Oxidative cleavage and rearrangement of aryl epoxides using iodosylbenzene: On criegee's trail

Article abstract of DOI:10.1002/hlca.201200444

Aryl epoxides undergo rearrangement and oxidative cleavage when reacted with in situ prepared hydroxy-λ³-iodane complexes. The presence of H₂O plays a decisive role in steering the reaction path. A mechanistic scheme is proposed that accounts for the observed chemoselectivities. Copyright

Full text of DOI:10.1002/hlca.201200444

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Helvetica Chimica Acta – Vol. 95 (2012)
Oxidative Cleavage and Rearrangement of Aryl Epoxides Using
Iodosylbenzene: on Criegeeꢀs Trail
by Nizam Havare and Dietmar A. Plattner*
Institut fꢀr Organische Chemie und Biochemie, Albert-Ludwigs-Universitꢁt Freiburg, Albertstraße 21,
D-79104, Freiburg im Breisgau
(phone: þ 49-761-2036013; fax: þ 49-761-2038714; e-mail: dietmar.plattner@chemie.uni-freiburg.de)
To D. S. – with admiration and gratitude
Aryl epoxides undergo rearrangement and oxidative cleavage when reacted with in situ prepared
hydroxy-l3-iodane complexes. The presence of H2O plays a decisive role in steering the reaction path. A
mechanistic scheme is proposed that accounts for the observed chemoselectivities.
Introduction. – Iodosylbenzene (PhIO) is a pale-yellow amorphous powder, and
practically insoluble in inert solvents. Iodosylbenzene reacts with several reagents to
form unstable hydroxy-l3-iodane complexes. They decompose with different half-lives
resulting in black tar and/or PhI. The decomposition rate is highly dependent on the
bond lengths ꢂaꢃ and ꢂbꢃ (Fig.) [1 – 3].
Figure. Hydroxy-l3-iodane complexes
Oxidative CꢀO and CꢀC cleavage reactions of epoxides with various oxidizing
agents have long been established and are used widely in industrial processes. Some of
the commercial reagents in use for this purpose are: NaIO4 [4], HIO4 [5], LiClO4 [6], or
oxo-metal compounds in high oxidation states: RuCl3/K2SO5 [7], OsO4 [8], WO3/H2O2
[9], etc. The stereo-, regio-, and enantioselective aspects of the oxidative cleavage of
epoxides were widely studied, too [10].
Hypervalent iodine reagents have been in use for oxidation, rearrangement, and
fragmentation reactions of organic substrates. Historically, the first application of a
hypervalent iodine reagents for the oxidative cleavage of 1,2-diols was developed by
Criegee and Beuker. They showed that 1,2-diols react with (diacetoxyiodo)benzene and
AcOH in benzene to yield the corresponding aldehydes [11]. Later, Chen et al. found
that 1,2-diols undergo the same reaction with (polystyrene)ꢀI(OAc)2 in CH2Cl2 at
room temperature. Protecting groups such as AcO, RO, BnO, BzO, and acetonides do
not react under these reaction conditions [12]. The oxidative cleavage of olefins to the
corresponding carbonyl compounds using complex 3 [2a] and the rearrangement of
ꢄ 2012 Verlag Helvetica Chimica Acta AG, Zꢀrich

