagent
(64)
(14) prepared from BBr3 (eq 61).148
Ph
CF3
F3C Ph
Carboxylic acids can be converted to acyl bromides in the pres-
1. PhMe, TEA
O
O
2. i-PrCHO
O ence of BBr3/Al2O3 (eq 65).152 BBr3 also can act as a bromi-
N S
+
S N
90%
nating agent for the conversion of alcohols to bromides. In this
Br
B
O
OtBu
O
case, tertiary alcohols are more reactive than secondary alcohols,
Br
CF3
F3C
and primary alcohols are the least reactive towards this reagent
14
(eq 66).153
OH O
59 86%
RCOOH + BBr3/Al2O3
(61) RCOBr (65)
OtBu
Br
CH2Cl2, 0 °C
ROH + BBr3 R-Br (66)
50 99%
It is reported further that the Diels-Alder reaction of cyclopen-
tadiene and 2-methacrolein is catalyzed by a chiral Lewis acid
prepared from BBr3 and a chiral phenolic amine to form the exo-
adduct in 96% yield and 96% ee (eq 62).149
1. Bhatt, M. V.; Kulkarni, S. U., Synthesis 1983, 249.
2. McOmie, J. F. W.; Watts, M. L.; West, D. E., Tetrahedron 1968, 24,
Ph
2289.
3. Meyers, A. I.; Nolen, R. L.; Collington, E. W.; Narwid, T. A.; Strickland,
Ph
N Br
R. C., J. Org. Chem. 1973, 38, 1974.
B
96%
O
4. (a) Benton, F. L.; Dillon, T. E., J. Am. Chem. Soc. 1942, 64, 1128.
+
+
O
(b) Manson, D. L.; Musgrave, O. C., J. Chem. Soc. 1963, 1011.
H
(c) McOmie, J. F. W.; Watts, M. L., Chem. Ind. (London) 1963, 1658.
(d) Blatchly, J. M.; Gardner, D. V.; McOmie, J. F. W.; Watts, M. L., J.
Chem. Soc. (C) 1968, 1545.
5. (a) Vlattas, I.; Harrison, I. T.; Tökés, L.; Fried, J. H.; Cross, A. D., J.
CHO
(62)
Org. Chem. 1968, 33, 4176. (b) Taub, D.; Girotra, N. N.; Hoffsommer,
Me
R. D.; Kuo, C. H.; Slates, H. L.; Weber, S.; Wendler, N. L., Tetrahedron
1968, 24, 2443.
96% ee
6. (a) Stetter, H.; Wulff, C., Chem. Ber. 1960, 93, 1366. (b) Locksley, H.
D.; Murray, I. G., J. Chem. Soc. (C) 1970, 392. (c) Bachelor, F. W.;
In Trost s formal synthesis of roseophilin, a diastereoselective
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an asymmetric boron reagent prepared in situ from BBr3 and the
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Chem. Soc. 1969, 91, 5675.
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Ph Ph
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C7H7O2SHN NHSO2C7H7
Soc. 1976, 98, 1612.
SnPh3
BBr3 14. (a) Grieco, P. A.; Hiroi, K.; Reap, J. J.; Noguez, J. A., J. Org. Chem.
H
+
1975, 40, 1450. (b) Grieco, P. A.; Reap, J. J.; Noguez, J. A., Synth.
80%
Commun. 1975, 5, 155.
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H
1411. (b) King, P. F.; Stroud, S. G., Tetrahedron Lett. 1985, 26, 1415.
OH
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(63)
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96% ee
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Avoid Skin Contact with All Reagents
12 BORON TRIBROMIDE
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A list of General Abbreviations appears on the front Endpapers