H2TPP organocatalysis in mild and highly regioselective ring opening of epoxides to halo alcohols by means of halogen elements

Article abstract of DOI:10.3390/molecules17055508

We found that elemental iodine and bromine are converted to trihalide nucleophiles (triiodine and tribromide anion, respectively) in the presence of catalytic amounts of meso-tetraphenylporphyrins (H₂TPP). Therefore a highly regioselective method for the synthesis of β-haloalcohols through direct ring opening of epoxides with elemental iodine and bromine in the presence of H2TPPs as new catalysts is described. At room temperature a series of epoxide derivatives were converted into the corresponding halohydrins resulting from an attack of trihalide species anion atoms at the less substituted carbon atom. This method occurs under neutral and mild conditions with high yields in various aprotic solvents, even when sensitive functional groups are present.

Full text of DOI:10.3390/molecules17055508

Molecules 2012, 17, 5508-5519; doi:10.3390/molecules17055508
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
H2TPP Organocatalysis in Mild and Highly Regioselective
Ring Opening of Epoxides to Halo Alcohols by Means of
Halogen Elements
Parviz Torabi 1,*, Javad Azizian 2 and Shahab Zomorodbakhsh 1
1
Department of Chemistry, Mahshahr Branch, Islamic Azad University, Mahshahr 63519, Iran
2
Department of Chemistry, Faculty of Science, Science and Research Branch,
Islamic Azad University, Tehran 11365, Iran
* Author to whom correspondence should be addressed; E-Mail: p.torabi@mahshahriau.ac.ir;
Tel.: +98-652-232-7070; Fax: +98-652-233-8586.
Received: 10 January 2012; in revised form: 6 April 2012 / Accepted: 12 April 2012 /
Published: 9 May 2012
Abstract: We found that elemental iodine and bromine are converted to trihalide
nucleophiles (triiodine and tribromide anion, respectively) in the presence of catalytic
amounts of meso-tetraphenylporphyrins (H2TPP). Therefore a highly regioselective method
for the synthesis of -haloalcohols through direct ring opening of epoxides with elemental
iodine and bromine in the presence of H2TPPs as new catalysts is described. At room
temperature a series of epoxide derivatives were converted into the corresponding
halohydrins resulting from an attack of trihalide species anion atoms at the less substituted
carbon atom. This method occurs under neutral and mild conditions with high yields in
various aprotic solvents, even when sensitive functional groups are present.
Keywords: oxirane; ring opening; nucleophilic addition; elemental halogen;
meso-tetraarylporphyrine; halohydrine
1. Introduction
Oxiranes are among the most versatile intermediates in organic synthesis, as they can be easily
prepared from a variety of other functional groups [1] and due to their ring strain and high reactivity,
their reactions with various nucleophiles lead to highly regio and stereoselective ring opened
products [2–4]. Vicinal halohydrins have found wide applications in organic transformations and in the

Molecules 2012, 17
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synthesis of marine natural products [5,6]. The availability of some epoxides in an optically active
form has enhanced their use as synthetic intermediates; a reaction sequence allows an impressive
access to a large variety of compounds in an optically active form [7,8]. However, their direct
conversion to halohydrins remains a reaction of considerable interest [9].
A variety of reagents are known to convert epoxides to halohydrins; the ring openings of
unsymmetrically substituted epoxides with Li2(NiBr4) [10], LiX-(Bmim)PF6 [11], haloborane
reagents [12], Br2/PPh3 [13], SmI2 [14], Ti(O-i-pr)4 [15], chlorosilanes [16], Lewis acids [3,17,18] and
BF3-Et2O [19] have been reported. In particular metal halides such as Li/Ti [2], Sn [20], P [13],
Cu [21], and Ni [10] easily induce epoxide-opening, in which the use of a stronger Lewis acid and a
metal ion in structure of catalyst often results in low yields of the halohydrins when other sensitive
functional groups are present [22]. However, in these approaches we encountered with some
limitations, such as the need for strong Lewis acid and protic media that certainly are unsuitable
conditions for complex epoxide compounds.
Recently, it has been found that epoxides can be converted into iodoalcohols and bromoalcohols by
elemental iodine and bromine, in the presence of some specific compounds such as Mn(II) salen
complexes [23], 2-phenyl-2-(2-pyridyl)imidazolidine [24], thiourea [25] and diamines [26] as efficient
catalysts. Among these catalysts, the Mn(II) salen complexes are more efficient and effective, but in
this method, the oxidation of metal(II) in complex catalyst is an important limitation and reduced the
activity of catalyst for next reusability.
We would like to describe herein that H2TPP’s are highly reactive catalysts for the cleavage of
epoxide rings to relative vicinal halohydrins in the presence of elemental iodine and bromine, more
efficiently and regioselectively and in high yield under mild conditions that are highly desirable. The
catalysts are easily recovered and can be reused several times.
2. Results and Discussion
In this study, the reaction of styrene oxide with iodine and bromine in the presence of some
derivatives of H2TPP as the catalyst were carried out (Scheme 1).
Scheme 1. Catalytic conversion of epoxides to halohydrins.
HO
R
Y
Y
O
Cat. (2a-e), r. t.
+
X2
aprotic solvent
R
X
3a -p
N
1a -h
Cat:
2a-e
H N
R of products
X=I X=Br
N H
R of epoxides
Ph
N
a
a
i
j
b
c
PhOCH2
b
c
k
l
m
n
p-Cl-PhOCH2
p-Me-PhOCH2
t-Bu-OCH2
3-Buten
Y
Y
d
e
d
e
f
Y:
H
CH3
a
b
c
f
OMe
i-pr
g
g
o
p
CH3
d
e
h
h
cyclohexene oxide
Cl