Helvetica Chimica Acta – Vol. 95 (2012)
2037
olefins to the corresponding aldehyde using [hydroxy(tosyloxy)iodo]benzene (4;
commonly referred to as the Koser reagent) were also described [13].
Results and Discussion. – Derivatives of (diacetoxyiodo)benzene such as I,I-
bis(trifluoracetoxy)iodobenzene were already employed for the reaction with epoxides
by Spyroudis and Varvoglis [14]. They demonstrated for the first time that addition of
I,I-bis(trifluoracetoxy)iodobenzene led to oxidative CꢀO and CꢀC cleavage and/or
rearrangement reactions. However, the yield of the products was just moderate-to-fair,
and it turned out that the different reaction pathways were difficult to control.
Here, we describe the oxidative cleavage and rearrangement reactions of alkyl- and/
or aryl-substituted substrates using PhIO and HBF4 (in H2O or in Et2O; Table 1). We
found that, by using PhIO, CꢀC cleavage or a rearrangement reaction can be selectively
achieved by setting the reaction conditions. Independent from the nature of the
substrate, the CꢀC cleavage reaction of epoxides in the presence of H2O is faster
compared to the rearrangement reaction (Entries 5 and 6, and Entries 9 and 10,
Table 1). When the substituents have a low migratory aptitude (e.g., Me group;
Entries 4 – 6), the reaction proceeds via CꢀC oxidative cleavage, even when no H2O is
present. The rearrangement reaction proceeds when the substrate has substituent(s)
with a reasonable migratory aptitude (e.g., H or aryl group) under anhydrous reaction
conditions (Entries 8 and 10, Table 1).
We assume that the CꢀC cleavage reaction of epoxides is catalyzed by H2O added
to or formed during the reaction. The pathway to the rearranged products can be
chosen by setting anhydrous reaction conditions (molecular sieves (4 ꢅ), HBF4 · OEt2
50 – 55% in Et2O; Entries 8 and 10, Table 1). The results compiled in Tables 2 and 3
(substrates trans-stilbene oxide and styrene oxide, resp.) show that the yields of
rearranged and oxidative CꢀC cleavage products are strongly dependent on the
reaction conditions. The yields of PhCHO under anhydrous reaction conditions (using
molecular sieves (4 ꢅ) are much lower compared to conditions without molecular
sieves (Entries 1 – 4, Table 2). The CꢀC cleavage product is the major product if
aqueous HBF4 (48% in H2O) is the reagent (Entry 7, Table 2).
Whether styrene oxide reacts under oxidative CꢀC cleavage or H rearrangement to
PhCHO cannot be deduced from the substrates listed in Table 3 (Entries 1 and 2). Thus,
we decided to conduct the reaction with the 2H-substituted analog rac-(b,b-2H2)styrene
oxide as substrate (Scheme 1). Under H2O-free conditions (using molecular sieves
(4 ꢅ), the ratio PhC(O)2H (rearrangement product)/PhC(O)H (product of CꢀC
cleavage) turned out to be 2 :1. Bearing in mind that 2H-rearrangement is discriminated
against H rearrangement by a kinetic isotope effect, and that neither the degree of
deuteration of the substrate nor exclusion of H2O will be 100% complete, it is safe to
declare that styrene oxide reacts predominantly under rearrangement.
In the light of these results and precedence for reactions using similar reagents [14],
we propose the reaction mechanism outlined in Scheme 21). Complex 1, prepared in
1
)
Further corroboration for this mechanistic scheme comes from an experiment with rac-(b,b-
2H2)styrene oxide in the absence of molecular sieves (4 ꢅ). Because of the H2O formed during the
reaction, CꢀC cleavage becomes the dominant reaction leading to undeuterated PhCHO as the
major product.

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Helvetica Chimica Acta – Vol. 95 (2012)
Table 1. Oxidative Rearrangement and CꢀC Cleavage of Epoxides
Entry
1
Substrate
Reagent HBF4 Drop rate CH2Cl2 Time Product(s)
Yield
[%]
[ml/h]
[ml]
[h]
1
1
20
3
no reaction
2
2
–
30
4
PhCHO
65
3a)
4c)
1
1
3.5
2
50
12
7
1
PhCHO
PhCHO
54
16
5b)c)
1
5
20
3.5
PhCHO
62
6a)c)
2
1
–
10
20
24
4
PhCHO
86
60
7
2.5
MeC(O)Ph
8
1
5
20
3.5
Ph2CO, PhCHO 51, 3
9
2
1
–
3
20
30
1
3
PhCHO, Ph2CO 64, 24
Ph2CO, PhCHO 71, 27
10
a) The yield was determined by GC analysis with 1,4-dichlorobenzene or mesitylene as internal
standards. b) The reaction was carried out at ꢀ 408. c) Formation of MeCHO was not observed.
situ from PhIO and HBF4 · OEt2 (50 – 55% in Et2O), is first attacked by the epoxide
substrate to form the epoxide-l3-iodane 5, which is in equilibrium with the open-chain
carbocation 6. In the presence of H2O, the carbocation reacts to intermediate 7 which
then under CꢀC cleavage yields the corresponding carbonyl compounds, PhI, and H2O.
However, under anhydrous reaction conditions, the reaction proceeds under rear-
rangement of the R’ substituent (R’ ¼ H, aryl) to the intermediate 8 which, under
proton loss, gives rise to intermediates 9 and 10. The formation of a cyclic intermediate
similar to 10 was postulated earlier in [15]. The cyclic intermediate then undergoes
cleavage in a concerted fashion to the carbonyl product, HCHO, and PhI. Form-
1
aldehyde is short-lived and could not be detected by H-NMR spectroscopy in our
experiments. We assume that HCHO is first oxidized to HCOOH, and further to CO

