Study on the reactive transient α-λ3-iodanyl- acetophenone complex in the iodine(III)/PhI(I) catalytic cycle of iodobenzene-catalyzed α-acetoxylation reaction of acetophenone by electrospray ionization tandem mass spectrometry

Article abstract of DOI:10.1002/rcm.6145

RATIONALE Hypervalent iodine compounds are important and widely used oxidants in organic chemistry. In 2005, Ochiai reported the PhI-catalyzed α-acetoxylation reaction of acetophenone by the oxidation of PhI with m-chloroperbenzoic acid (m-CPBA) in acetic acid. However, until now, the most critical reactive α-λ³-iodine alkyl acetophenone intermediate (3) had not been isolated or directly detected. METHODS Electrospray ionization tandem mass spectrometry (ESI-MS/MS) was used to intercept and characterize the transient reactive α-λ³- iodine alkyl acetophenone intermediate in the reaction solution. RESULTS The trivalent iodine species was detected when PhI and m-CPBA in acetic acid were mixed, which indicated the facile oxidation of a catalytic amount of PhI(I) to the iodine(III) species by m-CPBA. Most importantly, 3·H⁺ was observed at m/z 383 from the reaction solution and this ion gave the protonated α-acetoxylation product 4·H⁺ at m/z 179 in MS/MS by an intramolecular reductive elimination of PhI. CONCLUSIONS These ESI-MS/MS studies showed the existence of the reactive α-λ³-iodine alkyl acetophenone intermediate 3 in the catalytic cycle. Moreover, the gas-phase reactivity of 3·H⁺ was consistent with the proposed solution-phase reactivity of the α-λ³-iodine alkyl acetophenone intermediate 3, thus confirming the reaction mechanism proposed by Ochiai. Copyright

Full text of DOI:10.1002/rcm.6145

Research Article
Received: 10 November 2011
Revised: 21 December 2011
Accepted: 22 December 2011
Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2012, 26, 616–620
wileyonlinelibrary.com) DOI: 10.1002/rcm.6145
(
3
Study on the reactive transient a-l -iodanyl-acetophenone
complex in the iodine(III)/PhI(I) catalytic cycle of iodobenzene-
catalyzed a-acetoxylation reaction of acetophenone by electrospray
ionization tandem mass spectrometry
Hao-Yang Wang*, Juan Zhou and Yin-Long Guo**
Shanghai Mass Spectrometry Center, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling
Road, Shanghai, 200032, P.R. China
RATIONALE: Hypervalent iodine compounds are important and widely used oxidants in organic chemistry. In 2005, Ochiai
reported the PhI-catalyzed a-acetoxylation reaction of acetophenone by the oxidation of PhI with m-chloroperbenzoic acid
3
(
m-CPBA) in acetic acid. However, until now, the most critical reactive a-l -iodine alkyl acetophenone intermediate (3) had
not been isolated or directly detected.
METHODS: Electrospray ionization tandem mass spectrometry (ESI-MS/MS) was used to intercept and characterize the
3
transient reactive a-l -iodine alkyl acetophenone intermediate in the reaction solution.
RESULTS: The trivalent iodine species was detected when PhI and m-CPBA in acetic acid were mixed, which indicated
+
the facile oxidation of a catalytic amount of PhI(I) to the iodine(III) species by m-CPBA. Most importantly, 3ÁH was
+
observed at m/z 383 from the reaction solution and this ion gave the protonated a-acetoxylation product 4ÁH at m/z
1
79 in MS/MS by an intramolecular reductive elimination of PhI.
3
CONCLUSIONS: These ESI-MS/MS studies showed the existence of the reactive a-l -iodine alkyl acetophenone
+
intermediate 3 in the catalytic cycle. Moreover, the gas-phase reactivity of 3ÁH was consistent with the proposed
3
solution-phase reactivity of the a-l -iodine alkyl acetophenone intermediate 3, thus confirming the reaction
mechanism proposed by Ochiai. Copyright © 2012 John Wiley & Sons, Ltd.
Hypervalent iodine compounds are an important class of
reagents and they are efficient alternatives to metal-based
oxidants and transition-metal catalysts in many organic trans-
of [hydroxy(mesyloxy)iodo]benzene (HMIB) and [hydroxy
[11]
(tosyloxy)iodo]benzene (HTIB), while Silva and co-workers
studied the disproportionation reaction of DIB in acetonitrile
[
1–7]
[12,13]
formations.
Some transformations can be carried out by
and methanol by high-resolution ESI-MS/(MS).
using hypervalent iodine compounds as catalysts or recyclable
reagents under mild conditions with high selectivity and good
yield. The most widely used trivalent and pentavalent aryl
In recent years, the catalytic hypervalent iodine oxidation
[
6,14–18]
reaction has been studied by using co-oxidants.
In
this present study the chemical transformation catalyzed
by the cycle of ArI(III)XY/ArI(I) (where X and Y are the
anionic ligands) in the presence of a co-oxidant has been
investigated by electrospray ionization tandem mass spec-
trometry (ESI-MS/MS). In 2005, Ochiai and co-workers first
reported the direct iodobenzene-catalyzed a-acetoxylation of
acetophenone by using m-chloroperbenzoic acid (m-CPBA) in
[5]
iodine compounds are shown in Scheme 1.
In the past decade, efforts have been made to study the
chemical reactivity and the oxidation mechanism of these
[1,2,8,9]
hypervalent aryl iodine compounds.
These studies
revealed the presence of some reactive hypervalent iodine
species in the reaction solution. In 1995, Sam et al. first used
electrospray ionization mass spectrometry (ESI-MS) to study
[19,20]
acetic acid as the co-oxidant.
The speculated step-wise
[
10]
of oxidation reaction of PhIO in methanol
and they
reaction mechanism is shown in Scheme 2: (1) PhI (I) was first
oxidized to diacetoxy iodobenzene (DIB) 2 by m-CPBA in
acetic acid; (2) acetophenone 1 was activated to its enol form–
in acid conditions; (3) the ligand substitution of an OAc
moiety on 2 with the enol form of 1 occurred to generate the
3
+
observed reactive l -iodine species: [PhIOMe] at m/z 235,
PhIO+PhI OMe] at m/z 455 and [PhI(OMe) +PhI OMe] at
+
+
[
2
m/z 501 in the ESI-MS spectra. In 1997, Richter et al. applied
NMR and UV-visible spectroscopy to characterize the active
+
+
3
species, [PhIOH] and [PhIOI (OH )Ph], in the aqueous phase
transient reactive a-l -iodine alkyl acetophenone intermediate
2
–
3
; and (4) finally the OAc moiety of 3 initiated an intramolecu-
lar S 2 substitution reaction to form the corresponding product
N
of a-acetoxy acetophenone by reductive elimination of PhI.
The most critical reaction intermediate, a-l -iodine alkyl
acetophenone (3), had not previously been isolated or directly
detected due to its relatively weak non-covalent bonding and
its short lifetime. ESI-MS/MS is a valuable tool for reaction
*
Correspondence to: H.-Y. Wang and Y.-L. Guo, Shanghai
Mass Spectrometry Center, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling
Road, Shanghai 200032, P.R. China.
3
E-mail: haoyangwang@mail.sioc.ac.cn; ylguo@mail.sioc.ac.cn
Rapid Commun. Mass Spectrom. 2012, 26, 616–620
Copyright © 2012 John Wiley & Sons, Ltd.