Molecules 2012, 17
5510
Derivatives of H2TPP and metal-TPP’s have been recognized as being among the most promising
catalysts for various reactions [27,28]. These compounds show wide applicability and are now used as
catalysts for a variety of regio and enantioselective reactions, such as CO2/epoxide coupling [29],
acetolysis, hydrolysis and alcoholysis [30]. In all of these transformations, the coordinated metal ion in
catalyst has a key role in the reaction process and this necessity causes some destruction of sensitive
functional groups. After a solution of styrene oxide and a catalyst in CH2Cl2 was stirred in room
temperature, a solution of elemental halogen in CH2Cl2 was added dropwise. The amount of the
catalyst was a 0.05 molar amount of the styrene oxide used. The reaction product was
2-halo-1-phenylethanol (3a, 3i), and the yield was determined by GC analysis (Table 1). In each case,
cleavage of the epoxide ring occurs and, upon thiosulphate workup, iodo- and bromoalcohol are
obtained. The catalysts are easily recovered and can be reused several times. Tetraphenylporphyrin
derivatives were prepared and metallated according to the literature [31,32].
Table 1. Addition of Iodine (1 mmol) and Bromine (1 mmol) to Styrene Oxide (1 mmol)
in the Presence of Various Catalysts in CH2Cl2 at 25 °C.
Iodination
Time /h
Bromination
Time /h
Entry
Catalyst
Yield a /%
Yield a /%
>95
1
2
3
2a
2b
2c
2d
2e
-
2.1
2.1
2.2
2.3
2.5
>95
90
87
88
76
0
1.7
1.7
1.8
2.1
2.0
1
>95
90
82
4
5
75
6 b
Several days
31 c
a
b
c
GC yield, based on epoxide; In the presence of excess of halogen [29]; The only one isomer,
2-bromo-2-phenyl-ethanol was formed.
To ascertain the scope and limitation of the present reaction, a wide range of structurally diverse
epoxides were subjected to cleavage by this method to produce the corresponding halohydrins. These
results are summarized in Table 2. For comparison, a number of methods for the conversion of
oxiranes to the corresponding halo alcohols are given in entries 10–14 (Table 2).
However, other factors can exert a controlling influence such as: (1) steric hindrance of the
epoxides (for example, compare in Table 2, entry 7 with entry 8); (2) the nature of the solvent; (3) the
rate of admixing the reagents; and (4) the order in which the reagents are combined. Each one can have
a pronounced effect on the observed ratio of -halohydrin isomers and the overall yield.
The order and rate in which the reagents are combined were found to exert a subtle influence on the
yield and regioselectivity in both bromohydrin and iodohydrin formation. However, if bromine is
added to the epoxide before the catalyst, two isomeric bromoalcohols are produced, but if the epoxide
is added to catalyst and then bromine is added dropwise over a period of time, only one isomer is
formed. Furthermore, the rapid addition of bromine reduced the regioselectivity.