Helvetica Chimica Acta – Vol. 95 (2012)
2039
Table 2. Oxidative CꢀC Cleavage and Rearrangement Reaction of rac-trans-Stilbene Oxide
Entry Reagent CH2Cl2 [ml] HBF4 Drop rate [ml/h] Time
Conversion [%] Yield [%]a)
Ph2CO PhCHO
1
2
3
4
5b)
6b)
7
1
1
1
1
1
1
2
40
100
100
30
30
30
3
6
7.2
3
–
7 h
8 h
7 h
3 h
100
57
84
36
46
46
71
44
50
24
13
11
14
13
6
100
10 min 100
3 h 95
10 min 100
3
–
2
64
30
a) Yields were determined by GC analysis with mesitylene as internal standart. b) With molecular sieves
(4 ꢅ).
Table 3. Oxidative CꢀC Cleavage and Rearrangement Reaction of rac-Styrene Oxide
Entry
Reagent
CH2Cl2
[ml]
HBF4 Drop rate
[ml/h]
Time
[h]
Conversion
[%]
Yield of PhCHO
[%]a)
1b)
2b)
3
1
1
2
40
40
20
3
3
–
7
7
4
100
100
100
19
54
65
a) Yields were determined by GC analysis with mesitylene as internal standard. b) With molecular sieves
(4 ꢅ).
Scheme 1. The Reaction of rac-(b,b-2H2)Styrene Oxide with PhIO and HBF4 · OEt2 (50 – 55% in Et2O)
and CO2. The formation of CO and CO2 was detected by PdCl2/HCl reagent [16] and by
saturated Ba(OH)2 [17], respectively. Independent experiments with HCHO or
HCOOH confirmed our assumption that these are converted rapidly to CO and CO2
under these reaction conditions.
In summary, we have developed a protocol that allows selective CꢀC cleavage or
rearrangement to carbonyl compounds of aryl epoxides.

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Helvetica Chimica Acta – Vol. 95 (2012)
Scheme 2. Proposed Reaction Mechanism for Oxidative CꢀC Cleavage and Rearrangement Reactions
Experimental Part
General. All used substrate epoxides were prepared according to published procedures [18]. PhIO
was prepared from (diacetoxyiodo)benzene according to [19]. (Diacetoxyiodo)benzene, rac-trans-
stilbene oxide, (a,a,a-2H3)acetophenone, and solvents were obtained commercially from ABCR, Sigma-
Aldrich, Fluka, Alfa Aesar, and Acros. CH2Cl2 was dried over K2CO3, freshly distilled, and stored under
Ar prior to use. TLC: Silica gel 60 F254 25 aluminium sheets, 20 ꢁ 20 cm, from Merck KGaA Co., D-
Darmstadt. 1H- and 13C-NMR spectra: Varian 300 and Bruker 500 spectrometer; d in ppm rel. to Me4Si as
internal standard, J in Hz. GC/MS: Thermo TSQ 700; GC (Varian 3400, achiral GC column Macherey-
Nagel optima 5MS, 30 m ꢁ 0.25 mm, 0.25 mm film); in m/z.
General Procedure for the Rearrangement Reaction of Epoxides (Table 1, Entries 1, 3 – 5, 7, 8, and 10;
Table 2, Entries 1 – 6; Table 3, Entries 1 and 2). In a 100-ml two-necked flask equipped with two bubble
counters, which are connected and filled with the CO2 detection reagent (Ba(OH)2) and with the CO
detection reagent (PdCl2/HCl in H2O), resp., epoxide (4.1 mmol, 1 equiv.) was dissolved in abs. CH2Cl2
(15 ml) under Ar. If necessary, 100 ml of mesitylene or 100 mg of 1,4-dichlorobenzene as internal
standards were added (Table 1, Entries 3 and 10; Table 2, Entries 1 – 6; Table 3, Entries 1 and 2).
Iodosylbenzene (1.9 g, 8.63 mmol, 2.1 equiv.) and ca. 5 g of molecular sieves (4 ꢅ) were added (except for
Entry 3, Table 1). Under stirring, 780 ml (2.89 mmol, 0.7 equiv., 50 – 55% in Et2O) of HBF4 · OEt2, diluted
with abs. CH2Cl2 to the total volume given in Tables 1 – 3, was added dropwise to the suspension using a
syringe pump. During the reaction, formation of BaCO3 and Pd was observed in the bubble counters.
After the reaction time given in Tables 1 – 3, the mixture was first washed with sat. NaHCO3 soln. and
extracted twice with Et2O. The org. phase was then washed with sat. Na2S2O3 soln., dried (Na2SO4), and
filtered. The solvent was removed under reduced pressure. In the case of Entries 4 and 5 (Table 1), the
product was isolated by column chromatography (CC; cyclohexane/AcOEt). In the case of Entry 8
(Table 1), the products were derivatized to hydrazones and isolated by CC (cyclohexane/AcOEt).
The spectroscopic data obtained for benzophenone, PhCHO, and acetophenone matched the
published data [20].
General Procedure for CꢀC Oxidative Cleavage Reaction of Epoxides (Table 1, Entries 2, 6, and 9;
Table 2, Entry 7; Table 3, Entry 3). In a two-necked flask equipped with two bubble counters, which are
connected and filled with the CO2 detection reagent (Ba(OH)2) and with the CO detection reagent