PhI-catalyzed a-acetoxylation reaction
H C
F3C
3
The reaction of m-CPBA with PhI in the presence of HOAc
m-CPBA (235 mg, 3 equiv), PhI (6 mL, 0.1 equiv) and HOAc
O
O
O
O
O
I
O
I
(
2 mL) were added together with stirring.
O
O
H C
3
F3C
DIB
BTI
Iodobenzene-catalyzed a-acetoxylation reaction of
acetophenone
(
diacetoxyiodo)benzene
(Bis(trifluoroacetoxy)iodo)benzene
O
O
m-CPBA (235 mg, 3 equiv) and HOAc (2 mL) were placed in a
dry Schlenk flask with stirring, then water (90 mL, 5 equiv),
acetophenone (50 mL, 1 equiv), PhI (6 mL, 0.1 equiv) and
O
O
I
I
AcO
BF
3
ÁEt
2
O (0.15 mL, 3 equiv) were added. These reactions were
O
OAc
[19]
OH
AcO
carried out at room temperature.
IBX
DMP
2
-iodobenzoic acid
Dess-martin periodinane
Mass spectrometry
Scheme 1. Hypervalent iodine compounds: DIB, BTI, IBX
and DMP.
The ESI-MS and subsequent MS/MS experiments were per-
formed on a Finnigan TSQ Quantum Access triple-quadrupole
™
mass spectrometer (Thermo Scientific, San Jose, CA, USA)
equipped with a standard ESI ion source. Nitrogen was used as
the sheath gas and auxiliary gas, and argon as the collision gas.
m-CPBA (2 equiv), BF -Et O (3 equiv)
3
2
O
O
ArI (0.1 equiv), H O (5 equiv)
2
OAc
–
6
Ph
Me
O
Ph
The basic ESI conditions were: vacuum, 2.8 Â 10 Torr; spray
o
AcOH, 25-30 C,24h, Ar
ꢀ
voltage, 3000 V; capillary temperature, 270 C; sheath gas
pressure, 20 arbitrary units; auxiliary gas pressure, 5 arbi-
trary units; while the selected collision energy depended on
the dissociation capability of the precursor ion. Data acquisi-
tion and analysis were carried out with the Xcalibur software
package (version 2.0, Thermo Scientific).
OAc
PhI
m-CPBA
Ar
4
O
OAc
I
+ AcOH
Ar
Ph
3
Study of reactions by ESI-MS/(MS)
OAc
m-CBA
A volume of 2 mL of the reaction solution at a certain reaction
time was diluted with 98 mL of acetonitrile, and the diluted
reaction solutions was injected into the ESI source at a flow
rate of 10 mL/min.
PhI
OAc
2
O
H+
OH
Ar Me
Ar
1
Scheme 2. The reaction mechanism of iodobenzene-catalyzed
a-acetoxylation of acetophenone proposed by Masahito Ochiai.
RESULTS AND DISCUSSION
ESI-MS/(MS) study of the reaction of m-CPBA with PhI in
HOAc
mechanism studies in solution, with the transient reaction inter-
mediates being transferred from solution to the gas phase.
Thus, the reactivity and structures of reactive intermediates
can then be further studied by MS/MS in order to investigate
Acetonitrile was selected as the solvent for diluting the highly
concentrated reaction solution in the ESI-MS/(MS) studies as
[13,27,28]
[21–26]
it is not easily oxidized by trivalent iodine compounds.
and explain the reaction mechanism.
The direct capture
and study of the a-l -iodine alkyl acetophenone intermediate
by mass spectrometry would provide valuable confirmation
3
In order to verify whether trivalent iodine compounds, such
as DIB 2, were generated by the oxidation of a catalytic amount
of PhI with m-CPBA, m-CPBA and PhI (2:1 equivalent) in acetic
acid were mixed. Signals were detected of the trivalent
3
of its existence in the reaction solution and help to understand
the reaction mechanism.
+
+
iodine-containing ions: PhIOH at m/z 221, [2–OAc] at m/z
2
+
6 4
63 and PhIOOC(m-ClC H ) at m/z 359 in the acetonitrile-
diluted reaction solution of PhI with m-CPBA and acetic
EXPERIMENTAL
acid (Fig. 1).
+
The first important intermediate 2 was detected as [2–OAc]
at m/z 263. In the MS/MS process, [2–OAc] at m/z 263 gave
Materials and chemicals
+
•
+
Acetonitrile (purity of HPLC grade) was purchased from
Dikma Technology Inc. (Richmond Hill, ON, Canada). Acetic
acid, iodobenzene (PhI), acetophenone, m-chloroperbenzoic
acid (m-CPBA) and BF
pharm Chemical Reagent Co. Ltd. (Shanghai, China) and
used without further treatment.
two main product ions: PhI at m/z 204 by loss of anÁOCOCH
3
+
3
radical and Ph at m/z 77 by neutral loss of IOCOCH (Fig. 2
and Scheme 3). This information showed that the initiating
step of the catalytic reaction was the oxidation of a catalytic
3
ÁEt
2
O were purchased from Sino-
2
amount of PhI(I) to PhI(OAc) (III) 2 by m-CPBA in acetic
acid (Scheme 2).
Rapid Commun. Mass Spectrom. 2012, 26, 616–620 Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/rcm