Molecules 2012, 17
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Table 2. Reaction of various epoxides with elemental I2 and Br2 in the presence of catalyst 2a.
Yield a
/%
Entry
Epoxide (1a–h)
Conditions
Time/h
Product (s) (3a–p)
HO
O
1
I2, 2a, r.t., CH2Cl2
2.1
81
Ph
Ph
I
HO
O
PhO
2
3
4
5
6
״
״
״
״
״
3.9
4.3
4.5
6.5
4.6
80
81
82
78
82
PhO
I
OH
O
p-Cl-C6H4O
p-Me-C6H4O
p-Cl-C6H4O
p-Me-C6H4O
I
I
OH
O
I
O
O
HO
O
I
HO
O
O
O
7
9
״
5.8
3.5
2
77
72
83
I
HO
O
״
I
O
O
O
10 b
I2, r.t., acetone
Ph
Ph
OH
Br
[n-Bu4N]Br/Mg(NO3)2,
CHCl3
78
(5:1)
75
11 c
12 d
13e
14 f
15
״
5
12
OH
Br
ph
ph
(Me2N)2BBr/CH2Cl2,N2
atm.
״
״
(1:4.5)
I
I
SmI2 (2 eq.),
״
>5 min
1.3
93
OH
I
ph
OH
THF, −78 °C
87
״
NH4+X−/M+., CH3CN
OH
ph
ph
(1:2)
HO
Ph
״
Br2, 2a, r.t., CH2Cl2
1.7
91
Br
HO
O
PhO
16
17
״
״
2.0
2.4
84
PhO
Br
OH
O
p-Cl-C6H4O
p-Cl-C6H4O
82
Br

Molecules 2012, 17
5512
Table 2. Cont.
Conditions Time/h
Yield a
/%
Entry
Epoxide (1a–h)
Product (s) (3a–p)
OH
O
p-Me-C6H4O
p-Me-C6H4O
18
״
2.8
83
Br
Br
O
19
20
״
״
2.7
2.5
80
76
OH
O
O
O
Br
HO
O
O
21
״
3.5
78
Br
HO
O
22
״
2.2
73
Br
a Isolated products yields based on epoxide; b Ref. [29]; c Ref. [17]; d Ref. [33]; e Ref. [14]; f Ref. [34].
The results of the ring opening of styrene oxide in the presence of catalyst 2a in various solvents are
summarized in Table 3. The iodination and bromination reactions can cleanly proceed in
dichloromethane, while those performed in THF, DMSO, chloroform, diethylether and acetonitrile
lead to a lower yield of the -halohydrins. Thus, these reactions appeared to be heavily dependent on
the nature of the solvent.
Table 3. Halogenative Reaction of Styrene Oxide in the Presence of Catalyst 2a in Various
Solvents at 25 °C.
Entry
Solvent
Iodination
Bromination
Time /h Yield /%
Time /h Yield a /%
1
2
3
4
5
6
CH2Cl2
CHCl3
CH3CN
DMSO
2.1
2.2
2.8
3.2
3.5
3.5
>95
93
90
85
83
1.7
2.0
2.5
3.0
3.2
3.2
>95
95
91
88
85
85
THF
Diethyl ether
83
a GC Yield.
As shown in Table 2, an anti Markovnikov-type regioselectively [34] is generally observed in these
reactions. An attack of the nucleophile preferentially occurs at the less-substituted oxirane carbon atom
that this type of regioselectively appears to be the opposite of that observed in ring opening of the
same epoxides with aqueous hydrogen halides under classic acidic conditions [29] (entry 13, Table 2).
The cyclic epoxides (entries 5, 18) always produced trans-halohydrins as indicated by the observed
coupling constants of the ring of the hydrogens in their 1H-NMR spectra. When catalyst is not present,
the cleavage of epoxides can occur via two limiting mechanistic pathways, either an electrophilic
attack by halogen, behaving as a Lewis acid, giving the more-stable carbenium ion-like transition state