Helvetica Chimica Acta – Vol. 95 (2012)
2041
(PdCl2/HCl in H2O), the epoxide (4 mmol, 1 equiv.) was dissolved in 20 ml of abs. CH2Cl2 .
Iodosylbenzene (1.32 g, 6 mmol, 1.5 equiv.) was added. In the case of Entry 6 (Table 1), 100 ml of
mesitylene as internal standard was added to the suspension. Under stirring, 630 ml of HBF4 (4.82 mmol,
1.2 equiv., 48% in H2O) was added dropwise within 10 min (TLC control). No precipitation was
observed in the bubble counters. After the reaction time indicated in Tables 1 – 3, the reaction mixture
was worked up depending on the substrate as follows.
Isolation of Products by CC (Table 2a, Entry 7). The reaction mixture was first washed with sat.
NaHCO3 soln. and extracted twice with Et2O. The org. phase was dried (Na2SO4) and filtered. The
solvent was removed under reduced pressure. The products were isolated by CC (cyclohexane/AcOEt).
Isolation of Product as Hydrazone Derivative (Table 1, Entries 2 and 9; Table 2b, Entry 3). The
reaction mixture was slowly concentrated to a total volume of ca. 4 – 5 ml. 2,4-Dinitrophenylhydrazine
soln. was added, whereby a precipitate formed which was filtered and dried. In the case of Entry 9
(Table 1), the products were isolated by CC (cyclohexane/AcOEt). Preparation of 2,4-dinitrophenylhy-
drazine soln.: 2.26 g of 2,4-dinitrophenylhydrazine (8 mmol, 2 equiv., 70%), 8.7 ml of conc. H2SO4, and
13 ml of H2O were mixed. To the orange-colored soln., 42 ml of tech. EtOH was added. The soln. was
then stirred for 10 min at r.t.
Benzaldehyde 2-(2,4-Dinitrophenyl)hydrazone. 1H-NMR (300 MHz, (D6)DMSO): 11.7 (br. s,
CHNNH); 8.88 (d, J ¼ 2.7, 1 arom. H); 8.73 (s, CHNNH); 8.43 – 8.36 (dd, J ¼ 2.70, 9.70, 1 arom. H);
8.15 – 8.10 (d, J ¼ 9.70, 1 arom. H); 7.85 – 7.75 (dd, J ¼ 4.2, 2.06, 2 arom. H); 7.54 – 7.46 (m, 3 arom. H).
Benzophenone 2-(2,4-Dinitrophenyl)hydrazone. 1H-NMR (300 MHz, (D6)DMSO): 11.56 (s,
CHNNH); 8.88 (d, J ¼ 2.7, 1 H of (NO2)2C6H3); 8.66 (d, J ¼ 7.3, CHNNH); 8.41 – 8.35 (dd, J ¼ 2.70,
9.70, 1 H of (NO2) C6H3); 7.86 (d, J ¼ 9.70, 1 arom. H); 7.46 – 7.27 (m, 10 arom. H).
2
Synthesis of rac-1-Phenyl(2,2,2-2H3)ethan(2H)ol. LiAlH4 (3.39 g, 89.4 mmol, 1.1 equiv.) was sus-
pended in 40 ml of abs. THF under Ar. The suspension was cooled in an ice bath, and 10 g of (a,a,a-
2H3)acetophenone (81.3 mmol, 1 equiv.) in 40 ml of THF were added dropwise. The mixture was stirred
for 2 h at r.t. (TLC control). Subsequently, cooled D2O was added dropwise to the mixture cooled in an
ice bath. The mixture was then diluted with DCl soln. (35% in D2O) until dissolution of the precipitated