H.-Y. Wang, J. Zhou and Y.-L. Guo
Figure 1. ESI-MS spectrum of the products of the reaction of m-CPBA with PhI
in HOAc.
ESI-MS/(MS) study of the iodobenzene-catalyzed a-
acetoxylation reaction of acetophenone
The solution of PhI-catalyzed a-acetoxylation reaction was
then monitored by ESI-MS/(MS) and a signal was found
3
+
of protonated a-l -iodine alkyl acetophenone 3ÁH at m/z
83 together with signals of other trivalent iodines:
[
3
+
+
6 4
2–OAc] at m/z 263 and PhIOOC(m-ClC H ) at m/z 359.
The ESI-MS spectrum of the reaction solution at a reaction
+
time of 10 min is shown in Fig. 3. MS/MS of the 3ÁH ion
at m/z 383 was performed to assign its structure and probe
+
+
Figure 2. ESI-MS/MS spectrum of [2–OAc] at m/z 263 with
its gas-phase reactivity (Fig. 4), and this showed that 3ÁH
a collision energy (in the lab frame) of 10 eV.
could easily dissociate even without applying extra
collision energy (Fig. 4(a)). The proposed structure of
+
3
ÁH at m/z 383 is shown in Scheme 4 as a six-membered
ring hydrogen-bonding complex with a proton connecting
–
the O atom of the OAc group and the O atom of CO-Ph.
O
+
I +
I+
The proposed fragmentation pathways of 3ÁH at m/z 383
are shown in Scheme 4.
O
When the collision energy was 0 eV, the main dissociation
O
O
+
–
m/z 204
pathway of 3ÁH was the initiation by the OAc moiety of a
I+
O
gas-phase intramolecular S 2 substitution reaction to the
N
O
+
O
corresponding protonated a-acetoxy acetophenone 4ÁH at m/z
I+
179 by reductive elimination of a molecule of PhI (Fig. 4(a)).
m/z 263
+
+
O
The formation of 4ÁH at m/z 179 from 3ÁH at m/z 383
might proceed via a five-membered ring transition state
O
+
m/z 77
(Scheme 4). Interestingly, the gas-phase chemistries of 3ÁH
I
O
at m/z 383 were consistent with the proposed solution-phase
+
3
Scheme 3. The proposed dissociation pathways of [2–OAc]
at m/z 263.
reactivity of a-l -iodine alkyl acetophenone 3, which could
further undergo an intramolecular S 2 substitution reaction
N
Figure 3. ESI-MS spectrum of the products of the iodobenzene-catalyzed a-acet-
oxylation reaction of acetophenone.
wileyonlinelibrary.com/journal/rcm
Copyright © 2012 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2012, 26, 616–620