Molecules 2012, 17
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a, or via nucleophilic attack by a halide ion on the epoxide-halogen complex, giving the more stable
transition state b (Scheme 2). However, this new method appears to be highly competitive with the
other methods reported in the literature. The reaction occurs under neutral and mild conditions on the
acid sensitive substrates and vicinal halohydrins were obtained in high yields and with high
regioselectivity.
Scheme 2. Tow possible way for nucleophilic ring opening.
X+
O
(a)
-
+
X2
O+
R
O
X
R
R
X+
O
(b)
+
R
X
Based on our study on the complexation of porphyrins and other works reported on the different
compounds with elemental halogen [23,24,26], it seems that halogenative cleavage of epoxides occurs
−
via trihalide ion, X3 as the nucleophile. In support of this suggestion, the electronic absorption spectra
of catalyst (1), iodine (2) and complex formation between iodine and bromine in the presence of 2a as
catalyst (3) in dichloromethane solution at 25 °C are shown in Figures 1 and 2.
Figure 1. Absorption Spectra of: (1) Catalyst 2a; (2) Iodine (3) Catalyst 2a:I2 with Molar
ratio 0.2:1 in Dichloromethane Solution.

Molecules 2012, 17
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Figure 2. Absorption Spectra of: (1) Catalyst 2a; (2) Bromine (3) Catalyst 2a:Br2 with
Molar ratio 0.2:1 in Dichloromethane Solution.
The electronic absorption spectra of the related addition of H2TPP to iodine and bromine has shown
strong absorption band at 365 nm for I2 addition and 272 nm for Br2 addition (Figures 1 and 2)
respectively, presumably due to the complex formation of this ligand with I2 and Br2. It should be
−
−
noted that the bands of 364 nm and 272 nm are characteristic of the formation of I3 and Br3 ions,
respectively [35,36], in the process of complex formation of different electron-pair donor ligands with
iodine and bromine, while none of the initial reactants show any measurable absorption in these
regions. Thus we suggested a four-step mechanism for halogenative cleavage of epoxides in the
presence of catalytic amounts of porphyrin:
Scheme 3. A four-step mechanism for halogenative cleavage of epoxide.
−
H2TPP + 2X2 → (H2TPP ····X+) X3
(1)
(2)
(H2TPP ····X+) X3 → (H2TPP ····X)+ + X3
−
−
O
O
(3)
−
R
R
X
X3 +
→
+ X2
X+
O
R
O
(4)
(H2TPP ····X+) +
→
+ H2TPP
X
X
R
The first step [Equation (1), Scheme 3] involves the formation of a 1:2 or 1:1 molecular complex
−
between the catalyst and elemental halogen, in which the halogen ion (X3 ) exists as a contact ion pair.
−
In the second step [Equation (2)] this complex is further decomposed to release the X3 nucleophile ion
into the solution. Therefore, in this way, molecular iodine or bromine is converted to a nucleophilic
halogen species in the presence of H2TPP and, in the third step Equations (3) and (4), this ion

Molecules 2012, 17
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participates in the ring opening reaction of the epoxides, and the catalyst is reproduced and is used in
the first step again. These steps occur continuously until all of the epoxides and halogen are consumed,
however, in each case, cleavage of the epoxide ring occurs, after work-up with thiosulfate the catalyst
can be easily recovered and could be reused several times. One of the advantage of H2TPP as catalyst
for promoted the elemental halogen in the ring opening of epoxides, is activation of nucleophile
without any metal ion, and therefore the activity of catalyst is constant, and also in reusability of the
catalyst, we observed a negligible decrease of the activity.
The reusability of catalyst was tested in the reaction of styrene oxide with I2 in the presence of the
catalytic amounts of H2TPP in CH2Cl2 media at room temperature (Table 4). After the reaction was
completed, the reaction mixture was worked up as mentioned in the Experimental section; the resultant
solid containing the catalyst was reused with a fresh charge of epoxide and iodine. H2TPP reused
catalyst exhibited good activity rather than that in its original use even after five reuses. Yields of
related iodohydrin in each process were decreased slowly because the amounts of the catalyst reused
after every run is trifling decreased.
Table 4. Reusability of Catalyst a.
Entry
Run
Time (h)
2.1
Yield (%) b
1
2
3
4
5
0
1
2
3
4
>95
91
2.1
2.1
88
2.1
82
2.1
77
a Addition of 1 mmol iodine to 1 mmol styrene oxide in the presence of the catalyst 2a in CH2Cl2 at
25 °C; b Based on GC yields.
3. Experimental
3.1. Materials
Chemicals were purchased from the Merck Chemical Company in high purity. All of the
materials were of commercial reagent grade. The epoxides and used solvents were purified by
standard procedures.
3.2. Apparatus
IR spectra were recorded as KBr pellets on a Perkin-Elmer 781 Spectrophotometer and an Impact
1
13
400 Nickolet FTIR Spectrophotometer (Tehran, Iran). H-NMR and C-NMR spectra were recorded
in d6-DMSO on a Bruker DRX-400 spectrometer (Tehran, Iran) for samples as indicated with
tetramethylsilane as internal reference. Mass spectra were recorded on a Finnigan MAT 44S (Tehran,
Iran), by Electron Ionization (EI) mode with an ionization voltage of 70 eV. Melting points were
obtained with a Yanagimoto micro melting point apparatus (Tehran, Iran) and are uncorrected. The
purity determination of the substrates and reactions monitoring by the solvent system were
accomplished by TLC on Polygram SILG/UV 254 silica-gel plates (from Merck Company, Tehran, Iran).