salt, and extracted several times with Et2O (abs.). The org. phase was dried (Na2SO4) and filtered. The
solvent was removed under reduced pressure. Yield: 98%. 1H-NMR (300 MHz, CDCl3): 7.40 – 7.28 (m, 5
arom. H); 4.92 (s, PhCHODCD3).
Synthesis of (b,b-2H2)Styrene. In a 10-ml two-necked flask, equipped with a microdistillation
apparatus and a dropping funnel, a pressure of 60 mbar was adjusted. KHSO4 (370 mg, 2.72 mmol,
0.17 equiv.) was heated to 1808. A mixture of 2.00 g of rac-1-phenyl(2,2,2-2H3)ethan(2H)ol (16 mmol,
1 equiv.) and of 5 mg of p-(tert-butyl)catechol (0.03 mmol) was then added dropwise to the heated
KHSO4 slowly, and the product formed was distilled fractionally. Yield: 59%. 1H-NMR (300 MHz,
CDCl3): 7.46 – 7.22 (m, 5 arom. H); 6.72 (s, PhCHCD2).
Synthesis of rac-(b,b-2H2)Styrene Oxide. rac-(b,b-2H2)Styrene (376 mg, 3.5 mmol, 1 equiv.) was
dissolved in 20 ml of CCl4, and the soln. was cooled in an ice bath. meta-Chloroperbenzoic acid (m-
CPBA; 1.3 g, 5.3 mmol, 1.5 equiv., 70 – 75%) was added portionwise over 3 h to the soln. The mixture was
then slowly warmed to r.t. and stirred for 1 h (TLC control). m-CBA and unreacted m-CPBA were
removed by washing the soln. with NaHCO3 soln. and four times with 1m NaOH soln., resp. The org.
phase was extracted with Et2O, dried (Na2SO4), and filtered. The solvent was removed under reduced
pressure. Yield: 94%. 1H-NMR (300 MHz, CDCl3): 7.39 – 7.24 (m, 5 arom. H); 3.86 (s, PhCH(O)CD2).
Oxidative Rearrangement and Cleavage of rac-(b,b-2H2)Styrene Oxide. rac-(b,b-2H2)Styrene oxide
(234 mg, 2.2 mmol, 1 equiv.) was dissolved in 10 ml of abs. CH2Cl2 under Ar. PhIO (622 mg, 2.82 mmol,
1.3 equiv.) and ca. 5 g of molecular sieves (4 ꢅ) were added to the soln. Under stirring, 738 ml (2.91 mmol,
1.3 equiv., 50 – 55% in Et2O) of HBF4 · OEt2, diluted with abs. CH2Cl2 to a total volume of 5 ml, was
added dropwise to the suspension using a syringe pump with a drop rate of 2.5 ml/h. The mixture was
stirred for another h and then washed with sat. NaHCO3 soln. The org. phase was washed with sat.
Na2S2O3 soln. and extracted twice with CH2Cl2, dried (MgSO4), and filtered. The products were isolated
by CC (petroleum benzine (30/50)/Et2O 30 :1). The ratio PhCHO/PhCDO was determined by 1H-NMR
spectroscopy as 1:2.

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Helvetica Chimica Acta – Vol. 95 (2012)
(2H1)Benzaldehyde (¼(formyl-2H)Benzaldehyde). 1H-NMR (300 MHz, CDCl3): 7.89 (d, 2 arom.
H); 7.67 – 7.46 (td, J ¼ 7.19, 1.39, 1 arom. H); 7.57 – 7.49 (t, J ¼ 7.30, 2 arom. H).
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Received August 10, 2012