PhI-catalyzed a-acetoxylation reaction
+
of AcOH from 4ÁH at m/z 179 (Scheme 4). When the higher
+
collision energy of 5 eV was applied, the 3ÁH ion yielded
+
other two product ions (Fig. 4(b)): [2–OAc] at m/z 263 and
+
an ion at m/z 323. The [2–OAc] ion at m/z 263 could originated
+
3
from 3ÁH by the retro-formation of a-l -iodine alkyl aceto-
+[19]
phenone 3ÁH
by the coupling of acetophenone with [2–
+
OAc] at m/z 263 (Schemes 2 and 4). The product ion at m/z
+
3
(
23 was formed by neutral loss of HOAc from
Scheme 4).
3ÁH
CONCLUSIONS
The direct interception and unambiguous characterization
3
by ESI-MS/(MS) of the reactive a-l -iodine alkyl acetophe-
none intermediate in the catalytic cycle of the PhI-
catalyzed a-acetoxylation reaction provided valuable
evidence for the existence of these reactive species in
solution and enabled the reaction mechanism to be eluci-
dated. Interestingly, the MS/MS results revealed that the
+
intrinsic gas-phase reactivity of 3ÁH at m/z 383 is essential
3
in the product-formation step to the protonated a-acetoxy
Figure 4. ESI-MS/MS spectra of protonated a-l -iodine alkyl
+
+
acetophenone 4ÁH at 179. This occurred by the reductive
acetophenone, 3ÁH at m/z 383, with collision energies (in the
elimination of PhI via an intramolecular five-membered-ring
lab frame) of: (a) 0 eV and (b) 5 eV.
transition state, which was consistent with the proposed
3
reactivity of the a-l -iodine alkyl acetophenone intermediate
[
19]
to a-acetoxy acetophenone by loss of PhI.
The other
3. The information generated from this study confirmed the
+
[19]
product ion of 3ÁH at m/z 137 was generated by loss of
reaction mechanism proposed by Ochiai and co-workers.
+
CH
2
CO (ketene) from 4ÁH at m/z 179. The ion at m/z 119
Moreover, our results demonstrate two important driving
forces in such a reaction: (1) PhI is a good leaving group and
could be formed by loss of H
2
O from m/z 137 or by direct loss
Ph
I
O
Ph
O
O
H
+
Proposed structure of 3
·
at m/z 383
H
H
O
H
O
O
O+
O
O
Ph
Ph
Ph
Ph
-
AcOH
I
m/z 179
O
O
O+
Ph
Ph
-
PhI
Ph
O+
2
-CH CO
m/z 119
H m/z 383
H
H
O
O
Proposed
transition state
O+
-H O
2
OH
H
m/z 137
Ph
O
I
O+
Ph
Ph
Ph
I+
-
HOAc
Ph
O
m/z 323
O
H
O
I
Ph
Ph
O
H+
O
I
I+
+
Ph
OAc
3
·
at m/z 383
Ph
O
-
PhCOCH3 m/z 263
O+
H
O
Scheme 4. The proposed structure and the dissociation pathways of proto-
3
+
nated a-l -iodine alkyl acetophenone 3ÁH .
Rapid Commun. Mass Spectrom. 2012, 26, 616–620 Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/rcm

H.-Y. Wang, J. Zhou and Y.-L. Guo
3
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Acknowledgements
The authors acknowledge financial support from the SRF
for ROCS of SEM, NSFC (20875097, 20902104, 21072215,
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2
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Copyright © 2012 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2012, 26, 616–620