Molecules 2012, 17
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3.3. General Procedure for Conversion of Epoxides to β-Halohydrins
Epoxide (1 mmol) in CH2Cl2 (5 mL) was added to a stirred solution of catalyst (0.1 mmol) in
CH2Cl2 (5 mL) at room temperature. Then a solution of elemental halogen (1 mmol) in CH2Cl2 (5 mL)
was added dropwise (10 min) to the above-mentioned mixture. The progress of the reaction was
monitored by TLC analysis. After complete disappearance of the starting material, the reaction was
poured to the 10% aqueous Na2S2O3 (20 mL) and extracted with CH2Cl2 (3 × 10 mL). After the
evaporation of CH2Cl2 under vacuum, the organic phase has been dried on anhydrous sodium sulphate.
The product was purified by a short column chromatography through silicagel using CCl4/Et2O/EtOH
(3:1:1) as eluent (note: at room temperature the catalyst is nearly insoluble in Et2O/EtOH mixture.
Therefore, the catalyst has been washed easily from the column using CH2Cl2 as eluent). The halo
alcohols were identified by a comparison with authentic samples prepared in accordance with literature
procedures [15,23,26,29].
2-Iodo-1-phenylethanol (3a). IR (neat): 748 (m), 915 (m), 1032 (s), 1121 (w), 1243 (s), 1365 (m), 1492
(m), 1602 (s), 2885 (m), 2930 (s), 3061 (m), 3398 (br s) cm−1. 1H-NMR: δ = 2.02 (s, 1 H), 3.76 (d, 2 H,
J = 5.5 Hz), 4.78 (t, 1 H, J = 5.0 Hz), 7.17−7.35 (m, 5 H). 13C-NMR: δ = 54.96, 66.90, 128.22, 129.10,
129.21, 138.17.
1-Iodo-3-phenoxy-2-propanol (3b). IR (neat): 650 (w), 678 (w), 760 (m), 823 (m), 1038 (s), 1113 (w),
1
1240 (s), 1375 (m), 1494 (s), 1588 (s), 2877 (m), 2927 (s), 3050 (m), 3418 (br s) cm−1. H-NMR:
δ = 3.1 (s, 1 H), 3.48 (d, 2 H, J = 5.0 Hz), 4.06 (tt, 1 H, J1 = 7.0, J2 = 5.0 Hz), 4.13 (d, 2 H,
13
J = 5.6 Hz), 6.78−6.90 (m, 3 H), 7.36 (m, 2 H). C-NMR: δ = 67.18, 69.67, 70.01, 114.98, 116.87,
121.79, 129.89, 132.86.
1-Iodo-2-octanol (3f). IR (neat): 725 (m), 1015 (br s), 1105 (m), 1130 (m), 1185 (s), 1385 (s), 1425 (s),
1
1465 (s), 1475 (s), 2870 (vs), 2940 (vs), 3400 (br s) cm−1. H-NMR: δ = 0.89 (t, 3 H, J = 7.0 Hz),
13
1.26−1.58 (m, 10 H), 2.24 (s, 1 H), 3.24−3.55 (m, 3 H). C-NMR: δ = 14.09, 16.45, 22.62, 25.56,
29.12, 31.70, 36.89, 70.91.
2-Iodocyclohexanol (3h). IR (neat): 690 (s), 790 (w), 870 (m), 948 (s), 1038 (w), 1082 (br s), 1123 (m),
1189 (s), 1372 (m), 1462 (s), 2882 (s), 2960 (br s), 3425 (br s) cm−1. 1H-NMR: δ = 1.26−1.44 (m, 3 H),
1.75−1.95 (m, 3 H), 2.15−2.3 (m, 1 H), 2.3−2.35 (m, 1 H), 2.72 (s, 1 H), 3.58−3.62 (m, 1 H), 3.9−4.0
(m, 1 H). 13C-NMR: δ = 24.51, 26.56, 32.75, 35.40, 59.84, 71, 59.
2-Bromo-1-phenylethanol (3i). IR (neat): 689 (m), 766 (m), 823 (m), 1036 (s), 1115 (w), 1233 (s),
1
1375 (m), 1494 (m), 1600 (s), 2875 (m), 2935 (s), 3064 (m), 3405 (br s) cm−1. H-NMR: δ = 1.98 (s,
1 H), 4.01 (m, 2 H), 4.98 (t, 1 H, J = 5.0 Hz), 7.19−7.39 (m, 5 H). 13C-NMR: δ = 57.39, 67.97, 128.32,
129.30, 129.37, 138.98.
1-Bromo-3-phenoxy-2-propanol (3j). IR (neat): 641 (w), 688 (m), 756 (m), 823 (m), 1038 (s), 1112 (w),
1
1239 (s), 1375 (m), 1494 (s), 1588 (s), 2878 (m), 2925 (s), 3059 (m), 3415 (br s) cm−1. H-NMR:
δ = 2.75 (s, 1 H), 3.61 (d, 2 H, J = 5.3 Hz), 4.03 (tt, 1 H, J1 = 7.1 Hz, J2 = 5.0 Hz), 4.11 (d, 2 H,

Molecules 2012, 17
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J = 7.0 Hz), 6.78 (d, 1 H, J = 5.0 Hz), 6.94 (d, 2 H, J = 8.0 Hz), 7.35 (m, 2 H). 13C-NMR: δ = 69.58,
69.77, 69.93, 115.01, 116.82, 121.86, 129.99, 132.79.
1-Bromo-2-octanol (3n). IR (neat): 720 (m), 830 (m), 1050 (s), 1075 (s), 1125 (m), 1225 (m), 1265 (m),
1385 (m), 1425 (m), 1470 (s), 2860 (vs), 2935 (vs), 2970 (vs) 3380 (br s) cm−1. 1H-NMR: δ = 0.89 (t,
3 H, J = 6.5 Hz), 1.25−1.63 (m, 8 H), 1.86 (q, 2 H, J = 7.1 Hz), 2.22 (s, 1 H), 3.42 (d, 2 H, J = 7.1 Hz),
3.75−3.84 (m, 1 H). 13C-NMR: δ= 14.01, 22.52, 25.58, 29.14, 31.68, 35.05, 40.73, 71.02.
2-Bromocyclohexanol (3p). IR (neat): 690 (s), 793 (w), 865 (m), 960 (s), 1038 (m), 1075 (br s), 1123
1
(m), 1189 (s), 1372 (m), 1460 (s), 2882 (s), 2960 (br s), 3425 (br s) cm−1. H-NMR: δ = 1.26−1.42
(m, 3 H), 1.78−1.98 (m, 3 H), 2.18−2.32 (m, 1 H), 2.32−2.38 (m, 1 H), 2.68 (s, 1 H), 3.58−3.64
(m, 1 H), 3.82−3.92 (m, 1 H). 13C-NMR: δ = 24.48, 27.02, 33.95, 36.59, 62.13, 75.66.
4. Conclusions
In conclusion, we have found that epoxides are cleaved regioselectively to vicinal haloalcohols
under neutral conditions by elemental halogens in the presence of meso-tetraphenylporphyrins
as catalyst. The products are obtained in high yields and after short reaction times relative to
other procedures; fiurthermore, this methodology can be applied for acid sensitive substrates in
aprotic solvents.
Acknowledgments
We gratefully acknowledge the support for this work by the Islamic Azad University of Mahshahr
branch research council.
References and Notes
1. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 2nd ed.;
McGraw-Hill: New York, NY, USA, 1977; p. 1328.
2. Shimizu, M.; Yoshida, A.; Fujisawa, T. Regioselective conversion of epoxides to halohydrins by
titanium-(IV) halide-lithium halide complex. Synlett 1992, 23, 204–206.
3. Bonini, C.; Righi, G. Regio- and chemoselective synthesis of halohydrins by cleavage of oxiranes
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Sample Availability: Samples of the H2TPP compounds 2a−e are available from the author.
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